Application Note

2 Fluke Corporation Verifying hydraulic control system operation with Fluke ScopeMeter
®
120 Series test tools
When the voltage level
changes to -4 V, the position of
the valve changes and the flow
direction of the oil is reversed (P
to A and B to T), so the rod
moves at high speed to the out-
ermost position. After 2 seconds,
the rod reaches its outermost
position, the valve is closed (0 V)
and the cylinder holds this posi-
tion for about 1 second.
The voltage level then
changes to +3 V to re-open the
valve and retract the rod. Finally,
just before the innermost position
is reached again, the valve open-
ing is reduced slowly by chang-
ing the voltage level from +3 V
to +2 V, so the cylinder rod
moves gently into its innermost
position, where it is slowly
forced to its mechanical limits.
Using the ScopeMeter test tool
cursor function, it is easy to
determine the time taken to
move the cylinder rod from one
position to the other, and to
measure the control voltage lev-
els during the various stages.
From this picture, the engineer
can see which changes he can
make to adjust the behavior of
the system. For instance: he
could increase the retraction
speed by increasing the control
signal voltage from +3 V to +4 V,
or he could reduce the retention
force by decreasing the retention
voltage level from +2 V to +1 V.
Sensor signals
Most systems use sensors to
determine the position of various
moving components, such as an
“
end-of-stroke
” detector
. Fig
ure 2
illustrates how sensors can be
used to detect a “cylinder rod”
end-of stroke position.
The relatively low repetition
rate of the rod movement
requires a storage osc
illosc
ope to
store the measured sig
nal char-
acteristics over a long period of
time. With the Sc
opeMeter test
tool, signals from the sensors can
easily be measured and stored.
When the measurements are
complete, the cursor function can
be used to measure the time
required for the rod to move from
one sensor the other. Figure 3
shows the signals measured on
sensor A and B. The left cursor is
positioned on the falling edge of
the sensor A signal. This is
where the cylinder rod leaves
the sensor A position. The right
cursor is positioned on the rising
edge of the sensor B signal. This
is where the cylinder rod
reached the sensor B position.
The cursor read-out indicates the
time duration (dt) between both
cursor positions. When position-
ing information is required, a
resistive linear displacement
transducer is often used for short
rod movements. These give an
output signal directly related to
position.
For accurate positioning on
long rod movements, servo cylin-
ders are used. Here a sensor is
mounted in the c
ylinder head,
where it is protected from water,
dirt and other environmental
influenc
es
. The sensor operates
by mag
netically sensing a pat
-
tern of grooves cut into the base
of the piston rod. The change in
the magnetic field caused as the
grooves pass the cylinder head
generate a signal in the sensor,
which is converted by separate
electronics into a pulse. When
the direction of the rod is
reversed, a phase shift in the
sensor signal output occurs
enabling the direction of the rod
to be determined.
By counting pulses, and with
knowledge of the direction of
travel, highly accurate rod posi-
tioning can be realized.
During initial installation of the
system, or if any of the sensors
have to be replaced when carry-
ing out maintenance, the sensor
output signals must be aligned to
ensure that they are correctly
processed by the system’s elec-
tronics. In particular, the succes-
sive pulses in the output
wave-train must be symmetrical
and of equal amplitude. Figures 4
and 5 show incorrect and correct
alignment of this sensor output.
Conclusion
To verify correct operation of
electro-hydraulic systems, visual
storage of the control signals over
time is essential. The ScopeMeter
120 Series’ test tool’s low-speed
signal display capabilities make
these instruments ideal for meas-
uring and analy
zing hydraulic
c
ontrol sig
nals, and their rugged
battery powered capabilities
enable them to work in all the
environments where hydraulic
systems are employed.
Fig
ure 3. End-of-stroke sensor signals.
(Fluke 124)
Figure 4. Incorrect sensor alignment.
(Fluke 123)
Fig
ure 5. Correct sensor alignment.
(Fluke 123)
Fluke Corporation
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PO Box 1186, 5602 BD
Eindhoven, The N
etherlands
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ax (425) 446-5
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ax (3
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ax [phone]
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©2004 Fluke Corporation
. All rights reser
ved
.
Printed in U.S.A. 3/2004 2140244 A-ENG-N Rev A
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