GB2478423A - Anti-lock brake with piezo actuator and independent optical measurement of the instantaneous wheel velocity - Google Patents

Abstract

A strong piezo-actuator is inserted between brake piston and back side of the brake pad and is included in a regular anti locking brake system's feedback loop normally acting only on the hydraulic brake pressure system. Furthermore a laser scanner velocimetry is proposed to achieve a more accurate measure of the vehicle speed or respectively the longitudinal and lateral (drift) speed of each wheel axis. Both concepts together should enable a feedback response time roughly of the order of 10 microseconds which is much shorter than the nowadays achievable 10 milliseconds response time of solely hydraulic brake force generating systems which should make the anti locking brake system much more effective. Upon emergency breaking a vehicle can be held much closer to the limit of the elastic slip regime of the wheel slip (static friction regime) where the highest deceleration can be reached just before the vehicle transitions into sliding friction wheel slip.

Description

Concept for a piezo-supported anti locking brake comprising a very fast feedback response with an independent measurement of the instantaneous wheel velocity.

Introduction and problem:

For an anti locking brake the time constant of the braking force regulatorTs feedback control is an essential factor, as to how closely the braking force can be kept exactly at the static friction limit, i.e. just before the point where static friction would experience the transition into sliding friction and thus the friction coefficient would experience a discrete jump to a drastically lower value and thus the brake path of a vehicle, in particular a car or motorcycle or airplane, would be drastically extended. The smoother the road surface, the more critical this behavior is and the more inaccurate this feedback control will work the longer its time constant is. This braking force point of maximum deceleration is located so to speak at the maximum of the range of the mere elastic wheel-"slip", i.e. at that point of the braking force, at which the tire rubber is maximally elastically deformed, i.e. at which the forward velocity of the wheelTs axle is already faster than the vehicle speed calculated from wheel revolutions per time and the undeformed wheel circumference, but the wheel is not yet sliding on the asphalt. In simple words, the read-out vehicle velocity from a calibrated speedometer during braking is slower than the actual forward velocity of the wheelsT axles and thus of the vehicle already in the static friction regime. The equivalent holds vice versa for acceleration phases and thus for the traction control. Once the tire rubber begins to slide, this difference in velocity between the value as read-out from the speedometer and the actual forward velocity of the wheelsT axles will discontinuously and drastically increase, until the wheels will lock up completely and the speedometer then will show zero speed of course.

Current anti lock brake regulators are realized purely hydraulic, such that the time constant of the feedback control is in the range of several 10 to 100 milliseconds. Thus it is obviously impossible, especially under the condition of black-ice or hard-packed snow (when the jump of the friction coefficient from static to sliding friction is particularly high), to maintain the feedback controlTs working point just at or just slightly before that transition as described above, where the elastic wheel slip (mere tire deformation) ends (i.e. the point of maximum braking deceleration) and the sliding wheel slip begins -the brake will thus "stutter" and there will be significant periods of time, during which the wheel will briefly lock up and the braking deceleration then will be significantly reduced due to the then much smaller sliding friction being in effect and the brake path will roughly double, as compared to the situation if the braking force could continuously be maintained at the static friction limit. Hereby it is assumed, that during each feedback control cycle, i.e. during each "stutter/cadence-pulse" the friction coefficient and thus the braking force roughly lies in the adverse sliding friction regime during half of such a cycle time.

The problem of a conventional anti-lock-brake is thus on one hand the comparatively slow feedback response time of the hydraulic anti lock braking system and on the other hand the inaccurate determination of the actual forward velocity of the wheel axis during deceleration or acceleration as well.

State of the art: Anti-lock-brakes nowadays are solely hydraulic, where, however, additional brake servo pumps put an brake fluid under excess pressure, especially to enable a traction control, but eventually, the braking force in an anti lock brake is controlled merely by opening of rapid valves which causes to release the brake force at single wheels in a pulsating manner, in order to prevent these wheels from extended locking and thus in order to achieve a left-right compensation -where the wheel with the least grip dominates though -of the braking deceleration and particularly thus to maintain the tractability of the vehicle. The response time of these valves is roughly in the millisecond range and the feedback response of the entire system is further slowed by the visco-elasticity of the brake fluid and since the system thus suffers phase shifts, the feedback response needs to have an integral gain, which causes a further retardation of the feedback response (time constant). This all sums up to quite a few 10 -100 milliseconds response time of the brake forceTs feedback control. "Break by wire" -systems (EP1138564A2) are, however, already in preparation, but these concern only the side of the brake pedal, in order to eliminate the slightly disturbing vibration of the brake pedal during a braking phase when the anti-lock-brake feedback control regulation is active.

Further, US5067778, US4705323, and US6213564B1 disclose piezo-supported anti locking brake systems, where in US5067778 and US4705323 the piezo actuator is enhancing or releasing the hydraulic pressure in the brake fluid and thus only indirectly acts on the brake piston pressing the brake pads against the brake discs finally defining the brake force. In contrast, US6213564B1 comprises a purely piezo-driven anti locking brake system without an hydraulic system at all.

Hereby, US5067778, US4705323, and US6213564B1 but especially US 4705323 may be considered as the closest state of the art for the present invention.

Furthermore, for measuring the actual velocity of the wheelTs axle, only the usually not even calibrated speedometer and several acceleration sensors (accelerometers) are available.

Even a calibrated speedometer provides, as described above because of the "elastic wheel slip" a false (too small) value for the vehicleTs velocity during braking; mathematical integration of the acceleration, which is provided by an acceleration sensor, in principle gives an exact value though, but this value is only as exact as the starting initial value as provided by the calibrated speedometer, which can only be measured accurately as long as the wheel turns (elastic and sliding) slip-free. This in turn only functions, if the wheel is momentarily almost let loose by the brake, in order to again be able to rotate slip-free momentarily, in order to again record an/another exact starting initial value for the integration of the accelerometerTs signal. This release of the brake again extends the shortest achievable brake path significantly. Furthermore, even a calibrated speedometer can only be as accurate as the wheel diameter remains constant, which also is only approximately the case, since the tire rubber wears in time during driving.

Summary of the invention:

It is the objective of the invention to provide a piezo-supported anti-locking brake system comprising a very fast response time of the brake force regulating feedback loop with an independent measurement of the instantaneous wheel velocity. As a main improvement over customary purely hydraulic anti locking brake systems it is intended to significantly enhance the speed of the feedback loop's response regulating the brake force in an emergency braking situation and thus achieve a much shorter brake path and better tractability control when employed in ESP-systems.

Solution: Two essential changes on an anti locking break are planned in the present invention, in order to be able to reduce the braking distance significantly: 1. Piezo-supported brake force generation incorporated into a regular hydraulic system and 2. an independent optical measurement of each wheel's velocity.

Firstly a strong piezo actuator is introduced into the brake force generating system which usually is purely hydraulic. This piezo actuator could for instance be placed into the master cylinder -for instance for the case of an anti locking hand brake of a motor cycle -thus providing an additional (positive or negative) hydraulic pressure which adds to the hydraulic pressure generated by the brake pump's piston in the master cylinder. This additional "piezo brake pump1' should already be faster -especially when thinking of nowadays very fast piezo fuel injection pumps -than the customary pressure release valves in an ordinary anti-locking brake system but should be of the same order of magnitude with respect to the achievable time constant if this piezo actuator is actively controlled by the anti-locking brake system electronics. The piezo actuator of this piezo brake pump will have to be a stack of piezo discs to enable large travel range (to achieve sufficient pressure differences) which is thus relatively slow due to the electrically parallel connection of all the piezo plates in the stack and the system is further slowed by the visco-elasticity and the inertia of the hydraulic system as usual (mentioned above). US5067778 and US4705323 basically do the same, i.e. using a piezo actuator to generate additional influence on the hydraulic pressure (positive or negative, i.e. increasing the total brake pressure by some amount or releasing the total brake pressure by some amount) but there those piezos (piezo stacks) are located in the cylinder (brake fluid chamber) of the brake piston in the brake caliper, which is of course primarily related to a car where only one brake pedal engages all four brakes on all 4 wheels but could of course as well be applied in a motor cycle, where each wheel's brake has its own activating lever (hand brake lever and foot brake lever) usually.

However, in the present invention, in particular between brake piston and back side of the brake pad a strong piezo-actuator disk or layer shall be introduced and or optionally also behind the opposing brake pad (Fig. 1), which performs a fine and minute expanding or contracting movement and fast braking force regulation. Such a piezo can generate and hold and regulate the necessary force of kilo-Newton and over a iim range of motion time constants of lOllsec can be achieved. Of course, this achievable shortest time constant of the feedback response depends on the amount of force that has to be generated or regulated which in turn is correlated to the amplitude by which the piezo has to extend or contract: If smaller piezo-travel amplitude and thus smaller applied maximum braking force (for instance when braking on very slippery road conditions like on ice) are required a smaller feedback response time constant can be achieved, and vice versa. These short time constants are made possible by the above mentioned visco-elasticity of the hydraulic fluid (i.e. the hydraulic fluidT compressibility increases dynamically upon very rapid compression or expansion, in other words it stiffens) which slows the hydraulic part of system: When for instance trying to rapidly change -e.g. using a large travel piezo actuator -the hydraulic pressure more or less far away from the brake piston -e.g. at the master cylinder of a motor cycle connected to the hand brake lever -through narrow hoses it will take longer until this increased or decreased pressure has established/equilibrated all the way down at the brake piston due to the hydraulic fluidTs visco-elasticity. However here, the braking force is suggested to be rapidly varied directly between the brake piston and the back side of the brake pad without any brake fluid in between, just by rapid expansion/contraction of the said piezo directly: In this case, the visco-elasticity of the -eventually specialized -brake fluid works in favor of achieving short feedback response time constants since the brake fluid "stiffens" upon rapid compression/expansion, i.e. its compressibility increases until at some extremely short compression/expansion pulses it would even almost turn into a "quasi-solid" (Just like in the high school experiment of the liquid starch suspension/solution that turns practically into a trampolino when jumping on it). Thus in the case of additional braking pressure applied by the piezo (i.e. the piezo extends), "actio=reactio" is also provided via this viscous "quasi-solid" by the bottle necks and the side walls of the hydraulic system, not only by the brake pedal pressure and the master cylinder alone as it would be in the case the brake fluid were purely liquidous. But the usual case will be that the piezo rapidly contracts in order to intermittendly release the brake force, which will be possible by the inerrtia of the visco-elastic hydraulic system alone. The slow hydraulic system cannot extend the brake piston as quickly as the piezo can contract to compensate for the force release by the contracting piezo. Moreover, for slower time constants, the said piezo can have larger travel and simply add or subtract hydraulic pressure to/from the system by expanding or contracting. The more rapid movements of the piezo are required, the more symmetric the visco-elastic hydraulic system will behave though upon compression and expansion.

These piezo-actuators sandwiched directly (except for solid thermal isolation) between the back-side of the brake pad (the front side being the brake padsT friction layers which are in contact with the brake discs) and the front side of the brake piston (Fig. 1) may be in form of a piezo disc having the same cross section as the brake piston or may comprise several smaller piezo cubes or cylinders. All piezo actuators may also be formed of piezo plate stacks for achieving larger travel at smaller voltages. Hereby sizes, materials and geometry of the piezo actuators have to be optimized for the required forces, the thus required piezo travel and the required response time constants: Thin piezo discs with large surface area can generate high forces but exhibit slow time constants due to large capacitance of the thin piezo disc. Small cross section piezo discs are thus faster but can generate less force. Piezo stacks increase the actuator's travel but since all piezos (capacitors) in the stack are basically electrically connected in parallel, the time constant of such a stack goes up linearly with the number of piezo plates in the stack. Finally, there is the trade-off of more sensitive piezo electric materials (larger travel at lower voltages) versus their Curie-temperature: More sensitive piezo ceramic materials have lower Curie temperatures at which they will lose their piezo electric polarization while piezo electric crystals do not have a Curie temperature.

Suitable piezo ceramic materials for this application are "hard" piezo materials that can generate high forces (piezo electric constant "g33" large), exhibit smaller travel amplitudes (piezo electric constant d33 smaller) though but very importantly here comprise a high Curie temperature (of more than 300°C) such as for instance the proprietary piezo ceramic materials EBL#25, EBL#9, EBL#7, EBL#1 (EBL Products, East Hartford, CT, USA) or P1C300 (Physik Instrumente, Karlsruhe, Germany).

A piezo-supported anti locking brake system comprising regular feedback response time constants as nowadays achieved with valve-controlled anti-locking hydraulic systems could be realized by incorporating a large travel small cross-section piezo actuator (e.g. a stack of small piezo plates) into the master cylinder in the brake pump or a large travel large cross-section piezo actuator in the brake piston caliper in the brake fluid chamber behind the brake piston as disclosed in US5067778 or in US4705323 where in all cases these piezo actuators are acting on the hydraulic pressure in the hydraulic system when they contract or expand and thus are indirectly acting on the brake pad via the brake piston and thus eventually altering the brake force. Suitable piezo ceramic materials for such large travel piezos (piezo stacks) are soft" piezo ceramics which exhibit large piezo electric constants d33 (larger travel range per Volt applied Voltage) but exhibit lower Curie temperatures such as for instance EBL#23, EBL#3 (EBL Products, East Hartford, CT, USA) or P1C153 (Physik Instrumente, Karlsruhe, Germany).

But in all these cases, normally, the brake fluid's visco-elasticity slows the system's smallest achievable feedback response time constant, unless perhaps very fast and sharp pulses are applied that propagate in form of a wave packet through the hydraulic system towards the back side of the brake piston. But this latter case is most likely extremely difficult to control in a stable feedback loop.

This fast and sensitive regulation shall additionally support the "normal" hydraulic anti-locking-brake and shall maintain the wheel deceleration much more accurate at the upper limit of the elastic wheel-slip. It is hereby noted, that two coupled feedback loops (hydraulic and piezo-mechanic brake force, the manipulated variable), which are acting upon and are regulating the same control parameter (i.e. the elastic wheel slip), are very difficult to master and have to be adjusted very precisely.

For the experienced driver, the slow and rougher hydraulic regulating anti-locking feedback control should have the option to be turned off, such that the driver himself can maintain the hydraulic braking force very close within the optimal regime just using the brake pedal or lever, now only supported by the piezo-anti-locking brake system, which can regulate much faster, than it ever would be possible with the foot on the brake pedal, or much faster than also a brake by wire system could ever realize with solely hydraulic brakes.

The proposed piezo actuator between the front side(s) of the brake piston(s) -note that there may be up to 3 pairs of brake pistons in a caliper as in race brakes -and the back side of the brake pad and or optionally between the back side of the opposing brake pad and the brake caliper (Fig. 1) can be broken up into several piezo actuator elements which can be electrically controlled separately and wherein each piezo actuator element comprises an integrated, in particular piezo electric -pressure sensor which is also fed into a controlling anti-locking brake computer which thus allows to distribute the brake force/pressure evenly across the whole brake pad with its friction layer firmly engaged with the brake disc. Several small piezo elements at some lateral separation additionally have the advantage that due to their smaller capacitance they will be able to move faster as compared to a large piezo disc having the same size of the brake piston, or even as the brake pad, although it can generate higher forces. Practical would of course be to place one piezo actuator element on top of each brake piston in a brake caliper carrying 4-6 brake pistons and thus balancing the brake forces exerted by each brake piston on the brake pad, which is of further advantage since uneven brake forces applied by the different brake pistons is a known problem.

A measurement of the forward velocity of the 4 wheel axles shall be performed independently of the (calibrated) speedometer and independent of the acceleration sensors in the car: Underneath each of the 4 (half-) axles essentially an optical computer (laser-) mouse is to be installed, which is able to exactly record the momentary forward speed of each wheel axle. Even lateral drift can be recorded this way. The principle of operation is the recording of a 2-dimensional cross-correlation function of the minute road surface structural details which are migrating through the laser beam and cast a migrating shadow onto the detector. Repeatedly recorded is the quadrant detector signal as it varies in time during a certain time span delta t and this time-dependent signal is cross-correlated with the next signal-"train" in the consecutive time span delta t which is shifted by the infinitesimal time span 6t with respect to the first signal-"train". From the calibrated geometrical shift of the corresponding part of the distance on the road, by which the second signal-"train" has to be shifted, in order to obtain with the first signal-"train" the normalized Cross-correlation integral of almost 1 respectively to maximize the cross-correlation integral, divided by t, the instantaneous velocity can be obtained. Also a optical laser mouse primarily measures a velocity from which!bonly by integration a position is computed. The optical PC-mouse has to be altered to that extent -since it cannot sit directly on the road surface -that laser diode and quadrant detector have to be mounted and adjusted, such that the signal reflected from the road surface can readily be recorded. Possibly a continuous re-adjustment is necessary, e.g. by tilting of the laser in such a way, that the quadrant detector always provides maximized sum-and minimized difference-signals. Instead of the quadrant detector also a 2-dimensional CCD-array can be read-out and then always 2 pictures between the time difference St are recorded, which are then shifted such that their cross-correlation (integral) comes to normalized 1 or is maximized respectively.

Instead of the cross-correlation of a quadrant detector signal also a 2-dimensional CCD-array can be used, in which all pixel detectors are connected in parallel and the frequency of a point shadow, i.e. a scatterer on the road surface, migrating across the CCD-array illuminated by the reflection from the road surface provides a signal proportional to the velocity of the vehicle. This frequency multiplied with the separation of the middle points of the pixels just results in the speed of the vehicle at a laser beam reflected vertically from the road surface. In principle, a complete 2-dimensional array would not be needed, one linear CCD-array in direction of the vehicle heading and another one perpendicular would be sufficient. Thereby, the size of the arrays has to be chosen such that the pixel distance roughly equals the size of the expected point shadow in the laser speckle spot on the detector. The array length has to be as large, as the expected mean separations of the point shadows caused by the road surface structures in the reflected laser speckle spot. Intelligent computer software could even solve the problem, if for instance two road surface structural details are migrating simultaneously through the CCD-array -in that case a roughly doubled frequency component would also be measured, which would initially lead to a falsely doubled vehicle speed, but for instance through real-time software-comparison with the speedometer and combined with the said cross-correlation method of above, the computer would recognize immediately this measurement error of a factor of 2, since the speedometer aberration is not that large.

For simplicity, initially customary but expensive commercial heterodyne laser surface velocimeter (Polytek Waldbronn, see also DD232360A1 and DE102008038642A1) shall be employed for the accurate vehicle velocity measurement, which somewhat more simply can reach the here required velocity and accuracy ranges.

Also the wheel circumferential speed shall be determined via such a laser scanning system for enhancing the accuracy of the wheel slip feedback control/regulation, for instance by means of a laser in the wheel house which scans the tire on its upper side, in order to eliminate the errors of the (calibrated) speedometer measurement due to varying wheel circumferences (different tires/rims, worn tires etc.).

The anti-locking-brake computer also will compare the that way measured wheel axle velocity relative to the road surface with the speedometer or with the equivalently optically measured wheel circumferential speed respectively, and at that brake force magnitude point, where the difference will begin to rise very steeply (or where its derivative will steeply, almost discontinuously jump up respectively, i.e. at that point at which the "elastic wheel slip" will begin to transition into the sliding wheel slip, the computer will reduce the braking force until this difference will return to linear behavior with brake force magnitude again or until its derivative will have a plateau again respectively -but all this in the presently invented concept much faster and much more accurately than a solely hydraulic system Alternatively at each wheel suspension a x-y-z acceleration sensor is mounted, which upon braking measures the negative acceleration at each wheel, upon increasing the vehicle speed of course measures the positive accelerations. Hereby, the anti locking brake feedback control responds that way, that the deceleration in driving heading direction x is always kept as close as possible at its maximum value at each wheel, where for maintaining tractability of course that wheel dominates which is braking/decelerating the least. In a turn, however, the negative lateral acceleration is maximized, whenever the longitudinal deceleration in heading direction cannot be maximized. That means, in a turn, the feedback regulator will gradually release the break, either if the deceleration in heading direction x moves over its maximum value (and decreases again) or (logical or) the deceleration in lateral direction y. In the vertical z-direction, also the acceleration as well as the tilt of the wheel suspension is recorded, in order to achieve a "look-ahead-gain" for the digital feedback control, since at higher dynamical pressure of the wheel onto the road also higher braking decelerations are to be expected.

Alternatively, each of the 4 wheel suspensions possesses in x-and y-direction an additional counter bearing which ends into a piezoelectric pressure sensor (similar to a wheel balancing machine), which records the longitudinal and lateral acceleration forces; advantage would be a shorter response time as compared to the x-y-micro-electromechanical acceleration sensor. In z-direction again a acceleration sensor combined with a tilt sensor would be installed, in order to equivalently through a "look-ahead-gain" be able to further reduce the time constant of the feedback control.

For all velocimetry coherent or non-coherent illumination sources, instead of a visible laser, also -coherent or non-coherent -UV, visible, infrared, microwave or even ultrasound radiation sources can be used alternatively, which might be necessary as the undercarriage of the car might become covered with dirt from the road.

The same system can of course just like the normal anti lock brake system be used, in order to realize an electronic lane departure correction/warning system (ESP) and a traction control. Especially the independent exact optical measurement of the forward and sideways velocity of the 4 wheel suspensions is then of great advantage besides the faster brake force control at the single wheels.

Figures: Figure 1: Brake caliper showing in this embodiment only one brake piston (while there may be in particular up to 3 pairs of brake pistons in one brake caliper) where a piezo actuator is placed directly between the front side of the brake piston and the back side of the brake pad as well as on the other side of the brake disk an optional second piezo actuator is located between the brake caliper and the back side of that second brake pad. Various embodiments are of course possible with the piezo actuator on either or on both sides of the brake disc as well as embodiments comprising a plurality of brake pistons and thus a plurality of corresponding piezo actuators as well as embodiments where those piezo actuators per brake piston are broken up in several smaller piezo actuator elements depending on the requirements on the amount of force that has to be generated in connection with the required speed of the feedback response of the anti-locking brakeTs force regulating feedback loop.