Page 601 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 587
Power distribution
control signals
Implement
hydraulics
P 1
Steering
hydraulics
Power
P 2 distribution
valve
Brakes
hydraulics

P 3
Pump
control Cooling fan
signals hydraulics
Load
sensing
signals
Pump group

FIGURE 7.115: Programmable power allocation in multi pump multi circuit hydraulic
systems. Instead of dedicating a pump to each circuit in hardware, the output of all pumps is
brought into a controllable distribution valve. The valve directs the desired amount of flow to
each sub-system based on demand.

The valve is the main critical component in a hydraulic system from a control system
perspective. All of the valves we have discussed so far have a single spool for each stage.
One spool geometry defines the orifice areas between the four ports of the valve: pump
(P), tank (T), A and B side of the cylinder. The single variable, spool displacement x spool ,
determines the orifice areas
A (x ), A (x ), A (x ), A (x ), A (x ) (7.762)
PA v PB v AT v BT v PT v
These geometric relationships between the spool displacement versus the orifice areas are
designed and physically machined into each valve spool. Once the valve is machined, its
orifice characteristics are fixed. In order to accomodate many different application specific
requirements on orifice functions, many different spool geometry variations are often
needed. This requires machining many variations of basically the same spool geometry for
different applications. For instance, it has been estimated that one of the major construction
equipment manufacturers alone machines over 1600 different valve spool geometries.
It would be desirable to reduce the number of different spool geometries that must be
physically machined. This idea had led to the development of the independently metered
valves (IMV) concept. The idea is to define the orifice areas in software by actively
and independently controlling each orifice area by a separate spool (Figure 7.116). The
IMV valve has up to six independently operated spools and solenoids, one for each port
connection orifice area,
A (x ), A (x ), A (x ), A BT (x ), A (x ), A AB (x ) (7.763)
5
PT
4
1
AT
2
PB
6
3
PA
where each spool position is proportional to the associated solenoid current. Therefore, the
orifice area functions can be equally expressed as a function of solenoid currents.
(i )
A (i ), A (i ), A (i ), A BT 4 PT 5 AB 6 (7.764)
(i ), A (i ), A
PB 2
AT 3
PA 1