Controls
Deadman Switch
A deadman switch is the three-position enabling switch on the back of a teach pendant that has to be held at its middle position for the robot to move under manual control; releasing it or squeezing it hard stops the robot.
What it is
The deadman switch is the hold-to-run device behind manual motion. With the pendant enabled, the robot only jogs or steps through a program while you hold the switch at its middle position. Let go and motion stops. Clench it past the middle detent and motion stops too.
That third position is the point of the design. Someone startled by an unexpected move tends to let go or grip harder, and both reactions stop the robot. FANUC pendants carry two deadman switches on the back, one for each hand, and holding either at its middle position enables motion.
How it actually works
The deadman feeds the controller's safety circuit on two redundant channels. Release it with the pendant enabled and the controller drops servo power and posts SRVO-003 Deadman switch released; squeeze back to the middle and press RESET to clear it. In T1 the manual-mode speed limit applies at the same time, 250 mm/sec at the tool center point and flange, so the switch and the speed cap work together while you teach.
Because the circuit is redundant, the controller can catch the switch itself failing. SRVO-248 means the two channels disagree, most often because the switch was pressed only halfway. FANUC's field test is to release both deadman switches at once: if SRVO-236 Chain failure is repaired appears, there's no hardware fault. SRVO-206 is more serious. The deadman was released but the emergency stop line didn't open, a failed safety function that keeps the robot out of production until a qualified technician has repaired it.
How it differs
- Pendant enable switch · Terminology gets tangled here. Safety standards call the three-position device an enabling device, and some parts catalogs call it the enable switch. On a FANUC pendant, the enable switch is the separate ON/OFF switch on the pendant face that gives the pendant control of the robot. Manual motion needs both: pendant enabled and deadman held at the middle. With the pendant enabled in T1 or T2, releasing the deadman posts SRVO-003; switching the pendant off in T1 or T2 posts SRVO-211.
- Emergency stop · An e-stop is a latching red button that stops the robot in any mode and stays down until someone twists it to release. The deadman only acts while the pendant is enabled, and it doesn't latch: squeeze back to the middle and press RESET to continue. Each covers a situation the other doesn't, which is why the pendant carries both.
Where you meet it in the field
- SRVO-003 Deadman switch released · The everyday stop: the pendant is enabled and the deadman was released or squeezed past the middle.
- SRVO-248 Deadman switch status abnormal · The deadman's two channels disagree, usually a half-press, sometimes a pendant, cable, or E-stop board fault.
- SRVO-206 Deadman switch (SVEMG abnormal) · The deadman was released but the emergency stop line didn't disconnect. Treat it as a failed safety function.
- Teach Pendant Enable Switch · The three-position switch as a part: how it wears and what a worn middle position does to jogging.
Common questions
- Why do I get SRVO-003 even though I'm holding the deadman?
- Most often the grip has gone past the middle detent, which stops motion the same as letting go. Release fully, squeeze back to the middle, and press RESET. If it still alarms with correct technique, the switch may have worn and lost its middle position, which points to the pendant.
- Can the deadman be defeated so I can jog with one hand free?
- No. It's the safeguard that protects a person in the cell during manual motion, and defeating it removes that protection. If holding it is the problem, swap hands: FANUC pendants have a deadman on each side of the back for exactly that reason.
- What's the difference between SRVO-003 and SRVO-248?
- SRVO-003 is the normal stop when the deadman is released or squeezed too hard. SRVO-248 means the switch's two channels disagree, usually from a half-press. Releasing both switches at once and watching for SRVO-236 tells you whether there's a hardware fault behind it.