Page 604 - The Mechatronics Handbook

0066_Frame_C20 Page 74 Wednesday, January 9, 2002 5:49 PM

TABLE 20.11 Main Typical Differences Between Servovalves and Proportional Hydraulic Valves
Servovalve Proportional Valve
Electromechanical Bidirectional torque motor (0.1 ÷ 0.2 W) Unidirectional servosolenoid
converter with nozzle-flapper or jet pipe (10 ÷ 20 W)
Input current 100 ÷ 200 mA <3 A
Flow rate 2 ÷ 200 l/min (two stage type) with valve 10 ÷ 500 l/min (single stage type)
pressure drop = 70 bar with valve pressure drop = 10 bar
Hysteresis <3% (<1% with dither) <6% (<2% with electric feedback)
Bandwidth >100 Hz <100 Hz
depending on the amplitude of the input depending on the amplitude of
and of the supply pressure the input
Radial clearance of 1 µm (aerospace) 2 ÷ 6 µm
the spool 4 µm (industrial)
Dead band of the spool <5% of the stroke Overlap 10–20% of the stroke, less
if compensated

characteristic of the dynamic type. This is because it refers to a frequency diagram of the component
and defines the frequency at which the response drops by 3 dB below the low frequency value. Normally,
a bandwidth two to five times greater is required for continuous control valves compared with that
required by the system.
The main differences between servovalves and proportional valves are shown in Table 20.11. Except
for the traditional difference in the electromechanical conversion device, there are overlaps in the static
and dynamic characteristics in many components available on the market.
On the basis of the generically superior static and dynamic characteristics, servovalves are commonly
used in closed loop controls while proportional valves are used in open loop systems.
Servovalves
Two-stage models are very common in the context of servovalves, where a first pilot stage converts a low
power electric signal into a pressure difference capable of acting on the slide valve of the second stage,
usually four-way and symmetrical. Flow rate control servovalves are divided into two categories on the
basis of how they make the electric–hydraulic conversion:
• nozzle-flapper,
• jet-pipe.
An example of nozzle flapper servovalves is shown in Fig. 20.102. It comprises two stages: the former
consists of a torque motor, the flapper, and a system of nozzles and chokes, while the latter consists
mainly of the spool valve and output ports. The torque motor, constituted by the motor coil, the magnet,
the armature, and the polepiece, is capable of transmitting a torque to the flapper which undergoes an
angular displacement, thereby obstructing one of more calibrated nozzles to a greater degree. This
operation causes a pressure difference at the ends of the spool, thereby causing the latter to move until
the feedback wire, which connects the spool and the flapper, returns the flapper to the central position.
Through the flow metering slot carried out in the bushing, the spool thereby permits communication
between the various ports. The feedback wire is an elastic flexional element that provides the feedback
between the main power stage (spool valve) and the first stage (torque motor).
This type of valve usually requires a greater degree of oil filtration, as nozzle-flappers are more sensitive
to contaminants compared with the jet-pipe system.
Figure 20.103 shows a schematic section of an example of a servovalve of the jet-pipe type. It is
connected to orifice P (supply) of the servovalve by means of a filter and flexible hose. It should be noted
that, unlike the nozzle-flapper valve, it is not necessary to filter the entire incoming oil flow, but only
what is called the control flow (the one going through the jet-pipe); this is certainly an advantage in
terms of economic running and sensitivity to solid contamination. Starting with a standardized input
voltage, the amplifier produces a voltage increase in one torque motor coil and an identical reduction in

©2002 CRC Press LLC