Holding a grip without a second control loop

2026-06-20T00:00:00Z

The rehab-glove controller runs a proper PID loop while it inflates a finger — error, deadband, anti-windup, the whole thing. So the obvious instinct is to reach for a second PID loop on the vacuum side to hold the hand open. I didn’t, and the reason is worth writing down.

The hold state doesn’t need precision; it needs to stay put cheaply. The vacuum pump is switched by a relay, and a relay is a bad place to ask for fine, continuous modulation: it can’t really PWM, and rapidly toggling it just wears the contacts. A PID loop on top of a relay would chatter — constantly nudging an actuator that only has “on” and “off” to give.

So the open-hand hold (HOLD_VAC in the state machine) uses bang-bang hysteresis instead. The pump stays off until leakage drifts the flex reading past a tolerance band (~±100 ADC counts around the open-hand target); then it bumps on just long enough to pull the hand back, and switches off again. One comparison, a tolerance band, done.

The payoff is more than simplicity. There’s no integral term to wind up while the system sits idle at setpoint, the relay clicks a handful of times instead of thousands, and the controller spends most of the hold drawing almost no power. It’s a small decision, but it’s the kind that separates control logic that survives the bench from control logic that only works in simulation: match the controller to the actuator you actually have.

Rashid Al-Ma'awali — Writing

Holding a grip without a second control loop