Siemens SIPART DR21, Manual

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1 General Part -- Fundamental control technology terms Manual
SIPART DR21
C73000-B7476-C143-08
15
- Two-position controller with feedback
In modern two-position controllers with feedack -- such as the SIPART DR21 -- the
switching response is determined by the period, the system deviation and the para-
meters. The period T is set as a fixed value in the controller. The system deviation xd in
conjunctionw ith the parameters Kp/Tn/Tv determines the duty factor (ON/OFF ratio)
within the period. Thus the switching response of the controller is not only triggered by
changes in the controlled variable; appropriate selection of the parameters results in a
largely constant controlled variable x.
1 Control amplifier
2 Controlled system
3 Setpoint adjuster
4 Pulse/pause converter
3
w
xd
1
y
2
x
z
t
t
y
On
Off
-
-
+
-
4
*)
T
w
Duty factor 0 %
Duty factor
100 %
2
¢
100 %
Kp
- Period T
- System deviation xd
- Parameters Kp, Tn, Tv
w Command variable
x
Controlled variable
y Manipulated variable
Z Disturbance variable
T Period
*) Duty factor (in % if period)
Figure 1-11 a) Function diagram b) Switching output and resosne of controlled variable
Adjustment of the period (separately for heating/cooling) permits the controller to be
adapted to the special type of heater or the cooling unit. A compromise has to be made
beween the control quality and the degree of wear.
Short period: Improved control quality, but increased wear on contact/heating valve.
Prime use with electric heaters.
Long period: Low wear on contact/heating valve, but poorer control quality. Prime
application with gas heaters or coolers.