Fault diagnosis, testing, and commissioning of mechatronic systems
This domain tests systematic fault-finding, safe testing, and proving a system after repair or installation. Expect input-to-output tracing, half-split testing, insulation and continuity checks, sensor simulation, and functional commissioning.
Core rules
- Work from symptoms to root cause: gather information, reproduce the fault, define the boundary, test one variable at a time, and confirm the repair.
- Half-split the circuit: divide the signal path into input, controller, output, and load; test at the mid-point to halve the search.
- Verify safety functions first: E-stops, guards, interlocks, and pressure/energy isolation. Never bypass a safety device to prove a fault.
- Insulation resistance testing is done on an isolated, disconnected circuit; continuity testing is done de-energised.
- Commissioning follows pre-checks, cold tests, powered tests, functional tests, and documentation. Record baseline values.
Worked example
A conveyor does not start. The PLC input LED for the start button is on, but the motor contactor does not pull in. Half-split at the PLC output: the output LED is on, but no voltage appears at the contactor coil. The fault is in the wiring between output and coil, not the button or program. After repair, test the E-stop, guard interlock, overload reset, and motor direction before handover. The reasoning: evidence at each boundary localises the fault and avoids parts-swapping.
Traps
Replacing parts without evidence; bypassing interlocks; testing insulation with the circuit connected; assuming a PLC output LED means voltage is present at the field terminal; and failing to record the as-found condition. These practices hide the root cause and can create a repeat failure.