A limit switch is a simple sensor that tells a robot when a moving part has reached a specific position. In many machines, it is placed at the end of travel for a sliding carriage, rotating arm, elevator, or gantry axis. When the machine presses the lever, the switch changes its electrical state and sends a clear signal to the controller.
This matters because accurate end stops help robots home themselves, avoid crashes, and repeat motion safely.
Key Facts
- A lever limit switch changes state when a moving part pushes the roller lever far enough to actuate the internal contacts.
- Normally open means the circuit is open when not pressed and closes when actuated.
- Normally closed means the circuit is closed when not pressed and opens when actuated.
- For a pull-up input, Vinput is usually high when the switch is open and low when the switch connects the input to ground.
- Travel margin = stop distance available - stopping distance required, and it should be greater than 0 for safe motion.
- If speed is constant, stopping distance can be estimated by d = v t, where v is speed and t is stopping time.
Vocabulary
- Limit switch
- A limit switch is an electromechanical sensor that changes its electrical contact state when a machine part reaches a set position.
- Roller lever
- A roller lever is the hinged arm on a limit switch that is pushed by a moving part, often with a small roller to reduce friction.
- Normally open
- Normally open describes a switch contact that does not conduct current until the switch is actuated.
- Normally closed
- Normally closed describes a switch contact that conducts current until the switch is actuated.
- Homing
- Homing is the process of moving a robot axis until it detects a reference sensor so the controller can define a known position.
Common Mistakes to Avoid
- Confusing normally open with normally closed is wrong because the words describe the unpressed state, not what happens after the switch is pressed.
- Placing the switch exactly at the crash point is wrong because the robot needs extra distance to slow down after the signal is detected.
- Ignoring switch bounce is wrong because mechanical contacts can rapidly open and close for a few milliseconds, causing false multiple triggers.
- Wiring a limit switch without checking the controller input type is wrong because pull-up, pull-down, sinking, and sourcing inputs require different connections.
Practice Questions
- 1 A sliding carriage moves at 0.40 m/s and the controller takes 0.15 s to stop it after the limit switch triggers. What minimum stopping distance is needed?
- 2 A robot axis has 25 mm of space between the limit switch trigger point and the hard stop. If it moves at 0.10 m/s and stops in 0.18 s, how much travel margin remains?
- 3 A safety system uses a normally closed limit switch at the end of travel. Explain why normally closed wiring can be safer than normally open wiring if a wire breaks.