Emergency Stops and Safety-Rated Systems

Every industrial robot has an emergency stop, but stopping is not instant. How the robot stops matters: cutting power at once, braking under control first, or holding position with power on. Safety functions are also built to keep working when a part fails, which is why safety circuits use two independent channels that check each other.

Stop category Joint speed Dual-channel safety circuit Inject a fault (welded contact in channel 1)

Stop categories

IEC 60204-1 defines three stop categories. Category 0 removes power immediately, and the robot coasts to rest against its brakes. Category 1 first decelerates under motor control, then removes power. Category 2 decelerates under control and keeps power on, holding position. With deceleration $\alpha$ after a reaction time $t_r$, the angle traveled is

$$\theta_{\text{stop}} = \omega\,t_r + \frac{\omega^2}{2\,\alpha}$$

Redundancy

Safety functions are rated, for example by performance level in ISO 13849, partly by how they handle faults. A dual-channel circuit wires two independent paths that must agree, so a single welded contact is detected rather than silently disabling the stop.

What to notice

Stopping distance grows with the square of speed, so halving speed cuts it to about a quarter. Category 0 usually stops in more distance here, since mechanical brakes decelerate less hard than the motors, though it does not depend on the drive working. With a single channel, a welded contact means pressing the stop does nothing. With two channels, the mismatch is caught and the robot is stopped and blocked from restarting. Numbers on this page are illustrative.