An emergency stop circuit is a safety system that lets a person quickly stop a robot when motion becomes dangerous. In many robots, the most visible part is a red latching mushroom button that stays pressed after it is hit. The goal is not to shut down every computer, but to remove hazardous motor power as fast and reliably as possible.
This matters because software can freeze, sensors can fail, and a moving robot can injure people or damage equipment.
Key Facts
- An E-stop should remove power to actuators independently of the robot control software.
- A latching mushroom button stays open after being pressed and must be deliberately reset, often by twisting or pulling.
- A normally closed E-stop contact allows current during safe operation and opens the circuit when pressed or if a wire breaks.
- Ohm's law for checking circuit current is V = IR.
- Relay coil power can be estimated with P = VI.
- Stopping distance can be estimated with d = v^2/(2a) when deceleration is approximately constant.
Vocabulary
- Emergency stop
- An emergency stop is a safety function that rapidly removes hazardous energy from a machine when a dangerous situation occurs.
- Latching button
- A latching button is a switch that stays in its activated state until a deliberate reset action is performed.
- Normally closed contact
- A normally closed contact conducts current in its normal safe state and opens when the device is activated.
- Safety relay
- A safety relay is a device designed to monitor safety inputs and control outputs such as motor contactors with fault detection.
- Contactor
- A contactor is an electrically controlled high-current switch used to connect or disconnect power to motors and drives.
Common Mistakes to Avoid
- Putting the E-stop only in software is wrong because a frozen program or failed controller may not respond when the robot must stop.
- Using a normally open E-stop contact is unsafe because a broken wire can look the same as a safe unpressed button instead of causing a stop.
- Resetting the E-stop and allowing instant restart is wrong because a separate intentional start command should be required after the hazard is cleared.
- Cutting only logic power is incomplete because motor drives or stored energy may still move the robot unless actuator power is safely removed or controlled.
Practice Questions
- 1 A 24 V safety relay coil draws 0.10 A when energized. What is the coil power in watts?
- 2 A robot arm is moving at 1.2 m/s when the E-stop removes motor power and braking produces a constant deceleration of 3.0 m/s^2. Estimate the stopping distance.
- 3 Explain why an emergency stop circuit usually uses normally closed contacts and a safety relay instead of relying only on a microcontroller input.