22/69
Siemens Building Technologies
Electronic Air / Fuel Ratio Control System
CC1P7873.1en
HVAC Products
5 Description of functions
15.10.2002
5.7 Compensation of hysteresis
Mechanical play occurs in the actuator, in the mechanical link to the actuating device
and in the actuating device itself.
After a certain period of time, this mechanical play or hysteresis will increase due to
mechanical wear.
Function «Compensation of hysteresis» provided by the RVW26… is used to offset the
mechanical play within certain limits.
As a result of the hysteresis, one and the same
desired position
of an actuator has 2
different
actual positions
:
-
One actual position when approaching the desired position from below (output ad-
justment toward «OPEN»)
-
One actual position when approaching the desired position from above (output ad-
justment toward «CLOSED»)
Fig. 31 illustrates this:
1) The burner's output shall be increased from
to
, which means that the actuator
will have to travel along the curve from
«
a
» to
«
b
».
2) Then, the burner's output shall be reduced again to
, which means that the ac-
tuator will have to travel from
«
b
» to
«
a'
». For the same output 0, there is thus a
difference
«
H
» in the actuator's position.
2)
1)
78731w01e/1001
a
b
a'
Position of actuator
Burner output
(load)
H
2
1
Fig. 31
Effect of hysteresis
Due to the mechanical play, there is thus a difference between the actuators' real posi-
tion and the position ascertained by potentiometer «P».
Actuator
Mechanical play
Transmission of force,
linearization, etc.
Regulating unit
M
P
78731w02e/1001
Fig. 32
Basic diagram of hysteresis
The hysteresis can have a significant adverse effect on the combustion process.
It can lead to lack of air (formation of CO and soot) or to extreme amounts of excess air
(waste of energy).
Reasons for hysteresis
Effect of hysteresis
