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PWDXXA-40X_20 5715-677 UK.indd 26.01.17
Electronic for proportional DC valves
Series PWDXXA-40X
Operation Manual
Parker Hannifin Corporation
Hydraulics Group
Parker Hannifin Corporation
Hydraulics Group
5.5.2. Application: Closed loop systems for
pressure (via pressure control valves)
Introduction
The electronics uses a feedback control loop which
automatically adjusts the electrical input to the
valve amplifier that the commanded pressure ap-
pears. At the heart of this feedback loop is a digital
controller which computes and updates the signal
output (set value output) with a one millisecond
update time. The controller has adjustable coef-
ficients, which must be set by the user for the par-
ticular application.
The controller provides a standard PID control ca-
pability plus extra features which may be utilized to
improve the performance beyond the limits of PID.
Basic information for the control algorithm
Why tuning?
The controller can be used with valves that vary
greatly in flow capacity, frequency response, satu-
ration, dead band, load, and cylinders with a variety
of area and stroke. The user must adjust the control
coefficients for their specific system. There are no
fixed sets of values for the controller coefficients
that will handle every situation.
The electronics uses a “PID” controller for its basic
control action. The name PID comes from the fact
that the controller output is the sum of three terms,
called proportional (P), integral (I) and derivative
D), each with a user adjustable coefficient. The
user software provides therefore the parameters
P16 (P), P17 (I) and P18 (D).
P - P16
This term provides an immediate output signal,
proportional to the error between commanded and
measured pressure. If this parameter is set too
high, sustained oscillations may occur. If it is set too
low, accuracy and speed of response may be poor.
I - P17
This term causes the output to change at a rate
proportional to the error in measured pressure, in
a direction to drive the steady state error to zero.
P17 is active when a window will be leaving, which
might be adjustable by parameter P26. P26 shall
be adjusted in the way that the window lies near
the final pressure. A too low a value of P17 causes
low frequency oscillations, with a too higher value
the required pressure may be reached too slowly.
D - P18
The D-term provides an output proportional to the
rate of change of the measured pressure. Depend-
ing on the polarity of the parameter prefix, this
term causes damping or accelerating. For hydraulic
drives this term should be set to a very low value.
Parameter ranges
The controller provides two parameter ranges, of
course the operating modes
BASIC and EXPERT.
Herewith the
BASIC mode serves for adjustment
of the fundamental function parameters, while the
application specific setting of the control coefficients
may be done via the
EXPERT mode. The operating
mode may be selected via the menu
OPTIONS.
Simple tuning of a pressure control loop
Before trying anything complicated, you should ad-
just the bypass gain, accessible by P13. Herewith
a “feedforward” of the output will be achieved, the
PID-control loop will be “bypassed”. For this adjust-
ment all other control parameters P14 –19 as well
as P26 and P27 has to be set to zero. Then P13 will
be increased accordingly, until the feedback signal
appears 10...20% below the command signal .
Mostly a setup value of 40...50% is practicable.
Following is the adjusting of the proportional gain,
accessible by P16.
Adjusting the P-gain is done by increasing P16 to
the highest value that does not result in sustained
oscillations of the pressure. In many cases this two
coefficients will be all you need, and you will not
have to bother with any of the other control features.
If you do need the other features, you cannot adjust
them properly without first adjusting the P-gain.
Guarantee of control functionality
Before the tuning of the control loop may be start-
ed, the functionality of the control circuit must be
ensured. As previously mentioned, the control-
ler compares command and feedback signal and
adjusts the electrical input to the valve amplifier
to achieve the commanded pressure. To ensure
this functionality, the polarities of command and
feedback signal must be equal.
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