Page 466 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
452 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
100
P max 100
Q max
75
75
Setting range Pressure (%) 50 Set pressure Output flow (%) 50

25
25
0 0
10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100
Flow Q (%) Pressure (%)
(a) (b)
100

75

Pressure (%) 50

25

0
10 20 30 40 50 60 70 80 90 100
Flow Q (%)
(c)
FIGURE 7.37: Performance characteristics of a pump under various control methods: (a)
pressure regulated, (b) flow regulated, (c) flow regulated with pressure limit or pressure
regulated with flow limit.

where ΔP cmd is the desired pressure differential, P is the pump outlet pressure, P is
s
L
the load pressure, which is the larger of the two pressures sensed from the two ports of
the actuator (cylinder or hydraulic motor ports A and B). Typical values of ΔP cmd are
in the range of 10 bar to 30 bar. Notice that since this so-called load compensated (or load
sensing) pump control method maintains a constant pressure differential across the valve,
the speed of the cylinders is independent of the load (as long as maximum pump output
pressure limit is not reached) and proportional to the valve spool displacement. This can
be seen by examining the flow equations for a valve. The pump supplies the necessary
flow (up to its saturation value at maximum displacement) in order to maintain the desired
pressure differential between the pump outport pressure and the load pressure. Most load
sensing pump control mechanisms also include a valve to limit the pump output pressure
to a maximum value set by a spring loaded valve. This valve output flow is zero until the
pump pressure exceeds the maximum pressure limit.
Load sensing control is used to reduce energy waste when the pump is not needed to
operate at full system pressure and flow rate. It requires load pressure sensing and pump
output pressure sensing. Based on these two signals, the swash plate angle is controlled
by a valve and piston mechanism. Figure 7.38 shows a load sensing pump control system
with completely hydro-mechanical components. This system uses a completely hydro-
mechanical sensing, feedback, and actuation mechanism to perform a closed loop control
function on the swash plate angle in order to regulate a constant pressure drop across the