Automated machines act on what their sensors report, not on what is physically true. A false sensor state can make a healthy machine behave irrationally.
Five Principles to Use in the Field
- Check physical target first. A sensor may be electrically perfect but aimed at the wrong location, blocked by contamination, or seeing a bent flag.
- Understand normal state. Know whether the input is normally on or off, how fail-safe circuits are designed, and whether the program inverts the signal.
- Verify power and signal. For three-wire DC sensors, confirm supply, 0 V, and output. For analog devices, verify loop power and the actual signal value.
- Look for timing problems. A sensor can switch correctly but too late, too briefly, or with bounce. Sequence faults may be caused by timing rather than a dead device.
- Treat interlocks as clues. An interlock is not an obstacle to bypass; it tells you which condition the machine believes is unsafe or incomplete.
Pressure Mistakes to Avoid
- Adjusting sensor sensitivity before checking alignment and target.
- Bypassing a permissive because “everything looks clear.”
- Replacing a sensor when the input issue is actually a missing common.
- Ignoring intermittent cables in moving carriers and drag chains.
A Practical Breakdown Method
- Observe the mechanical target.
- Observe the sensor indicator.
- Observe the PLC input state.
- Compare expected timing with actual timing.
- Check wiring, power, and channel if states disagree.
- Correct the physical or electrical cause and verify repeated cycles.
Scenario: A case sealer faults randomly on “guard open.” The guard is closed and the safety switch appears aligned. Logging shows the signal drops for a few milliseconds during a nearby cylinder impact. The actuator bracket is loose and vibrates away from the coded switch. Tightening and realigning the bracket fixes the fault. The sensor was reporting reality—just a reality too brief to see by eye.
Train This Before the Next Callout
Choose one critical machine you know and apply the ideas from this chapter while it is healthy. For sensors, interlocks, and false reality, write down the normal sequence, the key evidence you would want to preserve, and the first three checks you would make if the machine stopped. Then compare your plan with the electrical drawings, PLC diagnostics, maintenance history, and the experience of another technician. A ten-minute rehearsal in daylight can remove several minutes of confusion at night.
3 AM Rule: When automation behaves irrationally, ask which input is giving the controller a false picture of the machine.
Chapter Checklist
- Can I describe the symptom without naming an unproven cause?
- Have I preserved the evidence that could disappear after a reset or restart?
- Do I know which system boundary I am testing next and why?
- Have I kept safety controls and temporary changes visible and controlled?
- Can I explain my current facts, hypothesis, and next action in under thirty seconds?