Berk Korucu - 030080104 - 7th Week Definitions

1) Feedforward Control (Control System)


Previous Definition

The strategy in feedforward control is to anticipate the effect of disturbances that will upset the process by sensing them and compansating for them before they can affect the process. As shown in Figure, the feedforward control elements sense the presence of a disturbance and take corrective action by adjusting a process parameter that compensates for any effect the

disturbance will have on the process. In the ideal case, the compensation is completely effective. however, complete compensation is unlikelybecause of imperfections in the feedback measurements, actuator operations, and control algorithns, so feedforward control is usually combined with feedback control, as shown in our figure. Regulatory and feedforward control are more closely associated eith the process industries than with discrete production manufacturing.

(Groover, M. P., Automation, Production Systems and Computer-Integrated Manufacturing, 3rd Edition, pg.93)

New Definition (Better)

In feedforward control there is a coupling from the setpoint and/or from the disturbance directly to the control variable, that is, a coupling from an input signal to the control variable. The control variable adjustment is not error-based. In stead it is based on knowledge about the process in the form of a mathematical model of the process and knowledge about or measurements of the process disturbances. Perfect or ideal feedforward control gives zero control error for all types of signals (e.g. a sinusoid and a ramp) in the setpoint and in the disturbance. This sounds good, but feedforward control may be diffcult to implement since it assumes or is based on a mathematical process model and that all variables of the model at any instant of time must have known values through measurements or in some other way. These requirements are never completely satisfied, and therefore in practice the control error becomes different from zero. We can however assume that the control error becomes smaller with imperfect feedforward control than without feedforward control.

If feedforward control is used, it is typically used together with feedback control. Figure 8.1 shows the structure of a control system with both feedforward and feedback control. The purpose of feedback control is to reduce the control error due to the inevitable imperfect feedforward control. Practical feedforward control can never be perfect, because of model errors and imprecise measurements.

(F. Haugen, Basic Dynamics and Control, p.97-98)




2) H-Bridge (Control System)


Previous Definition

The control of motors using PWM circuits typically performed by using the H-bridge configuration shown in Fig. 10.1. In this figure, the circuit has been implemented using four IGBT devices as the switches and four P-i-N rectifiers as the fly-back diodes. This is the commonly used topology for medium and high power motor drives where the DC bus voltage exceeds 200 V. When the H-bridge topology is used for applications that operate from a low DC bus voltage, it is typically implemented using four power MOSFET devices as the switches and four Schottky recfiers as the fly-back diodes. ( Fundamentals of power semiconductor devices - B. Jayant Baliga - page 1027 )

New Definition (Better)

An H-Bridge is an electronic power circuit that allows motor speed and direction to be controlled. Often motors are controlled from some kind of ”brain” or micro controller to accomplish a mechanical goal. The micro controller provides the instructions to the motors, but it cannot provide the power required to drive the motors. An H-bridge circuit inputs the micro controller instructions and amplifies them to drive a mechanical motor. This process is similar to how the human body generates mechanical movement; the brain can provide electrical impulses that are instructions, but it requires the muscles to perform mechanical force. The muscle represents both the H-bridge and the motor combined. The H-bridge takes in the small electrical signal and translates it into high power output for the mechanical motor.

(V. Sieben, A High Power H-Bridge, p.2)