1. Field of the Invention
This invention relates to a throttle control apparatus for an engine and, in particular, to such an apparatus having a fail-safe feature.
2. Description of Related Art
In state-of-the-art vehicles it is becoming common practice to control engine throttles electrically rather than by mechanical linkages. For example, an automotive vehicle having a traction-control system will typically include an electrically-controlled auxiliary throttle for reducing engine power whenever drive-wheel slippage is experienced. As another example, a vehicle having a drive-by-wire capability will typically include an electrically-controlled primary throttle for controlling engine power in response to a signal from engine-control electronics.
For any engine having an electrically-controlled throttle system it is desirable to have a safety mechanism for moving the throttle to a fail-safe position in the event that electrical power to the system is lost. Such safety mechanisms typically include respective springs for returning the primary throttle to a minimum engine-power position and for returning the auxiliary throttle to a minimum power-reduction position.
In FIG. 1, the solid line illustrates the force developed by an exemplary spring utilized for returning a throttle to its fail-safe position. In this example, the spring is a rotary spring, such as a torsion spring, disposed around a rotatable throttle shaft, and having distal ends attached to the shaft and to a fixed member (such as a throttle body), respectively. The spring force is plotted as a function of angular position of the throttle and is expressed as a torque for this rotary example. Throttle position .theta..sub.f corresponds to a fail-safe position while throttle position .theta..sub.m corresponds to a maximum operating position. In the example of a primary throttle, .theta..sub.f corresponds to a minimum engine-power position (e.g. throttle substantially closed), while .theta..sub.m corresponds to a maximum engine-power position (e.g. throttle fully open). Conversely, in the example of an auxiliary throttle for traction control, .theta..sub.f corresponds to a minimum power-reduction position (e.g. throttle fully open), while .theta..sub.m corresponds to a maximum power-reduction position (e.g. throttle substantially closed).
Note that the spring force increases steadily from a value L.sub.f to a value L.sub.m as the throttle is driven from the fail-safe position .theta..sub.f to the maximum operating position .theta..sub.m. Drive means for positioning the throttle must therefore be capable of producing a torque, at the throttle shaft, which is at least equal to the torque L.sub.m. This corresponds to the maximum operative counteracting torque exerted by the spring attached to the throttle shaft. Typically this driving force is produced by an electric motor which is connected to the throttle shaft through force-multiplying means, such as gearing, to provide a mechanical advantage. This reduces the size and power requirements for the motor. Unfortunately, however, it also reduces the spring advantage in acting against the motor (in a lost-power situation) to return the throttle to the fail-safe position. In such a situation, the spring must overcome frictional forces and any counteracting permanent magnet forces existing within the motor. Further, by providing such a mechanical advantage to the motor, the operating speed requirement of the motor is increased. Not only does this increase the cost of the motor, but in some types of motors the torque output of the motor actually decreases with increasing speed. Thus there is a limit to the mechanical advantage that can be provided.