User's Manual

Automation and Drives - SCE

T I A Training Document Page 9 of 64 Module

4. The Disturbance Variable z

The disturbance variable is the variable that unintentionally influences the controlled variable, and

moves it from the current setpoint value. A fixed setpoint control is necessary, for example, because

a disturbance variable exists. For the heating system considered here, this would be the outside

temperature, for example, or any other variable that changes the room temperature from its ideal

value.

5. The Setpoint Value w

The setpoint value at a point in time is the value that the controlling variable should ideally have at

that time. It should be noted that the setpoint value can change continuously under certain

circumstances if there is a slave value control. The measured value that the measuring device used

would establish if the controlled variable would have exactly the setpoint value is the instantaneous

value of the reference variable. In the example, the setpoint value would be the room temperature

desired at that time.

6. The Comparing Element

This is the point where the current measured value of the controlled variable and the instantaneous

value of the reference variable are compared. In most cases, both variables are measured circuit

voltages. The difference of both variables is the “control deviation“ e. It is passed on to the

controlling element, and evaluated there (see below).

7. The Controlling Element

The controlling element is the actual center piece of a control system. It evaluates the system

deviation -that is, the information about whether, how and to what extent the controlled variable

deviates from the current setpoint- as input information, and derives from this the

“Controller output variable“ Y

which, ultimately, influences the controlled variable. The controller

output variable would be, in the example of the heating system, the voltage for the mixer motor.

The manner in which the controlling element determines the controller output variable from the

system deviation is the main criterion of the control system. Part II discusses this topic in greater

detail.

Preface Fundamentals Discontinuous Action Controller Controller Block (S)FB41 Setting the System Appendix

Issued: 02/2008 Control Engineering with STEP 7