The field of the invention relates to speed control systems for motor vehicles.
Conventional speed control systems maintain vehicle speed at a reference speed by feedback control of the engine throttle in response to an error signal. The reference speed is generated by storing the vehicle speed existing at the time of operator actuation of a set button or switch. Typically, the error signal is provided by subtracting vehicle speed from the reference speed, and the feedback control then drives the error signal to zero. After speed control is temporarily disabled in response to operator application of the brakes, the reference speed becomes the previous stored reference speed after the operator actuates a resume switch.
A problem with such prior approaches is that upon speed control initiation, or resumption, feedback control responsive to the error signal must first "wind in" speed cable slack and then turn the throttle to require the reference speed. Thus, vehicle speed may initially droop upon initiation or resumption of speed control operation.
U.S. Pat. No. 4,591,986 issued to Nakajima et al addresses the above problem of speed droop. Nakajima et al looks up a target throttle position, at the time of speed control actuation, in a memory or table of throttle position versus vehicle speed. The engine throttle is then turned to this target throttle position and feedback control thereafter commenced.
The inventors herein have recognized numerous disadvantages of the above approach. For example, the look-up table is generated for a particular vehicle operating on flat road conditions. On the other hand, the amount of speed control cable slack which must be wound to achieve a desired or initial throttle position will be a function of cable slack and linkage slack which will vary among vehicles. Further, the relationship between the throttle position and vehicle speed varies among vehicles, vehicle equipment, and load conditions.