Process control is the ability to monitor and adjust a process to give a desired output. It is used in industry to maintain quality and improve performance. An example of a simple process that is controlled is keeping the temperature of a room at a certain temperature using a heater and a thermostat. When the temperature of the room is too low, the thermostat will turn on a heating source until the temperature reaches the desired level at which point the heater is turned off. As the room cools, this process repeats itself to keep the room at the desired set point. In this example, the set point is the temperature setting on the thermostat and the heater turns on and off to maintain temperature. Hence, this simplest form of process control is called on/off or deadband control. This type of process has a deadband which is the difference between the temperature where the heater is turned on and when it reaches the set point. This deadband is what keeps the heater from being turned on and off too often.
An example of a more sophisticated process control scenario would be where 2 fluids are being mixed in a particular ratio. As the flow of one fluid increases, the other has to increase proportionally. Typically the flows for both fluids would be controlled with actuated valves. A controller will calculate the amount of flow required from each valve based on the total flow required. Each valve is then controlled with a PID type controller that will vary the valve position based on the difference between the desired value (set point, as determined from the controller) and the measured flow rate (process variable). In simple terms, when the flow measurement is too low, the controller will determine how much the valve needs to open to increase the flow to the desired value. Most industrial processes are a combination of many smaller processes. It is important that each process is stable as instability in one process will often cause other processes to become unstable. This instability is often called variability.
In most processes, the ability to control close to the desired set point and minimize variability has a significant impact on costs. If the set point needs to be raised to insure that a minimum value is met due to variability, the amount that the set point is raised over the desired value often result in wasted resources. For example, if a flow rate needs to be 10 gallons per minute to achieve a desired process result and the variability is +/- 1 gpm, the set point would need to be 11 gpm to ensure that the flow rate never fell below 10 gpm. This average of 1 gpm is then wasted product. If the variability was reduced to 0.1 gpm, the set point could be readjusted to 10.1 gpm which would result in a product savings for the operator.