In many process control systems, microprocessor based programmable logic controllers are used for decision making due to the sophistication of the system and the large number of process variables involved.
The PLC can be configured to receive a small number of inputs (both analog and digital) and control a small number of outputs or the system can be expanded with plug-in modules to receive a large number of signals and simultaneously control a larger number of actuators, displays, etc.
In very sophisticated systems, PLCs have the ability to communicate with each other on a global basis and send operational data to and be controlled from a central computer terminal or they can be connected in a distributed control system like DCS.
The illustration in figure 1(a) shows a typical controller setup, monitoring a single process variable.
The output from the sensor is conditioned and transmitted to the input module of the controller; if the signal is an analog signal it is converted to a digital signal and compared to a reference signal stored in the PLC’s computer. A decision can then be made and the appropriate control signal sent via the output module to the actuator.
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The internal function of a furnace using analog loop is illustrated in figure 1(b).
The temperature of the furnace is transmitted to the computer in the PLC as an analog signal, where it is converted to a digital signal and recorded in the computer memory. The voltage reference signal Vref is converted to analog signal and the two signals subtracted as shown in an analog error differencing circuit. The amplified difference signal is then fed via control circuit to the furnace heater, making the control loop analog.
In figure 1(c) we have a digital control loop. Here the furnace temperature is converted into a digital signal and transmitted to the computer in the PLC.
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The digital temperature signal Ds and the digital reference signal Dref are then compared in a digital error detection circuit (comparator) and the difference signal is sent as a digital signal to the controller, where it is converted into a pulse width modulated (PWM) signal or a signal from a digital to analog converter (DAC) is used to control the heater.
Related articles:
- Proportional-Integral-Derivative (PID) Control Systems
- What is Process Control?
- The Basics of Ladder Diagrams for Programming PLCs
- How to Tune the PID Controller
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