Page 470 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
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control valve on top of the pump has three ports: load pressure sensing port, pump output
pressure sensing port, and output port to control the swash plate angle of the pump. The
load pressure is the maximum of the two pressures sensed between the cylinder axis and
rotary gear motor. Flow to each actuator is regulated by a manually operated proportional
flow control valve. Two flow control valves are typically built on a single frame valve block
with internal porting for P and T ports between them. Internal porting may connect P and
T lines to each valve either in a parallel or series connection. P and T connections are now
shown in the figure. Each valve has built-in ports to sense the maximum pressure at its
output ports. Using two resolver valves, maximum pressure is fed back to the pump control
valve. In this configuration, the smallest load pressure signal that can be fed back to the
control valve is the tank pressure, which is shown as one of the inputs to the resolver valve
next to the valve that controls the cylinder.
(4) Positive flow control (PFC) of pump: control the pump displacement such that it
matches the flow demand of the line. Let Q be the flow demand by the line, and w eng is
p
the input shaft speed of the pump. The necessary pump displacement to provide the desired
flow rate can be calculated from the pump performance characteristics,
Q = w eng ⋅ D ( ) (7.131)
P
P
−1
= D (Q ∕w ) (7.132)
P P eng
It is desirable to implement the PFC algorithm without a flow rate sensor. Therefore, let us
generate the desired pump displacement based on the predicted flow demand and using the
pump map,
Q Pcmd = Q (x , x , …) (7.133)
s1
P
s2
where (x , x , …) are the spool displacements of multiple valves supplied by the pump.
s1 s2
The desired pump displacement is determined as (an offset value is added to account for
initial nominal operating point),
−1
cmd = offset + D (Q Pcmd ∕w eng ) (7.134)
p
Notice that in order to implement the PFC method, we need the pump map function
−1
D (Q , w ) for the specific valve or valves supplied by the pump. The accuracy of this
P P end
pump map is important since mis-match between the flow demand and flow supply can
result in serious dynamic performance degradations. The price paid for the increased energy
efficiency benefit of the PFC closed center EH systems is the increased complexity of the
control algorithm.
Torque limiting and power limiting controls are implemented in pumps to prevent
them from stalling when both high pressure and high flow rate occur in the system at the
same time. In a mobile application the stalling will occur when the pump requires more
power than the diesel engine can output and this will eventually bring the diesel engine to
a dead stop. Torque limiting destrokes the pump to the point where the diesel engine is not
stalled, but simply accepts the available power from the engine instead of overloading it.

Transient Response of the Pump If the delay associated with the dynamic
response of the pump displacement controller is to be taken into account, instead of assum-
ing that the pump displacement is equal to the commanded pump displacement, a first or
second-order filter dynamics should be included in the pump model,
1
= ⋅ cmd (s) (7.135)
( s + 1)( s + 1)
p1
p1
where p1 and p2 are the time constants of the dynamic relationship between the pump
displacement command and the actual pump displacement. This effective time constant of