Detection and identification of pressure-sensor faults in electro-hydraulic (EHB) braking systems

Abstract: A method of detection and identification of pressure sensor faults in an electro-hydraulic braking system of the type comprising a brake pedal (10), respective braking devices (18a, 18b, 18c, 18d) connected to the vehicle wheels and which communicate with electronically controlled proportional control valves (14a, 14b, 14c, 14d) in order to apply hydraulic fluid under pressure to the braking devices, respective pressure sensors (24a, 24b, 24c, 24d) for measuring the hydraulic pressures at the individual braking devices, a hydraulic pump (16) driven by an electric motor (18), a high pressure hydraulic pressure accumulator (22) fed by the pump (16) for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves (14) in order to apply hydraulic fluid under pressure to the braking devices in proportion to the driver's braking demand as sensed at the brake pedal (10), and a supply pressure sensor (30) for monitoring the hydraulic pressure supplied to the electronically controlled proportional control valves. In accordance with the method, three or more of the pressure sensors (24a, 24b, 24c, 24d, 30) are arranged to be subjected to the same pressure and their readings measured and compared whereby to identify a sensor whose reading does not correspond to that of the others.

Description: The present invention is concerned with the detection and identification of pressure sensor faults within the context of electro-hydraulic (EHB) braking systems. A typical EHB system for a vehicle comprises a brake pedal, respective braking devices connected to the vehicle wheels and which are capable of being brought into communication with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in so called "brake by wire" mode in proportion to the driver's braking demand as sensed at the brake pedal. The EHB system is controlled by an electronic controller (ECU) which, inter alia, controls the hydraulic pump to keep the pressure in the hydraulic pressure accumulator within specified limits. The hydraulic pressure supplied to the electronically controlled valves (the "supply pressure") is monitored by a supply pressure sensor. The hydraulic pressure at the various braking devices is measured by individual pressure sensors at these braking devices. An isolating valve is included whereby the hydraulic pressure accumulator can be selectively isolated from the pump and the remainder of the system. Pressure sensors are key components of such EHB systems, being used in the process of controlling pressure as well as in the detection of system faults. Pressure sensors of the type in question convert hydraulic pressure into electrical signals which are supplied to the system's ECU for control purposes. If one of these sensors develops a fault, such that incorrect signals are supplied to the ECU, then control errors could result. Incorrect pressure readings therefore have the potential to cause uncomfortable and/or inconsistent control of the normal braking function, to impair the efficiency of control systems such as ACC (Autonomous Cruise Control) or ABS, to trigger false warnings of system failure, or to overlook the occurrence of genuine system faults. For these reasons one wishes to detect even relatively small sensor errors so that effects such as the above can be mitigated by effective countermeasures. Some types of fault can be detected by the sensor's individual on-board electronics, but others are most easily (and economically) found by comparison with the signals of another sensor subjected to the same hydraulic pressure. Sensor duplication is, however, uneconomic. Our earlier European patent No. EP-A-0743 234 describes an electro-hydraulic braking system in which a first master control valve is provided to supply substantially equal pressures to all brake actuators and additional control valves are provided to regulate pressures applied to respective brake actuators. The described system allows correlation between the brake site pressure sensors to be checked at frequent intervals. By operating the individual valves fully open during normal braking and controlling brake pressure by means of the master control valve signals from the brake site pressure sensors can be compared against each other by the control unit. Any significant deviation from this level between axles or individual brakes is suggestive of a fault which can be indicated to the driver. A signal from each of the sensors is sent to an adder, with the aggregate being divided by a divider to give an average value. The average is sent as a signal to a subtracter where the average is deducted from the individual signals from the brake site sensors. A low pressure is supplied specifically for the purpose of testing the system. The test is conducted either during normal braking (vehicle moving) or as part of the start-up procedure (vehicle stationery but not having been braked immediately previously). The test is conducted with all control valves fully opened and with the "master control valve" opened at least partially to apply a nominal low pressure to the brakes via the open control valves. In order to maintain this low pressure, the motor M needs to run to establish the necessary pressure. This involves a "motor start" with the consequential effect of significantly increasing the number of pump motor starts required over the vehicle's lifetime. GB-A-2317663 describes a brake system for an automotive vehicle which comprises a front hydraulic circuit including front wheel cylinders and a pressure sensor, a rear hydraulic circuit including rear wheel cylinders and a pressure sensor, a master cylinder and associated pressure sensor, a pump for normally supplying pressure fluid to the circuits, and a common return pipe, for the two circuits. When the vehicle is started, the ECU initially tests the hydraulic piping for leaks and then tests the pressure sensors for malfunction. For testing the sensors, a rear cut valve and pressure-decrease valves are opened, so that m/c pressure is applied to both circuits, and malfunction in the sensors is detected by comparing the pressure values detected thereby. It is an object of the present invention to enable faults to be detected and identified by sensor signal comparison but without requiring sensor duplication. In accordance with a first aspect of the present invention there is provided a method of detection and identification of pressure sensor faults in an electro-hydraulic braking system of the type comprising a brake pedal, respective braking devices connected to the vehicle wheels and which communicate with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, respective pressure sensors for measuring the hydraulic pressures at the individual braking devices, a hydraulic pump driven by an electric motor, a high pressure hydraulic pressure accumulator fed by the pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in proportion to the driver's braking demand as sensed at the brake pedal (10), and a supply pressure sensor (30) for monitoring the hydraulic pressure supplied to the electronically controlled proportional control valves, characterised in that, with an accumulator isolating valve closed and with the vehicle having been brought to a stationary condition by operating the brake pedal depending upon the brake pressure used for that stop three or more of said pressure sensors are arranged to be subjected commonly to the pressure then existing within the braking system downstream of the accumulator isolating valve by opening all proportional valves and their readings measured and compared whereby to identify a sensor whose reading does not correspond to that of the others. In accordance with a second aspect of the present invention there is provided an electro-hydraulic braking system comprising a brake pedal, respective braking devices connected to the vehicle wheels and which communicate with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, respective pressure sensors for measuring the hydraulic pressures at the individual braking devices, a hydraulic pump driven by an electric motor, a high pressure hydraulic pressure accumulator fed by the pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in proportion to the driver's braking demand as sensed at the brake pedal, and a supply pressure sensor for monitoring the hydraulic pressure supplied to the electronically controlled proportional control valves, characterised by control for identifying a sensor whose reading does not correspond to that of the others, which control means closes an acumulator isolating valve and, when the vehicle has been brought to a stationary condition by operating the brake pedal depending upon the brake pressure used for that stop, is arranged to measure the readings from three or more of said pressure sensors subjected to the common pressure then existing within the braking system downstream of the accumulator valve by opening all proportional valves. Preferably, after identification of a pressure sensor fault in the case of a four or more wheeled vehicle, all pressure sensors associated with the braked wheels, and the supply pressure sensor, are connected together, with all of the proportional control valves opened fully or substantially fully. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic illustration of one embodiment of an electro-hydraulic braking system to which the present invention is applicable; Fig. 2 is a sequence flow diagram illustrating one embodiment of a system for detection and identification of sensor faults; and Fig. 3 is a sequence flow diagram illustrating one technique for detecting but not identifying the presence of a faulty sensor. Referring first to Fig. 1, the illustrated EHB system comprises a brake pedal 10 with an associated sensor 12 for the acquisition of the driver's braking demand. The driver's demand is transferred to an electronic control unit (ECU), evaluated there, and used as the source for the generation of electrical control signals for proportional solenoid control valves 14a, 14b, 14c, 14d, a hydraulic pump 16, wheel brakes 18a, 18b of one axle supplied with hydraulic fluid by electrically actuated brake channels 20a, 20b and wheel brakes 18c, 18d of the other axle supplied by electrically actuated channels 20c, 20d. Under normal braking conditions, brake pressure modulation in the electrically actuated brake channels 20a, 20b, 20c, 20d is effected in a known manner by means of the proportional solenoid control valves 14a, 14b, 14c and 14d, the brake pressure being provided by a pressure accumulator/reservoir 22 whose pressure is maintained by the pump 16 operated by an electric motor 18. Pressure sensors 24a and 24b monitor the hydraulic pressure at the wheel brakes 18a, 18b of the front axle and pressure sensors 24c and 24d monitor the hydraulic pressure at the wheel brakes 18c, 18d of the rear axle. Further pressure sensors 26, 28 monitor the pressure within push-through circuits 27a, 27b for the right and left front wheel brakes and a pressure sensor 30 monitors the supply pressure in the circuit of pump 16. The operation of the aforegoing system is briefly as follows. The accumulator 22 is maintained by the pump 16 within a specified pressure range, as observed by the supply-pressure sensor 30. Braking demands by the driver result in actuation of the proportional solenoids 14 associated with each wheel. Energisation current will be increased if the pressure observed by the associated sensor 24 is less than that demanded, and reduced if it is greater than the demanded pressure. The normal requirement will be for identical pressures to be present at the left and right wheels of each axle, but the pressures at the front and rear axles may well be different in order to match the desired braking distribution. For this type of braking the master-cylinder isolation valves 31a, 31b will be closed, the accumulator's isolation valve 32 will be open, and the cross-axle-balance valves 34a, 34b can each be opened to ensure pressure equality left-to-right. However, some operating modes require individual control of the pressure at each wheel, e.g. during ABS or transverse pressure apportioning, and the cross-axle-balance valves 34a, 34b will be closed at such times. This hydraulic layout has been devised in order to permit most sensors to be compared with at least one other sensor under most operating conditions. For example, the pressure signalled at the front left wheel can be compared with that signalled at the front right wheel whenever the front-axle balance valve is open, and the sensors for the master-cylinder's primary and secondary circuits are normally subjected to the similar pressures. However, comparison of at least three sensor signals is necessary in order to diagnose which signal is incorrect. Mere knowledge that a fault exists is insufficient for situations which require a fault-management (fall-back) strategy to be adopted. Also there is no convenient partner for the supply-pressure sensor 30. Thus, for example, whenever the balance valves 34a, 34b are open it is possible to detect errors by comparing the sensor readings from opposite ends of each axle, e.g. front left with front right. However this does not identify which sensor is in error, so any fault management strategies triggered by this test must take this uncertainty into account. If more effective strategies are required then the diagnosis must be more specific. A similar situation exists for the accumulator pressure sensor 30. During pump operation, one can compare the sensor signal with a signal representing motor torque (proportional to pump delivery pressure) which can be obtained from observations of the motor behaviour, but this will not identify whether the fault is in the sensor or the motor. Likewise during periods without braking, all four brake pressure sensors 24a, 24b, 24c, 24d will be connected to the reservoir 39 and it is possible to compare their signals at zero pressure, but this will not capture faults which are only apparent under pressure. Also, the pressure at the supply-pressure sensor 30 is uncertain at such times. If the accumulator isolating valve 32 is open then the supply-pressure sensor 30 will normally be at accumulator pressure if the valve is closed then it will be at some intermediate pressure due to inevitable pressure decay via internal leakage at the proportional control valves 14. The hydraulic layout of the system of Fig. 1 provides a solution to these problems in that all four brake pressure sensors 24a, 24b, 24c, 24d are connected together and to the supply pressure sensor 30, if all of the proportional valves 14a, 14b, 14c, 14d are opened fully. This arrangement provides for the following test possibilities. 1. Open all proportional valves (14a to 14d) with accumulator valve (32) open. This allows detection of zero drift and gain errors, including those which might be caused by a sharp change (knee) part way along the sensor's characteristic of pressure to voltage. However, the high pressures reached (accumulator pressure) mean that there are issues of safety and reliability. This, for example, with the vehicle stationary and accumulator isolating valve 32 open, a five-sensor (the four brake pressure sensors 24 and the supply pressure sensor 30) comparison is possible under pressure. The high pressure involved will means however, that frequent checks in this manner could lead to premature caliper failure due to fatigue. The test is thus useful for diagnosis, once a fault has been detected by other means (e.g. the routine cross-axle check as described hereinafter with reference to Fig. 3) and could form part of a system-initialisation check but is not suitable for frequently invoked fault detection. It must be made sure that this test is only used when the vehicle is stationary. The main use of this test would be:- to diagnose brake-pressure sensor faults already detected, so that the correct fault-management procedure can be implemented, to detect and diagnose accumulator pressure sensor faults. 2. Open all proportional valves (14a to 14d) with accumulator valve (32) closed. This improves the safety and reliability concerns associated with 1. and can be used routinely every time the vehicle comes to rest. It can therefore diagnose faults already detected (by the simple cross-axle comparison test) at the earliest possible moment after detection. Such a test can be used frequently and has the additional advantage that it can use a range of pressures depending upon the brake pressure used for that stop, so that sensor abnormalities in the normal working range are more easily noted. However, because end-of-stop pressure is not usually very high (rarely > 30 bar) this test is not effective for faults which occur only at higher pressures, e.g. a knee in the pressure/voltage characteristic. 3. Cross-axle and inter-axle sensor (24a to 24d) plausibility check with seal-friction offset. This is a method of diagnosing which brake pressure is faulty by comparing the pressures across the axle and between the front and rear axles. This may appear to be a straight forward exercise to perform. However systems with isolating pistons (50a, 50b) for the push-through circuit have special problems because of the seal friction. Comparing pressure signals across the axle is straight-forward under the assumption that the friction at each piston is the same. When operating in "one-valve-in-hold" mode there will be a pressure drop across the balance valve during rapid pressure rise or dump, but any difference during slow rates of pressure change will be due to sensor errors. Diagnosis requires a comparison between more than two sensors, but the seal friction of the isolating pistons complicates an inter-axle plausibility check. Seal friction can be as much as 6 bar which is larger than the errors which should be detected. However when the pressure is changing it becomes possible to diagnose small errors if the friction effect is taken into account. Depending upon the sense of the error (+ ve or - ve) it may not be recognised until the demand pressure changes by an amount greater than the seal friction. This technique can be used with any of 1. or 2. described above. Fig. 2 is a sequence flow chart illustrating the aforegoing two options 1 and 2. Fig. 2 includes the following sequence steps 40 - start 41- Is V Ref = 0 ? 42 - Is a sensor fault awaiting diagnosis ? 43 - Is start-up check in progress? 44 - Was V Ref > 0 in preceding cycle ? 45 - Close accumulator isolating valve 46 - Open all proportional control valves 47 - Open all proportional control valves 48 - Open accumulator isolating valve 49 - Read signal from 4 x brake and 1 x supply pressure sensors 50 - Are all signals same +/- x bar ? 51 - Note sensor identity, error magnitude and sense 52 - Restore previous valve control status 53 - Finish An example of a brake pressure sensor plausibility check to establish whether a faulty brake pressure sensor exists (but which is unable to identify which particular sensor is faulty) is illustrated in Fig. 3. Fig. 3 includes the following sequence steps: 54 - Start 55 - Read all 4 brake pressure sensors 56 - Balance valves open ? 57 - Cross-axle threshold exceeded at either axle ? 58 - Set "detection" flag for faulty axle 59 - Set "waiting for diagnosis" flag 60 - Safe to begin diagnosis ? 61 - Begin diagnosis test 62 - Calculate inter axle threshold = seal friction + cross-axle threshold 63 - Compare sensor readings between sensors on each diagonal or along each side of the vehicle 64 - Inter-axle threshold exceeded at either side ? 65 - Set "diagnosis" flag for faulty axle at faulty side 66 - Start

Claims: A method of detection and identification of pressure sensor faults in an electro-hydraulic braking system of the type comprising a brake pedal (10), respective braking devices (18a, 18b, 18c, 18d) connected to the vehicle wheels and which communicate with electronically controlled proportional control valves (14a, 14b, 14c, 14d) in order to apply hydraulic fluid under pressure to the braking devices, respective pressure sensors (24a, 24b, 24c, 24d) for measuring the hydraulic pressures at the individual braking devices, a hydraulic pump (16) driven by an electric motor (18), a high pressure hydraulic pressure accumulator (22) fed by the pump (16) for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves (14) in order to apply hydraulic fluid under pressure to the braking devices in proportion to the driver's braking demand as sensed at the brake pedal (10), and a supply pressure sensor (30) for monitoring the hydraulic pressure supplied to the electronically controlled proportional control valves, characterised in that , with an accumulator isolating valve (32) closed and with the vehicle having been brought to a stationary condition by operating the brake pedal depending upon the brake pressure used for that stop, three or more of said pressure sensors are arranged to be subjected commonly to the pressure then existing within the braking system downstream of the accumulator isolating valve (32) by opening all proportional valves (14a, 14b, 14c, 14d) and their readings measured and compared whereby to identify a sensor whose reading does not correspond to that of the others. A method as claimed in claim 1, wherein after identification of a pressure sensor fault in the case of a four or more wheeled vehicle, all pressure sensors (24) associated with the braked wheels, and the supply pressure sensor (30), are connected together, with all of the proportional control valves (14) opened fully or substantially fully. A method as claimed in claim 1, wherein a comparison is made of the brake pressures as measured by the pressure sensors, both across the axles and between front and rear axles. An electro-hydraulic braking system comprising a brake pedal (10), respective braking devices (18a, 18b, 18c, 18d) connected to the vehicle wheels and which communicate with electronically controlled proportional control valves (14a, 14b, 14c, 14d) in order to apply hydraulic fluid under pressure to the braking devices, respective pressure sensors (24a, 24b, 24c, 24d) for measuring the hydraulic pressures at the individual braking devices, a hydraulic pump (16) driven by an electric motor (18), a high pressure hydraulic pressure accumulator (22) fed by the pump (16) for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves (14) in order to apply hydraulic fluid under pressure to the braking devices in proportion to the driver's braking demand as sensed at the brake pedal (10), and a supply pressure sensor (30) for monitoring the hydraulic pressure supplied to the electronically controlled proportional control valves, characterised by control means for identifying a sensor whose reading does not correspond to that of the others, which control means closes an acumulator isolating valve (32) and, when the vehicle has been brought to a stationary condition by operating the brake pedal depending upon the brake pressure used for that stop is arranged to measure the readings from three or more of said pressure sensors subjected to the common pressure then existing within the braking system downstream of the accumulator valve (32) by opening all proportional valves (14a, 14b, 14c, 14d). A system as claimed in claim 4, wherein after identification of a pressure sensor fault in the case of a four or more wheeled vehicle, the control means can connect together all pressure sensors (24) associated with the brake wheels, and the supply pressure sensor (30), with all of the proportional control valves (14) opened fully or substantially fully. A system as claimed in claim 4, wherein the control means is arranged to effect a comparison of the brake pressures as measured by the pressure sensors, both across the axles and between front and rear axles. A system as claimed in claim 4, 5 or 6, wherein said control means comprises a system ECU.

Inventors: HARRIS ALAN LESLIE, HOWELL MARK LEIGHTON, PHILLIPS MARK IAN, HARRIS, ALAN LESLIE, HOWELL, MARK LEIGHTON, PHILLIPS, MARK IAN
Applicants: LUCAS IND PLC, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1999
Main classification: B60T8
IPC codes: B60T17, B60T8, B60T13