English (GB)
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15. Appendix
15.1 Measuring and control technology
15.1.1 Terminology
Below you find explanations on the terminology used
within the measuring and control technology.
The examples are based on dosing of chlorine:
Controlled variable
The controlled variable is the concentration of
chlorine.
Actual value of the controlled variable
The actual value of the controlled variable is the
measured concentration of chlorine in the water.
Measuring element
The measuring element is the device which is used
to measure the actual setpoint of the controlled
variable. In this case, it is the measuring cell and the
measuring amplifier.
Setpoint of controlled variable
The setpoint of the controlled variable is the required
concentration of chlorine.
Disturbance variable
The disturbance variable is everything that can
change the controlled variable e.g. impurities in the
water.
Control deviation
The control deviation is the difference between the
measured chlorine concentration and the required
chlorine concentration.
Manipulated variable
The manipulated variable is the dosed quantity of
chlorine.
Final control element
The final control element that changes the
manipulated variable e.g. a chlorine gas dosing
device or a dosing pump for chlorine bleaching
leach.
Controller
The controller is the device which calculates the
manipulated variable on the basis of the control
deviation.
Dead time
The dead time is the time between the modification
of the dosed quantity of chlorine and modification of
the measured chlorine concentration.
15.1.2 Open-loop and closed-loop control
Open-loop control
In an open-loop control, the manipulated variable is
modified independent of the result. An example is
the dosing of chlorine proportional to the flow.
A water meter as input element signals the flowing
quantity of water as input variable to the controller.
The dosed quantity of chlorine is set proportional to
the water flow. The actual concentration of chlorine
is not checked. An open-loop control is only
meaningful, if all the disturbance variables are
known and if they do not change simultaneously. The
manipulated variable can be assigned accordingly.
Automatic corrections cannot be made, if unforeseen
changes occur.
Fig. 35
Open-loop control
TM
03
70
91
17
13
Input
element
Open loop
controller
Final control
element
Controlled
variable
Disturbance
variable
Control
factor
Manipulated
variable
Input
variable
