Troubleshooting SIMODRIVE 611 Position Counting Errors

The axis reports negative position as soon as it reverses during low-speed homing, even though the head remains inside its physical travel. The same axis now loses position at higher speed. Treat this as a feedback-chain fault until signal, wiring, configuration, and controller-input checks prove otherwise.

Reject the usual quick fixes

Trace the real position-counting path

The mechanical axis does not generate the displayed position directly. The measuring device produces feedback signals, the feedback cable carries them through connectors and shields, the servo control module receives and interprets them, and the machine control applies direction, scaling, reference offsets, and travel limits.

An immediate negative count when reverse motion begins points first to signal interpretation rather than physical overtravel. Common mechanisms include a missing feedback channel, an intermittent connection that appears during cable flex, contamination or poor contact at a connector, shield or reference-potential problems, an incorrect direction or feedback configuration, and a defective feedback receiver in the servo controller.

The progression from low-speed homing trouble to position loss during faster operation raises the priority of dynamic faults. Higher speed increases feedback edge rate, while machine movement can flex the cable more aggressively. A marginal signal may count correctly while stationary tests pass, then lose transitions in motion.

Separate motion faults from displayed-position faults

1. Place the axis where unexpected motion cannot strike a hard stop, tool, or workpiece. Use the machine's approved service mode and reduced-motion controls.
2. Mark or independently measure the physical position before commanding movement. Do not use the suspect displayed position as the only reference.
3. Jog a short distance forward and record physical direction, displayed direction, and displayed change.
4. Return to the starting point, then repeat in reverse. Stop immediately if the displayed count runs away, changes without proportional motion, or approaches a software limit while the mechanism remains in range.
5. Compare the same test with the healthy axis. Compare behavior, not raw count values; the two axes may have different mechanics or configuration.

If physical motion is smooth but the position jumps, reverses unexpectedly, or accumulates error, concentrate on feedback acquisition. If the shaft or load also jerks, stalls, or reverses unexpectedly, inspect both the feedback loop and the power stage because corrupted feedback can destabilize motion.

Prove the feedback path before replacing the controller

1. Remove power and follow the machine's approved isolation and discharge procedure. Verify the safe state before touching drive connectors.
2. Inspect the feedback connector at the motor and at the control module. Look for loose locking hardware, pushed-back contacts, bent pins, contamination, corrosion, damaged backshells, and cable strain.
3. Trace the cable through the moving route. Pay close attention to bends, clamps, drag-chain transitions, oil exposure, and places where the cable can rub against the frame.
4. Check conductor continuity and isolation with the feedback device and electronics disconnected as required by the applicable service procedure. Flex the cable during the test; a static continuity result can miss an intermittent break.
5. Check shield termination and grounding against the machine drawings. Repair the termination rather than adding an improvised parallel ground that may create another current path.
6. Reseat connectors only with power removed. Restore power and repeat the controlled forward-and-reverse position test.
7. If test equipment and signal documentation are available, compare feedback signals at the controller end on the faulty and healthy axes during slow motion. Look for a missing channel, unequal signal quality, noise, or dropout during cable movement.

Do not probe an unknown feedback interface by trial and error. Use the connector pinout and signal limits from the machine or Siemens documentation before attaching a meter or oscilloscope.

Use a controller swap as a controlled diagnostic

The suspected module is 6SN1118-1NJ01-0AA1. Before exchanging it with the other axis, compare both module order numbers and all visible option or hardware identifiers. Matching appearance is not proof of interchangeability.

1. Back up or record the complete axis configuration through the machine's established service method. Photograph connector locations and label every plug.
2. Record the present fault behavior and the exact test that reproduces it.
3. Determine where axis identity and addressing are assigned. They may reside in module settings, machine wiring, higher-level control configuration, or a combination. Do not change addresses merely because the modules are being exchanged.
4. If both assemblies are confirmed interchangeable, move one item at a time. Swapping the control modules while leaving motors and fixed cables with their original axes gives the clearest result.
5. Power up in the approved service mode. Check diagnostics before enabling motion, then use a short, low-speed jog with room to stop.

If the counting fault follows 6SN1118-1NJ01-0AA1, the module or its stored configuration becomes the leading cause. If the fault stays on the original axis, return to its fixed feedback cable, connectors, grounding, machine-control inputs, reference logic, and mechanics. If both axes develop faults, restore the recorded configuration and recheck interchangeability before further testing.

Verify the repair under the original conditions

1. Repeat homing at the low speed that first produced the immediate negative count.
2. Confirm that displayed position changes smoothly in the correct direction during forward and reverse movement.
3. Return repeatedly to the same physical reference and check for accumulating position error.
4. Run through the normal speed range while monitoring position and drive diagnostics. A repair is incomplete if low-speed homing passes but higher-speed operation still loses position.
5. Inspect the cable while the axis traverses its full normal route. Position-dependent failure usually points to flexing cable or connector damage.
6. Restore guards and normal operating controls only after direction, reference acquisition, travel limits, and repeatability all pass.

Get production moving only after the repeatable fault test passes. Then document the failed component, configuration state, connector work, and final verification so the temporary substitution does not become an undocumented permanent repair.

FAQ