A safe state must be maintained after stopping
Aliases: category 0/1 stop · brake hold · safe stop state
What it is
Fail-safe hold after emergency stop requires more than halting motion when the stop is triggered: the robot must stay in a state that cannot change on its own — no drooping under gravity, no coasting on residual momentum, no re-energizing or resuming from a controller reset or power fluctuation. Stopping is a state that must be actively maintained, not a one-time action that ends the story.
Why it happens
Once power is cut, a system can still move from stored mechanical energy (a drooping joint, a tensioned belt), a software restart, or an external trigger. This kind of "second motion after the stop" tends to occur exactly when a person has relaxed and is stepping into the hazard zone to deal with the problem — a moment riskier than the original operating phase. A safe state therefore has to be actively locked by brakes, latches, or interlock logic; it cannot rely on the default assumption that nothing moves without a command.
Where it stops holding
Different stop categories — controlled stop then power removal, immediate power removal, controlled stop with power retained — suit different mechanisms. Power removal is the most complete in one sense, but on a cantilevered structure it introduces a drop hazard, so a holding brake is needed instead of bare power cutoff. What counts as a safe state therefore depends on the mechanism type, the load, and the installation, and no single stop category fits every configuration.
Applying it
- Choose the stop category by mechanism type; fit a holding brake rather than relying on power removal alone for cantilevered or heavily loaded axes, so the stop itself does not introduce a new drop or fling hazard.
- Make sure reset or power reconnection after a stop cannot resume motion automatically; any path that could re-energize the system needs an explicit unlock step.
- Periodically measure how long the stopped state holds under full-load and low-battery conditions to rule out slow drift or slippage.