ABSTRACT
A computer-controlled system consists of the process, the controlling elements, and the digital processing units (microprocessors or computers). The transducer, controller, and control valve are the controlling elements or instruments with appropriate communication capabilities. The transducer is loosely understood as the sensing element, which generates an output signal, which can be transmitted, processed, and recorded. Usually, the signal is generated (by the sensor itself or with the help of an additional instrument called the signal conditioner) as electrical voltage or current. This signal will be continuously produced by the transducer and will vary with the change in the process variable it measures. However, it is desirable that when the transducer is not in contact with the process it will send a bias signal indicating its active condition. Thus, the absence of the bias signal will indicate the failure of the instrument, which may be located in a remote control room where it communicates with the controller, and action can be taken. The controller in the computercontrolled system is either a microprocessor, a desktop or laptop computer, or a big work station consisting of a network of computers. Usually, the transducer generates an electrical signal in a continuous manner, which is therefore known as the analog instrument, while the microprocessor controller has to convert this electrical signal into a digital signal with the help of an in built analog-to-digital (A/D) converter. The control logic is programmed and resides in the memory of the controller. For a PID controller, parameters, such as the set point, bias value, proportional band, reset time, rate time, sampling time, modes of action (direct or reverse), etc., are accessed and can be changed as and when required. The program then computes the output signal, which is a digital signal and needs to be converted to an analog signal as the ›nal control element, such as the motorized or solenoid control valve that can be operated only by electrical current or voltage of a desired level. In case the ›nal control element is a pneumatic control valve, an additional instrument known as the electric-to-pressure or -pneumatic (E/P) converter or current-to-pressure (I/P) converter will be required to actuate the control valve. A microprocessor controlled loop is shown in Figure 9.1.
