U.S. patent number 3,585,473 [Application Number 04/841,263] was granted by the patent office on 1971-06-15 for dynamic braking system for electric drive.
This patent grant is currently assigned to Caterpillar Tractor Co.. Invention is credited to Joachim Horsch, James T. Huxtable, Jay J. Murphy, Darrell E. Stafford.
United States Patent |
3,585,473 |
Huxtable , et al. |
June 15, 1971 |
DYNAMIC BRAKING SYSTEM FOR ELECTRIC DRIVE
Abstract
A system for dynamically braking an electrically driven vehicle
wherein a retarding means is directly coupled to the
engine/generator drive and employed to absorb the major portion of
the energy produced during power regeneration. Running at near
rated engine speed, the retarding means generates substantially
constant braking torque during vehicle deceleration and associated
control systems automatically operate the retarding means in
response to command signals and employ circuits to maintain the
excitation of generator below that of the wheel motors during
retarding whereby better power transfer is effected.
Inventors: |
Huxtable; James T. (Peoria,
IL), Murphy; Jay J. (Peoria, IL), Stafford; Darrell
E. (Peoria, IL), Horsch; Joachim (Lombard, IL) |
Assignee: |
Caterpillar Tractor Co.
(Peoria, IL)
|
Family
ID: |
25284432 |
Appl.
No.: |
04/841,263 |
Filed: |
July 14, 1969 |
Current U.S.
Class: |
318/140; 318/147;
290/14; 318/376 |
Current CPC
Class: |
H02P
29/0022 (20130101) |
Current International
Class: |
H02P
29/00 (20060101); H02p 005/22 () |
Field of
Search: |
;318/140,147,158,258,269,370,376,382 ;290/11,14,17,75 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rader; Oris L.
Assistant Examiner: Crosson; K. L.
Claims
What we claim is:
1. In combination with an electric vehicle drive having a prime
mover, an electric generator, a mechanical drive connecting the
prime mover with the electric generator, electric motors directly
connected to drive ground engaging members of the vehicle and leads
coupling the electrical output of the generator to the motors, a
dynamic control system comprising
a mechanical retarder connected to the mechanical drive connecting
the engine to the electric generator,
a retarder control connected to said mechanical retarder, said
retarder control having a transducer connected to said drive to
monitor its speed which provides an output signal to said retarder
control proportional thereto, and
control circuit means coupled to said electric generator and said
electric motors operable to cause said motors to convert to
generators and said electric generator to convert to a motor when
power regeneration occurs whereby energy motorizing said generator
will be absorbed by said mechanical retarder means when said
mechanical drive overspeeds.
2. The combination as defined in claim 1 wherein the prime mover
drive is a constant speed drive design.
3. The combination defined in claim 2 wherein the retarder control
means is provided with a control having at least two separate modes
of operation, one mode being responsive to the output signal
provided by the transducer, and the second mode being disposed to
control the actuation of said retarder control means independent of
said output signal.
4. The combination defined in claim 2 wherein the retarder control
has at least two separate control outputs, one of said control
outputs being coupled to place said retarding means in a partial
operating state the other of said control outputs being coupled to
said retarding control means to place said retarding means in a
full operating state.
5. The combination defined in claim 4 wherein the two separate
control outputs are responsive to a signal in the control circuit
means when power regeneration occurs and said other control output
is responsive to the output signal of the transducer.
6. The combination defined in claim 4 wherein said retarder is a
hydraulic retarder and one control output from the retarder control
initially charges said hydraulic retarder with fluid for partial
operation and said other control output supplies fluid to said
retarder for full operation.
7. The combination defined in claim 6 wherein the control circuit
means includes a generator control means and a motor control means,
said generator control being operably coupled to said generator to
cause said generator to operate as a generator to supply power and
as a motor when power regeneration occurs, said motor control being
operably coupled to said motors to cause said motors to operate as
motors when said vehicle is being driven and as a generator when
power regeneration occurs.
8. The combination defined in claim 7 wherein said generator and
motor controls are disposed to control the excitation level of
fields of said generator and motors respectively.
9. The combination defined in claim 8 wherein the control circuits
include a power pedal control, coupled to the generator control,
said power pedal control having two positions, one position being a
run position, the other a braking position which provides a signal
to the retarder control means to place the retarding means in a
partial operating state, said power pedal also including a switch
means for switching said generator control from a first mode where
it controls the generator as a generator to a second mode where it
controls said generator as a motor by controlling its field
excitation level relative to the field excitation level of the
motors.
10. The combination defined in claim 9 wherein an override means
responsive to a maximum mechanical drive speed disables the
retarder, circuit means and excitation of the generator and motors.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a dynamic braking system which is
uniquely adapted for use on large vehicles that are driven
electrically through electrical motors driving their wheels or
tracks. In small vehicles wherein the wheels are directly geared to
the engine, controlling the speed during power regeneration is
normally accomplished by a combination of engine braking and
service brakes. While this combination suffices for small, gear
driven vehicles, larger vehicles having high gross weights require
some means of dynamically braking the vehicle in addition to normal
engine braking and service brakes even when a gear drive is
employed. Thus, in large vehicles with electric drives where there
is no direct mechanical couple between wheels of the vehicle and
the engine, the problems become compounded due to a partial loss of
engine braking.
Many prior art systems have been suggested for supplying dynamic
braking for electrically driven vehicles including the use of
hydraulic or electric retarding devices which are mechanically
connected in the wheel drives of the vehicle. In some electrically
driven vehicles the drive motors are used as generators during
dynamically braking and the power generated by the generator is
dissipated in a grid of resistors or similar devices, but these
systems require expensive high power switching devices. In the
unique case of electrically driven vehicles which are powered from
external power lines or sources, the overall system can be used as
an electric power absorbing means and the electrical power
generated by the drive motors during power regeneration can be
merely returned to the general power distribution system. This
latter arrangement, however, is not suitable in the case of a
self-powered work vehicle having an engine driving a large
generator electrically connected to wheel drive motors.
While several systems for dynamically braking electrically driven
vehicles are known as indicated, they all have problems which have
rendered them unsatisfactory, especially for work or earthmoving
vehicles having electric drives which are continually decelerated
and accelerated. For example, suitable control is severe in the
case of earthmoving loaders which are usually required to stop at
least four times and then accelerate for each bucket load
transported. When hydraulic retarders are employed in low speed
operations, it poses a problem in that the torque generated by the
retarder is directly related to vehicle ground speed because they
are connected to the wheel drive train. Thus, as the drive vehicle
slows the dynamic braking provided by the retarder will
proportionally decrease which is serious during low speed
operations because of high energy dissipation required of the
service brakes. Electrical retarding devices have this same
characteristic and in large vehicles pose an additional
complication because of their large physical size. Further,
electric retarders in large sizes are relatively expensive and as
indicated like the hydraulic one tend to lose effectiveness as the
speed of the vehicle slows.
SUMMARY OF THE INVENTION
The present invention solves many of the above problems by
providing a novel dynamic braking system for a vehicle utilizing an
electric drive. More particularly, in an electric drive where the
prime mover of the vehicle drives the generator at a constant speed
so the electrical power output to the wheel drive motors is
controlled by excitation of the field of the generator, a hydraulic
retarding device is directly coupled to the prime mover and
generator drive so that it is driven at the same constant speed
which arrangement makes the speed of the retarder independent of
the vehicle speed at all times allowing it to generate a
substantially constant braking torque during power regeneration
regardless of the vehicle speed. Control circuitry maintains the
generator voltage below that of the wheel drive motors during power
regeneration to insure improved operation at lower vehicle
speeds.
Retarder control for the vehicle is provided with two levels of
operation, one being based on engine/generator speed and the other
being a general throttle position or other operator-initiated
condition in which the retarder is armed or partially charged with
hydraulic fluid. Thus, as the operator releases the power pedal the
retarder will be placed in a semioperating condition. As energy is
taken from the vehicle and transferred to the generator, the speed
of the generator and retarder will increase thus causing the
hydraulic retarder to activate and prevent engine overspeed. The
retarder serves to convert generator shaft horsepower (braking)
into heat in the retarder cooling system.
The use of the above arrangement provides several advantages over
previous retarding systems, such as quicker retarder response since
the retarder is always operating in a very efficient speed range.
In addition, running the drive at constant speed ensures better
cooling of the engine and its related auxiliary equipment. Finally,
the use of a constant retarder speed provides a constant dynamic
braking torque which in turn provides a faster response in a system
which is independent of the vehicle speed.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more easily understood from the
following detailed description of a preferred embodiment when taken
in conjunction with the attached drawings in which:
FIG. 1 is a block diagram arrangement of the mechanical drive
components of the present invention;
FIG. 2 is a block diagram showing the electrical arrangement and
the electrical control system;
FIG. 3 is a vertical section of the hydraulic retarding device
shown in FIG. 1;
FIG. 4 is a flow diagram showing the hydraulic controls for the
retarder;
FIG. 5 is a graph illustrating the relationship between the vehicle
speed and the generator voltage during retarding;
FIG. 6 graphically shows the relationship between the vehicle speed
and the retarding torque when the system is employed;
FIG. 7 illustrates the relationship between the engine r.p.m. and
horsepower; and,
FIG. 8 graphically shows the relationship between the power pedal
position and available motoring or retarding horsepower for pedal
position.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIG. 1, the overall arrangement of the mechanical
drive of the vehicle is illustrated and while no particular vehicle
type is shown, the drive system is adapted to any type of electric
drive vehicle. The system is particularly suitable for use in an
electrically driven earthmoving vehicle such as a large loader.
Both auxiliary and wheel drive power is provided by an engine or
prime mover 10 which may be either a conventional internal
combustion engine or a gas turbine. As explained above the engine
is operated at a constant speed preferably by means of a governor
system (not shown). The engine governor may include separate
positions so that the engine can be idled or set to run at rated
speed. Directly coupled to the engine output shaft is a generator
11 which supplies power over leads 13 to the wheel drive or
traction motors (not shown in FIG. 1) of the vehicle. Connected in
the drive train with the engine and the generator is a retarder 12
which is preferably a hydraulic retarder, such as shown in detail
in FIG. 3 and it should be appreciated that the retarder may be
located any place in the engine/generator drive train so long as it
is directly mechanically coupled thereto. The engine also drives
two auxiliary alternators 14 and 15 which supply electrical power
for various electrical systems, as well as electrical power for
exciting the field of the main generator 11 and the main drive
motors.
The drive train through the generator shaft is also illustrated
coupled to the input of a drive box 20 which contains a series of
auxiliary shafts for driving auxiliary equipment on the vehicle.
More particularly, the auxiliary equipment illustrated consists of
two hydraulic pumps 22 and 23. The hydraulic pumps are a steering
pump and a retarder charging pump. This drive box is also connected
to the input shaft 24 of a second drive box 25. As shown in FIG. 1
the second drive box is arranged for location on an articulated
portion of a vehicle, although all of this auxiliary equipment if
desired could be mounted on one unit. The second drive box 25
drives three hydraulic pumps 26, 27 and 28, designated as an
implement pump No. 1 and implement pump No. 2 and a brake pump. The
implement pumps, of course, supply hydraulic fluid for operating
the various earthmoving implements on the vehicle while the brake
pump supplies fluid for operating the service brakes on the
vehicle.
Referring now to FIG. 2 the electrical and hydraulic controls for
the dynamic braking system of the present invention are shown. The
engine 10 drives the retarder 12 and the generator 11 while the
alternators 14 and 15, shown separated from the engine, are driven
by the engine as explained above. Normally, the engine is operated
at a constant r.p.m., and more particularly, at rated r.p.m. which
is at or near a full throttle position. In this arrangement, the
engine governor has no mechanical or electrical connection with the
remainder of the control system described herein. The actual
vehicle speed and motoring condition is controlled by varying the
excitation of the generator field and the fields of the drive
motors, as is explained more fully below, and also through the
employment of the novel retarder system.
The excitation of the generator field is controlled by a control
circuit 31 that is connected to rectify the three phase power
supplied by the alternator 14 and control the excitation level of
the generator field in response to control signals. This alternator
is controlled by a regulator 16 (shown in FIG. 2) which regulates
its output on a constant volts-per-cycle basis. For example, the
alternator may be regulated to supply 400 cycle three-phase
alternating current power to the field control 31. The field
control circuit 31 also contains signal conditioning and switching
circuitry and is responsive to various conditions such as engine
speed, the selected vehicle speed, as indicated by the position of
the power pedal, and the generator and motor current. More
particularly, the field control circuit 31 is supplied with a
signal representing engine speed by means of a tachometer generator
41 on lead 40, a signal representing the generator current flow by
means of the measuring device 35 on lead 34 and a signal
representing the motor current flow by means of a measuring device
37 on lead 36. The generator and motor current measuring devices
may be any well known device which is capable of measuring current
flow in the generator and motor armature circuits, respectively.
The field control circuit 31 is also supplied with a reference
voltage via lead 32 from a battery 33.
A signal from the power pedal 44 over a lead 42 is also received by
the generator field control circuit for supplying the operator's
selected speed in the control loop. In addition, a separate
switching signal is supplied from a switch 45 on the power pedal
over lead 46 to the field control circuit and over lead 50 to the
retarder control circuits 54 and via lead 51 to the motor field
control circuits 53. This switching signal is used to condition the
controls for either a motoring mode or a retarding mode, including
those of the motor control circuits for either a motoring or a
retarding mode of operation. More particularly, in the motoring
mode the field control circuit 31 excites the field of the
generator to supply power to the drive motors while in the
retarding mode it limits generator field excitation sufficiently
below the level of motor field excitation so that the generator
will act as a motor. Of course, the reverse control relationships
are provided by the motor field control.
The power pedal 44 is provided with two potentiometers 47 and 48
and a single wiper 43. More particularly, the potentiometer 47 acts
to control the motoring mode of operation and supplies a signal to
the field control circuit indicating the desired vehicle speed.
Similarly, the potentiometer 48 supplies a signal to the field
control circuit indicating the desired amount of dynamic
braking.
A retarder control 54 includes both electric and hydraulic controls
to control the initial filling or arming of the retarder, as well
as the actuation of the retarder. The hydraulic fluid for filling
and operating the retarder is supplied by the pump 23 shown in FIG.
1. A retarder control receives a signal from an engine speed
reference 55 which is preferably a frequency signal related to the
engine speed. This signal can be easily provided by using the
frequency of the alternator 14 since it is directly related to the
engine speed or a tach generator input as illustrated. This
retarder control also includes frequency sensitive means as, for
example, frequency sensitive relays 144 and 148, one of which
operates at a first predetermined engine speed (i.e., relay 144
could operate at 2,120 r.p.m.) to actuate the retarder 12 and a
second relay which operates at a second higher preselected speed
representative of maximum safe engine speed (i.e. relay 148 could
operate at 2,250 r.p.m.) to disconnect and inactivate both the
generator and the motor field controls so that serious damage will
not occur. In this condition the vehicle must be stopped by service
brakes or other means. Disconnecting circuits for the generator and
motor field controls 31 and 53 are provided to prevent damage to
either the engine or generator due to an overspeeding condition
since normally the engine governor will maintain its speed.
Disconnect signals from the relay are supplied over lead 52 to the
generator and motor controls, respectively.
The motor field control 53 is similar to the generator field
control circuit 31 and utilizes rectifying circuits for rectifying
the AC power supplied by the alternator 15. More particularly, the
alternator 15 is controlled by a regulator 17. Normally, this
alternator will supply 400 cycle three phase power to the motor
field control 53 over leads 60.
A reference voltage from a battery 62 over the lead 61 is also
received by the motor field control which is summed with a signal
related to the output voltage of the generator 11 over lead 63 from
a sensing unit 64.
Two drive motors 70 and 71 are shown although additional
parallel-connected motors could be used. In the case of a four
wheel drive vehicle such as an earthmoving loader, the two motors
would drive differential units of the several axles which in turn
drive wheels mounted thereon. The armatures of the two motors 70
and 71 are coupled in parallel across to leads 13 from the
generator 11 while the field windings 72 and 73 are coupled in
parallel across the lead 74 from the motor field control 53.
Referring now to FIG. 3, a hydraulic retarder is shown which is
suitable for use with this invention. Other similar types of
hydraulic retarders could be employed as well. In the illustrated
arrangement shaft 80 of the prime mover is coupled to a flywheel 81
which in turn is coupled to the generator shaft 83 by means of a
flexible coupling 82. This flexible coupling may be any of several
conventional coupling designs.
Mounted on a spline portion of the generator shaft is a rotor 84 of
the retarder which is disposed in the housing 85. The housing is
securely attached to the main frame structure 86 of the prime
mover. Hydraulic fluid is introduced to the retarder through the
inlet opening 90 and flows into an annular space 91 surrounding the
blades on the outer periphery of the rotor and through a series of
axial holes 92 which are provided near the center of the rotor to
provide fluid communication to the opposite side of the rotor.
Hydraulic fluid is exhausted through an opening in the outer
periphery of the housing not shown in FIG. 3. A more complete
description of the construction and operation of the hydraulic
retarder may be found in U.S. Pat. No. 3,352,385 issued to
Johnson.
Referring now to FIG. 4 a detailed schematic diagram of the
retarder electric-hydraulic control 54 is shown. More particularly,
the hydraulic controls employ a retarder control valve 100 for
controlling the supply of hydraulic fluid to the retarder and a
two-stage reducing valve 101 which controls the operation of the
retarder. Hydraulic fluid from the pump 23 is supplied to several
solenoid operated valves 104 and 105 by a supply line 103, and both
these valves are connected by supply lines 106 and 110 respectively
to the two-stage reducing valve 101. A pump by a drain line 107 is
provided for one valve and a similar drain line 111 is provided for
the other.
On the output side of the hydraulic pump is a reducing valve 112
which limits the maximum pressure supplied to the retarder control
valve 100 via line 113 to a somewhat lower pressure than supplied
to valves 104 and 105. The retarder control valve 100 is connected
to a heat exchanger or a cooler 115 by means of a line 114 while
the outlet of the cooler is connected to circulate the hydraulic
fluid back to the control valve via line 116. This control valve
supplies hydraulic fluid to the retarder 12 through line 120 which
returns to the valve through a line 121. The two-stage reducing
valve 101 is connected by a line 122 to the left-hand end of the
control valve 100, as illustrated and operates to sequence the
retarder control valve.
The reducing valve includes a lower piston or valve spool 123 and
an upper piston or valve spool 126 both of which are positioned in
its bore 124. A compression spring 125 is positioned between the
two valve spools to control their positioning and an outwardly
projecting flange 127 on the upper valve spool 126 limits its
downward travel by contacting in a shoulder formed on the body of
the valve. In this construction spool 123, when pressurized through
line 106, will move toward spring 125 until a modulated pressure
output of a predetermined value occurs in line 122 due to hydraulic
fluid entering chamber 123a. For example, a modulated pressure of
20 p.s.i. may be employed. However, if pressure thereafter occurs
in line 110 it will compress the spring 125 causing maximum system
pressure to occur in line 122, due to increased spring pressure on
spool 123. This valve is somewhat conventional and provides for
initial partial filling (arming) of the retarder 12 and subsequent
full actuation.
A spool 130 in the retarder control valve 100 is controlled by
pressures in line 122 and initially acts as a differential valve
partially filling the retarder 12 with hydraulic fluid when line
122 has approximately 20 p.s.i. therein. In the partially filled
state the retarder will be absorbing approximately 100 horsepower.
If thereafter, pressure in line 122 is increased, the retarder will
be filled as spool 130 is shifted by the increased pressure and
will circulate hydraulic fluid through heat exchanger 115. An
identical retarder control valve is described in U.S. Pat. No.
3,386,540 issued to Horsch and further discussion is not warranted
of the valve itself.
The two valves 104 and 105 are supplied with electrical power from
a battery 140 over leads 145, 146 and 147. A normally closed speed
relay 144 is disposed in leads 145 and 146 and is controlled by the
engine speed reference 55, as described below. Another normally
closed fill relay 143 is disposed in the lead 147 to control the
operation of the valve 104. Relay 143 is opened to cut off power
circulation to valve 104 when the power pedal wiper 43 is in the
decelerating mode through a signal via lead 50. It may also be
opened by an auxiliary, operator-initiated signal means via lead
50'; an example of this mode of control would be appropriate
switching means (not shown) which provides a signal via lead 50'
whenever a directional transmission shift is made.
OPERATION OF A PREFERRED EMBODIMENT
The operation of the above-described dynamic braking system can be
more easily understood by referring to FIGS. 2 and 4 and the
operating curves shown in FIGS. 5--7. Operation of the circuit of
the control system will be explained by considering the motoring
mode followed by the retarding or braking mode. Considering the
motoring mode, it is assumed that the governor control is in a run
(rated r.p.m.) position so that the electrical controls are
energized and the engine is operating on the straight line portion
of the power curve shown in FIG. 7. In this condition, an
illustrative engine may have a no load engine speed of 2,080 r.p.m.
which will then decrease as the horsepower increases along the
straight line portion of the horsepower curve to a speed of
approximately 1900 at which point the engine will develop its
maximum rated horsepower. When the engine governor is in the run
position and depressing the power pedal will move the slider 43
along the potentiometer 47 increasing its negative output, the
amplitude of the signal generated at the wiper 43 will then be
related to the desired power and thus vehicle speed. This signal is
fed to the generator field control circuit 31 where it is summed
with the engine speed signal as supplied by the tachometer 41.
Since the engine is operating on the straight portion of its
horsepower curve, its actual speed will be linearly related to its
actual developed horsepower which, in turn, is related to the
desired power indicated by the position of the power pedal.
The generator field control circuit responds to the two signals by
supplying the proper rectified excitation level from the alternator
14 to the generator field 30. This generator field excitation level
will cause the generator to supply the necessary voltage output
over the lines 13 to the armature circuit of the two drive motors
70 and 71. The generator output voltage which is sensed by sensing
device 64 is also used as a feedback to the motor field control 53.
The motor field control will compare the generator voltage with a
fixed reference voltage 61 to control the full wave rectifier unit
in the motor field control for a proper excitation level for the
drive motor fields.
If during the motoring operation the power pedal is depressed still
more, the excitation level of the generator field and drive motor
fields will be controlled and as the power demand is increased, the
engine will slow further but will remain on the straight line
portion of its power curve. Since the engine speed changes from
2,080 r.p.m. at no load to 1,900 r.p.m. at full load, the above
system provides for extremely fast response from no load to full
load operation by converting some of the inertia to power. Further,
since the generator field control circuit 31 compares the actual
engine operating speed which is related to the power developed with
the power setting of the power pedal 44 and controls the generator
field excitation in response thereto, auxiliary mechanical loads
placed on the engine will have priority over the generator load.
For example, if a hydraulic implement places a heavy load on the
engine and the engine tends to slow, the generator field control
circuit will reduce the field excitation so as to reduce the load
placed on the engine by the generator to compensate for this
auxiliary load. This results from the fact that a particular
setting of the power pedal is related to engine speed and the
generator control will attempt to adjust the excitation of the
generator field to obtain this engine speed.
In normal operation if the vehicle tends to overspeed, the wheels
will increase the speed of wheel drive motors so they act as
generators. When the wheel drive motors act as generators, they
will reverse the flow of power and supply power to the generator 11
causing it to act as a motor and drive the engine. As the generator
attempts to drive the engine above its maximum governed speed of
2,080 r.p.m., the governor reduces the fuel flow to the engine and
compression braking, along with braking from the electrical losses
in the generator and the friction losses in the engine will absorb
some of the energy from power regeneration. If this amount of
braking is sufficient to permit the engine to maintain a speed
below the first predetermined speed of 2,120 r.p.m., the system
will tend to stabilize. If, however, the braking requirement is
such that the engine speed reference 55 indicates a speed above
2,120 r.p.m., a signal to the retarder control system 54 will
activate it.
The retarder electrical controls consist of three relays 143, 144,
and 148 as shown in FIG. 4. Relays 144 and 148 are frequency
sensitive so that they operate at a frequency representing engine
speeds of approximately 2,120 and 2,250 r.p.m. Relay 143 operates
as a function of pedal position or other anticipatory,
operator-initiated signal means as mentioned above. These relays
are usually of the type that open their associated contacts when
their operating frequency is exceeded for failsafe reasons. When
the relay 143 opens positive battery voltage is interrupted to the
valve 104 causing valve 104 to open, which will initiate partial
filling of the retarder with hydraulic fluid, as previously
indicated, thereby consuming approximately 100 horsepower as a
result of valve spool 123 throttling the hydraulic fluid supplied
to supply line 122 connected to the retarder control valve.
Pressure of the hydraulic fluid acting on the retarder control
valve will be sufficient to move the valve spool 130 to the right
so that the edge 132 of one of the projecting lands on the valve
spool will uncover the inwardly projecting land 133 on the valve
body. This will permit hydraulic fluid to flow into the retarder
and partially fill it. Normally, the flow of hydraulic fluid will
be sufficient to fill the retarder to approximately 80 percent of
its maximum capacity as a result of the first stepwise sequence.
This partial filling of the retarder will create the additional
loading indicated and thus additional braking on the vehicle. If
this additional braking load is sufficient to maintain the engine
below said first predetermined speed of 2,120 r.p.m., the system
will tend to stabilize. If the operator again depresses the power
pedal, relay 143 will be reenergized and relay contacts closed to
reenergize the valve 104 thus permitting the hydraulic fluid to
drain from the left hand of the piston 130 and return the system to
its normal operating condition as the retarder is drained of
hydraulic fluid.
If the above retarding is no sufficient frequency sensitive relay
144 will open its contacts at 2,120 r.p.m. which will not only cut
off battery voltage in lead 146 and open the valve 105 to the
hydraulic supply, but also cut off battery voltage to lead 147 and
open valve 104. When the valve 105 is opened it will supply
hydraulic fluid through the line 110 to the top of the two-stage
reducing valve which will force the valve spool 126 downwardly and
cause the lower valve spool 123 to travel towards the bottom of the
valve. This in turn will permit the full pressure of the hydraulic
fluid to be supplied over the line 122 from valve 104 to the
left-hand end of the retarder control valve which will cause the
retarder to completely fill from supply line 120. As a result the
retarder will be charged to 100 percent capacity at the pressure
set by the relief valve 112 and the drain line 121 of the retarder
will be coupled by the control valve to the inlet 114 of the oil
cooler 115. Similarly, the outlet 116 of the oil cooler will be
connected to the drain lines at the left-hand end of the control
valve so that the hydraulic fluid will drain back to the sump of
the pump 23. Thus, the hydraulic retarder 12 will be placed in full
operation to absorb the maximum horsepower in a typical case up to
1,200 horsepower. If this is sufficient to hold the engine speed
below a second or maximum allowable speed of 2,250 r.p.m., the
vehicle will be continuously dynamically braked to a speed at which
the drive motors no longer generate sufficient power to drive the
generator. Once the retarder is partially full which happens
whenever the power pedal is released or a directional change is
made, it can provide maximum power absorption in one-half second so
the engine cannot overspeed appreciably.
Under typical conditions, the retarder is partially filled as the
operator releases the power pedal which causes the slider 43 to
move upwardly from the potentiometer 47 to potentiometer 48. This
will adjust the generator and motor field controls so drive motors
will operate as generators to supply power to the generator, which
will then operate as a motor. The switching of the drive motors to
generator operation and the generator to motor operation will be
provided by the switch 45 that is also operated by the power
pedal.
If the above-described dynamic braking consisting of the retarder,
engine compression braking, engine friction losses and electrical
losses are insufficient to control the speed of the vehicle, the
retarder electrical control 54 at the second predetermined engine
speed will send a shutdown signal over lines 52 to the generator
field control 31 and the motor field control 53, respectively. A
warning device such as a light or bell would advise the vehicle
operator of this condition. In response to the shutdown signal the
generator and motor field controls will cut the excitation from the
field of the drive motors and the generator. This will permit the
vehicle to freewheel and the vehicle service brakes then must be
used to slow and stop the vehicle--but the fully loaded retarder
still serves to slow the power plant. When the power plant speed
drops below the second predetermined speed by approximately 75
r.p.m. (to allow a deadband) the safety relay disconnects the
shutdown signal in lines 52 and excitation of the fields for
electrical braking is resumed.
When the vehicle is operating on normal terrain and the operator
desires to slow the vehicle to perform a different operation, he
releases some of the pressure on the power pedal 44, so the wiper
43 will shift from the accelerating potentiometer 47 to the
decelerating or braking potentiometer 48. As the transfer of the
wiper takes place from one potentiometer to the second, it also
sends a signal to the accelerator switch 45 to the retarder
hydraulic controls. This signal will cause fill relay 143 in the
lead 147 to open. This in turn will actuate the valve 104 and
initially charge the retarder to 80 percent of capacity as
described above. The switching signal from the switch 45 is also
supplied by a lead to the generator field control circuit 31 and to
the motor field control 53. This switching signal will cause the
generator engine speed reference from the tachometer to be removed
as a reference in the generator field control circuit and adjust
the generator field excitation relative to the retarding signal
from the potentiometer 48 and the motor current signal 34. The
generator field control circuit will then control the generator so
that it operates as a motor to absorb the power produced by the
drive motors. The switching signal supplied to the motor field
control will remove the fixed reference from the control and
replace it with a second fixed reference which will be matched with
the generator voltage signal 63. This will have the effect of
causing the motor field control to reduce the motor field current
versus the generator volts to permit the drive motors to act as
generators down to a very low vehicle speed since the generator
excitation will be reduced relative to that of the motors, to allow
constant reverse current flow to the generator, depending on the
signal from the retarding potentiometer.
The vehicle will then transfer regenerative power from its drive
train to the drive motor and causing them to act as generators and
power from the drive motors will drive the main generator as a
motor. The retarding rate will be controlled by the position of the
wiper along potentiometer 48 although in all cases the generator 11
will be driven as a motor while the drive motors will act as
generators and the engine and retarder 12 will absorb the
regenerative power, as necessary.
As described above under motoring conditions if this reverse power
flow plus initial charging of the retarder is sufficient to control
the speed of the vehicle, the system will stabilize. If this amount
of power absorption is not sufficient the retarder hydraulic
controls will then fully activate at approximately an engine speed
of 2,120 r.p.m. At this engine speed speed relay 144 is energized
and signal in leads 146 and 147 cut off. This will have the effect
of placing the retarder in full operation, as described above,
making maximum braking available. If the full retarding is
sufficient to maintain the engine speed below its maximum allowable
speed of 2,250 r.p.m., the system will stabilize. As explained
above, if the engine speed tends to increase beyond this maximum
speed, the engine speed reference 55 will supply a shutdown signal
to line 52 through safety relay 148.
Referring to FIG. 5 there is shown a relationship between vehicle
speed and the generator voltage during retarding operation. As
indicated, the voltage will remain relatively constant until the
vehicle reaches approximately one-half speed at which time the
generator voltage will then proportionally decrease to 0 relative
to speed.
Shown in FIG. 6 is the relationship between the retarding torque
and the vehicle speed during retarding. As indicated between
maximum speed and one-half speed, the retarding torque will
increase from a minimum level, while from one-half speed to
substantially 0 the retarding torque will remain relatively
constant regardless of the speed of the vehicle as long as the
excitation levels of the generator and wheel drive motor are
adjusted to achieve constant reverse current flow. FIG. 8 shows the
relationship between the power pedal position and the horsepower
developed, and as indicated, at mid position neither retarding nor
motoring horsepower is demanded. In a fully depressed position the
maximum generator output will be developed while in the fully
released position the maximum retarding horsepower will be
absorbed.
While the above-described described embodiment has been with
relation to an earthmoving vehicle, obviously the invention can be
applied to any vehicle using an electric drive system. Also, it
would be possible to adapt the retarder arrangement to other types
of drives as, for example, hydraulic drives, wherein the drive
motors can act as energy generators in order to feed power back to
the normal energy generating source. Also while a particular
control system has been described, obviously other types of control
systems can be utilized. The important feature of this invention is
the use of a constant torque retarding means in combination with a
control system which operates to switch the retarder from a normal
condition to a full operating condition in a relatively short time.
Further, the switching is accomplished in response either to the
actual vehicle speed or command of the vehicle operator, and in
addition, the excitation of levels of the generator and motor
fields are adjusted to achieve substantially constant reverse
current flow from the wheel drive motors to the generator down to
almost zero vehicle speed in the retarding mode. This is an
advantageous feature when compared to conventional full-electric
dynamic brake systems where reverse current drops substantially and
braking torque reduces proportionally with decreasing vehicle
speed.
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