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BACKGROUND OF THE INVENTION
This invention relates generally to electric propulsion systems for
self-propelled traction vehicles, and it relates more particularly to such
a system for driving a traction vehicle at controllable speeds.
In one embodiment of the invention to be disclosed hereinafter, the
electric propulsion system is intended to drive earthmoving machines known
technically as "wheel loaders" (standard J1057 of the Society of
Automotive Engineers) and popularly as "front end loaders." A front end
loader comprises a self-propelled vehicle with an integral front-mounted
bucket supporting structure and linkage that loads earth and other
materials into the bucket through forward motion of the vehicle and then
lifts, transports, and discharges the load. Such a machine typically
includes an articulated frame and a four-wheel drive. Both front and rear
axles can be driven by an electrical system comprising a pair of variable
speed reversible d-c motors (each having an armature and a field) which
are energized by a generator coupled to a diesel engine or other suitable
prime mover, and the bucket and its boom can be powered by hydraulic means
including lift cylinders which derive their hydraulic pressure from the
same prime mover. By appropriate manipulation of a speed-control pedal and
a forward-reverse selector lever, an operator can control the electric
drive system so as to determine, respectively, the machine's speed and
direction of movement. The various operating modes of such a system
include propulsion (motoring) or dynamic retardation (braking) in either a
forward or reverse direction, with the bucket either loaded or unloaded;
propelling the machine forward with the bucket down to penetrate a pile of
earth ("crowding"); and lifting the bucket while the wheels are either
stationary or moving forward or backward.
Propulsion systems for front end loaders should preferably have certain
characteristics including: (1) smooth control of torque, (2) minimal wheel
slip for improved tire life, (3) high tractive effort at low speeds to
permit the loader bucket to readily penetrate the pile, termed "full crowd
tractive effort," (4) relatively constant prime mover engine speed to
permit rapid cycling and response of the bucket and boom assembly and to
facilitate engine smoke control, (5) controllable vehicle speed, and (6)
simplified control, such as, for example, to facilitate the repeated
reversals in direction required during operation. Whereas the present
invention will be described in connection with a propulsion system having
the above characteristics and particularly adapted for front end loaders,
it may be utilized in other types of electrical drives including those for
other types of vehicles and those providing certain alternative
characteristics.
SUMMARY OF THE INVENTION
The general objective of the present invention is the provision, in an
electric propulsion system for a traction vehicle including a thermal
prime mover which drives a generator supplying electrical energy to
traction motors propelling the vehicle, of a relatively simple scheme for
controlling the vehicle speed in accordance with the manual setting of a
speed controller.
In carrying out the invention in one form, the electric traction motors of
a traction vehicle are energized by the output of an electric generator
driven by a suitable thermal prime mover. The traction motors are of a
type whose speed, during propulsion of the vehicle, depend on the voltage
output of the generator. The prime mover normally drives the generator at
a relatively constant rate, and a variable excitation source is arranged
normally to supply a predetermined maximum level of excitation to the
generator field. The propulsion control system includes means for
determining voltage and power limits of the electrical output supplied by
the generator to the traction motors and for reducing the generator field
excitation in the event detected levels of voltage and power exceed their
respective limits. A manually adjustable vehicle speed controller is
provided, and the aforesaid voltage limit is modified as a function of the
setting of the speed controller so as to vary with desired vehicle speed
between zero and a predetermined maximum level without changing the
maximum power limit of the generator.
The invention will be better understood and its various objects and
advantages will be more fully appreciated by the following description
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of an electric propulsion system
incorporating the present invention, which system includes a pair of
motors energized by a generator driven in turn by a prime mover;
FIG. 2 is a graphic representation of the relationship between output
voltage and output current of the generator used in the propulsion system
of FIG. 1;
FIG. 3 is a schematic circuit diagram illustrating a preferred embodiment
of the means shown in block form in FIG. 1 for regulating the generator
output; and
FIG. 4 is a schematic circuit diagram illustrating a preferred embodiment
of the means shown in block form in FIG. 1 for controlling motor field.
DESCRIPTION OF PREFERRED EMBODIMENT
FIG. 1 shows a preferred embodiment of an electrical propulsion system
useful for driving a front end loader or the like. To facilitate an
understanding of the main parts of this system and of its overall
operation, the following description is organized under separate headings
and preferred means for implementing certain functions in the system are
shown in greater detail in other figures. After this description, the
specification will conclude with claims pointing out the particular
features of the system that are regarded as the subject matter of the
present invention. Other features of the described system are claimed in
co-pending patent applications filed concurrently herewith and assigned to
the assignee of the present invention. The co-pending patent applications
and their titles are as follows:
______________________________________
S.N. 553,864
Lifting Force Responsive Load Control
For Electrically Propelled Earthmoving
Vehicles
S.N. 553,862
Prime Mover Speed Responsive Load
Control For Electrically Propelled
Traction Vehicles
S.N. 553,861
Field Boost Arrangement For Separately
Excited D-C Traction Motors Of A
Vehicle Propulsion System
S.N. 553,866
Field Tapering Arrangement For Separately
Excited D-C Traction Motors Of A Vehicle
Propulsion System
______________________________________
With particular reference now to FIG. 1, the illustrated propulsion system
includes a front motor 2 and a rear motor 4 which are intended to propel,
or to retard, the front and rear axles, respectively, of the front end
loader by a coupling arrangement schematically indicated by lines 6 and 8.
In lieu of a single motor axle, multiple motors may of course be utilized,
such as, for example, separate electrically powered traction wheels having
their armatures connected in parallel or in series during propulsion, i.e.
motoring operation. The electrically powered traction wheels may be of the
general type disclosed in U.S. Pat. No. 2,899,005 -- Speicher.
Each of the traction motors 2 and 4 is a variable speed, reversible d-c
motor having an armature and a separately excited field. The armatures of
the two motors are connected in parallel for energization by the same
voltage, and as is shown more clearly in FIG. 4, the motor fields are
connected in series with each other for separate excitation by the same
field current. A thermal prime mover 10, such as, for example, a diesel
engine, drives, as is indicated by dashed line 14 in FIG. 1, electrical
generating means 12 which in turn provides an electrical output to the
parallel connected armature means of motors 2 and 4, as schematically
indicated by line 16. A source of field current, field supply means 18,
provides field current, I.sub.F, on line 20, to the series connected field
windings of motors 2 and 4. In the preferred embodiment, the field supply
means 18 is a rotary d-c generator which is also driven by the prime mover
10, as schematically indicated by line 22. The described arrangement
provides substantially identical armature voltages and field currents, and
thus field flux, to the separately excited traction motors so that each
motor maintains identical rotational speeds. This feature assists in
minimizing wheel slip.
Motor performance, i.e. motor torque and speed, is a function of the
magnitude of applied armature voltage, and thus armature current, and
magnitude of field flux, i.e. applied field current. In the preferred
embodiment these parameters are controlled by applying appropriate
excitation to the fields, respectively, of the electrical generating means
12 and of the field supply generator 18. In the illustrated embodiment
field excitation for generating means 12 is provided by exciting generator
24, whose output V.sub.E is coupled through switch 26 to the field circuit
of generating means 12. The output of exciter 24 constitutes an amplified
output of the signal I.sub.F applied on line 28 to excite the field of the
exciter generator. This signal I.sub.F is regulated by a regulating system
described subsequently.
MOTOR FIELD EXCITATION
The magnitude of motor field flux is determined by the field excitation of
field supply generator 18, i.e. the magnitude of field current supplied on
line 30. Preferably the motor field, and thus the field current on line
30, is of predetermined constant magnitude under normal operating
conditions, and this magnitude is selected so that motor flux is above the
knee of its saturation curve. The field current 30 may, for example, be
supplied from a constant current source, such as a battery, connected
serially with a resistance and a field winding of generator 18. As
subsequently described, however, improved performance is obtainable by
automatically modifying the field current under special conditions. One
occurs when unusual tractive effort is required, such as needed for the
bucket to crowd into a pile. A field current boost circuit responsive to
traction motor currents in excess of a predetermined magnitude
automatically provides additional field excitation to increase motor
torque. This permits attainment of adequately high motor torque while
maintaining motor armature current within desired limits. The other
condition occurs when additional vehicle speed is required when maximum
armature voltage is supplied to the traction motors. The field current is
then automatically reduced to provide field weakening and extended speed
operation. The summation circuit 32 of FIG. 1 functionally presents an
arrangement for thus modifying field energization. The previously
referenced normal state field current, I.sub.E, is illustrated as being
supplied from forward-reverse switch 34 and line 36 to summer 32. Under
normal conditions this constitutes the sole excitation of the field supply
generator. Double pole switch 34 functionally illustrates an arrangement
for reversing the rotational direction of the traction motors for
reversing the vehicle. Motor reversal is obtained by reversing the field
excitation current, such as by reversing the connections between the field
winding and the source of potential supplying the field current. Obtaining
reversal by switching the relatively small field current supplied to the
field supply generator, permits use of switching devices, such as
contactors, having limited current carrying capability.
In order to obtain the above-referenced field current boost, a current
boost signal, I.sub.BOOST, is applied from the field current boost
circuit, comprising OR gate 38 and hold off gate 40, by line 42 to summer
32. Motor armature current signals on lines 56 and 58 are supplied to OR
gate 38 which supplies the current signal of highest amplitude I.sub.M on
line 39 to hold off gate 40. Gate 40 provides an output I.sub.BOOST on
line 42 in the event signal I.sub.M exceeds a predetermined threshhold.
Preferably I.sub.BOOST increases proportionately with further increases of
signal I.sub.M.
In order to obtain the above-referenced field weakening, a field weakening
signal, I.sub.T, is applied from a field weakening circuit, comprising
hold off gate 44, by line 46 to summer 32. Hold off gate 44 receives an
input V.sub.G representative of traction motor voltage or output voltage
of generating means 12. In the event this signal exceeds a predetermined
magnitude, preferably near the maximum rated voltage, gate 44 supplies an
output signal I.sub.M on line 46.
As indicated in FIG. 1, the boost current on line 42 is combined
additively, and the field weakening current on line 46 is combined
subtractively, with the normal field current I.sub.E on line 36. In the
preferred embodiment the above-described summation function of summer 32
is in fact achieved by utilizing plural field windings on the field supply
generator 18.
GENERATOR REGULATING SYSTEM
In order to explain additional features of the system of FIG. 1, reference
is now made to the regulating system which controls the output of the
electrical generating means 12. As is known, the voltage output of means
12 must be maintained within a predetermined magnitude, primarily to
protect the field windings of generating means 12 and to prevent
dielectric breakdown of the insulation of the entire traction system.
Further the current output of means 12 must be maintained within a
predetermined magnitude to protect the armature of generating means 12 and
other circuit components. In addition the power output of the generating
means 12 must be kept within a predetermined power, e.g. "horsepower,"
limit to prevent overloading the prime mover engine 10 and to prevent
stalling of the engine. Essentially the voltage and current output of the
generating means 12 are dependent on the load, i.e. the performance of the
traction motors, and are independent of each other. For example, when the
vehicle accelerates from standstill the high torque requirements result in
high armature current, i.e. the load impedance is very low, and the
current must be limited. On the other hand at high speed and minimal
torque operation, the traction motors develop substantial counter voltage,
i.e. back emf. This is equivalent to increasing the load impedance.
Accordingly the voltage output of the generating means increases at high
speeds and voltage limiting is required. At intermediate levels of
operation, the power output of the generating means must be limited. The
horsepower output limit, being a function of the products of voltage and
current outputs, is hyperbolic in form. The resulting idealized operating
envelope is illustrated in FIG. 2, a plot of the output voltage vs. the
output current of generating means 12. Line F--G represents the voltage
limit, line H--I represents the current limit, and hyperbolic curve
portion G--H represents the horsepower limit portion of the envelope. The
regulating system assures that the output of the generating means does not
exceed the limits prescribed by the abovedescribed envelope. This is
achieved essentially by deriving signals representative of the voltage and
current outputs of the generating means, processing these voltage and
current signals to provide a signal which is a function of the power
output of the generating means, and comparing these signals with
appropriate reference signals to derive a control, or error, signal. The
control signal controls the field excitation of the generating means to
maintain output within the desired generator voltage and current envelope,
i.e. within predetermined maximum voltage, power, and current parameters.
In the preferred embodiment illustrated in FIG. 1, line 16 provides
voltage feedback signals representative of the actual voltage V.sub.G
applied from the output of the generating means to the armature circuit of
the traction motors, and line 52 provides current feedback signals
representative of the actual current output supplied by the generating
means to the armature circuit of the traction motor means. As illustrated
in FIG. 1, this current feedback signal may be derived by detecting
signals representative of the armature currents of the front motor means
2, on line 56, and of the rear motor means 4, on line 58, and summing
these signals, I.sub.M1 and I.sub.M2, in summation device 60 so as to
provide the above-referenced current feedback signal, I.sub.G, on line 52.
The voltage feedback signal on line 16, and the current feedback signal on
line 52 are processed by devices 62 and 54 to generate a signal on line 66
which varies appropriately as a function of the power output of the
generating means, and thus may be termed a power feedback signal. The
voltage signal on line 16 is supplied to device 46 where it is subject to
modification in a manner to be described subsequently. The voltage signal
output of device 46, I.sub.VMR, on line 48, and the power feedback signal
I.sub.CMR on line 66 are supplied to a comparison circuit comprising
devices 50 and 70. The comparison circuit compares the voltage feedback
signals and the power feedback signals with a reference signal, I.sub.REF,
applied by line 72 to device 70, to provide, at its output line 74, a
control signal, I.sub.CONT. The control signal is appropriately modified
by devices 76 and 78 to provide on line 28 an exciter field current
I.sub.F which is supplied to the field of the exciting generator 24. The
control signal produced by the comparison circuit thus modifies the field
excitation of the generating means to limit its output within the
predetermined maximum voltage, power, and current limits which were
described with reference to FIG. 2. The above-described aspects of the
generator regulating system of the preferred embodiment are for the most
part disclosed in U.S. Pat. No. 3,105,186 and in Parts 12 through 14 of
"Electronics on the Rails" by Robert K. Allen published in "Railway
Locomotives and Cars" about 1966-1967.
Reference is again made to FIG. 1 for a further description of the
regulating system. The voltage feedback signal on line 16 is supplied to
one input of summer 46, a voltage measuring reactor (VMR). As subsequently
described a speed control member 92, such as a foot pedal, controls the
output of voltage control circuit 94, voltage control signal I.sub.VC,
which is applied by line 96 to a second input of summer 46. The output of
device 46, a current I.sub.VMR, is supplied by line 48 to one input of
gate 50. The current feedback signal I.sub.G on line 52 is supplied to one
input of summer 54, a current measuring reactor (CMR). The output of
summer 54, a current I.sub.CMR on line 66, is supplied to a second input
of gate 50. The output of the gate 50, consisting of the input signal
having the larger amplitude, is supplied by line 68 to one input of summer
70.
A reference current signal, I.sub.REF, on line 72, is applied to another
input of summer 70 so as to be subtractively combined with the signal on
line 68. Under normal conditions of vehicle operation, the signal
I.sub.REF corresponds to signal I.sub.PM on line 98 which is generally
representative of normally available horsepower output of the prime mover
10 and in the preferred embodiment has a predetermined constant magnitude.
However, as subsequently explained, the reference current signal is
subject to modification such as when the vehicle is engaged in penetrating
and lifting earth matter. Under such conditions the reference current
signal I.sub.REF is subject to modification responsive to the lifting
force applied to the earth moving means, e.g. boom and shovel. The
arrangement for thus modifying the reference current signal comprises
devices 100, 102 and 106.
The gate 50 and summer 70 thus constitute a comparison circuit which
selected the greater one of the output signals of the VMR summer and of
the CMR summer and compares the greater of these signals with the
reference signal I.sub.REF to produce a control current signal I.sub.CONT,
on line 74. In the preferred embodiment the control current signal is
produced only if the larger one of the VMR and CMR output signals has a
greater amplitude than the reference current signal. In the preferred
embodiment the comparison circuit comprising the gate 50 and summer 70 is
a reference mixer bridge circuit of the type disclosed in U.S. Pat. No.
3,105,186.
The above-described arrangement assures that the output of the generating
means is within predetermined maximum voltage and predetermined maximum
current limits. It assures that the output voltage of the generating means
cannot, for example, exceed the voltage level defined by line F--G of FIG.
2, and that the output current cannot, for example, exceed the current
level defined by line H--I of FIG. 2. For example, if the traction vehicle
operates at high speed the traction motors develop a substantial counter
emf, thus causing a high generator output voltage. If the generator output
voltage approaches the predetermined maximum limit, the voltage feedback
signal will exceed the current feedback signal. In the event the voltage
feedback signal exceeds the reference current signal, comparison of this
voltage feedback signal with the reference current signal produces a
control current signal which reduces excitation and prevents further
increase of the generator output voltage. Similarly under high load
current conditions, as encountered during low vehicle speeds, the current
feedback signals, exceeding the voltage feedback signals and reference
current signals, produces control current to reduce excitation.
In addition to the above-described arrangement for limiting voltage and
current output of the generating means, it is necessary to assure that the
generator output does not exceed a desired power level, such as for
example, the power output defined by segment G-H of FIG. 2. For this
purpose the preferred embodiment utilizes function generator 62. Its input
is the voltage feedback signal V.sub.G on line 16. The output of the
function generator, current I.sub.FG, is supplied by line 64 to another
input of summer 54, the current measuring reactor CMR. Current I.sub.FG
modifies the output of the CMR summer 54, i.e. the current I.sub.CMR on
line 66, which would otherwise be solely proportional to the current
reference signal I.sub.G on line 52 and thus to the armature current of
the traction motor means. Operation of the function generator 62 is now
described in connection with FIG. 2. If the generator output voltage is
within the voltage amplitude defined, for example, by segment I--H of FIG.
2, the function generator 62 provides no output voltage and the CMR output
signal, I.sub.CMR, is unaffected by the function generator. Under these
circumstances the regulator limits output current to within the magnitude
defined by segment H--I. However, as the generator output voltage, and the
voltage feedback signal on line 16 increase, an increasing signal is
applied by line 64 to CMR summer 54 and thus is added to the current
feedback signal I.sub.G, such that the output signal of summer 54, i.e.
current I.sub.CMR on line 66 is greater than that which would have been
produced solely by the current feedback signal. This increase of the CMR
summer output with increasing generator voltage causes the maximum
generator output current to decrease with increasing generator output
voltage. Therefore the current limit of the generating means output
approximates segment H--G of FIG. 2 instead of being maintained at a
constant value such as defined by segment I-H. The summer 54 signal
I.sub.CMR, under such conditions, limits the output power of the
generating means and therefore constitutes a power feedback signal.
The output of the comparison circuit, i.e. the control signal I.sub.CONT
output of summation circuit 70, is applied by line 74 to an amplification
system which provides an appropriate excitation signal for the generating
means 12. In the preferred embodiment illustrated in FIG. 1, the control
signal in line 74 is applied to one input of summer 76, whose output
current I.sub.PWM is applied by line 80 to function circuit 78. The output
of function circuit 78, the excitation control current I.sub.F, is as
previously described, applied by line 28 to the exciter 24 so as to
energize the field of generating means 12.
In the preferred embodiment the summer 76 comprises a pulse width modulated
(PWM) amplifier, and in particular a magnetic PWM, of the general type
disclosed in U.S. Pat. Nos. 2,886,763 and 3,105,186. Such a device
produces a train of square wave pulses whose duty cycle is varied, i.e. by
modifying the time duration or width of the respective pulses. It
comprises a saturable transformer excited by a square wave oscillator with
a tapped secondary winding connected in a full wave rectifier circuit to
the function circuit 78. The main windings of a controlled saturable
reactor are connected in circuit with the end terminals of the secondary
windings. Control windings of the controlled saturable reactor apply
controlling signals, including the control signal I.sub.CONT, to summer
76, as illustrated by lines 74, 84, and 90 in FIG. 1. Line 74 supplies the
previously described control signal I.sub.CONT. Line 84 supplies a rate
feedback signal I.sub.RP, preferably derived by coupling the output signal
V.sub.E of exciter 24 through rate feedback circuit 82. This provides
system stability by limiting accelerating rates and compensating for the
long time constant of the generator field in respect to the control system
response time.
Additionally, as subsequently described in the section "Prime Mover Speed
Responsive Load Control," an electrical signal .omega. responsive to the
shaft speed of prime mover 10 is preferably supplied to the load control
circuit 88. In the event the prime mover is overloaded such that its
rotational speed decreases below its rated speed, load control circuit 88
produces a load control signal I.sub.LC which is coupled by line 90 to
summer 76 so as to reduce excitation.
The presence of control signals on lines 74, 84, or 90 varies the core
saturation of the previously described saturable reactor such that the d-c
signal output of summer 76, which is applied to the function circuit 78,
varies inversely with the summation magnitude of the control signals
applied to summer 76. When no control signals are applied to the control
windings, the reactor cores are saturated such that a maximum positive
signal is applied to the input of the function circuit, permitting up to
maximum excitation of the generating means. The application of control
signals, such as control signal I.sub.CONT, will reduce the flux in the
cores. Thus the output signal, I.sub.PWM, is reduced proportionately to
the sum of the amplitudes of the applied control signals, and the
excitation of the generating means is reduced. Block 78 of FIG. 1
illustrates the preferred transfer function of the function circuit 78.
For purpose of explanation it is assumed that the control signals applied
to summer 76 are zero, e.g. the reference current on line 72 exceeds the
larger of the feedback signals applied to the input of gate 50. In this
case the output of summer 76, current I.sub.PWM is at a minimum amplitude
and the excitation field current I.sub.F is at a maximum positive value.
As the control currents, e.g. I.sub.CONT, applied to summer 76 increase,
the current I.sub.PWM increases proportionately and the excitation field
current I.sub.F falls off rapidly. With further increase of the control
currents, and resulting decrease of current I.sub.PWM, the excitation
field current decreases to zero and subsequently reverses in polarity
until it levels off at a predetermined negative amplitude. This reversal
of excitation field current provides a fast reduction of generating means
output, and provides for overcoming the residual flux of the generating
means so as to permit operation, if desired, at substantially zero output
voltage. As previously stated the output of summer 76, as used in the
preferred embodiment constitutes a train of sequential pulses whose time
duration is minimum when the reference current on line 72 exceeds the
larger of the feedback signals applied to gate 50, i.e. under conditions
when the output of the generator need not be reduced. The above-recited
increase of I.sub.PWM is accomplished by increasing the time duration of
the individual pulses, e.g. increasing the average value of the signal on
line 80.
PRIME MOVER SPEED RESPONSIVE LOAD CONTROL
Reference was made in the preceding section "Generator Regulating System"
to a load control arrangement wherein overloading of the prime mover
produces a load control signal I.sub.LC which is coupled by line 90 to
summer 76 so as to reduce excitation. Such an arrangement is advantageous
in traction vehicles where the prime mover must supply a variable
auxiliary load, i.e. a load additional to the traction motor propulsion
system. For example, in front end loaders, the prime mover, e.g. diesel
engine, also energizes the hydraulic system for moving the boom and bucket
assembly. The load thus imposed on the prime mover varies considerably
being maximum when the bucket penetrates the pile of earth matter and the
hydraulic system is utilized to lift the boom and bucket. Under the latter
conditions the engine is subject to bogging and speed reduction. When the
prime mover is overloaded under such conditions, the excitation of the
generating means, and thus the electrical load, is reduced in the manner
described below.
The prime mover 10 normally operates at a predetermined relatively constant
speed controlled by known types of governor systems (not illustrated). The
shaft speed .omega. of the prime mover 10 is detected, and a load control
means 88, responsive to a shaft speed signal, is arranged to generate the
load control signal I.sub.LC whenever the shaft speed is abnormally low,
i.e. is below a first predetermined angular velocity .omega..sub.a. As the
shaft speed increases above .omega..sub.a, the load control signal
I.sub.LC varies as a suitable inverse function of speed until the load
control signal is reduced to zero at a second angular velocity
.omega..sub.b which, depending on the particular application of the
propulsion system, can be either lower or higher than normal. In one
embodiment, for example, the normal, loaded prime mover speed as
determined by the governor, is approximately 2100 rpm, and the load
control signal I.sub.LC is zero until the prime mover looses speed to
2,050 rpm. I.sub.LC then increases with decreasing speed to 2,000 rpm and
thereafter, for lower speeds, remains at a predetermined substantially
constant level. The load control signal I.sub.LC is applied from load
control 88 by line 90 to summer 76 so as to reduce excitation of the
generator 12 when the shaft speed is below .omega..sub.b. The signal loop
comprising prime mover 10, load control 88 and components 76, 78, 24, and
12 constitute a closed loop circuit which if desired can maintain
operation along the slope of the load control signal within the range of
speeds defined by .omega..sub.a and .omega..sub.b. Thus when auxiliary
loads are applied, the electrical load of the prime mover is modified to
minimize engine bogging.
The prime mover shaft speed is preferably detected by a speed sensor
providing an analog output. For example, a magnetic speed sensor can be
used to provide a pulse output whose frequency is proportional to engine
speed. The pulse signal is applied to a digital to analog converter.
Arrangements of this type are well known in the art, including for
example, peak clipping circuits, such as a saturating transformer,
providing input signals to a single shot trigger circuit. The output
pulses of the trigger circuit are integrated to provide an analog signal
having an amplitude proportional to the prime mover speed.
The analog signal is supplied to a transistor amplifier, in load control
circuit 88. The amplifier is biased to normally conduct and to produce a
predetermined output current with no applied input signal. The amplifier
is biased such that conduction is decreased, and output current reduced,
when the analog signal is proportional to speed in excess of
.omega..sub.a. Conduction is cut off and the output current is zero when
the analog signal is proportional to a speed .omega..sub.b. Thus load
control circuit 88 has a sharp cut off characteristic such that the load
control signal I.sub.LC is strongly increased as a result of a relatively
small reduction in prime mover speed.
It should be noted that this arrangement for modifying excitation does not
in any manner modify the maximum available traction motor voltage and
current limits as established by I.sub.REF, I.sub.VMR, and I.sub.CMR (as
described in the section entitled "Generator Regulating system"). For
example, as a front end loader penetrates a pile it exerts maximum torque
but operates at a very low speed. The horsepower output of the generating
means is a function of the product of speed, i.e. armature voltage, and
torque, i.e. armature current. Therefore under such conditions, the
horsepower output of the generating means is generally below the maximum
available horsepower. However, armature current is maximum under such
conditions. When the hydraulic system is concurrently activated to move
the boom and bucket, the load on the prime mover is suddenly increased. In
response to the resulting reduction of prime mover shaft speed, the
above-described load control system reduces the load of the generating
means. This is accomplished, however, independently of the parameters
(I.sub.REF, I.sub.VMR, and I.sub.CMR) of the regulating system which
produces control signal I.sub.CONT. Accordingly, the maximum available
current limit is not modified, and the traction motors can utilize maximum
armature currents. Similarly at high speed operation, the prime mover
speed responsive load control system does not reduce the maximum available
armature voltage.
VOLTAGE LIMIT SPEED CONTROL
Operation of certain off-highway traction vehicles, such as front end
loaders, is subject to sudden and substantial modifications of propulsion
torque. For example, front end loaders may travel under conditions
requiring relatively low propulsion torque, but may suddenly and
repeatedly penetrate piles of earth matter so as to be subjected to
repeated major increases of propulsion torque. These repeated rapid and
substantial variations in propulsion torque make it desirable to provide
for automatic regulation of torque and to make operator control of the
traction vehicle substantially independent of torque, while assuring that
the previously described predetermined maximum voltage, current, and
horsepower output limits of the generating means 12 are not exceeded. The
previously described regulating system compares feedback signals
representative of generator voltage and current to derive a power feedback
signal and compares these with reference signals to generate control
signals to limit the output of the generating means within such
predetermined maximum voltage, current, and horsepower limits. Operator
control of the vehicle is attained by a moveable control member connected
to produce a voltage control signal representative of the position of the
control member. This voltage control output signal is coupled in circuit
with the regulating means for comparing the voltage feedback and reference
signals so as to reduce the maximum voltage output of the generating means
below the predetermined maximum voltage limit, with minimal modification
of the maximum horsepower and current limits of the generating means. In
the arrangement of FIG. 1, the position of the moveable control member 92
modifies the output, I.sub.VC, of the voltage control circuit 94. A
preferred embodiment of the latter circuit will be described below in the
section "Voltage Control Circuit." The voltage control signal I.sub.VC is
added by summer 46 to the voltage feedback signal V.sub.G, such that
signal I.sub.VMR, the output of summer 46, limits the maximum generator
output to an output voltage, and thus to a vehicle speed, determined by
the position of the control member. Member 92 preferably is a foot pedal
normally spring biased in its upper most position. When the pedal is in
this position, the level of signal I.sub.VC corresponds approximately to
the signal V.sub.G which is produced without presence of an I.sub.VC
signal with maximum predetermined voltage output of the generating means.
Signal I.sub.VMR, the output signal of VMR summer 46, in such case would
normally exceed signal I.sub.CMR, the output of CMR summer 54, and would
exceed reference current signal I.sub.REF, such that summer 70 of the
comparison circuit would produce a control signal I.sub.CONT sufficient to
reduce the maximum voltage output of the generating means to a
predetermined minimum level, e.g. slightly above zero volts. As the pedal
is depressed by the operator, the signal I.sub.VMR is reduced and the
maximum available voltage output of the generating means increases
proportionately with the amount of pedal depression.
Operation of the vo | | |
