Page 501 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 487
Connector A Connector B

Solenoid B
Solenoid A
Valve spool

T A P B T

A B

P T
FIGURE 7.63: Single stage EH valve: two solenoids directly move the valve spool. Courtesy
Moog Inc.

There are three main types of two-stage valve designs:

1. spool-spool (double spool) design,
2. double nozzle flapper design, and
3. jet pipe design.

In a two stage valve, the electrical current moves an intermediate spool, which then amplifies
power using the pilot pressure line to move the second main spool (Figure 7.64). In multi
stage valve cases, the valve is so large that the electrically generated force by the solenoid
(or electric actuator) is not large enough to move the main spool.
The same concept applies to the three stage valves (Figure 7.65) where there are three
spools in the valve, the first two acting as the amplifiers to move the final third stage main
valve spool. The great majority of valves used in mobile equipment applications are two
stage valves. From a control system perspective, the functionality of single stage and multi
stage valves are the same: input current is translated proportionally into the main spool
displacement (with some dynamic delay effects, of course) which is then proportional to
the flow rate under constant pressure drop across the valve.
Notice that the second stage spool position (main spool) of a two-stage valve is
integral of the first stage spool position (pilot spool) if there is no feedback from the main
spool position to the pilot spool position. A two-stage spool valve is basically a single stage
direct acting valve connected to a second valve spool. The second valve can be viewed
as a small cylinder connected to a single-stage valve. Therefore, the current–main spool
displacement relationship is not proportional, but an integral relationship.
1
x (s) = K ⋅ x (s) (7.231)
main mp pilot
s
1
= K mi ⋅ i (s) (7.232)
sol
s