admin Position deviation alarms (also called following error alarms) occur when the actual slide position differs from the commanded position by more than the allowed tolerance. On a servo press, this is one of the most common alarms during production. ## What Causes Position Deviation The servo system continuously compares commanded position (from the motion controller) with actual position (from the encoder). If the difference exceeds the deviation limit, the alarm triggers. **Mechanical causes:** - Gib clearance too tight ??increases friction, motor cannot follow command - Lubrication failure ??same effect as tight gibs - Die binding ??tooling interference preventing slide movement - Brake not fully releasing **Electrical/drive causes:** - Motor torque limit set too low - Acceleration/deceleration ramp too aggressive for load inertia - Encoder signal noise causing position jumps - Drive gain parameters not tuned for press inertia **Parameter causes:** - Deviation limit set too tight for the application - Speed feedforward gain too low ## Diagnostic Approach 1. **Check when alarm occurs**: At start of stroke? During forming? During return? - Start of stroke ??brake or lubrication issue - During forming ??torque limit or die binding - During return ??regenerative energy issue 2. **Check actual vs commanded position**: Use drive monitor to view position error in real time. Normal position error during forming: 0.1-0.5mm. Error above 2mm indicates a problem. 3. **Check motor current**: If current is at limit during alarm, the motor is torque-limited. Either increase torque limit or reduce forming speed. ## Tuning Parameters For Siemens S120: - p2546: Position deviation monitoring tolerance (increase if alarm is too sensitive) - p1082: Maximum speed limit - p1520: Torque upper limit For Yaskawa Sigma-7: - Pn520: Position deviation overflow level - Pn100: Speed loop gain - Pn102: Position loop gain **Tuning rule of thumb**: Position loop gain (Kp) should be 1/3 of speed loop gain (Kv) for stable operation on a press.
admin Related: see also [homing procedure after power loss](https://servopress.club/d/143-143-servo-press-homing-procedure-after-power-loss-step-by-step) for complementary detail.
mike_chen_eng For position deviation on Yaskawa Sigma-7: if the alarm only occurs during the forming stroke and not during approach or return, check Pn522 (position deviation overflow at stop). Sometimes this is set tighter than Pn520 (deviation during motion) and triggers unexpectedly when the press dwells at BDC.