Differential Behaviour; General Control Variable Function For Pid Controllers; Switching Performance - Ecolab MULTRONIC Notice Technique

8.6.3

Differential behaviour

With differential-action controllers, the speed at which the state deviation changes is assigned
to a specific value of the control variable.
The characteristic quantity for the D controller is the rate time, T V . The rate time is the time it
takes for the ramp response of a P controller to arrive at the value which a D controller would
reach without any delay.
A pure differential-action controller is technically not feasible. Because the controller reacts not
to the state deviation but to the changes in the state deviation, the differential action of the
controller is not sufficient to adjust the directly controlled variable to match the command
variable. A D controller cannot be used on its own to correct disturbance variables, as it will
only produce a measured variable if its input signal is changing. A D controller would therefore
not correct a constant state deviation because it only outputs a control variable when the state
deviation is changing.
It is for this reason that D controllers are only used in
combination with P- and I-controllers.
In combination with the other basic types, the D
component has the effect of correcting disturbance
variables more quickly.
Control variable function for PD controller: Characteristic value Tv
NOTE
8.6.4

General control variable function for PID controllers

8.6.5

Switching performance

The outputs are switched on and off in accordance with the control algorithm and the internally
calculated control variable. The effect is that the output switches off before the setpoint value
is reached because, in properly set control, the actual value approaches setpoint value in
small steps.
Example:
Actual: 10.0
Value
NOTE
417101163_Multronic.doc
D controllers are not useful as stand-alone controllers as they only respond to abrupt
changes in the input signal. The D-action is therefore best used in combination with P-
and I-controllers to improve their dynamic response.
pH value control, setpoint value of 7 pH: Representation of the control approach
of the actual value.
9.0 8.0
Relay switching performance
In the standard version, the measuring unit and the control unit are combined.
If control functions are superimposed on more than one measuring module, then the unit
must be occupied with an additional relay module.
7.5 7.2
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7.1 7.0
Rev: 5.07-08