China
Africa
Explore chapters and articles related to this topic
Computerized Control Systems: Basics
Published in Gauri S. Mittal, Computerized Control Systems in the Food Industry, 2018
Deciding how to set the manipulated variable can be done in one of two ways. In open loop or feed-forward control, the disturbances are sensed and the process set to an appropriate operating condition before the process variable changes. This can provide effective control; however, unmeasured disturbances will not be corrected. Closed loop or feedback control operates by measuring the process variable and taking corrective action based on the departure from its setpoint (process error). Since this corrective action also affects the process variable, a loop is formed, as shown in Fig. 1. All of the disturbances that can affect the process variable are included in the loop. Indeed, minor design shortcomings are also corrected by the feedback action. Feedback control is, therefore, the most used type of control system.
PID Control in the Industry
Published in Stamatios Manesis, George Nikolakopoulos, Introduction to Industrial Automation, 2018
Stamatios Manesis, George Nikolakopoulos
A feedback controller is designed to produce an “output”, which acts correctively in one process, in order to lead a measured process variable to the desired value, known as the set point. In Figure 9.1, a typical feedback control loop is shown, where the blocks represent the dynamics of the whole system (controller and controlled process) and the arrows represent the flow of information either in the form of electrical signals or in the form of digital data. All the feedback controllers determine their output by taking into account the error between the desired and the measured actual value of the controlled variable. For example, a home thermostat is a simple ON-OFF controller that activates the heating system when the difference (error) between the actual and the desired room temperature value exceeds a threshold. A PID (proportional, integral, derivative) controller implements the same function as a thermostat, but determines the output with a more complex control algorithm. In particular, it takes the current value of the error in the series, the integral of the error in the latest time period, and the current value of the derivative of the error into account, in order to determine not only the size of the correction that should apply, but also the time duration of the corrective action. These three quantities are multiplied by three different gains (P, I, and D) with their sum as the final controller’s output (CO(t)) according to the following equation:
Controls
Published in Carl Bozzuto, Boiler Operator's Handbook, 2021
Start with a parameter. It is a quantity, value, or constant whose value varies with the circumstances of the system. The controller does not know what the parameter is, and it does not care. It can be pressure, temperature, level, count, pH, oxygen content in percent, differential pressure, a flow of any fluid, a weight, etc. The controller basically deals with parameters that are called inputs. They are used to create an output or out-puts. Inputs are assigned names that indicate what they are in relation to the controller. The two most important ones are process variable and set point. The process variable is a value representing the measurement of whatever it is that is trying to be maintained. If it is a pressure controller, it is the pressure. If it is a level controller, it is the level. It is the control system’s representation of the actual value of the parameter that is trying to be controlled. The set point is a value representing what the process variable should be. If the boiler pressure should be 100 psig (pounds per square inch gauge), the set point must be adjusted until the parameter represents 100 psig. When properly applied, the controller will indicate that it is set at 100 psig. The actual value is an electronic signal. There is some math going on to convert that signal to a value that corresponds to 100 psig. The same is true for a digital readout on a sensor. The sensor actually measures milliamps. It has to convert that measurement into the required units to produce a readout that is meaningful. Set points are not always set by the operator. A set point can be the output of another controller.
Tuning rules for feedforward control from measurable disturbances combined with PID control: a review
Published in International Journal of Control, 2021
A regulation control problem is classically known as the design of a feedback control law to reduce the disturbance effects on the process variable. In that problem, it is assumed that the load disturbance is unmeasurable and therefore no information is available about the disturbance signal. However, in many cases, disturbances can be measured and this information can be incorporated into the feedback loop to contribute to the disturbance rejection. This is the main idea of the feedforward control approach, as its name indicates. That is, the disturbance signal is measured and fed in advance (forward) into the loop before affecting the process output. Thus, feedforward control is proactive against load disturbances, while a feedback control scheme is a reactive since it acts once the process output has been modified by the disturbance signal (Liu et al., 2019).