Speed averaging for variable effort power steering

Abstract: A variable effort power steering unit (10) under control of a microcomputer (16) is able to change steering effort in response to control current. The control current is developed as a function of average vehicle speed. In the case of vehicle deceleration, a long averaging time is used to inhibit rapid steering effort changes during braking. In the case of vehicle acceleration, a short averaging time is used for rapid response of steering effort to speed increases.

Description: This invention relates to power steering, and more particularly, to controlling the steering effort as a function of average vehicle speed. The conventional hydraulic power assist steering system comprises a hydraulic actuator for moving the steering linkage in relation to the fluid flow supplied thereto, and a rotary hydraulic control valve assembly for controlling fluid flow to the actuator in relation to the operator exerted steering torque. The control valve generally includes a cylindrical valve body rotatable within the valve housing and a spool rotatably disposed within the valve body. Hydraulic fluid is supplied to a cavity formed in the spool, and the valve body is grooved to receive fluid flow in relation to the amount of relative rotation between spool and valve body. The fluid so received is then directed to the actuator so that steering assist is developed in relation to the relative rotation of the valve body and spool. The spool is manually rotated by the operator of the vehicle and is connected to mechanically drive the steering linkage through a lost motion coupling. A resilient element, such as a torsion bar, couples the spool and valve body to provide a centring force for aligning the spool and valve body, and to permit relative rotation therebetween in relation to operator exerted steering torque, at least within the limitations of the lost motion coupling. In systems of the type described above, the level of driver steering effort required to produce a given level of power assist depends primarily on the compliance of the torsion bar. If the torsion bar has relatively high compliance, a relatively low level of driver steering effort is required. This is generally desirable in, low speed operation of a vehicle where relatively high steering forces are required. If the torsion bar has relatively low compliance, a relatively high level of driver steering effort is required. This is generally desirable in high speed operation of a vehicle where relatively low steering forces are required. The need to accommodate different steering levels at different speeds has been met by US Patent No. 4,886,138. This patent describes a hydraulic power assist steering system having conventional relatively rotatable spool and valve body elements for flow regulation, and an integral electromagnetic mechanism operating in conjunction with the torsion bar to define a coupling of variable resilience between the spool and valve body for adjusting driver steering effort required to produce a given level of power assist. The current to the electromagnetic mechanism determines the steering effort and the current is varied as a function of vehicle speed. A method in accordance with the preamble of claim 1 is disclosed in DE-A-3504561. In the event that the vehicle speed changes rapidly, the steering effort may also change rapidly. In some cases, such as during vehicle acceleration, this is desirable since the better road feel becomes available when needed. On the other hand, steering effort may decrease rapidly during deceleration. It is preferred, however, that the current be controlled to enhance drivability and not cause sudden decrease in steering effort. It is therefore an object of this invention to limit the rate of change in steering effort in response to vehicle speed, especially during deceleration. To this end, a method in accordance with the present invention is characterised over DE-A-3504561 by the features specified in the characterising portion of Claim 1. Preferably, the present invention provides a different steering response to speed for acceleration and deceleration. The invention is carried out in a variable effort power steering system responsive to a steering effort control current, and having a control unit for varying control current as a function of vehicle speed by the method as disclosed in Claim 1. The present invention will now be described, by way of example, with reference to the following description, taken in conjunction with the accompanying drawings, in which:- Figure 1 is a block diagram of a variable effort power steering system for carrying out the method of the invention; Figure 2 depicts the variation in driver steering effort achieved by the system of Figure 1; Figure 3 is a graph of current versus effective vehicle speed; and Figure 4 is a flow chart illustrating the program steps for carrying out the invention. The ensuing description is directed to a particular steering system with electromagnetic variable resilience coupling and a speed responsive current control for that system. However, the invention is not limited to use with that mechanism. The speed responsive control applies as well to other variable effort power steering systems which respond to an electrical speed signal. One such alternative system utilizes a variable orifice in the power steering hydraulic system to change the steering effort, and the orifice is electrically controlled as a function of vehicle speed. Figure 1 shows a system and a variable effort power steering unit 10 which are further described in detail in the above mentioned US Patent No. 4,886,138, which details are incorporated herein by reference. The steering unit 10 determines the steering position of the wheels 12. The speed of the wheels 12 is sensed by a speed sensor 14 and the signal therefrom is used as a measure of vehicle speed. A (computer based) control unit is supplied with operating power from the vehicle storage battery (not shown), and comprises a microcomputer 16, an input/output device (I/O) 18, an input counter 20, and a pulse-width-modulation driver (PWM) 22, all of which may be conventional devices. The microcomputer 16 communicates with the remainder of the system via the I/O device 18; in response to various input information, microcomputer 16 executes a series of program instructions for developing an output command pertaining to the required energization of the steering unit 10. The primary control unit input is an oscillatory speed signal from the speed sensor 14. The speed signal is applied to the I/O device 18 through the input counter 20, which divides the frequency of the speed signal by a predetermined factor. The PWM command for exciting the steering unit 10 is applied to the PWM driver 22 (which is bidirectional) for correspondingly modulating the steering unit 10 with control current from the vehicle storage battery. According to the embodiment described in US patent no. 4,886,138, the relative displacement of a valve spool and a valve body is resisted by a torsion bar and a magnetic centring force, and the magnetic force is dependent on actuator or control current supplied to the steering unit 10. For a zero control current, the combined effect of the torsion bar and the magnetic centring force in the steering unit 10 produces an intermediate level of steering assist, represented by the trace 30 in Figure 2. This level of assist is most suitable for an intermediate vehicle speed such as 48 kph (30 mph). Current of one polarity increases the driver steering effort per unit relative displacement of the valve spool and body, with more current requiring greater effort, as indicated by the traces 32. This is generally suitable for higher speeds. Current of the opposite polarity decreases the driver steering effort, as indicated by traces 34, and is desirable at lower speeds. Thus variable effort is effected by varying the control current. While the actuating or control current may be generated as a linear function of speed, it may also be tailored to optimize the benefit of the steering effort adjustability. Figure 3 shows one example of how the control current may vary with speed. Maximum current of one polarity occurs at zero vehicle speed to give maximum steering assist. The current decreases gradually between 0 and 16 kph (0 and 10 mph) and remains constant up to about 48 kph (30 mph) to continue a greater level of assist than is available with no current. Then between 48 and 80 kph (30 mph and 50 mph), the current changes linearly to the maximum value of the other polarity occurring at speeds of 80 kph (50 mph) and higher where the most steering effort is required and the best road feel is attained. A zero crossing of the current occurs just below 64 kph (40 mph) where the effort represented by trace 30 in Figure 2 prevails. Thus, it is apparent that the steering effort changes during acceleration or deceleration, especially in certain speed ranges. Of course if the control current were a linear function of speed, the steering effort would change during speed changes at all speeds. To determine the vehicle speed to be used in calculating the actuating current, the vehicle speed signals are sampled periodically to calculate speed samples. These speed samples can be used directly in the current calculations, but a moving average can be advantageously used for preventing sudden changes in the control current and the steering effort. It is particularly helpful to retard the steering effort change during vehicle deceleration. Thus, the speed samples can be averaged in different ways or over different time periods according to the type of speed change, if any. The flow chart of Figure 4 illustrates the method used within the microcomputer 16 to derive actuator or control current as a function of speed. The numbers nn in angle brackets 〈nn〉 refer to the reference numerals in the chart, and are used to describe the functions of each block. The input from the speed sensor 〈40〉 is used to calculate vehicle speed 〈42〉. A new speed sample is calculated for each control loop of the microcomputer to produce a speed sample, say, every 40 msec. Consecutive speed samples are compared to detect a change of speed 〈44〉. If the vehicle speed is decreasing 〈46〉, the average speed is calculated over a first time period such as three seconds 〈48〉. If the vehicle speed is increasing 〈50〉, the average speed is calculated over a second time period 〈52〉 which is preferably shorter than the first time period. The second time period may even equal the sampling period of 40 msec so that, in effect, the last speed sample is adopted as the "average speed". If the vehicle speed is neither decreasing 〈46〉 nor increasing 〈50〉, the previous time period is invoked as the averaging period for calculating the average speed 〈54〉. For example, if the last speed change was decreasing, the first time period is used and will continue to be used until a speed increase is detected. Then, the actuator or control current is calculated based on the calculated average speed 〈56〉, the actuator or control current is output 〈58〉 to the steering unit 10 via the PWM driver 22, and the loop is repeated. It will thus be seen that the current control according to this invention is able to independently manage speed averaging under acceleration and deceleration conditions by utilizing different averaging periods for each case, thereby changing the amount of steering assist less rapidly for deceleration then for acceleration.

Claims: A method of calculating vehicle speed for use in a variable effort power steering system responsive to a steering effort control current and having a control unit (16-22) for varying control current as a function of vehicle speed, the method comprising the steps of providing (40) a vehicle speed input signal; periodically (42) calculating vehicle speed samples from the speed input signal; detecting (44,46,50) a change in vehicle speed; calculating (48,52,54) an average speed; and generating (56,58) a steering effort control signal as a function of the calculated average speed; characterised by calculating the average speed by averaging the calculated speed samples over a given first time period when a vehicle speed decrease is detected (46); and calculating said steering effort control current as a function of said calculated average speed. A method as claimed in Claim 1, wherein when a vehicle speed increase is detected (50), the step (52) of calculating the average speed comprises averaging the calculated speed samples over a second time period shorter than said first time period. A method as claimed in Claim 1 or Claim 2, wherein when no speed change is detected (46,50), the step (54) of calculating the average speed comprises averaging the calculated speed samples over the time period which was invoked just prior to attaining constant speed. A method as claimed in Claim 1 wherein when speed increase is detected (50), the step of calculating the average speed comprises adopting the last calculated speed sample as the average speed, whereby the control current can respond rapidly when vehicle speed increases.

Inventors: SHORKEY MICHAEL JOHN, SHORKEY, MICHAEL JOHN
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION
Application year: 1992
Main classification: B62D6
IPC codes: B62D6