The present invention relates to dc motor controllers and, more particularly,
to control systems for dc motors having separately excited armature and field windings.
Material handling trucks fall into one of several power plant categories.
One such category is the electric vehicle, the energy source for which is a lead-acid
battery that can weigh many thousands of pounds. Besides providing the energy
source to the vehicle, in many instances the battery also provides vehicle counterbalance.
The ratio of the load weight to the gross unloaded vehicle weight
of industrial lift trucks is extremely important. For example, if an unladen vehicle
weighs 12,000 lbs, and the maximum load weight it can carry is 4,000 lbs, then
the gross unladen/laden weight may vary from as little as 12,000 to as much as
16,000 lbs. This represents a change of 33% in motor torque requirements. Moreover,
the vehicle must be able to maneuver on loading ramps, further increasing the
motor torque requirements. For these and other reasons, it is desirable to have
a control system capable of extracting precise and efficient work from the vehicle.
The main motive element of this type of vehicle, referred to as the
traction system, conventionally consists of a series-wound dc motor coupled to
a gear reducer and drive wheel. Some electric vehicles utilize a single "steer-drive"
traction system, while others employ a "dual-drive" (differential) traction system.
The rotational direction of the series-wound dc motor is controlled
by the polarity orientation of the field winding with respect to the armature.
Under conventional control, the field winding orientation is controlled through
a pair of contactors, such that when power is applied across the field-armature
combination, the motor is caused to rotate in the desired direction.
The series-wound dc motor, heretofore used extensively in industrial
lift trucks, displays one very important characteristic: it has extremely high
torque at zero speed. This is extremely important, because it provides the necessary
Under conventional control, the field-armature combination is controlled
as a single unit. Motor speed regulation is achieved through voltage switching
typically utilizing such power semiconductor technologies as silicon-controlled-rectifiers
(SCR). The voltage drop associated with the SCR as well its duty cycle limit impose
a speed limit on the motor. To extract the maximum speed from the motor and reduce
overall system power loss, a bypass contactor is utilized across the SCR, thereby
placing the motor's field-armature combination in series with the battery.
Under such a control scheme, however, the series dc motor does have
one major drawback: it may operate only along its characteristic commutation curve
limit. This results in motor speed variations due to changing torque loading arising
from variations in load capacities, travel path conditions and grade variations.
With the proper controls, the use of a shunt-wound dc motor under
independent field and armature control can provide distinct advantages over conventional
series-wound dc motors for lift truck applications. The control method of the
present invention provides the shunt-wound dc motor with the ability to simulate
a series-wound dc motor, hence developing the necessary starting torque.
The separately excited dc motor represents a highly coupled multi-input,
multi-output, non-linear, dynamic system or plant. It is highly coupled in the
sense that, when one of its inputs is changed, all of the outputs are affected.
This is undesirable, since the purpose of control is to knowingly and intentionally
affect the desired output(s) only, without altering other output states.
United States Patent No. 4,079,301 issued to Johnson, III discloses
a dc motor control circuit having separately excited armature and field windings.
The control circuit is operable in both the constant torque and constant horsepower
modes. The transfer characteristics of the circuit provide high gain at low frequencies
and low gain at higher frequencies. The circuit can further reduce the gain at
low frequencies when motor operation switches from the constant torque mode to
the constant horsepower mode.
United States Patent No. 3,694,715 issued to Van Der Linde et al
discloses a contactless dc motor reversing circuit. The current from a variable
frequency, pulsed dc source is applied to the series field by a pair of solid
state switching devices for forward motor rotation. A second pair of solid state
switching devices applies current for reverse motor rotation. Common to both switching
devices is a third switching device which normally carries the induced armature
current between pulses. It is de-energized during transfer of conduction between
both pairs of switching devices, assuring that the blocking state of one pair
occurs before the second pair is turned on.
United States Patent No. 4,264,846 issued to Sauer et al discloses
a speed control braking circuit for a dc motor. The field and armature currents
are independent of each other to allow motor operation in the field weakening
region. The armature current is set by a pulsing dc element. The field winding
is contained in a series circuit with a switch which is connected in parallel with
the dc element. Shunted across the field winding is a field current bypass diode.
It would be advantageous to provide a motor control system capable
of producing variable torque while maintaining constant speed.
It would also be advantageous to provide a system in which the characteristics
of a series-wound dc motor could be simulated using a shunt-wound dc motor.
It would further be advantageous to provide a system in which a traction
motor's field and armature windings are separately excited and controlled.
It would also be advantageous to provide a system in which the motor
can be controlled by a decoupling controller.
It would still further be advantageous to provide a system in which
the decoupling controller is achieved using software.
Independent field and armature control enables control of a motor
anywhere along, and below its characteristic commutation curve limit. While a bypass
contactor may be employed across the armature voltage switching device to reduce
power losses, independent field control extends controllability of the motor, thereby
making the system less sensitive to variations in load capacities, travel path
conditions and grade variations.
A shunt-wound dc motor is the main motive mechanism replacement for
the traditional series-wound dc motor. The shunt-wound dc motor's field windings
require far less current than its series-wound counterpart, thereby making it
economically feasible to apply full variability (voltage switching) field control.
Field and armature voltage switching is achieved through the utilization
of power transistors as opposed to the traditional SCR's. Although SCR's provide
an inexpensive means of voltage switching, they are limited in switching speed
and require additional circuitry due to their non-self-commutating characteristics.
In accordance with the present invention, there is provided a system
for controlling separately excited shunt-wound dc motors, where control is achieved
through microprocessor-based independent pulse-width-modulation (PWM) control
of a chopper (armature) and an H-bridge (field). Connected to the armature is an
armature voltage amplifier for varying the applied armature voltage. A field voltage
amplifier is also provided for varying the voltage applied to the field winding.
A first sensor is connected to the motor armature in order to determine the motor
rotational speed. A second sensor is connected to the armature circuit in order
to determine the armature current. A third sensor is connected to the field circuit
in order to determine the field current. A decoupling controller uses the motor
speed and armature current information and adjusts the armature voltage and the
The use of such a system results in many benefits including, but
not limited to, precise velocity control, precise torque control, optimized efficiency,
increased performance, increased reliability and decreased cost-of-ownership.
A complete understanding of the present invention may be obtained
by reference to the accompanying drawings, when taken in conjunction with the detailed
description thereof and in which:
- FIGURE 1 is a graphical representation of a typical speed-torque relationship
for series-wound dc motor;
- FIGURE 2 is a block diagram of a multi-variable coupled system representation
of a shunt-wound dc motor showing its particular internal channel transfer functions;
- FIGURE 3 is a schematic diagram of the preferred embodiment of a dc motor control
circuit in accordance with the present invention;
- FIGURE 4 is a block diagram of the decoupling control system; and
- FIGURE 5 is a flow chart of decoupling controller operation.
Before describing the preferred embodiment of the present invention,
it is desirable to discuss briefly the speed-torque characteristics of a series-wound
Referring now to FIGURE 1, there is shown a typical speed-torque
graph for a series-wound dc motor showing the characteristic commutation limit,
field current If lines, and the armature current Ia lines.
As discussed above, under conventional control a dc motor is restricted
to operation along its characteristic commutation limit as represented by the motor
rotational speed Wm and motor shaft torque loading /L. Hence,
as can be seen from FIGURE 1, a change in /L results in a change in
However, under independent and fully variable field and armature
control, a change in /L may not necessarily result in a change in Wm.
Rather, a control system in accordance with the present invention, and described
in greater detail hereinbelow, can select a new motor operating point through
Ia and If under the commutation limit resulting in an unchanged
Wm for the new torque loading value /L.
Referring now to FIGURE 2, there is shown a block diagram of a multi-variable
coupled system representation of a series- or shunt-wound dc motor.
An applied armature voltage Va and an applied field voltage
Vf are input to a motor 10. The physical transformations within the
motor 10 may be suitably represented by cross channel transfer functions 12, to
produce the motor rotational speed Wm and armature current Ia
From an analysis of a series- or shunt-wound dc motor, it can be
seen that the motor 10 consists of two first-order and two second-order dynamic
systems. Such a motor system may be represented in the s-domain (Laplace transformation)
by the transfer functions G&sub1;&sub1;(s), G&sub1;&sub2;(s), G&sub2;&sub1;(s)
These transfer functions 12 are representative of the particular
channels of the motor system defined as follows,
G&sub1;&sub1;(s) = Wm(s)/Va(s)
G&sub1;&sub2;(s) = Wm(s)/Vf(s)
G&sub2;&sub1;(s) = Ia(s)/Va(s)
G&sub2;&sub2;(s) = Ia(S)/Vf(s)
where G&sub1;&sub1;(s) and G&sub1;&sub2;(s) are first-order systems, and G&sub2;&sub1;(s)
and G&sub2;&sub2;(s) are second-order systems. Determination of these transfer
functions 12 is analytical as well as experimental.
Referring now also to FIGURE 3, there is shown a schematic diagram
of a dc motor control circuit, shown generally as reference numeral 14, which provides
independent control of a series- or shunt-wound dc motor by independently controlling
its armature winding 44 and field winding 42. A load (not shown) is driven by the
A suitable means for providing a feedback signal proportional to
the motor rotational speed is indicated by encoder 43, which is connected to armature
44. It will, of course, be obvious to those skilled in the art that encoder 43
is merely exemplary and that other devices or methods can be employed to perform
the same speed sensing function.
The primary components of motor control circuit 14 are a chopper
circuit 18 which controls armature winding 44 and an H-Bridge circuit 16 which
controls field winding 42. Two pairs of transistors 20, 24 and 26, 22 are connected
to field winding 42, as shown.
Power is supplied to motor control circuit 14 by a dc battery 48.
A main power contactor 56 is connected to battery 48 and chopper circuit 18 and
H-Bridge circuit 16. Main contactor 56 enables system shut down should any system
A chopper circuit fuse 52 is connected between main contactor 56
and chopper circuit 18 to limit excessive current to chopper circuit 18. An H-Bridge
circuit fuse 54 is connected between main contactor 56 and H-Bridge circuit 16
to limit excessive current to H-Bridge circuit 16.
Power regulation through armature winding 44 and field winding 42
is achieved through transistors 19 (in chopper circuit 18) and transistors 20,
22, 24 and 26 (in H-Bridge circuit 16). Control of transistors 19, 20, 22, 24 and
26 is achieved through driving circuits 17, 21, 23, 25 and 27, respectively. Motor
rotation direction is dictated by the field winding 42 orientation with respect
to the armature winding 44. Field winding 42 orientation is controlled by transistor
pairs 22, 26 and 20, 24.
The ON-OFF ratio of transistors 19, 20, 22, 24 and 26 results in
an average applied terminal voltage to armature winding 44 and field winding 42,
respectively. As such, totally independent and fully variable control of armature
winding 44 and field winding 42 is achieved.
Polarized snubber circuits 36 and 38 are provided in H-Bridge circuit
16 and chopper circuit 18 respectively to:
- a) absorb switching power losses of transistors 19 (in chopper circuit 18),
and transistors 20, 22, 24, 26 (in H-Bridge circuit 16);
- b) prevent secondary breakdown due to localized heating effects during turn-on
and turn-off of transistors; and
- c) prevent spurious turn-on of transistors due to dV/dt.
Free wheeling diodes 28, 29, 30, 32 and 34 provide a path for current
upon turn-off of transistors 19, 20, 22, 24 and 26, respectively. Another free
wheeling diode 35 is provided across armature 44, also to provide a current path
when chopper circuit transistor 19 is turned off.
A dI/dt limiting inductor 37 is provided between H-Bridge circuit
fuse 54 and H-Bridge circuit 16 to restrict the rate of rise of current through
the H-Bridge circuit 16. This dI/dt limiting inductor 37 protects the H-Bridge
circuit transistors 20, 22, 24 and 26 from armature voltage spikes. A pair of
back to back breakdown diodes 47, 49 and a resistor 45 form a tranzorb 40 across
field winding 42 to limit the field voltage.
A regeneration diode 46 connected across transistor 19 provides recirculation
of load current back to battery 48 during part of the motor deceleration cycle.
A bypass contactor 50 connected across transistor 19 eliminates the
power loss in transistor 19 during sustained high speed travel.
Referring now also to FIGURE 4, there is shown a schematic block
diagram of the separately excited dc motor decoupling control system.
Armature voltage control amplifier 86 adjusts armature voltage Ia
94, which causes the speed of motor 44 to vary. Encoder 43 is connected to motor
44 to sense rotational speed thereof and to generate a continuous signal representative
of such new motor rotation speed Wm.
A programmable, microprocessor-based decoupling control system is
shown generally at reference numeral 71. The functions of control system 71 can
be accomplished by a processor such as a Model No. 68HC11 microprocessor manufactured
by Motorola Corp. The unfiltered motor rotational speed reference Wmref
and the unfiltered armature current reference Iaref are input via respective
lines 68 and 70 to control system 71 and more specifically to a filter 72.
Within filter 72 is a 2x2 filter matrix Q 74. Using basic matrix
algebra, the desired motor rotational speed reference Wmref and desired
armature current reference Iaref
are transformed to produce filtered input
references R&sub1; and R&sub2; applied to lines 76 and 78.
Also within processor control system 71 is a controller 100. Motor
outputs of armature current Ia and motorz rotational speed Wm
are input via respective lines 96 and 98 to controller 100. Within controller 100
is a 2x2 feedback controller matrix F 99. Since 1x2 matrix y consists of signals
Ia and Wm, by performing basic matrix algebra, controller
100 produces conditioned motor outputs Fy&sub1; and Fy&sub2; applied over lines
101 and 103.
Filtered input references R&sub1; and R&sub2; enter summers 80 and
82 over lines 76 and 78, respectively. Also entering summers 80 and 82 over lines
101 and 103 are conditioned motor outputs Fy&sub1; and Fy&sub2;, respectively.
Summer 80 produces an armature control voltage Varef applied to line
84. This can be represented mathematically as Varef = R&sub1; - Fy&sub1;.
In the same manner, summer 82 produces a field control voltage reference Vfref
applied to line 88. Together, armature control voltage Varef and field
control voltage Vfref form the system control effort as a function of
filtered input references R&sub1;, R&sub2; and conditioned motor outputs Fy&sub1;,
Fy&sub2;. The existing control effort is then applied to the motor 44 and field
42 as follows.
Armature control voltage reference Varef enters an armature
voltage control amplifier 86, which amplifies armature control voltage reference
Varef to produce armature voltage Va 94, which is then applied
to motor 44, which provides an armature current Ia over line 96. While
the motor is generally referred to as reference numeral 44, also included in the
motor are armature resistance Ra and armature inductance La.
Field control voltage reference Vfref enters a field voltage
control amplifier 90, which amplifies field control voltage reference Vfref
to produce a field voltage Vf 92, which is then applied to field 42.
While the field is generally referred to as reference numeral 42, also included
in the field are field resistance Rf and field inductance Lf.
Referring now also to FIGURE 5, there is shown a flow chart of decoupling
controller operations. It should be noted that the diagram represents only one
of a series of repeating cycles.
Data representative of armature current Ia and motor rotational
speed Wm is entered, step 110.
Desired output matrix R and motor output matrix y are read, step
112. Desired output matrix R is a matrix of unfiltered input armature current and
motor rotational speed references, Ia and Wm. Matrix y is
the actual motor rotation speed Wm and armature current Ia.
Filter matrix Q filters input references, step 114, to result in
filtered input references R&sub1; and R&sub2;. Feedback controller matrix F, step
116, conditions motor outputs Ia, Wm to result in Fy&sub1;,
Fy&sub2;. The applied control voltage (effort) Vref is then calculated
(summed) as the difference between filtered input reference R and conditioned motor
output Fy. Matrix Vref is an armature control voltage reference Varef
and a field control voltage reference Vfref.
The applied control voltage effort Vref is then applied
to motor, step 120, and the cycle repeats.
Since other modifications and changes varied to fit particular operating
requirements and environments will be apparent to those skilled in the art, the
invention is not considered limited to the example chosen for purposes of disclosure,
and covers all changes and modifications which do not constitute departures from
the true spirit and scope of this invention.