Page 464 - Mechatronics with Experiments

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(1) Pressure compensated pump control: control the pump displacement such that
the actual pump output pressure (P ) is regulated about a desired pump output pressure
s
(P cmd ),
cmd = offset + K ⋅ (P cmd − P ) (7.128)
s
where cmd is the commanded (desired) swash plate angle, offset is the offset value of the
swash plate angle, K is the proportional gain for the pressure regulator. For simplicity, we
illustrate a proportional control logic here. Clearly, more advanced control logic can be
implemented in the relationship between the desired variables, measured variables, and the
output of the controller.
This type of pump control is called a pressure compensated pump control, and
provides constant pressure output. The flow rate will be determined by whatever is needed
to maintain the desired pressure, up to the maximum flow capacity (Figure 7.37a). The
commanded pressure does not have to be constant and typically can be set between a
minimum and a maximum value for a given pump. As the flow rate increases, the regulated
pressure tends to reduce a little in hydro-mechanically controlled pumps. But this effect
can be eliminated by software control algorithms in digitally controlled pumps. When the
external load is so large that it cannot be moved, the pump under pressure compensated
control will provide the desired regulated output pressure at almost zero flow rate. This is
called the dead head condition.
Figure 7.36 shows the hydraulic diagram for a pressure-compensated pump control
system. The valve has a spring through which the desired output pressure is set by preloading
the spring. The pump output pressure is compared to that pressure setting by the valve.
Based on the difference, either the pump or the tank ports are connected to the control
piston line to up-stroke or down-stroke the swash plate of the pump. When p = p desired
s
with a certain error margin, where the allowed error margin is defined by the overlapped
part of the orifice opening of the valve, the output flow to the control piston is zero and
the pump’s swashplate maintains that angular position. If the pump output pressure is less
than the desired pressure setting, the valve moves to the left and the control piston port
is connected to the tank, hence as the flow moves out from control piston to tank, the
swash plate stokes up, which should result in increased output flow and increased pump
pressure. If the pump pressure is higher than the desired pressure setting, the valve moves
to the right, connecting the control piston port to the pump pressure port. As the flow goes
from pump port into the control piston, the swash plate destrokes, and the pump output
flow decreases as does the pump output pressure. As a result, the pump output pressure
is regulated to be equal to the desired pressure setting by the preloaded spring. Typical
hydro-mechanical pump controls have a pressure control valve where the preloads on the
springs are adjustable by a screw, which allows the user to adjust the desired pressure. If the
desired output pressure is variable (i.e., based on operator command or control logic), then
the preloaded spring is replaced by a remote pilot pressure which is variable and controlled
by operator commands or control logic.
Pressure limiting control is established by feeding back the pressure in the outlet to
a check valve. When a preset pressure limit value is reached, the check valve opens to let
oil in to the control piston and therefore destroke the pump swash plate to lower the output
pressure. Pressure limiting is installed, as a safety mechanism to make sure that the pump
does not output a pressure higher than the maximum allowed pressure for the system.
(2) Flow compensated pump control: control the pump displacement such that the
actual pump output flow rate (Q ) is regulated about a desired pump output flow rate (Q cmd ),
p
cmd = offset + K ⋅ (Q cmd − Q ) (7.129)
P