Process control systems monitor industrial processes to make sure they don't vary from pre-set limits. They start corrective action if they find unwanted deviations or abnormalities.
Basic closed-loop control
The key element of a process control system is the basic closed-loop control. Systems usually include many hundreds of individual control loops, each of which controls one aspect of a particular process - for example, the temperature of an oven. The control loop's job is to make sure that a particular variable or parameter is maintained at its pre-set value, or 'set-point', and doesn't vary from this by more than an acceptable amount.
The control loop must also react quickly to any changes in set-point so that production isn't interrupted.
To achieve this, a control loop is made up of three main components - the measurement device, the controller and the regulator.
To stop any unwanted variation in a production process, a control loop monitors the process regularly to check it's performing as it should. This is usually done by a sensor that measures a particular property like temperature. A transmitter converts the sensor's output into a signal which is sent to a controller. The signal can be sent individually or with other signals through a special network - called a 'fieldbus'.
The controller compares the measurement recorded by the sensor against the pre-set value. If there's an unacceptable difference it initiates appropriate action. For example, if an oven's temperature has fallen too far, it instructs the regulator to send more fuel to the burners to increase it up to the required set-point.
Most of the control tasks required by an industrial process can be handled by a well-designed and well-tuned single-loop controller. These normally monitor a single measurement and adjust one regulator, but they can also be linked to another controller to adjust another related set-point. This is known as a 'cascade system'.
The regulator controls the throughput of the process. It responds to commands from the controller and makes adjustments where necessary. Control valves are the most common type of regulator - these adjust the flow of a fluid in response to messages from the controller. A variable speed pump is an alternative type of regulator which controls the flow of a fluid more accurately and with greater energy efficiency. Variable speed drives can also be used where the controller regulates the movement of solid materials rather than fluids or gases.