GB2478368A

GB2478368A - Anti-lock brake with piezo actuator and independent optical measurement of the instantaneous wheel velocity

Info

Publication number: GB2478368A
Application number: GB1016791A
Authority: GB
Inventors: Frank Michael Ohnesorge
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-03-04
Filing date: 2010-10-06
Publication date: 2011-09-07
Also published as: GB201103641D0; DE102011013153A8; DE102011013153A1; GB201016791D0; GB2478423A

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 1Otsec feedback response and an exact independent measurement of the instantaneous forward velocity of each wheel's axle

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 behaviour 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 force's feedback control. "Break by wire1' -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.

Furthermore, for measuring the actual velocity of the wheel's 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 vehicle's 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 accelerometer's 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.

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. 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 "only' 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 t 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 slip1' 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.

2. Between brake piston and back side of the brake pad a piezo-actuator disk or layer shall be introduced, which performs a fine and minute movement and fast braking force regulation. Such a piezo can generate the necessary force of kilo-Newton and over a Iim range of motion time constants of lOi.isec can be achieved. 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.

3. 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.

4. The same system can of course just like the normal anti lock brake system be used, in order to realize an electronic lane departure warning system and a traction control.

Especially the independent exact optical measurement of the forward and sideways velocity of the 4 wheel suspensions is of great advantage besides the faster brake force control at the single wheels.