Page 463 - Mechatronics with Experiments

October 9, 2014 8:41 254mm×178mm
Printer: Yet to Come
JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 449
The swash plate angle is limited to the range
≤ ≤ (7.127)
min max
where is the minimum and is the maximum angular displacement of the swash
min max
plate. If the pump has two output ports, it is called a bidirectional pump. Output flow is
directed to one or the other port by the swash plate angle control. Such pump control is
referred to as over-center pump control. If the swash plate displacement is on one side of
the center, output flow is in one direction. If the swash plate displacement is on the other
side of the center, then output flow is in the opposite direction. Hence the flow direction can
be changed by pump control alone in a closed-circuit hydraulic systems such as hydrostatic
transmission applications.
Figure 7.38b shows the logical relationship between the load-sensing valve and
pressure limiting valve. The same control is shown in Figure 7.39 as a cross-sectional view
of a variable displacement piston pump with a load sensing control valve and a pressure
limit valve. If the order of the two valves were swapped (load sensing valve and pressure
limiting valve in Figure 7.38), the pump would not be able to destroke when the cylinder
hits a load that it can not move (stalled load condition). Therefore, it is important to note that
the pressure limiting valve is placed between the load sensing valve and swash plate control
piston. For further discussion, if we eliminate the load sensing valve and signal, and connect
the pressure limiting valve to pump output and tank ports as well as the swash plate control
piston port (three port connection), the pump control becomes a pressure regulating (also
called pressure-compensated or pressure controlled) pump where the output pressure of
the pump is set by the spring (Figure 7.36). In almost all hydro-mechanical pump controls,
the preloads on the springs are adjustable by a screw, which allows the user to adjust
the desired pressure limit and load-sensing pressure differential. Typical load pressure
differential setting (the pump output pressure minus the load pressure feedback signal) is
about 150 psi to 200 psi range in construction equipment applications.
In most pumps, the maximum and minimum swash plate angles can be mechanically
adjusted by a set screw. In hydro-mechanically controlled pumps, the pressure feedback
signals are provided by hydraulic lines with orifices. The command signal (i.e., desired
output pressure) is implemented by an adjustable spring and screw combination. The
actuator which moves the swash plate is called the control cylinder or control piston. In some
pumps, the actuator provides the power to move the swash plate in both directions under
the control of the proportional valve, whereas in others, the actuator provides power for one
direction and the power for the other direction is provided by a preloaded spring. In most
cases, at startup the preloaded spring will move the swash plate to maximum displacement.
As the output pressure of the pump builds up, the compensator mechanism (proportional
valve and control piston) provides control power to reduce the swash plate angle.
Pump control, that is the control of the pump displacement (D ( )) which is a function
p
of the swash plate angle, may be based on different objectives such as (Figure 7.33)
1. pressure compensating and limiting,
2. flow compensated,
3. load sensing,
4. positive flow control (matched flow supply and demand),
5. torque limiting,
6. power limiting.
Among different pump control methods, pressure compensated valve control and
load sensing valve control are the two most common methods.