Control Systems and Automation
Core concepts
Understand open-loop versus closed-loop control, feedback, setpoints, error signals and disturbance rejection. Know PID control: proportional action reduces error, integral action removes steady-state offset, derivative action improves damping. Candidates should interpret transfer functions, block diagrams, and the meaning of poles and zeros, plus stability concepts such as gain and phase margin. Study sensors and actuators in a control loop, sampling and the Nyquist criterion, and PLC fundamentals including ladder logic, scan cycles, and input/output modules. Familiarity with industrial networks and safety interlocks is expected.
Traps
A frequent error is confusing integral and derivative roles-integral eliminates offset but can cause windup; derivative reacts to rate of change and amplifies noise. Questions may ask about stability: a system with poles in the right half-plane is unstable. Watch sampling questions-sampling below twice the highest frequency causes aliasing. In PLC scenarios, remember the scan cycle reads inputs, executes logic, then writes outputs, so fast pulses may be missed.
Worked example
A controller with proportional gain 4 has an error of 0.5 units, giving output 2.0 units. Rule of thumb: increase proportional gain to reduce error, add integral to eliminate steady-state offset, and add derivative only when noise is low. Always verify stability margins after tuning.