PMA KS 98, Manual

Page: 175 / 246

Three-point stepping controller
Switching controller for control of a valve (e.g. temperature control by means of motorized valve and gas-air mixture)
1.
Open valve;
r
outputs Y1 = 1 , Y2 = 0
2. Don’t move valve;
r
outputs Y1 = 0, Y2 = 0
3. Close valve;
r
outputs Y1 = 0, Y2 = 1
To validate the adjusted X
p1
for the actuator response time, response time T
m
must be adjusted. The smallest position-
ing step is 100ms.
Adjusting the neutral zone
Neutral zone X
sh
can be increased in case of excessive output switching. However, note that an increase the neutral
zone will reduce the control sensitivity.
For this reason, we recommend optimizing switching frequency (actuator wear) and control sensitivity.
Output Y1 (open)
Output Y2 (closed)
Three-point stepping controllers can be operated with or without position feedback Yp .
Schritt
3-point stepping controller
SchrittYP
3-point stepping controller with position feedback
whereby
Yp
is not used for control. The static characteristic of a three-point stepping controller is shown in Fig. : 3 .
The hysteresis shown in this diagram is practically unimportant, but can be calculated from the adjustable min. pulse
length T
puls
? 100ms.
X
Tpuls Xp
Tm
sh
= × ×
2
0 1 ,
g
With
Tpuls
switched off, the shortest positioning step
Tpuls
is dependent of
Tm
,
Xsh
and
Xp
.
By varying
Xsh
, a required min. pulse length
Tpuls
can be realized:
X Xp
Tpuls
Tm
sh
= × × -
12 5 0 75 ,
'
,
9499 040 50611
Controller
175 Controller behaviour
100%
0%
100%
0%
Y1
Â
Y2
Â
Xn100 Xn0 W0 Weff W100
Xeff
Xp1
Xsh
Fig.: 3
Static
operating principle of a three-point
stepping controller
Set-point
Set-point
process value
process value