A speed control system for a vehicle

Abstract: A self-tuning speed control system is disclosed. The gain of the system controller is tuned to provide optimal performance based upon a sensitivity point analysis and gradient optimisation of a prescribed cost index.

Description: The present invention relates generally to a speed or cruise control for a vehicle and more particularly to an adaptive or self-tuning speed control which continuously tunes controller gain so as to optimise performance. Speed control systems are well known today. Each such system maintains the vehicle at a substantially constant speed selected by the operator. One significant objective in the design of a speed control system is acceptable performance over a wide range of vehicle lines and over a wide range of operating conditions (often referred to as "robustness"). In this context, performance is measured in terms of a speed tracking ability, throttle movement, steady state speed error, and frequency of recalibration. In an attempt to achieve the foregoing objective, and to meet increasingly stringent performance requirements, speed control systems have become increasingly more complex through the years. The earliest systems simply held the throttle in a fixed position (J.T. Ball, "Approaches and Trends in Automatic Speed Controls," SAE Technical Paper #670195, 1967). In the late 1950's, speed control with feedback appeared. These systems used proportional feedback of the speed error, and the controller gain typically provided full throttle in response to an error in the range of six to ten miles per hour (mph) (W.C. Follmer, "Electronic Speed Control," SAE Technical Paper #740022, 1974). The next enhancement was proportional control with an integral preset. Then came proportional integral control systems, often referred to as P.I. systems (B. Chaudhare et al , "Speed Control Integrated into the Powertrain Computer," SAE Technical Paper #860480, 1986). The combination of proportional and integral feedback, with appropriate controller gains, substantially reduced speed droop when the system was initialised. Further enhancements are described in United States Letters Patent Nos. 4,803,637; 4,870,583; and 4,893,243. In general, system performance is improved by switching controller gains to predetermined values in accordance with specific operating conditions. With the recent availability of inexpensive microcontrollers, more sophisticated control strategies have been attempted. These include proportional integral derivative, or P.I.D., control; optimal linear quadratic regulation (T. Tabe et al , "Vehicle Speed Control System Using Modern Control Theory," IEEE IECON '86 Proceedings, 1986); fuzzy logic control (M. Uriubara et al , "Development of Automotive Cruising Using Fuzzy Control System," Journal of SAE of Japan, Vol. 42, No. 2, 1989); and self-tuning control (T. Tsijii et al , "Application of Self-Tuning to Automotive Cruise Control," American Control Conference Proceeding, May, 1990). US Patent No 4 893 243 describes a control device for maintaining a constant speed of an automobile having an internal combustion engine controlled by a throttle valve and an actuator for the valve. The speed of the automobile is corrected to a set speed by cyclically repeating an integral calculation at a first rate when the running speed is less than the set speed. The integral calculation proceeds at a different rate when the running speed is greater than the set speed. The objective of the present invention is a robust stable speed control system. The potential saving from a single generic system capable of providing acceptable performance across a wide range of vehicle lines and over a wide range of operating conditions is enormous. According to the invention there is provided a speed control system for a vehicle, having a movable throttle operated by a throttle command for controlling vehicle speed, an operator-actuated control including a source of a set speed signal indicative of a desired vehicle speed, a speed sensor providing an actual speed signal representative of actual vehicle speed, means for generating an error signal having a magnitude related to the difference between said actual speed signal and set speed signal, and an adaptive controller for receiving said set speed signal and said actual speed signal and for issuing said throttle command in response to said set speed signal, said actual speed signal and at least one gain value that is controllable in accordance with the vehicle operation, characterised in that, to adaptively modify said gain value, the speed control system further comprises, in combination: sensitivity filter means responsive to said error speed signal for generating a partial derivative value indicative of the rate of change of said actual vehicle speed with respect to said gain value, said sensitivity filter means modelling (i) a predetermined throttle-position-to-speed transfer function of the vehicle and (ii) a fixed gain controller for the vehicle; and gradient means responsive to the partial derivative value for repeatedly adjusting said gain value by an incremental amount according to an integral of the sum of a value dependent on the throttle command signal and a value dependent on the error signal. throttle command changes as said integral gain value changes. The system embodying the present invention is a robust, stable self-tuning speed control system which provides good speed tracking and disturbance rejection, without excessive throttle jitter or steady state error. The system dynamically tunes controller gain based upon a sensitivity points analysis and a slow adaptation procedure. The system is premised upon the identification of a vehicle model and a compatible controller model, i.e. , a fixed gain controller which, when properly tuned, will produce the desired performance in the modelled vehicle. These models form the basis for a sensitivity filter subsystem which provides a partial derivative of the speed signal with respect to the gain of the controller. The sensitivity filter subsystem is coupled to a gradient subsystem which defines a cost index or function related to controller gain. Minimisation of the cost index during vehicle operation, based upon the speed/gain partial derivative signal, determines an optimal gain for the controller, i.e. , the gain which substantially optimises speed control performance. The system embodying the invention has an advantage that it is a self-tuning speed control system. The controller gain is tuned to provide substantially optimal performance. Further the system is capable of use over a complete range of vehicles. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : FIGURE 1 is a schematic block diagram of a vehicle including a preferred embodiment of the present invention in the form of a self-tuning speed control system; FIGURE 2 is a partial schematic block diagram of the self-tuning speed control; FIGURE 3 is a schematic block diagram of a vehicle and controller of the type necessary to implement the present invention; FIGURE 4 illustrates a linear closed loop model of the vehicle and controller shown in FIGURE 3; FIGURE 5 is a block diagram of the sensitivity filter derived from the model shown in FIGURE 4; FIGURE 6 is a block diagram of a high pass filter; and FIGURE 7 is a schematic diagram illustrating the algorithm of the gradient means forming a part of the self-tuning speed control. A preferred embodiment of the present invention is shown in the drawing as a self-tuning speed control system 10 for a vehicle, generally designated 12. The vehicle 12 includes an automatic (or manual) transmission 14, an engine 16 with a torque converter (or clutch), and a chassis 18. With reference to FIGURE 1, the speed control system 10 controls the engine throttle 20 in a conventional manner to provide a substantially constant speed. For example, the system 10 includes a control assembly 22 mounted on the steering wheel 24. The assembly 22 includes the following operator controllable monitoring switches: ON/OFF 26, COAST 28, SET/ACCELERATE 30, and RESUME 32. One input from the control assembly 22 is a speed set pulse which establishes the desired vehicle speed "Yset." The system 10 further includes a speed sensor 34, controller 36, processor 38 and throttle actuator 40, such as a stepper motor. The speed sensor 34 provides a speed signal "Y" representative of actual vehicle speed, and the error, i.e. , Y - Yset, is designated herein as "e." As is well known in the art, the controller 36 monitors actual vehicle speed and stores the desired speed Yset in response to the speed set pulse. The controller 36 further issues a throttle position command "U" and the throttle actuator 40 responsively sets the throttle 20. The system 10 additionally includes conventional deactivation, safety and back-up subsystems (not shown). Whenever the system 10 is deactivated, the controller 36, via the throttle actuator 40, sets the throttle 20 to an idle position. In this preferred embodiment, and as best shown in FIGURE 2, the controller 36 is a microcontroller programmed to operate in the manner of a proportional integral controller, i.e. , having a proportional gain "K p " and an integral gain "K i ." The controller 36 may further include any number of conventional enhancements, such as shown in the above-identified patents. For example, the controller 36 may be speed selective, such that the initial settings of the gains K p and K i or other controller parameters are determined by vehicle speed Y at the time of system initiation. The self-tuning speed control system 10 continuously tunes the controller gains K p and K i to substantially optimise system performance. In general operational terms, the self-tuning system 10 regulates performance based upon a system sensitivity, i.e. , system response with respect to a particular system parameter. In this preferred embodiment, a partial derivative of the system output with respect to controller gain is obtained through a sensitivity filter. Gain adaptation, driven by the vehicular response to unmeasured torque load disturbances, follows through minimisation of a predetermined, gain-related cost index or function during real time operation of the vehicle 12. Referring to FIGURE 2, the system 10 includes sensitivity filter means 42, coupled to the control assembly 22 and speed sensor 34, and gradient means 44, coupled to the controller 36. Separate and discrete subsystems may be utilised; preferably, however, the function and operation of the sensitivity filter means 42 and gradient means 44, respectively, are performed by the microprocessor 38, as depicted. The controller 36, sensitivity filter means 42 and gradient means 44 may also be incorporated into a single microprocessor. In this preferred embodiment, the system sensitivity is related to vehicle speed Y. The sensitivity filter means 42 receives the error signal e from the controller 36 and responsively provides at least one partial derivative thereof (which is identical to the partial derivative of the speed signal Y). More particularly, the sensitivity filter means 42 provides a partial derivative of the speed signal Y with respect to each tunable parameter of the controller 36. In this preferred embodiment, the sensitivity filter means 42 includes a first or K p sensitivity filter 46 and a second or K i sensitivity filter 48. The sensitivity filter means 42 is based upon a predeterminable nominal model of the vehicle 12 and an identifiable nominal fixed gain P.I. controller compatible therewith. That is, the system 10 is premised upon the development of (i) an acceptable nominal vehicle model (expressed as a vehicle or more particularly a throttle position-to-speed transfer function) and (ii) a nominal fixed gain P.I. controller model which will provide acceptable speed control performance when applied to the selected vehicle model (expressed as a controller transfer function). FIGURE 3 illustrates simple vehicle and controller models of the type necessary for implementation of the present invention. The unmeasured torque disturbance input to the vehicle 12 is represented as "Tdis"; the transfer function for the vehicle dynamics is "G o (s)"; powertrain gain is shown as "G 1 "; and the transfer function for the compatible P.I. controller is "C(s,P)," P being a vector representation of K p and K i . Based thereon, the output Y is expressed as follows: Y = - G o (s) 1 - C(s,P)G 1 G o (s) Tdis. Differentiation with respect to P reveals: ∂Y ∂P = G 1 G o (s) 1 - C(s,P)G 1 G o (s) ∂C ∂P Y. Thus, the partial derivative of the speed signal Y with respect to any controller parameter, ∂Y/∂P, is generated in real time by passing the system output through a sensitivity filter which is a copy of the system modified by the partial derivative of the controller transfer function with respect to the same parameter, ∂C/∂P. As is well known, the transfer function for a conventional fixed gain P.I. controller the form: C(s,p) = U(s) Y(s) = - (K p + K i s ) Thus, ∂C/∂K p and ∂C/∂K i are -1 and -1/s, respectively. The resulting conventional linear closed loop model for the vehicle 12 and compatible controller is shown in FIGURE 4, wherein "Tw" represents drive torque at the wheels of the vehicle 12, and K 1 , K 2 and K 3 represent the powertrain gain, the inverse of the time constant for the vehicle 12, and a speed conversion factor, respectively. The corresponding sensitivity filter means 42 is shown in FIGURE 5. Implementation of the sensitivity filter means 42 need only take a nominal or approximate form to substantially ensure convergence of the gains within the speed control system 10. Preferably, the vehicle and controller models are empirically derived from an average or mid-range vehicle with a mid-range engine so as to substantially enhance the robustness of the system 10. The system sensitivities, ∂Y/∂K p and ∂Y/∂K i , are received by the gradient means 44, and the controller gains K p and K i are responsively tuned using a gradient optimisation technique. The gradient means 44 is preprogrammed with a quadratic cost index which, when minimised, establishes the optimal gains. In this preferred embodiment, the cost index, given below, contains a speed error component and a high frequency throttle component: where P is again a vector representation of the controller gains K p and K i and β 1 and β 2 represent and define a vector weighting. A cost index based solely on speed error could result in unduly high controller gains and excessive throttle movement. The term represents the control command due to disturbance inputs and is obtained by passing the throttle position command through a high pass filter 50 shown in FIGURE 6. Use thereof penalises high frequency throttle movement. In this preferred embodiment, the high pass filter 50 forms a part of the sensitivity filter means 42. To minimise J(P), the following gradient algorithm is used: where ε 1 and ε 2 are adaptation gains. This algorithm requires additional sensitivity filtering of ∂ /∂P to generate ∂ /∂K p and ∂ /∂K i . In this preferred embodiment, the necessary filtering is provided by the sensitivity filter means 42 in a conventional fashion under control of the processor means 38. The gradient algorithm is shown schematically in FIGURE 7. To implement this adaptive law in the processor 38, conversion to a discrete form is necessary, preferably utilising first order low pass filters of the form a(z+1) z-y This implementation minimizes the necessary memory and coding requirements and avoids the numerical issues of higher order filters. The adaptation gains ε 1 and ε 2 determine the speed of adaptation and are preferably based upon the slowest dynamics within the vehicle plant. The dominant dynamics result from vehicle inertia and have a time constant on the order of thirty (30) to fifty (50) seconds. To substantially avoid interaction between the states of the adaptive system 10 and plant, adaptation is at least an order of magnitude slower, and preferably convergence is achieved in the range of two (2) to ten (10) minutes, depending on the initial conditions and road load disturbances. The products ε 1 β 1 , ε 1 β 2 , ε 2 β 1 and ε 2 β 2 , are experimentally derived to provide this system time constant. There is a tradeoff with respect to the weightings of the cost index, β 1 and β 2 . To decrease limit cycle or surging behavior, the ratio β 1 /β 2 is increased. To improve tracking and transient speed error, the ratio is decreased. The choice of weightings is preferably made iteratively based upon the predetermined and compatible fixed gain P.I. controller. For the K p and K i assigned to the controller model and the selected ε 1 and ε 2 , the values of β 1 and β 2 are iterated until the adaptation converges to the assigned gains. Final refinement of the parameters β 1 and β 2 the vehicle line and over the range of operating conditions. In this preferred embodiment, the processor 38 monitors the throttle position and "freezes" the self-tuning gains K p and K i at predetermined limits thereof. While the assumption of a linear plant is sufficient under most conditions, nonlinearities such as those due to the throttle control limitations ( i.e. , wide open or fully closed throttle) may occur. Tuning is halted in any conventional fashion under control of the processor 38. Filter updating continues, however, such that gains are immediately tuned when the throttle limit is cleared. The processor 38 also monitors and limits a rate of change for the gains K p and K i . Generally, the choice of ε 1 and ε 2 provides sufficiently slow rates of change, but a problem may occur should the vehicle experience a large disturbance ( e.g. , when the driver overrides the speed control by rapidly and fully depressing the accelerator pedal). During events of this nature, the speed error and throttle commands become very large and may cause large gain changes. In this preferred embodiment, the rate change of K i is limited to a rate within the range of one (1) to four (4) (degrees/mph second)/second, while change K p is held to a rate within the range of ten (10) to twenty (20) (degrees/mph)/second. If exceeded, the processor 38 adjusts ε 1 and ε 2 through simple division by the smallest power of two (2) necessary to provide an acceptable rate of change. The processor 38 also preferably monitors and limits the values of K i and K p . As with any physical system, there are performance limitations. Here, for example, there is an upper limit on the frequency and amplitude of hills over which speed can be properly maintained. These limitations are not embodied in the adaptive algorithm, such that gain limits are appropriate. The upper gain limits are chosen to allow adequate tracking performance over the range of vehicles. The lower limits are set such that the low speed stability ( i.e. , elimination of noticeable limit cycles) is achieved whenever possible with the compatible fixed gain P.I. controller. Finally, two conditions are required for stability of the self-tuning speed control system 10: a persistently exciting input condition and a phase condition on the sensitivity filter means 42. The persistently exciting condition is satisfied by the varying road disturbance input and by the inherent speed limit cycle due to system nonlinearities. To satisfy the second condition, the phase of the nominal sensitivity filters 44, 46 must be within ninety degree (±90°) of the phase of the actual sensitivity at the dominant frequencies. To substantially ensure that this phase condition is met over the vehicle line and over the range of operating conditions, the nominal filter values are, as described above, preferably selected for a mid-sized car with an average power engine. Furthermore, the gains K p and K i of the controller model are chosen near the centre of the preferred ranges, respectively.

Claims: A speed control system for a vehicle, having a movable throttle (20) operated by a throttle command for controlling vehicle speed, an operator-actuated control including a source of a set speed signal indicative of a desired vehicle speed, a speed sensor (34) providing an actual speed signal representative of actual vehicle speed, means for generating an error signal having a magnitude related to the difference between said actual speed signal and set speed signal, and an adaptive controller (36) for receiving said set speed signal and said actual speed signal and for issuing said throttle command in response to said set speed signal, said actual speed signal and at least one gain value that is controllable in accordance with the vehicle operation, characterised in that, to adaptively modify said gain value, the speed control system further comprises, in combination: sensitivity filter means responsive to said error speed signal for generating a partial derivative value indicative of the rate of change of said actual vehicle speed with respect to said gain value, said sensitivity filter means modelling (i) a predetermined throttle-position-to-speed transfer function of the vehicle and (ii) a fixed gain controller for the vehicle; and gradient means responsive to the partial derivative value for repeatedly adjusting said gain value by an incremental amount according to an integral of the sum of a value dependent on the throttle command signal and a value dependent on the error signal. A speed control system as claimed in claim 1, wherein the controller (36) generates a throttle command in response to the combination of (a) a proportional component related to the product of said error signal and a proportional gain value, and (b) an integral component related to the product of said error signal and an integral gain value, said proportional gain value and said integral gain value being independently adjustable during vehicle operation to adaptively vary the dynamic response of the said controller (36), and said speed control system further comprises; sensitivity filter means for modelling the dynamic characteristics of the combination of said vehicle and said controller to generate; a first sensitivity value having a magnitude indicative of the rate at which said actual speed signal changes as said proportional gain value changes, and a second sensitivity value having a magnitude indicative of the rate at which the magnitude of said throttle command changes as said proportional gain value changes. A speed control system as claimed in claim 2, further comprising; sensitivity filter means for modelling the dynamic characteristics of the combination of the vehicle and controller to generate; a third sensitivity value having a magnitude indicative of the rate at which said actual speed signal changes as said integral gain value changes, and a fourth sensitivity value having a magnitude indicative of the rate at which the magnitude of the throttle command changes as said integral gain value changes. A speed control system as claimed in claim 3, wherein said gradient means operates according to an integral function of the form: where P is a vector representation of said integral and proportional gains, β 1 and β 2 define weighting values, is said throttle command modified by a high pass filter, and e is the error signal.

Inventors: LIUBAKKA MICHAEL K, WINKELMAN JAMES R, LIUBAKKA, MICHAEL K., WINKELMAN, JAMES R.
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1992
Main classification: G05D13
IPC codes: B60K31, G05B13, B60G17