Encoder does not count fault resolution - Database & Sql Blog Articles

Photocoupler

Recently, a colleague came to me with an issue they were facing. They mentioned that their encoders often fail to provide any data. The problem occurs when the encoder value remains fixed and doesn't change during normal operation. The encoder in question is a Turck incremental encoder, and it's connected to an FM450-1 module. The wiring includes 1A*, 1B*, 1N*, 1M, and DC24V. The 24V power supply for the module is provided by an external Phoenix power source.

The colleague mentioned that they have already replaced the encoder, the module, and the cables. Additionally, the 24V power supply was checked and found to be functioning properly. However, the issue still persists. Since the device itself seems to be working fine, the next logical step was to investigate grounding and potential electrical interference.

I recalled a similar situation I had encountered before where an encoder wasn’t working correctly due to signal level issues. I referred to the FM450-1 manual, which clearly states: “The circuit of the counter input (encoder power supply, encoder signal) is electrically isolated from the CPU ground. Therefore, terminal 4 (1M) must be connected to the CPU ground with low impedance!”

Based on this information, the colleague decided to measure the voltage of the encoder power supply relative to the CPU ground both under normal conditions and during the fault. What they found was that during the fault, the voltage was only around 10V, which was insufficient for the module to correctly interpret the encoder signal. As a result, the counting function failed.

After connecting terminal 4 (1M) to the CPU ground, the issue was resolved. This experience highlights the importance of proper grounding, especially in systems where signal isolation is involved. It also serves as a reminder that even if all components appear to be functioning correctly, grounding problems can lead to unexpected behavior and should not be overlooked.