This invention relates to the field of industrial process control, and, more particularly, to a system for braking an AC induction motor utilized as part of an industrial process control system.
Unlike synchronous-inductor motors wherein the rotor quickly stops when the motor is de-energized, due to the natural "cogging" or "detenting" action between the rotor and stator, AC (squirrel cage) induction motors have no natural "cogging" or "detenting" action. Thus, when an AC induction motor is de-energized, it has a tendency to coast. In routine applications this coasting action (over-travel) is not a problem, but in precision applications it can be quite undesirable.
For example, in the semiconductor wafer processing equipment marketed by the assignee of the present invention, an AC induction motor is used to drive (through a gear train) the lead screw of a direct drive elevator which vertically positions a cassette used for transporting and storing the semiconductor wafers. During operation of the elevator, a photo-electric sensing mechanism is used to detect the proper position of the elevator lead-screw at which point the motor is de-energized. Any over-travel of the motor will cause the cassette to be positioned slightly high or slightly low, depending on the direction of travel. This positioning error can prevent proper transport of the wafers due to mechanical interference with the "lands" of the cassette which separate and support the wafers. Accordingly, it is quite undesirable.
An optimal braking system for AC induction motors typically utilized in the field of industrial process control would possess certain important attributes. In particular, it should be capable of quickly causing the motor to come to a complete stop in a minimum number of revolutions. Moreover, it should be relatively inexpensive to implement and should be versatile, i.e., capable of being used with various types of AC induction motors. And finally, it should be reliable, simple and easy to attach.
Prior to the present invention it is believed that the above needs have gone unfulfilled.
It is accordingly a general object of the present invention to overcome the aforementioned limitations associated with AC induction motors and to fulfill the needs mentioned by providing a braking system for an AC induction motor having characteristics more nearly approaching the optimal attributes noted above.
It is specific object of the present invention to provide a safe, relatively inexpensive braking system for causing an AC induction motor to quickly come to a complete stop within several revolutions.
Other objects will be apparent in the following detailed description and practice of the invention.