Improved Weight Estimation for Vehicles with Air Suspensionuu.diva-portal.org/smash/get/diva2:1315123/FULLTEXT01.pdf · 2019-05-11 · An early pneumatic suspension for vehicles was

UPTEC F 19014

Examensarbete 30 hpMaj 2019

Improved Weight Estimation for Vehicles with Air Suspension

Erik Klavebäck

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Improved Weight Estimation for Vehicles with AirSuspension

Erik Klavebäck

The aim of this thesis is to explore and improve the accuracy ofweight estimation in vehicles with air suspension. The main focus hasbeen to understand the hysteresis and characteristics of differentsuspension types used in Scania NCG trucks. The thesis has led toinsights into the attributes affecting hysteresis and the accuracy ofweight estimation. As a proof of concept a software implementationhas been developed from the knowledge derived trough out the thesisto account for hysteresis. The result of this work has proven it tobe possible to increase the accuracy and reduce the error to lessthan half compared to today's method.

ISSN: 1401-5757, UPTEC F19 014Examinator: Thomas NygrenÄmnesgranskare: Hans RosthHandledare: Henrik Svensson

II

Sammanfattning

Att uppskatta vikten av ett fordon har stor betydelse för tung trafik som till exempellastbilar. Trafikförordningen reglerar både den tillåtna totalvikten av fordon på allmännavägar samt vikten på enskilda axlar på fordonet. Idag finns ett antal stationära vägn-ingsplatser längs vältrafikerade vägar som finns tillgängliga för kontroll av att ett fordonär korrekt lastat och inte bryter mot lagen. Det finns även andra sammanhang där ennoggrann uppskattning av vikten är intressant, exempelvis för att uppskatta nyttolasten.Det finns en stor efterfrågan för inbyggda funktioner i fordonen för att kunna uppskattavikten utan tillgång till en vägningsplats.

I luftfjädrade fordon kan vikten uppskattas med hjälp av att mäta lufttrycket i fjädringen,som sedan konverteras till en viktuppskattning. Noggrannheten av uppskattningen äridag sämre än vid stationära vägningsplatser och en förbättrad precision är efterfrågad.Examensarbetet har studerat och undersökt luftfjädrade fordon i syfte att utveckla en nymetod för att förbättra precisionen av den inbyggda viktuppskattningen hos fordon.

Lufttrycket i fjädringen beror ej enbart på vikten av fordonet, utan även hur fjädringenhar rört sig innan tryckmätningen utförs. Rörelsen av fjädringen påverkas av eftersläpn-ing som kallas hysteres. Hysteres påverkar noggrannheten hos viktuppskattningen påså vis att en bestämd vikt kan orsaka olika tryck i fjädringen beroende på hur fjädernkomprimerats precis innan mätningen utförts.

Under arbetets gång har tester och mätningar utförts på lastbilar med olika typer avluftfjädringar, för att förstå hur de olika fjädringarna beter sig och för att identifieravilka komponenter som ger upphov till hysteres i systemet. Den samlade mätdatanhar legat till grund för utvecklingen av en ny metod för att uppskatta vikten. Metodenanvänder både trycket i luftfjädringen, samt chassihöjden för att uppskatta vikten påvarje luftfjädrad axel. Metoden tar hänsyn till hysteres genom att basera uppskattningendels på det nuvarande trycket och höjden, samt deras förändring över tid.

Resultatet av arbetet visar att den nya metoden kompenserar för hysteres och förbättrarviktuppskattningen jämfört med en metod enbart baserad på det momentana tryckethos fjädringen.

Uppsala University Erik Klavebäck

III

Acknowledgements

There are many people that have supported and helped me throughout this thesis. Iwould like to thank you all, especially my supervisor Henrik S. for being there for meevery day, supporting and guiding me throughout my work. Stefan S. who always madetime for my questions and discussions. Robert S. and Johan P. who have introducedme to the suspension and brought valuable insights to the thesis. Lennart and Mårtenwith their colleagues down in the workshop who has helped me throughout all of mytests. My subject reader Hans R. for his support and guiding. The group at ESCA formaking me a part of the team from day one. I would also want to thank my family fortheir support every step of the way.


Table of Contents IV

Table of Contents

Abstract II

Acknowledgements III

1 Introduction 11.1 Background and application of pneumatic suspension . . . . . . . . . . . 11.2 Air-Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Weight Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4 Axle Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.5 Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Outline of report 4

3 Theory 53.1 Previous research in hysteresis and air springs . . . . . . . . . . . . . . . 53.2 Suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2.1 Air Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2.2 Suspension Type A . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2.3 Suspension Type B . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.4 Suspension Type C . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.5 Bushings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.6 Shock Absorber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2.7 Pressure Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.2.8 Level Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 Electronic Level Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Testing 144.1 Testing Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2 Acquisition Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.3 Suspension Specific Testing . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.3.1 Suspension Type A . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.3.2 Suspension Type B . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3.3 Suspension Type C . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5 Implementation of New Method 19


Table of Contents V

6 Results 206.1 Current Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.2 Hysteresis Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6.2.1 Height dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . 226.3 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6.3.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276.3.2 Live test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

7 Discussion 327.1 Minimum Suspension Height . . . . . . . . . . . . . . . . . . . . . . . . . 327.2 Air spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327.3 Bushings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347.4 Shock absorbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347.5 Anti-roll Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347.6 Conclusion From Modifications . . . . . . . . . . . . . . . . . . . . . . . . 347.7 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7.7.1 Type A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.7.2 Type B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.7.3 Type C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.8 Sources of error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.8.1 Measurement data errors . . . . . . . . . . . . . . . . . . . . . . . 377.8.2 Data fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

8 Conclusion 408.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

List of Figures 42

Literature 45


Introduction 1

1 Introduction

1.1 Background and application of pneumatic suspension

An early pneumatic suspension for vehicles was patented as early as 1901 [1] in the USAand the notion of using air in springs goes back more than 50 years earlier. An earlypatent from 1847 describes a pneumatic spring for machinery [2]. Since the early 20thcentury, air springs have been used in an increasing number of applications and vehiclessuch as air crafts, buses, trucks and machinery. A simple type of pneumatic suspensionconsists of a flexible bellow filled with a pressurized gas that acts like a spring. Thesepneumatic springs can be made lighter than the traditional steel counterpart and thebellows can be designed in a way so that the length can be changed when inflated ordeflated. This is very useful for vehicles such as buses that can lower the chassis to allowpassengers to embark or disembark more easily, commonly called kneeling. Advantageswith inflating or deflating the air spring include keeping a constant height of the chassisduring load changes. When the load is increased the air spring is compressed, inflationof the air-spring can compensate and return the vehicle to nominal height. The same forunloading. Level control during load changes were traditionally managed by mechanicallink arms operating valves for inflation and deflation. Today it is electronically managedin modern vehicles and this functionality is referred to at Scania as ELC, Electronic LevelControl.

1.2 Air-Spring

The basic principle of an air spring is show in 1.1. The flexible bellow encloses pressur-ized air. The relative pressure to the surroundings gives rise to a force exerted on thewalls of the bellow like an inflated balloon. When the spring is compressed, the pressureincreases inside the bellow, the increased pressure will counteract the compressing forceand a force equilibrium is achieved at a stationary state.


Introduction 2

Figure 1.1: Principle of an air spring with flexible bellow filled with a pressurized gas inshaded blue with pressure P and external force F compressing it. Theeffective area A is approximately the cross section of the bellow.

1.3 Weight Estimation

The pressure inside the air spring is directly related to the force exerted by it as a spring,thus it is also related to the sprung weight supported by them compressing it. So,if the pressure is measured inside the air spring the weight could be estimated. Anestimation of weight is useful in many applications varying from total weight of thevehicle, brake force distribution to ensure consistent braking, cargo estimation, axleload verification, traction control, and even lowering fuel consumption by selectingoptimized gear shifting patterns based on the total weight of the vehicle. Some of theseapplications are safety oriented such as the brake force distribution. Others are for lawand regulations conformity such as legal axle load. For applications such as cargo weighttracking is desirable with a high degree of accuracy to increase profits.

1.4 Axle Load

The axle load of a vehicle is defined as the load transferred by each individual axleonto the road. This load is relevant in several aspects, especially in compliance withregulations and law. Since these rules change by region and country as well as at specificlocations such as particular roads that are designed to handle a certain load, it is usefulfor drivers to have an easy way of estimating the current axle load. This estimation isalso important to avoid overloading scenarios that potentially could damage the vehicleitself. Relying on road-side weighing often means a detour that is both time consumingand adds unnecessary driving, in some areas there might not even be scales availablewithin a reasonable distance. Thus, a built-in function in the vehicle that can offer an


Introduction 3

estimated axle load is useful. Today this functionality exists in some vehicles equippedwith air suspension, but it is unreliable compared to road-side scales and suffers fromhysteresis that causes large deviations.

1.5 Hysteresis

Hysteresis is the phenomenon that the state of the system not only depends on itslocation and the momentary force acting upon it, but the history of where it has beenpreviously. This appear in both natural systems and in engineered systems. In naturalsystems it can take the form of internal friction, where energy is dissipated internally ina solid material that is deformed, like a rubber band heating up when being stretched.In focus of this work, hysteresis and friction contribute to the phenomenon that onespecific pressure in the suspension can carry a wide range of loads depending on thehistory of the system. It is important to understand this behavior and evaluate methodsto eliminate the effect of hysteresis on the axle load estimation. Hysteresis is evidentwhen a vehicle is raised or lowered while carrying a certain load, the end pressure isdifferent if it is raised up to nominal drive level compared to if it is lowered down to it.During loading, hysteresis also affects the resulting pressure change depending on if theload has increased or decreased. Meaning that if the vehicle is loaded to half capacity orun-loaded to half capacity, the air spring pressure will differ due to hysteresis. Thus, aspecific pressure can carry a wide range of loads, and a specific load can be carried by awide range of pressures.

Figure 1.2: Illustrating hysteresis of an air spring with the possible system states S inshaded blue. Particular states depicted as dots with lines representing thehistory of the system to reach those states by loading or unloading thespring.


Outline of report 4

2 Outline of report

The report is divided into eight chapters, each accounting for different parts of the thesis.The chapter 1 gives an introduction on the topic with a background to the applicationof weight estimations and the focus of the thesis. Chapter 2 is this outline of the report.Chapter 3 brings up previous research in the field and outlines the underlying theory forweight estimation and the designs of air suspensions researched in this thesis. Chapter4 describes how tests were performed to gather data to analyze the behavior of thedifferent suspension types and to investigate their total hysteresis as well as individualcomponents contribution. The next chapter, number 5, gives a brief account of thenew method developed in in the thesis and how it is implemented. Due to companyconfidentiality an exact account is excluded. Chapter 6 begins with presenting measuredhysteresis characteristics of the different suspensions as well as the impact of individualcomponents. The second part of this chapter presents the results of the proposed methodthat take hysteresis into account when calculating an estimated weight. Chapter 7contains the discussion of the hysteresis characteristics as well as a discussion aroundthe results of the proposed method, designed to compensate for this. The last part of thischapter describes error sources and uncertainties encountered during the thesis. Thefinal chapter, number 8, contains a conclusion of the thesis work as well as a proposalsof focus areas for future work.


Theory 5

3 Theory

3.1 Previous research in hysteresis and air springs

Air springs and their hysteresis have been studied in several research papers. Sortti[3] derived in a thesis a model for simulation of the air springs used in trucks. Thethesis proposes an equation for calculating the force exerted by the air spring. Thisequation relies on knowing the mass of air inside the spring bellow. The thesis takes noconsideration for hysteresis in the proposed equation but points to it as a source of error.Lartén [4] wrote a thesis on modeling of the air suspension used in heavy duty vehicles.In this thesis the author does not consider hysteresis in the model and it is concluded inthe end that hysteresis might be part of the errors and the deviation seen between thetest data and the simulations. Florian Löcken [5] investigates the dynamic characteristicsand hysteresis effects of air springs in passenger cars. A thermodynamic model thatreplicate hysteresis measured on an air spring is presented in this work. Another thesisfrom 1991 by Nygren [6] discusses the origin of hysteresis in rolling sleeve air springs,the same type as considered in this thesis. The thesis did a comprehensive study of airsprings and presents many useful relations in modelling them. The thesis also presents acomprehensive "handbook" to understand the air springs and the use of them in differentapplications.

3.2 Suspension

At Scania there are three types of air suspensions used on the trucks considered inthis thesis. Each with numerous variations that could affect the weight estimation.With the possibility of combining these suspensions with numerous axle configurationsyields hundreds of combinations. The variations can be different types of brakes, shockabsorbers, tyres, driven or non-driven, liftable and or steerable axle. A selection ofaxle configurations can be seen in figure 3.1, at Scania it is currently possible with airsuspension on up to five axles. Most of the variations affect the un-sprung weight andare considered to only cause an offset to the estimated weight. Different variations ofcomponents such as shock absorber, bushing or air spring could also affect the estimation.


Theory 6

To limit the thesis, three suspension types representative for a large number of vehicleswere chosen, hereafter called suspension type A [7], B [8], and C [9].

Figure 3.1: Selection of axle configurations. Black wheels represent driven axles.Inclined wheels represent steerable. Non-driven axles can be liftable.

3.2.1 Air Spring

The air bellows used in the suspension types A, B, and C are constructed as seen infigure 3.2. A flexible rubber sleeve is sealed to an end plate in the top and at thebottom to a piston. The rubber sleeve can roll up onto the piston, changing the lengthof the air spring and thus the enclosed volume. The rubber sleeve has cord layers ofinelastic cords that prevent elastic changes (stretching) due to relative pressure. The forceexerted by the spring is proportional to the relative air pressure, between the bellowand surroundings, and the effective area. The effective area that relates the pressure to aforce is approximately the size of the horizontal cross sectional area of the bellow seefigure 1.1. With a straight piston, the effective area is constant compared to extensionto a certain point, this results in a linear pressure to force relation. Using pistons ofdifferent shapes changes the effective area due to extension, thus giving rise to a heightdependence of the pressure to force relation. The shaping of the pistons will directlyaffect the spring characteristics and the force to pressure relation. As seen in figure 3.3the effective area can be designed to be constant or increase and even decrease for certainextensions. In suspension types A, B, and C are all straight as in figure 3.2.

Previous work at Scania [10] investigates the differences between samples of bellowswith the same parts number produced on different tooling (molds). The results show aspread between samples and sets a lowest attainable precision, still there remains roomfor improvement. On trucks a straight piston is used that gives the air-spring an almostlinear behavior. On buses there are several different shapes of pistons used [11] and thatcould have a large impact on behavior. This thesis was limited to the suspensions A, B,and C, all with straight pistons and as shown by Nygren [6] the effective area is almostconstant, see figure 3.3.


Theory 7

Figure 3.2: Rolling sleeve bellow in cross section. From left to right, normal drive level,low and high level. Enclosed air volume in blue. Source: N.G.Nygren [6]L.Sortti [3]

Figure 3.3: Rolling sleeve bellow with different pistons and constant mass of air.Positive Z direction corresponds to compression from nominal height. Arepresent the effective area at compression Z, F represents the spring force ofthe air spring. Source: N.G.Nygren [6]

3.2.2 Suspension Type A

A principal design of suspension type A [7] can bee see in figure 3.4. The suspensioncombines both a leaf spring and an air spring to support the load on the axle. Thisdesign includes several pivot points with rubber bushings to let the leaf spring deformunder stress, for example when the suspension is raised or lowered. It also has shockabsorbers and optional anti-roll bar. The potential candidates for hysteresis include theleaf spring and its three associated bushings, shock absorber, reaction rod, air spring,and the anti-roll bar with the associated bushings.


Theory 8

Figure 3.4: Suspension type A with one half of the suspension drawn. In grey are leafspring with shackle, reaction rod, anti-roll bar, shock absorber and air spring.Source: Scania

3.2.3 Suspension Type B

A principal design of suspension type B [8] is shown in 3.5. This type of suspension usesa stiff air spring link with the axle placed between the pivot point and the air spring. Inthe pivot point there is a load bearing rubber bushing. The spring is extended alongan arc drawn in the figure. The candidates for hysteresis are the main bushing, shockabsorber, and the air spring itself.

Figure 3.5: Suspension type B with one half of the suspension drawn. In grey are springlink, shock absorber, and air spring. Swing of the base of the piston shown inred. Source: Scania


Theory 9

3.2.4 Suspension Type C

A principal design of type C [9] is shown in figure 3.6. This type of suspension usesa 4-bellow suspension, two per side, enabling an almost vertical raising and loweringmotion compared to type B. Besides the shock absorber and air springs as potentialsources of hysteresis this suspension type requires the use of several reactions rods andlinkage to prevent movement and twisting around the horizontal plane.

Figure 3.6: Suspension type C with one half of the suspension drawn. In grey are springlinkage, reaction rod, shock absorber and air springs. Source: Scania

3.2.5 Bushings

Bushings are plane bearings for rotary applications similar to ball bearings. Rubberbushings are usually used at pivot points where vibration isolation or stiffness is needed.Rubber bushings in load bearing applications are often constructed of two concentricallyaligned metal cylinders with a rubber filling in between. See figure 3.7. The rubber canbe both pressed in place or joined with either or both shells by the means of for exampleglue. When the rubber is strictly joined, twisting and loading the shells in relation toeach other causes shearing stress and load strain deformation of the rubber. Whenrubber is deformed, internal friction dissipates energy and when force is plotted againstthe angle of a cyclic rotation a hysteresis loop is formed encircling an area equivalentto this energy. This is called Elastic Hysteresis. When the rubber is not joined with oneor both the shells, the static friction between metal and rubber can be overcome whena large enough torque is applied. This causes a slip and can exaggerate the hysteresiscurve [12] as shown in figure 3.8 as horizontal parts with constant torque. This is likelyto contribute to the overall hysteresis in the suspension if large suspension travel causesslip in the bushings.


Theory 10

Figure 3.7: Example of a load bearing rubber bushing with two metal cylinders joinedby a rubber filling.

Figure 3.8: Example curve of hysteresis and slippage in bushings for cyclic torsion.

3.2.6 Shock Absorber

Shock absorbers are used to dampen the suspension and absorb oscillations. The forcefrom the shock absorber is proportional to the speed of compression or extension asseen in figure 3.10. At very low speeds the force from the shock absorber is almostzero. At fast changes, the shock absorber can exert large forces and thus counteract highfrequency oscillations.

3.2.7 Pressure Sensor

Mounted on each air spring circuit there is a piezoresistive pressure sensor measuringabsolute pressure. The location of the sensor varies between axle setups. During valveactivation when letting in or out air, the pressure will fluctuate rapidly. It is likely thatthe placement of the sensor can affect the pressure measured during these events due tothe nature of the system. At static conditions when the pressure fluctuations from lettingair in or out has equalized the location of the sensor will not affect the measurement in


Theory 11

Figure 3.9: Showing one half of type B suspension. 1: Vehicle frame. 2: Spring lingshackle. 3: Bushing. 4: Air spring link. 5: Axle. 6: Shock absorber. 7: Airspring. 8: Drive axle. 9: Level sensor

any considerable way. High frequency changes in pressure from vibrations add noise tothe pressure signal.

3.2.8 Level Sensor

The level sensor on the vehicles consists of rotary angle sensors connected to the axlewith stiff linkage arms as seen in figure 3.9 point 9. The placement of the sensor andlength of linkage arms can differ between configurations, this leads to different sets ofconversion factors to convert the sensor angle to a chassis height.

3.3 Electronic Level Control

The current Electronic Level Control, ELC, is provided by a supplier to Scania in theform of an ECU, Electronic Control Unit. The hardware and software is developedand maintained by the supplier. The demand specifications are written by Scania andimplemented by the supplier in their own proprietary ways.

The purpose of the ELC is to control the air suspension and its functions. The airsuspension ELC system consists of the air bellows fitted to the axle, valves for emptyingand filling bellows. Pressure sensor to measure the bellow pressure. Height sensor tomeasure the chassis height. A park brake release valve, to relieve tensions in the chassis.For liftable tag axles there is also a lifting air bellow.


Theory 12

Figure 3.10: Example curve of shock absorber force as a function of extension orcompression speed. Source: Scania

Functions the ELC perform include:

• Level Control - automatic and manual height adjustment• Lifting and lowering tag axle - automatic and manually controlled• Traction help - adjusting load on driven axles for traction control• Pressure ratio - adjusting load distribution between axles by pressure control• Kneeling - lowering for embarking or disembarking the vehicle• Axle load - estimating weight of each axle on the road• Side walk detection

Figure 3.11 shows possible setups of air control circuits for vehicles with air suspension.Each blue circuit is one group of connected air springs that are emptied and filled atthe same time. The first depicts a vehicle with a two-bellow air suspension on the rearaxle, suspension type B. The middle depicts a vehicle with a four-bellow suspensiontype C, and to the right a vehicle with two rear axles with two bellow type B suspensionon both, with a shared air circuit.

Figure 3.11: Examples of different combinations of suspension mixes where blue circuitsrepresent an air suspension circuit.

The code that executes the weight estimation is proprietary. A list of parameters is giventogether with the dimensions and a brief explanation of formulae in the specificationsissued by the supplier.


Theory 13

The current implementation takes the pressure in each air spring circuit and convertsaccording to a linear relationship. The linearity relation of the pressure to weight ratiois empirically derived from tests [13] where a certain axle type is analyzed and thecoefficients from this are then used. Correction factors are also derived to compensatefor vehicle height. See figure 3.12 for illustrations of pressure to load and chassis heightto pressure dependence used. The correction factors for height are stated in %/mm, onefor above normal drive height and one for below. The application of this correction tothe calculation is unclear. The set of parameters are type-of-axle specific [14].

Figure 3.12: Left: Linear pressure to load relation. Right: Height dependence of thepressure for a fixed load.


Testing 14

4 Testing

4.1 Testing Hysteresis

To analyze the hysteresis of the suspension an array of tests was designed to determinethe sources of hysteresis and the characteristics of the different types of suspensions. Firsta base line had to be established for each type of suspension. By recording the pressurein the air springs for different heights a visualization of the hysteresis can be plotted as aloop encircling an area of all possible states of the system. This is repeated for differentloads to form the boundary of a three-dimensional representation of the hysteresis. Toestablish the base line for the different suspension types appropriate vehicles with thecorrect set of suspension was located within Scanias test fleet. To be able to easily testdifferent loads, the vehicles were fitted with a load frame to load weights onto. Thevehicles were then fitted with external sensors, complementing the on-board sensors ofthe vehicle. By using external, calibrated sensors mounted on the vehicle the height andpressure could be recorded independent of the vehicles production mounted sensors.Any deviations introduced by the sensors, mechanical setup of the height linkage arms,or software conversion of the signals in the ECU could therefore be detected.

The tests were carried out by recording the chassis height and air spring pressure duringraising and lowering of the vehicle. To form a complete hysteresis curve the heightwas changed in very small steps of approximately 1-2mm each, holding the height fora few seconds to let the pressure and chassis level to settle. Changing the height fromminimum to maximum and vice versa with no direction changes to ensure a full outlineof the hysteresis. The reaching the bump stop the bellows were emptied further toensure full suspension travel.

To ensure a known axle load, the tests were carried out on-top of the scale in By-106 atSTC with an individual scale for each tyre.

4.2 Acquisition Setup

During the tests the vehicles sensor data was recorded from the CAN-bus along withthe external sensors and scale output in a software called DEWESoft X3 by the means of


Testing 15

DEWESoft Sirius STG+ data acquisition instruments. The calibrated conversion factorsfor the sensors were acquired from the instrument register by their unique serial number.To convert the analogue scale output a two-point calibration was made using knownweights.

4.3 Suspension Specific Testing

To determine sources of the hysteresis an array of modifications to the suspensions wasmade. For each modification a full series of load cases were tested to form the outlineof a 3D hysteresis volume. By comparing the measurements from modified tests to thebase line any differences could be visualized.

4.3.1 Suspension Type A

Due to the geometry of the suspension, when the height is adjusted there is significantrotation in the bushings in the air spring link. The torque exerted by these bushings onthe suspension is thought to affect the hysteresis curve. The shape of hysteresis for thebushings can be seen in figure 3.8 [12].

Another unique effect for this suspension is the air spring link that acts as a leaf spring.This spring is under tension at drive height, exerting a lifting force on the vehicle chassis.When lowered the force is gradually increased and when raised the spring passes itsfree spring position and for high chassis heights, inverting the direction of the springforce to pulling the chassis downward [15].

The tests derived are presented in the following subsections.

1: Shock Absorber

Firstly, the shock absorbers were removed to eliminate any hysteresis caused by them.The effect of removing the shock absorbers should theoretically be evident at fast changesin height, due to the slow changes in these tests the expectation was that this wouldyield little or no effect.

2: Anti-Roll Bar

Secondly, while the shock absorbers were removed the anti-roll bar was also discon-nected. The effect of this was thought to yield a notable impact on the hysteresis. The


Testing 16

Figure 4.1: Suspension Type A: Pointing out the shock absorber (1), anti-roll bar (2) andrubber bushings (3).

anti-roll bar was very stiff to turn because of the rubber bushings clamping it to the axle.On this vehicle they were so stiff that they were not possible to turn by hand.

3: Bushings

The last modification to be stacked on-top of the previous changes was to loosen thebolts in the bushings. This was to release any clamping force that would cause the outerand inner shell to turn in relation to each other, twisting the rubber filling. There are intotal three rubber bushings in the leaf spring and the shackle holding it in place per halfof the suspension. These were suspected to contribute significantly to the hysteresis.

4.3.2 Suspension Type B

This type of suspension has a simple design with a stiff spring link pivoting in one endon a rubber bushing and in the other the air spring. Due to the swing of the spring linkthe piston moves in an arc causing shearing in the bellow as seen in figure 4.2.

The tests derived are presented in the following subsections.


Testing 17

Figure 4.2: Showing the deformation of the bellow of the air spring in type B suspensionat full extension.

Figure 4.3: Suspension type B: (1) Shock absorber, (2) Air spring, (3) Rubber bushing

1: Shock Absorber

Firstly, as with suspension type A, the shock absorbers were removed to eliminate anyhysteresis caused by them. Expected effect on the hysteresis was little or none.

2: Air Spring Bellow

The next modification was to exchange the air spring to see if there are any measurabledifferences in hysteresis between a new bellow and one a few years old. The change of


Testing 18

bellow was thought to be of measurable size due to results from previous work at Scania[10].

3: Bushings

The next modification was to loosen the bushing in the spring link. This bushing wassuspected to be one of the major contributors to hysteresis in this type of suspension.Due to unforeseen circumstances it was not possible to carry out this modification andtest it.

4.3.3 Suspension Type C

Due to the design of this type no options for modifications were available withoutheavily rebuilding and retrofitting the suspension. The only possible modification to testwas to remove the shock absorber. Due to the results from this modification on types Aand B it was ruled excessive. Thus, only a baseline was made established.


Implementation of New Method 19

5 Implementation of New Method

Matlab was used to analyze the gathered data. The measurement series for each test wasfirst divided into two parts, one for raising the vehicle, and one for the lowering. Whenlevel adjustments were performed the pressure spikes and quickly settles. To attainonly the data points collected at stationary states, all values during valve activation andseveral milliseconds after were disregarded. Using polynomial fitting, surfaces werefitted to the test data and tweaked until a satisfactory fit was produced. Simplified, themethod uses two surfaces representing the boundary of the hysteresis to estimate thesystem state of the vehicle.

The new method was coded in Matlab Simulink as a state machine and is based on thesurfaces from the polynomial fitting of the test data. A representation of the implemen-tation is shown in figure 5.1. Where the normal state is calculating the weight, duringadjustments or out of bounds conditions, it is suspended. As seen in figure 1.2, the stateof the system is not confined to the outer bound of the hysteresis, it can be anywherewithin the outer bounds of the curve. Based on the previous changes in pressure andheight, a compensation for hysteresis is calculated.

The method was simulated in Simulink with measured data from previous tests to verifythe model for the type B suspension. After model verification it was compiled and testedon a test rig to verify that the code ran on the intended ECU, the coordinator, in thevehicle. Finally it was tested in a vehicle [16] that were modified to run the calculation inits coordinator ECU. The testing of the implementation was done in the same manner asfor the previous described tests for the base line, with no modifications to the suspensionand for fully loaded.

Figure 5.1: Block schematics of the implementation representing the new method.


Results 20

6 Results

All data in this chapter are subject to alterations due to confidentiality. The data presentedin this section are representations of real data.

6.1 Current Estimation

In figure 6.1, the current weight estimation from a vehicle with type A suspension on thefront axle and type B (two bellow) suspension on the rear axle is presented. The test wasdone on both axles at the same time, requiring that both axles were raised and loweredin unison, keeping the vehicle horizontal to avoid a redistribution of the load as far aspossible. From T0 the level of the vehicle was lowered from drive level down to bumpstop, reached for the front axle (type A) at T1 and the rear axle (type B) at T2. From T2,the rear axle was raised to the same height as the front, to level out the vehicle. At T3both axles were raised off their bump stops, passing drive level at T4. Around T5 thereis a sharp increase in the estimated weight for the rear axle (type B) when passing theheight H1. The front axle (type A) takes a turn just above the drive height and at T6changes direction sharply at height H2. After reaching the maximum height of each axleat T7 and T8 respectively the vehicle was lowered. At T10 and T11 the front and rearaxles reach heights H2 and H1 respectively which is evident on the estimate. Note thatthe noise in the shape of spikes in the estimates are momentary pressure spikes duringlevel adjustment.

6.2 Hysteresis Characteristics

The hysteresis is seen in figure 6.1 of the weight estimation from time T4 to T12 wherethe vehicle is at drive level with different weight estimations. This section presents andvisualizes the results from tests on the hysteresis of the chosen suspension types.


Results 21

Figure 6.1: Figure shows the current performance of weight estimation on a vehicle withone type A and one type B suspension. Note the load redistribution aroundtime T2 and T8 due to a difference in designed height of the front and rearsuspension on this vehicle.


Results 22

Figure 6.2: Air spring pressure plotted against chassis height. P1 and P2 indicate bumpstop for the suspensions. P3 for type B and P4 for type A are addressed insection 6.2.1 under suspension type A and B.

6.2.1 Height dependence

From the tests on the vehicles the height dependence for a certain axle load is presentedin figure 6.2. The lower part of the loops corresponds to the pressure in the suspensionwhen lowering the vehicle, and the upper to the pressure while raising. The area enclosedrepresent the hysteresis. The curve is the boundary to possible combinations of pressureand height for this axle load. As previously known [17] there is a height dependence thatis clearly visible in figure 6.2. The hysteresis differs between the suspension types. Allsuspensions exhibit the same behaviour at the lower end of suspension height, aroundpoints P1 and P2. At this height of the suspension reaches the bump stop and unloads theair springs, and no more suspension travel is achieved. In the higher end of suspensionheight both type A and B exhibits a clear break point in the upper range of chassis height.

Height Dependence - Suspension Type A

The measurements on type A shows a significant dependence on suspension height. Theupper break point of the curve, seen as P4 in figure 6.2 and P2 in figure 6.3, coincides with


Results 23

Figure 6.3: Axle type A subjected to multiple sequential elimination of components.

the strain to force relations of the leaf spring. The Leaf spring is in its neutral unloadedstate at this very point and exhibits a break point in the stress to force relation here. Infigure 6.3 the effects of modifying the suspension is clearly visible. One after anothermodifications were made to show how the accumulated effects changes the behavior.In the normal case, no changes were made. In the next case the shock absorbers wereunmounted. In the third case the anti-roll bar was disconnected. And in the final case,all the accessible bushings were loosened. For the shock absorber, there were almost novisible change. There is a slight change when the anti-roll bar is disconnected. Whenbushings were loosened there is a significant change in hysteresis visible. The breakpoint P2 in figure 6.3 moves slightly.

Looking at the rotation in the bushings compared to chassis height in figure 6.4 it can beseen that the angle is non-linear for bushings 2 and 3.

Load Dependence - Suspension Type A

The load dependence is roughly linear as seen in figure 6.5. The break point P2 seen inin figure 6.3 moves slightly at increasing loads.


Results 24

Figure 6.4: Axle type A: bushing angular movement of outer vs inner shell over fullsuspension travel.

Figure 6.5: Hysteresis curves for type A suspension plotted in 3D for several loadscenarios.


Results 25

Figure 6.6: Suspension type B subjected to multiple sequential elimination ofcomponents.

Height Dependence - Suspension Type B

Compared to type A, the height dependence of type B is less, as seen in figure 6.6. Whenreaching point P2 the pressure sharply rises. Disconnecting the shock absorbers changesthe hysteresis only to the slightest degree. Exchanging the bellows to new one’s doeshave an effect on the hysteresis. The curve is slightly raised in the lower end.

Load Dependence - Suspension Type B

Looking at the load dependence of this suspension in figure 6.7, an apparent linearrelation to load is seen with one exception. At low loads the pressure sharply increasesat chassis height H1, this point correlates with point P2 in figure 6.6, and for heavierloads this point moves down to point H2.

Height Dependence - Suspension Type C

Looking at type C suspension there were a larger hysteresis compared to type B. Only aunmodified suspension was tested since it was not possible to rebuild the suspensionwith in the time frame of the thesis.


Results 26

Figure 6.7: Hysteresis curves for type B suspension plotted in 3D for several loadscenarios.

Figure 6.8: Axle type C height dependence


Results 27

Figure 6.9: Hysteresis curves for type C suspension plotted in 3D for several loadscenarios.

Load Dependence - Suspension Type C

At the maximum height where the pressure spikes, point P2 in figure 6.8, is less pro-nounced at lower loads for this type of suspension but stationary with respect to at whatchassis height the pressure increases more rapidly for different load cases.

6.3 Implementation

6.3.1 Simulation

Simulating the new method on collected data from tests shows a large improvementin counteracting hysteresis as seen in figures 6.10, 6.11 and 6.12. The figures show therecorded weight estimation of the old method along with the response from the newmethod simulated on data collected during that test. Sharp spikes in the estimates aredue to insufficient filtering of pressure spikes from valve activation. The height of thesuspension is plotted above as reference, since the rate of change of the chassis height is


Results 28

Figure 6.10: New method for type A simulated on recorded data. The recorded estimatefrom the old method is plotted as reference for performance improvement.

not constant, it serves as a reference when looking at points where the estimates changebehaviour or diverge from the scale measured value.

Looking at figure 6.10, the results of simulating the new method adapted for type Asuspension can be seen. It can clearly bee seen that the old method suffers from hysteresisand its estimate is quite accurate when raising the vehicle, a height close to the nominaldrive level. While lowering it suffers from hysteresis and severely underestimates theweight. The simulated response of the new method shows an improvement of accuracywhile raising the vehicle, extending the span of vehicle height where the estimation isaccurate compared to the old method. While lowering there is a great improvementover the old method, it is clearly seen that the new method can handle the hysteresisdifference quite well. Note the error in the beginning of the simulated estimate, theseresults are discussed in section 7.7.1.

Looking at figure 6.11, the results of simulating the new method adapted for type Bsuspension can be seen. It is clear that the old method is only accurate for nominal drivelevel when the vehicle has been raised to it. The hysteresis can be seen when comparingthe old estimate at drive level for raising and lowering of the vehicle. The hysteresisis as seen in previous results much smaller in type B suspension compared to type A.The new method simulated on the recorded data show a clear improvement of accuracy


Results 29

Figure 6.11: New method for type B simulated on recorded data. The recorded estimatefrom the old method is plotted as reference for performance improvement.

over suspension height, it also handles the hysteresis difference between raising andlowering the vehicle. See discussion under section 7.7.2.

In figure 6.12, the simulation results of type C adapted method is shown. As seen fromthe recorded estimate from the old method the greatest accuracy is achieved whilelowering the vehicle, as opposed to both type A and B that has the greatest accuracywhen raising. The effect of the hysteresis is also clearly visible on the old method.Looking at the simulated response of the new method it handles the hysteresis very wellespecially while raising the vehicle. These results are discussed under section 7.7.3.

6.3.2 Live test

The new method adapted to type B suspension was tested live in the coordinator ECUof a vehicle with type B suspension. Due to the modifications necessary to run the teststhe old method estimation was not available to record. In figure 6.13 the results of thelive test are presented. As seen there is a clear offset not seen in the previous simulation.Besides the offset the method holds true over a wide range of heights just as seen in thesimulation in figure 6.11. Looking at time T3 in figures 6.11 and 6.13 the correspondingheight H1 is reached. Note the estimates rate of change is uneven in the middle span


Results 30

Figure 6.12: New method for type C simulated on recorded data. The recorded estimatefrom the old method is plotted as reference for performance improvement.

between times T3 and T4 due to a decreased rate of change of the chassis height. Theseresults are discussed and analyzed in section 7.7.2.


Results 31

Figure 6.13: Live test of the new method for type B in a vehicle.


Discussion 32

7 Discussion

The first section in this chapter discusses and analyzes the components tested for af-fecting the hysteresis and weight estimation, in the second section the focus lies on theproposed new method of calculating the axle weight and the results from testing it.

7.1 Minimum Suspension Height

The hysteresis characteristic curve for the suspension types are compared in figure 6.2.As clearly seen in the figure, when reaching the bump stop the suspension travel islimited but the springs bellows can still be depressurized further. This sets an absoluteminimum height for any weight estimation, while resting in part or fully on the bumpstop makes it impossible to estimate the weight and must therefore be considered as outof bounds and no estimation made here will be accurate.

7.2 Air spring

Close to maximum extension of the bellows in the air spring the pressure makes a sharpincrease, this phenomenon is pronounced for types A and B but is visible for type Cas well. One possible and probable reason could be "over extension". When the airspring is fully rolled off the piston the non-elastic cords embedded in the sleeve wallsare extended and under normal tension due to the relative pressure pushing on thewalls. Extending the spring further will put a higher tension on the cords. Due to theirin-elasticity they will need to straighten, changing their angle within the wall. Illustratedin figure 7.1 where the angle of the inelastic cords are affected by extreme extension. Atlow extensions when the bellow is partly rolled onto the sleeve the tension in the cordswould most likely be proportional to relative pressure only. If the sleeve is fully rolledoff and stretched further the tension in the cords may be dependent on both the relativepressure and the further extension. If the suspension travel is limited to below the pointof the sleeve being fully rolled off this effect should be reduced, and a smaller pressurehike occur at full extension. This could be the reason why it is not as pronounced in thetype C suspension tested.


Discussion 33

Figure 7.1: Illustrates the change in the cord layer when the sleeve is stretched furtherafter it has rolled off the piston. The inelastic cords need to change anglewithin the sleeve wall to accommodate further extension.

In the case of type A the air spring also needs to work against the leaf spring thatis pulling the vehicle downwards after point P2 in figure 6.3 as well as the sleeve isstretched off the piston. The air spring links spring force is non-linear around this pointand could contribute to the sharp increase in pressure needed to raise the vehicle further.This point is relatively stable for all load cases and should in theory be possible toaccount for this break point in software considering the movement of the break point.Looking at the upper break point for axle type B it is more load dependent and notvery stable, thus making it more complicated to account for. Comparing measurementdata to visual inspection of the air-springs, see figure 4.2, shows a clear deformationof the bellow at high chassis heights, especially for type B suspension. The bellow isfully rolled off the piston and due to the geometry of the suspension it is pulled alongan arch. The deformation of the bellow at high loads appear to affect the behavior ofthe hysteresis. For axle type A and C there is no arching of this type occurring since thesuspension movement is vertical as opposed to axle type B, but the sleeves are still rolledoff the pistons and causing a pressure increase. When the air-spring was replaced on thetype B suspension there was a clear change in hysteresis shape visible over the wholeaxle load spectrum. This points to a dependency on the condition of the individualbellows. If the bellows properties and behavior changes with aging, or worse, betweenindividual samples the gain in accuracy from the new estimation can be greatly reduced.


Discussion 34

7.3 Bushings

The movement in rubber bushings has a clear impact on the hysteresis as seen on thetests on axle type A. Note that not all bushings were loosened due to inaccessibilityand time constraints, if all were loosened a larger decrease of hysteresis is expected.Depending on the type of bushing the result may prove different. Using lubricatedmetal bushings reduces the friction in a joint but inevitably lose the vibration isolatingproperties of having rubber ones. The aging of the bushings is thought to influence thehysteresis as they become less stiff and the rubber degrades and is worn out. As seen intests of rubber bushings [18][19] the bushings change behavior when worn. In figure 6.4the sum of the movement in bushings on axle type A is presented. As a comparison thetotal movement in a rubber bushing for axle type B is comparable to either bushing oneor two in that graph. Due to difficulties and obstacles the time was insufficient to testthe effect of the bushings on axle type B. However, the expected result of such a test is tosee a decrease of hysteresis with minimal shape changes of the curve.

7.4 Shock absorbers

Little to no effect of eliminating the shock absorbers were seen. Theoretically the effectshould be small for the types of tests performed in this thesis since the vertical velocitywas kept very low. At high vertical velocity’s the effect of the shock absorbers mayincrease.

7.5 Anti-roll Bar

Little or no effect was seen from eliminating the anti-roll bar of the type A suspension. Itis possible it affects the hysteresis, but the amount is minimal compared to the rest inthis type of suspension.

7.6 Conclusion From Modifications

As seen in the results the modifications change the behavior of the hysteresis mainly bydecreasing it. The shape remains the same and thus it is probable that is stems from thedesign of the suspension and components such as rubber bushings adds hysteresis butdo not affect the general behavior of the height to pressure relation. The dependence onload is only evident at high suspension height and a main contributor to this would bethe deformation of the air spring when the sleeve is deformed or stretched off the piston.


Discussion 35

7.7 Implementation

7.7.1 Type A

Because of the non-linear behavior of the hysteresis on the height of the suspension thereare interesting phenomena visible in the weight estimation. These appear in both theold and new method and originate from the change seen slightly above chassis heightzero, drive height, in figure 6.3. Above this point the pressure increases slower thanbelow causing the estimates to decrease. This is compensated for to a degree by thenew method as seen in figure 6.10 but it still drifts off while approaching the height H1where the bellow is deformed off the piston and the leaf spring is non-linear. The initialerror in the simulation could be because of insufficient initialization of the model, sincethe immediate history before is unknown. Spikes that are visible in the data originatefrom pressure spikes during valve activation that is not sufficiently filtered out, somepropagate through the simulation and appears as spikes on the simulated response.

7.7.2 Type B

The proposed method handle this type of suspension very well in simulation as seen infigure 6.11. As expected the accuracy is lost at height above H1 since it is consideredas out of bounds by the implementation. In the vehicle test the method was loadedinto an ECU on the vehicle and tested live, the results is seen in figure 7.2. The heightdependence is corrected for during the live test as expected but there is an unexpectedoffset. The source to the offset were believed to be one of two. Firstly, since the vehiclethe live tests are performed on is another than the one used to develop the method,and thus it is possible that the un-sprung weight is higher for the vehicle the model isbased on. This should cause an offset seen in the results. The second possible reasoncould be with the conversion of the pressure. In the simulations and the old method, thepressure sensor is connected to and the value calculated by the ECU responsible for theELC functions. In the live tests this ECU is bypassed, and the calculation of the pressureis performed elsewhere and is not available to be recorded. Since this was known, anexternal pressure sensor was connected to the vehicle making it possible to record thepressure and simulate the method afterward. Using the same height information as inthe live test but with an external pressure sensor the offset appears to be corrected asseen in figure 7.2. This points to a possible offset error of the conversion of the measuredpressure or conversion of it from absolute to relative pressure and that it is probably nota large difference in un-sprung weight between these two vehicles.


Discussion 36

Figure 7.2: Live test of the new method on type B suspension in a vehicle, along withsimulated response of the method on height data recorded from theCAN-buss and pressure information form an external sensor.

7.7.3 Type C

As expected the simulation results points to an accurate method of compensating forheight dependence and hysteresis. This type of suspension allows for an accurate andreliable weight estimation with the new method. An interesting question arose, sincethe hysteresis appear very similar for types B and C, can the same model be used forboth? To find this out the model adapted for type B suspension was simulated on datarecorded from a type C suspension. The results are shown in 7.3. As seen, part from anoffset, the model for type B suspension does a pretty good job of compensating for heightchanges and the hysteresis of a type C suspension. It even follows the slight changeof axle load recorded on the scale that appear due to a load redistribution between thefront and rear axle as this vehicle only has air suspension on the rear axle.

7.8 Sources of error

Here follows a listing and discussion of errors and uncertainties uncovered in thisthesis. Certain errors will affect the results and implementation directly and have a largepotential impact compared to others deemed less important.


Discussion 37

Figure 7.3: Simulated response of using the new method adapted for a type Bsuspension on recorded data from a type C suspension.

7.8.1 Measurement data errors

On-board height sensor

The on-board length sensor is a rotation sensor with a linkage arm connected to theaxle. When the vehicle is raised or lowered the suspension movement to degrees onthe sensor is not linear. The output of the sensor is a PWM signal that is converted intodegrees by an ECU. The conversion from degrees to mm is linear in the ECU, meaningthe non-linearity of the real system is lost. This will affect the extreme ends of heightmeasurements and therefore also any weight estimation done here.

On-board pressure sensor

The total error of the pressure sensors used on production vehicles causes an unknownerror to the weight estimation. This uncertainty between samples of sensors could inthe worst-case amount to an small error in the weight estimation. As with all hardwarecomponents, variations between sensors could lead to a small variation in accuracybetween vehicles.


Discussion 38

When conducting the tests of the hysteresis for different load cases the pressure wascalculated in the ECU provided by a supplier, for the implementation testing in a vehiclethe sensors and calculation was moved to another ECU. The pressure available for theimplementation were the absolute pressure in the bellows, so a constant of 100kPa wassubtracted in the calculations. If the resulting relative pressure differs from the relativepressure provided from the other ECU, it could contribute to a difference betweensimulated and tested response of the method. Since the old method and the simulationsuse the pressure calculated in the intended ECU and the new implementation uses apressure calculated by another ECU there is a slight possibility that the conversion isdifferent causing a discrepancy between live tests and simulated response.

Sampling error

The sampling error from the sensor input to the ECU and later conversions will introducean error in height and pressure values that will propagate to the weight estimation. Thesize of these errors is unknown. The resolution of the pressure and height readings willdirectly affect the resolution of the weight estimation. Even the smallest step sizes forpressure and height swing the estimation considerably.

External height sensors

The external string length sensor placed on the vehicle to evaluate the behavior of theon-board height sensor may hide some of the non-linearity due to its placement. Theend of the string was placed on the top of the axle visually at the center. This point doesnot follow a vertical movement compared to the fixed point on the chassis where theother end of the sensor was placed. On a 2-bellow rear axle this point moves in an arc.This is considered to be of little effect. On a 4-bellow drive axle and a 2-bellow front axlethe placement of the sensor was quite difficult and resulted in a slight tilt from vertical,the travel of the axle compared to the chassis is considered to be vertical for these twoaxles. To eliminate these errors, it would be possible to measure from the ground up tothe chassis frame.

7.8.2 Data fitting

When using Matlab to analyze the data, if was filtered to remove pressure spikes duringvalve activation. The spikes seem to decay exponentially, therefore the pressure takes arelatively long time to stabilize. This results in a variation in pressure and height seenas clusters of points with a small spread. This is visible in figure 6.2. The spread of the


Discussion 39

clusters corresponding to "one measuring point" might push the fitted surfaces apartand the calculated hysteresis to slightly larger.

When fitting polynomial surfaces to the data, the order in pressure and height determinesthe range of height for one type of suspension where the calculations will be accurate.Choosing a lower order will decrease the accuracy of the estimation, especially forsuspension type A, during height changes. With a higher order a better accuracyis theoretically possible, at the expense of increased baseline data necessary and theincreased load on the ECU estimating the weight.


Conclusion 40

8 Conclusion

8.1 Conclusion

The suspension types evaluated in this thesis show unique behaviors that need to beconsidered for an accurate weight estimation. A linear pressure to weight conversion isonly valid with a reasonable accuracy under a strict set of circumstances and a narrowband of heights. Compensating for the height of the chassis gives an improved accuracyfor a wider range of chassis heights and should be considered essential if the estimationis to be trusted outside of predetermined states, especially for axle type A where thehysteresis and height dependence is very large. The potential accuracy improvementshown in the simulations of the new method is large and the proof of concept in a vehiclepoints to the same results.

As with compensating for the hysteresis it was evident that rubber bushings contributevery much. The introduced element of driving the vehicle will inevitably impact theaccuracy of this method since it will try to evaluate every change in pressure and height.Suspensions with low hysteresis should retain a better accuracy even during estimationswhile for example driving. The travel of the suspension allowed needs to be controlledto ensure a good estimate, this holds for all suspension types, especially important fortype A and B.

When designing a method of estimating the axle load it has been shown to be importantto know the height dependence and size of hysteresis. By introducing boundaries andlimitations of how and when the axle weight is estimated both the old and new methodcan yield a reasonable accuracy. If the limitations of when estimations are available isto be relaxed, the results show that the history of the system, including pressure andheight are keys for doing this.

8.2 Future work

As seen in the results, figure 6.2, different axles behave very differently. This togetherwith how different parts age it is quite easy to "over determine" the system making itvery adept to a particular individual vehicle in a specific condition and with certain


Conclusion 41

parts. As soon as one of those are changed the estimation will be off. As shown inthe results the bellows have a considerable individual spread, possibly due to aging.These variations and wear and tear can considerably affect the precision of the method.Any change of parts can have an impact on accuracy, thus further work looking atthe variations between individual vehicles is necessary to understand the final gain inaccuracy for production vehicles.

It would be interesting to explore the aging of parts to see if they constantly changefrom aging or if all conform to a "worn in" state that individual samples of parts behavethe same after a certain time. The temperature impact on the system is relevant andcould contribute significantly since the rubber in bushings and bellows may changetheir characteristics at extreme temperatures.

To tackle the offset of some of the implementation, the pressure calculated in the ELCrunning the implementation should be audited to see if there are any discrepanciesbetween those readings and the actual relative pressure. The offset could possibly becaused by a different un-sprung weight between the vehicle tested with the implementa-tion and the one the method is based upon. Both have the same type of suspension but,there are many other parts affecting the un-sprung weight that could be different.

The number of parameters that could be considered is vast and the ones believed tohave the greatest impact on the weight estimation are an accurate understanding of thehysteresis, measurement of pressure and height. The numerous parameters affecting theun-sprung weight will in the end only cause an offset in the estimation. To minimize thecomplexity of the estimation it would be prudent to focus on an area of the suspensionheights where the behavior is consistent and predictable. The upper and lower regionsshould not be included in the estimation or construction of the model. It is interesting toget a better understanding of how the hysteresis develops over time, is it constant overthe life span of the parts or is it changing rapidly? Changes over time can be hard topredict and thus calibrations will be necessary to ensure accuracy. Perhaps the hysteresisis too unpredictable to consider over time without regular re-calibrations. Since themethod does not use a linear relation for pressure to load for the estimations, a two-pointcalibration would not be sufficient.

If the hysteresis is handled, the next step could be to verify the resolution of pressureand height measurements, these will set the smallest step-size for the estimation andtheir accuracy will directly propagate to the accuracy of the estimation.


List of Figures 42

List of Figures

Figure 1.1: Principle of an air spring with flexible bellow filled with a pressurizedgas in shaded blue with pressure P and external force F compressingit. The effective area A is approximately the cross section of the bellow. 2

Figure 1.2: Illustrating hysteresis of an air spring with the possible system statesS in shaded blue. Particular states depicted as dots with lines repre-senting the history of the system to reach those states by loading orunloading the spring. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 3.1: Selection of axle configurations. Black wheels represent driven axles.Inclined wheels represent steerable. Non-driven axles can be liftable. 6

Figure 3.2: Rolling sleeve bellow in cross section. From left to right, normaldrive level, low and high level. Enclosed air volume in blue. Source:N.G.Nygren [6] L.Sortti [3] . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 3.3: Rolling sleeve bellow with different pistons and constant mass ofair. Positive Z direction corresponds to compression from nominalheight. A represent the effective area at compression Z, F representsthe spring force of the air spring. Source: N.G.Nygren [6] . . . . . . 7

Figure 3.4: Suspension type A with one half of the suspension drawn. In greyare leaf spring with shackle, reaction rod, anti-roll bar, shock absorberand air spring. Source: Scania . . . . . . . . . . . . . . . . . . . . . . 8

Figure 3.5: Suspension type B with one half of the suspension drawn. In grey arespring link, shock absorber, and air spring. Swing of the base of thepiston shown in red. Source: Scania . . . . . . . . . . . . . . . . . . . 8

Figure 3.6: Suspension type C with one half of the suspension drawn. In grey arespring linkage, reaction rod, shock absorber and air springs. Source:Scania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 3.7: Example of a load bearing rubber bushing with two metal cylindersjoined by a rubber filling. . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 3.8: Example curve of hysteresis and slippage in bushings for cyclic torsion. 10Figure 3.9: Showing one half of type B suspension. 1: Vehicle frame. 2: Spring

ling shackle. 3: Bushing. 4: Air spring link. 5: Axle. 6: Shock absorber.7: Air spring. 8: Drive axle. 9: Level sensor . . . . . . . . . . . . . . . 11

Figure 3.10: Example curve of shock absorber force as a function of extension orcompression speed. Source: Scania . . . . . . . . . . . . . . . . . . . . 12


List of Figures 43

Figure 3.11: Examples of different combinations of suspension mixes where bluecircuits represent an air suspension circuit. . . . . . . . . . . . . . . . 12

Figure 3.12: Left: Linear pressure to load relation. Right: Height dependence ofthe pressure for a fixed load. . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 4.1: Suspension Type A: Pointing out the shock absorber (1), anti-roll bar(2) and rubber bushings (3). . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 4.2: Showing the deformation of the bellow of the air spring in type Bsuspension at full extension. . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 4.3: Suspension type B: (1) Shock absorber, (2) Air spring, (3) Rubberbushing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 5.1: Block schematics of the implementation representing the new method. 19

Figure 6.1: Figure shows the current performance of weight estimation on avehicle with one type A and one type B suspension. Note the loadredistribution around time T2 and T8 due to a difference in designedheight of the front and rear suspension on this vehicle. . . . . . . . . 21

Figure 6.2: Air spring pressure plotted against chassis height. P1 and P2 indicatebump stop for the suspensions. P3 for type B and P4 for type A areaddressed in section 6.2.1 under suspension type A and B. . . . . . . 22

Figure 6.3: Axle type A subjected to multiple sequential elimination of components. 23Figure 6.4: Axle type A: bushing angular movement of outer vs inner shell over

full suspension travel. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 6.5: Hysteresis curves for type A suspension plotted in 3D for several load

scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 6.6: Suspension type B subjected to multiple sequential elimination of

components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Figure 6.7: Hysteresis curves for type B suspension plotted in 3D for several load

scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Figure 6.8: Axle type C height dependence . . . . . . . . . . . . . . . . . . . . . 26Figure 6.9: Hysteresis curves for type C suspension plotted in 3D for several load

scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 6.10: New method for type A simulated on recorded data. The recorded

estimate from the old method is plotted as reference for performanceimprovement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Figure 6.11: New method for type B simulated on recorded data. The recordedestimate from the old method is plotted as reference for performanceimprovement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Figure 6.12: New method for type C simulated on recorded data. The recordedestimate from the old method is plotted as reference for performanceimprovement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30


List of Figures 44

Figure 6.13: Live test of the new method for type B in a vehicle. . . . . . . . . . . 31

Figure 7.1: Illustrates the change in the cord layer when the sleeve is stretchedfurther after it has rolled off the piston. The inelastic cords need tochange angle within the sleeve wall to accommodate further extension. 33

Figure 7.2: Live test of the new method on type B suspension in a vehicle, alongwith simulated response of the method on height data recorded fromthe CAN-buss and pressure information form an external sensor. . . 36

Figure 7.3: Simulated response of using the new method adapted for a type Bsuspension on recorded data from a type C suspension. . . . . . . . 37


Literature 45

Literature

[1] W. W. Humphreys, “Pneumatic spring for vehicles”, 1901.

[2] J. Lewis, “Pneumatic spring”, 1847.

[3] L. Sortti, “Modellering av luftbälgar för simulering i adams car”, Master Thesis,Kungliga Tekniska Högskolan, 2017.

[4] C.-P. Lartén, “Modeling and identification of air suspension in heavy-dutyvehicles”, Master Thesis, Department of Electrical Engineering, 2017.

[5] M. W. Florian Löcken, “The dynamic characteristic and hysteresis effect of an airspring”, 2015.

[6] N.-G. Nygren, “Luftfjädrar”, Master Thesis, Maskinteknik, 1991.

[7] Scania, Axle Type A.

[8] Scania, Suspension type B.

[9] Scania, Suspension type C.

[10] Scania, Air spring variations.

[11] Scania, Air spring pistons.

[12] Scania, Bushing.

[13] Scania, ALG extraction.

[14] Scania, ALG constants.

[15] Scania, Leaf Spring.

[16] Scania, ELC Basic.

[17] Scania, Height dependence.

[18] Scania, Bushing Durability.

[19] Scania, Spring link fatigue test.


Documents

Improved Weight Estimation for Vehicles with Air Suspensionuu.diva-portal.org/smash/get/diva2:1315123/FULLTEXT01.pdf · 2019-05-11 · An early pneumatic suspension for vehicles was