Setpoint (control system)

This article is about setpoints in control theory. For other uses, see Set point.

Block diagram of a negative feedback system used to maintain a setpoint in the face of a disturbance using error-controlled regulation. Positive error means feedback is too small (controller calls for an increase), and negative error means feedback is too large (controller calls for a decrease).

In cybernetics and control theory, a setpoint (also set point or set-point) is the desired or target value for an essential variable, or process value of a system. Departure of such a variable from its setpoint is one basis for error-controlled regulation using negative feedback for automatic control. The set point is usually abbreviated to SP, and the process value is usually abbreviated to PV.

Examples

Cruise control

The SP-PV error can be used to return a system to its norm. An everyday example is the cruise control on a road vehicle; where external influences such as gradients cause speed changes (PV), and the driver also alters the desired set speed (SP). The automatic control algorithm restores the actual speed to the desired speed in the optimum way, without delay or overshoot, by altering the power output of the vehicle's engine. In this way the SP-PV error is used to control the PV so that it equals the SP. A widespread use of SP-PV error control is the PID controller.

Industrial applications

Special consideration must be given for engineering applications. In industrial systems, physical or process restraints may limit the determined set point. For example, a reactor which operates more efficiently at higher temperatures may be rated to withstand 500°C. However, for safety reasons, the set point for the reactor temperature control loop would be well below this limit, even if this means the reactor is running less efficiently.

Human body weight

Human body weight is theorized to be influenced by a biological set point mechanism set at an organism-specific level. Maintenance of human body weight is known to involve the hormone leptin; the specifics of the maintenance mechanisms involved are an area of continued research among endocrinologists. One known mechanism is that fat tissue produces leptin, which then enters the blood stream. Higher leptin levels decrease appetite, so as fat deposits increase the organism eats less overall. Similarly, if fat deposits decrease then less leptin enters the blood stream and appetite increases, thus pushing body fat levels towards a set point.

See also

• Process control