An SDE servo system controlled by a Renu PLC reportedly develops a 3–4 mm positioning error on nearly one out of every two jobs. The servo drives communicate with the PLC over Ethernet, while the available description of the Wecon HMI connection is ambiguous: it mentions both an Ethernet port and Renu RS485 communication. Do not assume that Ethernet causes the error until command and feedback data identify where the deviation begins.
Establish the Position Error Boundary
Determine whether the error exists inside the servo positioning loop or after the motor has reached its commanded position. When the drive operates in internal positioning mode, record the position value assigned to the processed PR and compare it with the drive's feedback pulse value during both correct and failed cycles.
| Comparison result | Diagnostic direction |
|---|---|
| PR position equals feedback pulse position | The drive reports that it reached the requested position. Inspect the mechanical transmission and load. |
| PR position differs from feedback pulse position | The error remains in the positioning or command path. Continue checking the selected PR, commanded value, and drive response. |
Run a Controlled Cycle Comparison
- Identify the PR processed during a correct cycle and during a cycle showing the 3–4 mm error.
- Record the PR position value and feedback pulse value for both cycles.
- Confirm whether the same PR and commanded position were used in each cycle.
- If command and feedback agree during the failed cycle, inspect the mechanics. If they disagree, isolate the command or servo-positioning path before changing mechanical components.
Because the fault occurs intermittently, compare records from successful and failed jobs under the same machine sequence. A static value observed only after a successful cycle cannot identify the intermittent deviation.
Inspect Mechanics When Feedback Matches
If the processed PR position and feedback pulse position match while the machine remains 3–4 mm out of position, check the mechanical elements between the motor and the working point. The available evidence does not identify the transmission type, so inspect the installed coupling, transmission, load connection, and any element that can slip or introduce lost motion without assuming a specific mechanism.
Evaluate Servo Tuning
Auto-tuning is a supported follow-up action, but it is not proof that tuning causes the fault. First preserve the PR-to-feedback comparison results and mechanical findings. Apply the drive's documented auto-tuning procedure only after confirming the machine can move safely, then repeat identical correct-versus-failed cycle measurements to verify whether the positioning error is eliminated.
FAQ
Why does my SDE servo miss position by 3–4 mm?
Compare the processed PR position with the feedback pulse value. Matching values direct the investigation toward the mechanics; different values direct it toward the positioning or command path.
How do I test an intermittent servo position error?
Record the selected PR, commanded position, and feedback pulse during both correct and failed cycles. Confirm that the same PR and command were used before comparing drive response.
Should I auto-tune the servo to fix the position error?
Auto-tuning is a possible corrective action, not a confirmed root cause. Preserve baseline measurements, check command-to-feedback agreement and mechanics, perform the documented tuning procedure safely, and repeat the same cycle test.