Parker PWD Series, Operation Manual

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Electronic for proportional DC valves
Series PWDXXA-40X
Operation Manual
PWDXXA-40X_20 5715-677 UK.indd 26.01.17
Parker Hannifin Corporation
Hydraulics Group
Parker Hannifin Corporation
Hydraulics Group
• 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 fre-
quency response.
• Pressure transducer
Some analog output transducers will present
problems due to the use of filters intended to
smooth the output. To ensure best system dy-
namics, sensors with integrated D/A-converter
should operate with a high sampling rate. Mag-
netostrictive transducers with digital output often
have a low sampling rate, causing an interaction
with the achievable resolution. Please obtain
detailed information from the transducer sup-
plier.
• System
The cutoff frequency will be determined by mass
and compressibility of the hydraulic media in the
entire system (cylinder, piping, valves). Mostly
the value is quite low, particularly for the usage
of large system volumes.
Pay attention for pressure peaks!
High pressure peaks may occur particularly at
closed loop pressure controls systems. Therefore
the dynamic pressure behavior should be inves-
tigated after the controller tuning utilizing a data
aquisition device such as a storage oscilloscope. If
the measured pressure peaks exceed the system
design hardware specification, reductions may be
achieved by adjustment of the control parameters.
Update rate of the controller
The controller updates the electrical output to the
valve at 1000 Hz. 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.
Pressure transducer accuracy and resolution
A control system is no better than its transducer.
Here are points to watch for:
• Absolute accuracy
The controller adjusts the system as closely as
possible to the pressure measured by the feed-
back device. The absolute accuracy of the
measured pressure is also determined by the
transducer. On the other hand, die signal span
of the transducer should be oversized to achieve
a safe operation even if pressure peaks ap-
peared. For example a system pressure of 300
bar requires a sensor with 400bar nominal pres-
sure. This results in a value of 75 % for the
feedback scaling P20 (300 bar means 75 % of
400 bar).
• Resolution
The electronics is unable to set a pressure 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 a pressure transducer plays
an important role in the proper functionality of a
closed loop pressure control system. The pres-
sure transducer should be located as close to
the point of control as is practical.
Example: A closed loop pressure control is re-
quired for a given hydraulic cylinder. In this case
the pressure control valve and transducer should
be mounted close together, otherwise poor
closed loop control will result.