Servo travel limits sound like a checkbox on the commissioning checklist. Set the PWM range, test full throw, done. But a packaging machine integrator skipped one step last month. He set the electronic limits in the controller but left the mechanical end-stops at the factory default. When the controller glitched during a power sag, the servo drove past its software boundary and rammed the linkage into the frame. The steel gears survived. The aluminum housing cracked at the mounting flange.
Setting end-stops correctly means understanding how mechanical and electronic limits work together. One without the other is a liability.
Mechanical vs Electronic Travel Limits
A mechanical limit is a physical stop — a set screw, a pin, or a hard feature in the linkage that blocks the servo horn from rotating past a certain angle. It works even when the controller is dead, the firmware crashes, or the position feedback loop fails open. Mechanical servo travel limits are your last line of defense, not your primary control method.
Electronic limits set the PWM range in software. The controller clamps the output signal so the servo never commands a position beyond the allowed window. This works perfectly in normal operation and fails perfectly when anything goes wrong — a firmware bug, a noise spike on the signal line, or a watchdog reset that sends a garbage pulse. Every single failure mode bypasses the electronic limit.
Because of this, you set the mechanical stop just outside the electronic boundary. If the software limit is 1000-2000 µs, the physical stop should engage at roughly 950 and 2050 µs of equivalent travel. The gap leaves room for normal operation while catching runaway motion before the gears strip.
Where to Place the Mechanical Stop
Do not put the end-stop on the servo horn. Put it on the load side of the linkage. A stop on the horn protects the servo but leaves the linkage free to jam against the frame. A stop on the load arm protects both. The linkage rod acts as a fuse, absorbing the impact before it reaches the gear train.
The stop material also matters. A steel pin driven into an aluminum bracket deforms the bracket over repeated impacts. Use a steel-on-steel interface with a Delrin or urethane bumper between them. The bumper compresses on impact and spreads the deceleration over a few milliseconds instead of a single instantaneous spike. This small detail triples the life of the gear train.
Testing the Limits Under Power
After setting both limits, test them at full speed. Run the servo into the mechanical stop under power at the system’s maximum supply voltage. If anything cracks, bends, or makes a noise you do not like, the stop is not strong enough. Better to find out on the bench than at a customer site at 2 AM.
Then test the electronic limit recovery. Drive into the software boundary, kill power, manually push the linkage past the limit, and power back on. The controller should detect the out-of-range position and move the servo back into bounds at a safe speed. If it snaps back at full torque, it will tear the linkage apart. Tune the recovery ramp to prevent this.1