sampled-data control system


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sampled-data control system

[′sam·pəld ¦dad·ə kən′trōl ‚sis·təm]
(control systems)
A form of control system in which the signal appears at one or more points in the system as a sequence of pulses or numbers usually equally spaced in time.

Sampled-data control system

A type of digital control system in which one or more of the input or output signals is a continuous, or analog, signal that has been sampled. There are two aspects of a sampled signal: sampling in time and quantization in amplitude. Sampling refers to the process of converting an analog signal from a continuously valued range of amplitude values to one of a finite set of possible numerical values. This sampling typically occurs at a regular sampling rate, but for some applications the sampling may be aperiodic or random.

While the device to be controlled is usually referred to as the plant, sampled-data control systems are also used to control processes. The term plant refers to machines or mechanical devices which can usually be mathematically modeled by an analysis of their kinematics, such as a robotic arm or an engine. A process refers to a system of operations such as a batch reactor for the production of a particular chemical, or the operation of a nation's economy. The output of the plant which is to be controlled is called the controlled variable. A regulator is one type of sampled-data control system, and its purpose is to maintain the controlled variable at a preset value (for example, the robotic arm at a particular position, or an airplane turboprop engine at a constant speed) or the process at a constant value (for example, the concentration of an acid, or the inflation rate of an economy). This input is called the reference or setpoint. The second type of sampled-data control system is a servomechanism, whose purpose is to make the controlled variable follow an input variable. Examples of servomechanisms are a robotic arm used to paint automobiles which may be required to move through a predefined path in three-dimensional space while holding the sprayer at varying angles, an automobile engine which is expected to follow the input commands of the driver, a chemical process that may require the pH of a batch process to change at a specified rate, and an economy's growth rate which is to be changed by altering the money supply. See Analog-to-digital converter, Process control, Servomechanism

The analog-to-digital converter changes the sampled signal into a binary number so that it can be used in calculations by the digital compensator. Since a digital controller computes the control signal used to drive the plant, a digital-to-analog converter must be used to change this binary number to an analog voltage. The digital compensator in the typical sampled-data control system takes the digitized values of the analog feedback signals and combines them with the setpoint or desired trajectory signals to compute a digital control signal, to actuate the plant through the digital-to-analog converter. A compensator is used to modify the feedback signals in such a way that the dynamic performance of the plant is improved relative to some performance index. See Control systems, Digital computer, Digital control