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Cascade control is very useful when you require highly
efficient control in the event of noise or when the secondary
part of the process involves a long delay.
Cascade control has two controllers (a primary and a
secondary); normally, the choice of control actions, based
on process speed, is made as follows:
•
Generally fast processes: for precise control, integral
action in the primary and only proportional in the
secondary is sufficient (primary controller PI, secondary
controller P).
•
Generally, very slow processes: for best system
readiness, precision, and stability, configure the primary
controller PID and the secondary controller PI.
The simplest example of a cascade control is a controller on
a valve positioner: in this application the positioner is used
to overcome hystereses and to reduce valve time constants.
Cascade control is normally not required in fast control loops
(flow rates, pressures, etc.) and is more useful in temforature
controls.
On series 850, 1650, 1850 controllers, the PID.1 control
output is the setpoint for PID.2.
5.10.7.1. Tuning two PIDs configured for cascade control
If you need to tune two PIDs configured for cascade control
(parameter APP.t=CAS.HE\CAS.CO\CAS.HC on EN.FUN
menu), do as follows:
1.
Set the primary PID to Manual (for example with the
Automatic\Manual button on home page Home.1), and
keep the secondary PID in Automatic
2.
Set the value of power delivered by the primary PID
(secondary PID setpoint).
3.
Start the Self-Tuning procedure for the secondary PID
(see paragraph "5.10.3. Self-Tuning" on page 203)
4.
When the Self-Tuning procedure for the secondary
PID is done, return the primary PID to Automatic (for
example with the Automatic\Manual button on home
page Home.1)
5.
Start the Self-Tuning procedure for the primary PID
(see paragraph "5.10.3. Self-Tuning" on page"5.10.3.
Self-Tuning" on page 203).
80209C_MHW_850-1650-1850_01-2020_ENG_pag. 206
5.10.8. Ratio control
In ratio control, the variable to be controlled is not a physical
quantity but instead its ratio with another quantity, whose
value must obviously be available.
This type of control is commonly used, for example, in
processes where a reactor has to be fed with two reagents
in a fixed ratio.
In practical applications, the primary variable is not controlled
or externally controlled, as in the case of mixing two fluids
(Fluid1/Fluid2).
The control is obtained by simply calculating the setpoint of
substance A (Fluid1), which can be controlled, as a product of
substance B (Fluid2) multiplied by an appropriate coefficient
(RAT.CO), which expresses the ratio to be maintained
between the two substances.
SETP1
IN.2
RATIO
yB
IN.1
RATIO is the ratio required between IN1 (PV1) and IN2 (or
IN3) (range from 0.01 to 99.99), i.e.
RATIO = IN1 / IN2 (or IN3)
This ratio is automatically calculated in the transition from
manual -> automatic and can be changed on the User menu.
The PID control controls IN1 so that it is always:
IN1 = SETP1 = IN2 (or IN3) x RAT.CO.
5.10.8.1. Activating the ratio controller
Activate ratio controller work mode as follows:
•
Enable the remote setpoint (parameter SP.REM on
MODE menu = On).
•
Configure the auxiliary input function (FUNC in INPUT.2)
or the auxiliary input 2 function (FUNC in INPUT.3)
as the reference for the ratio controller for PID.1.
e
PID
yA
P(s)
u
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