A proximity sensor can prove that this motor is rotating without measuring exact speed. Follow the signal from the shaft target through the sensor, input circuit, input filter, PLC image, and logic.
Which rotation-detection approach fits the requirement?
The required result is rotation present or rotation absent, not a precise rpm value. That favors increasing the proximity target's angular width before adding an encoder.
| Approach | Signal characteristic | Best use | Limitation |
|---|---|---|---|
| Enlarge the proximity target | One long pulse per revolution | Binary rotating/stopped indication | Does not provide high-resolution position or speed |
| Use a PLC high-speed input | Captures pulses outside the normal cyclic input path | A target pulse remains shorter than the standard input can capture | Requires a compatible input and correct high-speed configuration |
| Fit an encoder and high-speed counter | Multiple pulses per revolution | Speed, direction, position, or accurate pulse counting | Adds hardware and configuration unnecessary for a simple rotation flag |
Use a wider target first. A target covering approximately half the rotating circumference produces a much longer sensor-on interval than a bolt head. Move to a high-speed input when the standard path still cannot acquire that pulse. Select an encoder only when the control requirement extends beyond rotation presence.
Where can the signal stop?
The data path starts mechanically. The rotating target must enter the sensor's usable sensing zone long enough to change its output. The output must then match the PLC input electrically. After that, the input circuit and its filter must pass the transition, the PLC must copy it into the input image or high-speed subsystem, and the program must read the correct address.
| Path segment | Failure indication | Deciding check |
|---|---|---|
| Target to sensor | Sensor indicator never changes | Jog slowly and watch the indicator as the target crosses the sensing face |
| Sensor to input terminal | Sensor changes, input indicator does not | Measure the output at the PLC terminal and check output/input compatibility |
| Input filter | Terminal pulse exists but the controller omits it | Compare measured pulse width with the module's documented filter setting |
| Cyclic input image | Short pulses appear intermittently or never appear in logic | Compare pulse width with update and task sampling intervals |
| Program | Raw input changes but the rotation flag does not | Monitor the raw input, edge logic, and missing-pulse timer separately |
A continuously off PLC indication therefore has two main branches: no electrical transition reaches the input, or a valid but narrow transition disappears before cyclic logic samples it. Layer one first.
How narrow can the pulse become?
With one target per revolution, pulse frequency equals shaft revolutions per second. The revolution period follows T = 60 / rpm. Target angular coverage determines the approximate on-time; a half-circumference target gives about half of each revolution period.
| Motor speed | Pulse frequency | One-revolution period | Approximate on-time with half coverage |
|---|---|---|---|
1,800 rpm |
30 Hz |
||
3,000 rpm |
50 Hz |
||
3,600 rpm |
60 Hz |
The reported PLC scan time is about 3.6 ms, but scan time alone does not prove capture. A bolt head may occupy only a small fraction of the circumference, so its pulse can be shorter than one scan. The input filter and the phase relationship between the pulse and input-image update add further opportunities to miss it. Do not convert the revolution period into pulse width unless the target coverage has been measured.
Which checks isolate the fault?
- Jog or rotate the shaft slowly. Confirm that the target enters the sensor's sensing zone and that the sensor indicator changes. Correct target size, alignment, or sensing gap if it does not.
- Check the sensor supply and output at the PLC terminal. Match the sensor output type and wiring to the installed input circuit; obtain those details from the sensor and module documentation.
- Compare the physical input indicator with the online raw-input state. If neither changes, stay on the electrical path. If the indicator changes but the program does not, inspect filtering, input updates, addressing, and task execution.
- Measure pulse width at the operating speeds with equipment capable of displaying short transitions. A general-purpose meter may average the waveform and hide individual pulses.
- Read the input module's filter setting and minimum detectable pulse specification. Use the fastest setting appropriate for the installation if the wider pulse still falls below the current filter requirement.
- If the standard input path still misses a verified electrical pulse, move the signal to a compatible high-speed input. Configure that channel for pulse acquisition and test the raw high-speed status before adding rotation logic.
How should the wider target be implemented?
- Isolate the shaft before modifying the rotating target.
- Replace the narrow bolt-head dog with a target that covers approximately half the circumference seen by the sensor. Maintain a stable sensing distance throughout rotation.
- Check that the target is mechanically secure and does not create unacceptable imbalance or interference.
- Run at
1,800 rpmand confirm an on-time near . Run at3,000 rpmand confirm an on-time near . Actual values depend on the target's measured angular coverage and sensor switching points. - Use the raw input transition to reset a missing-pulse timer. Base the timeout on the slowest valid speed: one revolution at
1,800 rpmis . Add the required number of missed revolutions and machine response allowance rather than using scan time as the timeout.
This arrangement stretches the signal before it reaches the PLC. It directly addresses the sampling problem while retaining a simple proximity sensor.
How is the final result verified?
- At the lowest operating speed, observe several consecutive raw-input on and off transitions without missing a revolution.
- Repeat at the highest operating speed, where the target produces its shortest pulse.
- Monitor the rotation flag while speed changes across the operating range. It must remain true without scan-dependent flicker.
- Stop the shaft and confirm that no residual input transition resets the missing-pulse timer.
- Restart the motor and confirm that the first valid pulse restores the rotation indication according to the programmed startup behavior.
FAQ
What happens if the proximity target is only a bolt head?
The input filter or cyclic scan can reject or miss that pulse, leaving the PLC indication off.
What happens if the PLC scan time is 3.6 ms?
A 3.6 ms scan captures only pulses that survive the input filter and overlap the relevant input update. It does not prove that a narrow proximity pulse will be read.
What happens if the target covers half the circumference?
Approximate sensor-on time becomes at 1,800 rpm and at 3,000 rpm. Those longer pulses give the standard input path more opportunity to acquire the signal.
What happens if the normal input still misses the wider pulse?
Measure the pulse at the input terminal and compare it with the module's filter specification. If the electrical pulse is valid but cyclic acquisition still misses it, use a compatible PLC high-speed input.
What happens after the target and input are corrected?
Test at 1,800 rpm and 3,000 rpm, watch the raw input and rotation flag, then stop the shaft and confirm that the missing-pulse timer removes the rotation indication.