Limit Switches in Motion Control

A limit switch is a device that produces a signal used to halt the motor. Limit switches are usually used to indicate an end of travel, or to prevent a motor from travelling too far in one direction. Some applications have motors that cannot move any more than a specified distance without doing damage to the motor or the surroundings. When a limit switch is activated in the direction of travel, a controller needs to stop motion in that direction as soon as possible. To accomplish this, the National Instruments Motion Controllers use a halt command to stop immediately. The halt command acts like a brake and causes the motor to decelerate faster than it normally would.

One example of a situation where limits are frequently used is on an XY table. When a table has moved to the farthest position it can move to, the motor must stop trying to push it any further. If there were no limit switches on the XY table, the motor would reach the end of travel, but still continue to turn, causing damage to either the motor or the XY table.

Limit inputs on the National Instruments Motion Controllers accept a TTL signal which, when it is triggered, causes the motor to stop any further movement in that direction. To get further movement in the direction in which the limit is active, the limit must be disabled.

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