WO2013038329A2

WO2013038329A2 - A controller and method for energy dissipation

Info

Publication number: WO2013038329A2
Application number: PCT/IB2012/054698
Authority: WO
Inventors: Alexander Fraser
Original assignee: Protean Electric Limited
Priority date: 2011-09-13
Filing date: 2012-09-10
Publication date: 2013-03-21
Also published as: GB2483375B; WO2013038329A3; GB201115798D0; GB2483375A

Abstract

A controller for a vehicle having an electric motor for driving the vehicle and a friction brake for inhibiting movement of the vehicle, the controller comprising means arranged to generate a control signal for actuating the electric motor to consume current for generating a drive torque and means for actuating the friction brake to dissipate energy resulting from the drive torque produced by the electric motor while the drive torque is being generated.

Description

A CONTROLLER AND METHOD FOR ENERGY DISSIPATION

The present invention relates to a controller, in particular a controller for dissipating regenerative energy.

With increased interest being placed in environmentally friendly vehicles there has, perhaps unsurprisingly, been a corresponding increase in interest in the use of electric vehicles .

Electric vehicles typically use an electric motor to provide both mechanical drive for the vehicle and

regenerative braking for stopping the vehicle. To effect regenerative braking rotary motion of a drive wheel

connected to an electric motor is converted into electric energy. This involves consumption of kinetic energy that provides a braking force to the drive wheels by applying a braking torque in an opposite direction to the rotation of the drive wheels.

The electric energy generated during regenerative braking results in current flow within coil windings of the electric motor, where the current is typically directed to an energy storage device. The recovered energy can then be used, when required, to drive the electric motor, thereby increasing the operational efficiency of the electric motor.

However, if a condition should occur that prevents regenerative current being stored within the energy storage device this can result in a reduction in regenerative braking torque.

One solution to this problem has been to use dump resistors, which are used within a vehicle to absorb

regenerative currents that cannot be stored within an energy storage device, thereby ensuring that regenerative braking torque is not compromised should a condition occur that prevents regenerative current being stored within the energy storage device. However, dump resistors can be bulky and expensive .

It is desirable to improve this situation.

In accordance with an aspect of the present invention there is provided a controller, a vehicle and a method according to the accompanying claims.

This provides the advantage of allowing regenerative current to be dissipated by redirecting the current to drive a second motor with the resultant drive torque energy being absorbed by a friction brake system.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which :

Figure 1 illustrates a vehicle according to an

embodiment of the present invention;

Figure 2 illustrates an exploded view of an electric motor as used in an embodiment of the present invention;

Figure 3 illustrates an exploded view of the electric motor shown in Figure 2 from an alternative angle;

Figure 4 illustrates an embodiment of a friction brake in accordance with an embodiment of the present invention;

Figure 5 illustrates a graphical representation of variation in current during a braking event according to an embodiment of the present invention; Figure 6 illustrates a graphical representation of variation in torque during a braking event according to an embodiment of the present invention. Figure 1 illustrates a vehicle 100, for example a car or lorry, having four wheels 101, where two wheels are located in the vehicle's forward position in a near side and off side position respectively. Similarly, two additional wheels are located in the vehicle's aft position in near side and off side positions respectively, as is typical for a conventional car configuration. However, as would be appreciated by a person skilled in the art, the vehicle may have any number of wheels. For the purposes of the present embodiment,

incorporated within each wheel 101 is an in-wheel electric motor that is arranged to provide mechanical drive and brake torque to the respective wheel, where the in-wheel electric motor is described in detail below.

The vehicle includes a battery 120 that acts as a power source for the in-wheel electric motors. The battery 120 also acts as an energy storage device for holding charge generated by the in-wheel electric motors when the electric motors operate in a regenerative braking mode.

The in-wheel motors are arranged to operate in a drive mode for providing mechanical drive power in the form of drive torque to their respective wheels to enable the vehicle to move in a forward or reverse direction and a brake mode for providing a brake torque to inhibit movement of the vehicle.

Additionally, the vehicle includes a vehicle controller 150 for controlling the operation of the in-wheel motors. For the purpose of illustration, the in-wheel electric motor is of the type having a set of coils being part of the stator for attachment to the vehicle 100, radially

surrounded by a rotor carrying a set of magnets for

attachment to a wheel. However, as would be appreciated by a person skilled in the art, the present invention is applicable to other types of electric motors.

As illustrated in Figure 2, the in-wheel electric motor 40 includes a stator 252 comprising a rear portion 230 forming a first part of the housing of the assembly, and a heat sink and drive arrangement 231 comprising multiple coils and electronics to drive the coils. The coil drive arrangement 231 is fixed to the rear portion 230 to form the stator 252 which may then be fixed to a vehicle and does not rotate during use. The coils themselves are formed on tooth laminations which together with the drive arrangement 231 and rear portion 230 form the stator 252. A rotor 240 comprises a front portion 220 and a

cylindrical portion 221 forming a cover, which substantially surrounds the stator 252. The rotor includes a plurality of magnets 242 arranged around the inside of the cylindrical portion 221. The magnets are thus in close proximity to the coils on the assembly 231 so that magnetic fields generated by the coils in the assembly 231 cooperate with the magnets 242 arranged around the inside of the cylindrical portion 221 of the rotor 240 to cause the rotor 240 to rotate. The rotor 240 is attached to the stator 252 by a bearing block 223. The bearing block 223 can be a standard bearing block as would be used in a vehicle to which this motor assembly is to be fitted. The bearing block comprises two parts, a first part fixed to the stator and a second part fixed to the rotor. The bearing block is fixed to a central portion 233 of the wall 230 of the stator 252 and also to a central portion 225 of the housing wall 220 of the rotor 240. The rotor 240 is thus rotationally fixed to the vehicle with which it is to be used via the bearing block 223 at the central portion 225 of the rotor 240. This has an advantage in that a wheel rim and tyre can then be fixed to the rotor 240 at the central portion 225 using the normal wheel bolts to fix the wheel rim to the central portion of the rotor and consequently firmly onto the rotatable side of the bearing block 223. The wheel bolts may be fitted through the central portion 225 of the rotor through into the bearing block itself. With both the rotor 240 and the wheel being mounted to the bearing block 223 there is a one to one correspondence between the angle of rotation of the rotor and the wheel. Figure 3 shows an exploded view of the same assembly as

Figure 2 from the opposite side showing the stator 252 comprising the rear stator wall 230 and coil and electronics assembly 231. The rotor 240 comprises the outer rotor wall 220 and circumferential wall 221 within which magnets 242 are circumferentially arranged. As previously described, the stator 252 is connected to the rotor 240 via the bearing block at the central portions of the rotor and stator walls.

Additionally shown in Figure 2 are circuit boards 80 carrying control electronics, otherwise known as motor drive controllers or inverters, for controlling current flow in the coils in response to receiving a torque demand control signal from the vehicle controller 150. In particular, each inverter is arranged to provide PWM voltage control across the respective coils to provide a required torque in

response to a control signal output by the vehicle

controller 150, where the vehicle controller 150 controls operation of the respective electric motors via the

application of control signals. As such, the electric motors are arranged to provide mechanical drive power via the consumption of electrical power. A V shaped seal 350 is provided between the

circumferential wall 221 of the rotor and the outer edge of the stator housing 230. The rotor also includes a focussing ring and magnets

227 for rotor position sensing, which in conjunction with sensors mounted on the stator allows for an accurate

position determination of the rotor relative to the stator to be made.

To supplement the braking torque that can be provided by the electric motors, there is incorporated within the vehicle 100 a brake system in which a friction brake, for example a disc or drum brake, is associated with at least one of the wheels 101 to allow a brake force to be applied to the at least one of the wheels 101. For the purposes of the present embodiment, respective friction brakes are mounted to provide a friction braking torque to each of the wheels located in the vehicle's forward position, thereby allowing a braking force to be imparted to each of the front wheels. However, a friction brake can be associated with any number of the vehicle's wheels.

Each friction brake includes a brake assembly and a brake disc.

The brake assembly is mounted to a mounting point on the vehicle 100, for example a mounting point on a part of the vehicle's suspension system.

The brake disc is mounted to a portion of a bearing block that is arranged to rotate relative to the vehicle and to which a wheel is mounted. Each brake assembly is mounted in a position that allows the brake assemblies to apply a brake force on the brake disc, as is well known to a person skilled in the art. The application of a brake torque is typically

initiated via a brake pedal 130 located within the vehicle 100. The brake torque provided may be a combination of friction brake torque provided by the brake assemblies and electric motor brake torque, as described below.

The brake pedal 130 is coupled to the vehicle

controller 150, where the vehicle controller 150 is arranged to apply a friction and electric motor brake torque

dependent upon the force being applied to the brake pedal and whether regenerative current can be provided to the battery . To actuate the friction brake system, the vehicle controller 150 is coupled to an actuator that in turn is coupled to a master cylinder assembly 140 having a piston assembly, where the piston assembly is arranged to vary hydraulic pressure in hydraulic lines that form part of the friction brake system, dependent upon a force being applied to the brake pedal 130. However, as stated above, the friction brake torque can be generated via other means, for example the use of pneumatic pressure. The hydraulic fluid pressure in hydraulic lines, where the hydraulic lines are coupled to friction brakes, is used to apply a force between a piston and a calliper that forms part of the brake assembly to cause brake pads to be brought into contact with the brake disc, thereby applying a braking force to the brake disc.

Preferably, to reduce space requirements the brake assembly has a sliding calliper arrangement for applying a braking force to the brake disc, as is well known to a person skilled in the art. However, a fixed calliper arrangement can be used. One embodiment in which a friction brake is arranged to provide a braking force to a wheel that is also coupled to an in-wheel motor is illustrated in Figure 4, where a brake assembly is preferably mounted to the stator with a brake disc being mounted to the rotor.

Figure 4 illustrates a friction brake embodiment for use with an in-wheel electric motor where each brake

assembly 270 includes a carrier 310 mounted to the vehicles suspension system. The carrier 310 is arranged to support a brake calliper 320. The brake calliper 320 is a U shaped element arranged to surround the outer edge of the brake disc 260. The brake calliper 320 is mounted to the carrier 310 in a manner that allows the brake calliper 320 to slide relative to the carrier 310 in an axial direction.

A recess is formed on an inner face of the calliper 320 facing the outer surface of the brake disc, which is mounted axially in-board of the wheel. A piston is mounted in the recess. A first brake pad 340 is mounted on the piston with a second brake pad 360 being mounted on an inner surface of the calliper 320 that faces an opposite surface of the brake disc 260 to that of the piston 330. This allows both brake pads 340, 360 to be separated from each other by the brake disc 260 with one brake pad facing one side of the brake disc and a second brake pad facing the other side of the brake disc. The brake pads 340, 360 are arranged to impart a tangential force (i.e. a braking force) to the annular disc 260 when the brake pads 340, 360 are pressed against the annular disc 260.

Upon activation of the brake pedal 130, for a given brake torque demand the vehicle controller 150 is arranged to control the electric motors and the friction brake system to provide the required brake torque, where the relative amount of regenerative brake torque and friction brake torque will be dependent upon predetermined criteria. If the torque demand can be handled by the regenerative braking torque capabilities of the electric motor and the battery 120 has sufficient storage capacity to accept and store the regenerative current generated by the electric motors the vehicle controller 150 is arranged to provide all the required braking torque via regenerative braking. For greatest braking stability typically a larger proportion of the braking torque will be directed to the front wheels of the vehicle.

If the torque demand exceeds the braking torque

capabilities of the electric motors the vehicle controller 150 is arranged to provide the required braking torque by supplementing the regenerative braking with friction braking torque via the friction brake system.

If, however, the battery 120 is not capable of

receiving all the current that would be generated by the electric motors when in regenerative braking mode, for example when the charge in the battery 120 is above a predetermined level, the temperature of battery cells are above a threshold value, or a fault has occurred with the battery 120, upon application of a force to the brake pedal 130 the vehicle controller 150 is arranged to place the electric motors coupled to the rear wheels in a regenerative braking mode to provide a brake torque to the rear wheels with the current generated by the rear electric motors being made available to the electric motors mounted at the front of the vehicle. The vehicle controller 150 controls the electric motors mounted at the front of the vehicle 100 via one or more control signals to consume electrical power using the current generated by the rear electric motors to generate mechanical drive power.

To counteract the drive torque generated by the

electric motor located at the front of the vehicle 100 the vehicle controller 150 is arranged to increase the braking torque generated by the friction brakes located at the front of the vehicle 100 to react the drive torque generated as a result of the regenerative current being used to drive the front electric motors. Additional friction brake torque may also be provided at the front wheels to provide the overall require brake torque.

In the preferred embodiment, the front electric motors, which are arranged to consume the electrical power generated by the rear electric motors and generate drive power, and the respective friction brakes, which dissipate the

mechanical drive power as heat, are preferably located on the same wheel. In this configuration, the respective electric motors and brakes form a closed system, where the road-tyre interface does not part of the mechanical power transmission path between them. This results in there being no net torque change at the respective front wheels. Although it would be possible to mount an electric motor and brake to separate wheels, this would result in the tyre-road interfaces forming part of the mechanical power transmission path, which would result in, amongst other effects, a lowering of the overall available grip.

Figure 5 and Figure 6 illustrate a graphical

representation of a braking event in accordance with an embodiment of the present invention. Figure 5 illustrates variation in current generated during a braking event with the y-axis representing current in amps and the x-axis representing time.

Line A in Figure 5 represents the battery current limit, that is to say the maximum current that can be provided to the battery 120. Line B in Figure 5 represents - li the net battery current, that is to say the amount of current provided to and by the battery 120.

Figure 6 illustrates variation in torque during a braking event with the y-axis representing torque in Nm and the x-axis representing time.

Line C in Figure 6 represents the motor torque

generated by the rear electric motors. Line D in Figure 6 represents the motor torque generated by the front electric motors. Line E in Figure 6 represents the braking torque generated by the front friction brakes. Line F in Figure 6 represents the overall torque at the front wheels. It should be noted that negative torques and positive currents correspond to braking torque and regenerative current respectively; while a positive torque corresponds to a drive torque. To simplify the graphical representation inefficiencies within the system are not illustrated.

At t=0, the vehicle is braking with regenerative torque at each of the front and rear electric motors of 700 and 600Nm respectively. Each of the front friction brakes are braking with a torque of 1300Nm. This results in a total braking torque of 600Nm at each rear wheel and 2000Nm at each front wheel. In this embodiment this results in a net regenerated current of 200A.

At t=10seconds the battery current limit begins to drop from the 500A starting point. The drop in current can be a result of any reason, for example thermal restrictions or state of charge. While the battery current limit is above the net regenerated current no action is required and the electric motors continue to provide the same braking torque. However, after t=21seconds to maintain the net battery current within the battery current limit the net battery current is reduced to keep it in line with the reducing battery current limit by reducing the amount of regenerative braking provided by the electric motors.

To prevent a reduction in rear braking torque the vehicle controller reduces the front regenerative torque resulting in the required lowering of the net battery current. The front friction braking torque is increased to compensate for the reduction in front regenerative torque. At t=24 seconds, to maintain the net battery current within the battery current limit the vehicle controller 150 stops the front electric motors from providing any

regenerative brake torque with the friction brakes providing all required braking torque for the front wheels.

After t=24 seconds, to accommodate the continuing reduction in battery current limit while preventing a reduction in rear braking torque provided by the rear electric motors, the vehicle controller 150 issues a control signal for controlling the front electric motors to provide mechanical drive power in the form of drive torque with the front electric motors being arranged to consume current regenerated by the rear electric motors. By controlling the front electric motors to provide mechanical drive power using the current generated by the rear electric motors the vehicle controller 150 is able to keep the net battery current within the battery current limit .

To compensate for the drive torque provided by the front electric motors the vehicle controller 150 controls the braking system actuator to increase the friction braking torque .

To maintain the net battery current within the battery current limit the drive torque generated by the front electric motors is increased with a corresponding increase in braking torque being provided by the friction braking system until t=30 seconds, where the system reaches a steady state with the battery current limit having been reduced to zero amps.

After t=30seconds the front electric motors provide a drive torque of 600Nm with the front friction brakes providing a braking torque of 2600Nm, where 600Nm of the friction braking torque is for the purposes of compensating for the 600Nm drive torque generated by the front electric motors .

By the vehicle controller 150 controlling the electric motor system to re-route excess energy within the electric motor system produced by a first electric motor to drive a second electric motor, where the resultant drive torque energy is dissipated through the front friction brakes, the present embodiment allows the vehicle controller 150 to maintain a required braking torque for a vehicle, where the rear electric motor torque does not need to be reduced and without changing the total front wheel braking torque, while also ensuring that battery current limits are observed and without affecting the dynamics of the vehicle.

Claims

1. A controller for a vehicle having an electric motor for providing mechanical drive power to a wheel of the vehicle and a friction brake for dissipating mechanical power, the controller comprising means arranged to generate a control signal for actuating the electric motor to consume electrical power for generating mechanical drive power;

means for actuating the friction brake to dissipate energy resulting from the drive power produced by the electric motor while the drive power is being generated; and means for providing electrical power to the electric motor when the control signal is generated, wherein the electrical power is generated by a second electric motor.

2. A controller according to claim 1, wherein the electrical power generated by the second electric motor is regenerative electrical power generated when the second electric motor is placed in a braking mode.

3. A vehicle comprising a controller according to any preceding claim.

4. A vehicle according to claim 3, further comprising a first electric motor arranged to drive a first wheel and a second electric motor arranged to drive a second wheel and a friction brake arranged to apply a braking torque to the second wheel, wherein the first electric motor is arranged to generate electrical power when placed in a brake mode and the controller is arranged to generate a control signal for actuating the second electric motor to consume electrical power for generating drive power and is arranged to actuate the friction brake to dissipate energy resulting from the drive power produced by the second electric motor while the drive power is being generated.

5. A vehicle according to claim 4, wherein the first wheel is located at the rear of the vehicle and the second wheel is located at the front of the vehicle.

6. A method of dissipating electrical power in a vehicle having an electric motor for providing mechanical drive power to a wheel of the vehicle and a friction brake for dissipating mechanical power, the method comprising generating a control signal for actuating the electric motor to consume electrical power for generating mechanical drive power and actuating the friction brake to dissipate energy resulting from the drive power produced by the electric motor while the drive power is being generated, wherein the electrical power is generated by a second electric motor.

7. A method according to claim 6, wherein the

electric motor is actuated to consume electrical power and the friction brakes are actuated to maintain a required braking force while maintaining regenerated current to a battery located in the vehicle to the battery' s current limit .