Tuning Two Pids Configured For Cascade Control; Ratio Control - gefran 3850T Use And Installation Manual

7. EXAMPLES AND APPLICATION NOTES
secondary has an output that controls the process.
The main advantages of cascading control are:
• Disturbances occurring within the secondary loop are
corrected by the secondary controller before they can
affect the primary variable.
• Existing delays in the secondary part of the process are
considerably reduced by the secondary loop and this
increases the response speed of the primary loop.
• The gain variations in the secondary part are compen-
sated for within the respective chain.
• The secondary loop allows the primary controller to
work accurately on the flow of matter or energy.
Cascading control is of utmost importance when imposing
high efficiency in controlling noise or when the secondary
part of the process involves a high delay (phase-out).
In cascade control there are two controllers, one primary
and one secondary. The choice of control actions, de-
pending on the speed of the process, must normally be as
follows:
• Medium fast processes:To achieve precision in con-
trol, proportional integral action on the primary variable
and proportional action on the secondary variable will
be sufficient (primary controller PI, secondary controller
P).
• Very slow processes on average: for faster reaction,
precision and stability, the system set the primary con-
troller PID and secondary controller PI.
The simplest example of a cascade control is a controller
on valve positioner. Cascade control is not normally provid-
ed in fast control loops (flow rates, pressures, etc.) and is
most useful in temperature control.
7.5.3.1. Tuning two PIDs configured for cascade
control
If you need to tune two PIDs configured for cascade con-
trol, we recommend that you follow these steps:
1. Set the primary PID to Manual, keeping the secondary
PID in Automatic.
2. Set the value of the power delivered from the primary
PID (setpoint del PID secondario, multiplied by the
Trim Range parameter to become the setpoint of the
secondary PID).
3. Activate the self-tuning procedure of the secondary
PID.
4. After completing the self-tuning procedure on the sec-
ondary PID, return the primary PID to Automatic.
5. Return the system to idle.
6. Activate the self-tuning procedure of the primary PID.
110
7.5.4.

Ratio control

In ratio control, the variable to be controlled is not a physi-
cal value, but its ratio to another value, whose value must of
course be available.
This type of control is commonly used, for example, in pro-
cesses in which it is necessary to feed a reactor in which
we wish to control the ratio between two reactants.
In practical applications the primary variable is not checked
or controlled externally, as in the case of mixing between
two fluids (Fluid1/Fluid2).
The control is obtained by simply calculating the setpoint of
the substance A (Fluid1), on which you can exercise control,
as a product of other substance B (Fluid2) multiplied by an
appropriate coefficient (RATIO), which expresses the ratio
that you want to keep between the two substances.
SP1
IN2
RATIO
yB
RATIO is the value of ratio you want between 1 (PV1) and
IN2 (PV2 range from 0.01 to 99.99). i.e.
RATIO = IN1 / IN2
This ratio is automatically calculated in manual -> automatic
change and can be edited.
Use of two input signals (PV1 and PV2) with the same num-
ber of decimal points is recommended.
Example: how the Ratio calibration procedure might take
place on the basis of delivered power.
– Activate manual mode
– Keep PV2 as constant as possible
– Modify manual power until PV1 reaches the desired
point leading to balance in the process.
– The PV1\PV2 ratio will be calculated automatically.
– Switch back to automatic mode.
– The RATIO will be maintained invariable at the last
value calculated at the end of the manual regulation
process. Output power in the final conditions will be
the balanced power.
80703B_MHW_2850T/3850T_02-2019_ENG
e
PID
IN1
yA
P(s)
u