Parker PID00A-40 Series, Operation Manual, Page: 32 / 44

32
Electronics for Closed Loop Control
Series PID00A-40X Operation Manual
PID00A_10 5715-616 UK.indd CM 14.11.14
Parker Hannifin Corporation
Hydraulics Group
D - P18
The D-term provides an output proportional to
the rate of change of the measured pressure. De-
pending 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 ap-
plication specific setting of the control coefficients
may be done via the
EXPERT mode. The operat-
ing 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 – 28 has to be set to zero. Then P13 will
be increased accordingly, until the feedback sig-
nal 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 ad-
just 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 controller
compares command and feedback signal and ad-
justs the electrical input to the valve amplifier to
achieve the commanded pressure. To ensure this
functionality, the polarities of command and feed-
back signal must be equal.
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 digi-
tal controller which computes and updates the sig-
nal output (set value output) with a one millisecond
update time. The controller has adjustable coeffi-
cients, 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, sat-
uration, dead band, load, and cylinders with a va-
riety 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 coeffi-
cients that will handle every situation.
The electronics uses a “PID” controller for its ba-
sic 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 de-
rivative D), each with a user adjustable coefficient.
The user software provides therefore the parame-
ters 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.