Page 489 - Mechatronics with Experiments

October 9, 2014 8:41 254mm×178mm
Printer: Yet to Come
JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 475
where p is the pressure at the input port of the compensator valve, p is the load pressure
c
l
feedback signal. Again, if we eliminate the Δx from the above equations, and note that
cs
K vp ≫ k spring , and let A = A ,(k spring ∕K ) ≈ 0.0, it can be shown that
vp
1
2
k spring
p = p + ⋅ x preload (7.204)
c
l
A 1
which is the same relationship as the previous case. However, there are functional differ-
ences between pre-pressure compensators and post-pressure compensators in multi function
circuits. In post-pressure compensated systems, the pump is controlled to maintain a margin
pressure (pressure difference) between the pump output pressure and load pressure,
p = p +Δp margin (7.205)
l
s
The pressure differential across the main flow control valve is
( )
k spring
p − p = p +Δp margin − p + ⋅ x preload (7.206)
l
c
s
l
A 1
k spring
p − p =Δp − ⋅ x (7.207)
s c margin preload
A
1
which shows that the pressure drop across the main flow control valve is independent of the
load pressure. It is important to emphasize that pressure compensator valves are primarily
used to balance the flow distribution in multiple parallel hydraulic functions supported by
a single pump.
In short, the compensator valve adds restriction (pressure drop) between the pump
line and the output line (A or B) as a function of the difference between the pump pressure
and the load pressure.
In multiple circuits, a set of shuttle valves are used to select the maximum load
pressure among all circuits. The maximum load pressure is needed for two purposes: (i)
to control the pump displacement so that the pump is able to support the circuit with
the most load (worst case scenario), (ii) to control multiple compensator valves and try
to equalize the flow distribution among circuits. There are two major types of pressure
compensation in multi function circuits: the pre-compensator type and post-compensator
type (Figures 7.56, 7.57).
Figure 7.56 shows a multi function hydraulic circuit supplied by a variable dis-
placement pump. Each circuit has a pre-compensated valve along with the proportional
directional flow control valve. Each compensator valve uses two pressure feedbacks to
regulate the pressure differential across the main valve ahead of it. The pressure feedback
signal for each compensator valve is local to that circuit: output pressure of the compen-
sator and load pressure of the cylinder (maximum pressures of cylinder sides A and B are
selected by a shuttle valve). Finally, the pump displacement is controlled by the maximum
load pressure among all circuits so that the pump runs at a displacement that can support
the largest load among all circuits.
Figure 7.57 shows a similar multi circuit. The only difference is the location of
the compensator valves: in post-compensator configuration. In each circuit, the sources of
the two pressure feedbacks to the compensator valve are not all local: one of the pressure
feedbacks is the pump pressure line which is the input line pressure to the post-compensator
and the second load pressure feedback is the maximum load pressure among all circuits.
Hence, in the post-compensated configuration, the load pressure feedback (the second
pressure feedback) to each compensator valve is determined by the rest of the circuits. It
is always the maximum of all load pressures. The same maximum load pressure signal is
used to control the pump displacement.