Control loop is automation essence

Vance J. VanDoren, consulting editor -- Control Engineering, 6/1/1998

The control loop is the essence of automation. By measuring some activity in an automated process, a controller decides what needs to be done next and executes the required operations through a set of actuators. The controller then remeasures the process to determine if the actuators' actions had the desired effect. The whole routine is then repeated in a continuous loop of measure, decide, actuate, and repeat.

Common industrial controllers include programmable logic controllers (PLCs), distributed control systems (DCSs), stand-alone loop controllers, and more recently, personal computers (PCs). Heating coils, robot arms, pumps, other motor types, and conveyor belts are some of the actuators that a controller can use to operate an automated process.

Discrete control
In discrete control applications, control loops automate the production of individual objects, such as computer chips, automobiles, and light bulbs. Activities to be controlled generally occur in a step-by-step manner where each step starts only after its predecessor finishes.

An automatic car wash that produces clean cars from dirty ones is a familiar example of discrete control. When the controller detects the departure of the previous vehicle, it signals the next one to enter the bay. When that vehicle reaches the stopping point, the controller displays the STOP NOW sign. When each step of the washing process finishes, the controller starts the next operation. When all of the required operations have been completed, the controller displays the EXIT NOW sign.

The controller measures the progress of the washing process with a variety of sensors. An electric eye detects the departure of the previous vehicle. A proximity switch detects the arrival of the next one. Actuators for this automated process include valves to regulate the water flow through the sprayers, conveyors to position the sprayers, and instruction signs to direct movements of incoming and outgoing vehicles.

Continuous control
In continuous control applications, the controller and its actuators operate constantly. Continuous control is also commonly known as process control even though many automated processes are discrete.

A continuous controller measures flow rates, temperatures, pressures, and other continuous variables that can change at any time. It then decides if those variables are at acceptable levels and uses its actuators to change them if necessary.

Continuous control loops generally cycle through the measure-decide-actuate routine much faster than discrete control loops do. In fact, most continuous controllers will make a whole series of control decisions before the results of the first one are completely evident.

Considerable analytical effort is sometimes required to program a continuous controller. Its decision making algorithm has to consider not only the current activity of the process, but the on-going effects of all of its previous decisions.

The water heater that provides hot water to the car wash is a continuous process subject to continuous control. A thermocouple measures the water temperature in the tank and signals the controller to turn on the heating coil whenever the actual temperature drops below a specified level. The tricky part is deciding how long the heating coil should remain activated each time a temperature drop is detected. If it is shut off too soon, it will just have to be reactivated as the water temperature begins dropping again. If it is kept on too long, the tank could boil over.

Continuous control loops are especially common in industries where the product flows in a continuous stream--petrochemicals, foods, pharmaceuticals, pulp and paper, etc. The proportional-integral-derivative (PID) algorithm is the most common method by which continuous controllers decide what to do next.

Vance J. VanDoren, Ph.D., P.E., is president of VanDoren Industries, West Lafayette, Ind.