Direction Interlock
A logical or electrical measure that prevents two opposite direction commands from being accepted at the same time. Without an interlock, if both directions are triggered together the upper and lower switch of the same leg in the H-bridge can conduct together and a short circuit forms between supply and chassis. In relay-based solutions two relays strain the mechanics.
An interlock can be built in three layers and a good design contains all three:
- In software: When a reverse direction command arrives, the driver definitively terminates the previous direction. If both commands arrive together it applies neither and stays on the safe side.
- Electrically: In relay arrangements the normally closed contact of one relay is wired in series with the coil circuit of the other; while one is pulled in, the other physically cannot pull in.
- Mechanically: On two-way control levers the fact that a single lever cannot go both ways at once is already a natural interlock.
The inseparable complement of the interlock is the direction change delay: if reverse voltage is applied while the back-EMF of the turning motor is still high, the current rises many times above nominal. That is why the driver typically applies a wait of a few hundred milliseconds or a ramped reset before changing direction.
In the field, always test what the system does when both direction buttons are pressed at the same time.
The context in which this term is used: DC Motor Driver Selection Guide