Animation Fundamentals: Windshield Wiper Control

I use this euphemism because it's a memorable way to think of the principle. We all know how windshield wipers behave: they always return to the same position no matter when we shut them off or how briefly we hit the button. This is not to be confused with limit switches, which set absolute stop points; instead, it's a process of providing a mechanism with an index, or starting point. Such mechanisms are almost always rotary, and almost always rotate in only one direction

Mechanisms can have more than one index point. For instance:

  • doors that open and close
  • semaphores that can be in one of three positions

Practical Application

This tiny mechanism opens and closes enginehouse doors. The two brass pins on the black spur gear engage the sub-microswitch to stop the motor in either the open or closed position. A SPST pushbutton activates the mechanism, which then runs until it reaches the other index point.

Wiring

There are multiple ways to wire self-indexing devices. Here are the two most common.

This pushbutton circuit can be used for devices having any number of index points, from one to whatever; it's all down to how many trip pins are involved. The motor will run on its own until it reaches the next index point.

The above circuit controls a two-position device, such as a doors-open/doors/closed animation. It requires two index switches, instead of one. The advantage of this circuit is that the control panel switch physically indicates the status of the device.

Examples

Also See...

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