1. Field of the Invention
This invention generally relates to a throttle control apparatus for an internal combustion engine. This invention specifically relates to an apparatus including a sensor for detecting the degree of opening of a throttle valve in an internal combustion engine, and a device for feedback-controlling the actual degree of opening of the throttle valve at a target degree in response to the detected degree of opening of the throttle valve.
2. Description of the Prior Art
In recent years, most automotive internal combustion engines have been equipped with a sensor for detecting the degree of opening of a throttle valve. The output signal of the throttle opening degree sensor (throttle position sensor) is used by various types of control.
A known throttle control apparatus includes a DC motor for moving a throttle valve, a sensor for detecting the degree of opening of the throttle valve, and a device for driving the DC motor in response to the detected degree of opening of the throttle valve to control the actual degree of opening of the throttle valve.
In some automotive vehicles with automatic transmissions, a sensor detects the degree of opening of a throttle valve, and the automatic transmission is controlled in response to the detected degree of opening of the throttle valve according to a predetermined transmission control map.
The characteristics of throttle opening degree sensors (throttle position sensors) tend to vary from sensor to sensor. In addition, the characteristics of throttle opening degree sensors tend to vary with ageing thereof. Such variations in the sensor characteristics cause errors in the sensor output signal.
As will be described hereinafter, there are various known apparatus for correcting an error in the output signal of a throttle opening degree sensor.
Japanese published unexamined patent application 58-10131 and Japanese published unexamined patent application 63-180755 disclose throttle control apparatus in which a switch serves to detect the fully-closed position of a throttle valve, and the output signal of a throttle opening degree sensor which occurs when the switch is turned on is used as an indication of the fully-closed position of the throttle valve to correct an error in the sensor output signal.
Japanese published unexamined patent application 58-122326 and Japanese published unexamined patent application 3-107561 disclose throttle control apparatus in which the detected value currently prodded by a throttle opening degree sensor is compared with a memorized idle value (fully-closed position value). When the current detected value is smaller than the memorized idle value, the current detected value is memorized as a new idle value so that the memorized idle value is updated. Otherwise, the memorized idle value is held as it is. The updating of the memorized idle value corrects an error in the sensor output signal.
In the throttle control apparatus of Japanese patent application 58-122326, a determination is made as to whether the detected value provided by the throttle opening degree sensor equals a same value a given number of times or for a given length of time. In cases where the detected value provided by the throttle opening degree sensor equals a same value the given number of times or for the given length of time, when the same value is smaller than the memorized idle value, the current detected value is memorized as a new idle value so that the memorized idle value is updated.
In the throttle control apparatus of Japanese patent application 3-107561, engine operating conditions corresponding to the throttle fully-closed position can be detected by a suitable device. In cases where such engine operating conditions are actually detected, when the memorized idle value is smaller than the detected value currently provided by the throttle opening degree sensor, the memorized idle value is corrected into an increased idle value. The correction of the memorized idle value is intended to prevent am adverse affection of noise components of the sensor output signal.
The above-mentioned known throttle control apparatus have problems as follows. Generally, operating conditions of engines (for example, the rates of air flow into the engines) which occur at the fully-closed position of a throttle valve vary from engine to engine. Specifically, air leaks through throttle valves of engines even when the throttle valves are fully closed, and the rates of air leakage vary from engine to engine. Therefore, the relations between the throttle opening degrees detected by throttle position sensors and the engine operating conditions vary from engine to engine. Such a variation in the relations causes another type of error in the sensor output signal which adversely affects control responsive to the sensor output signal and the engine operating conditions. The above-mentioned known throttle control apparatus can not correct this type of error in the sensor output signal. In addition, the relation between the throttle opening degree detected by the throttle position sensor and the engine operating conditions varies with ageing of the sensor and a change in the engine operating conditions. Such a variation in the relation causes a type of error in the sensor output signal which adversely affects control responsive to the sensor output signal and the engine operating conditions. The above-mentioned known throttle control apparatus can not correct this type of error in the sensor output signal.