ELECTRIC CAR

Abstract: A drive system (11) for an electric car has four wheels (12, 13, 14, 15) to each of which is connected an electric motor (18). The electric motors (18) are connected to a power supply (25) and are controlled by a supervisory control unit (20) and separate motor control units (19) for each motor (18). Control of the torques delivered by the motors (18) is dependent upon instructions received from the driver of the car through the accelerator pedal (21), brake pedal (22) and steering wheel (23) and upon the temperatures of the motors (18) detected by the sensors (24) located on each motor (18). In this manner, the torques drawn from the motors may be maximised without damaging the motors thus improving the electric car's performance and permitting the use of smaller motors than those previously used thereby reducing the total weight of the car.

Description: ELECTRIC CAR The present invention relates to electric cars and is particularly applicable to a drive system for an electric car. A known type of electric car is described in EP 0417326 which has a separate electric motor provided for each of the driven wheels of the car. A controller is also provided which is connected to and controls the operation of each of the motors. The controller determines the operation of each of the motors in dependence on information input from the accelerator pedal, the brake pedal and the steering wheel of the car and from sensors which detect the rotational speed of each of the wheels of the car. With electric cars of this type the control of the motors connected to the wheels is in part dependent on the rotational speed of the wheels. Thus, as the rotation of the wheels approaches the speed which the controller has calculated is the driver's desired speed, in response to the depression of the accelerator pedal, the responsiveness of the car to the driver's instructions can be reduced resulting in instructions from the driver being overridden. In this way the responsiveness of the car to the driver can feel very different to that of a conventional internal combustion engine vehicle. There is the further disadvantage that since the operation of the electric motors connected to the wheels of the car is dependent on the rotational speed of the wheels, efficient control of the motors cannot be achieved and such electric cars have been found to have poor performance characteristics and to be underpowered. In an attempt to overcome this problem, very large motors have been used which introduces a new problem of substantially increasing the total weight of the car, since there is a motor for SUBSTITUTESHEET each wheel. Therefore any increase in the power of the car is offset to a greater or lesser extent by the increased weight of the car. The present invention seeks to overcome the above identified disadvantages of known electric cars by more efficiently controlling the electric motors which drive the wheels of the car and therefore enabling smaller motors to be used. In addition, the present invention provides an electrically driven motor vehicle which dispenses with the need for a plurality of sensors on each wheel of the car which detect the rotational speed of each wheel. Moreover, the present invention seeks to provide an electrically driven motor vehicle which is controlled and responsive to the instructions from the driver and therefore feels to the driver very similar in response to that of a conventional internal combustion engine vehicle. The invention is particularly applicable to electric cars having three or more wheels at least two of which are driven. The drive system may provide for two or four wheel drive. The present invention provides an electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, at least one variable speed electric motor coupled to one offside wheel and one nearside wheel to drive said wheels, an electric power supply connected to said at least one motor, and central control means responsive to inputs from each of a vehicle speed control member, a vehicle braking member, a vehicle steering member, and one or more motor temperature sensors provided to detect the temperature of said at least one motor, and coupled to said at least one motor to control the current drawn by and the torque of said at least one motor in dependence on each of the inputs to the central control means. SUBSTITUTESHEET Thus, with the present invention the current drawn and hence the torque of the motor is controlled in response to the driver's instructions and the temperature of the motor whereby the motor may be controlled very efficiently to maximise the torque from the motor without detracting from the performance of the vehicle. This means, as a result, that a small motor may be be used with the vehicle whilst still providing an acceptable vehicle performance for the driver and moreover the responsiveness of the drive system to the control of the driver is closer to that of a conventional internal combustion engine vehicle. Preferably, separate electric motors are provided for each of said one offside wheel and said one nearside wheel which are independently controlled by said central control means. Since a separate electric motor is provided for each of the offside and nearside wheels, clutch and gear mechanisms may be dispensed with in the drive system. In a preferred embodiment the central control means comprises a supervisory control unit and a plurality of motor control units connected to the supervisory control unit, there being provided a motor control unit for each electric motor and connected thereto, Since a separate motor control unit is provided for each motor, separate control of each motor can be implemented in a particularly simple but effective manner. Moreover, the central control means may include an information display device for providing information on the motor vehicle to the driver whereby the driver can be given information on, for example, the power supply remaining, the efficiency of the motor vehicle, the speed of the motor vehicle and other such information. The information display device may also include an audible warning device to warn the driver using sound of potential problems. SUBSTITUTESHEET In a preferred embodiment there is further provided means for regenerative braking of the at least one electric motor whereby power may be conserved during the braking of the motor vehicle by supplying power from the electric motor to the power supply during regenerative braking. In addition, where the regenerative braking is implemented when the driver coasts the vehicle by actuating neither the vehicle speed control member nor the vehicle braking member the response of the vehicle is even more similar to that of a conventional internal combustion engine vehicle and so feels familiar to the driver. The electric motors may be connected to the wheels either directly or via a fixed ratio torque converter. In this manner a very simple means for driving the wheels through the motors is provided which therefore is less prone to breakdown and hence is more reliable. In addition, a directional control selector may be provided for permitting the driver to select whether the vehicle is to be driven in a forward or reverse direction, with the directional control selector being connected to the central control means and there may be further provided an inhibitor to prevent the directional control selector being actuated to select a new direction of travel when the vehicle is in motion and current is being drawn by said at least one motor from the power supply. Hence, damage to the drive system by the selection of a reverse direction when the car is still in motion in the forward direction or vice versa may be avoided. Additionally, the desired turning of the motor may be detected by a linear sensor located on supporting axles of the wheels which are in turn connected to the vehicle steering member. Ideally, the linear sensor is in the form of a potentiometer. In this manner a more accurate determination of the desired turning angle may be achieved, particularly in view of the fact that a conventional steering detector mounted on the column of a steering wheel is required to detect rotations of the column in excess of 360° which problem is avoided with the linear sensor. In a further aspect the present invention provides an electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, at least one variable speed electric motor for driving one offside wheel and one nearside wheel, an electric power supply coupled to said at least one electric motor, and switching means connected between said power supply and said at least one electric motor, the switching means including two complementary switches for the or each electric motor controlled by central control means for controlling the power supplied to the or each electric motor. With this further aspect of the present invention a very simple and effective means for controlling the power supplied to the motor can be achieved and moreover the complementary switches ensure that the power supplied to the motor is smooth and energy savings can be achieved which is of particular importance with electrically driven motor vehicles where the range of such vehicles can be very limited owing to the size of the power supply which may be supported by the vehicle. Preferably, the complementary switches are high speed FET switches. In a separate further aspect the present invention provides an electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, a first electric motor being provided for one offside wheel and a separate second electric motor being provided for one nearside wheel, each of said electric motors being a SUBSTITUTESHEET variable speed motor coupled to its respective wheel to drive the wheel, an electric power supply connected to drive each of said motors, and a central control means responsive to inputs from each of a vehicle speed control member, a vehicle braking member, and a vehicle steering member and coupled to each of said motors to control independently the current drawn by and the torque of each of said motors in dependence on each of said inputs to the central control means wherein in response to an input from said vehicle steering member indicating a turning of said motor vehicle said central control means digitally calculates a differential torque and independently controls the current drawn by each of said first and second electric motors such that the torque delivered by said first electric motor is different to the torque delivered by said second electric motor. With this further separate aspect of the present invention the turning of the motor vehicle is improved by virtue of the control of the different torques delivered to the nearside and offside wheels. In a fourth aspect the present invention provides an electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, a first electric motor being provided for one offside wheel and a separate second electric motor being provided for one nearside wheel, each of said electric motors being a variable speed motor coupled to its respective wheel to drive the wheel, an electric power supply connected to drive each of said motors and central control means coupled to each of said motors to control independently through a switching network the current drawn by and the torque of each of said motors, whereby the current drawn by each of said motors is synchronised such that there is a phase displacement between the current drawn by said first electric motor and the current drawn by said second electric motor. With this fourth aspect of the present invention the current energy losses of the drive system of the motor vehicle may be substantially reduced. The present invention is particularly applicable to vehicles having two offside wheels and two nearside wheels which may be driven as two wheel or four wheel drive. Additionally, the power supply of the electrically driven motor vehicle may be an electric battery system and preferably a gas turbine may be provided coupled to a generator to provide charging of the electric battery system. An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the drive system of an electrically driven motor vehicle in accordance with the present invention; Figure 2 is a more detailed diagram of one of the motor control units of the drive system of Figure 1; Figure 3 is a more detailed diagram of the supervisory control unit of the drive system of Figure 1; Figures 4a and 4b are control diagrams showing the starting and operating conditions of the drive system of Figure 1; and Figure 5 is a circuit diagram of the switching network between the power supply of the motor vehicle and the electric motors. SUBSTITUTE SHEET The drive system 11 of an electrically driven motor vehicle is shown in Figure 1. The drive system 11 has a front nearside wheel 12, a rear nearside wheel 13, a front offside wheel 14 and a rear offside wheel 15. Each one of the wheels 12, 13, 14, 15 has associated with it a connecting axle 16, a disk brake 17, a separate electric motor 18 and a separate motor control unit 19. The electric motor 18 is controlled by the motor control unit 19 and is connected to the wheel by the connecting axle 16 either directly or via a fixed ratio torque converter so that rotation of the electric motor is transmitted by the connecting axle 16 to the wheel. The construction of the disk brake 17 and the connecting axle 16 are of largely conventional design. Each electric motor 18 may be mounted in the hub of its associated wheel or, preferably, the electric motor may be attached to the chassis of the electric car so that the weight of the electric motors 18 is supported by the suspension system (not shown) of the electric car. The suspension system may also be largely conventional in design, however, in view of the increased weight of the vehicle, by virtue of the provision of four separate electric motors 18, the suspension system may be stronger than is commonly provided for in conventional systems. Each of the four motor control units 19 is connected to and controlled by a common supervisory control unit 20. The supervisory control unit 20 receives driving instructions from the driver of the electric car through a speed control member 21, in the form of an accelerator pedal, a braking member 22, in the form of a brake pedal, and a steering member 23, in the form of a steering wheel. Transducers mounted on the accelerator pedal 21 and the brake pedal 22 generate outputs the voltage of which correspond to the extent of depression of the pedals by the driver. These outputs are converted into digital signals by an analog-to-digital converter and are then fed SUBSTIT TESHEET to the supervisory control unit 20. In the case of the steering wheel 23 a transducer may be mounted on the steering column to detect rotation of the column or alternatively a linear transducer may be mounted on a steering transmission shaft which is connected to one of the wheels so that a linear measurement of the desired steering of the car may be determined. This too is converted into digital signals by an analog-to-digital converter and is transmitted to the supervisory control unit 20. The supervisory control unit 20 also receives information on the operation of the motors 18 from the motor control units 19 and from one or more motor temperature sensors 24 located on each of the electric motors 18. From the information received by the supervisory control unit 20, the supervisory control unit 20 determines the correct operation of each of the motors 18 and instructs the motor control units 19 to ensure correct operation of the motors 18 and hence the wheels 12, 13, 14, 15. The supervisory control unit 20 determines and supplies independent control instructions to each of the four motor control units 19 so as to provide speed and movement of the car in either a straight line or in a turn as desired by the driver. As each motor is controlled independently of the other motors the drive torque of each of the driven wheels can be independently controlled on instruction from the supervisory control unit 20 to compensate for the different turning circle of the inner and outer wheels when the vehicle is not driven in a straight line. The supervisory control unit 20 contains the algorithms which are used to calculate digitally the differential torque requirements of each of the drive wheels and these are achieved by the correct control signals supplied to each of the motor control units 19. In this way an automotive digital differential (ADD) is provided which provides an accurate differential effect using only the required SUBSTITUTE SHEET amount of energy by each of the wheels rather than absorbing unwanted energy as occurs in conventional friction operated differential systems. The supervisory control unit 20, the motor control units 19 and the motors 18 are connected to a power supply 25 through a switching network 26. The switching network 26 controls the supply of power from the power supply 25 to the motors 18 in response to signals from the supervisory control unit 20 and the motor control units 19. The switching network 26 is also connected to a directional control selector 27 which is actuated by the driver of the car and can assume, at any instance, one of three selectable conditions: forward direction of travel; reverse direction of travel; or neutral. The supervisory control unit 20 is capable of detecting the selected condition of the directional control selector 27 and of preventing the alteration of the selected condition unless the condition of the car so permits. The power supply 25 may be in the form of one or more batteries. Preferably, the one or more batteries can be recharged by a gas turbine 28. The gas turbine 28 is driven by fuel from a supply (not shown) and operates a voltage generator 29 which is connected through a voltage regulator 30 to the one or more batteries. The gas turbine 28 is controlled by a charge monitor 31. A secondary power supply 32 is also provided which is electrically isolated from the power supply 25 but may draw power from the power supply 25 and is used to power up the supervisory control unit 20 and the motor control units 19. Moreover, solar panels (not shown) may be provided on the car body connected to the power supply to recharge the batteries when the solar panels are exposed to sunlight. The braking member 22 is also connected (not shown), in a largely conventional manner, to each of the disc brakes 17 so that conventional hydraulic braking of the wheels 12, 13, 14, 15 may be achieved. A separate parking brake (not shown) may also be provided. Moreover, the steering member 23 is connected (not shown) , in a largely conventional manner, to either the front connecting axles 16 or the rear connecting axles 16 for causing movement of the wheels 12, 13, 14, 15 relative to the body of the car thereby steering the car when in motion. Turning now to Figure 2, one of the motor control units 19 is shown in greater detail. The motor control unit 19 has a CPU 40, a current sensor 41, an overcurrent trip device 42, a control output 43 and an input/output communications port 44. The control output 43 is connected to the switching network 26 for controlling the power supplied to the associated electric motor 18. A more detailed explanation of the operation of the switching network 26 may be found below with reference to Figure 5. The input/output communications port 44 is connected to a serial link 33 which is in turn connected to the supervisory control unit 20. All instructions from the supervisory control unit 20 are received by the motor control unit 19 and all information on the operating conditions of the electric motor 18 associated with the motor control unit 19 are sent to the supervisory control unit 20 from the motor control unit 19 at the input/output communications port 44 on the serial link 33. The current sensor 41 is connected to the motor 18 and monitors the current drawn by the motor 18 from the switching network 26. The current sensor 41 is preferably a magnetically coupled transducer capable of providing a low voltage signal (for example 10 volts) isolated from the power circuit. The output of the current sensor 41 is digitised by an analog-to-digital converter (not shown) SUBSTITUTE SHEET and is supplied to both the CPU 40 and the overcurrent trip device 42. The speed of sampling of the current is chosen so that the feedback factor is less than 1 i.e. the sampling speed should be greater than the motor time constant. The overcurrent trip device 42 is a high level, hard wired safety mechanism for inhibiting the supply of power to the motor 18 by direct deactivation of the switching network 26 through connection 45 if the sensed current exceeds a predetermined limit. The predetermined limit is greater than the software current limit of the CPU 40. The overcurrent trip device 42 is also connected to the CPU 40 of the motor control unit 19 whereby when a current exceeding the predetermined limit is sensed and the overcurrent trip device 42 is actuated the CPU 40 enters a "freeze" mode until the overcurrent trip device 42 is re-set. In the "freeze" mode the monitoring of the operation of the motor 18 is continued but the motor 18 is prevented from drawing current from the power supply 25. Preferably, in order to re-set the overcurrent trip device 42 the car must be stationary and all power turned off although this need not be necessary where automatic re-setting procedures can be implemented by the supervisory control unit 20. The CPU 40 includes a digital pulse width modulation (PWM) integrated circuit which controls the mark-space ratio of the output of the CPU 40 and controls the pulse width modulation of the switches 62 of the switching network 26 for the supply of power to the motor 18 and ensures smooth transitions between different pulse width modulation settings. Each motor control unit 19 controls two switches 62 in the switching network 26 which will be described in greater detail later. The CPU 40 receives instructions from the supervisory control unit 20 on the serial link 33 in the form of current demand, direction SUBSTIT T and synchronism instructions for the motor 18. The CPU 40 then calculates, using an algorithm involving the torque demand and the motor current, the pulse width modulation of the switches 62 in the switching network 26 necessary to meet the instructions received from the supervisory control unit 20 and to avoid exceeding the current limit set by the supervisory control unit 20. For reliable activation of the switches 62 in the switching network 26, the CPU 40 preferably provides a gate power to the switches in the region of 1 amp at 15 volts. In addition, for system security galvanic isolation is provided between each switch 62 and the CPU 40. The CPU 40 also includes means for preventing both switches 62 from being turned on simultaneously and for introducing a "dead time" between one switch being turned on after the other switch has been turned off. In order to avoid "jitter" noise at low speed the CPU 40 also provides for the pulse width modulation to be set to zero when the armature current is less than 1%. The control of the motor 18 employed by the motor control unit 19 is a proportional realtime corrective error control system, in other words a torque controlled system. Thus, the electric car, in response to the driver's instructions, is controlled through the torque applied by each of the motors to the wheels and is not controlled in response to the speed of the wheels. In Figure 3 the supervisory control unit 20 is shown in greater detail. The supervisory control unit 20 has a computational and command unit 50, a memory 51, a controller interface 52, a serial link interface 53 and an information display unit 54. The computational and command unit 50 is connected to the controller interface 52 and the serial line interface 53 which are in turn connected to a peripheral bus 34 and a serial link 33 respectively. The computational and command unit 50 is also connected to the memory 51, the information display SUBSTITUTE SHEET unit 54 and the switching network 26 and is used to determine the instructions to be sent to the motor control units 19 for controlling the operation of the electric motors 18 and the information to be displayed to the driver of the electric car. The controller interface 52 receives signals from the peripheral bus 34 from the transducers on the accelerator pedal 21, the brake pedal 22, the steering wheel 23, and from the temperature sensors 24 and the directional control selector 27 which are converted into digital signals by an analog-to-digital converter before being input into the computational and command unit 50. The controller interface 52 also outputs instructions on the peripheral bus 43 from the computational and command unit 50 to safety devices (not shown) provided on the electric car. The serial link interface 53 receives signals from the serial link 33. The signals are from the motor control units 19 and provide information on the operation of the motors 18 and in particular the current sensed by the current sensors 41 and the pulse width modulation on time being supplied by the motor control units 19 to the switching network 26. The serial bus interface 53 also outputs instructions on the serial link 33 to each of the motor control units 19 for the control of the motors 18. The information display unit 54 receives information from the computational and command unit 50 to be displayed to the driver of the electric car. Such information may include details on the running performance of the car or individual motors, the sensed temperatures of the motors, the currents drawn by the motors, the power remaining, the speed of the car and the distance travelled. The information display unit 54 may also display a warning to the driver if, for example, the operation of the motor is unsatisfactory, the remaining power is low, or the overcurrent trip device 42 has been activated. In addition, the information display unit 54 can be used to warn the driver if, for example, the car is being driven without the safety belt catch being actuated or the car is turned off with the car headlights still on. The information display unit 54 may incorporate a speech device to provide audible verbal warnings to the driver of the car. In order to determine the instantaneous speed of the car and the distance travelled by the car, a sensor is provided (not shown) on at least one of the wheels to detect the rotation of the wheel. The information from the sensor is transmitted on the peripheral bus 34 to the supervisory control unit 20. However, this information is not used in the calculation of the current to be drawn by each motor 18 and is, instead, used for diagnostic and driver information purposes. The memory 51 may be in the form of a ROM or EPROM and contains data and algorithms which may be accessed by the computational and command unit 50 for determining the instructions to be sent to the motor control units 19; to the information display unit 54 and via the controller interface 52 to the safety devices. In the case of the motor control units 19 the individual instructions regarding current demand, direction and synchronisation of the motors are sent via the serial link 33. With the supervisory control unit 20, since all the control of the car is computerised and the operation of the motors 18 monitored, diagnostic information on the car may be easily obtained from the supervisory control unit 20, thereby simplifying much of the work which may be needed' to maintain the car's performance. In addition, since the steering of the car is monitored, this ET information may be implemented in a navigational system which monitors the location of the car. With this drive system 11, the temperatures of the motors 18 are monitored in order that the maximum permitted torque which can be delivered by the motors at any moment may be calculated. For any given motor there are fixed maximum parameters for speed and torque and the power output of the motor is limited by its temperature rise which in turn depends upon its losses which are affected by several factors including the windage, resistive and iron losses and the cooling of the motor. Since the permissive temperature rise of a motor is proportional to the product of the length of time the current is supplied and the square of the current it is possible to permit very high currents to be drawn by the motor as long as this is for a very short period of time. Furthermore, since the torque supplied by the motor is proportional to the current drawn, very high torques may therefore be supplied by the motor without adversely affecting the motor. Thus, very high torques can be obtained by using currents as high as seven times greater than the long term thermal rating of the motor without damaging the motor as long as the very high torques are supplied for a limited period of time. It is for this reason that controlling the peak currents within the thermal boundaries is an essential element of the drive system. By using a drive system which is dependent on the temperatures of the motors, smaller motors than those formerly used in known electric vehicles can be used which reduces the total vehicle weight and ensures a greatly improved car performance or permits for a greater payload. Preferably, in order to take the greatest advantage of this drive system 11, cooling apparatus for cooling the motors 18 may be included in the drive system. For example, apparatus may be provided to deflect air passing under or over the car, as the car is in motion, to flow directly over the motors 18. Moreover, in view of the high currents which may be drawn from the power supply 25, care must be taken to minimise peak currents drawn from the power supply 25. This is achieved by synchronising the operation of each motor control unit 19 so that the pulse width modulation of the switches 62 in the switching network 26 have a phase displacement between different motor control units 19. Thus, assuming a preferred pulse width modulation frequency of 10kHz (periodic time lOOus) and a first motor control unit begining its cycle at t=0, the second motor control unit preferably begins its cycle at t=25us, the third motor control unit at t=50us and the fourth motor control unit at t=75us. Such synchronisation is controlled and monitored by the supervisory control unit 20 and when used in conjunction with the power capacitors 66 ensure a smooth demand for power from the power supply 25. By synchronising the motors 18, energy losses proportional to the product of the squared current with time (I T) can be noticeably reduced. Without phase displacement, using four motors, the losses are 2 proportional to (41) T whereas with phase displacement 2 the losses are reduced by a factor of 4 to 41 T. The switching network 26 is shown in Figure 5 and has a double pole power contactor 60, mechanical direction switches 61 and protection diodes 67 connected to four switch sub-networks 26a in parallel. Each sub-network 26a includes two high speed switches 62, which may be FETs or IGBTs for example, and two corresponding "snubber" networks each comprising a diode 63, a capacitor 64 and a resistor 65 which are connected such that when one of the high speed switches 62 is turned off the transient voltage SUBSTITUTE SHEET which would normally occur is suppressed by the corresponding snubber network. The high speed switches 62 and snubber networks of each of the sub-networks 26a are arranged to form a complementary switching arrangement. The high speed switches 62 of each of the sub-networks 26a, in this embodiment FETs, are connected at their bases to the control output 43 of a motor control unit 19. Each sub-network 26a is connected to a separate associated motor control unit 18 and an associated motor 18. The complementary switching results in a saving in energy which is of great importance when all the power for the car is taken from the same power supply. It will be understood that the provision of a complementary switching arrangement for controlling the power supplied to a motor is not limited to an electric car having individual motors for each wheel controlled in dependence on motor temperature and could be used with any electric vehicle having one or more motors to drive the wheels. The motor control unit 19 controls the high speed switches 62 and thereby the power supplied to the motor 18 in the following manner. The upper switch 62, as seen in Figure 5 controls the power supplied to the motor in the driven direction and the lower switch 62 controls the regenerative braking power. The diode 63 connected across the lower switch allows inductive current from the motor 18 to "freewheel" about the sub-network 26a when the upper switch 62 is turned off. When the vehicle is being reversed the lower switch is used to power the motor in the driven direction and the upper switch is used for the regenerative braking. The mechanical direction switches 61 are actuated by the directional control selector 27 and determine whether the motors are to drive the car in a forward or reverse direction. As discussed earlier, the switches 62 are controlled by the motor control units 19 SUBSTITUTESHEET using pulse width modulation and thereby the current drawn by the motors 18 is controlled. The switching network 26 also includes fuses which prevent damage to the drive system 11 in the event of a short circuit. One fuse 68 is connected within the power supply 25 and additional fuses 68 are provided in series with each motor 18. By complementary switching is meant that when a motor is being driven in either the forward or reverse directions the two switches 62 are continually being switched on and off alternately, with only one switch being on at any one moment, and with the switches being controlled by the motor control unit 19. Thus, for example, with the motor being driven in the forward direction and at a switching frequency of 10kHz (periodic time lOOus) the upper switch 62 may be turned on for 50us, for a 50% drive, and then turned off followed by a dead time of 2us before the lower switch 62 is turned on for 46us and then turned off followed by a dead time of 2us before the upper switch 62 is turned on again. During the period when the lower switch 62 is turned on the inductive current is allowed to "freewheel" around the circuit as mentioned above. With reference to Figures 4a and 4b, when in use, firstly a start or "ignition" key is turned, by the driver. The start key. connects the supervisory control unit 20 to the secondary power supply 32 to power up the supervisory control unit 20. The starting procedure is shown in Figure 4a. The supervisory control unit 20 then performs a self-check 100 and if the results of the checks are satisfactory the supervisory control unit 20 proceeds to the powering up 101 of the motor control units 19. If the checks are unsatisfactory safety devices are activated by the supervisory control unit 20 and the driver is informed by means of the information display unit 54 of the problem 102. The checks are then repeated until the results of the checks are satisfactory. The safety devices may SUBSTITUTE SHEET include a directional control selector inhibitor and a main power contactor enabled. The directional control selector inhibitor may be in the form of solenoid operated interlocks mounted on the directional control selector 27 to prevent the selection of forward or reverse drive positions. The motor control units 19 are in turn checked 103 and communicate through the serial link 33 with the supervisory control unit 20 confirming the state of each of the motor control units 19. If the results of the motor control unit checks are unsatisfactory the safety devices are activated and the driver informed 102, as discussed above. The checks are then repeated until the results are satisfactory. Once all the checks have been performed and the results are satisfactory the directional control selector 27 is checked by the supervisory control unit 20 to see if it is in its neutral position 104. If the directional control selector is in the forward or reverse drive positions, the driver of the car is informed 105 by means of the information display unit 54 and the directional control selector 27 is re-checked. If the directional control selector 27 is in the neutral position the supervisory control unit 20 instructs the main power contactor 60 to be energised 106 so the the power capacitors 66 are charged up 107 via a current limiting circuit. The directional control selector 27 is inhibited 108 until the voltage of the power capacitors 66 has a similar value to that of the power supply 25. Once the power capacitors 66 are fully charged up the directional control selector 27 is released 109 and the electric car is now ready to be driven by selection of forward or reverse drive positions with the directional control selector 27 and the application of pressure to the accelerator pedal 21. Turning now to Figure 4b which shows the control of the electric car once it is ready to be driven, the driver selects forward or reverse drive with the directional control selector 27 and the computational and command unit 50 of the supervisory control unit 20 checks 110 whether a signal has been received on the peripheral bus 34 indicating that pressure has been applied to the accelerator pedal 21. If a signal is received indicating that the driver of the car desires acceleration, the computational and command unit 50 checks 111 what the temperatures of the motors 18 are from the signals input on the peripheral bus 34 from the temperature sensors 24. The computational and command unit 50 then determines 112 what torques the motors 18 can deliver to the wheels 12, 13, 14, 15 to provide the acceleration desired by the driver in dependence on the sensed temperatures of the motors 18 and instructs the motor control units 19 accordingly on the serial link 33, to ramp the current and hence increase the torque to be delivered by the motors 18 up to the required amount. At the same time the supervisory control unit 20 sends instructions to inhibit alteration of the directional control selector 27. This control sequence is repeated as long as the driver continues to apply pressure to the accelerator pedal. If no pressure has been applied to the accelerator pedal 21, the computational and command unit 50 checks 113 whether a signal has been received on the peripheral bus 34 indicating that pressure has been applied to the brake pedal 22. If a signal indicating that the driver of the car desires the car to brake, the computational and command unit 50 checks 111 what the temperature of the motors 18 are, as mentioned above. The computational and command unit 50 then determines 114 what regenerative braking the motors 18 can deliver to the wheels 12, 13, 14, 15 in dependence on the sensed temperatures of the motors 18 and instructs the motor control units 19 SUBSTITUTE SHEET accordingly on the serial link 33, to regenerate energy from the motors 18 back to the power supply 25. If the pressure applied by the driver to the brake pedal 22 is hard 115 then the conventional hydraulic system is also implemented 116 using the disk brakes 17 on each wheel. This control sequence is repeated until the driver takes the pressure off the brake pedal 22. If no pressure has been applied to the brake pedal 22, the computational and command unit 50 checks 111 what the temperatures of the motors 18 are, as mentioned above. The computational and command unit 50 then instructs the motor control units 19 to ramp the torque delivered by the motors 18 to zero. This results in the car continuing under its own momentum with the motor 18 delivering no accelerating force. This control sequence is repeated until the driver applies pressure to either the accelerator pedal 21 or the brake pedal 22. Alternatively, if the driver "coasts" the car by depressing neither the accelerator pedal nor the brake pedal the supervisory control unit 20 may be programmed to implement regenerative braking as described above. This results in the car behaving in a similar manner to the breaking effect experienced when an internal combustion engine vehicle is coasted. The same control sequences are applied whether the forward or reverse drive is selected by the driver. However, a change from forward to reverse drive or vice versa by alteration of the drive selected with the directional control selector 27 is only permitted if the car is stationary and there is no current flowing in any of the motors 18. The powering down of the car, after use, is also controlled by the supervisory control unit 20. The car is brought to a halt and the directional control selector 27 is move by the driver to its neutral position. The "ignition" key is then turned to the "off" position. The supervisory control unit 20 then opens the power contactors 60, and the power capacitors 66 are discharged. The motor control units 19 are powered down. If, for example, the directional control selector 27 is not in the neutral position when the "ignition" key is turned off, the supervisory control unit 20 will inform the driver through the information display unit 54 and will not power down the drive system 11 until the driver has moved the directional control selector 27 to neutral. Thus, in the manner described above speed control of the electric car is effected by the balancing of a desired torque requested by the driver to the required torque. In the case of the driver wishing to accelerate, power is drawn from the power supply 25 and the car is accelerated until the torque provided by the four motors 18 balances the drivers' speed requirements at which stage the driver will remove or reduce the pressure applied to the accelerator pedal 21. The torque required to balance the speed requirements will vary on the load carried and the gradient of the road, ie. either uphill or downhill. The further the accelerator pedal 21 is depressed the more torque will be developed by the motors 18. With this drive system one of the main limiting factors to the motor torque is the thermal limits of the motor. It is for this reason that the supervisory control unit 20 monitors the temperature of each motor 18 sensed by the temperature sensors 24. The sensed temperature for any particular motor is used to determine the maximum permitted torque for that motor. With the drive system described above the car performs in a similar manner to that of an internal combustion engine vehicle and so feels familiar in its response to the driver. This is because, in part, unnecessary limitation SUBSTITUTESHEET of the performance of the car is not caused by making the control of the motors dependent on the rotational speed of the wheels. The invention is not limited to the details of the foregoing example. For instance each driven wheel may be provided with two or more electric motors. In such a case each electric motor for any one wheel will be controlled by the motor control unit for that wheel; under operation of the supervisory control unit. Additionally, the foregoing example describes an electric car, however, it wil be appreciated that any electrically driven motor vehicle could incorporate the features of the present invention having a plurality of wheels. For example the vehicle may have three wheels or eight wheels. Moreover, not all the wheels need be driven by an electric motor. Thus, a four wheel car could have only two driven wheels. In addition, the particular components of the supervisory control unit 20 and the motor control units 19 could be changed whilst still providing an electrically driven motor vehicle in accordance with the invention. Finally, the particular circuit of the switching network 26 is not intended to be limiting and other switching networks performing the same function of controlling the power supplied to the motors may be implemented. Although brushed d.c. motors are described in the foregoing example, it will be understood that other types of motor could be implemented such as brushless d.c. motors or a.c. induction motors without departing from the spirit of the present invention.

Claims: CLAIMS : 1. An electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, at least one variable speed electric motor coupled to one offside wheel and one nearside wheel to drive said wheels, an electric power supply connected to said at least one motor, and central control means responsive to inputs from each of a vehicle speed control member, a vehicle braking member, a vehicle steering member, and one or more motor temperature sensors provided to detect the temperature of said at least one motor, and coupled to said at least one motor to control the current drawn by and the torque of said at least one motor in dependence on each of the inputs to the central control means. 2. An electrically driven motor vehicle as claimed in claim 1, wherein separate electric motors are provided for each of said one offside wheel and said one nearside wheel which are independently controlled by said central control means. 3. An electrically driven motor vehicle as claimed in claim 2, wherein the central control means comprises a supervisory control unit and a plurality of motor control units connected to the supervisory control unit, there being provided a motor control unit for each electric motor connected thereto. 4. An electrically driven motor vehicle as claimed in any one of claims 1 to 3, wherein, the central control means includes an information display device for providing information on the motor vehicle to the driver. 5. An electrically driven motor vehicle as claimed in any one of the preceding claims, wherein means are SUBSTITUTESHEET provided for regenerative braking of the at least one electric motor. 6. An electrically driven motor vehicle as claimed in any one of the preceding claims, wherein a directional control selector is provided for permitting the driver to select whether the vehicle is to be driven in a forward or reverse direction, the directional control selector being connected to the central control means and there is further provided an inhibitor to prevent the directional control selector being actuated to select a new direction when the vehicle is in motion and current is being drawn by said at least one motor from the power supply. 7. An electrically driven motor vehicle as claimed in any one of the preceding claims, wherein the desired turning of the motor vehicle is detected by a linear sensor located on supported axles of the wheels which are in turn connected to the vehicle steering member. 8. An electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, at least one variable speed electric motor for driving one offside wheel and one nearside wheel, an electric power supply coupled to said at least one electric motor, and switching means connected between said power supply and said at least one electric motor, the switching means including two complementary switches for the or each electric motor controlled by central control means for controlling the power supplied to the or each electric motor. 9. An electrically driven motor vehicle as claimed in claim 8, wherein the complementary switches are high speed FET switches. 10. An electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, a first electric motor being provided for one offside wheel and a separate second electric motor being provided for one nearside wheel, each of said electric motors being a variable speed motor coupled to its respective wheel to drive the wheel, an electric power supply connected to drive each of said motors, and a central control means responsive to inputs from each of a vehicle speed control member, a vehicle braking member, and a vehicle steering member and coupled to each of said motors to control independently the current drawn by and the torque of each of said motors in dependence on each of said inputs to the central control means wherein in response to an input from said vehicle steering member indicating a turning of said motor vehicle said central control means digitally calculates a differential torque and independently controls the current drawn by each of said first and second electric motors such that the torque delivered by said first electric motor is different to the torque delivered by said second electric motor. 11. An electrically driven motor vehicle having a vehicle body with a plurality of wheels including at least one offside wheel and at least one nearside wheel, a first electric motor being provided for one offside wheel and a separate second electric motor being provided for one nearside wheel, each of said electric motors being a variable speed motor coupled to its respective wheel to drive the wheel, an electric power supply connected to drive each of said motors and central control means coupled to each of said motors to control independently through a switching network the current drawn by and the torque of each of said motors, whereby the current drawn by each of said motors is synchronised such that there is a phase displacement between the current drawn by said SUBSTITUTE SHEET first electric motor and the current drawn by said second electric motor. 12. An electric drive system for a motor vehicle as claimed in any one of the preceding claims. TE HEET

Inventors: JONES PERRY ADAM, MURPHY EDWARD GERARD, SIDDIQI KAMAL, STARNS GREGORY JOHN, WHITEHOUSE JAMES HENRY, JONES, PERRY, ADAM, MURPHY, EDWARD, GERARD, SIDDIQI, KAMAL, STARNS, GREGORY, JOHN, WHITEHOUSE, JAMES, HENRY
Applicants: FRAZER NASH TECHNOLOGY LIMITED, SIDDIQI KAMAL, FRAZER-NASH TECHNOLOGY LIMITED, SIDDIQI, KAMAL
Application year: 1992
Main classification: B60L11
IPC codes: B60L3, B60L11