Automation, Robotics, and Control Systems
This domain tests PLC scan behavior, PID control, robot motion concepts, and the separation of standard control from safety functions.
Core rules
- A PLC scan reads inputs, executes logic, then writes outputs. Scan time affects event detection: a pulse shorter than the scan may be missed unless it is latched or handled by an interrupt or high-speed input.
- PID output combines proportional, integral, and derivative terms: Kp·e + Ki·∫e dt + Kd·de/dt. Integral windup, derivative noise, and setpoint changes require filtering, clamping, or bumpless transfer.
- Robots use joint, world, tool, and user coordinate frames. Payload and inertia affect achievable speed; singularities cause unstable motion or faults.
- Safety functions are not ordinary control logic. Risk assessment determines required performance level or SIL; safety PLCs and guard interlocks must remain independent of the basic control system.
Worked example
A photoeye produces a 15 ms pulse, and the PLC scan is 25 ms. The pulse can occur entirely between input scans and be missed. The fix is not simply a faster CPU: use a latching input, a high-speed counter, or an interrupt routine. If the required stop response is 50 ms, also sum the input filter, detection latency, logic execution, and output delay. A 5 ms filter plus worst-case 25 ms detection plus 10 ms output already totals 40 ms, leaving little margin.
Traps
Using a standard PLC for safety; ignoring worst-case scan and filter delays; failing to clamp PID integral terms; commanding robots beyond payload/inertia limits; and assuming faster hardware fixes a non-deterministic network.
Key points
- PLC scan order is input, logic, output.
- Short pulses need latching, interrupts, or high-speed inputs.
- PID requires windup and noise management.
- Robot payload, inertia, frames, and singularities affect motion.
- Safety systems must be independent and risk-based.