TORQUE CONTROL SYSTEM

FIELD OF THE INVENTION

The present invention relates to vehicles of the type driven by electric motors, with a separate motor to drive each wheel.

Using electric wheel motors to power motor vehicles allows the torque at each wheel to be applied independently and changed almost instantaneously. This offers the opportunity to improve the handling and safety of the vehicle. This applies not only during extreme events, such as anti-skid functions, but also during normal driving.

BACKGROUND OF THE INVENTION

Technology exists to redistribute torque between front and rear wheels and between left and right wheels by mechanical means that may be used where a single motor is used to power the vehicle. Algorithms for redistributing the torque by estimating tyre slip or acceleration in order to avoid skidding are provided in, for example:

US Patent 5701247, "Integrated Control System for 4WD Vehicles for Controlling Driving Torque Distribution"

US Patent 5262950, "Torque Distribution Control Apparatus for Four Wheel Drive".

SUMMARY OF THE INVENTION

The present invention relates to the use of the calculated or measured normal forces to obtain an optimal torque distribution under normal driving manoeuvres, and the application to a vehicle driven by electric wheel motors.

A novel method for distributing torque independently to the four wheels of a vehicle powered by four electric wheel motors is presented. Since wheel traction is a function of the normal force between road and tyre, the torque is applied

according to the estimated or measured normal forces acting at each of the wheels, so as to supply, in total, the torque demanded by the driver. Torque may be distributed amongst the four wheels dependent on longitudinal acceleration or braking, cornering, road inclination and camber and speed related down-force. The results of the invention are that more torque may be applied for acceleration or braking without causing skid, and better handling of the vehicle.

The invention is defined in the claims to which reference is now directed.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a model of the forces on a 4 wheel vehicle;

Figure 2 shows a schematic view of a wheel and suspension arm;

Figure 3 is a schematic view of an electric motor; and Figure 4 shows a control arrangement.

DETAILED DESCRIPTION

Overview

The amount of traction between a tyre and the road surface is proportional to the normal force between the two, which is equivalent to the weight distribution amongst the four wheels of the vehicle. The invention utilises this fact to apply torque according to the weight distribution amongst the wheels, the total torque being that which is demanded by the driver at that instant. In this case braking is regarded as a demand for negative torque which is the mechanism for braking using electric wheel motors.

The weight distribution of the vehicle whilst static on a flat road, not inclined or cambered, may be regarded as the base weight distribution. During longitudinal acceleration (acceleration in the direction of travel) or braking weight is redistributed to the rear or front wheels respectively. Similarly, weight is redistributed to the outer wheels whilst cornering and to the lower wheels when the car is on a sloped road. Furthermore, the aerodynamics of some vehicles creates speed related down-force, creating additional normal forces at front and rear wheels. In response, according to the invention, torque is redistributed from a base distribution.

Description of preferred arrangements

The preferred method for calculating the torque to be applied comprises four main steps:

1. determination of static characteristics of the vehicle and expression in terms of parameters of a vehicle model;

2. determination of the vehicle's speed, longitudinal acceleration, lateral acceleration, inclination and tilt;

3. a calculation of the instantaneous normal force at each of the wheels;

4. a calculation of the torque to be applied at each wheel given the estimated normal forces.

The calculation of the instantaneous normal force at each wheel is carried out with a simplified model of the vehicle based on the "equivalent roll stiffness model" presented in "The Multibody Systems Approach to Vehicle Dynamics", Blundell and Harty (Elsevier Ltd, ISBN-13: 978-0-7506-5112-7). In this model the front and rear axles are separate mass-less rigid bodies that are connected to a massive body that can revolve around an axis running from the front to the rear of the vehicle at a specified height above the ground. The connections are revolute joints with given torsion coefficients representing the resistance to roll of the vehicle's body. The centre of mass is positioned at a given height above the axis of revolution, and at a given point along its length. The geometry of the model of the vehicle is shown in Figure 1.

The XYZ coordinate system is defined with respect to the vehicle such that the X direction is the straight line forward direction of motion of the vehicle and the Z- axis is vertical when the vehicle is standing on level ground. CoM is the position of the centre-of-mass of the body of the vehicle and the coordinate system is such that its position satisfies .

In this model the roll-axis of the body of the vehicle is assumed to be in the plane, as is the centre-of-mass. The revolute joints at the front and rear axes are positioned above the centre-points between the front and rear wheels respectively. Longitudinal acceleration and forces refer to the X direction, lateral acceleration and forces refer to the Y direction.

For any given vehicle the parameters of the model must be determined, namely: 1. the length of the wheel-base, a + b, 2. the track width at the front and rear, t _f and t _b ,

3. the height of the revolute joint at the rear axle above ground, h _b ,

4. the position of the revolute joint above the front axle, h _f ,

5. the torsion coefficient of the revolute joint at the rear axle, K _b ,

6. the torsion coefficient of the revolute joint at the front axle, K _f

7. the height of the centre-of-mass above the roll-axis of the body of the vehicle, h ,

8. the position of the centre-of-mass along the wheel-base of the vehicle, a , b ,

9. the mass of the body of the vehicle, m .

In order to account for aerodynamics and rolling resistance some dynamic quantities associated with the vehicle are also required:

1. the rolling resistance coefficient, relating the rolling resistance force, F to the normal force, , according to 2. an air resistance coefficient, C _ar , relating the drag force, F _{x ar} , to the vehicle speed, v , according to ² (note that this is different from the standard definition of "drag coefficient" as it encapsulates also the projected frontal area and the air density)

3. a height above ground level at which air resistance acts, on average, h _ar ,

4. a front-axle down-force coefficient, relating the aerodynamic down- force, F _f at the front wheels (in total) to the vehicle speed according to

5. an equivalent rear-axle down-force coefficient,

Determination of Vehicle's Orientation, Speed and Acceleration

In the preferred method the vehicle's speed, longitudinal acceleration and lateral acceleration are calculated from the measured wheel speeds by well-known methods that do not form part of the invention. Inclination and tilt, which characterise the slope and camber of the road, will be ascertained using sensors and this is also not part of the invention.

Calculation of Normal Forces at Wheels

The formulae for the normal forces at each wheel are derived using the model, starting from a base weight distribution based on the vehicle being static on a flat road, and adding corrections separately according to:

1. the longitudinal acceleration (or deceleration);

2. the lateral acceleration due to cornering;

3. aerodynamic down-force, drag and rolling resistance;

4. the inclination (up-hill or down-hill) and tilt (left or right).

The model of the vehicle and the equations derived are a simplification of the real problem, ignoring some details of the vehicle statics and dynamics and certain effects. Nevertheless the formulae are preferred for the torque distribution problem as they provide a good compromise between the tractability of the problem and accuracy of the solution. This analytical approach also provides an insight into the main causes of weight distribution of the vehicle and their dependencies.

Base Weight Distribution

The base weight distribution represents the normal forces (force in Z direction) at each wheel when the vehicle is stationary on a level surface. They are denoted a _n d _for front-left, front-right, back-left and back-right wheels respectively.

In this scenario there are no lateral forces on the wheels and the centre-of-gravity is directly above the roll-axis so that there is no moment of force acting around this axis. Balancing moments of force about the roll-axis at the rear and front axes requires

Balancing moments of force about the Y axis requires Finally balancing vertical forces gives where g * 9.81 ms ^"2 is the gravitational acceleration.

Solving equations i), ii) and iii) yields

Longitudinal Acceleration

Here the weight redistribution due to accelerating or braking is calculated. The driver is assumed to make a torque demand of the vehicle which is positive to achieve acceleration and negative to achieve braking. In the calculation presented here it is further assumed that all wheels have the same radius so that a total torque demand of can be related to a total longitudinal force demand of w _{nere r} is the wheel radius.

No moment of force is generated around the roll-axis due to the application of longitudinal torque so there is no weight redistribution from right to left or left to right. The total weight redistribution from front to rear wheels is denoted * which would normally be a negative quantity under braking. Based on this quantity, corrections to the normal forces on each of the wheels for longitudinal acceleration are

The weight redistribution can be calculated by considering the moments of force around an axis through the centre-of-mass in the Y-direction. The longitudinal p forces generated between the tyres and road surface sum to ^x - ^d and act perpendicular to the roll-axis at a distance given by the height of the centre-of- mass above the ground,

The moment of force about the axis due to the longitudinal forces is which needs to be balanced by the weight redistribution according to

This quantity is positive for acceleration, representing a shift of weight to the rear wheels, and negative for braking, representing a shift of weight to the front wheels, as expected.

Lateral Acceleration

In this section the weight redistribution due to lateral acceleration, , is calculated. Such acceleration arises from cornering with a turning radius given by where is the vehicle's speed. Following the geometry illustrated in

Figure 1 , a positive value of ^y corresponds to a left turn, and a negative value to a right turn.

In such a manoeuvre no new moments of force are generated around the Y-axis and so there is no weight redistribution from front to rear wheels nor from rear to front wheels. Two quantities are sought, and , representing the weight shift from left wheel to right wheel at the front axle and rear axle respectively. Using these two quantities the changes in normal forces at the four wheels due to lateral acceleration are

Acceleration acting through the centre-of-mass in the Y-direction of ^y requires a moment of force about the roll-axis of approximately , where is thse an tgleh ofe ef incli vnateionh ofi thce vleehicle b body roll axis, which is provided by the torsion coefficient, i.e. stiffness, of the revolute joints. The approximation here is that the roll angle is assumed insignificantly small, or equivalently that the torsion coefficient is very large. The ratio of moments of force at the front and rear revolute joints is and therefore the moments of force at these joints are given by

at the front and rear respectively.

Additionally, each wheel pair provides the lateral force required to produce the acceleration without causing a moment of force about the Z-axis. These forces are given by

at the front and rear respectively.

The combination of the moments of force around the revolute joints resulting from the lateral forces given in equation x) and the normal force imbalances must balance the moments required to accelerate the car body given in equation ix). This is expressed by the equations

Expanding to express the sought weight redistribution quantities explicitly

As expected, a left-hand turn shifts weight to the right-hand wheels, and a right- hand turn shifts weight to the left-hand wheels.

Aerodynamic Effects and Rolling Resistance

In the above calculations of weight re-distribution, aerodynamics and rolling resistance have been ignored, but they do change the normal forces at the wheels when the vehicle is in motion. In order to account for rolling resistance, air resistance (drag) and down-force the parameters required are described in herein.

Down-force is generally deliberately generated to improve traction, and results directly in an increase in the normal forces proportional to the square of the speed of the vehicle.

Air resistance is likewise proportional to the square of the speed of the vehicle and is assumed to act purely in the X-direction to oppose the direction of travel. Furthermore, it is assumed to act on average through a centre-of-resistance given by This configuration results in a moment about an axis in the Y-direction through the centre-of-mass of the vehicle body of where being the vehicle speed in the positive X-direction. Note that is negative if the vehicle is moving forward, representing opposition to the movement of the vehicle. The moment generated is balanced by a weight shift between front and rear wheels.

Rolling resistance acts in the Z _p | _{ane t0} oppose motion in the X-direction and is proportional to the normal force. In the preferred method it is assumed that the rolling resistance coefficient is equal on all wheels and therefore the total rolling resistance, summed over the four wheels, is proportional to the total normal force, The total normal force is calculated ignoring inclination and tilt of the vehicle but accounting for down-force, so that

The moment generated about an axis in the Y-direction through the centre-of- mass is then which must be balanced by a front-rear weight shift.

Combining the above effects, the total normal force increases at front and rear axles respectively are

Distributing these normal force increased to the four wheels, yields

Inclination and Tilt

In this section the weight redistribution of a static vehicle on a slope or camber is derived. The XYZ reference frame is considered the local frame and is fixed with respect to the vehicle. A global reference frame, the xyz frame, is defined which is coincident with the XYZ frame if and only if the vehicle is on level ground. The orientation of the vehicle is represented by the orientation of the local frame within the global frame. An inclination of the vehicle (up-hill orientation) of angle and a tilt, right-to-left, of angle ^φ are represented by rotating the XYZ (local)

frame by around the Y-axis and then by around the X-axis within the global frame.

Numerically, a vector, , in the local frame is seen in the global frame as v (G) according to

and the inverse operation is

The effect of inclination and tilt is that the gravity vector is no longer in the negative Z-direction in the local frame of the vehicle. Using equation xvii), the gravity acceleration vector in the XYZ frame is

The resultant moment of force about the roll-axis is

where η ^(L) is a unit vector in the direction of the roll-axis in the XYZ local reference frame:

Combining equations xix) and xx) the moment of force generated around the roll- axis by gravity acting on the mass of the vehicle body is

The moments of force at the front and rear revolute joints are proportional to the respective torsion coefficients:

In the presence of tilt there is a component of gravitational force in the Y-direction that is balanced by lateral forces on the front and rear wheels according to

These lateral forces contribute a moment of force around the axis of revolution and these, together with the moments in equation xxii), are balanced by a left-to- right weight redistribution at the front and rear of the vehicle according to

A positive tilt angle, corresponding to a right-to-left tilt, results in a negative weight redistribution which corresponds to a right-to-left weight shift as expected.

Similarly, the moment of force due to gravity about the Y-axis at ,

This is balanced by a front-to-rear weight redistribution given by A positive inclination, i.e. pointing up-hill, corresponds to a positive front-to-rear weight redistribution.

These effects are combined to express the normal force changes on each wheel as a result of the vehicle orientation:

Summary of Normal Force Distribution

The static, level weight distribution and weight redistributions as a result of acceleration or braking, cornering, aerodynamic and rolling resistance effects and vehicle orientation are combined to give the normal forces at each of the four wheels separately in the simplified model presented above:

The derived formulae exhibit the expected features, namely that the weight is transferred to the back wheels during acceleration, to the front wheels during deceleration, to the outer wheels during cornering and to lower wheels when the vehicle is on a slope or camber.

Distribution of Torque

The final step is to distribute the required torque amongst the four wheels based on the normal forces at each of the four wheels. In the preferred method this is done directly proportional to the normal forces calculated at each wheel in step 2. The total torque is that demanded by the driver, normally by use of the throttle or

T brake. It may be positive or negative and is referred to here as ^d . The torques to be delivered at each wheel: front-left, front-right, back-left and back-right respectively are

As an alternative to the preferred method, four new parameters: and are introduced that can be set by the user to specify the degree to which torque is redistributed as a result of accelerating, cornering, vehicle orientation and aerodynamics respectively. This approach allows the user to tune the vehicle handling behaviour to his requirements and reduces the theoretical model dependency. The torque at each wheel is then defined as

where F

The tuning parameters, , are set to zero to turn off torque redistribution according to the related manoeuvre or effect, and to one to turn on the full redistribution as in the preferred method. Values between zero and one scale the torque redistribution.

As a further variation the base torque distribution may be chosen differently to that implied by the base weight distribution, and the torque redistribution scaling parameters may be different for front and rear wheels. In this case limits may be set to avoid a situation in which some wheels have torque opposing the sense of the demanded torque.

Variations on the Preferred Method

The variations represent alternatives to the method presented above. Any or several of longitudinal acceleration, longitudinal deceleration, speed and lateral acceleration of the vehicle may be determined by sensors rather than being derived from wheel speeds. Such sensors may be accelerometers, steering-wheel position sensors or other sensors.

The normal forces at each wheel may be determined directly by sensors rather than being calculated arithmetically. If arithmetic calculations are employed they may differ from those presented in the preferred method. A different vehicle model may be employed as a framework for the calculations and different calculations for the drag, down-force and rolling resistance may be employed. In particular calculations may have a different functional form and require more parameters, and may be implemented as look up tables relating forces to vehicle speed. The normal forces may be calculated as a numerical, rather than analytical, solution to a set of equations.

Further effects on the normal forces at the wheels may be included, beyond those in the preferred method.

The relationship between the normal forces, determined either by calculation or measurement, to the applied torque may not be directly proportional, but may be modified as follows:

1. torque redistribution is only applied within or beyond prescribed limits, for instance only beyond a certain acceleration or deceleration,

2. torque redistribution is limited such that the wheel speed differences are maintained below defined thresholds,

3. torque distribution is applied in response to some but not all of acceleration, braking, cornering, inclination and tilt.

4. the torque distribution is not proportional to the weight distribution but is related to the weight distribution according to some other relationship,

5. the torque redistribution is related to the weight redistribution differently according to whether the weight redistribution is due to acceleration, braking, cornering, inclination or tilt,

6. the manner in which the calculated normal forces are related to the torque distribution may be programmable so that the user may tune the vehicle handling according to his requirements,

7. the torque distribution accounts for lateral forces on the tyres, reducing torque relatively on wheels with relatively higher lateral forces since they tolerate less longitudinal force before losing traction.

In a further variation on the preferred method, the vehicle may be powered by means other than by electric wheel motors, providing that a method for distributing torque amongst the wheels is available. Furthermore the vehicle may have a different number of wheels, and a different number of driven wheels which may not be the same as the total number of wheels. In this case the invention applies to the driven wheels.

Example Arrangement

An arrangement embodying the invention is shown in Figures 2 to 4. As shown in Figure 2, the torque drive and control system includes a wheel and suspension arrangement, each wheel 330 in this example can be mounted on a suspension arm 340.

In Figure 2, the normal direction of travel of the vehicle with respect to the surface 350 is shown in Figure 2 by the arrow labelled Z. Accordingly, as shown in Figure 2, the wheel 330 is rotated in the direction indicated by the arrow labelled X. If additional torque is applied to the wheel in the direction indicated by the arrow labelled X, this will tend to impart a force on the suspension arm 340, whereby the suspension arm 340 will tend to rise up in the direction indicated in Figure 2 by the arrow labelled Y. In this example, the wheel 330 is a front wheel of a vehicle such as a car. The rising up of the suspension arm 340 in the direction indicated by the arrow labelled Y in Figure 2, would therefore cause the front of the car in locality of the wheel 330 to rise up also. In this way, by adjusting the torque to the wheel of a vehicle, a suspension control system for a vehicle can be implemented.

As shown in Figure 3, the arrangement includes an electric motor of the type that is mounted within each wheel. The motor 40 in this example is a three phase motor. Again, it will be appreciated that motors according to this invention can

jnclude an arbitrary number of phases (N = 1, 2, 3...). Being a three phase motor, the motor 40 includes three coil sets. In this example, each coil set includes two coil sub-sets. The coil sub-sets of each coil set are labelled 44, 46 and 48, respectively. The coil sub-sets 44, 46 and 48 are arranged around a periphery of the motor 40. In this example, each coil sub-set is positioned opposite the other coil sub-set in that coil set, although such an arrangement is not strictly essential to the working of the invention. Each coil sub-set includes one or more coils.

The motor 40 can include a rotor (not shown in Figure 3) positioned in the centre of the circle defined by the positioning of the various coils of the motor, thereby to allow rotation of the rotor within the rotating magnetic field produced by the coils. Preferably, though, the rotor is arranged around the coils. The rotor may typically comprise one or more permanent magnets arranged to rotate such that their poles sweep across the ends of the coils of the motor 40. Appropriate switching of currents in the coils of the coil sub-sets allows synchronized attraction and repulsion of the poles of the permanent magnet of the rotor to produce the rotating action of the motor 40. It will be appreciated that Figure 3 is highly schematic and, in practice, the coil sub-sets will be arranged at the outer periphery of the stator with the rotor magnets surrounding the coils.

The calculation of the torque to be provided by each motor may be in software on a processor or on a dedicated processor with calculations in firmware. The control circuit may be distributed, with a separate control circuit for each wheel, or a common controller used.

As shown in Figure 4, a common control device 92 can be used to coordinate the operations of the multiple control devices 80 provided in the motor. In prior motors, in which synchronization of the magnetic fields produced by the coils of each coil sub-set is automatically achieved by virtue of the fact that they are connected in series. However, where separate power control is provided for each coil sub-set, automatic synchronization of this kind does not occur. Accordingly, in accordance with an embodiment of this invention, a common control device 92 such as that shown in Figure 4 can be provided to ensure correct emulation of a polyphase system incorporating series-connected coils. Terminals 86 can be provided at the multiple control devices 80 to allow interconnections 90 to be

formed between the multiple control devices 80 and the common control device 92.

The interconnections 90 can pass signals between the common control device 92 and the control devices 80 such as timing/synchronization signals for appropriate emulation of a polyphase series-connected system.