What is a servo press?

A servo press uses servo motors for precise force and position control during forming operations.

How do I calculate servo press tonnage?

Use our free tonnage calculator at servopress.club/tonnage-calculator.html

Position deviation alarm - causes and tuning help

anna_plc

Getting position deviation alarms intermittently. Not every cycle, maybe 1 in 50. Parts are still good. Is this a tuning issue or something mechanical?

**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.

anna_plc

Position deviation alarms during forming are almost always a sign that the motor is torque-limited at that point in the stroke. Check the motor current during the fault. If it is at or near the drive current limit, the motor simply cannot produce enough force. Solutions: reduce forming speed to give the motor more time, reduce the forming force by adjusting the die, or accept that you need a larger press.

pressroom_tom

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.

toolndye_dave

Position deviation alarms that only appear on heavy jobs point to a torque capacity issue at the specific slide position where forming occurs. Remember that servo press force capacity varies with crank angle. A 200T press might only deliver 140T at 30mm above BDC. If your forming happens at that position and requires 160T, you will get position deviation every time regardless of parameter settings.

jim_apps

1-in-50 intermittent deviation with good parts — that pattern is almost always thermal. Here's how to confirm: log the deviation alarm timestamps for a shift. If they cluster 45-90 minutes after cold start and then again after lunch restart, it's thermal growth changing your BDC position by 0.01-0.03mm, just enough to occasionally exceed the deviation window.

Quick diagnostic: put a dial indicator on the slide at BDC, run 200 cycles, and watch the reading drift. On our Komatsu H1F the slide drops 0.025mm over the first hour as the frame warms up. That's enough to trigger Pn520 if it's set tight.

The real fix depends on your drive. On Yaskawa, widen Pn520 from the default (usually 200 pulses) to 400-500 for press work — the default is meant for CNC positioning, not forming. On Fanuc, it's parameter 2064. But don't just crank it open — if you're seeing >0.05mm deviation, something mechanical is actually moving and you want to know about it.

One more thing — check if the alarm correlates with specific die numbers. We had a progressive die where station 4 (a heavy coin) would occasionally spike the deviation because the nitrogen springs were losing pressure over the shift. Topped them off and the alarms disappeared. Sometimes it's not the press at all.

raj_pm

1 in 50 with good parts = thermal. We see the exact same pattern. Goes away after the press warms up for about an hour.