Limit Switches

A limit switch detects the physical motion of an object by direct contact with that object.
An example of a limit switch is the switch detecting the open position of an automobile door,
automatically energizing the cabin light when the door opens.The “normal” status of a switch is the condition of minimum stimulus. A limit switch will be in its “normal” status when it is not in contact with anything (i.e. nothing touching the switch actuator mechanism).

Limit switches find many uses in industry, particular in robotic control and CNC (Computer
Numerical Control) machine tool systems. In many motion-control systems, the moving elements
have “home” positions where the computer assigns a position value of zero. For example, the axis
controls on a CNC machine tool such as a lathe or mill all return to their “home” positions upon
start-up, so the computer can know with confidence the starting locations of each piece. These home
positions are detected by means of limit switches. The computer commands each servo motor to
travel fully in one direction until a limit switch on each axis trips. The position counter for each
axis resets to zero as soon as the respective limit switch detects that the home position has been
reached.

A typical limit switch design uses a roller-tipped lever to make contact with the moving part.
Screw terminals on the switch body provide connection points with the NC and NO contacts inside
the switch. Most limit switches of this design share a “common” terminal between the NC and NO
contacts like this: