The Self-Energising Hydraulic Brake Integrated Braking Torque Control without Electrical Energy Driver assistance systems require for low-power electrical interfaces for braking control. At the same time for safety reasons a mechanical fallback level should be provided. At the Institute of Fluid Power Drives and Controls of the RWTH Aachen University (Germany) an innovative Self-Energising Hydraulic Brake is being developed to solve this conflict of objectives. Besides its low power consumption it features a closed loop control of the actual braking torque. The presented measuring results demonstrate the brake’s development potential for cars and util- ity vehicles. RESEARCH ATZ 10I2008Jahrgang 110 46 Brakes
Transcript
The Self-Energising Hydraulic BrakeIntegrated Braking Torque control without electrical energyDriver assistance systems require for low-power electrical interfaces for braking control. At the same time for safety reasons a mechanical fallback level should be provided. At the Institute of Fluid Power Drives and Controls of the RWTH Aachen University (Germany) an innovative Self-Energising Hydraulic Brake is being developed to solve this conflict of objectives. Besides its low power consumption it features a closed loop control of the actual braking torque. The presented measuring results demonstrate the brake’s development potential for cars and util-ity vehicles.
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Brakes
1 Introduction
Conventional hydraulic brakes not only have a mechanical-hydraulic fallback lev-el; they also achieve a high degree of comfort and active safety by means of electronic closed-loop control systems. Today’s conventional brakes do have weak points, however, such as the space required to accommodate the necessary infrastructure – including brake booster and hydraulic control unit – the high power demand and the way in which braking intervention by the vehicle con-trol system affects pedal feel. Research is therefore being concentrated on finding alternative braking concepts, which re-quire less power, offer better dynamic performance and comfort and can be driven electrically, as well as complying with the stringent safety requirements.
The hydraulic energy required to actu-ate the brake cannot be generated by the driver alone. Brakes that are not self-rein-forced therefore generally need brake boosters, which are mostly vacuum-fed. The necessary vacuum is no longer sup-plied by modern engines free of charge, however. Turbo-charged spark-ignition engines and diesel engines require a vac-uum pump, which not only incurs addi-tional costs for the components, but also increases fuel consumption, and the space required to accommodate a vacu-um brake booster constitutes another disadvantage: Power demand is deter-mined by the driving dynamics require-ment, which means that the actuating forces can only be reduced by self-rein-forcing or self-energising brakes that use the kinetic energy of the vehicle.
Braking intervention by a driver as-sistance system should have as little feed-back to the driver as possible for reasons relating to both comfort and safety. This leads to the demand for the ability to sep-arate the mechanical drive from the hy-draulic drive, at least temporarily. This is already the normal situation as far as hy-brid vehicles are concerned, as the vehi-cle controller is required to divide the braking torque between the friction brake and the regenerative brake (brake blending). Nevertheless, a pedal simula-tor is needed here in order to communi-cate the sense of deceleration via the ped-al. An extension of the functions of a con-ventional hydraulic brake therefore inev-
itably leads to parallel system structures, which incur costs.
An innovative Self-Energising Electro-Hydraulic Brake (SEHB), which is charac-terised by low energy consumption and a high degree of ruggedness and comfort, has been developed on the basis of the (supercritical) self-energising effect at the Institute of Fluid Power Drives and Controls (IFAS) of the RWTH Aachen Uni-versity [1]. This first implementation of the hydraulic self-energising principle was realised within the framework of a research project funded by the German Research Committee DFG in the setting of a railway application with electrically driven valves [2, 3]. The electrical control system offers many advantages, particu-larly for long vehicles with many brakes. A hydraulic control concept offers a feasi-ble alternative for shorter vehicles, how-ever, and appeals by virtue of its simplic-ity and reliability. The concept was there-fore adapted to the operating conditions in cars and utility vehicles, was simpli-fied to a great extent at the same time and the design was made even more com-pact in terms of installation space. The result is the Self-Energising Hydraulic Brake (SHB), which enables integrated closed-loop braking torque control and therefore offers a means of achieving even deceleration independent of chang-es of the friction coefficient between disc and pads, using hydraulic components without electronics. The principle of the concept is such that it offers the possibil-ity of a hydraulic through-drive, as well as interfaces for energy-minimised con-trol using electric actuators.
2 Self-Energising Brakes
Self-Energising brakes use the generated braking power to boost their own actuat-ing force by using suitable gearing to di-vert the braking power to the application force of the brake pads. There are many mechanical solutions for this. The most well-known are the drum brake and the wedge brake shown in Figure 1 left and middle. As far as the drum is concerned, the braking force Fbrake causes extra torque to act on the brake lever to ampli-fy the braking effect. The amplification is defined by the friction coefficient and angle α. The smaller the angle α, the
The Authors
Dr.-Ing.Matthias Liermann is Leader of the re-search group systems and control enginee-ring at the Institute for Fluid Power Drives and controls (IFas) at RWTh aachen Univer-sity (Germany).
Dipl.-Ing. Julian Ewald is member of the aca-demic staff at IFas at RWTh aachen Univer-sity (Germany).
Dipl.-Ing. Jan Elvers is member of the aca-demic staff at IFas at RWTh aachen Univer-sity (Germany).
Univ.-Prof. Dr.-Ing. Hubertus Murrenhoff is Director of the IFas at RWTh aachen Uni-versity (Germany).
Dr.-Ing. Christian Stammen is Director of Research and systems at Montan-hydraulik Gmbh at holz-wickede (Germany).
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greater the supporting force at the ful-crum of the lever required to compen-sate the friction force Fbrake. The amplifi-cation achieved by this is subject to cer-tain limits, however, as the process may be destabilised by a shift in the frictional contact point or a change in the coeffi-cient of friction. The amplification factor of a simplex drum brake usually amounts to around 2.0 to 2.3.
The wedge brake, Figure 1 middle, us-es a similar operating principle, whereby the friction force is supported by an in-clined wedge plane. In an effort to achieve a balance with the supporting force on the wedge plane, the wedge is retracted into the friction contact while compres-sion increases. The shallower the wedge, the greater the blocking tendency of the
brake. Brake-by-wire developers are pin-ning great hopes on the electromechani-cal wedge brake, which is characterised by its operating point being in the critical self-energising range. The necessary actu-ating forces are relatively small here; however, the braking process is unstable. The unstable operating point can be cor-rected using a high-performance closed-loop control system [4].
The hydraulic self-energising princi-ple is illustrated in Figure 1 right. The braking force is supported by cylinders fitted on either side. A suitable hydraulic link routes the generated pressure back to the brake actuator cylinder and sup-plements the normal force. An appropri-ate area ratio between supporting cylin-der and brake actuator ensures that the
supporting pressure is always higher than the causative braking pressure. This means that the entire hydraulic energy required for braking can be generated by the braking process itself. This is one of the main points that differentiate this principle from that of the electrome-chanical wedge brake: While the Self-En-ergising Hydraulic Brake is theoretically capable of supplying any amount of brak-ing force for the same amount of drive energy, the power demand of the electro-mechanical wedge brake grows as the braking force increases.
3 The Hydraulic Self-Energising Concept
Figure 2 shows the hydraulic circuit of the Self-Energising Hydraulic Brake. Apart from the conventional disc and pads, it comprises a frame, which can be moved from side to side and is capable of rout-ing the braking power into the support-ing cylinder. The valve unit belonging to each brake actuator comprises three check valves (CV1-3) and hydraulically ad-justable seat valves (generator/relief valve). The brake has two outgoing hy-draulic connections, one depressurised line leading to the reservoir and one line connecting it to the actuating unit.
3.1 Braking Force Build-UpIf the actuating unit dictates a pressure value while the brake is open, the high-pressure valve is opened by the effect of the actuating pressure on one side of the valve. It can be closed by the supporting pressure, but this is not effective initially. Figure 2: SHB circuit concept
The valve’s seat element has two active sur-faces with an area ratio of 1:12. It is bal-anced when the supporting pressure is twelve times higher than the actuating pressure. The construction of the low-pres-sure valve is analogous, with an area ratio of 1:13. The volume flow into the brake ac-tuator leads to the brake pad being ap-plied. As soon as braking force has built up as a result of this, the load causes one of the two supporting cylinders to be pressed in according to the direction of rotation. The generated pressure is applied to the input of the high-pressure valve via the check valve. The pressure in the support-ing cylinder is always higher than the pressure in the brake actuator because of the small piston area, which means that the brake is at the supercritical self-ener-gising operating point. The initial applica-tion pressure increases very rapidly. The supercritical self-energising principle pro-vides for a rapid build-up of braking force, even if the coefficient of friction is low, un-til such time as the supporting pressure is so high that the high-pressure valve is closed against the actuating pressure. This is the point at which the brake has achieved the specified target setpoint.
3.2 Controlled Brake TorqueUnlike any other brake, the closed-loop hydraulic control system of the SHB real-ises a brake torque setpoint rather than a compression setpoint. Figure 3 illustrates the fact that the brake torque setpoint is adjusted autonomously, irrespective of the current frictional coefficient. The dia-gram shows the actuating force of the SHB required for a specified brake torque of 2000 Nm compared with the necessary
frictional coefficient-dependent compres-sion force according to the design of the prototype. The brake torque is adjusted automatically, irrespective of fluctuations in the coefficient of friction, so that the driver is not taken by surprise by a mea-gre braking effect when the frictional co-efficient of the brake is drastically re-duced by moisture or de-icing salt.
3.3 Braking Force ReductionIf the actuating pressure decreases, or the frictional coefficient increases to-wards the end of a braking operation, the low-pressure valve opens and the brake fluid escapes from the brake actu-ator into the reservoir. The elastic seal of the brake piston pulls it back slightly at the end of the braking operation in ex-actly the same way as a conventional brake. As soon as the braking force has been reduced, the supporting cylinder is returned to its original position by its in-tegrated spring, while drawing brake flu-id in from the reservoir via a check valve. Apart from the initial application of the brake pads, the energy required for the
braking operation is completely supplied by the braking torque itself.
3.4 Is the SHB Exhaustible?The volume of the supporting cylinder constitutes the pressure supply to the brake. It is designed and dimensioned in such a way as to provide sufficient pres-sure for maximum braking operations and all regulating interventions on the part of the control system. If it is almost exhaust-ed, in spite of this, the brake is released briefly and returned to its original posi-tion. During this time, the braking torque can be distributed between the remaining brakes in order to stabilise the vehicle.
3.5 ScalabilityIn principle, the brake can be scaled up and down at will, as the energy required for braking is produced by the braking torque. The active surfaces of the brake piston and the supporting cylinder are the only dimensions that need to be adapt-ed to the requisite compression forces. The SHB therefore offers a particularly in-teresting solution for utility vehicles and
Figure 4: Space requirement for first prototype (brake in rim as CAD image)
Figure 5: Valve unit for purely hydraulic closed-loop control, effect of the positive overlap through un-equal surfaces
Figure 3: SHB actuating force for 2000 Nm brake torque compared with the necessary compression force
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buses, which require the generation of high, closed-loop-controlled compression forces.
4 The SHB Prototype
Although use of the hydraulic self-ener-gising principle requires additional com-ponents in the vicinity of the wheels and more space to accommodate them than a simple hydraulic brake, there is no longer any need for a brake booster to be fitted in the engine compartment. The realisable degree of compactness is there-fore an important issue, as well as the amount of extra weight. The IFAS de-signed, developed and built a first proto-type in order to demonstrate the con-cept’s feasibility for cars and utility vehi-cles. A 17 inches rim was chosen as the setting. Figure 4 shows a three-dimension-al Computer Aided Design (CAD) image of the prototype in the installed posi-tion.
4.1 Brake Actuator and Supporting CylinderFigure 4 shows the floating calliper, the frame to guide the pads and the support-ing cylinders, positioned symmetrically on either side. The floating calliper is guided across the brake disc by two guide rods that are connected to the frame. The frame is guided in the direction of the supporting cylinders by guide rods, which are not visible in the figure. The element that accommodates the sup-porting cylinders is permanently con-nected to the axle support.
4.2 Valve UnitThe valve unit is the heart of the brake. It is accommodated in the running gear at the side to keep the unsprung weight low, and is connected by means of hoses. As the electric actuators for the superim-posed brake controller can also be inte-grated here, another argument in favour of fitting the valve unit in the running gear is the associated reduction in vibra-tory load. Figure 5 shows the SHB valve unit. The two valve spools are clearly visi-ble, the high-pressure valve having an ar-ea ratio of 1:12, while the low-pressure valve has an area ratio of 1:13. The actuat-ing pressure always acts on the larger sur-face and the supporting pressure always acts on the smaller surface, Figure 2, so that the valve unit performs the function of a brake booster. A front wheel must be braked with around 95 bar braking pres-sure (2000 Nm brake torque, 0.115 mm mean brake radius, a frictional coeffi-cient of 0.4 and 54 mm brake actuating piston diameter) to achieve full braking of a mid-range car weighing 1.5 metric tons. The area ratios of the valves are such that 35 bar actuating pressure is required with low volume flow. As the hydraulic energy for brake compression is supplied by the supporting cylinder, the actuation process only has to supply a very small amount of energy to adjust the valve.
5 Test Results
The brake actuator unit with supporting cylinders has already been built and tested, Figure 6. In an effort to verify the function-ality of the self-energising brake actuator with compact plunger support, it was ini-tially tested with an electrically driven con-
trol valve as work is still in progress on the mechanical/hydraulic closed-loop control system. Figure 7 shows the hydraulic circuit for the prototype tests.
The valve connects the supporting cyl-inder to the brake actuator when the lower position is active. The brake actua-tor is otherwise connected to the reser-voir. The pressure setpoint input acti-vates the self-energising function, which is made available via the brake pedal. A regulated setpoint input facility is used for the measurement to ensure compara-bility between different measurements.
The measurement results for a rapid increase in braking force from 0 bar to 300 bar supporting pressure psup are shown in Figure 8. They demonstrate the high dynamic performance of the self-energising effect.
The valve initially opens proportional to the growing system deviation. The route from the pressure supply to the brake actu-ator is released and brake fluid flows into the brake actuator at low pressure. Once hoses, seals and pads have been pre-com-pressed, the pressure begins to build up in the brake actuator with a delay of ta = 47 ms. As the compression rises, the brak-ing torque increases and, with it, the sup-porting force acting on the supporting cyl-inders. The self-energising effect triggers as soon as the pressure generated in the sup-porting cylinder exceeds 15 bar at tSE = 139 ms. This is achieved after 139 ms. The slight kink in the supporting pressure characteristic indicates the closing of the check valve. The compression increases with a steep slope from this torque value. The pressure-build-up time amounts to ts = 239 ms. The closed-loop brake control sys-tem reaches its setpoint value without any delay. The response delay ta, added to half of the pressure build-up time ts, constitutes a measure of the dynamic braking per-formance. This means that the value ta + ts/2 = 193.5 ms achieved by the SHB is lower than the value usually achieved by car brakes today, which is 200 ms [5].
6 Electrical Control Concepts for SHB
The SHB is a rugged mechanical-hydrau-lic implementation of a closed-loop braking torque control system, which operates independent of an electric pow-er supply. Nevertheless, it must offer ap-
Figure 6: Prototype during the brake test
Figure 7: Hydraulic circuit for prototype measurement and testing
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Brakes
propriate interfaces for an electrical control function so that it can be driven by driver assistance systems or to enable brake force blending in hybrid vehicles. Three different control concepts are shown in Figure 9.
In principle, the closed-loop hydrau-lic brake control process may be influ-enced by the use of additional electro-magnets on the control valves. This con-cept offers a means of influencing all as-pects of the control process during a braking operation, enabling the realisa-tion of an Antilock Braking System con-cept. A solution like this cannot trigger a braking operation alone as required for the Electronic Stability Programme (ESP), however. The control valves exert no influence at all when the brake is open and in the absence of actuating pressure. A pump is needed to initiate a braking operation. Two solutions for this problem are shown in the Figure. The first one provides for a single-piston piezoelectric pump combined with a re-lief valve. The second solution uses a bi-
directional pump, which is powered by an electric motor in a closed-loop pres-sure control circuit. The important thing about this is that the pressure level and flow rate are much lower in this concept than in the current ESP systems, where the pump is required to deliver the com-plete compression volume at full brak-ing pressure. In the SHB concept, the pump only delivers the volume flow re-quired to build the actuating pressure up at a much lower pressure level.
7 Outlook
The SHB is completely self-sustaining thanks to the closed-loop-controlled su-percritical self-energising concept. It is predestined for applications, which have limited control capacity, but are still re-quired to regulate large quantities of braking power. This is the case in the automotive sector, but more particularly in commercial vehicles and buses. Al-though a hydraulic-mechanic through-
drive concept from pedal to brake is easy to realise, it is not absolutely essential, as the actuating pressure can also be pro-duced by a small pump operating at a low pressure level. The IFAS has demon-strated that the concept can be realised in an automotive setting on a compact test rig prototype and is continuing to pursue this approach. The list of possible applications for the self-energising hy-draulic brake with purely hydraulic closed-loop control is a long and diversi-fied one and they are by no means limit-ed to motor vehicles. They could conceiv-ably be used in aircraft, to the same ex-tent as in stationary industrial plants. The RWTH Aachen University has ap-plied for a patent for the SHB.
[2] hermanns, M.; hennen, M.; Liermann, M.; stützle, T.: Intelligentes, Integriertes einzelrad-antriebs-Brems-Modul [Intelligent, integrated single wheel traction and braking module] (eaBM), eTR 04/2008, pp. 222-228
[4] Gombert, B.; Gutenberg, P.: Die elektronische Keil-bremse, Meilensteine auf dem Weg zum elek-trischen Radantrieb [The electronic wedge brake, milestones along the road to the electric wheel drive], aTZ 11/2006, s. 905-913
[5] Breuer, B.; Bill, K. h. (Publishers) Bremsenhand-buch, Grundlagen-Komponenten-systeme-Fahr-dynamik [Brake handbook, basic principles – components, systems – driving dynamics], Friedrich Vieweg & sohn Verlag / GWV Fachverlage Gmbh, 2003
Figure 8: Measurement results
Figure 9: Integrating the SHB into closed-loop vehicle control systems
