Note: Descriptions are shown in the official language in which they were submitted.
Description
Load Responsive System
Background of the Invention
This invention relates generally to load
responsive fluid power and control systems in which
the flow out of the pump is automatically varied to
maintain a constant pressure differential between the
discharge pressure of the pump and the maximum system
load pressure.
In more particular aspects this invention
relates to load responsive fluid power and control
systems, in which the flow of the pump is varied by a
bypass control.
In still more particular aspects this
invention relates to a load responsive fluid power and
control system, in which the flow out of the system
pump is varied by variation in the rotational speed of
the prime mover driving the pump.
Load responsive fluid power and control
systems are very desirable, since they provide an exact
and accurate proportional control of system loads.
Such systems may use an inexpensive fixed displacement
system pump, usually driven at a constant speed and
provided with a bypass type output flow control, as
disclosed in USE Patent 3,488,953, issued to
Haussler. Although such a system provides high
performance at low cost, it is comparatively
inefficient especially with a duty cycle utilizing low
system flows at high pressure. This drawback can be
overcome by a system as disclosed in US. Patent
3,444,689, issued to Budzich, in which the flow output
of the pump is varied by change in the pump
displacement, in response to a load pressure signal
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Such a system is very efficient but, since it uses a
variable displacement pump, it becomes relatively
expensive.
Summary of the Invention
In one aspect of the present invention a
fluid power and control system includes load actuating
means subjected to load pressure, pump means driven by
a variable speed prime mover selectively communicable
with said actuating means, and rotational speed
control means of said prime mover, means operable to
transmit load pressure signal from said actuating
means to said rotational speed control means, means
operable to transmit a pump discharge pressure signal
from said pump means to said rotational speed control
means, outlet flow bypass means in said pump means,
control means of said bypass means having means
operable to vary the bypass flow to maintain a
relatively constant pressure differential between said
discharge pressure and said load pressure when said
prime mover works at a certain minimum rotational
speed, and means in said rotational speed control
means operable to vary the rotational speed of said
prime mover to maintain a relatively constant pressure
differential between said discharge pressure of said
pump means and said load pressure while said outlet
flow bypass means remain inactive above said certain
minimum rotational speed.
It is therefore the principal object of this
invention to provide a highly efficient low cost load
responsive system using a fixed displacement pump, the
flow output of which, in the range of higher pump
flows, is controlled by the variation in the speed of
the prime mover, driving the pump, in response to the
load pressure signal
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Another object of this invention is to
provide a highly efficient, low cost load responsive
system, in which, in the range of lower pump flows,
associated with the minimum idling speed of the prime
mover, the flow out of the pump is varied by a bypass
flow control, in response to the load pressure signal.
It is a further object of this invention to
vary the flow out of a fixed displacement pump by
change in its rotational speed, to maintain a constant
lo pressure differential between the pump discharge
pressure and the maximum system load pressure.
It is a further object of this invention to
provide a bypass flow control, to vary the flow out of
a fixed displacement pump in the range of low
rotational speeds of the pump and to make said bypass
flow control inactive and to control the flow out of
the pump, by change in the rotational speed of the
pump, in the range of higher rotational speeds of the
pump, associated with higher pump flow.
It is a further object of this invention to
provide variable speed control of a fixed displacement
pump responsive to the control output from the bypass
flow control.
It is a further object of this invention to
dissipate the flow peaks associated with sudden
reduction in the flow control, while the rotational
speed of the pump is being lowered.
briefly the foregoing and other additional
objects and advantages of this invention are
accomplished by providing a novel highly efficient,
flow changing control of a fixed displacement pump, in
which, in the idling speed range of the pump, at low
lo horsepower levels the less efficient bypass flow
control is used, while in the range of higher pump
speeds, equivalent to higher horsepower outputs, the
flow output of the pump is controlled in a very
efficient way, by change in the rotational speed of the
pump. The flow out of the pump in those two modes of
control operation, is varied to maintain a constant
pressure differential between the pump discharge
pressure and maximum system load pressure, which is
characteristic of a load responsive system.
Additional objects of this invention
become apparent when referring to the preferred
embodiments of the invention as shown in the
accompanying drawings and described in the following
detailed description.
Description of the Drawings
Fig. l is a longitudinal sectional view of a
differential pressure bypass flow control with the
hydraulic system, load signal transmitting circuit,
signal generating circuit, prime mover speed control,
prime mover, mechanical drive, fixed displacement pump
and system reservoir shown diagrammatically;
Fig. 2 is a longitudinal sectional view of a
differential pressure bypass flow control, together
with the actuating mechanism of the control of prime
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mover speed and leakage flow control with the hydraulic
system, load signal transmitting circuit, prime mover
speed control, prime mover, mechanical drive, fixed
displacement pump and system reservoir shown
5 diagrammatically;
Fig. 3 is essentially the arrangement of Fig.
1 with the components of the hydraulic system shown in
greater detail and including a partial longitudinal
sectional view of the direction control valve.
Description of the Preferred Embodiments
Referring now to jig. 1, an embodiment of a
load responsive bypass valve assembly) generally
designated as 10, is interposed between fixed
displacement pump 11 and schematically shown load
responsive system 12, provided with schematically shown
load sensing circuit 13, operable to -transmit maximum
load pressure signal to the bypass valve 10 through
line 14. The fixed displacement pump 11, driven by a
prime mover 15, through a mechanical drive 16, is
connected by a discharge line 17 with the inlet core
18, of the bypass valve 10, which in turn is connected
to the load responsive system 12. The bypass valve 10
has a housing 19, provided with a bore 20, slid ably
guiding a bypass spool 21, provided with throttling
slots 22, terminating in throttling edges aye,
controlling by throttling the bypass flow between the
inlet core 18 and exhaust core 23. The bypass spool 21
defines in respect to bore 20 spaces 24 and 25. Space
25 is connected with inlet core 18 through lines 26 and
27 and therefore communicates directly with the
discharge pressure of pump 11. space 24 is connected
by line 14 to the maximum load pressure of load
responsive system 12 and contains control spring 28,
biasing the bypass spool 21 towards position, in which
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communication between inlet core 18 and exhaust core 23
is disrupted The bypass spool 21 is provided with
extension 29, which selectively engages an actuator rod
30, of a position signal generator 31. The actuator
rod 30 is biased towards position as shown by a spring
32. The position signal generator 31 is connected
through a signal transmitting mechanism 33 with a speed
control 34 of the prime mover 15. The pump 11 and the
load responsive system 12, in a well known manner, are
connected to a system reservoir 35.
Referring now to Fig. 2, like components of
Figs. 1 and 2 are designated by like numerals. A
bypass valve, generally designated as 36, is interposed
between the fixed displacement pump 11 and
schematically shown load responsive system 12, provided
with schematically shown load sensing circuit 13,
operable to transmit maximum load pressure signal to
the bypass valve 36. The pump 11 is connected through
line 17 and line 37 with core 18, while also being
connected by line 17 with space 25. A bypass spool 38
is provided with a timing surface 39, selectively
communicating space 24 with control core 40, which in
turn is connected by line 41 with an actuating control,
generally designated as 42. The actuating control 42,
provided with a piston 43, slid ably guided in bore 44
and biased by a spring 45, defines spaces 46 and 47.
Space 46 is connected by line 17, 37 and 48 to the pump
discharge pressure. Space 47 is connected by line I
with the control core 40 and also connected by line 49
with a constant leakage control, generally designated
as 50. The constant leakage control 50 is provided
with a metering spool 51, guided in bore 52, which
defines spaces 53, 54 and 55. The metering spool 51 is
biased by a spring 56 and is provided with throttling
slots 57 and metering orifice 58.
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Referring now to Fig. 3, like components of
Figs. 1, 2 and 3 are designated by like numerals The
diagrammatically shown load sensing circuit 12 of Figs.
1 and 2 is shown in detail in fig. 3 and consists of a
fluid power actuator 59 controlling a load We a
direction control valve, generally designated as 60,
and another schematically shown load responsive system
61. The direction control valve 60 is provided with a
housing 62, slid ably guiding, with bore 63, a direction
control spool 64, provided with throttling slots 65,
selectively interconnecting inlet core 66 with load
core 67. Load pressure sensing port 68 is connected
through line 69, a shuttle valve 70 and line 14 with
space 24. The shuttle valve 70 is also connected by
line 71 with the load sensing circuit of load
responsive system 61. Inlet core 66 of the direction
control valve 60 is connected by line 72 and a load
check 73 with inlet core 18, which in turn is connected
by line 74 and a load check I with load responsive
system 61.
Referring back now to Fig. 1, the load
responsive system 12, well known in the art, may be
composed of a number of fluid power actuators,
controlling the system loads, each actuator being
controlled in turn by a load responsive direction
control valve, provided with load pressure sensing
ports. Load pressure signals, from such load sensing
ports, are connected by a load sensing circuit, which
through a series of check valves in a manner, well
known in the art, transmits the maximum load pressure
signal to the pump flow control. Such a maximum load
pressure signal is transmitted from the load sensing
circuit 13 through line 14 to the load responsive
bypass valve 10. The bypass spool 21, of the bypass
valve 10, is subjected on one end to the maximum system
load pressure in space I and the biasing force of
spring 28, while on the other side being subjected to
the force, generated by pump discharge pressure in
space 25. In a well known manner, while subjected to
those forces, the bypass spool 21 will automatically
assume a certain throttling position, in which it will
throttle, by throttling slots 22, the bypass flow
between the inlet core 18 and exhaust core 23, to
maintain a constant pressure differential between the
pump discharge pressure and the maximum system load
pressure As is well known in the art, this constant
pressure differential will be proportional to the
reload of the spring 28. With the flow demand of the
load responsive system 12 rising the bypass spool 21
will move from left to right, progressively throttling
a smaller bypass flow. With the flow demand of the
load responsive system 12 equal to the output of the
fixed displacement pump 11, the throttling edges aye
will isolate the inlet core 18 from the exhaust core 23
and the full Lowe of pump 11 will be delivered to the
load responsive system 12.
Assume that under those conditions the fixed
displacement pump 11 is driven through the mechanical
drive 16 by the prime mover 15 at its minimum or idling
speed. Any increase in flow demand of load responsive
system 12 will, by exceeding the flow output of the
pump 11l automatically lower the pump discharge
pressure in space 25. The bypass spool 21, biased by
spring 28, will move further from left to right to a
point, at which the extension 29 will engage the
actuating rod 30. The displacement of the actuating
rod 30 will generate, through position signal generator
31, a proportional control signal, which will be
transmitted through the signal transmitting mechanism
33 to the speed control 34, of the prime mover 15. It
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should be noted that the position signal generator Cowan be of a mechanical, fluid power or electrical type
and that it will transmit a control signal proportional
to the displacement of the actuating rod 30, through
the signal transmitting mechanism 33, which can be of
any type well known in the art, to the speed control
34. The prime mover 15 can be an internal combustion
engine, or a variable speed electric motor and the
speed control 34 can be of any type, capable of
proportionally changing rotational speed of the prime
mover, in response to an external control signal and
maintaining the speed at any specific level,
proportional to the signal. Therefore, once the
maximum flow capacity of the pump 11, driven at minimum
idling speed, is reached, the bypass action of the
bypass valve 10 ceases and the control of the pressure
differential, between the discharge pressure of the
pump and the maximum system load pressure is
accomplished by variation in the pump RIP The
displacement of the actuating rod 30 from left to
right, in a manner as described above, will gradually
increase the rotational speed of the prime mover and
the pump from minimum idling speed to maximum speed.
Therefore in the zone of small pump flow, equivalent to
idling speed of the prime mover 15, the flow delivered
to the load responsive system 12 is regulated by the
bypass action of the bypass valve 10, to maintain a
relatively constant pressure differential between the
pump discharge pressure and the maximum system load
pressure. In the range of higher flows than those
equivalent to the idling speed of the pump, this
pressure differential is maintained relatively constant
by variation in the flow output of the pump, caused by
the change in the rotational speed of the prime mover r
since the output flow of a fixed displacement pump is
directly proportional to its rotational speed
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As is well known in the art the control of
pump flow through a bypass operation is comparatively
inefficient, with a large amount of fluid power energy
being converted to heat. On the other hand the
variation in the pump flow output by a change in its
speed of rotation is extremely efficient, since none of
its output flow is throttled.
Assume that the idling speed of the prime
mover is equal to 25~ of its maximum working speed.
Then the inefficient bypass control will only be used
in the small horsepower range ox the system, while in
the highest horsepower range the control of the
constant pressure differential is accomplished in the
most efficient way, by control of the rotational speed
of the fixed displacement pump.
In the system of Fig 1 the response to a
sudden increase in the demand of the load responsive
system 12, at pump flows higher than those equivalent
to its idling speed will strictly depend on the
response of the prime mover to its speed control. A
sudden reduction in the flow demand, of the load
responsive system 12, will put the bypass valve 10 into
bypass condition, while the speed of the prime mover is
being lowered, producing a much faster responding
control. This bypass condition will cease as soon as
the rotational speed of the prime mover is reduced to
the level, equivalent to the output flow of the pump,
equal to the system demand.
Referring now to Fig. 2, the performance of
the control system of Fig. 2 is identical to that of
Fig. 1 and the system is using similar control
components. The operation of the system of Fig. 2,
while bypassing flow at idling speeds of pump 12, is
identical to that of Fig. 1. The bypass valve 36
regulates the bypass flow to maintain a constant
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pressure differential between pump discharge pressure
and maximum system load pressure. Once the flow demand
of the load responsive system 12 exceeds the capacity
of the system pump, driven at idling speed, the bypass
spool 38 moves into position, in which it isolates by
throttling edges aye inlet core 18 from exhaust core
23, while connecting, by timing surface 39, the control
space 24 with the control core 40. Under those
conditions the maximum load pressure from space 24 is
connected through line 41 with space 47, while space 46
is connected through lines 48, 37 and 17 with the pump
discharge pressure. The piston 43 will then control
the speed control 34 and the rotational speed of the
prime mover 15, to maintain a constant pressure
differential between pump discharge pressure and
maximum system load pressure, as dictated by the
reload in the spring 45. Space 47 is also connected
through constant leakage control 50 with system
reservoir 55. In a well known manner constant leakage
control 50, with its metering spool 51, having metering
slots 57, throttles the fluid flow from space 54, to
maintain space 55 at a constant pressure level, as
dictated by the reload of the spring 56. In a well
known manner constant flow will pass from space US,
through orifice 58, to space 53 and therefore the
system reservoir 35. Therefore with control core 40
isolated by bypass spool 38, subjected to pump
discharge pressure the piston 43 will move all the way
to the left, compressing the spring 45 and reducing the
rotational speed of the prime mover 15 at a constant
speed, equivalent to the constant rate of flow through
the constant leakage control 50.
Referring now to Fig. 3, the bypass valve 36
and the actuating control 42, of the speed control 34,
are identical to that of Fig. 2. The system of Fig. 3
it
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performs in an identical way as the systems of Figs. l
and 2. Fig. 3 shows the components of the
schematically shown load responsive system 12 and load
sensing circuit 13 of Figs. l and 2. A direction
control valve 60 is interposed between bypass valve 36
and the fluid actuator 59. The displacement of
direction control spool 64 to the left creates a
metering orifice through throttling slot 65, between
load core 67 and inlet core 66. In a manner as
lo previously described, the control system of Fig. 3 will
maintain a constant pressure differential between the
load core 67 and inlet core 66 and across the orifice
created by displacement of the throttling slots 66,
either by bypassing action of the bypass valve 36, or
by change in rotational speed of the fixed displacement
pump ho The maximum load pressure signal, either from
direction control valve 60 or load responsive system
61, in a well known manner, will be transmitted through
the action of the shuttle valve 70 to space 24, of the
bypass valve 36.
There are two basic types of load sensing
systems known in the art. In one system a variable
displacement pump automatically varies the output flow
in response to maximum load pressure signal to maintain
a constant pressure differential between pump discharge
pressure and the maximum load pressure. In the other
system a fixed displacement pump driven, at a constant
maximum speed of rotation, provided with a bypass flow
control, is used. The bypass flow control is made
responsive to the maximum load pressure signal and
controls the flow delivered to the hydraulic power
circuit to maintain a constant pressure differential
between pump discharge pressure and the maximum system
load pressure. From a performance standpoint both of
those load responsive systems are identical. The basic
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difference between those two load responsive systems is
in their efficiency. The system using a variable
displacement pump is one of the most efficient systems
known, while the load responsive system using fixed
displacement pump is comparatively inefficient. The
load responsive system, using a fixed displacement
pump, is commonly used, in spite of its inefficiency,
because of the low cost and high reliability of fixed
displacement pumps.
In the system of this invention a fixed
displacement pump is provided with a bypass control
which, as previously described, operates only in the
flow range, corresponding to low horsepower, producing
comparatively small throttling losses. At higher flow
outputs the pump flow is varied by the rotational speed
of the prime mover, to maintain a constant pressure
differential between -the pump discharge pressure and
the maximum load pressure. In this mode of operation,
corresponding to high horsepower range, this system
efficiency exceeds the efficiency of the system using a
variable displacement pump. The power unit consisting
of a variable speed prime mover and fixed displacement
pump, operates in a load responsive system at this
maximum efficiency level throughout its entire speed
range from idling to maximum RIP which corresponds to
the zone of maximum generation and utilization of power.
Although the preferred embodiments of this
invention have been shown and described in detail it is
recognized that the invention is not limited to the
precise form and structure shown and various
modifications and rearrangements as will occur to those
skilled in the art upon full comprehension of this
invention may be resorted to without departing from the
scope of the invention as defined in the claims.
