LiTH, IEI
Fluid and Mechanical Eng. Systems
2008-10-14
Characteristics of Servo Valves
Laboratory exercise in the course TMHP51: Hydraulic Servo Systems
Introduction
Servo valves can be broadly classified either as single-stage, two-stage or three-stage.
Single-stage servo valves consist of a torque motor or a linear force motor directly
attached for positioning of the spool. Because torque or force motors have limited
power capability, this in turn limits the hydraulic power capacity of single-stage servo
valves. In some applications the single-stage concept may also lead to stability
problems. This is the case if the flow forces acting on the spool are close to the force
produced by the electro-magnetic motor. Flow forces are proportional to the flow and
the square root of the valve pressure drop, which gives a limitation in hydraulic power.
A single-stage servo valve with a linear force motor is shown in Figure 1. The valve
illustrated in the figure is a valve, which employs just one linear force motor
(proportional magnet) to move the spool either side of the central position. The electric
signal from a position transducer is then used for closed loop control of the spool
position.
Figure 1: Single-stage servo valve with electrical position feedback
One of the most common types of servo valve is the two stages one. Either a flapper
nozzle pilot stage or a jet pipe stage can be used in conjunction with an electric torque
motor to control the main spool in the valve as illustrated in Figure 2.
1
LiTH, IEI
Fluid and Mechanical Eng. Systems
2008-10-14
2
Figure 2: Two-stage servo valve with jet-pipe pilot stage, MOOG
A current through the coil displaces the jet pipe from neutral position. This
displacement combined with the special shape of the nozzle direct focussed fluid jet
more into one receiver than the other. The jet now produces a pressure difference in the
control ports. This pressure difference results in a pilot flow, which in turn causes a
spool displacement.
Exercises
1.
Blocked line pressure gain
The actual servo valve is a four-way critical center valve with two stages. You have to
measure the pressure gain of the valve around zero operation and when the load ports
are blocked, as shown in Figure 3. The pressure gain Kpi0 is measured by stroking the
valve and recording load pressure, pL versus input current (iv).
pL
ps
iv
Kpi0
ps
1
pL
iv
-ps
Figure 3: Blocked line pressure gain for a critical center valve
LiTH, IEI
Fluid and Mechanical Eng. Systems
2008-10-14
3
Task 1
Make measurements of the load pressure for two different supply pressures: ps = 75 bar
and ps = 150 bar. Calculate the pressure gain Kpi0 for the two supply pressure levels and
discuss the result.
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Task 2
Study the hysteresis of the pressure curves and try to make a general comparison
between the actual two-stage valve and a single-stage valve.
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2.
Dynamic valve response
The dynamic behaviour of a servo valve can be described by its step response, as
demonstrated in Figure 4. The step response is measured by recording the spool
displacement xv versus time for a step input in iv. By varying the amplitude of the input
step saturation in the spool control also can be studied.
ps
xv
Input signal step [%]
100
iv
75
50
25
0
Time
Figure 4: Step response for a servo valve at different input step amplitude
Task 1
Measure the step response for two different input step amplitudes, Δiv = 5% and Δiv =
50% of ivmax and for two different supply pressures, ps = 75 bar and ps = 150 bar.
Calculate the time constant and bandwidth for all step response curves.
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LiTH, IEI
Fluid and Mechanical Eng. Systems
2008-10-14
4
Task 2
Compare the results from task 1 and discuss saturation and the influence from supply
pressure.
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