Regenerative braking apparatus and method for direct current motors

A regenerative braking control apparatus for use with a motor driven by a D.C. power source. In response to a series of operating parameters, the control apparatus causes the motor to transition between a drive mode, a transient mode, a regulated current regenerative braking mode, and a constant PWM regenerative braking mode. In particular, the motor enters the drive mode in response to a request for an increase in speed, the motor enters the transient mode in response to a request for a decrease in speed, the motor enters the regulated current regenerative braking mode in response to a lapse of a predetermined time delay or in response to a request for a reduction in the magnitude of the previously requested decrease in speed, and the motor enters the PWM regenerative braking mode in response to a request to continue braking when the requested braking torque cannot be maintained.

FIELD OF THE INVENTION 
The present invention relates generally to regenerative braking in an 
electric motor and, more specifically to transitioning from a motor's 
normal drive mode to a regenerative braking mode and to controlling 
regenerative braking once current is flowing through a regenerative 
braking circuit. 
BACKGROUND OF THE INVENTION 
Regenerative braking is the conversion of kinetic energy created by a 
system powered by a motor into electrical energy, and the storing of that 
electrical energy to power the motor. 
Zapi Corporation of Italy manufactures direct current motor controllers 
using a regenerative braking circuit, as illustrated in FIG. 1. The 
essential element of the Zapi regenerative braking circuit is a 
single-pole, double-throw regenerative braking contactor 10. The 
regenerative braking contactor 10 is energized in drive mode. When 
energized, the regenerative braking contactor 10 allows current to flow 
from the positive terminal of battery 20 through armature winding 30, 
field winding 40, and motor drive FETs 50. 
The regenerative braking contactor 10 is de-energized in braking mode. When 
the regenerative braking contactor is de-energized its contacts connect a 
top connector A1 of the armature winding 30 to the negative terminal of 
the battery 20. This connection enables regenerative braking. 
While the Zapi circuit is somewhat effective, it may be utilized only under 
limited conditions. In fact, three conditions must be met in order for 
regenerative braking to occur: first, the motor field winding 40 must 
contain sufficient flux to induce an opposing current in the armature 30; 
second, the motor field winding 40 must be electrically connected such 
that the magnetic interaction of the armature 30 with the induced field 
generates a regenerative current (i.e., the direction contactors 42 and 44 
must be reversed so the motor current flows in a direction that opposes 
the existing direction of motor rotation); and finally, a complete path 
for current flow must exist from the motor field winding 40 to the battery 
20. 
These conditions for operation make reversing the direction contactors 42 
and 44 very difficult. For example, to switch from the forward driving 
direction to the reverse driving direction, the motor current must be off. 
Otherwise, if the direction contactors 42 and 44 are reversed while a 
substantial current is flowing through the motor, there is a danger the 
contactors will be welded in place. Moreover, even if the direction 
contactors 42 and 44 are successfully switched while current is flowing 
through the motor, the contactor switch times are generally longer than 
the time constant (the time required for flux dissipation) of high powered 
direct current motors. Thus, there will be little or no flux left in the 
field winding 40 after the contactors are reversed. 
For regenerative braking to be safely induced using the available Zapi 
circuit, a secondary mechanism must be included for exciting the 
regenerative braking current in the armature 30. This is accomplished by 
using a weak magnetic field induced in the iron of the electric motor case 
(not shown). If a flux path is in the proper direction in relation to the 
magnetic field, a high rotational armature speed will cause a small 
current to begin to flow through the motor drive FETs 50. Once the flow of 
current is detected, it can be regulated through controlled pulse width 
modulation (PWM) at the motor drive FETs 50. 
One primary disadvantage of the Zapi regenerative braking circuit is that 
high motor speed is required to initiate the regenerative braking current. 
The Zapi circuit requires a four-step process to begin regenerative 
braking: 
1. Disengage forward contactor 42 and engage reverse contactor 44. 
2. Initiate plug braking by turning on the motor drive FETs 50 to "set" the 
flux direction in the field winding 40. 
3. Drop out the main contactor 10 to connect the negative terminal of the 
battery 20 to the top of the armature 30. 
4. Monitor the circuit for initiation of a regenerative braking current and 
signal the motor drive FETs 50 to control the regenerative breaking 
current. 
In short, the Zapi regenerative braking circuit depends upon a weak 
magnetic field in the iron of the motor case to excite the armature 
current for regenerative braking. This eliminates the possibility of using 
non-ferrous metals in the motor case. Even more importantly, the armature 
current can only be generated in the Zapi circuit if the speed of the 
armature is great enough to overcome the voltage drops of the active and 
passive components in the circuit. This may require hundreds, or thousands 
of motor revolutions per minute, depending upon the motor and gearing 
ratios utilized, to begin regenerative braking. 
There is, therefore, a need in the art for a more efficient regenerative 
braking apparatus and method which addresses the shortcomings of the 
available art. However, the applicant knows of no prior art which 
satisfies all of the aforementioned problems. More specifically, the 
following is the most relevant prior art known to the applicant. 
U.S. Pat. No. 4,479,080 to Lambert is directed to an electrical braking 
control for DC series traction motors that initiates braking in a plug 
mode, transitions to regeneration mode, and returns to plug mode when 
regeneration braking is no longer efficient, with all switching carried 
out smoothly and efficiently without unduly wasting regenerative power. A 
series wound traction motor includes an armature winding, a field winding, 
a mechanical arrangement, and a battery source. A power regulating circuit 
is used and the field winding is arranged to be connected in either a 
forward or reverse direction by means of a plurality of contacts. A 
current shunt is connected in series with the motor and power source to 
supply a signal to the motor controller. Included in the circuit is a 
plugging diode, a free-wheeling diode, and a regenerative braking diode. 
Upon initiation of braking by the operator, the direction and brake logic 
circuit will switch from propulsion to plug braking mode. Signals from the 
shunt and the percent on-time controller are monitored by the regeneration 
control circuit and, if efficient, will switch into regenerative braking 
mode. When the motor speed decreases to the point that the regulator is 
operating at 100% duty cycle and the desired motor torque cannot be 
maintained, the braking operation will switch back to plug mode. While the 
Lambert device is somewhat effective, it has several disadvantages. One 
primary disadvantage is that, in the Lambert device the contactor 22 is 
switched from a closed position to an open position to force the 
transition from plug mode to regenerative braking mode. Using the 
contactor 22 as a switch between these modes exposes the contactor 22 to a 
welding risk since the contactor 22 cannot instantaneously go from the 
closed position to opened position. Another disadvantage in the Lambert 
device is its transition from regenerative braking mode to plug mode when 
a required motor torque cannot be maintained due to low motor rotation 
speed. As can readily be appreciated, this transition drains of the charge 
the battery stored during regenerative breaking. A further disadvantage in 
the Lambert device is its inability to regulate regenerative braking at 
high speeds. 
U.S. Pat. No. 5,332,954 to Lankin is directed to a solid state electronic 
control for DC traction motors having a series or separate field. The 
control provides for regenerative braking at low motor speeds. The optimum 
configuration of a DC motor controller includes a plurality of MOSFET 
devices to connect the series wound or separately excited traction motor 
to the power source for propulsion or braking. During vehicle operation, a 
control logic circuit continuously pulls a brake sensor to determine if 
conditions are suitable for regenerative braking. When selected, the field 
circuit with the bridge arrangement of MOSFET devices can provide strong 
regenerative braking by supplying full field as required. 
U.S. Pat. No. 4,730,151 to Florey, et al., is directed to a continuous 
field control of series wound motors, particularly for use in regulating 
electrical braking current in a direct current electric traction motor. A 
DC series motor with an armature and field winding is supplied power 
either in series or in parallel for operation in the running or braking 
mode. In normal, forward or reverse operation, the motor is connected in 
series mode with the battery supplying power to the armature connected in 
series with the field. Control is provided by a microprocessor controlling 
a chopper control in series with the motor. Field control mode, with the 
armature across the battery and the field in series with the chopper 
control, is used for maximum power in the running mode or for full control 
in regenerative braking mode. 
U.S. Pat. No. 4,422,021 to Schwarz is directed to highly efficient 
recuperation of energy stored in a dynamo electric machine system using a 
control system that includes a controlled switch to initiate field current 
to start generation. The motor is controlled by a circuit including field 
reversing switches and a control switch. Upon braking, the field is 
reversed using the field switches and the controlled switch. A thyristor 
or transistor, is turned on, along with turn-off thyristor, to supply 
energy stored in capacitor to cause a field current in a winding. After 
the release of an impulse from the capacitor, the field current is 
maintained by a field diode until the generator action of the dynamo 
starts. A control unit will control the duty cycle of the cycling period, 
maintaining the current flow through the armature at an appropriate value. 
Finally, U.S. Pat. No. 4,124,812 to Naito, et al., is directed to a braking 
control for a battery operated forklift that executes a regeneration mode, 
then plugging. Voltage from a battery is controlled by a chopping circuit 
and applied to the series circuit comprised of a motor field coil and 
motor armature. For braking, a regeneration contractor is switched and the 
armature and field are connected in parallel with one another, and in 
series with the chopping control across the battery for regenerative 
braking. After the velocity detector determines that the speed is down to 
a predetermined value, the braking is changed over to plugging. 
SUMMARY OF THE INVENTION 
Presently, electric vehicles driven by direct current motors use 
regenerative braking systems which require the motor to turn at a 
relatively high rate of speed prior to transition into a regenerative 
braking mode. The higher speed is necessary to excite the armature winding 
of the motor with the residual magnetism present in the motor housing. 
This speed requirement narrows the useful regenerative braking range of 
most D.C. motor speed controllers, thereby lowering the overall efficiency 
of the controllers and the range of vehicles to which the controllers can 
be attached. 
To address the shortcomings of the prior art, the present invention 
provides an apparatus and method for regenerative braking in direct 
current motors. A preferred embodiment is designed for, but not limited 
to, application in electric vehicles. The inventive regenerative braking 
apparatus comprises a MOSFET transistor positioned to quickly alter the 
path of motor current in a regenerative braking circuit and means for 
utilizing a MOSFET transistor switch to initiate the regenerative braking 
sequence and maintain the dynamic braking torque of the motor by 
maintaining the desired field winding current. 
The inventive method comprises first entering a transient mode from a drive 
mode. In the transient mode, the current driven by the DC battery flows 
through the field winding in a direction opposing the existing direction 
of motor rotation for a limited period of time. This current causes the 
armature to act as an electrical generator. The armature acts as an 
electrical generator due to the opposing flux in the armature and field 
windings. The generator action causes current to flow in a transient 
circuit which preferably includes a power diode normally reverse-biased 
across the armature of the motor and a MOSFET transistor in series with 
the power diode. 
Two important benefits are realized by establishing the transient mode of 
operation prior to entering a regulated current regenerative braking mode. 
First, current flows from the battery through the field and armature 
windings of the motor in a direction which sufficiently excites the 
armature winding to generate a maintainable armature current. Second, a 
steady-state regulated current is established in the motor during the 
transient mode prior to entering the regulated current regenerative 
braking mode. In particular, a regulated current is present in the field 
and armature windings prior to the transition from the transient mode to 
the regulated current regenerative braking mode. After the regulated 
current reaches a predetermined magnitude, the controller turns off the 
drive signal to the Regen FET, thereby altering the path of the current in 
the motor circuit (i.e., the transition from transient mode to regulated 
current regenerative braking mode occurs). The current path is altered 
while the regulated current is flowing through the motor circuit. The 
abrupt change in the current path generates a regulated regenerative 
braking current. 
Thus, a notable feature of the present invention is the substantially 
instantaneous transition, permitted by the incorporation of an FET switch, 
from the drive mode to the regulated current regenerative braking mode, 
without diminishment of the current through the motor windings or the 
degradation of the flux interaction between the motor windings. 
Another notable feature of the present invention is the low speed 
implementation of regulated current regenerative braking. A key element of 
the low speed implementation of regulated current regenerative braking is 
the control circuit's anticipation of the difference between the power 
derived from the motor circuit in the transient mode and the power which 
would be extracted from the motor in the regulated current regenerative 
braking mode. In the transient mode, the power is the product of the 
established current and the voltage drops across the motor windings, 
conductors, and the reverse biased diode. The voltage may be from a few 
volts to tens of volts in typical electric vehicle applications. However, 
in the regulated current regenerative braking mode, the voltage from the 
armature circuit must be greater than the sum of the battery voltage, the 
diode voltage drops, and the voltage drop across other conductors. Also, 
the voltage generated in the armature circuit must be substantially 
greater than the battery voltage to achieve battery currents in the tens, 
or hundreds of amps. 
This implementation of regulated current regenerative braking anticipates 
the pulse width modulation (PWM) of the motor voltage which would be 
required to successfully transition from the transient mode to the 
regenerative braking mode and very rapidly establishes the necessary PWM 
in the motor circuit as the transition is made from the transient mode to 
regulated current regenerative braking mode. 
Another notable feature of the present invention relates to the ability to 
continue extracting power from the motor system in a constant PWM 
regenerative braking mode, as the vehicle brakes to a stop. In a preferred 
embodiment, the controller would regulate the current in the motor system 
to sustain a braking torque, as requested by depression of the brake 
pedal, until the PWM approaches 100%. In the constant PWM regenerative 
braking mode, the controller would maintain the PWM at some high level 
less than 100%, such as 96%, while allowing the current present in the 
regenerative braking circuit to diminish as the vehicle slows to a stop. 
Although the current and braking torque would continue to diminish, energy 
would continue to be extracted from the motor system. In operation, the 
driver would experience a steady reduction of braking torque during the 
constant PWM regenerative braking mode. The driver would compensate for 
the perceived decrease in braking torque by increasing brake pedal 
pressure. The net effect of the decreasing braking torque due to 
regenerative braking and the driver's increase in brake pedal pressure is 
the seamless transition into hydraulic braking as the vehicle slows to a 
stop. 
In contrast to the above feature of the present invention, the regenerative 
braking device disclosed in Lambert, U.S. Pat. No. 4,479,080, alternates 
between regenerative braking and plug braking. In Lambert, when the 
current generated in the armature circuit falls below the current 
necessary to achieve the braking torque requested by the driver, the 
controller switches the motor system to a plug mode of operation. The plug 
mode of operation has the undesirable characteristics of power consumption 
and heat dissipation. In particular, power from the battery is consumed to 
maintain the requested plug braking current and the kinetic energy of the 
vehicle is dissipated as waste heat. 
A first advantage of the present invention is therefore the enablement of a 
transition to regenerative braking at a much lower motor speed than in the 
prior art. 
Another advantage of the present invention is that as the vehicle slows and 
the energy available from the motor for regenerative braking diminishes to 
a point where the requested battery charging current can no longer be 
maintained, a smooth transition to a constant PWM regenerative braking 
mode occurs. During the constant PWM regenerative braking mode, energy 
continues to be extracted from the motor as the motor approaches very low 
motor speeds. 
Briefly stated, a regenerative braking control apparatus is provided for 
use with a motor driven by a D.C. power source. In response to a series of 
operating parameters, the control apparatus causes the motor to transition 
between a drive mode, a transient mode, a regulated current regenerative 
braking mode, and a constant PWM regenerative braking mode. In particular, 
the motor enters the drive mode in response to a request for an increase 
in speed, the motor enters the transient mode in response to a request for 
a decrease in speed, the motor enters the regulated current regenerative 
braking mode in response to a lapse of a predetermined time delay or in 
response to a request for a reduction in the magnitude of the previously 
requested decrease in speed, and the motor enters the constant PWM 
regenerative braking mode in response to a request to continue braking 
when a requested braking torque cannot be maintained. 
A feature of the invention includes a control apparatus for controlling the 
regenerative braking of a motor electrically connected to a D.C. power 
source, the control apparatus having control means for generating at least 
one of a drive mode signal, a transient mode signal, a first regenerative 
braking mode signal and a second regenerative braking mode signal in 
response to a plurality of operating parameter signals, drive mode means 
responsive to the drive mode signal for causing the motor to operate in a 
drive mode such that the motor is driven in a first direction of rotation 
by the D.C. power source, transient mode means responsive to the transient 
mode signal for causing the motor to transition from the drive mode to a 
transient mode such that a flux builds up in an armature winding and a 
field winding of the motor, the flux generating a current flow through a 
transient circuit path running through the motor, and first regenerative 
braking mode means responsive to the first regenerative braking mode 
signal for causing the motor to transition from the transient mode to a 
first regenerative braking mode such that the current flow switches from 
the transient circuit path to a charging circuit path interconnecting the 
motor and the D.C. power source, the flux charging said D.C. power source 
and generating a requested braking torque in the motor in a second 
direction of rotation opposing the first direction of rotation, the 
requested braking torque having a constant magnitude. 
Another feature of the invention includes a control apparatus for 
controlling the regenerative braking of a motor electrically connected to 
a D.C. power source, the control apparatus including control means for 
generating at least one of a drive mode signal, a transient mode signal, a 
first regenerative braking mode signal and a second regenerative braking 
mode signal in response to a plurality of operating parameter signals, 
drive mode means responsive to the drive mode signal for causing the motor 
to operate in a drive mode such that the motor is driven in a first 
direction of rotation by the D.C. power source, transient mode means 
responsive to the transient mode signal for causing the motor to 
transition from the drive mode to a transient mode such that a flux builds 
up in an armature winding and a field winding of the motor, the flux 
generating a current flow through a transient circuit path running through 
the motor, first regenerative braking mode means responsive to the first 
regenerative braking mode signal for causing the motor to transition from 
the transient mode to a first regenerative braking mode such that the 
current flow switches from the transient circuit path to a charging 
circuit path interconnecting the motor and the D.C. power source, the flux 
charging the D.C. power source and generating a requested braking torque 
in the motor in a second direction of rotation opposing the first 
direction of rotation, the requested braking torque having a constant 
magnitude, and second regenerative braking mode means responsive to the 
second regenerative braking mode signal for causing the motor to 
transition from the first regenerative braking mode to a second 
regenerative braking mode such that the flux continues to charge the D.C. 
power source while generating a diminishing braking torque, the 
diminishing braking torque having a maximum magnitude that is less than 
the constant magnitude of the requested braking torque. 
Yet another feature of the invention includes a control apparatus for 
controlling the regenerative braking of a motor electrically connected to 
a D.C. power source, the control apparatus including control means for 
generating a drive mode signal in response to a request for an increase in 
motor speed, a transient mode signal in response to a request for a 
decrease in motor speed, a first regenerative braking mode signal in 
response to one of a lapse of predetermined time delay or a request for a 
reduction in the decrease in motor speed, and a second regenerative 
braking mode signal in response to an inability to maintain the requested 
decrease in motor speed, drive mode means responsive to the drive mode 
signal for causing the motor to operate in a drive mode such that the 
motor is driven in a first direction of rotation by the D.C. power source, 
transient mode means responsive to the transient mode signal for causing 
the motor to transition from the drive mode to a transient mode such that 
a flux builds up in an armature winding and a field winding of the motor, 
the flux generating a current flow through a transient circuit path 
running through the motor, first regenerative braking mode means 
responsive to the first regenerative braking mode signal for causing the 
motor to transition from the transient mode to a first regenerative 
braking mode such that the current flow switches from the transient 
circuit path to a charging circuit path interconnecting the motor and the 
D.C. power source, the flux charging the D.C. power source and generating 
a requested braking torque in the motor in a second direction of rotation 
opposing the first direction of rotation, the braking torque having a 
constant magnitude, and second regenerative braking mode means responsive 
to the second regenerative braking mode signal for causing the motor to 
transition from the first regenerative braking mode to a second 
regenerative braking mode such that the flux continues to charge the D.C. 
power source while generating a diminishing braking torque, the 
diminishing braking torque having a maximum magnitude that is less than 
the constant magnitude of the requested braking torque. 
A further feature of the invention includes a method of regenerative 
braking a motor electrically connected to a D.C. power source including 
the steps of generating at least one of a drive mode signal, a transient 
mode signal, a first regenerative braking mode signal, and a second 
regenerative braking mode signal in response to reception of a plurality 
of operating parameter signals, operating the motor in a drive mode in 
response to reception of the drive mode signal such that the motor is 
driven in a first direction of rotation by the D.C. power source, causing 
the motor to transition from the drive mode to a transient mode in 
response to reception of the transient mode signal such that a flux builds 
up in an armature winding and a field winding of the motor, the flux 
generating a current flow through a transient circuit path running through 
the motor, causing the motor to transition from the transient mode to a 
first regenerative braking mode in response to reception of the first 
regenerative braking mode signal such that the current switches from the 
transient circuit path to a charging circuit path interconnecting the 
motor and the D.C. power source, the charging circuit path permitting the 
flux in the motor to charge the D.C. power source and to generate a 
requested braking torque in the motor in a second direction of rotation 
opposing the first direction of rotation, the requested braking torque 
having a constant magnitude, and causing the motor to transition from the 
first regenerative braking mode to the second regenerative braking mode in 
response to reception of the second regenerative braking mode such that 
the flux continues to charge the D.C. power source while generating a 
diminishing braking torque, the diminishing braking torque having a 
maximum magnitude that is less than the constant magnitude of the 
requested braking torque. 
A still further feature of the invention includes a control apparatus for 
controlling the regenerative braking of a motor electrically connected to 
a D.C. power source, the control apparatus including a controller that 
generates at least one of a drive mode signal, a transient mode signal, 
and a first regenerative braking mode signal in response to reception of a 
plurality of operating parameter signals, drive mode circuitry responsive 
to the drive mode signal to cause the motor to operate in a drive mode 
such that the motor is driven in a first direction of rotation by the D.C. 
power source, transient mode circuitry responsive to the transient mode 
signal to cause the motor to transition from the drive mode to a transient 
mode such that a flux builds up in an armature winding and a field winding 
of the motor, the flux generating a current flow through a transient 
circuit path running through the motor, and first regenerative braking 
mode circuitry responsive to the first regenerative braking mode signal to 
cause the motor to transition from the transient mode to a first 
regenerative braking mode such that the current flow switches from the 
transient circuit path to a charging circuit path interconnecting the 
motor and said D.C. power source, the flux charging the D.C. power source 
and generating a requested braking torque in the motor in a second 
direction of rotation opposing the first direction of rotation, the 
braking torque having a constant magnitude. 
An additional feature of the invention includes a control apparatus for 
controlling the regenerative braking of a motor electrically connected to 
a D.C. power source, the control apparatus including a controller that 
generates at least one of a drive mode signal, a transient mode signal, a 
first regenerative braking mode signal, and a second regenerative braking 
mode signal in response to reception of a plurality of operating parameter 
signals, drive mode circuitry responsive to the drive mode signal to cause 
the motor to operate in a drive mode such that the motor is driven in a 
first direction of rotation by the D.C. power source, transient mode 
circuitry responsive to the transient mode signal to cause the motor to 
transition from the drive mode to a transient mode such that a flux builds 
up in an armature winding and a field winding of the motor, the flux 
generating a current flow through a transient circuit path running through 
the motor, first regenerative braking mode circuitry responsive to the 
first regenerative braking mode signal to cause the motor to transition 
from the transient mode to a first regenerative braking mode such that the 
current flow switches from the transient circuit path to a charging 
circuit path interconnecting the motor and the D.C. power source, the flux 
charging the D.C. power source and generating a requested braking torque 
in the motor in a second direction of rotation opposing the first 
direction of rotation, the braking torque having a constant magnitude, and 
second regenerative braking mode circuitry responsive to the second 
regenerative braking mode signal to cause the motor to transition from the 
first regenerative braking mode to a second regenerative braking mode such 
that the flux continues to charge the D.C. power source while generating a 
diminishing braking torque, the diminishing braking torque having a 
maximum magnitude that is less than the constant magnitude of the 
requested braking torque.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Drive Mode 
Referring now to FIG. 2, a control circuit 60 for regenerative braking is 
shown in the drive mode. The drive mode exists when the motor is being 
operated to propel a vehicle forward or backward. We will assume a forward 
contactor coil 62 is energized and a reverse contactor coil 64 is not 
energized. Motor Drive FETs 66 have a controlled pulse width modulation 
(PWM) duty cycle in which the Motor Drive FETs 66 alternate between being 
on and being off. A controller 100, such as a 32 bit microprocessor, 
controls the PWM duty cycle in response to a detected depression on the 
accelerator pedal (not shown). The current path through a field winding 68 
when the Motor Drive FETs 66 are turned on is marked as path "A" (solid 
line). The current path through the field winding 68 when the Motor Drive 
FETs 66 are turned off is marked as path "B" (dashed line). Current path 
"A" extends from the battery 72 through the regenerative braking contactor 
74, the armature 70, the field winding 68, the current sensor 76 and the 
motor drive FET 66, and back to the battery 72. Current path "B" extends 
from the armature 70 through the field winding 68, the current sensor 76, 
the free wheel diode 78 and the regenerative braking contactor 74, and 
back to the armature 70. 
Transient Mode 
Referring now to FIG. 3, when the controller 100 detects a depression of a 
brake pedal (not shown) by the operator of the electric vehicle, the 
controller 100 initiates a transient mode as shown. The controller 100 
terminates a signal to the Motor Drive FET 66 thereby turning off the 
Motor Drive FET 66 and interrupting the motor current "A" (shown in FIG. 
2). The controller 100 then reverses the direction of the contactors 62 
and 64 by energizing the reverse contactor 64 and de-energizing the 
forward contactor 62. This sets up the polarity of current through the 
field winding 68 and the armature 70 that is needed to generate a braking 
torque in the motor and to develop the counter electro-magnetic force 
(EMF) in the armature necessary for regenerative braking. 
The controller 100 then de-energizes the regenerative braking contactor 74 
and turns on the Regen FET 80 by an enabling signal to its gate drive 
input pin. At the time the Regen FET 80 is turned on, there is no current 
flowing through the transient circuit. When the contactors 62, 64, and 74 
are determined to be stabilized in their new conducting states by either 
the lapse of a predetermined delay or by a closure sense circuit, the 
controller 100 initiates current in the transient circuit by applying a 
PWM signal to the motor drive FETs 66. For example, after a delay, 
preferably on the order of 50 milliseconds, to allow the contactors 62, 
64, and 74 to reach their new states, the controller 100 reactivates the 
Motor FET Drive 66. This permits current to flow through the armature 70 
and field winding 68 along a current path "C". As current begins 
developing along the current path "C", the interaction of the flux in the 
field winding 68 and the armature winding 70 increases. The voltage 
generated in the armature 70, due to the flux interaction, causes an 
armature current to flow along a transient current path "D" which extends 
through the now forward-biased diode 82. As discussed in further detail 
below, a lower armature voltage can cause current to flow along transient 
current path "D" than can cause current to flow along the regenerative 
braking path, shown in FIG. 4, since the voltage generated by the armature 
70 only needs to forward-bias the diode 82 to start current flow through 
current path "D". Preferably, an armature voltage of about 1 to 1.5 volts 
is enough to forward-bias diode 50. Current path "D" also extends through 
Regen FET 80 which is connected in parallel with regenerative braking 
contactor 74. Since Regen FET 80 only carries the transient current 
flowing through path "D" for a relatively short time period, preferably on 
the order of 100 milliseconds, Regen FET 80 can preferably be a large 
inexpensive MOSFET. However, the MOSFET used must be able to carry a 
current of 50 to 75 amps without malfunctioning for the short time period 
the transient mode exists. 
Regulated Current Regenerative Braking Mode 
Referring now to FIG. 4, once the current in path "D" is established, the 
Regen FET 80 is abruptly turned off. Since the regenerative braking 
contactor 74 is off (in the open position), the only remaining path for 
current flow from the armature 70 is current path "E". Current path "E" 
extends through the battery 72 in the direction from the positive terminal 
to the negative terminal. With an initial current flowing along path "D", 
the transition to the regulated current regenerative braking mode from the 
transient mode is achieved under substantially optimal conditions, i.e., 
flux present in the field winding 68, an existing current flow along path 
"D", and a current path "E" through the battery 72 from the armature 70. 
As a result, the controller 100 can anticipate the optimal PWM of the 
motor drive FETs 66 necessary to charge the battery 72 and generate the 
requested braking torque prior to causing the motor to transition into the 
regulated current regenerative braking mode. 
The following is an example of how the controller anticipates the optimal 
PWM of the motor drive FETs 66 necessary to charge the battery 72 and 
generate the requested braking torque prior to causing the motor to 
transition into the regulated current regenerative braking mode. As 
explained above, a transient current begins to flow along current path "C" 
after the controller 100 applies a PWM signal to the motor drive FETs 66. 
The transient current increase at a rate dependent on the time constant of 
the motor windings and the rate of change (between "on" and "off") of the 
PWM drive signal to the motor drive FETs 66. The controller 100 monitors 
the transient current flow via the current sensor 76. After the transient 
current has reached a minimum current level necessary for transition to 
the regulated current regenerative braking mode, the controller 100 can 
shut off Regen FET 80. To optimize this transition, the controller 
calculates the PWM signal to the motor drive FETs 66 necessary to 
establish the battery charging current (which flows along current paths 
"E" and "G") prior to initiating the transition. It should be noted that 
the PWM drive signal to the motor drive FETs 66 during the transient mode 
is different than the PWM drive signal to the motor drive FETs 66 during 
the regulated current regenerative braking mode. This difference is due to 
a couple of reasons. In the transient mode of operation, the voltage 
output of the armature is determined by the voltage drops of the motor 
windings, the conductors, and the diode drops in circuit path "D". As a 
result, it may take a few volts to tens of volts to generate the transient 
current in circuit path "D". However, in the regulated current 
regenerative braking mode, the voltage output of the armature must be 
greater than the battery voltage to forward-bias the diodes 82 and 88 in 
circuit path "E". Therefore, during regenerative braking, the armature 
provides a much larger voltage than previously provided during the 
existence of the transient circuit. For example, in a 144 volt system, the 
ratio of the regenerative braking mode PWM signal to the motor drive FETs 
66, to the transient mode PWM signal, to the motor drive FETs 66 is on the 
order of 10-20 to 1. Therefore, when the transition to regenerative 
braking occurs, the PWM signal may have to change from 4% PWM to 80% PWM 
to sustain a current through current path "E". In the prior art, it would 
take more than 100 milliseconds for the prior art controllers to gradually 
increase the PWM signal from 4% PWM to 80% PWM. However, a delay of this 
magnitude would cause the current initially generated along transient path 
"D" to decay long before it could flow along current path "E". Therefore, 
in the present invention, the controller 100 calculates the required PWM 
necessary to cause current to flow along current path "E" prior to causing 
the control system 60 to transition from the transient mode to the 
regulated current regenerative braking mode. Accordingly, when the 
transition takes place, the controller instantly applies the calculated or 
anticipated PWM signal to the motor drive FETs 66. 
Although the only remaining current path is current path "E" through 
battery 72, the diode 82 can no longer be forward biased with just 1 to 
1.5 volts of potential at anode 84 with respect to armature terminal A2, 
since the cathode 86 of diode 82 is at the same potential as the positive 
battery terminal. As a result, the output of the armature will increase to 
at least 1 volt above the battery voltage to forward bias the plug diode. 
Furthermore, the armature voltage will increase to a voltage which permits 
the armature current to charge the battery 72. 
The current through field winding 68 is preferably regulated to maintain 
the desired braking torque. The current sensor 76 monitors the magnitude 
of the field winding current and provides current magnitude data to the 
controller 100 which, in turn, modulates the PWM signal to motor drive 
FETs 66 to maintain the desired field winding current. In particular, the 
controller 100 pulse width modulates the current going through the motor 
drive FETs 66 by altering the PWM duty cycle of the motor drive FETs 66. 
When the motor drive FETs 66 are turned "on" by the controller 100 the 
current flows along current path "F", building the field winding flux to 
the level required to generate the field current necessary to maintain the 
requested braking torque. When the motor drive FETs are turned "off" by 
the controller 100, the current flows along current paths "E" and "G", 
charging the battery 72. As the battery 72 is charged the maximum flux 
level possible in the field winding 68 is reduced. Over time, the motor 
drive FETs must be turned "on" for longer periods of time to build the 
field winding flux to the level required for generating the field winding 
current necessary to maintain the requested braking torque. If permitted, 
the motor drive FETs would eventually remain in the "on" state so 100% of 
the current would travel along current path "F", thereby maintaining the 
flux in the field winding at the level necessary to generate the requested 
field winding current. However, even with the motor drive FETs in a 
continuous "on" state (i.e., 100% duty cycle), the field winding flux 
would eventually drop below the level necessary to generate the requested 
torque and field winding current. It is at this point in the prior art 
when the battery 72 would be used in a plug braking mode to build the 
field winding flux. 
Constant PWM Regenerative Braking Mode 
In the preferred embodiment of the present invention, when the vehicle 
slows to the point where the output of the armature can no longer supply 
sufficient current to generate the requested braking torque, controller 
100 will run the motor drive FETs at a 96% duty cycle. This means that the 
motor drive FETs 66 will be "on" 96% of the time and "off" 4% of the time. 
Afterwards, the controller 100 will monitor the field winding current via 
the current sensor 76 to determine if the current has increased to a level 
where the motor could deliver the requested current at 96% PWM. For 
example, as the vehicle slows to a speed where 98% PWM is required for the 
motor to provide a current which generates the requested braking torque, 
the controller 100 will regulate the current at some value less than the 
requested current such that the PWM of 96% is not exceeded. If the motor 
speed increases, as would happen if the vehicle starts to accelerate down 
a slope, the available current which could be generated by the motor at 
96% PWM would increase. If the vehicle continued to increase its speed on 
the downward slope, the motor current would rise to the point where the 
motor could deliver the requested current at 96% PWM. Any further increase 
in motor speed would cause a transition back to regulated current 
regenerative braking mode, which would hold the current at the requested 
level by varying the PWM. However, if vehicle continued to slow to a speed 
where even a PWM of 100% would not provide the requested current, the 
controller would continue to hold the PWM constant at 96% while allowing 
the current traveling along paths "E", "F", and "G" to diminish as the 
vehicle slows to a stop. Although the braking torque and the current 
traveling along paths "E", "F", and "G" would continue to diminish, energy 
would still be extracted from the motor system. In operation, the driver 
will experience a steady reduction of braking torque during the constant 
PWM regenerative braking mode. The driver will compensate for the 
perceived decrease in braking torque by increasing brake pedal pressure. 
The net effect of the decreasing braking torque due to regenerative 
braking and the driver's increase in brake pedal pressure is the seamless 
transition into hydraulic braking as the vehicle slows to a stop. This not 
only provides a smooth torque to nearly zero RPM, but, more importantly, 
there is preferably no skipping into and out of regenerative braking at 
low speeds as experienced by most prior art controllers. 
A significant benefit of maintaining the constant PWM regenerative braking 
mode after the regulated current regenerative braking mode has terminated 
is the ability to re-establish the regulated current regenerative braking 
mode at a lower level of current without having to transition to normal 
drive mode first. For example, if the regulated current regenerative 
braking mode is initially established with 100 amps of field current while 
the vehicle is moving at 30 miles per hour and the vehicle slows to 5 
miles per hour, the regulated current regenerative braking mode may 
terminate, since the requested braking torque requiring 100 amps of field 
current cannot be maintained by a motor speed which is propelling the 
vehicle at 5 miles per hour when a large portion of the energy in the 
motor windings is being used to charge the battery 72. However, in the 
constant PWM regenerative braking mode the current primarily flows along 
current path "F" to generate the requested braking torque. If the driver 
then decreases the brake pedal pressure to a position which requires only 
50 amps of field current, the regulated current regenerative braking mode 
will then be re-established automatically, since the field winding can 
maintain a current that induces the necessary braking torque with the 
armature winding, and the flux present in the motor windings can provide 
the necessary current to charge the battery 72. The apparatus of the 
present invention thereby allows for a very smooth transition between the 
constant PWM regenerative braking mode and the regulated current 
regenerative braking mode without abrupt changes in the vehicle's braking 
torque, while allowing maximum extraction of battery charging energy from 
the vehicle's motion at low speeds. It should be noted that although all 
of the energy in the motor windings can be used to generate a braking 
torque in the constant PWM regenerative braking mode, a small amount of 
energy from the motor windings is preferably used to charge the battery. 
In other words, the Motor drive FETs 66 are preferably run at a 96% duty 
cycle such that some regenerative braking occurs. 
The preferred method of the present invention for regenerative braking may 
therefore be summarized as follows: 
1. In response to depression of the vehicle's brake pedal, controller 100 
shuts off the motor drive. 
2. Contactors 62 and 64 are reversed, allowing the motor to be driven in a 
direction opposing the vehicle's motion. 
3. Regenerative braking contactor 74 is de-energized, breaking the path of 
current from battery 72 to the armature winding 70 and the field winding 
68. 
4. Regen FET 80 is turned on to provide a path for battery current to the 
motor. 
5. As motor drive FETs 66 are energized, current begins to flow in a 
direction which tends to drive the motor against its previous direction of 
rotation. 
6. The motor acts as a generator due to the opposing flux in the armature 
windings 70 and field windings 68. 
7. The motor/generator induces current flow from armature connection A2 
through plug diode(s) 82 and back to the armature connection A1. 
8. While field winding current is flowing and the armature current is 
flowing, the Regen FET 80 is abruptly turned off. 
9. The instantaneous change in the current path from the battery 72 to the 
motor causes motor current to flow through battery 72 as charging current, 
thus inducing regenerative braking. 
10. The charging current level is maintained by pulse width modulating 
(PWM) at motor drive FETs 66. During the "on" time of the PWM cycle the 
field winding current builds to the desired level, and during the "off" 
time of the PWM cycle regenerative braking current flows through the 
battery 72 and Regen diode 88. 
11. Flux in field winding 68 is maintained by the current from the armature 
70 while motor drive FETs 66 are turned on during the "on" time of the PWM 
cycle. 
A general description of the apparatus and method of the present invention, 
as well as a preferred embodiment of both, has been set forth above. One 
skilled in the art will recognize and be able to practice many changes in 
many aspects of the apparatus and method described above, including 
variations which fall within the teachings of this invention. The spirit 
and scope of the invention should be limited only as set forth in the 
claims which follow.