Technology evaluation report for electrical actuated mechanical

Responsible (Name, Organisation) Dawid Szkucik, SOLARIS BUS & COACH S.A.

DELIVERABLE REPORT

Page

1(39)

Issuer (Name, Organisation)

Paravizzini Pier Paolo, CRF

Dawid Szkucik, SOLARIS BUS & COACH S.A.

Date

May 2013

WP No

2300

Report No

D2300.1

Subject:

Auxiliaries for chassis function – Technology evaluation

Dissem. Level

PU

HCV Hybrid Commercial Vehicle – D2300.1, Rev_1 page 1 of 39

HYBRID COMMERCIAL VEHICLE (HCV)

DELIVERABLE D2300.1

TECHNOLOGY EVALUATION REPORT FOR ELECTRICAL ACTUATED MECHANICAL

BRAKES AND ELECTRICALLY POWERED STEERING SERVO


Summary This report contains a general overview of today’s possible solutions for the electrification of braking auxiliaries as well as the electric and electro-hydraulic steering. First a general description of the operation principle of a normal production (NP) hydraulic braking system is presented. Afterwards, several supplier solutions are reviewed in order to get a global overview what is the state of art for the braking system’s electrification. Concerning the steering systems different options are compared: the hydraulic, the electro hydraulic and the electric one. Their installation, working principles, effect on fuel consumption and other advantages and disadvantages were investigated.

Conclusions Many solutions have been investigated for the electrification of braking auxiliaries. To choose the best one many aspects must be taken into account, the most important factor is the system’s functionality but also costs and dimensions have to be considered. Obviously the costs cannot be well-defined until the number of pieces that will be manufactured is determined. In general, all solutions fulfil the functional and safety requirements for braking electrification, but there are many differences between the systems regarding their complexity. The simplest solution is the implementation of an Electric Vacuum Pump in order to remove the one driven by the crankshaft. This permits driving in pure electric mode for hybrid vehicles with both: engine and electric motor. Using more complex electrical braking architecture a full by-wire solution that provides the maximum flexibility concerning the component’s positioning and the braking strategies can be implemented. Nowadays, most commonly used is the hydraulic power steering system. However, after an analysis of available solutions it can be concluded that they do not seem to be state of the art anymore and should be replaced by more sophisticated solutions. Advanced electro-hydraulic power steering systems have many benefits. The main advantage of these systems is that they are independent of the vehicle’s motor operation (possibility of providing effort assist when the engine is switched off - towing, manoeuvres at petrol stations). Other advantages include e.g. smaller size, simplicity of construction, lower price and lower fuel consumption (2 - 5%). The vehicle’s combustion engine does not have to power the hydraulic pump, which means that during straight driving the electric motor does not need to work and therefore, it does not consume energy during these time periods.


Abbreviations ABS anti-block system ECU electronic control unit EDIB Electrically-Driven Intelligent Brake EHCU electro-hydraulic control unit EHB electro-hydraulic brake EMB electro-mechanical brake ESC electronic stability control ESC-R electronic stability control regenerative ESP electronic stability program EVP electronic valve program HAS hydraulic apply system ICE Internal Combustion Engine MC master cylinder NP normal production PCA Pressure Controlled Actuation PFS Pedal Feel Simulator RBS Regenerative Braking System SCB slip control boost TMC tandem master cylinder TV(V) thermo vacuum (valve) HPS Hydraulic power steering NEDC New European Driving Cycle


Table of content Summary ............................................................................................................................... 2

Conclusions ........................................................................................................................... 2

Table of content..................................................................................................................... 4

Table of figures ..................................................................................................................... 5

List of Tables ......................................................................................................................... 5

1 Electric actuated mechanical brakes .............................................................................. 6

Introduction ............................................................................................................. 6 1.1

Power Supply .......................................................................................................... 8 1.2

Supplier solutions .................................................................................................... 9 1.3

Electro-Hydraulic Brake - EHB ......................................................................... 9 1.3.1

Electro-Pneumatic Brake .................................................................................15 1.3.2

Electro-Mechanical Booster.............................................................................19 1.3.3

Electro-Mechanical Brake EMB .......................................................................22 1.3.4

Cost evaluation ......................................................................................................27 1.4

2 Electric and electro-hydraulic steering ..........................................................................28

Introduction to power steering system solutions .....................................................28 2.1

Hydraulic power steering system ............................................................................29 2.2

Working principle of the system .......................................................................29 2.2.1

System installation ..........................................................................................31 2.2.2

Cost analysis ...................................................................................................31 2.2.3

Fuel savings ....................................................................................................32 2.2.4

Electro-hydraulic powered steering system ............................................................32 2.3



Cost analysis ...................................................................................................36 2.3.3

Fuel savings ....................................................................................................37 2.3.4

Electric power steering system ...............................................................................37 2.4



Cost analysis ...................................................................................................38 2.4.3

Fuel savings ....................................................................................................39 2.4.4


Table of figures Figure 1-1: Typical braking actuation system ......................................................................... 6 Figure 1-2: Possible braking power supply systems (Electric Vacuum Pump / Standard pneumatic vacuum booster) .................................................................................................. 6 Figure 1-3: Typical ABS/ESC electro-hydraulic modulator ..................................................... 7 Figure 1-4: Typical disc-brakes scheme ................................................................................ 7 Figure 1-5: Electrified Vacuum Pump .................................................................................... 8 Figure 1-6: Main components of Bosch Hydraulic Apply System ..........................................10 Figure 1-7: Hydraulic architecture of Bosch Hydraulic Apply System ....................................11 Figure 1-8: Left: Braking system architecture equipped by TRW ESC-R with external simulator unit. Right: TRW ESC-R electro-hydraulic modulator ............................................12 Figure 1-9: Left: TRW Slip Control Boost Electro-Hydraulic Control Unit EHCU Right: TRW Slip Control Boost Modified Master Cylinder ................................................................12 Figure 1-10: Hydraulic architecture of TRW SCB Braking System ........................................13 Figure 1-11: Hydraulic architecture of ADVICS Braking System (Toyota Prius) ....................14 Figure 1-12: Hydraulic architecture of the Bosch ESP HEV Braking System ........................15 Figure 1-13: Continental Regenerative Braking System components ...................................16 Figure 1-14: Continental RBS – Simulated braking actuation ...............................................17 Figure 1-15: Bosch regenerative braking architecture (EVP + PCA + ESP hev) ...................18 Figure 1-16: Bosch electro-mechanical actuation (iBooster + standard Tandem Master Cylinder) ...............................................................................................................................19 Figure 1-17: 3D drawing of Bosch iBooster ..........................................................................20 Figure 1-18: Basic characteristic and software adjustability of Bosch iBooster .....................20 Figure 1-19: Bosch iBooster size (left) in comparison with standard pneumatic booster (right) .............................................................................................................................................21 Figure 1-20: Hitachi electro-mechanical actuation (Electrically Driven Intelligent Brake) .......22 Figure 1-21: Schematic Electro-Mechanical Braking System ................................................23 Figure 1-22: (a) – Standard braking circuit layout modification (b) – Functional architecture of Hybrid Electro Mechanical Braking System ..........................................................................24 Figure 1-23: Brembo electro mechanical braking actuator (EM caliper) ................................25 Figure 1-24: Siemens electro mechanical braking actuator (EM caliper) ..............................26 Figure 1-25: Comparison between conventional brake and wedge brake technology ...........26 Figure 2-1: Heavy vehicle power steering system .................................................................29 Figure 2-2: Drawing showing the working principle of a power steering system ....................30 Figure 2-3: Example of bus power steering system ..............................................................31 Figure 2-4: Example of electric power steering system (side view) .......................................33 Figure 2-5: Example of electric power steering system (top view) ........................................33 Figure 2-6: Electrical diagram of a 3x400 VAC three phase asynchronous motor and a 600 VDC/3x400 VAC converter ...................................................................................................34 Figure 2-7: Example of a power steering system installation in a bus ...................................36 Figure 2-8: The decrease in fuel consumption achieved by the usage of an electro-hydraulic power steering system..........................................................................................................37 Figure 2-9: An example of electric power steering system solution .......................................38

List of Tables Table 1: Cost analysis ..........................................................................................................36


1 Electric actuated mechanical brakes

Introduction 1.1

Last year’s fuel-saving and pollution-reduction objectives have led to the idea of directing carmakers innovative research towards vehicle electrification. The key drivers for that are: first the diminishing of natural resources such as oil; then the eminent importance to save our environment e.g. by continuing reduction of CO2 emission. Vehicle electrification helps to reduce or even eliminate several components that are usually mechanically driven and therefore it enables to save much energy. The first step towards braking electrification is to know how the brake system works. Most vehicles that are on the road today are equipped with hydraulic brakes. Normal production (NP) architecture can be divided in four well-defined subsystems:

Actuation: Allows the driver to apply the necessary force on the system. Usually there is a master cylinder that translates the mechanical force of the brake pedal into hydraulic pressure. A typical braking actuation system is shown in Figure 1-1:.

Figure 1-1: Typical braking actuation system

Power supply: The subsystem that amplifies the driver’s braking request in order to

obtain a high brake pressure against a low effort on pedal. As an example a braking power supply system is shown in Figure 1-2.

Figure 1-2: Possible braking power supply systems (Electric Vacuum Pump / Standard

pneumatic vacuum booster)

Modulation: Has to guarantee the vehicle stability by means of the modulation of brake pressure at each wheel (ABS/ESC). A typical modulation unit can be seen in Figure 1-3.


Figure 1-3: Typical ABS/ESC electro-hydraulic modulator

Brakes: A subsystem that has to physically actuate the braking torque on the wheels

in order to fulfil the deceleration requested by the driver. Almost all vehicles today are equipped with disk brakes (see Figure 1-4).

Figure 1-4: Typical disc-brakes scheme


Power Supply 1.2

Up to now all brake boosters are pneumatic because in these systems vacuum can easily be obtained by using the pressure difference between two chambers of a piston. In one chamber the vacuum is always present and in the other one the pressure is modulated between vacuum and ambient pressure. The vacuum can be provided in two ways:

In gasoline engines the intake system’s vacuum (caused by engine suction) downstream to the throttle can be exploited.

In diesel engines there is no vacuum available therefore, a vacuum pump driven by the crankshaft is used.

In both cases the booster doesn’t work when the engine is off. The first step towards electrification of the braking system is the replacement of the conventional solutions to achieve vacuum, by an Electrified Vacuum Pump (see Figure 1-5). This enables the improvement of the system’s energy efficiency. The pump is not always active, it is just switched ON if the vacuum level is too low. Another crucial benefit of electric vacuum pump is the possibility to have the full braking servo-assistance also when the engine is OFF. This is a primary requirement for hybrid and full electric vehicles. Moreover, it is indispensable for the implementation of Stop-and-Go strategies in vehicles with ICE.

Figure 1-5: Electrified Vacuum Pump


Supplier solutions 1.3

Most supplier solutions for braking electrification are intended for hybrid and electric vehicle so they are completely independent from the engine. Apart from the electrified vacuum pump, the other solutions can be divided into four main categories:

Electro-Hydraulic Brake – EHB Electro-Pneumatic Brake Electro-Mechanical Booster Electro-Mechanical Brake - EMB

Electro-Hydraulic Brake - EHB 1.3.1

The goal of this design is the removal of the pneumatic booster replaced by a hydraulic active booster that provides all the power requested by the driver. In general the brake pedal is decoupled from the hydraulic braking circuit and the driver’s intention to slow down is detected by a sensor installed on the pedal. The power is usually provided by a hydraulic pump and modulated by several valves. The hydraulic system is able to function as a back-up in case that the electronic part of the system fails. EHB can be seen as a more acceptable initial step towards full electronic brake-by-wire because of retention of the conventional hydraulic system.


Bosch Hydraulic Apply System - HAS 1.3.1.1 This system totally replaces the pneumatic booster by a hydraulic accumulator, a pump to generate the power and a modified master cylinder to increase the brake pressure (see Figure 1-6). A stroke simulator is introduced to decouple the pedal and the brake circuit (passive feeling simulation).

Figure 1-6: Main components of Bosch Hydraulic Apply System

In normal operation mode, when the driver presses the brake pedal, two on-off valves are energized: by that the pedal is isolated from the master cylinder and connected to a feel simulator (PFS). By means of a stroke and a pressure sensor, the corresponding brake pressure is computed, and it is actuated by the power unit: the accumulator pressure is modulated by two proportioning valves (boost valves). In case of failure the pedal can push directly the master cylinder and a degraded braking action is achieved.


Figure 1-7: Hydraulic architecture of Bosch Hydraulic Apply System

The hydraulics permit high dynamic intervention and low power dissipation in steady state situations. When a constant pressure is requested, there are no boost valves switching (see Figure 1-7). Downstream of the booster a normal production electronic stability program (NP ESP) can be introduced with the common functions of a stability program. Furthermore, in case of regenerative braking in hybrid/electric vehicles, due to the pedal decoupling, it is possible to reduce the brake pressure requested by the driver and obtain the deceleration gap by means of the electric motor. Limiting is that this reduction can only be done on both axles at the same time; the split shall be done by ESP at pressure level after the actuation command performed by the Master Cylinder section. The size of the Brake Operating Unit is similar to a NP master cylinder plus a pneumatic booster, but the Actuated Control Module is added.

TRW ESC-R 1.3.1.2 This system includes a modified electronic stability control (ESC), maintaining the NP braking architecture (Figure 1-8).


Figure 1-8: Left: Braking system architecture equipped by TRW ESC-R with external

simulator unit. Right: TRW ESC-R electro-hydraulic modulator

Outwardly, the only modification is the introduction of a stroke sensor on the brake pedal, whereas the normal pneumatic booster is maintained. Inside the ESC (or outside if preferred) a simulator unit is introduced to decouple the pedal from the brake circuit permitting regenerative braking. Internal valves are modified to resist the continuous activations and deactivations in regenerative strategies. The system is able to reduce the brake pressure on the four wheels independently in order to achieve the required deceleration by means of the electric motor. The size of the ESC-R is slightly larger than a normal ESC, due to the pedal feel simulation and the modified valves.

TRW Slip Control Boost – SCB 1.3.1.3 It is a complete braking architecture (Master cylinder (MC) + electro-hydraulic control unit EHCU) for blending and boosting and simultaneously it has ESP functionality.

Figure 1-9: Left: TRW Slip Control Boost Electro-Hydraulic Control Unit EHCU

Right: TRW Slip Control Boost Modified Master Cylinder


It consists of two different modules: a modified master cylinder that is normally decoupled from the brake circuit an electro-hydraulic control unit (EHCU) that generates and modulates the pressure

requested by the driver (see Figure 1-9)

Inside the EHCU a pedal simulator senses the pressure imposed by the driver. A power unit consisting of a hydraulic pump, accumulator and a set of valves, modulate the pressure on the wheels. In this way every regeneration strategy is possible because front and rear axles are decoupled (see Figure 1-10). In case of failure the simulator is disconnected and the driver directly brakes the wheels.

Figure 1-10: Hydraulic architecture of TRW SCB Braking System

The new master cylinder totally replaces the NP one and the EHCU replaces the ESP unit however the new unit is larger in size.


ADVICS 1.3.1.4 The ADVICS solution is currently included in the production of the Toyota Prius and its function is very similar to the SCB one. The only difference is that in case of failure the driver can directly brake only the front wheels.

Figure 1-11: Hydraulic architecture of ADVICS Braking System (Toyota Prius)


BOSCH ESP HEV (Hybrid Electric Vehicle) 1.3.1.5Bosch ESP HEV is a solution that allows decoupling only the rear axle pressure from the brake pedal (see Figure 1-12). It substitutes the NP ESP, so that the classic pneumatic booster is maintained.

Figure 1-12: Hydraulic architecture of the Bosch ESP HEV Braking System

Equal to every other architecture, which permits regeneration, a stroke sensor is introduced on the brake pedal. The module is very similar to a classic ESP, but the rear valves are modified. The brake pedal is directly connected to the front axle callipers in every situation, whereas the TV valve permits isolating the rear callipers. Therefore, the rear pressure can be modulated without a disturbing feeling on the pedal for the driver. This architecture is intended to be integrated in vehicles with electric motors on the rear axle so that the deceleration share, which cannot be regenerated, is achieved by rear axle friction brake, and the front axle contribution remains constant. Nevertheless, it can also be used on the front axle of vehicles with electric motor. Since the only modifications are the integration of internal valves and the introduction of the stroke sensor, the size of the whole architecture remains the same as the one of the standard vehicle.

Electro-Pneumatic Brake 1.3.2

The goal of this architecture is maintaining the pneumatic booster, but with a contribution of electrification.


Two solutions are presented; both of them use an almost standard pneumatic booster with an active vacuum control. The driver’s intention to brake is detected by a stroke sensor and the corresponding brake pressure is achieved modulating the vacuum inside the first chamber of a normal booster. Continental – Regenerative Braking System RBS It is similar to a normal pneumatic booster but several modifications have been included (see Figure 1-13).

Figure 1-13: Continental Regenerative Braking System components

A pushrod gap has been introduced in such way that during normal operation no longer any mechanical link between the pedal and the master cylinder exists. A pedal feel simulation guarantees a constant pedal feeling, and a stroke sensor detects the driver’s intention to brake. When the driver presses the brake pedal down, the pedal feel simulator provides the optimum pedal feel for the driver, while the pedal angle sensor measures the brake demand of the driver. The pedal angle is converted into a desired brake torque request according to a predefined project specific curve and afterwards into a desired brake pressure which is achieved by commanding the active booster. In contrast to a conventional braking system the booster is not activated mechanically (this is prevented by the incorporated gap) but only controlled electrically. Therefore, the vacuum pump provides the correct vacuum level, and an active control modulates the pressure inside the booster chamber.


In case of a failure, the active booster cannot provide the pressure modulation, so when the driver pushes the pedal, the gap is filled and the rod presses directly on the master cylinder, allowing a degraded but safe braking. Even if not very bulky, this architecture shown on Figure 1-14 is quite complicated, and in any case it does not permit the split of the front and rear axle pressure. The ESP module should be maintained and a place for the vacuum pump must be found in the vehicle, so the size of the system is not reduced compared to the normal production system.

Figure 1-14: Continental RBS – Simulated braking actuation


Bosch – Pressure Controlled Actuation PCA 1.3.2.1 This architecture is very similar to the previous one, but a modified ESP is implemented (see Figure 1-15). It has different valves, which are more suitable for frequent activations requested by regenerative braking.

Figure 1-15: Bosch regenerative braking architecture (EVP + PCA + ESP hev)


Electro-Mechanical Booster 1.3.3

These components are intended to substitute the NP booster by removing all the pneumatics from the system. They can be handled as an electric motor that generates the booster action instead of the pneumatics: they amplify the force imposed by the driver via a factor that can be adjusted by the software. In case of failure, the mechanical connection between pedal and brake circuit is guaranteed. Normal boosters use the intake system’s under pressure or the pumps driven by the crankshaft in order to obtain the vacuum. This means some energy is always wasted when the engine is running. This further leads to the fact that Stop-and-Go strategies cannot be implemented, because when the engine is off, the booster capability would be compromised. Electro-mechanical boosters solve these kinds of problems, because they are electrically driven and only require energy when the pedal is pressed.

Bosch iBooster 1.3.3.1 It is arranged by an electric motor, a worm gear for translating circular into longitudinal motion and a stroke sensor on the pedal, which is implemented to detect the driver’s intention to brake (see Figure 1-16 and

Figure 1-17).

Figure 1-16: Bosch electro-mechanical actuation (iBooster + standard Tandem Master

Cylinder)


Figure 1-17: 3D drawing of Bosch iBooster

From a functional point of view it is a “position follower” because the motor tries to follow the brake pedal position with a force gain that can be adjusted via software. Several parameters can be adjusted as shown in

Figure 1-18.

Figure 1-18: Basic characteristic and software adjustability of Bosch iBooster

The electric motor can be highly dynamic, therefore it can be used for manoeuvres like emergency braking because the hydraulic flow rate at low pressure is much higher than with ESP. Possibly high pressure application can be started by iBooster and boosted by support of the ESP unit. For two main reasons this system does not permit a real regenerative braking:


it cannot split between front and rear axle but it presses directly the master cylinder the brake pressure cannot be released for regenerative braking without compromising

the pedal feeling Dimensions are similar to a normal pneumatic booster, with removed crankshaft driven pump, if one was present (see Figure 1-19).

Figure 1-19: Bosch iBooster size (left) in comparison with standard pneumatic booster (right)


Hitachi - Electrically-Driven Intelligent Brake EDIB 1.3.3.2The EDIB architecture is quite similar to the iBooster: there is an electric motor that amplifies the driver’s effort when the pedal is pressed (see Figure 1-20).

Figure 1-20: Hitachi electro-mechanical actuation (Electrically Driven Intelligent Brake)

Contrary to the iBooster system in this case there is the possibility to introduce regeneration strategies without affecting the pedal feeling. This is the fact because between the pedal and the master cylinder there are some springs that simulate the hydraulic circuit behaviour and that are able to maintain the same pedal feeling even if brake pressure is different from the one requested by the driver (in particular when pressure is released). For example in case of regenerative braking, the fluid pressure produced by the master cylinder is reduced to an extent corresponding to the vehicle deceleration resulting from energy regeneration by the motor. To accomplish that, the primary piston is moved to a retracted position and the target brake torque relationship is maintained in a way that it provides the braking performance demanded by the driver. The reduction of the master cylinder pressure at this time reduces the reaction force acting on the input rod. However, that reduction of reaction force is balanced by increasing one of the springs according to the relative positional relationship between the input rod and the primary piston. This design facilitates cooperative regenerative braking while suppressing any variation in the driver's brake pedal force. So EDIB enables modulating the brake pressure according to regenerative strategies, but it does not allow splitting front and rear axle pressure: for this function an ESC system is needed.

Electro-Mechanical Brake EMB 1.3.4

In Electro-Mechanical Braking (EMB) systems, the complete conventional hydraulic system (master cylinder and vacuum booster, hydraulic pipes and brake callipers) is replaced by an electro-mechanical system. The hydraulic actuators are also exchanged by motor units driven electrically (see Figure 1-21).


A value of force can be set and the electric motor control will provide the correct achievement. Due to the absence of a mechanical back-up, the system is very critical in case of brake failure (electric malfunction) concerning matters of safety. If the electric power is not sufficient, no braking will take place. Therefore EMB systems have completely changed requirements compared to the previous hydraulic and EHB systems. Many advantages can be obtained due to the EMB implementation, especially from the designing point of view:

All pneumatic boosters, dragged pumps and connections to the intake system are removed leading to high space savings.

Master cylinder and pipes towards callipers are removed causing high space savings. Very modular architecture can be obtained according to different vehicle sizes. The

only difference is the wire length. The system is environmentally friendly due to the lack of brake fluid and also requires

little maintenance (only pads and disks). Its decoupled brake pedal can be mounted in a crash compatible and space saving

manner for the passenger compartment. There are no restraints to the design of the pedal characteristic, so ergonomic and safety aspects are easy to consider.

Electric Parking Brake is integrated in the design without introducing other modules. It is the best solution for regenerative braking: the force can be applied on every

wheel in a different way optimizing the ground forces.

Figure 1-21: Schematic Electro-Mechanical Braking System

Hybrid Electromechanical Brake 1.3.4.1 To meet the rigid safety requirements given by the absence of mechanical back-up it is possible to introduce an intermediate solution in which EMBs are used on the rear axle, whereas at the front axle a classical hydraulic architecture is maintained (see Figure 1-22 (a) and (b)). The following considerations are taken into account:

Rear axle pressure can be easily modulated for regenerative strategies Rear axle can be braked independently from front axle, permitting easy EBD

strategies Rear pipes are totally removed and substituted by wires


Brake fluid quantity is halved EPB is integrated in the architecture without implementing other modules The booster can be downsized because absorption levels are even more than halved ESP needs only two channels

For safety reasons it is necessary to use a bigger amount of sensors: a stroke sensor connected to the brake pedal to detect the driver’s intention to brake and also to permit regenerative strategies; additionally a pressure sensor on the master cylinder is needed for redundancy. However, one pressure sensor is already integrated in the ESC module, so this one can be used.

a)

b) Figure 1-22: (a) – Standard braking circuit layout modification (b) – Functional architecture of

Hybrid Electro Mechanical Braking System


Brembo EMB 1.3.4.2 This is a possible solution for Hybrid Electro-Mechanical braking that allows having a maximum clamping force of 13 kN. It is driven via a CAN and an EPB is integrated in the system (see Figure 1-23).

Figure 1-23: Brembo electro mechanical braking actuator (EM caliper)


Siemens VDO Normally EMB requires a quantity of energy that is not insignificant. Siemens VDO tries to solve this problem with a solution based on a wedge principle that exploits the kinetic energy of the car to amplify the clamping force: the braking force is auto-engaged by the speed of the disc (see Figure 1-24).

Figure 1-24: Siemens electro mechanical braking actuator (EM caliper)

The difference between a conventional braking system and the wedge braking technology concerning the supply of the needed force is shown in Figure 1-25.

Figure 1-25: Comparison between conventional brake and wedge brake technology


Cost evaluation 1.4

To do a comparative cost analysis is complicated because of diverse reasons. First of all, the costs of a specific component depend on many factors related to the specific vehicle’s platform application. In particular it is not possible to define a component’s price and the connected costs of engineering development and tooling (done by the suppliers) without information about production volumes and the component’s requirements specifications. In any case the suppliers are not willing to provide this information if no specific business reason is defined. Moreover, most of the technological solutions presented are used on competitors’ vehicles, and therefore, the costs are sensitive data that cannot be published. Finally, some solutions are not yet installed in any vehicle or their development phase has not been finished too, thus there is no information on their costs at all.


2 Electric and electro-hydraulic steering

Introduction to power steering system solutions 2.1

Nowadays classic combustion engine vehicles are more and more replaced with alternative energy powered vehicles or with hybrid ones. This is mainly due to the implementation of strict car fume standards and of the pursuit of minimising the fuel consumption. The emission of exhaust gases can be eliminated totally by using electric motors to drive vehicles. There are also systems which help to achieve the wanted results through changes in the vehicle engine. An example of such solution may be the Start-Stop system, which interferes directly with the vehicle’s functions. This system switches off the engine whenever it is not used to propel the vehicle, e.g. during a stop in a traffic jam, and instantly switches it on again when the driver wants to start. Power steering systems in vehicles serve the purpose of minimising the effort needed to move the steered wheels. Such hydraulic power systems first appeared in the 1950s. They were appreciated as a solution which highly increased the driving comfort, in particular in heavy vehicles. At high speed, however, the functioning of the power steering system proved to be too soft and inaccurate, often leading to dangerous situations. Therefore, further development of these systems was inevitable. One of the most important aspects taken into consideration in their design is the energy needed for proper functioning of the system as well as its efficiency and, as a result, its influence on fuel consumption. Modernisation of the system was a consequence of increasingly high requirements concerning vehicle safety and comfort. Nevertheless, this advancement mainly consists of further improvement of already available systems, designing new solutions and seeking for new applications. ZF has developed the Servotronic system, which fully adjusts the power assistance to the vehicle speed. At low speed (e.g. parking, which requires many moves with the steering wheel) the power assist is maximal. At higher speed it is reduced, which makes maintaining high precision of the system possible and enables the driver to keep full control of the vehicle. Power steering systems, which are used nowadays in heavy vehicles, may be divided into two main groups characterized by the type of booster pump used:

mechanically powered by the combustion engine

powered by the electric motor Power steering systems are used in:

heavy vehicles (enables manoeuvring)

passenger cars (facilitating manoeuvring)


1

6

3

2

4

52

Hydraulic power steering system 2.2

Working principle of the system 2.2.1

The scheme in Figure 2-1 shows the components of a heavy vehicle power steering system:

oil tank (1)

booster pump (4)

steering gear (3)

hydraulic couplers (5)

hydraulic pipes (2)

hydraulic hoses (6)

Figure 2-1: Heavy vehicle power steering system

In hydraulic power steering systems the needed oil is fed by a high pressure oil pump (sliding-vane or gear pump) powered by the combustion engine. A pump powered like that must have the required delivery rate at idle condition. At higher rotational speed of the combustion engine a flow limiter must be used, which prevents the maximum permissible delivery rate from being exceeded. The pressure created by the pump gets to a mechanical gear with hydraulic assist. Nowadays there are two types of steering gears used in heavy lorries:

worm-and-nut gears

toothed gears (gear train) with various variants and structures The world market of steering gears for lorries is dominated by ZF and RTW, which offer a wide range of solutions. Also ThyssenKrupp, Presta, SteerTec and Luk actively operate on the market.


Figure 2-2: Drawing showing the working principle of a power steering system

A – oil tank, B – pump, C – feeding conduit, D – return conduit, E – thrust washer, F – bearing,

G – box, H – piston, I – throttle valve, J – ball nut, K – worm shaft, L – torsion bar, M – pin, N – safety valve, O – by-pass valve, P – input shaft, Q –distribution valve core,

R – distribution valve box, S – main shaft with a toothed segment, T – cover

Steering gears enable steering even when the effort assist is lost. In such case the working mechanism is the same as for a regular mechanical gear. The input shaft (P) is connected to the hollow worm shaft (K) via the torsion bar (L). Excessive mutual angular displacement of these elements is prevented by the pin (M). The rotary motion of the steering wheel is transmitted onto the input shaft. After the torsion bar (L) rotates around a small angle (about 7°), the transmission of the torque is transferred to the pin (M). The worm shaft (K) with rightward rifling is fit tightly into the box (G) via two linear needle bearings (F) and a thrust washer (E). Through the nut (J), caused by closed circulation of the balls, the turn of the input shaft (P) is induced. Because of its coupling with the steering wheel the rotational force is transformed into linear motion of the piston (H). The piston is toothed and the gear interlocks with the main shaft (S) connected to the drop arm. The angular motion of the drop arm is transmitted to the steering linkage of the steered axle by the linear shaft. Steering gear transmission and the number of revolutions between the extreme positions of the wheels depend on the meshing radius between the piston and the main shaft and also on the angle of the screw line (helix). In a proper working steering gear the process of mechanical torque transfer is hydraulically assisted as a result of pressure created by the external oil pump. The hydraulic part of the


gear consists of the piston (H) and the distribution valve, embedded in the input and worm shaft. The valve core (Q) constitutes one unit together with the input shaft (P). A segment of the worm shaft (K) functions as the valve box (R). The input shaft is located inside the worm screw. There are six linear valve seats on the surface of the core and the distribution valve box. Valve seats in the core are alternately connected to the feeding conduit (C) and the return conduit (D) through oil passages. Three of the six seats in the valve box are connected with the left side of the piston, the others with the right side. Connection of these seats is also alternate. If there is no force put on the torsion bar, the distribution valve is in its neutral position. Calibration of this valve position takes place during the production stage of steering gears. In this position the seats in the core (Q) and in the box (R) are positioned in relation to one another in such manner that spacing on the piston's (H) left and right are at the same time connected with the feeding conduit (C) and the return conduit (D). In other words, the feeding conduit is connected with the return conduit. When the engine is started oil pumped by the hydraulic pump flows through the distribution valve and is directed back to the tank. Forces applied to both sides of the piston equalise. It is the so-called hydraulic central position.

System installation 2.2.2

There are many inconveniences installing the system. A characteristic feature of the hydraulic power steering system is the hydraulic pump, which must be powered directly by the combustion engine, from the power take-off. Another issue is related to the hydraulic hoses and pipes, which stretch out over the entire vehicle (Figure 2-3), from the combustion engine (11), located in the back, to the steering gear (3), located in the front. This solution generates flow losses and a problematic transition through the vehicle joint. During installation of the hydraulic systems there are also problems related to requirements concerning the placing of hydraulic hoses the radiuses of bends etc.

Figure 2-3: Example of bus power steering system

Cost analysis 2.2.3

Costs were calculated for a prototype unit. Because of the common use of the listed components the cost is exactly the same as the price of the standard system.


NAME WEIGHT [kg] PRICE [EUR]

Oil tank 0.5 60.2

Pump LUK LF77 ca. 8

Steering gear ZF 8098.955.698 ca. 40 496.38

Hydraulic couplers 152.28

Hydraulic pipes and hoses Operating fluid

125.41

TOTAL 834.27

Fuel savings 2.2.4

The described type of power steering system is the simplest solution that does not reduce energy consumption, hence fuel consumption. The pump is constantly consuming energy from the engine, and at high rotational speed oil with too high pressure is directed to the tank by overflow valves.

Electro-hydraulic powered steering system 2.3

Because it is steadily required to decrease the emission of harmful substances and also to diminish the fuel consumption of motor vehicles, more and more hybrid and electric drives are created. In hybrid vehicles the combustion engine does not need to operate all the time during vehicle motion. Using a classic power steering system with a pump powered by the vehicle’s engine in a hybrid drive (combustion engine not running) is therefore impossible - there would be periodical lacks of pressure in the steering system. To energize the power steering system pump from the vehicle’s engine is also impossible in fully electric vehicles and in trolleybuses.


The working principle in the electro-hydraulic power steering system is very similar to that of the just hydraulic one. The main structural change is the replacement of the mechanical hydraulic pump by an electrical one. Figure 2-4 and Figure 2-5 show a power steering system used in a hybrid bus by Solaris Bus & Coach from a different view. The working principle of this system is based on an electric motor working at constant rotational speed, connected directly to a hydraulic pump.


Figure 2-4: Example of electric power steering system (side view)

Figure 2-5: Example of electric power steering system (top view)

Drive is provided by a 3x400 VAC three phase asynchronous motor, which is powered by a 600 VDC/3x400 VAC converter (see Figure 2-6). Such solution enables easy control of the whole system. Further possibilities include the usage of a 3x400 VAC motor, but this one is powered by the bus existing boardnet installation (24 VDC) through a converter. There is also the possibility to use a 24 V motor, fed directly by the bus existing boardnet installation. Using constant rotational speed of the electric motor it is possible to benefit from a pump having significantly smaller capacity.

Electric motor TAMEL 2,2kW

BOSCH-REXROTH pump PGF2-22 008RE01VE04-8CCM

Check control Steering gear

ZF 8098.955.806


The greatest reduction of energy losses may be achieved by a system in which the engine powering the pump operates at variable rotational speed rates. During manoeuvring in a parking situation the pump produces high pressure, which increases the steering effort assist. On the other hand when driving on a straight road, the pump produces very low pressure, which causes energy savings. In an electric steering system this pressure change between manoeuvring in park situation and driving at higher speeds is controlled by the rotational speed rate of the electric motor, which powers the hydraulic pump. The electric motor speed depends on:

vehicle speed

angular velocity of the steering shaft

vehicle load (total weight)

The power steering system is controlled by CAN-BUS.

Figure 2-6: Electrical diagram of a 3x400 VAC three phase asynchronous motor and a 600 VDC/3x400 VAC converter

Power steering system electric pump without adjustable turning angle This is the most effective way of using the power steering system electric pump. It has at least three connections - two for powering and one for receiving information of the motor operation. Optionally an additional connection sends information on the vehicle speed to the pump, enabling steering effort assistance based on the vehicle speed. This pump only

CONTROL

Electric motor of hydraulic pump


operates when the motor is active and because of having no connection to the turning angle sensor it must perform without this support. Power steering system electric pump with adjustable turning angle This system contains an electric pump without a turning angle sensor. The differences are the following:

If a vehicle has a turning angle sensor, the pump may operate accordingly and the vehicle benefits from an increased steering effort assist.

This pump may be fully integrated within the CAN-Bus system therefore it can communicate with other electrical steering units of the vehicle.

This pump can additionally be programmed to work in an ecological mode turning the power off when the power steering system is not needed. Power steering system electric pump with adjustable turning angle The difference between this system and the one described above lies in the hardware and software of the pump’s electronic control unit. In this system the electric pump can log into the vehicle electronic control system but can only be adjusted to the vehicle via a specific diagnostic system obtained by the manufacturer of the vehicle. This means that the steering system is interacting more with the vehicles electronic. The solutions described above enable the usage of advanced comfort and safety systems, such as Park Assist, Steering Torque Control and Lane Keeping ones.


As far as installation is concerned the electro-hydraulic system is less problematic compared to the hydraulic one. Figure 2-7 shows the power steering system installation in the Solaris Urbino electric bus. All components of the system are located near the left front wheel arch. The system is compact and there is no need to run hydraulic hoses through the entire length of the bus. The described type of a power steering system is the most convenient in terms of installation because it enables placing the components in convenient places in the bus. It does not require a reworking of the dashboard.


Figure 2-7: Example of a power steering system installation in a bus

Cost analysis 2.3.3

Costs were calculated for a prototype unit. Because of common use of listed elements the cost is exactly the same as the standard system price.

Table 1: Cost analysis

NAME WEIGHT [kg] PRICE [EUR]

Oil tank 0.5 60.2

Electric motor SLG-100L4A 230/400V WTO-067 104.75

Hydraulic pump PGF-2-22/008 RE01 VE4 ca. 8 348.31

Coupling linking the electric motor

with the hydraulic pump 19.23

Steering gear ZF 8098.955.698 ca. 40 496.38

Hydraulic couplings 17.08

Hydraulic pipes and hoses 72.2

Clamps, seals and other 41.88

TOTAL - 1160.03


Fuel savings 2.3.4

Using the presented solution may decrease fuel consumption between 0.3 to 0.4 dm3/100 km outside of urban areas and in urban areas fuel savings reach 1 dm3/100 km.

Figure 2-8: The decrease in fuel consumption achieved by the usage of an electro-hydraulic

power steering system

The decrease in fuel consumption achieved by the usage of an electro-hydraulic power steering system in function of a vehicle’s speed with a ZS motor is shown in Figure 2-8.

Electric power steering system 2.4

In electric power steering systems hydraulic components have been totally eliminated -. All steering and powering components are fully electric. The electric power steering system is the most innovative solution so far. It is quite widespread in cars but due to its principle of operation it is still problematic to use it in heavy vehicles. The pioneer in using the electric power steering system is TRW. This company also participate in research and development to implement the system in heavy lorries and buses.


Solely electric power steering is enabled by an electric motor connected with the steering column or the toothed bar. It is controlled by a microprocessor taking into consideration turning directions, torque value transmitted by the steering shaft, vehicle speed and


combustion engine rotational speed rate. The value of steering effort assist is calculated based on this data, but in some systems the driver can increase it by pressing a button, e.g. during manoeuvring at low speed. The systems which are closest to realization in heavy vehicles are those offered by TRW. Their Electric Power Steering System with Belt Drive incorporates a brushless motor, a toothed belt and a bearing nut unit with a roller. It also enables a more direct feel of the steering system and its faster reaction. With the development of this technology, the Electric Power Steering System with Belt Drive may be used in all applications that need a very compact construction and have very limited installation space. This is a prototype solution proposed by TRW which is under development phase.


Installation of a fully electric power steering system (without belt drive) using an electrical motor in buses or trolleybuses is quite problematic due to large sizes of the electric motors. This system is located on the steering shaft and there is no room for placing a large electric motor (See Figure 2-9). This solution is very popular in automobile industry and should be adapted to the buses applications which will be a big issue due to the electric motor size.

Figure 2-9: An example of electric power steering system solution

Cost analysis 2.4.3

Due to the lack of such solutions on the market it is impossible to calculate the costs of a fully electric power steering system for busses. It is only possible to estimate the costs based on a comparison with electro-hydraulic systems.


The obvious advantage of the electric system is that there are neither hydraulic hoses nor operating fluids. Therefore, the costs of couplings, fluids and hoses are eliminated. Other advantages of this system are the significantly lower operating and maintenance costs. During a system’s inspection there is no need to check the fluid pressure, or the system bleeding etc. The system can be analysed easily by just using a diagnostic tool.

Fuel savings 2.4.4

The solution closest to being implemented into buses is the electric power steering system with belt drive. This is due to the higher efficiency compared to classic systems that are placed on the pinion shaft and powered by a hydraulic pump. This system also only uses less energy whenever it is necessary, that means an electric solution will work when a driver will try to turn steering wheel. Without putting an additional load on the motor, the Electric Power Steering System with Belt Drive enables fuel savings up to 0.33 l/100 km correlated to this is a decrease of carbon dioxide emission by about 8 g/km.

Documents

Technology evaluation report for electrical actuated mechanical