Process control loops keep industrial systems such as tanks, pipes, reactors, and boilers operating safely and consistently. A control loop compares what the process is doing to what the operator wants, then adjusts equipment to reduce the difference. This matters because temperature, pressure, level, and flow must often stay within narrow limits for quality, efficiency, and safety.
A common loop uses a sensor, controller, actuator, and control valve connected by a feedback signal.
Key Facts
- Error signal: e(t) = set point - process variable
- Negative feedback reduces error by adjusting the manipulated variable in the opposite direction of the disturbance.
- PID control law: u(t) = Kp e(t) + Ki integral e(t) dt + Kd de(t)/dt
- Proportional control reacts to present error, integral control reacts to accumulated error, and derivative control reacts to the rate of change of error.
- A control valve changes flow rate, pressure drop, or heat input to move the process variable toward the set point.
- Closed-loop control measures the output and feeds it back, while open-loop control acts without measuring the result.
Vocabulary
- Set point
- The set point is the desired value of a process variable, such as 80 degrees Celsius or 250 kPa.
- Process variable
- The process variable is the measured condition being controlled, such as temperature, pressure, flow rate, or liquid level.
- Controller
- A controller is a device or program that compares the process variable to the set point and calculates a control action.
- Actuator
- An actuator is the mechanism that converts the controller output into physical motion or action, such as opening a valve.
- PID control
- PID control is a feedback method that combines proportional, integral, and derivative actions to reduce control error.
Common Mistakes to Avoid
- Confusing the set point with the process variable is wrong because the set point is the target, while the process variable is the measured value.
- Using positive feedback in a stabilizing control loop is wrong because it increases deviations instead of reducing them, which can make the process unstable.
- Assuming a valve position is the controlled variable is wrong because valve position is usually the manipulated variable, while temperature, pressure, level, or flow is controlled.
- Increasing PID gains without checking stability is wrong because overly aggressive tuning can cause oscillation, overshoot, or unsafe actuator movement.
Practice Questions
- 1 A tank temperature set point is 75 degrees Celsius and the sensor reads 68 degrees Celsius. Find the error e = set point - process variable and state whether a heater controller should increase or decrease heat input.
- 2 A flow control valve is 40 percent open and the controller output increases the valve opening by 15 percentage points. What is the new valve opening, and what is the likely effect on flow if the pressure drop stays approximately constant?
- 3 A pressure vessel experiences a sudden increase in inlet flow. Explain how a negative feedback control loop using a pressure sensor, controller, and outlet valve can bring the pressure back toward its set point.