What should the watchdog (communication monitoring) time be?
On wireless remote controls the receiver waits for a valid signal to be refreshed within a time window; if the window expires, a stop is initiated. This time is not a fixed regulatory figure: the manufacturer declares it, the risk assessment on the machine determines it, and in wireless remote control applications it is expected not to exceed approximately 0.5 seconds.
The watchdog is the heartbeat monitoring of the wireless remote control. The transmitter sends a valid packet periodically; the receiver resets its counter on every valid packet. If the counter exceeds the declared time, the receiver initiates an active stop.
There are two opposing pressures when setting the time:
- Too short a time: motion is cut even on the loss of a single packet. In an industrial environment momentary packet loss is normal; an excessively short window produces constant stalling and operator complaints.
- Too long a time: the machine keeps moving after communication is lost. On hoisting and slewing movements this is a direct risk; even a few hundred milliseconds of extra movement means a collision.
For this reason, evaluate the value through the machine's stopping distance: how far does the load travel during the watchdog time plus the brake response time? If this distance is not acceptable, the time must be shortened or the mechanical brake improved.
IEC/EN 62745 addresses the establishment, indication and loss of communication and the behaviour on loss of power in wireless control systems; however, it does not specify the communication protocol, the frequency band or the EMC requirements. Take the numerical value from the product manual and from the manufacturer's declaration, do not make it up.
The full topic: 433 MHz or 2.4 GHz?