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Example "PID control"

4.1 Introduction

STEP 7 Basic V10.5

136 Getting Started, 12/2009, A5E02651459-01

The setpoint "

" has been predefined. The setpoint in the following example is a

temperature setting of 75°C in the heating chamber. The error (

) is calculated from the

setpoint (

) and the actual value (

). The error will be converted by the controller (K) into a

controlled variable (

). The controlled variable changes the actual value (

) via the controlled

system (G). The controlled system (G) in this example is the temperature control in the

heating chamber, for example, by increasing or decreasing the energy supply.

In addition to the controlled system (G), the actual value (

) can be altered through

disturbance variables (

). A disturbance variable in this example may be an unwanted

temperature change in the heating chamber, for example, caused by a change in the outside

temperature.

Use of the PID controller

In the example project you use the PID controller to reach the desired temperature of 75°C

as quickly as possible and to maintain the setpoint as constant as possible.

In this example, the setpoint can be exceeded because the heating element continues to

emit heat even after it has been switched off. This effect is known as "overshoot"; it occurs

when there is a time delay between the control and the measurement of the actual value.

The following figure shows the possible characteristic of the temperature curve after the

initial switching on of the machine:

7HPSHUDWXUHLQWKH

KHDWLQJFKDPEHU

DFWXDOYDOXH

5HTXLUHGWHPSHUD

WXUHVHWSRLQW

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Siemens v1.0 Manual de servicio Pagina 136