Page 486 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
472 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
Adjustable needle valve orifice
Valve body
P S
Inlet
P L
port
Outlet
port

Compensator spool Bias spring
(a) (b)

P = P - P
out in Spring
Outport
P
in
Inport

P
Spring
Spring

To tank
(c)

FIGURE 7.53: Flow control valves: (a) needle type flow control valve, (b) pressure
compensated needle type flow control valve, (c) by-pass type pressure compensated flow
control valve. The needle position may be manually or remotely controlled.

be proportionally controlled by an electric actuator, such as a proportional solenoid, which is
the case in many pressure compensated EH flow control valves (see www.HydraForce.com).
In a pressure compensated valve, a compensator spool moves in proportion to the
difference between two pressure feedback signals. It is also called adjustable pressure
compensated flow control valve. In the so-called pre-compensator valve configuration, one
pressure is fed back from the output of the compensator valve and the other pressure is
fed back from the output of the flow control valve (Figure 7.54). In the so-called post-
compensator valve configuration, one pressure feedback is from the input pressure to the
compensator valve, and the other pressure feedback is from the load pressure (Figure 7.55).
The objective of the compensator valve is to maintain a constant pressure drop across
the main flow control valve. Hence, when the main flow control valve spool is at a certain
position, the flow rate across it would not be affected by the pressure variations at the pump
supply or load side as long as the pressure variations do not reach the saturation levels. The
function of compensator valve is to add restriction (pressure drop) in a hydraulic circuit. It
is the analogy for variable resistance in series with another resistance in electrical circuits.
A directional flow control valve in series with a pressure compensator valve is called
a pressure compensated directional flow control valve or just a pressure compensated
directional valve. Compensator valves are used in

1. circuits where load is variable and that the flow rate should not vary as a function of
the load as long as the valve is not saturated (fully open or fully closed), and
2. multiple parallel hydraulic circuits where all circuits share the same pump supply
pressure, but the load pressure in each cylinder may be different. If a compensator