Parker PID00A-40 Series, Operation Manual, Page: 31 / 44

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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
Repeatable position error too large because of
valve dead band or hysteresis
Utilization of the integrated dead band compensa-
tion coefficient enables this effect to be reduced
or eliminated. There are separate adjustable pa-
rameters P7 and P8 for both valve sides. This fea-
ture generates constant correction signals which
will be active for the dedicated valve site if a sig-
nal occurs on the command signal input. As a re-
sult this will cause the valve to “Jump“ through the
overlap region of the spool.
Increasing load stiffness
“Load stiffness” is the term given to the resistance
of the servo loop to deflection of the drive by ex-
ternal forces. Adjusting P16 to the highest practi-
cal value is important.
Disappointing control loop function
The higher the value of P-gain that can be used,
the better is the static and dynamic performance of
the system. You may not have any direct need for
fast response, but may still need a high value of P
to reduce static error, reduce following error or in-
crease actuator stiffness. High values of P cause
faster system response, whether you need it or not.
As the system frequency response gets too close
to the frequency response of one of your compo-
nents, the system response becomes oscillatory.
•
Valve
The frequency response of the valve can be
obtained from the datasheets. As a first guess, it
needs to be at least twice the system frequency
response.
• Position transducer
Some analog output transducers will present
problems because of filtering intended to smooth
the output. To ensure the best system dynamic,
sensors with integrated D/A-converter should
operate with a high sampling rate. Magnetostric-
tive transducers with digital output often have a
low sampling rate, because an interaction with
the achievable resolution. Please obtain detailed
information from the transducer supplier.
• Drive
There is a mode of mechanical vibration created
by the mass of the load and the compressibility
of the hydraulic fluid in the whole system. This
frequency is often surprisingly low when using
long stroke cylinders.
Update rate of the controller
The controller updates the electrical output to the
valve at 1000Hz. This rate is fast enough to have
negligible effect in almost all hydraulic applications,
but it does set an absolute limit on the response
of the system.
Position transducer accuracy and resolution
A control system is no better than its transducer.
Here are points to watch for:
•
Absolute accuracy
The controller positions the load as closely
as possible to the position measured by the
feedback transducer. The absolute accuracy
of the measured position is determined by the
transducer.
• Resolution
The electronics cannot position the load more
accurately than the resolution of the transducer.
Be sure the resolution of your sensor is adequate.
When using a true analog transducer (analog
measurement principle and analog output), the
resolution is limited by the utilized A/D-converter,
which is 12 bit (1/4096 = 0.025 %).
• Installation
The installation of the position transducer plays
a most important rule for the proper functionality
of the closed loop control. By all means it has
to be secured, that the mounting of transducer
and actuating device provides absolute freedom
from vibration and clearance effects. This is
most important even for the commonly expected
velocities and accelerations. Also, the transducer
has to be laid out for the dynamic requirements.