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Open-Loop and Closed-Loop Control Conceptsin Wheeled Loaders

Open-Loop and Closed-Loop ControlConcepts in Wheeled Loaders

• Flexibility through ancillary devices and theimplementation of LS (load sensing) andLUDV (flow shaning) technology

• Precise control of the service motions withfeed and return controls

• Reduced assembly times thanks to modulartechniques

• High availability and service friendlinessthrough monobloc construction methods and

• Improved operating convenience up to andincluding open- and closed-loop electro-hydraulically controlled sequencing

2. Kinematics and hydraulic controlVarious kinematic lifting gear geometries are em-ployed in order to achieve smooth lifting and low-ering operations and optimum break-out mo-ments, (Fig. 1).The best known and most common are the „Z“and the parallel kinematic designs and a mixtureof the two. All three attempt, in accordance withthe requirements profiles in each case, to ensureoptimum positional and force profiles. The „Z“kinematic geometry is particularly impressive dueto its enormous break-out force on the ground,while the parallel kinematic system offers advan-tages in terms of the consistency of the bucket orfork tilt angle.

As regards the control technology, it is particularlyimportant to comply with the requirements of oilflow demand in order to achieve harmoniousmotions, without any interruption to the motionprofile. This also has to be ensured under chang-ing load conditions, or even with varying levels ofvolume flow, including the possibility of anti-saturation to the hydraulic cylinders.

(Fig. 2): In wheeled loaders up to approx. 60 kW(approx. 7 tonnes total service weight and 1 cbmbucket capacity), fixed-displacement pump systemsare exclusively employed in conjunction with so-called open centre valves. In vehicles over 60 kW,hydraulic systems with variable-displacementpumps and so-called closed centre valves havebecome the norm.The difference between open and closed centre isdefined by the design of the centre port or pas-sage at the neutral position. An open centre meansthere is an open connection between the pumpport and the tank, with a large cross section. Inthe closed centre pattern, the pump port is closedwhen the valve is in its neutral (centre) position.The connection to other loads - such as the steer-ing system or brakes - is effected, depending onthe valve system, on the basis of either a parallelor series arrangement.

Helmut FischerRexroth HydraulicsLohrPhone +49 (0) 93 52 18-24 34Fax +49 (0) 93 52 18-21 59E-mail: [email protected]

1. IntroductionToday, the wheeled loader is one of themain pillars within the wide range ofconstruction machinery available on themarket. It stands alongside the excavatorand backhoe-loader as a popular machineemployed for the loading, lifting andtransportation of bulk materials andstackable items. This paper describes thecurrent state of the art in open-loop andclosed-loop control technology, and intro-duces various hydraulic and electro-hydrau-lic components for new integrated conceptsin these vehicles and associated applications.

While a heavy-duty kinematic geometry of thefront lifting gear means that this vehicle is ideallyadapted to the arduous operating environmentinvolved in the handling of goods, the great ma-noeuvrability of this wheel-driven loading machineis achieved through the generally prevalent cen-tre pivot steering system and a hydrostatic drivetransmission. The duty profiles involved extendfrom pure loading operations on building andmotorway (freeway) sites to applications in thesteel, quarrying and wood industries, right up tothe many different applications that occur in thelocal government/municipal service sector.

The development objectives for wheeled loadersas „all-round machines“, and the associated re-quirements placed on the travel drive and hydrauliccontrol system read as follows:• Maximum traction at low diesel engine speed• High torque conversion range without

traction interruption• High productivity/cost-efficiency through the

employment of variable-displacement pumpsfor the open circuit

Fig. 1: Wheeled loader kinematics

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2.1 Open centre systems (OC)While wheeled loaders over 5-7 tonnes serviceweight are extensively equipped with variable-dis-placement pump systems, in the case of smallerunits the classic throttle control system employingthe so-called 6/3-way principle is still used.An example of this is provided by the SM 12 typecontrol block which is usually equipped with threevalve axes and can serve all the loads (actuators)concurrently via the parallel connection configu-ration (Figs. 3, 4).The flow position can be regarded as a special fea-ture of the load control system, this being attainedby means of a fourth main spool operating positionwhich connects actuator ports A and B to the tank.As in the case of the larger vehicles, in loaders witha service weight upwards of 3 tonnes an actua-tion system employing hydraulic joysticks has be-come widely used as compared with the purelymechanical actuation system.

2.1.1 Steering system, brakesThe steering system of these small and compactloader units also feature a gear pump for fluidpower supply, and operate - similar to the direc-tional valve - in accordance with the open centreprinciple, i.e. with neutral circulation when thesteering system is not being operated (Fig. 5).The high level of manoeuvrability of wheeled load-ers is the direct result of the fact that they offer allthe kinematic advantages of a centre pivot steer-ing system, although this also means that evenwith relatively small loaders, larger oil capacitiesare required for the steering cylinders than is thecase in other similar construction and agriculturalmachines which feature a knuckle steering system.As, in open centre devices, the complete pump oilflow is delivered via the steering unit, absorptioncapacity is limited by factors such as noise andthermal loading. The LAGC low noise steering sys-tem (Fig. 6) takes all the associated requirementsinto account and is characterised by enlarged andoptimised cross sectional sequences. Noise im-provements of 3 to 10 dBA are achieved, with re-ductions of approx. 5 dBA being regularly attainedunder normal operating conditions at 40 rpm.Steering systems employing the closed centre prin-ciple are already employed at service weights ofup to 2 tonnes. Further details in this regard areprovided in the section below dealing with theclosed centre system.

In Germany, legislation demands that there be 3independent brakes provided for deceleration, withthe hand brake counting as one - albeit with re-duced effect.

Fig. 2: Components overview

Fig. 3: Service hydraulics, 1 - 3.5 tonnes service weight

Fig. 4: Service hydraulics, 3 - 5 tonnes service weight

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While in the case of vehicles up to 3 tonnes serv-ice weight, main brake cylinders are used, withheavier vehicles, the actuation forces applied areno longer sufficient in order to effectively meter thebraking action. More scope is available if the hy-drostatic travel drive is incorporated in the brak-ing strategy with engine braking being providedby the diesel unit.Here, an inch brake valve of the type LT 31 (Fig. 7)is used, which represents a combination of a sin-gle-circuit power brake valve (3-way pressure re-ducing valve) and an inch valve (2-way pressurereducing valve) with infinitely variable actuatingaction. The inch brake valve has been derived fromthe modular system of power brake valves and, inthis case, from the LT 07 dual circuit valve.Operation of the pedal reduces the pilot pressureof the travel drive, the travel pump displacementangle is reduced and the vehicle undergoes hydro-static braking. After a certain excursion distance,the plunger then acts on the brake valve and al-lows the control or braking pressure to rise. Thebraking pressure now increases proportionally upto the adjustable stroke stop, so ensuring that thevehicle is evenly decelerated to a halt.

There are several significant advantages in this sys-tem:• Reduced piping and installation requirement• Overlapping of inching and braking possible• Direct, proportional actuation at pedal• No need for preload valves

2.1.2 Suspension dampingsystem (Fig. 8)Owing to the bucket mass and the distance be-tween the bucket and the front wheel, which hasthe effect of a tilting fulcrum, wheeled loaders areparticularly susceptible to rocking motions, particu-larly as they have no shock absorber system be-tween the wheels and the chassis such as thedampers which form part of the state of the art incar and truck technology. For technical reasons,they have instead only a limited wheel dampingcapability via the tyres. One effect of this is that,depending on the load and ground conditions,rocking oscillations can occur. The vehicle, the loadand the driver are all exposed to these sometimesheavy movements. Aside from the mechanicalstress placed on the machine, which can greatlyincrease wear in the long term, it has also beenproven that this motion also harms the health ofthe driver. Other consequences of rocking includereduced handling efficiency, increased braking dis-tances, impaired steering response and lowertransportation speeds.

Fig. 5: LAG steering unit

Fig. 6: LAG low-noise steering unit

Fig. 7: Inch brake valve LT31

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Rexroth was quick to recognise the importance ofstabilisation and oscillation neutralisation (dy-namic ride control) in the wheeled loader. We cannow offer a full range of stabilisation modules (Fig.9) in the standard sizes of 12 to 32. The designsinclude both OEM models and also retrofit devices,although virtually all modern wheeled loaders aresupplied from the factory ready-equipped withthese systems. Such devices are also becomingmore and more popular in other mobile plant suchas backhoe-loaders and skid-steer loaders.

The RSM (Rexroth Stabiliser Module) operates as apassive system and provides damping for the lift-ing cylinder via pressure accumulators. It is imple-mented in the form of a compact block in which allthe necessary functions are integrated. The accumu-lators are charged to the maximum pressure whichoccurs e.g. during lifting of the load, via a 2-waycartridge valve. The pressure in the accumulators ismaintained via a pressure sequencing and a pres-sure relief valve within a defined range. At low pres-sures, the 2-way cartridge valve remains closed sothat the accumulators are not exposed to the pres-sure fluctuations that occur under varying loads. Thisincreases their service life. The RSM remainsswitched off during operations (e.g. loading). Thesystem is activated by a solenoid valve respondingto speed. The bottom end of the lifting cylinder isthen connected to the pressure accumulators.

I would like at this juncture to highlight the newgeneration of Rexroth Stabiliser Modules which arecharacterised by their compactness and simple,low-cost design. These devices - like today’s currentmodules - are also all subject to the approval re-quirements of the German Technical Inspectorate(TÜV) and the health and safety authorities em-ployed by employers’ liability insurance companies.The new generation not only offers cost advantagesin relation to existing solutions, but can also beemployed for new, high-volume applications suchas backhoe-loaders, stackers and telehandlers.

2.2 Closed centre systems (CC)Closed centre systems (Figs. 10, 11, 12, 13) areessentially based on variable displacement pumpsystems with parallel-connected valves and aclosed pump port. Virtually all vehicles upwards of6 tonnes service weight can be expected shortlyto be equipped with this type of configuration.Multi-pump strategies are being replaced by acentralised pump system, with the interlinkingconnection from the steering system, brake andservice hydraulics being provided by hydraulic cas-cade circuits as in the case of other constructionand agricultural machinery.

Fig. 9: RSM stabiliser module


Fig. 8: Rexroth stabiliser module (RSM)

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The closed centre directional valve technologieswhich have become widely accepted include boththe classic load sensing and also the so-calledload-independent flow distribution (LUDV) sys-tems. LUDV controls are characterised by the factthat all the actuator flows are uniformly reducedwhen the pump oil flow lies below the demandlevel of the actuators. Advantages offered by LSsystems include the following:

• Low heat loss (reduction of oil temperature,cooling capacity, etc.)

• Diesel fuel savings• Small oil tanks (procurement cost, mainte-

nance, disposal)• Higher engine power output available for

travel drive• Improved precision control characteristics,

proportional technology

Undefined conditions such as those occurring inload-sensing controls include, for example, the situ-ation where the feed oil flow is insufficient in or-der to satisfy the demand of all the pressure com-pensators. This occurs when a low operating speedof a diesel engine coincides with high demandflows (parallel operation of the steering system,lifting gear and bucket). Due to the insufficientcontrol pressure differential of the pressure com-pensators, the oil flows are divided in an uncon-trolled manner and are always attracted to thelowest load pressure - possibly leading to thestandstill of one or several actuators.Such malfunctions are particularly undesirable inthe case of wheeled loaders.

It is worth noting that there is no individual loadcompensation per section in this case, but rathera common load compensation system per valveblock. For this reason, the differential pressurevalve only has a weak spring for home position -the control pressure differential is determined asa rule by the variable-displacement pump. Thevalve operates in a manner similar to a flow divider.

2.2.1 Variable displacementaxial piston pumpThe integration of the variable displacement pumpfor the service hydraulics is either effected in theform of a central unit or in combination with fixeddisplacement or other variable displacementpumps which serve the steering system. The basicpump employed takes the form of the variable-displacement axial piston pump type A10VOwhich, owing to its inherently wide range of ap-plications, is found in a broad spectrum of differ-ent duty situations.

Fig. 11: Service hydraulics, 6 -12 tonnes service weight



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2.2.2 New range of directionalvalvesThe M6 valve range with standard sizes of 15 (Figs.14, 15) and 22 has been especially developed tosatisfy the requirements of lifting kinematics suchas those found in wheeled loaders and similarmachines, e.g. dozers, crawler dozer-loaders etc,and contains all the necessary functions incorpo-rated in a monobloc housing:

• flow sharing (LUDV)• Integrated LUDV system functions• Integrated load holding function for all axes• Separate tank chambers with regenerative power

feedback capability (high-pressure capable)• Loss-free priority switching in the parallel

mode (for bucket)• Secondary pressure limiting valves• Electric float position control for lifting gear• Threaded or flanged connections available• Expandable through the addition of sandwich

valve elements/sections• Leakage-free service ports available on

request (M6 15)• Optional priority valve (M6 15) for steering• Optional integrated pilot pressure supply system

Both sizes can be provided in the hydraulically orelectro-hydraulically actuated design.

The development work performed on the new M6control blocks has thus brought enormous func-tional improvements for the machines in whichthey are incorporated. Owing to the monoblocdesign, the universal demand for no external leak-age in mobile machinery has now been fully ac-commodated. Other outstanding control featuresare particularly worthy of detailed mention. The M6valve family covers all the essential functions whichare required and expected of an advanced tech-nology wheeled loader control system:

The electrically activated float position controlenables the bucket to follow the ground contourin spite of its deadweight. In addition, the actua-tor port is protected by pressure limiting valves.The float position valve is also activated parallel tothe RSM, as a result of which the bucket cylindersare directly linked to the stabilisation module andits accumulators. The cylinders can now transfer loadpeaks to the RSM and, after yielding to dynamicloads, can be returned to their initial position.

Fig. 14: M6-15 control block

Fig. 15: M6-15 control block (circuit diagram)

Fig. 16: Step-change response and load compensation of the sequential control system

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Regenerative power feedback circuit for thecontrol of negative or pulling loads. Here, the oilflowing from the cylinders to the tank is preloadedby means of a pressure valve and returned to theinlet. This feature ensures the avoidance of cavita-tion even under conditions of reduced pump inletflow. Critical movements in wheeled loaders caninclude, for example, the lowering of the lifting gearand tip-out of the bucket - and even bucket inser-tion in the material - depending on the kinematicsof the system and the cylinder arrangement.

Load pressure independent sequence con-trol for lifting cylinder lowering: In the case of vir-tually all common directional valves, the inlet flowsare nowadays load-compensated (LS, LUDV). How-ever, particularly in the case of pulling loads, loadcontrol can only be effected by the outlet-end con-trol system. In a manner similar to that of a pure„throttle control“, the oil flow driven by the pres-sure differential - and the associated cylinder speed- are heavily load-dependent. Light loads lowervery slowly, heavy loads fall quickly and are almostimpossible to decelerate.For these reasons, the series M6 control blocksfeature flow force compensation (Fig. 16) at themain spool, to facilitate stable load pressure bal-ancing, so ensuring that the oil flow returning fromthe cylinder remains constant independent of theload. This means that both light and heavy loadscan be controlled with a very high degree of preci-sion. Fig. 16 shows, for example, two different loadsituations with constant oil flows which yield avolume flow variation of less than 10%. In addi-

Fig. 17: THF6 loader joystick

tion, the step response in this diagram reveals asettling behaviour with virtually optimum damping.

Individual priorities for selected actuatorscan be implemented on the basis of LUDV direc-tional valves. By appropriately selecting the pres-sure compensator characteristics, prioritisation canbe hydraulically implemented in the parallel modewithout undue complication.In particular, the combination of lowering the lift-ing gear with simultaneous emptying of the bucketrequires, owing to the low lowering pressure, apriority circuit with the usual LS situation of anunsaturated system. The standard LUDV servesboth actuators on an equal basis, i.e. although themotion sequence is performed at a slower rate, themotions are nevertheless synchronous. The „LUDVpriority circuit“ takes into account the fact thattipping out the bucket while performing a lower-ing operation slows down the speed of the latterduring the overlap phase.

Leakage-free shut-off of each actuator port forthe lifting gear and bucket axes with the aid ofpilot-controlled poppet valves, which are openedparallel to the main spool, enables the holding ofloads without the lifting gear „sinking“. This de-sign ensures maximum opening stability of thepoppet valves as the pilot pressure is held constant.Negative loads are no longer able to cause insta-bilities at the lifting gear. A generously dimen-sioned opening ratio ensures early opening - evenwith low pump inlet pressures.This design also precludes any chance of the mainspool becoming stuck as leakage is prevented not

by a tight fit between the spool and bore but ratherby the poppet valves.

2.2.3 Hydraulic joystickA hydraulic joystick is used as standard for theoperation of LS directional valves, and also forthrottle control blocks in vehicles upwards ofapprox. 3.5 tonnes service weight. The „fingertip“operability of this system increases both user con-venience and productivity. Such a hydraulic joystickalso eliminates the need for mechanical linkagesystems, i.e. the number of high-maintenance com-ponents is reduced and reliability is correspond-ingly increased.Joystick command units of the THF6 series (Fig. 17)operate on the principle of directly controlled pres-sure reducing valves and, for wheeled loaders, fea-ture all the necessary electric end position inter-locks, a fifth port for auxiliary functions - usuallythe float position control - and the possibility offlanging on a further command section for controlof an additional function.

The hydraulic joystick is thus characterised by thefollowing features:• Progressive, fingertip operation• Precise and backlash-free control• Tactile resistance point on actuation close to

the lever end position• Ergonomic handle design (registered utility

model) with various electrical contacts• Electro-magnetic end position interlock for

each port• Optional 5th pilot pressure port• Possibility of flange-mounting additional

functional sections• DEUTSCH or AMP central connector

2.2.4 Steering unitsHydrostatic steering systems of the LAGU series areemployed in wheeled loaders between 3 and 6tonnes service weight where emergency operationis required, i.e. in the event of failure of the pumpedoil supply system, so as to ensure that the maximumemergency steering force is not exceeded. This isachieved in that, if the oil supply system fails, halfof the displacement volume is short-circuited andthus de-activated. With the swept volume of Vgmax/2 now effective, the actuation force required for thesame steering pressure is reduced. Transmissionratios of both 2:1 and 3:1 are possible.

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With a manual steering force of 5 daNm and aspeed of 20 rpm, the pressures required are asfollows:

If a higher pressure is required for emergency steer-ing, a steering unit from the LAGZ series can beused as the next step up:

While steering units of the LAGU series (withchamber shut-off) are very simple and inexpensiveto manufacture owing to their design principle,steering systems of the LAGZ series serve to reducethe swept volume in order to limit the actuationforce by switching over from 2 to 1 displacementunits. The efficiency in this case is more favourableand the resultant actuating forces are lower, al-though this is accompanied by the disadvantageof an increase in the associated dimensions.Steering units operating according to the chambershut-off principle can, in contrast, be directly inter-changed as they all have the same dimensions.All units are available as open and closed centresystems with static and dynamic load signalling.

2.2.5 Compact brake control blockThe power brake valve LT17 (Fig. 18) has also beendeveloped for wheeled loaders as a complementto the multi-component braking system. This notonly features the usual brake valve functions butalso all the essential individual functions such asservice, holding and auxiliary braking, all in a sin-gle compact unit. The advantages already availablewith a hydraulic power brake system have thusbeen further expanded to include the following:

• Simple and fast installation• Minimised piping requirement• Reduced space requirement• Integration possible within existing hydraulic

systems• Integrated actuating pedal• Direct mounting of accumulators possible• Electric hand brake valve• Shorter response time• Precision metering• Low maintenance

Like steering components, power brake systems aredefined as safety components. Rexroth Hydraulics

will be pleased to assist prospective customers inboth their design work and in relation to safetymatters.

3. Electro-hydraulic controltechnologyOwing to the constantly increasing requirementsin relation to productivity, efficiency and the reduc-tion of emissions, the degree of interaction of vari-ous function-determining signals and operatingsequences in the wheeled loader is constantly in-creasing. As user convenience continues to im-prove, fatigue-free working and thus better pro-ductivity are becoming more readily possible.The incorporation of mobile-capable measuringequipment has facilitated the development ofclosed-loop control circuits, and thus the introduc-tion of semi-automatic and automatic operations

which not only reduce the workload on the driverbut also produce a higher quality of work.The integration of modern bus techniques (pre-dominantly CAN) not only saves on cabling - themodular design and „plug and play“ systems whichhave become possible as a result have also greatlyreduced the time required to assemble a machine.Thanks to the possibility of data transmission, andthe check functions which are additionally trans-mitted with the signals, fault-immune and reliabletransfer systems have become possible (Fig. 19).Thus, the various devices and components operat-ing on the bus can, through the provision of statusand fault signals, actively contribute to simplifyingand accelerating the commissioning, diagnosis andparameterisation of the system.The maximum benefit of a bus system is only real-ised once all the components that exist in the sys-tem are connected to the bus.

Size 125/60 160/80 200/100 250/125 320/160P in bar 40 30 24 19 15

Size 160/50 200/65 250/80 320/105P in bar 40 31 24 17

Size 050 063 080 100 125 160P in bar 90 85 80 60 50 40

Size 200 250 320 400 500 630P in bar 30 25 20 15 12 10

Fig. 18: LT17 compact brake control block

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3.1 Service hydraulicsThe service hydraulics of a wheeled loader em-ployed to control the lifting gear consist essentiallyof the variable displacement axial piston pump, thecontrol block and the hydraulic joystick. In the caseof the electro-hydraulic circuit, the control blockoperates electro-hydraulically, the command unitstake the form of electric signal transmitters (elec-tric joysticks) and an electronics module ensuresthe coordination of signals and electrical poweroutput values. With the addition of sensors, oper-ating variables such as position and load can bedetermined and this information incorporated inthe closed-loop control circuit.

3.1.1 Control blocksThe core components on the service hydraulics sideare the series M6 control blocks which have beenspecifically developed for wheeled loaders (sizes15 and 22), and these operate essentially eitherhydraulically or electro-hydraulically. Both methodsof operation follow the barometric principle basedon pressure reducing valves, and are the associ-ated systems are thus extremely easy to convertfrom hydraulic to electro-hydraulic actuation or viceversa. In order to reduce the piping requirementand interface problems, two electrically operatedpressure reducing valves are provided at the con-trol block for each spool axis. Individual flange-mounted sections can also be provided with eitherform of actuation. Mixed solutions are also possi-ble. The basic functions of the control blocks - cf.Section 2.2.2 - remain the same.

3.1.2 MicrocontrollersAs the electronic heart of the system, a PCB of thetype MHVD - equipped with an 8 bit microcontroller- acquires all the relevant data such as the com-mand and actual value signals, and also statusvalues relating to the machine such as enginespeed, vehicle velocity, etc., and enables the cor-responding power outputs for on/off and propor-tional functions. The CAN bus board also locatedon the motherboard enables the incorporation ofa bus network for bi-directional data interchange.The power outputs for actuation control of the valvesolenoids likewise pass through a software loop,with the temperatures that arise under variousoperating conditions being duly compensated out.The advantage of a central electronics systems lies- as will be apparent in the following - in the factthat the signals and power outputs can be cen-trally coordinated and no additional componentsare required.

3.1.3. Electric joysticksThe operator of the machine controls the liftinggear by means of an electric joystick of the typeM-Control, the output values of which are gener-ated contact-free and thus wear-free on the basisof the Hall principle via an angle algorithm, andthen directly converted into a CAN bus commandstring. With the corresponding integration withinthe network via definition of the CAN protocol, thejoystick can be duly defined as a bus device andincorporated in the system.

3.1.4 Angle encoderThe system incorporated in Rexroth’s own experi-mental wheeled loader includes the function ofparallel control of the bucket or fork relative tomachine attitude. This is possible by virtue of the factthat a non-contacting angle sensor is provided bothat the lifting arm and at the bucket cylinder, and thecurrent position is continuously signalled back to thecontrol. Of particular interest in this regard is the factthat both sensors, like the command value genera-tor, exhibit a CAN bus interface and can thus beeasily incorporated within the bus concept.

3.1.5 Function modules / SoftwareAs already mentioned, an electro-hydraulic conceptsuch as that already described above enables a highlevel of functionality offering a wide range of addi-tional benefits without the need to expand the scopeof the hardware. Aside from the control strategiesof the travel drive, the following useful and effi-ciency-enhancing functions can also be incorporatedwithin the service hydraulics of wheeled loaders:

Basic functions• Lifting height limitation• Electric limit stops• Shake-out function• Automatic teach-in function• Return to load• Automatic engine acceleration• Mode selection functions• Diagnostics; machine data monitoring

• Expansion modules• Parallel control• Dynamic ride control• Variable contact pressure control in the float

position• Communication with the travel drive (vehicle

management)

It is possible in part both to adapt solutions fromsoftware modules existing in similar machines, andto transfer software solutions from wheeled load-ers to these other machines. By way of example,the following describes experience gained withpositional and force controls employed in the lift-ing gear control system on tractors and combineharvesters.

The structure of a control system can be describedin more detail on the basis of the parallel controlfunction exhibited in the Rexroth wheeled loader:

Fig. 19: Electro-hydraulic loader control system with automatic parallel control

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The parallel control system operates according tothe master-slave principle with the command valuefor the angular position of the bucket or the at-tached tool being determined from the currentangle signal of the lifting gear, taking into accountthe kinematic relationships. The lifting gear is con-trolled on the basis of a pure open-loop system,i.e. without taking into account the position feed-back signal. When activating the „parallel control“function, initially the actual position of the bucketor of the attached tool is determined from the twosensor signals, and on actuation of the lifting gearthis is adjusted to the calculated command value.There are various control algorithms available forthe closed-loop control system.If the calculated position cannot be attained bycorrection of the bucket cylinders, the engine man-agement system can raise the engine speed. If thisis still not sufficient, once an angle deviation (con-trol error) of more than 4° has been reached, theoil flow for the lifting cylinders is also reduced.The figure (Fig. 20) shows the control error of theattached tool in relation to a command value step-change for the lifting gear, with a subsequentquasi-steady phase of movement at a low pres-sure of approx. 100 bar and slow retraction witha short intermediate stop. Both in the case of the„lift“ command value step-change and duringretraction, the control error lies within a windowof +/- 2°. Where the lifting gear moves slowly, orwhere there are no load pressures, the control er-ror is further reduced.In addition, an electric end position cushioningsystem for the lifting and bucket cylinders is pro-vided.Other software functions are in development.

3.2 Electrohydraulic SteeringhydraulicsFurther vehicle requirements include the incorpo-ration of the steering system within the overallelectro-hydraulic concept (Fig. 21) as a furtherdevelopment stage. Already today, there is a de-sire for more all-round view, individual wheelsetsteering control and „oil-free“ cabins.To this end, the Rexroth wheeled loader featuresan electro-hydraulic steering axis connected inseries to the power brake system via a priorityvalve, and connected in parallel to the steering unit.This means that safe steering continues to be pos-sible even in the event of one unit failing. A pres-sure switch monitors the operation of the steer-ing unit and disconnects the pilot pressure supply

Fig. 20: Control error in the parallel control mode

Fig. 21: Electro-hydraulic steering system

of the directional valve if required so that no acci-dental operation can occur.The command unit of an electro-hydraulic steer-ing system (Fig. 22) is still under discussion, as isthe preparation for the road traffic licensing of suchcircuits. Fast steering speeds can be implementedat standstill by means of the joystick and a speed-dependent steering pressure intensifier as can limi-tation of the steering speed as travel velocity in-creases. The travel dynamics of the vehicle can thusnever be over-stretched. The safety considerationsin respect of all components and the integratedcircuit need to be thoroughly reviewed - both interms of the hardware and the software - particu-larly in view of the fact that, as previously de-scribed, electro-hydraulic systems can, under cer-tain circumstances, increase the level of safety ofsuch systems.

4. Summary and outlookThe development of the functional and design fea-tures indicated here provides an example of thelatest possibilities available in hydrostatic traveldrives and hydraulic control technology. Within theareas of application represented by constructionmachinery, these features can bring about a fur-ther increase in machine performance and cost-efficiency.Increased integration of the individual functionswithin complete modules can only be implementedon the basis of an evaluation the system technol-ogy as a whole. Hydraulic components such aspumps, motors, steering system, brakes, traveldrives etc. can no longer be regarded in isolation.Functionality and the associate costs are now be-ing considered on the basis of the overall, inte-

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grated system, and interfaces are being eliminatedas a result. The application of the know-how ac-quired combined with a properly aligned strategyis likely to yield even more effective systems andcomponents.

The future focus of development work will lie inthe further integration of electronics within thehydraulics system. The process of separation of theelectronics between power-related and informa-tion-related components through bus systems hasalready begun.Vehicle management systems will, in the future,not only control individual components such asengines, but will also perform all the essential func-tions of a construction machine relating to thetravel drive and lifting gear within an integratedapproach.The increasing demand for automatic process con-trol capabilities such as the closed-loop control ofthe parallel motion at the lifting gear, safety cir-cuits and enhanced ergonomics, e.g. in the formof active anti-oscillation systems, and even theincorporation of complete vehicle managementsystems represent further potential for develop-ment work.

Fig. 22: Operating concepts for electro-hydraulic steering

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