Brake blending strategy for a hybrid vehicle

A hybrid electric powertrain system is provided including a transmission for driving a pair of wheels of a vehicle and a heat engine and an electric motor/generator coupled to the transmission. A friction brake system is provided for applying a braking torque to said vehicle. A controller unit generates control signals to the electric motor/generator and the friction brake system for controllably braking the vehicle in response to a drivers brake command. The controller unit determines and amount of regenerative torque available and compares this value to a determined amount of brake torque requested for determining the control signals to the electric motor/generator and the friction brake system.

FIELD OF THE INVENTION 
The present invention relates generally to a hybrid electric vehicle and, 
more particularly, to an electric motor/regenerator and friction brake 
torque distribution control strategy for a hybrid electric vehicle. 
BACKGROUND AND SUMMARY OF THE INVENTION 
Since the invention of power vehicles, many different powertrain systems 
have been attempted, including a steam engine with a boiler or an electric 
motor with a storage battery. It was, however, the discovery of petroleum 
in 1856 and the fourstroke internal combustion engine invented by Otto in 
1876, that provided the impetus for the modern motor vehicle industry. 
Although fossil fuel emerged as the fuel of choice for motor vehicles, 
recent concerns regarding fuel availability and increasingly stringent 
federal and state emission regulations have renewed interest in 
alternative fuel powered vehicles. For example, alternative fuel vehicles 
may be powered by methanol, ethanol, natural gas, electricity, or a 
combination of these fuels. 
A dedicated electric powered vehicle offers several advantages: electricity 
is readily available, an electric power distribution system is already in 
place, and an electric powered vehicle produces virtually no emissions. 
There are, however, several technological disadvantages that must be 
overcome before electric powered vehicles gain acceptance in the 
marketplace. For instance, the range of an electric powered vehicle is 
limited to approximately 100 miles, compared to approximately 300 miles 
for a similar fossil fuel powered vehicle. Further, the costs of batteries 
are significantly more than that of a comparable fossil fuel powered 
vehicle. 
Hybrid powered vehicles, powered by both an internal combustion engine and 
an electric motor, have been widely proposed for overcoming the technical 
disadvantages of a dedicated electric vehicle while still offering an 
increased efficiency. The performance and range characteristics of a 
hybrid powered vehicle is comparable to a conventional fossil fuel powered 
vehicle. However, a great deal of development is still necessary in order 
to provide a hybrid electric vehicle which would be widely accepted by the 
consuming public. 
The present invention deals with the issue of determining a desirable 
amount of braking torque distribution by an electric motor/generator and a 
friction brake system of a hybrid electric vehicle in order to provide 
efficient regeneration of braking energy into stored energy. 
Accordingly, it is an object of the present invention to provide an 
improved brake blending strategy for a hybrid powertrain system. 
To achieve the foregoing object, the present invention provides a hybrid 
electric powertrain system for a vehicle, including a transmission for 
driving a pair of wheels of the vehicle. A heat engine and an electric 
motor/generator are coupled to the transmission. A friction brake system 
is provided for applying a braking torque to the vehicle. A controller 
unit is provided for generating control signals to the electric 
motor/generator and the friction brake system for controllably braking the 
vehicle in response to a driver's brake command. The controller unit 
determines an amount of regenerative torque available and compares this 
value to a determined amount of brake torque requested for determining the 
control signals to the electric motor/generator and the friction brake 
system. 
Further areas of applicability of the present invention will become 
apparent from the detailed description provided hereinafter. It should be 
understood however that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are intended for 
purposes of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a hybrid powertrain system 10, according to the 
present invention, is illustrated for a motor vehicle, generally shown at 
8. The hybrid powertrain system 10 includes a heat engine 14 operating on 
a hydrocarbon based or fossil fuel. In this example, the engine 14 is a 
compression-ignited engine fueled by a diesel fuel. Preferably, the engine 
14 is sized comparable to an engine for a non-hybrid motor vehicle. 
The hybrid powertrain system 10 also includes a clutch mechanism 16, as is 
known in the art, for operably interconnecting engine 14 and transmission 
18. The Clutch mechanism 16 compensates for the difference in rotational 
speed of engine 14 and transmission 18, to smooth engagement between 
engine 14 and transmission 18. 
Transmission 18 connects to engine 14 through clutch 16 and transmits 
engine rotation and power at various ratios to a pair of drive wheels 26 
of the motor vehicle. Thus, transmission 18 enables the motor vehicle 8 to 
accelerate through predetermined gear ratios, while engine 14 functions 
within a predetermined operating range. Examples of known transmission 
types include an automatic transmission, a manual transmission and a 
continuously variable transmission. It should be appreciated that in a 
preferred embodiment transmission 18 is a four or five-speed manual 
transmission as is well known in the art. 
Transmission 18 drives a differential unit 28. Differential unit 28 engages 
a pair of axle shafts 30 which are operably connected to the pair of 
wheels 26. 
The hybrid powertrain system 10 also includes an electric motor 32 operably 
connected to transmission 18 at the opposite end of an input shaft from 
clutch 16. Electric motor 32 is connected to the input shaft opposite from 
clutch 16 by a gear train 33. The electric motor 30 is able to provide 
both positive and regenerative torque, by functioning as a motor and a 
generator, respectively. An example of an electric motor 32 is an 
induction motor or a permanent magnet motor, such as manufactured by 
Delphi Electronics Corporation. 
As a generator, electric motor 32 produces a regenerative torque, 
preferably as an alternating current (A/C), which is transferred to a 
control mechanism, such as a motor controller 34. Motor controller 34 
changes the alternating current into a direct current (D/C), as is well 
known in the art. The direct current may then be transmitted to an energy 
storage apparatus 38, such as a battery. Alternatively, as a motor, the 
electric motor 32 produces a positive torque that is applied to the input 
shaft of the transmission 18 and is ultimately used to drive wheels 26. 
Motor vehicle 8 is provided with a regenerative braking system, capable of 
capturing kinetic energy from the momentum of the motor vehicle as it is 
slowing down and storing this energy as potential energy in the energy 
storage apparatus 38 to be described. Electric motor 32 is controlled to 
slow the motor vehicle down by applying a braking force that slows down 
the rotation of the input shaft. Electric motor 32 functions as a 
generator and captures the reverse energy flow. Motor vehicle 8 is also 
provided with a friction brake system which includes a brake controller 46 
and a plurality of friction brakes assemblies 48 which apply a braking 
force to the wheels 26 of the vehicle 8. 
Hybrid powertrain system 10 also includes a transmission controller 50, 
such as an electronic control unit. Transmission controller 50 enables 
electronic control of transmission 18 to enable the transmission 18 to be 
configured as a manual-style transmission, but to be operated from a 
drivers standpoint as an automatic transmission. To effect such operation, 
transmission 18 has a pair of actuators 52 and 54 which simulate 
positioning of the stick shift actuators as in a conventional manual 
transmission. Further, actuator 56 enables operation of clutch 16 in 
replacement of a clutch pedal as on a conventional manual transmission. In 
order to generate such control signals, transmission controller 50 
receives input signals from engine 14 or an engine controller 58. Examples 
of such information received from engine 14 or engine controller 58 
include vehicle speed, RPM, or the like. Similarly, transmission 
controller 50 generates output signals to control actuators 52, 54, and 56 
and also outputs diagnostic and other communication signals to engine 14 
and/or engine controller 58. Transmission controller 50 may also receive 
other vehicle condition signals, depending on a particular configuration 
of the transmission 18. 
In operation, as will be described in greater detail herein, transmission 
controller 50 receives input signals from engine 14, engine controller 58, 
clutch 16, clutch actuator 56, transmission 18, and from additional 
sensors. Actuator 56 is preferably a rotary actuator which causes linear 
movement to effect engagement and disengagement of clutch 16. With respect 
to actuators 52 and 54, these actuators combine to mimic movement of the 
shift lever in a conventional manual transmission. That is, in visioning 
the standard "H" shaped shift configuration, actuator 52 may operate as 
the cross over actuator, i.e., determining what leg of the "H" the shifter 
is in. Similarly, actuator 54 operates as a select actuator which mimics 
an upward or downward movement of the shifter within the leg of the H. The 
actuators 52, 54, and 56 receive control signals from transmission 
controller 50 to operate the shifting portion of transmission 18 as in a 
conventional manual transmission. Further, transmission controller 50 
sends control signals to electric motor 32 through motor controller 34, to 
effect activation and deactivation of electric motor 32 as determined by 
the control strategy described herein. Transmission controller 50 also 
sends control signals to the friction brake controller 46 as determined by 
the control strategy described herein. 
Hybrid powertrain system 10 includes an energy storage apparatus 38, such 
as battery, to store potential energy for later use by the motor vehicle. 
For example, the potential energy stored in the battery may be 
transferred, as DC current, to operate an accessory component 40. In a 
typical motor vehicle, engine 14 operably supplies a battery with 
potential energy. In this example, electric motor 32 operating as a 
generator supplies battery 38 with potential energy for storage. 
Hybrid powertrain system 10 includes at least one accessory component 40. 
An example of an accessory component may be a power steering pump, a water 
pump, a lighting system, and a heating and cooling system, which are all 
conventional and well known in the art. Accessory components 40 are 
usually mechanically driven by the engine 14 or electrically powered with 
energy from battery 38. For example, accessory component 40, such as the 
power steering pump, is operably connected to engine 14 and mechanically 
driven by engine 14. The lighting system relies on energy supplied by the 
battery 38, as a source of power. 
Upon command from the motor controller 34, battery 38 supplies potential 
energy, such as a D/C current, to motor controller 34, which converts it 
into an A/C current. The A/C current is directed to the electric motor 32, 
causing it to act as a motor and produce a positive torque. The positive 
torque is applied to the transmission 18, which in turn induces the 
rotation of the axle shaft 30 and the rotation of the drive wheels 26 of 
the motor vehicle. 
With reference to FIGS. 2-3, a brake blending strategy for a hybrid vehicle 
will be described. 
A braking command B.sub.pos from the driver is received as input 1 (100). 
The brake command B.sub.pos is multiplied by a gain K (102) to determine 
the total brake torque requested at the axle T.sub.r. The torque requested 
at the axle T.sub.r is supplied as a second input to multiplexer Mux1. 
In order to determine the regenerative torque available, the engine speed 
S.sub.e (input 3) is multiplied by the motor to engine gear ratio 
(mot.sub.-- gr) (104) which provides a motor speed S.sub.m for the 
regenerative torque available determination. As shown in FIG. 3, the 
regenerative torque available from the motor T.sub.rm is determined from a 
lookup table (106) based upon the motor speed S.sub.m. The torque T.sub.rm 
from the table is then multiplied by the motor to engine gar ratio (108) 
to determine the torque available at the transmission input shaft 
T.sub.is. 
Available regenerative torque to the final drive T.sub.fd is determined 
(110) by multiplying the torque available at the transmission input shaft 
T.sub.is by the gear ratio determined in the Look-Up Table of gear ratios 
(112) from the gear number G.sub.n. This torque T.sub.fd is then 
multiplied by the gear ratio of the final drive (114) to determine the 
regenerative torque available at the axle T.sub.ra. 
If a shift is commanded as determined from input 2, the regenerative torque 
available is set to zero by providing a boolean value of 0 or 1 and 
multiplying the boolean value by the regenerative torque available at the 
axle T.sub.ra (116). In this case, if a shift sequence is taking place, 
the boolean value is set to zero. Therefore, during a shift sequence, the 
regenerative torque available T.sub.ra ' is set to zero. 
If the motor speed S.sub.m is below a predetermined level as prescribed as 
a minimum motor speed for regeneration (118), then a boolean value of zero 
is multiplied by the total regeneration torque available value T.sub.ra ' 
at block (120). Accordingly, the total regeneration torque available 
T.sub.ra " would then be set to zero in order to provide a regeneration 
cutout for the minimum motor speed. If the motor speed S.sub.m is above 
the predetermined level, then a boolean value of one is multiplied by the 
value T.sub.ra '. The total regenerative torque available T.sub.ra " is 
then input into the first input of the multiplexer (Mux1). 
A motor command for regeneration is determined in block 122 labeled "regen 
blend." If no regenerative torque T.sub.ra " is available, then the 
command will be set to zero by the boolean multiplier u[1]&gt;0 which will be 
equal to zero if the value T.sub.ra " input into the first input of the 
multiplexer (Mux1) is not greater than zero. However, if the value 
T.sub.ra " is greater than zero, the boolean multiplier value would be 
one. 
If the requested braking torque T.sub.r (u[2]) is greater than the 
regeneration torque available T.sub.ra "(u[1]), then the motor commands MC 
for regeneration is set to 1. This is because the boolean multiplier 
u[1]&gt;u[2] would be false and therefore, the boolean multiplier (u[1]&gt;u[2]) 
in the equation ((u[1]&gt;u[2])*(u[2]/u[1])) would be equal to zero while the 
addition of the boolean value for (u[1]&lt;u[2]) would be one. Accordingly, 
the equation (u[1]&gt;0)*(((u[1]&gt;u[2])*(u[2]/u[1]))+(u[1]&lt;u[2])) would result 
in 0+1=1. If the requested braking torque T.sub.r is less than the 
regeneration torque available T.sub.ra ", then block 122 would provide a 
motor command MC for regeneration that is proportional to the difference 
of the requested braking torque and the regeneration torque available. In 
other words, the equation 
EQU (u[1]&gt;0)*(((u[1]&gt;u[2])*u[2]/u[1]))+(u[1]&lt;u[2])) 
would result in a motor command 
EQU MC=1*(1*u[2]/u[1])+0=u[2]/u[1]). 
This command MC is then sent to a motor controller 34. 
If the requested braking torque T.sub.r is in excess of the regeneration 
torque available T.sub.ra "(u[2]&gt;u[1]), then a brake signal BC 
representative of the difference between the requested braking torque 
T.sub.r and the regeneration torque available T.sub.ra "(u[2]-u[1]) is 
sent to the brake controller 46 at block 124. If T.sub.r is less than 
T.sub.ra ", then the boolean multiplier (u[2]&gt;u[1]) would be zero 
resulting in a brake signal BC of zero. 
According to the present invention, a brake blending strategy is provided 
for a hybrid electric vehicle in order to efficiently regenerate stored 
energy from braking energy. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.