Page 505 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 491
Pilot supply pressure Feedback Pressurized Cylinder head-end port Cylinder rod-end port Pilot supply pressure
spring
Hydrac
chamber
spring
Pilot pressure, (Pilot controlled) Centering (Right side)
meter-in area
Hydrac Stabilizing
spring
(Left side)

T P T Solenoid
Pilot spool
Pilot pressure,
Tank pressure (or stem) To tank From pump To tank proportional valve Tank pressure
meter-out area Main spool Main valve,
four-way, three
FIGURE 7.67: Two-stage proportional valve with two pressure reducing valves at the pilot
stage in line with the main spool. Pilot stage output is a proportional pressure to the solenoid
current. The main spool displacement is proportional to the pilot control pressure as a result of
the centering spring. The feedback between the main spool and pilot spools (one on each side
of main spool) has mechanical springs in addition to the hydraulic feedback.

lever (Figure 7.50). In the mechanically actuated pilot valve version, the pressure reducing
pilot valve pair is mounted right under the operator control lever. The output pilot control
pressure line is then routed to the main valve which is likely to be closer to the cylinder.
In the electrically controlled version (EH version), the main valve and the pilot valve
along with solenoid pair can all be located at the same location and control signals from
the operator lever sent to the solenoids electrically. Therefore, the EH version has fewer
transient delays associated with the pilot control pressure transmission.
A variation of the two-stage proportional valve with pressure reducing pilot valve
pair is shown in Figure 7.67. The differences between the two versions (Figure 7.66 and
Figure 7.67) are as follows:

1. The pilot valve pair (pressure reducing valve pair) is in line with the main spool and
there is a mechanical spring feedback between the pilot stage spools (one on each
side of main spool) in addition to the hydraulic pressure feedback.
2. The main spool is actuated by reducing the pilot control pressure on one side as
oppose to increasing it in the previous version. In order to move the main spool to
left, the solenoid on the left side is energized and pilot pressure is dropped to a lower
value proportionally on that side. Hence, the main spool moves to the left until the
centering spring on the left side balances the forces on both sides. When the current
to the solenoid is reduced, the pilot control pressure increases and pushes back the
main spool to neutral position. The operation for shifting the main spool to the right
follows a similar relationship.
3. The centering springs follow the main spool to the pilot side but not into the main
spool body side. As a result, when the main spool shifts to left, it works against only
the centering spool on the left side. When it shifts to right side, it works against only
the centering spool on the right side.