Method and apparatus for accurately controlling the position of a film

Abstract

Method and apparatus for use in a web transport system wherein the web is stopped at a position it occupied when an event occurs. In particular, a motor driven capstan is provided to drive a web, such as a filmstrip, between first and second reels. A tachometer, operatively coupled to the capstan motor, provides a first signal representing the actual film velocity. The first signal is applied to an integrator which is controlled in a manner whereby during normal operation of the web transport system the integrator is off and does not affect system performance. When the event occurs requesting the capstan drive motor to stop, the integrator is enabled and provides an output which is proportional to the distance traveled during deceleration of the capstan drive motor (overshoot). The output of the integrator is coupled to first summing junction whereat it is added to a signal representing the desired film velocity which is driven to zero upon occurrence of the event. The integrator output, if it were operable, is of opposite polarity to the desired film velocity signal. The combined signals are coupled through an acceleration limiter to a second summing junction and compared with the tachometer output. The output of the second summing junction causes the capstan motor to turn in the direction opposite to that before the occurrence of the event, the integrator signal being driven to zero by the tachometer output, the motor velocity and overshoot thereby being driven to zero simultaneously.