If you’re interviewing for an instrumentation role, technical questions about instruments and measurement systems are to be expected. Being able to discuss instrumentation concepts like accuracy, resolution, repeatability, and calibration will impress your interviewers and show your knowledge and experience. Here are some of the top instrumentation interview questions you should prepare for.
What Is Process Control?
Process control is the act of controlling a process variable to follow a desired trajectory. It aims to get an output of high quality, safety, and economy. The inputs, disturbances, and unmeasured perturbations are managed using sensors, controllers, and actuators to obtain the desired output. Process control is widely used in industrial applications like processing plants to monitor and control physical and chemical processes. Candidates appearing for instrumentation engineering roles should have a sound understanding of process control fundamentals.
What Is A Control Loop?
A control loop is a feedback mechanism that uses measurements to adjust the input to a system so that the output follows a desired trajectory. So it contains sensors, controllers, and actuators connected in a circular fashion. The sensor measures the output which feeds it back to the controller. The controller determines the difference between the actual output and the desired output and calculates how to adjust the input. It then signals the actuator to make the adjustment, and the cycle continues. Control loops are the fundamental building blocks used to automatically regulate levels, flows, temperature, pressure, ph, and other process variables in an industrial instrumentation program.
What Are On-Off, P, Pi, Pd, And PID Controllers?
Controllers are devices used to improve the performance of a control loop. So the most common types of controllers are:
- On-Off Controller: Switches abruptly between on and off states based on a threshold. Not suitable for precise control.
- Proportional (P) Controller: Produces an output that is proportional to the error signal. Fast but results in an offset.
- Proportional-Integral (PI) Controller: Proportional controller with an additional integral term to reduce the offset. So it gives a lower response.
- Proportional-Derivative (PD) Controller: Proportional controller with a derivative term to improve stability and response time. This may lead to overshoots.
- Proportional-Integral-Derivative (PID) Controller: Combines proportional, integral, and derivative actions to get a fast and stable response with minimal overshoot or offset. The most widely used controller in industrial control systems. Candidates should know how these controllers work and their applications.
What Is A Control Valve?
A control valve is a power-operated device used in control loops to modify the flow rate of fluids like gases, vapors, or liquids. It consists of an actuator, valve, and positioner. The actuator opens or closes the valve based on signals from the positioner. The positioner compares the input control signal from the controller with the actual valve position and sends a signal to the actuator to eliminate any difference between the two.
What Is A Transducer?
A transducer is a device that converts one form of energy into another form of energy. It takes energy from one system and converts it to energy that another system understands and can utilize. Transducers are used extensively in instrumentation and control circuits to convert non-electrical quantities like flow, level, pressure, temperature, etc. into proportional electrical signals that can be measured and used for digital control. Common examples include pressure transducers, flow transducers, level transducers, and temperature transducers. Knowledge of different types of transducers used in instrumentation is essential.