Troubleshooting position control faults

Example case A. (1) Tracking error originally occurs because motor output current gets saturated to maximum configured MMC parameter value of 2.0 Amps (first graph). (2) This leads to growing position tracking error (second graph) and fault gets triggered when error grows larger than 300 counts (set by parameter FPT). The third graph shows HV DC bus voltage which stays within valid range, so no reason for fault there. The reason is also apparent from Fault location 140401.

Example case B. Fault error occurs because HV DC bus voltage drops under high motor current load. This causes Undervoltage fault to activate, and possibly other faults (such as Tracking error) to follow. The reason is also apparent from Fault location 140405.

Position control fault can occur due to several reasons. This page gives tips how to troubleshoot the reason of the fault.

Using fault status information

Method for finding the root cause of the fault: check Fault location from Granity Testing tab (scroll down the page to find Fault location numbers). This indicates the fault that occurred first of all faults, and is likely the reason causing rest of the faults as well.

Some most typical reasons that may happen:

• Insufficient torque on motor: motor will lag behind if it is not allowed to produce enough torque to overcome load resistance (see scope examples below). Solutions: decrease acceleration/velocity rates or increase motor current limits to get more torque.
• Undervoltage fault: HV DC bus power supply dropped voltage under the load and caused drive to fault stop
• Tracking error occurred: too tight tracking error tolerance (see Position tracking error thresholdFPT) and Servo motor tuning guide

Using Granity scope

One of most effective ways to find reason for fault is to use scope capture in Granity. In this example, we capture signals revealing fault reason in example cases:

1. Choose Capture on fault as trigger
2. Choose quite low sample rate, such as 500 Hz
3. Choose channels Torque achieved, Torque setpoint, HV bus voltage and Tracking error
4. Untick Continuously repeating capture
5. Start capture
6. Perform the operation that causes the fault

If you can't reproduce the fault while Granity is connected, you may use the offline capture feature.

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