When a linear axis vibrates, overshoots, or produces inconsistent positioning, the controller is usually the first suspect.
Sometimes the PID values are wrong. Sometimes the mechanism is simply asking the controller to perform magic.
Before spending another hour tuning gains, I like to check three mechanical conditions.
- Check for binding
Move the axis slowly through its complete stroke and record:
timestamp
command_position
actual_position
motor_current
travel_direction
A current spike that repeatedly appears at the same position may indicate misalignment, rail binding, contamination, or an uneven mounting surface.
If the spike only appears in one travel direction, friction or preload deserves attention before the control loop does.
- Check the sensor mount
A sensor can be accurate while its bracket is not.
With the axis stationary, monitor the feedback signal and lightly disturb the cable or surrounding structure. If the reading changes, check:
Bracket stiffness
Fastener tightness
Cable strain
Sensor-to-target distance
Nearby vibration sources
Filtering unstable feedback may hide the symptom, but it does not make the measurement more trustworthy.
- Check the coupling and drive alignment
Misalignment between a motor, coupling, and lead screw can create periodic load changes. At low speed, watch for motor-current peaks that repeat once per revolution.
That pattern is usually more informative than random vibration. It points toward eccentricity, angular misalignment, a bent shaft, or poor support alignment.
A useful debugging order
When an axis behaves badly, try this sequence:
Mechanical freedom
→ Sensor stability
→ Drive alignment
→ Electrical noise
→ Control tuning
PID tuning works best when the mechanism is predictable. Otherwise, the controller is only being tuned around a moving target.
What mechanical problem has wasted the most control-tuning time in one of your projects?
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