| These valves
modulate hydraulic parameters according to
electronic reference signals. They can be
used in open or closed loop applications to
modify speed, direction, force and accurate
motion of a hydraulic cylinder. Open loop
applications typically involve the
acceleration or deceleration of a load and
are not required to stop accurately. For
this reason valving used to accomplish this
are not coupled with Temposincs position
transducers and usually have positive
overlapping in the center position of the
spool. This overlapping (deadband) is not
detrimental to the ability to accelerate or
decelerate. In these applications the use of
feedback devices internal to the valve is
not necessary. As the requirements for
accuracy are introduced, the type of valve
used to accomplish high repeatability,
changes. To close in on a specific pressure
or position you must have a valve that has a
high response time. I.E the time for the
valve to react to a change in reference
signal. This is expressed in the engineering
data of each valve as frequency response.
This response number must be measured at the
same degree of phase lag ( typically 90
degrees) in order to have a meaningful
comparison of valves. If for example, you
require the cylinder to return to the same
place within +- .002" of an inch, you would
require the system to be capable of several
things. First, to know where the cylinder is
at all times, secondly likewise to know the
position of the spool in the valve at all
times, and third to have no deadband in the
spool of the valve. The positive overlap or
“deadband” is detrimental to accuracy
because it allows spool movement around the
center position without a corresponding
change in flow. The requirements for this
type of accuracy would dictate the
following: A. Temposonics position feedback
in cylinder itself. B. Zero overlap (no
deadband) in the proportional valve and C.
Position feedback device on the main spool
of the proportional valve. All of these
parameter must then tie back to a Delta
motion controller to control the entire
operation. The valves must also be “tuned”
to the program either in the amplifier card
driving the valve or in the Delta motion
controller. The parameters that must be
tuned and their definitions are as follows:
Bias: Driving current required for
bringing the valve to a null point
Dither: The pulse frequency of the
driving current.
Scale: The relationship between the
driving current values and the values of
the reference signal
Ramps: The time required to change
driving current to the valve following a
step change in reference signal
Gain: The factor which modifies the loop
error, to correct the values of the
driving current I closed loop controls.
While all this may seem very complicated,
rest assured that the staff here at Flow
Dynamics and Automation are well versed in
setting up a variety of applications using
this technology. Once installed the
“program” is burned into a plug in chip that
may be replaced like any other part.
Therefore the technical maintenance
requirements are minimal and simple and
simply do not require much attention.
If you have an application that requires
the above described technology, give us a
call or e-mail us a
sales@flowdynamics.net and we will be
happy to assist you. |
| I couldn’t
help but chuckle a little upon reading Check
list for choosing a repair facility in the
October I & A department. Irving Weiner
makes good sense with his six important
points, and I agree with him. However,
customers should go a little beyond a few
bells and whistles in their search. Let’s
face it, there are many quality independents
out there – but some shoemakers as well. A
case in point: a local customer with about
100 Moog servovalves in the shop – who
formerly dealt with one of the large
independents – would send out three or four
of these servovalves for repair each and
every week on average. Our company, the Moog
distributor in this area, finally convinced
this customer to send the Moog servos back
to the manufacturer for evaluation and
repair. During a 6-month period, almost 100
previously “repaired” servovalves were sent
to Moog, evaluated, serviced, and returned
to the customer. During the ensuring six
months, only two servovalves had to be sent
out for repair by this customer. The savings
during that period: $72,800.
In addition, Moog initiated a tracking
system which provides the customer with a
complete record indicating when each and
every valve was repaired previously, how
long it was in service, when it next needed
repair, and what the possible cause of
failure was.
This is not to be construed as an
indictment of all independent service shops,
because there surely are some very good
ones; but among the problems many component
manufactures find is the use of non-factory
parts, substandard testing equipment, and
some times, poorly trained employees. My tip
to Mr. Customer: take a good, hard look at
who will be doing your service work and ask
the difficult questions.
Do we have an axe to grind? Nope! We are
an independent repair facility – and have
been for the past 40 years.
James D. Palmer Sr. | General
Manager Tri-State Hydraulics, Inc.
| Charlerio, Pa
Copyright© 1998 Penton Media, Inc.
Cleveland, Ohio 44114
Reprinted from HYDRAULICS & PNEUMATICS
November 1997
Moog Industrial Controls Division
Moog Inc., East Aurora, NY 14052-0018 |
