The fault code "F31118" on Siemens drives indicates an issue related to encoder feedback, specifically stating that the speed change detected by Encoder 1 is not considered plausible. This typically suggests a discrepancy between the expected speed change and the actual speed change as detected by the encoder.

Here's how you can address this issue:

1. Check Encoder Connections: Inspect the connections between the encoder and the drive. Ensure that all connections are secure and properly terminated. Look for any signs of damage or wear on the encoder cables or connectors.

2. Verify Encoder Functionality: Test the encoder to ensure that it is functioning properly. Check for any mechanical issues with the encoder such as misalignment, damage to the encoder wheel, or obstruction of the encoder's optical path.

3. Calibrate Encoder: If possible, recalibrate the encoder according to the drive's specifications. This process typically involves setting the correct parameters for the encoder type, resolution, and scaling factor in the drive's configuration settings.

4. Check Encoder Settings: Verify that the encoder settings in the drive match the specifications of the installed encoder. Ensure that the drive is configured to use the correct encoder type, resolution, and feedback scaling.

5. Inspect Motor and Mechanical System: Check the motor and mechanical system for any issues that could affect speed feedback. This includes mechanical wear or damage to the motor or driven equipment, as well as issues with belts, gears, or couplings.

6. Review Drive Settings: Check the drive's control parameters related to speed feedback and encoder monitoring. Adjust the settings as necessary to ensure accurate speed feedback and to minimize the occurrence of speed change errors.

7. Consult Technical Documentation: Refer to the drive's user manual, technical documentation, or contact Siemens technical support for specific guidance on troubleshooting and resolving encoder-related faults.

By following these steps and addressing the potential causes, you can troubleshoot and resolve the "F31118" fault related to encoder feedback on your Siemens drive.