Process Control Introduction

There are examples of feedback controllers everywhere. There are 3 essential elements of a feedback control system.

  1. Actuator that manipulates and causes an action
  2. Sensor that measures the response
  3. Controller that adjusts the actuator in response to the measurement

The controller receives the sensor measurement and compares it to the target (set point) to determine what adjustment is needed to the actuator. Some actuators are binary such as ON/OFF while others are continuous such as percent open (0-100%). Below are a couple examples of common feedback controllers.

Example 1: Building Temperature Control

One well-known example of a controller is the temperature control of a building in winter includes a heater (actuator), sensor (temperature thermistor), and controller (thermostat). The controller switches the heater on when the temperature drops below a certain level and switches the heater off when the temperature rises above a certain level. A disturbance variable may be changes in the outside air temperature or a change in indoor temperature due to a door opening.

ElementCruise Control
ActuatorValve or switch in heater, Fuel to the furnace
Controller Set PointDesired temperature indoors
SensorTemperature sensor such as a thermocouple or thermistor
DisturbanceDoors opening, wind, temperature outside

Example 2: Automobile Speed Control

Consider an automobile with an automatic cruise control. The driver may adjust the velocity set point for the controller. The controller adjusts the gas pedal position (actuator) in response to changes in the speedometer reading (sensor). A disturbance variable may be an approaching hill or wind that would cause a deviation of the speed from the desired set point.

ElementCruise Control
ActuatorGas Pedal Position, Fuel to the Engine
Controller Set PointDesired Speed (mph or km/sec)
SensorSpeedometer, measured velocity
DisturbanceHills, wind, other cars

Control Terminology

There is different terminology when talking about common controllers such as Proportional Integral Derivative (PID) or advanced controllers such as Model Predictive Control (MPC). Below is a table of some of the terminology and associated abbreviations.

ElementCommon Control (PID)Advanced Control (MPC)
ActuatorController Output (CO) or Output (OP)Manipulated Variable (MV)
ControllerSet Point (SP)Set Point (SP) or Range (SPHI/SPLO)
SensorProcess Variable (PV)Controlled Variable (CV)
DisturbanceDisturbance Variable (DV)Disturbance Variable (DV)

Block Diagrams