Digital Feedback Controllers

We have presented a number of different approaches for designing digital feedback controllers. Digital controllers that emulate continuous-time PID controllers can include a number of special features to improve operability. Controllers based on Direct Synthesis or IMC can be tuned in continuous or discrete time, avoid ringing, eliminate offset, and provide a high level of performance for set-point changes. Minimum variance control can be very effective if a disturbance model is available. 

With the advent of the microprocessor, digital technology began to be used for data collection, feedback control, and aU other information processing requirements in production facUities. Such systems must acquire data from a variety of measurement devices, and control systems must drive final actuators. 

Proportional plus integral plus derivative (PID) feedback controllers, 20 692-693. See also Digital PID controllers Proportioning, 26 226 batch, 26 249-251... 

There are three basic types of controllers that are commonly used for continuous feedback control. The details of construction of the analog devices and the programming of the digital devices vary from one manufacturer to the next, but their basic functions are essentially the same. 

Simulate the three-CSTR system on a digital computer with an on-off feedback controller. Assume the manipulated variable is limited to +1 mol of A/lt around the stcadystate value. Find the period of oscillation and the average value of for values of the load variable of 0.6 and 1. 

In a digital computer-control system, the feedback controller has a pulse transfer function. What we need is an equation or algorithm that can be programmed into the digital computer. At the sampling time for a given loop, the computer looks at the current process output x, compares it to a setpoint, and calculates a current value of the error. This error, plus some old values of the error and old values of the controller output or manipulated variable that have been stored in computer memory, are then used to calculate a new value of the controller output m,. 

Does not have a Sensor-Feedback Control Loop. Digital Setpoint Only. 

Another method is to apply feedback control as in Figure 7.1, but to periodically interrupt the circuit for a brief moment so that the magnitude of iRu can be determined and a correction applied. Several variations of such a control system have been published, and one version is available as an accessory for a potentiostat. The outstanding feature of these approaches is that Ru does not have to be known and it can vary during an experiment with only minor decrease in control accuracy. In many situations, the brief interruption of the feedback loop is a small price to pay for such convenience. Microprocessors will foster further developments in this direction the advent of digital potentio-stats is imminent. 

Discrete Time (Digital) Fixed Peremeter Feedback Controllers... 

Discrete time controllers will not normally be stand-alone units but will be simulated within the software of a digital computer. The capacity of the computer can be used if necessary to produce more complex forms of feedback control than those provided by the standard algorithms of the classical fixed parameter controller. 

Feedback control systems can be either analogue, where the controller is a mechanical device, or digital, where the controller is a computer or... 

A DC motor is feedback-controlled by a current sub-control loop and a primary speed control loop. In order to close the control loop, the actual current value is fed back to the current control loop and a speed signal to the speed control loop. While current is measured in the power converter, a shaft encoder on the motor is required for speed signal feedback. Either a tacho-generator or a digital encoder is used as a speed transmitter. If speed measurement accuracy is not very important, the speed feedback can be measured via armature voltage. In this case, this measurement can also be done within the power converter. Static control accuracy reaches... 

A related approach which has been used successfully in industrial applications occurs in discrete-time control. Both Dahlin (43) and Higham (44) have developed a digital control algorithm which in essence specifies the closed loop response to be first order plus dead time. The effective time constant of the closed loop response is a tuning parameter. If z-transforms are used in place of s-transforms in equation (11), we arrive at a digital feedback controller which includes dead time compensation. This dead time predictor, however, is sensitive to errors in the assumed dead time. Note that in the digital approach the closed loop response is explicitly specified, which removes some of the uncertainties occurring in the traditional root locus technique. 

Toijala (Waller), K. V. and Fagervik, K. C., "A Digital Simulation Study of Two Point Feedback Control of Distillation Columns," Kern. Teollisuus, 1972, 29, 5. 

The digital computer age has brought us sophisticated analytical instruments and computation abilities to delve even deeper into basic sciences. It also has enabled close monitoring and feedback control of processes, even in remote inhospitable atmospheres, to ensure that operations, and materials and products storage, are continuing as intended. 

The eantilever oseillation amplitude is kept constant within a few Angstroms by a digital proportional/integral feedback controller. A second feedback controller was used to keep the distance between tip and sample surfaee constant via keeping the frequeney shift of the cantilever oscillation at a preset value. 

The PID control results presented so far have been based on a continuous application of the controller. The digital apphcation of feedback control is applied at discrete points in time. DCSs use sequential microprocessors that perform control calculations for many control loops. Typical control loops are executed every 0.2 to 0.5 s for regulatory loops and 30 to 120 s for supervisory loops. The time between control applications is the control interval. At. PID control is apphed industrially on DCSs using digital formulas that are applied at discrete control intervals [Equations... 

In Chapters 16 and 18 we discussed the design of analog feedback controllers in the time and Laplace domains. Does the introduction of a digital controller change these design methodologies ... 

However, the intelligence of a digital computer offers the possibility for more advanced and computationally more complex feedback controllers. In this chapter we will study both the design of digital approximations to the analog P. PI, and PID controllers, and the design of more complex digital feedback controllers. 

What are the two classes of methods for designing digital feedback controllers Discuss their essential characteristics and identify their principal representatives. 

Digital feedback controllers, 634 approximation of analog controllers, 635-38... 

Hardware components computer systems, 552-57 control loops, 28-30, 32, 561-66 digital control loops, 557-61 Hardware instructions, 553 Heat exchanger cascade control, 399 control loops, 364 feedback control, 243 feedforward control, 413 modeling, 69-70... 

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