Electrical

# working principle of limit switch

## Introduction

A limit switch detects the physical movement of an object by direct contact with that object. An example of a limit switch is the switch that detects the open position of a car door, automatically activating the cabin light when the door is opened.

Remember that the “normal” state of a switch is the condition of the minimum stimulus. A limit switch will be in its “normal” state when it is not in contact with anything (ie nothing that touches the switch actuator mechanism).

## Theory

Limit switches find many uses in the industry, particularly in robotic control and CNC machine tool systems (Computer Numerical Control). In many motion control systems, mobile elements have “domestic” positions where the computer assigns a position value of zero. For example, the controls of the axis of a CNC machine tool such as a lathe or mill return to their “starting” positions at the time of commissioning, so that the computer can know with confidence the initial locations of each piece. These initial positions are detected by limit switches. The computer orders that each servo motor travels completely in one direction until a limit switch on each axis is triggered. The position counter for each axis is reset to zero as soon as the respective limit switch detects that the initial position has been reached.

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

This switch contact arrangement is sometimes referred to as a form-C contact set, since it incorporates both a form-A contact (normally-open) as well as a form-B contact (normally-closed).

A close-up view of several limit switches (used on a drum sequencer) shows the arrangement of connection terminals for form-C contacts. Each limit switch has its own “NO” (normally-open), “NC” (normally-closed), and “C” (common) screw terminal for wires to attach

• Works in almost all environments
• accuracy
• consume little electrical energy

• moving mechanical parts can wear out
• restricted to equipment working in very slow speed