The invention pertains generally to automatic speed controls for vehicles and is more particularly directed to such systems for use in trucks, off-road vehicles, construction equipment, and other vehicles that use a heavy duty diesel engine.
Cruise control has become an increasingly popular option on passenger cars in recent years. The cruise control or automatic speed control option permits the vehicle operator to maintain a predetermined speed while being able to remove his foot from the accelerator pedal. The cruise control option has an obvious advantage in reducing driver fatigue and also providing a marginal increase in fuel economy since a steady speed is maintained and the vehicle is not repeatedly accelerated and decelerated. Heavy duty vehicles such as trucks are commonly used on long-distance routes and this inherent advantage of cruise control is increased for these vehicles. However, until recently heavy vehicles have not been equipped with speed or cruise control devices. A cruise control for heavy vehicles is disclosed in a U.S. Pat. No. 4,286,685 filed in the names of Rudolph et al on May 23, 1979, and which is commonly assigned with the present application. The disclosure of Rudolph et al is expressly incorporated by reference herein.
One problem with providing heavy vehicles with a speed control system is that compared to passenger cars, trucks are relatively under-powered and do not have the capability to accelerate quickly in order to maintain the speed set by an automatic speed control. This is because the horsepower/mass ratio in trucks is much different than in passenger cars and therefore, the heavy vehicle speed control must compensate for this difference. Additionally, the horsepower/mass ratios for a truck may vary over a wide range whereby in one instance the truck may be required to pull a fully loaded trailer while in another instance may only be required to travel without a trailer attached. Still further, the number of gears in a heavy vehicle varies considerably compared with the normal three or four for the normal passenger vehicle. For these reasons the heavy vehicle speed control must be more sophisticated than the automotive speed control.
The heavy vehicle speed control must also cooperate with the fuel control of the particular engine environment in which it is incorporated. Most heavy vehicles today are equipped with diesel engines. When accompanying a diesel engine the control must interface advantageously with a fuel control and not just a throttle control as in a passenger car. The fuel control of a diesel engine is essentially a governor device that can be divided into two types. The first general type of diesel governor is a min-max governor and the second type is an all-speed governor.
The min-max governor does not initiate a governing or limiting action unless the engine of the vehicle is operated above its maximum speed or below its minimum speed. Therefore, a heavy vehicle automatic speed control has a wide range of engine speeds it can use to regulate the vehicle velocity without the fuel governor recognizing its presence. The min-max governor is therefore relatively compatible with an automatic speed control. More problematic is the all-speed governor which attempts to maintain a certain engine speed for a particular throttle setting. Normally, an automatic speed control will regulate the throttle position of a heavy vehicle in response to a control signal indicating an error between the actual vehicle speed and the desired or commanded vehicle speed. If the throttle position is moved too quickly the all-speed governor will constantly counteract the action and hunt to bring the system back into equilibrium. This produces an instability where the speed control and all-speed governor are mismatched. The cruise control must work with and not against these mechanical feedback systems when on diesel engines. Prior to this time there has not been a heavy vehicle speed control with a control theory that is compatible with the operation of the diesel engine which may use either of these governors.
Another factor making truck speed control desirable is the fact that heavy vehicles are often equipped with accessory devices that require a predetermined constant engine speed for their most efficient operation. These devices, such as power takeoff (PTO), are run generally in direct drive from the engines or through various transmissions. The major advantage of any heavy vehicle cruise control is that it maintains vehicle speed when driving, but the control also can regulate engine speed when the vehicle is stationary. This stationary throttle control mode feature provides an engine speed control for use when the vehicle engine is used to operate auxiliary devices. This feature is particularly advantageous in off-highway vehicles such as construction equipment and the like. The feature may also be used as a constant engine speed device when a heavy vehicle is stationary and being warmed up.
When operating in a throttle control mode it is important to have the throttle position and hence rpm of the engine remain relatively constant. A higher proportionally and a finer adjustment to the positioning of the throttle are necessitated by the throttle control mode. Thus, this feature requires a different control law and a separate mode of operation from the cruise control mode.