Vehicle power steering system with digital angle sensing device

Abstract: A power steering system for a vehicle which has one or more steerable wheels (10) there is provided a manually operable steering wheel (12), a first angle sensing device (16) connected to the steering wheel (12), a servo motor (13) connected to the steering wheel or wheels (10) and includes a second angle sensing device (15), and a control unit (17) for receiving signals from the first angle sensing device (16) and for energizing the servo motor (13), wherein the first angle sensing device (16) comprises a single digital angle encoder fed with a constant supply voltage (VCC) by the control unit (17) and generating electric pulses of a certain amplitude (ΔV 1 ), the control unit (17) includes a malfunction detecting device connected to the angle encoder (16), and the angle encoder (16) comprises a circuit for establishing a constant analogous voltage for reducing the pulse amplitude (ΔV 1 ) normally delivered by the angle encoder (16) to a suitable check amplitude (ΔV 2 ), wherein the bottom voltage level of each pulse is above zero and the top voltage level of each pulse is below the supply voltage (VCC), and the malfunction detecting device comprises a means for providing high and low trip voltage levels for each pulse delivered by the angle encoder, wherein the low trip voltage level (V t1 ) is below the bottom voltage level of each pulse but above zero, and the high trip voltage level (V th ) is above the top voltage level of each pulse but below the supply voltage (VCC).

Description: The invention relates to a power steering system for a vehicle having one or more steerable wheels, and which comprises a manually operable steering wheel, a first angle sensing device connected to the steering wheel, a servo motor connected to the steerable wheel or wheels and including a second angle sensing device, and a control unit for receiving signals from the first angle sensing device and for energizing the servo motor. In prior art systems e.g. EP-A-0885756, there are used analogous types of angle sensing means like potentiometers or DC-tachometers connected to the steering wheel. In those systems, it is fairly simple to obtain a reliable safety control of the angle sensing device operation. Such a safety control is highly important, since it is crucial for the safety of the vehicle operation. For different reasons it has become desirable to use digital angle sensing devices instead of the former analogous devices both as a steering wheel signal producer and as an angle controlling device at the steerable wheel of the vehicle, and according to one previously suggested solution to the safety problem two separate digital angle encoders have been used. A faulty operation of one of these encoders indicates a malfunction to be attended to, whereas the steering system would remain operative to enable a safe handling of the vehicle to stand still. It is desirable, however, to simplify the power steering system and to bring down its costs, and a solution to that would be to use just one digital angle encoder. There is a problem, though, to accomplish a reliable detection of a faulty operation of such system and to initiate an immediate stop to the vehicle operation. In a power steering system described in EP 0 885 756, there are used analogous type angle sensors in connection with the steerable wheels of the vehicle, and there is also described a method for faulty detection of these sensors. However, in this known system there is used a digital angle encoder as a signal generating means in connection with the steering wheel. However, there is not described or mentioned anything about how to accomplish a reliable function monitoring of this digital angle encoder. According to the invention, a power steering system for a vehicle having one or more steerable wheels comprises a steering wheel which is mechanically connected to a single digital angle encoder, a servo motor with an angle sensing device connected to the steerable wheels, and a malfunction detecting device connected to the digital angle encoder and arranged to apply a constant analogous voltage across the digital encoder to, thereby, alter the digital pulse amplitude from the encoder to a certain check amplitude, wherein a faulty angle encoder operation is detectable as a deviation of the pulse amplitude from that check amplitude. The above described problems are solved by the invention which provides an improved power steering system including a simplified but safely controlled digital angle sensing device. A preferred embodiment of the invention is described below with reference to the accompanying drawing. On the drawing: Fig. 1 shows schematically an example of a power steering system according to the invention. Fig. 2 shows unaltered amplitude pulse trains from the digital angle encoder. Fig. 3 shows pulse trains from the digital angle encoder with altered amplitude. The power steering system shown in Fig. 1 is applied to a vehicle having one steerable wheel 10, a steering wheel 12 for manual operation of the system, a servo motor 13 mechanically coupled to the steerable wheel 10 via a gear mechanism 14, an angle sensing device 15 associated with the servo motor 13, an angle encoder 16 mechanically connected to the steering wheel 12, and a control unit 17. The control unit 17 comprises electronic data processing components as well as power supply means and communicates with the servo motor 13 via wiring 18 for supply of electric power to the motor 13 and with the angle encoder 16 and the angle sensing device 15 via wirings 19 and 20, respectively. The angle encoder 16 connected to the steering wheel 12 is of a digital type delivering a train of electrical pulses when turning the steering wheel 12, and the angle sensing device 15 associated with the motor 13 is preferably arranged to deliver feed back pulses to the control unit 17 so as to obtain a correlation between the set pulse number and frequency delivered by the angle encoder 16 and the actual pulse number and frequency delivered by the angle sensing device 15. The pulse frequency corresponds to the rotation speed of the steering wheel 12 and the servo motor 13. The angle encoder 16 is of the double-channel type, wherein the two channels are arranged at a 90 degree phase displacement for obtaining a signal pattern indicating the actual direction of movement. The angle encoder 16 is fed with a constant supply voltage VCC by the control unit 17, and normally, the pulses delivered by an angle encoder of the digital type have an amplitude ΔV 1 extending from zero to a certain level, for instance 0 - 12 V. This means that if you are going to check the encoder device for discovering any malfunction you can get either 0 V or 12 V, which means that 0 V could be either o.k. or indicate a broken connection, and 12 V could indicate o.k. or short circuiting. However, by mixing the digital signal from the angle encoder 16 with a constant analogous voltage it is possible to obtain a check amplitude ΔV 2 of for example 2 - 10 V. This means that if you get a reading of at least 2 V when checking the encoder operation you know that there is no broken connection, and if you get a reading of no more than 10 V you can be ascertained that there is no short circuiting in the encoder circuit. Alternatively, the check amplitude ΔV 2 could be 0.5 - 4.5 V, which is reduction of the original pulse amplitude of 0 V - 5V. None of the two latter readings would give any information about the operation order of the angle encoder 16, unless the reduced check amplitude is provided by the addition of the constant analogous voltage. For accomplishing this, the system includes a circuit (not illustrated) for generating the constant analogous voltage. Preferably, this is accomplished by a circuit comprised in the angle encoder 16. Such an arrangement results in a simplified wiring between the angle encoder and the control unit 17. In Fig. 2, there are illustrated the delivered pulse trains of the two channels A and B of the digital encoder 16 where no mixing is made with an analogous constant voltage. In this case, the pulse amplitude ΔV 1 is the original amplitude delivered by the digital encoder 16. In Fig. 3, there is shown a diagram illustrating the delivered pulse trains from the two encoder channels A and B where an analogous constant voltage has been added, whereby the pulse amplitude ΔV 1 has been reduced to ΔV 2 which is used as a check amplitude for function check of the digital encoder 16. By mixing a constant analogous voltage with the digital pulses delivered by the angle encoder 16, as described above, the bottom voltage level of each pulse is raised from the zero level to a first voltage level V 1 and the top voltage level of each pulse is reduced from the supply voltage level VCC to a second voltage level V 2 . The malfunction detecting device included in the control unit 17 comprises a means for providing high and low trip levels outside the normal pulse amplitude extremes where a malfunction of the angle encoder 16 is to be indicated. Accordingly, between the top amplitude level V 2 of each pulse and the supply voltage VCC, there is provided a high trip level V th , and between the bottom amplitude level V 1 of each pulse and zero, there is provided a low trip level V tl . See Fig. 3. If the pulse amplitude voltage reaches either one of the trip levels, V th or V tl the malfunction detecting device will indicate a faulty angle encoder. By arranging a window type amplitude monitoring function there is obtained a safe automatic functional feature for checking the operation of the digital angle encoder. Thereby, it is made possible to use a modern digital type of angle sensing and encoding device without loosing the possibility to have a continuous function check of the encoder and to ensure a safe vehicle operation. Should the readings fall outside the window boundaries the operation of the vehicle is stopped.

Claims: Power steering system for a vehicle having one or more steerable wheels (10), comprising a manually operable steering wheel (12), a first angle sensing device (16) connected to said steering wheel (12), a servo motor (13) connected to said steerable wheel or wheels (10) and including a second angle sensing device (15), and a control unit (17) for receiving signals from said first angle sensing device (16) and for energizing said servo motor (13) , characterized in that said first angle sensing device (16) comprises a single digital angle encoder arranged to generate electric pulses of a certain amplitude (ΔV 1 ) in relation to changes in the angular position of said steering wheel (12), said control unit (17) providing said angle encoder (16) with a constant supply voltage (VCC) and including a malfunction detecting device connected to said angle encoder (16), said angle encoder (16) comprises a circuit for establishing a constant analogous voltage for reducing said certain pulse amplitude (ΔV 1 ) normally delivered by said angle encoder (16) to a suitable check amplitude (ΔV 2 ), wherein the bottom voltage level of each pulse is above zero and the top voltage level of each pulse is below said supply voltage (VCC) . Power steering system according to claim 1, wherein said certain pulse amplitude (ΔV 1 ) extends from zero to said supply voltage (VCC), and said constant analogous voltage being arranged to reduce said certain pulse amplitude (ΔV 1 ) to said check amplitude (ΔV 2 ), which thereby extends from a first voltage level (V 1 ) above zero to a second voltage level (V 2 ) below said supply voltage (VCC). Power steering system according to claim 1 or 2, wherein said malfunction detecting device comprises a means for providing a high trip voltage level (V th ) between the supply voltage (VCC) and said second voltage level (V 2 ), and a low trip voltage level (V tl ) between said first voltage level (V 1 ) and zero, said high and low trip voltage levels (V th ) and (V tl ) define limit values for indicating malfunction of said angle encoder (16). Power steering system according to anyone of claims 1 - 3, wherein said second angle sensing device (15) is arranged to deliver a feed back signal to said control unit (17) for obtaining a correlation between the set pulse number and frequency delivered by said angle encoder (16) and the actual pulse number and frequency delivered by said second angle sensing device (15).

Inventors: FOERBORGEN CARL ERIK MIKAEL, FOERBORGEN, CARL ERIK MIKAEL, Förborgen, Carl Erik Mikael
Applicants: INMOTION TECHNOLOGIES AB
Application year: 2000
Main classification: B62D15
IPC codes: G01D5, G01D3, B62D15