Application Note

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 k
inds:
•
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
Fig
ure 2 show
s 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 c
ylinder
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 “hea
vy duty” areas to operate faster and more accu-
rately. The service, maintenance and installation of these
electro-hydraulic systems require measurement tools that
pr
ovide 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
T
ank (to fluid reser
voir) retur
n
which is then c
onnected to the
non-pressurized A or B output. In
this wa
y the c
ylinder rod can b
e
moved left or right by controlling
the pressure fed to the A or B
outputs with the control signal.
Figure 2. Valve control.