Page 475 - Mechatronics with Experiments

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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 461
If we focus on the case where the cylinder inner diameter is twice the rod diameter,
d line 2 ∕4
V fwd = ⋅ V line (7.164)
d he 2 ∕4
d he 2
V rev = 2 2 ⋅ V fwd (7.165)
d − d
he re
4
= ⋅ V fwd (7.166)
3

7.5 HYDRAULIC VALVES

Valves are the main control components in hydraulic circuits. They are the metering com-
ponent for the fluid flow. The metering of the fluid is done by moving a spool to adjust the
orifice area. There are two main valve output variables of interest which can be controlled
by the spool movement:

1. flow (rate and direction) and
2. pressure.
If the movement of the spool is determined in order to maintain a desired pressure,
the valve is called a pressure control valve. If the movement of the spool is determined in
order to maintain a desired flow rate, it is called a flow control valve. If the flow direction
is changed between three or more ports, they are called directional flow control valves.
If the spool position of the valve is controlled only to two discrete positions, they are
called ON/OFF valves (Figure 7.44). If the spool position of the valve can be controlled to
be anywhere between fully open and fully closed position (i.e., in proportion to a signal),
then they are called proportional valves. The ON/OFF type valves are used in applications
which require discrete positions, that is open the door and close the door. Here, we are
interested primarily in proportional and servo control valves.
A valve is sized based on three major considerations:
1. flow rating: the flow rate it can support at a certain pressure drop across the valve port
when it is fully open (i.e., maximum flow at 1000 psi pressure drop for servo valves,
at 150 psi for proportional valves),
2. pressure rating: the rated pressure drop across the valve and the maximum supply
port pressure,
3. speed of response: the bandwidth of the valve from current signal to spool displace-
ment.

The main differences between ON/OFF valves and proportional valves are as follows,
1. Solenoid design: in proportional valves, the gain of the current–force relationship is
fairly constant in the travel range of the solenoid, whereas in ON/OFF valves, the
main thing is to generate the maximum force and the linearity current–force gain
relationship is not important.
2. Centering spring constant in proportional valves tends to be larger than than the
comparable size ON/OFF valves.
3. While the valve body of a proportional and ON/OFF valve may be almost identical,
the spool designs are different. The proportional valve spools are carefully designed
with desired orifice profiles to proportionally meter the flow.