The present invention relates to an electronically controlled actuator, and more particularly, to an electronically controlled actuator suitable for the positional control of the movable vanes of a variable-capacity turbocharger for an automotive internal combustion engine.
A prior art electronically controlled actuator disclosed in, for example, EP-A No. 109088/1999 includes a motor and a speed change mechanism. In this prior art electronically controlled actuator, one end of an output shaft included in the speed change mechanism is connected to a driven member and the other end of the same is provided with a position sensor. The position sensor measures the present working position of the actuator and provides data to be used for the feedback control of the working position. The output shaft is supported for rotation in two bearings. The motor and the speed change mechanism are held in a case having a body and a cover.
Since one end of the output shaft of the speed change mechanism of this prior art electronically controlled actuator is used for transmitting driving force to an external device and other end of the same is provided with the position sensor, both the two bearings supporting the output shaft for rotation are disposed in the body at a small interval and hence the inclination of the output shaft increases. Consequently, 1) vibration resistance is reduced and 2) errors are introduced in measured positions due to an increase in the vibration of the position sensor.
First, the increase in the inclination of the output shaft increases the vibration of the output shaft in directions perpendicular to the axis of the output shaft. Consequently, fretting abrasion occurs in surfaces in contact under pressure of the speed change mechanism comprising a worm gear and a worm wheel, which abrasion shortens the life of the actuator.
Secondly, if the output shaft of the actuator shakes while the driven member is at a standstill, the output of the position sensor varies. Although the driven member is at a standstill at a normal position, the variation of the output of the position sensor is considered to indicate that the driven member is moving and a control signal to drive the driven member is provided. Consequently, the driven member is operated wrongly. Thus, the reliability of the actuator is reduced.