Fluke 190 202 Application Note

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Hydraulic system

principles

An hydraulic system has a fluid

reservoir and a pump that pumps

hydraulic fluid into the various

hydraulic cylinders to create

movement of the cylinder rod. A

controller determines into which

cylinder the fluid is pumped by

opening a valve to that cylinder.

The amount the valve is opened

determines the speed of move-

ment of the cylinder rod.

The valve stays open and the

rod moves until a sensor (or

operator) feeds information back

to the controller, causing it to

close the valve. These feedback

sensors are of three main kinds:

•limit sensors that feed back a

position that has been

reached, for example the “end

of stroke” for the rod

Verifying hydraulic control

system operation with

Fluke ScopeMeter®

120 Series test tools

Figure 2 shows a valve control

signal that has been measured

using a ScopeMeter®test tool.

Initially the signal has a level of

+2 V. This means that the valve

is slightly open, connecting the P

to B (and A to T) and the cylinder

rod is retracted.

From the Fluke Digital Library @ www.fluke.com/library

Application Note

The electronic sensors, controllers and actuators or valves

used to control a hydraulic system enable cranes, presses,

injection molding machines, earth moving equipment and

other “heavy duty” areas to operate faster and more accu-

rately. The service, maintenance and installation of these

electro-hydraulic systems require measurement tools that

provide insight into the system circuit behavior.

Figure 1. Basic diagram.

•position sensors that feedback

the actual position of the rod,

for accurate positioning

•pressure valves that limit the

maximum pressure by venting

fluid to the reservoir

Measuring control and

sensor signals

Control signals

The motion and speed of the

cylinder rod are directly related

to the oil flow regulated by the

valve. Figure 1 shows a

schematic drawing of a cylinder

controlled by a 4-way propor-

tional valve.

This valve has a P for pressure

input that can be fed to either its

A or B outputs depending upon

the control signal, and an T for

Tank (to fluid reservoir) return

which is then connected to the

non-pressurized A or B output. In

this way the cylinder rod can be

moved left or right by controlling

the pressure fed to the A or B

outputs with the control signal.

Figure 2. Valve control.

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. Figure 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 oscilloscope to

store the measured signal char-

acteristics over a long period of

time. With the ScopeMeter 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 cylinder head,

where it is protected from water,

dirt and other environmental

influences. The sensor operates

by magnetically 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 analyzing hydraulic

control signals, and their rugged

battery powered capabilities

enable them to work in all the

environments where hydraulic

systems are employed.

Figure 3. End-of-stroke sensor signals.

(Fluke 124)

Figure 4. Incorrect sensor alignment.

(Fluke 123)

Figure 5. Correct sensor alignment.

(Fluke 123)

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