Electric motor and motor controller

Abstract

An electric motor controller having a front face and a rear face, the front face carrying a plurality of AC output couplings and the controller carrying a converter configured to convert a received DC supply into an output AC supply for controlling an electric motor, the AC output couplings being disposed symmetrically about an axis of symmetry of the controller on the front face of the controller. Also described is an apparatus comprising: a DC series motor; and a first current supply configured to supply a first current to an armature of the DC series motor; a second current supply configured to supply a second current to a field winding of the DC series motor; and a controller configured to control the first current supply to supply the first current based on a required torque output for the motor, and to control the second current supply to supply the second current based on the first current.

Claims

1. An electric vehicle motor controller system, the system comprising a first electric vehicle motor controller and a second electric vehicle motor controller, wherein the first electric vehicle motor controller comprises: a first power provider; a first health indicator configured to provide a first indicator signal only in the event that operation of the second electric vehicle motor controller is consistent with safe operation of the vehicle, and to check said safe operation by comparing a drive direction of the second electric vehicle motor controller with information obtained from a CANBUS of the vehicle; a first failsafe, comprising a signal receiver for receiving a second indicator signal from the second electric vehicle motor controller, wherein the first failsafe is configured to enable activation of the first power provider based on the received second indicator signal; and a first processor, wherein the first processor is arranged to control operation of the first power provider based on the second indicator signal received from the second electric vehicle motor controller, and wherein the second electric vehicle motor controller comprises: a second power provider; a second health indicator configured to provide a second indicator signal only in the event that operation of the first electric vehicle motor controller is consistent with safe operation of the vehicle, and to check said safe operation by comparing a drive direction of the first electric vehicle motor controller with information obtained from the CANBUS of the vehicle; a second failsafe, comprising a signal receiver for receiving the first indicator signal from the first electric vehicle motor controller, wherein the second failsafe is configured to enable activation of the second power provider based on the received first indicator signal; and a second processor, wherein the second processor is arranged to control operation of the second power provider based on the first indicator signal received from the first electric vehicle motor controller.

2. The electric vehicle motor controller system of claim 1 in which at least one of the first failsafe and the second failsafe is configured not to enable activation of the power provider if the indicator signal is not received.

3. The electric vehicle motor controller system of claim 1 in which at least one of the first failsafe and the second failsafe is configured to activate the power provider in response to receiving the indicator signal.

4. The electric vehicle motor controller system of claim 1 in which at least one of the first processor and the second processor is configured to determine at least one parameter of the indicator signal and to control the power provider based on the at least one parameter.

5. The electric vehicle motor controller system of claim 4 in which the at least one parameter comprises one of: a DC voltage level; a frequency; a phase; peak to peak amplitude; RMS amplitude; and a duty cycle.

6. The electric vehicle motor controller system of claim 1 in which at least one of the first indicator signal and the second indicator signal is a time varying signal.

7. The electric vehicle motor controller system of claim 6 in which the at least one time varying indicator signal comprises at least one of a pulsed output signal and a square wave.

8. The electric vehicle motor controller system of claim 1 in which at least one of the first health indicator and the second health indicator is configured to provide the indicator signal based on a determination of at least one operational parameter.

9. The electric vehicle motor controller system of claim 8 in which the at least one operational parameter is selected from a list comprising: the direction of a DC current of the first motor controller; the direction of a DC current of the second motor controller; a key switch voltage; the temperature of the power transistors (IGBTs) of at least one of the first controller and the second motor controller; the temperature of control logic of at least one of the first motor controller and the second motor controller; the voltage and/or temperature of DC link tracks associated with at least one of the first motor controller and the second motor controller; ADC calibration voltages; analogue inputs; one or more programmable or non-programmable supply voltage (Vcc) monitors; sin-cos encoder data; resolver measurements; speed feedback measurements; other digital inputs and motor PTC inputs.

10. The electric vehicle motor controller system of claim 8 in which the determination comprises comparing the at least one operational parameter with a threshold value.

11. The electric vehicle motor controller system of claim 1 wherein at least one of the first power provider and the second power provider is operable to provide a power supply for an electric traction motor.

Description

BRIEF DESCRIPTION OF FIGURES

(1) Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates a system comprising an electric motor and controller;

(3) FIGS. 2A and 2B illustrate the front surfaces of first and second controllers according to the invention;

(4) FIG. 3 shows a modular motor-controller assembly comprising two motors and two controllers with a heat sink serving both controllers and a cooling manifold;

(5) FIG. 4 shows first and second controllers in profile, each controller having an extended terminal for monitoring current on the other controller;

(6) FIG. 5 schematically illustrates a DC series motor;

(7) FIG. 6 schematically illustrates a DC motor and a circuit for motor reversal;

(8) FIG. 7a schematically illustrates an arrangement for power circuitry for a four terminal DC series motor;

(9) FIG. 7b schematically illustrates a further arrangement for power circuitry for a four terminal DC series motor;

(10) FIG. 8 schematically illustrates an arrangement for power circuitry for a three terminal DC series motor;

(11) FIG. 9 illustrates a control algorithm for generating armature and field currents according to embodiments of the invention;

(12) FIG. 10 illustrates a method of determining motor speed according to an embodiment of the invention; and

(13) FIG. 11 shows a very schematic view of an electric vehicle;

(14) FIG. 12 shows a motor controller;

(15) FIG. 13 shows a configuration of two motor controllers with an external control unit;

(16) FIG. 14 shows two interconnected motor controllers;

(17) FIG. 15 shows an example combining features of FIGS. 13 and 14;

(18) FIG. 16 shows a very schematic diagram of a vehicle; and

(19) FIG. 17 shows a functional block diagram of a control system used in the vehicle of FIG. 16.

(20) AC ELECTRIC MOTORS AND MOTOR CONTROLLERS

(21) In overview, the electric motor system 10 of FIG. 1 comprises motor 2 and a controller 20 to control operation of the motor. The controller 20 is configured to derive a three-phase alternating current (AC) supply for the motor 2 from a direct current (DC) power source 6, in this example a battery. The electric motor system 10 also comprises a cooling apparatus 4. Cooling apparatus 4 is coupled to both the motor 2 and controller 20 to remove heat produced by the operation of the motor 2 and controller 20. A drive output of the motor provides torque to drive a drive plate 8 coupled to a drive axle of a vehicle.

(22) The controller 20 is configured to perform inversion, filtering and conditioning processing on the DC output of the power source 6 to provide the motor with the required AC power source.

(23) The controller 20 comprises an inverter 28 to convert a DC input to a three-phase AC output for use by the motor 2. Any suitable form of inverter may be used. In a preferred example, an insulated gate bipolar transistor (IGBT) inverter is used. In some cases MOSFETs or other IGFETs may also be used. Preferred characteristics of the IGBT converter include high efficiency and fast switching.

(24) FIG. 2A shows a front view of one example of a controller 20 comprising a printed circuit board (PCB) 22. FIG. 2A also shows a controller casing 21 within which the controller is housed. The PCB 22 carries supply contact fittings 26a, 26b and 26c each fitting may comprise a current transducer for sensing current passed through the fitting. Supply contact fitting 26b comprises a supply contact B+ for coupling to a first terminal of a DC supply, not shown. Supply contact fitting 26c comprises supply contact B for coupling to a second terminal of the DC supply (not shown). The PCB 22 also carries AC output terminals 24a, 24b and 24c and an inverter 28 (shown in dashed lines in FIG. 2A). Supply contact B+ is electrically coupled by copper arm 35b to battery terminal 32a and 32b mounted to extension 33 of the casing 21 through which wires run to provide for connection to the power source 6. Supply contact B is electrically coupled by copper arm 35a to battery terminal 32b mounted to the extension of the casing 21.

(25) The DC supply contacts 26b, 26c are coupled to respective inputs of the inverter 28. The inverter 28 outputs are coupled to AC output terminals 24a, 24b, 24c. Each of the output terminals is operable to be coupled to a respective phase winding of a motor 2.

(26) The AC output terminals 24a, 24b, 24c are disposed symmetrically about an axis x-x of PCB 22. In this example, the DC supply contact fittings 26a, 26b, 26c are similarly symmetrically disposed.

(27) PCB 22 may comprise other functionality 30 which may include, for example, microprocessors, power supplies, capacitors and inductors configured for control filtering and conditioning.

(28) FIG. 2B shows a front view of a second controller 20 comprising PCB 22. FIG. 2B also shows controller casing 21 within which the controller 20 is housed. Controller 20 is the same as controller 20 except for the orientation of the PCB 22. Compared to PCB 22 and controller casing 21, PCB 22 is rotated through 180 degree relative to its controller casing 21. Compared to controller casing 21 and PCB 22, controller casing 21 is inverted relative to PCB 22 with respect to an axis normal to the axis x-x.

(29) The effect of the PCB rotation is that when the controllers are disposed with their edges 23, 23 aligned and their back faces towards one another, the output terminals 24a, 24b, 24c of controller 20 are aligned with the output terminals 24a, 24b, 24c. The supply contact fittings 26a, 26b, 26c of controller 20 are aligned with the supply contact fittings 26a, 26b, 26c of controller 20. The casing inversion means that the profiles of controller casings 21, 21 coincide. When the controllers are arranged in this way, a heat sink may be positioned between their respective back faces adjacent the power transistors of the inventors (the principal heat source is the controller). The heat sink operates to cool both controllers simultaneously, so that separate heat sinks need not be provided for the controllers. FIG. 3 illustrates an arrangement where first and second controllers are arranged on an axis 10 either side of a heat sink 12.

(30) The controllers described above enable formation of modular assemblies consisting of sets of motors and associated controllers. The option of having identical PCBs 22, 22 which can simply be rotated for assembly together in a back to back formation simplifies production of the PCBs and assembly requirements of modular stacks of motors and controllers.

(31) Referring in more detail to FIG. 3, there is illustrated a modular assembly 50 contained within an enclosure 40 and comprising first and second motors 2, 2, first and second controllers 20, 20 and common cooling apparatus comprising a heat sink 12 and a cooling manifold 14, and supply couplings 160 running through the extensions 33 to enable the battery terminals 32a and 32b to be coupled to power source 6.

(32) The motors and controllers are disposed on an axis 10 configured to be coupled to the drive plate 8. First motor 2 is disposed on a first end of the axis 10. First motor 2 is adjacent the front face 34 of the first controller 20. The back face 36 of the first controller 20 carrying the inverter is adjacent and in thermal contact with heat sink 12. Heat sink 12 is also adjacent and in thermal contact with the back face 36 of the second controller 20. The front face 34 of the second controller 20 is adjacent second motor 2 which is disposed on the second end of the axis 10. First and second motors and first and second controllers share a cooling manifold 14 which may be integrated with the enclosure 40. Cooling manifold 14 has an inlet 16 and an outlet 18. The cooling manifold 14 is in physical and thermal contact with heat sink 12 and is in thermal contact with first and second motors 2, 2.

(33) DC supply couplings 160 are coupled to the first and second controllers 20, 20. The first motor 2 is in electrical contact with the first controller 20. Electrical contact is achieved through an electrical coupling between each of the output terminals 24a, 24b, 24c and a phase winding in the motor 2. The second motor 2 is in electrical contact with the second controller 20. Electrical contact is achieved through an electrical coupling between each of the output terminals 24a, 24b, 24c and a phase winding in the motor 2.

(34) DC power is supplied to controllers 20 and 20 via DC supply couplings 160. The first controller 20 converts the DC supply to three-phase AC for the first motor 2. The AC signal produces a torque on the rotor of the first motor 2 which produces a drive at the first end of the axis 10. The second controller 20 and motor 2 operate in the same way to produce drive at the second end of the axis 10. The motor outputs may be coupled to respective drive coupling, e.g. drive plates, to drive respective drive shafts.

(35) In one possibility the first and second controllers 20, 20 are in electrical contact. That is, DC terminals B+, B of the first controller 20 are electrically coupled to the corresponding terminals B+, B of the second controller 20. Each controller has a B+ terminal on the front face (26b, 26b) and one B terminal on the rear face (26a for controller 20 and 26c on controller 20). The B terminals pass through a current sensor on the other unit allowing monitoring of performance of both controllers to be done by each controller in the pair.

(36) The cooling system operates as follows. Cooling manifold 14 receives fluid coolant via inlet 16. The coolant absorbs heat from the first and second motors 2, 2. The manifold 14 also provides fluid to the heat sink 12, which removes heat from the inverters 28, 28 disposed on the respective back surfaces 34, 34 of the first and second controllers 20, 20 (as shown in FIG. 4). In this example, fluid supplied to the heat sink circulates back to the cooling manifold. Cooling fluid leaves the cooling manifold via outlet 18.

(37) In a possibility, a plurality of motors and controllers in a ratio of one motor to one controller are disposed on the axis 10 and within the enclosure 40. Controllers arranged back to back may share a heat sink. At most, two controllers may share a given heat sink. All heat sinks are in physical and thermal contact with the cooling manifold 14. Generally the motors will stacked at the ends of the axis 10 in order that drive be produced at the ends of the axis 10.

(38) An advantage of the modular assembly 50 is that first and second controllers 20, 20 share a cooling unit, as described above, rather than each controller being provided with a cooling unit. Therefore, in any engine having more than one motor-controller pairing, the modular assembly substantially reduces the engine's weight and volume by eliminating at least one cooling unit 4.

(39) In a possibility, the enclosure 40 has integrated electrical connections (e.g. for phase to stator windings, rotor position encoder and DC input). These serve to further optimize size and weight of the combined apparatus.

(40) It will be appreciated that the relative rotation of the second PCB 22 results in the order of the electrical terminals being inverted compared to those of the first PCB 22. This means that, when the first and second controllers 20, 20 are arranged as shown in FIG. 3, their output terminals 24, 24 are aligned as follows. Terminal 24a is aligned with 24c. Terminal 24b is aligned with 24b. Terminal 24c is aligned with 24a. In this example, terminals 24a and 24a output a first electrical phase, terminals 24b and 24b output a second phase and terminals 24c and 24c output a third phase. When identical motors are connected to the controllers as shown in FIG. 3 and identical motor-controller couplings are used, motors 2 and 2 produce a torque in opposite directions. It may be desirable that the output terminals of the controllers line up so that terminals outputting like phases are aligned. Optionally, therefore, a switch may be provided on the first PCB 20 or the second PCB 20 to invert the phases output from terminals 24a and 24c and 24a and 24c, respectively. Alternatively, circuitry may be provided to switch the phases of those terminals automatically. In possibilities where multiple controllers and motors are stacked up alongside one another, such automatic switching may serve to coordinate the phase outputs across the whole stack of controllers.

(41) It follows from the rotation of PCB 22 that the supply contact fittings 26 are also inverted. The unused contact fitting can be used to perform an additional function, such as monitoring current through another controller. An arrangement for performing current monitoring is illustrated in FIG. 4, which shows first and second controllers 20, 20 arranged back to back either side of a common heat sink 12. In the example shown, the unused contact 26c of the first controller 20 is coupled to a current monitoring transducer of the second controller 20. The coupling is achieved by means of an extended terminal 38 passing through the inverter 22 and the heat sink 12. Similarly, the unused contact 26a of the second controller 20 is coupled to a current monitoring output of the first controller 20 via an extended terminal 38 passing through inverter 28 and heat sink 12.

(42) In another possibility, power supply 6 is connected to supply contacts of the first and second controllers 20, 20 such that a positive battery terminal is provided on each respective controller by supply contacts 26b and 26b, and a negative battery terminal is provided on each respective controller by supply contacts that are aligned when the controllers are arranged back to back. With reference to FIGS. 2A and 2B, that is to say, supply contact 24c provides the negative battery terminal on the second controller 20 when supply contact 24a provides the negative battery terminal on the first controller 20, and supply contact 24a on the second controller 20 provides the negative battery terminal when supply contact 24c provides the negative terminal on the first controller 20. This allows the controllers 20, 20 to be connected in parallel to the power supply 6. In some examples, the front face carries 26b (the B+ terminal) and the rear face carries 26c (B terminal) in these cases 26a is not a terminal, rather it is a hole which can carry a current sensor. Where a current sensor is disposed in this position it can be used to sense current passed through the hole to an adjacent controller.

(43) As described above, the motor is a three-phase motor and the supply produced by controller 20 is three-phase AC. In another example, the motor may be a single-phase motor and the supply produced by the controller single-phase AC. In another example the motor may be a two-phase motor and the supply produced by the controller two-phase AC. The motor may simply be a multi-phase motor, configured to operate with a plurality of phases. As described above, the power source 6 comprises a battery. In other examples the power source may be a fuel cell or an electric double-layer capacitor (EDLC) or other source of DC power.

(44) Although the inverter described employs IGBT other voltage controlled impedances may be used, such as MOS-FETs other types of IG-FET or BJTs.

(45) DC Series Motor Control

(46) Embodiments of the DC Series Motor Control invention aim to allow a commercially available DC series motor to be operated in a similar fashion to a separately excited motor, providing a greater level of control of the motor operating characteristics while maintaining the ability to provide a high starting torque associated with DC series motors. FIGS. 7a and 7b illustrate two arrangements for connecting a four terminal DC series motor to a three phase inverter. Three phase inverter circuits are known and recently inverters capable of managing high currents have become commercially available at reasonable cost, such as the Sevcon Gen4 AC motor controller. Each leg of the inverter provides the same current capacity allowing equal currents to be provided to the armature and field windings. In the arrangement of FIG. 7a, a first leg of the three phase inverter comprises transistors 1012a and 1012b coupled in series between positive (V+) and negative (V) supply voltages. The armature 1004 of the DC series motor is connected between the positive supply voltage and a first node between transistors 1012a and 1012b. Second and third legs of the three phase inverter comprise transistors 1014a and 1014b, and 1016a and 1016b respectively, coupled in series between the positive and negative supply voltages. Field winding 1006 is coupled between a second node between transistors 1014a and 1014b, and a third node between transistors 1016a and 1016b. While the armature 1004 is shown coupled to the positive supply voltage, it will be recognized that it could equally be coupled to the negative supply line.

(47) In operation, a control signal comprising a series of control pulses is applied to the gate couplings of transistors 1012a and 1012b to modulate the current flowing through the armature winding 1004. It will be recognized that for each leg of the three phase inverter, the two transistors (a and b) will be controlled in complementary fashion so that only one of the transistors will conduct at one time. Thus, pulse width modulating the control voltage applied to transistors 1012a and 1012b allows the armature current I.sub.a to be controlled. As the field winding 1006 is coupled between two legs of the three phase converter, it is possible to reverse the direction of current I.sub.f through the field winding 1006, to thereby control the direction of rotation of the motor, by controlling the transistors 1014a, 1014b, 1016a, 1016b. Similarly, the magnitude of the field current I.sub.f can be controlled by modulating the control voltage of the transistors 1014a, 1014b, 1016a, 1016b, for example by using pulse width modulation.

(48) In the arrangement of FIG. 7b the armature 1004 is coupled between the first node and the second node, while the field winding 1006 is coupled between the third node and the positive supply. As for FIG. 7a, the field winding could instead be coupled between the third node and the negative supply.

(49) In operation of the circuit shown in FIG. 7b, the current I.sub.a in the armature 1004 is reversed, by switching of appropriate ones of the transistors of the three phase inverter, in order to reverse the direction of rotation of the motor.

(50) As will be recognized, a three phase inverter circuit such as illustrated in FIGS. 7a and 7b will include further features that have been omitted to increase clarity of the figures. In particular, a freewheeling diode would be expected to be associated with each of the switches 1012a, 1012b, 1014a, 1014b, 1016a, 1016b.

(51) In some circumstances, the arrangement of FIG. 7b may have advantages over that shown in FIG. 7a. In particular, the field winding generally has a much higher associated inductance than the armature, and therefore it is generally quicker to reverse the armature current when attempting to reverse the direction of rotation of the motor. Furthermore, the arrangement of FIG. 7b ensures that it is always possible to control the armature current I.sub.a even if the back e.m.f. becomes negative, e.g. during freewheeling of the motor. In contrast, in the arrangement of FIG. 7a, a negative e.m.f. may result in a short circuit forming across the armature via a freewheeling diode associated with the transistor 1012a, resulting in the armature current becoming uncontrolled.

(52) FIG. 8 illustrates an arrangement in which the three phase inverter is used to control a DC series motor provided with only three terminals. In the arrangement of FIG. 8 the armature 1004 is coupled between the first node and the second node of the three phase inverter, and the field winding 1006 is coupled between the second and third nodes.

(53) FIG. 9 illustrates a control algorithm for providing the armature current I.sub.a and the field current I.sub.f based on a torque demand (.sub.Demand). The algorithm of FIG. 9 can be used in conjunction with any of the arrangements shown in FIG. 7a, 7b or 8. It will of course be appreciated that the switches will be controlled differently in the different examples. For example, in the case of FIG. 7a switches 1012a and 1012b will be controlled to control the armature current whereas in FIG. 7b switches 1012a, 1012b, 1014a and 1014b will be controlled to control the armature current.

(54) The following description with reference to FIG. 9 relates to the circuit shown in FIG. 7a.

(55) According to the illustrated algorithm, a torque demand is received at a first function element 1050 which limits the torque demand, e.g. so that the torque demand does not exceed a maximum forward or maximum reverse torque. The limits may be predetermined or calculated based on measurements of the motor, for example temperature to protect the motor from excessive heat, or based on other criteria. The output of the function element 1050 is a torque demand (.sub.lim) that is to be provided by the motor.

(56) The torque demand output (.sub.lim) is provided to a first amplifier 1052 which converts the torque demand to signal representing an armature current demand corresponding to the current required to provide the requested torque output. The current demand signal is then input to a feedback control loop consisting of a first differencer 1058, first PID controller 1054 and second amplifier 1056. The output of the feedback loop (from the second amplifier 1056) is configured to regulate the armature current by switching transistors 1012a, 1012b (this is true of the example shown in FIG. 7a but for other arrangements the precise switches controlled may be different, for example, in FIG. 7b it would be 1012a, 1012b, 1014a, 1014b for armature control) to provide the armature current I.sub.a for the motor based on the current demand signal from the first amplifier 1052.

(57) The armature current I.sub.a is also supplied to a second function element 1060. Second function element 1060 includes a torque-flux table, this is just one option and other methods are known in the art. The torque-flux table is used by the second function element as a look-up table to convert the supplied armature current value to a field current demand value. The torque-flux table may be generated empirically, by characterisation of the motor or may be based on calculated performance of the motor. The output field current demand value is then input to a further feedback loop comprising second differencer 1062, second PID controller 1064, and third amplifier 1066 which regulates the output field current I.sub.f based on the field current demand value. The output of this feedback loop (from the third amplifier 1066) is configured to regulate the field current by switching transistors 1016a, 1016b to provide the field current I.sub.f for the motor based on the current demand signal from the third amplifier 1066.

(58) It will be appreciated that the use of a torque-flux table is just one way of deriving I.sub.f and that other ways known in the art may be used to derive I.sub.f from I.sub.a and other parameters.

(59) By calculating the field current I.sub.f based on the armature current I.sub.a the control algorithm of FIG. 9 provides automatic field weakening. This is because as the back e.m.f. (electro-motive force) of the motor increases eventually the available supply voltage will not be able to provide the desired armature current to the motor. However, as the field current I.sub.f is calculated based on the armature current I.sub.a, the reduction in armature current will automatically lead to a proportional reduction in the calculated field current.

(60) As will be understood by the skilled reader in the context of the present disclosure, FIG. 9 is merely schematic and when implemented in apparatus, one or more functions illustrated as separate functional components may be provided by a single control element or may be further subdivided into multiple elements. In addition, the field current and the armature current for the motor may be regulated using pulse width modulated, PWM, control signals applied to the transistors. The signals passed between elements of the algorithm in FIG. 9 may be single values, or multi-valued parameters describing PWM signals or in some examples may comprise PWM signals.

(61) FIG. 10 illustrates a speed calculation algorithm for a DC series motor being controlled according to the algorithm of FIG. 5. For many applications, it is helpful to be able to accurately determine the rotational speed of the motor, however measuring the speed of the motor directly can be difficult. The algorithm of FIG. 10 provides a way of calculating the speed based on easily measured, or known, values such as the armature and field currents applied to the motor and the armature voltage V.sub.a and battery/supply voltage V.sub.bat.

(62) The speed control algorithm operates by determining a back e.m.f (electro-motive force) for the motor and dividing this by an estimate of the magnetic flux within the motor due to the field and armature windings. For a motor having independently controlled field and armature currents, the determination of the magnetic flux must take into account both of these values, providing an extra complication.

(63) In order to calculate the back e.m.f. the armature voltage is subtracted from the battery voltage at a first differencer 1068. The output of first differencer 1068 is then supplied to a second differencer 1070. The armature current is supplied to amplifier 1076, which multiplies the armature current with the resistance of the armature to determine the IR losses in the armature (i.e. the voltage dropped across the armature due to the armature resistance). This value is then input to the second differencer 1070 where it is subtracted from the output of the first differencer 68 to determine the back e.m.f value.

(64) The armature current value I.sub.a is also input to a first flux look up table 1078 which outputs a first flux value associated with the armature current I.sub.a. The field current I.sub.f is input to a second flux look up table 1080 which outputs a second flux value associated with the field current. The first and second flux values are then input to adder 1082 to calculate the total magnetic flux within the motor. The calculated back e.m.f. value and the total flux are then input to divider 1072 where the back e.m.f. value is divided by the total flux to generate a speed value which is then scaled in a function element 1074 to provide a calibrated speed value for the motor.

(65) Thus, the algorithm of FIG. 10 is able to calculate an accurate speed value based on easily measured electrical parameters of the electrical supply to the motor. Although the inverter has been described with reference to IGBT transistors this is merely exemplary and any voltage controlled impedance may be used, for example MOSFETs, IG-FETs or BJTs.

(66) FIG. 11 shows a vehicle 2100 having four wheels 2102, 2104, 2106, 2108. Wheels 2106 and 2108 are opposite each other towards the front of the vehicle. Wheels 2102 and 2104 are opposite each other towards the rear of the vehicle 2100.

(67) Wheel 2102 is coupled to electric motor 2101, which in turn is electrically coupled to motor controller 2002. Wheel 2104 is coupled to electric motor 2103 which in turn is electrically coupled to motor controller 2002. Vehicle control unit 2126 is coupled to both of the motor controllers 2002, 2002.

(68) The electric motors 2101, 2103 are operable to drive the wheels 2101, 2103 independently of one another under the control of controllers 2002, 2002. The vehicle control unit 2126 provides control signals to controllers 2002, 2002 to control movement of the vehicle by driving the motors 2101, 2103.

(69) Although shown as rear wheel drive the vehicle 2100 may be front wheel drive. In addition, although the vehicle 2100 is shown as a two wheel drive vehicle examples of the invention may be applied to four wheel drive vehicles. The vehicle control unit 2126 may be coupled to the controllers 2002, 2002 by a control bus such as a controller area network, CAN-bus.

(70) FIG. 12 shows a motor controller 2002 comprising a control means 2012 coupled to a local current monitor 2010 and a remote current monitor 2014. The motor controller comprises a power provider 2016 which is coupled to an external fail safe 2020 and to the control means 2012. The control means 2012 is also coupled to a health indicator 2018.

(71) In FIG. 12 power supply input connections (e.g. battery connections) and AC power output connections are not shown. Power supply connections are discussed in more detail below with reference to FIG. 14.

(72) The power provider 2016 includes an insulated gate bipolar transistor, IGBT, inverter and is operable to provide a pulse width modulated power output to an electric motor based on a direct current, DC, power supply. The power provider can be disabled by a shutdown signal from the control means 2012 or as a result of the failsafe 2020 disconnecting the power electronics (e.g. the PWM drive) of the power provider 2016. When the output of the failsafe 2020 is in the enabling state, it turns on a transistor circuit (not shown) which controls the supply of power to the PWM drive circuits of the power provider 2016. When the output of the failsafe 2020 is in the disabling state, it turns off the transistor circuit thereby deactivating the PWM drive.

(73) The local current monitor 2010 is operable to sense current drawn from the DC power supply by the power provider 2016 and provides a sense output signal to the control means 2012 based on the sensed current flow. This local current monitor 2010 is operable to provide an estimate of output torque from a motor driven by the power provider 2016 and can be used to check whether the controller is providing energy to the motor (e.g. operating in a drive mode) or if the motor is returning energy to the controller (as in a regenerative brake mode). The remote current monitor 2014 is operable to be coupled to sense current drawn by a power provider of another motor controller, similar to motor controller 2002.

(74) The health indicator 2018 is operable to provide a heartbeat output signal at its output coupling dependent upon a determination that the other components of the motor controller, 2010, 2012, 2014, 2016, 2018, 2020 are functioning correctly and that the signal from the remote current monitor 2014 is consistent with safe operation of the vehicle. The heartbeat signal is a sequence of square-wave pulses having a selected amplitude, frequency and duty-cycle. If the heartbeat signal is acceptable then the failsafe 2020 produces a signal which enables the power provider. If the heartbeat signal is not acceptable then the output of the failsafe circuit disables the power provider 2016.

(75) It will be understood that the output of the failsafe is combined with other signals in the system, for example the inverse of the shutdown signal from the control means 2012, so that all such signals must be in the enabling state to enable the power provider 2016 and so that any such signal in the disabling state will cause the power provider to be disabled. A means to prevent the power provider 2016 from re-enabling for a period of time after it has been disabled is provided. Determination of correct function is based on checking the range and in some cases transient behaviour of signals associated with the various component blocks.

(76) In the example of FIG. 12 the heartbeat signal comprises a pulsed (square wave) signal.

(77) The failsafe 2020 is operable to receive a heartbeat signal from an external system and to provide an enable signal to the power provider 2016. The failsafe 2020 is configured to provide an enable signal to the power provider 2016 in the event that it receives a heartbeat signal and the power provider is arranged such that it cannot be activated in the absence of an enable signal from the failsafe, for example the power supply to the power provider may be configured such that, in the absence of an enable signal the power supply to the power provider 2016 is disconnected. In addition to or as an alternative to a failsafe, the failsafe 2020 may also comprise a detecting means configured such that in the event that it does not receive a heartbeat signal it causes the power provider to taper or reduce or to otherwise modify its power output to a safe level. In some cases the power provider or the failsafe may include a timer configured to prevent the power provider from being re-enabled for a selected time after it has been disabled. Alternatively or in addition, the failsafe or the power provider may be arranged to signal to the control means 2012 that action has been taken to disable the power provider.

(78) The control means 2012 is a control device, such as a microprocessor, configured to control operation of the other components of the controller 2002. For example, the control means 2012 is configured to compare the local current sensed by the local current monitor 2010 with the current sensed by the remote current monitor 2014. Based on this comparison the control means 2012 is configured to control the power provider 2016. For example, in the event that the direction of the current sensed by the local current monitor 2010 does not match the direction of the current sensed by the remote current monitor 2014 the control means is configured to shut down the power provider. This could relate to a situation in which one of the motors is engaged in regenerative braking. Note that the reversal of battery current in the reversed motor will be transient. If the situation persists then regenerative braking will end. The reversed motor could start to drive in the opposite direction and battery current would then go back to the original sense.

(79) In operation the power provider 2016 provides an AC power output signal for an electric traction motor provided that: (a) the failsafe 2020 provides an enable signal to the power provider 2016 indicating that the failsafe 2020 is receiving a valid heartbeat signal; and (b) the control means does not provide a shut down signal to the power provider. The health indicator 2018 provides an output heartbeat signal (e.g. to be received by another controller) in the event that it determines that the other components 2010, 2012, 2014, 2016, 2018, 2020 of the motor controller are functioning correctly and that the signal from the remote current monitor 2014 is consistent with safe operation of the vehicle, e.g. to ensure that the motors are driving in the same direction (or that they are driving in a selected direction or to ensure that the speed or torque of the motor is within a selected range, for example a range based on the speed or torque of another motor and/or the current supplied thereto. In some cases the power provider is configured to check that the remote motor's direction of rotation is consistent with safe operation. For example, where the power provider is coupled to a controller area network bus (CANbus) of a vehicle then speed information from the remote monitor may be read from the CANbus to perform this check. Although the power provider 16 has been described as comprising an IGBT inverter other types of inverter may be used, for example the inverter may comprise IG-FETs, MOS-FETs, BJTs or other types of voltage controlled impedances. Coupling between components of the system may be direct or indirect and, where appropriate, may be provided by wireless couplings and/or through other components of the system. Although, for the purposes of explaining the invention components of the system have been shown as discrete units, this is merely exemplary and similar functionality may be provided in a smaller number of functional units, a single integrated unit, or the functionality may be further distributed/subdivided between a greater number of functional units.

(80) The failsafe circuit could be implemented using digital or analogue electronics. The heartbeat signal is described as a sequence of square-wave pulses having a selected amplitude, frequency and duty-cycle. However other waveforms such as sinusoids or sawtooth waveforms may be used. In addition, one or more of the amplitude, frequency and duty cycle may not be used. For example the heartbeat may simply be based on the frequency of the signal or its amplitude or the duty cycle. In some cases a combination of two or more of these parameters may be used.

(81) FIG. 13 shows the motor controller 2002 of FIG. 12 (in which like reference numerals indicate like elements) in use with a second similar motor controller 2002 and an external safety monitor 2026. To assist understanding of the example of FIG. 13, elements which are not directly relevant to the example are shown only in broken lines; these or other elements may be omitted. The external fail safe 2020 of the motor controller 2002 is coupled to the health indicator 2018 of the second motor controller 2002 by a switching unit 2024. The external fail safe 2020 of the second motor controller 2002 is coupled to the health indicator 2018 of the motor controller 2002 by the switching unit 2024. The switching unit 2024 is coupled to the external safety monitor 2026. The external safety monitor 2026 may be provided by a control unit such as the vehicle control unit 2126 of FIG. 11.

(82) The switching unit 2024 is controllable by the external safety monitor 2026 to couple/decouple the failsafe 2020 of the motor controller 2002 to the health indicator 2018 of the second motor controller 2002. The switching unit 2024 is also controllable by the external safety monitor 2026 to couple/decouple the failsafe 2020 of the second motor controller 2002 to the health indicator 2018 of the motor controller 2002.

(83) In operation the external safety monitor 2026 and/or the health indicator 2018 monitors safety parameters of the vehicle such as; the current sense of each motor controller 2020, 2002; the key switch voltage (e.g. the supply voltage to the control electronics, as opposed to the supply voltage of the main power stage); the temperature of the power transistors (IGBTs) of each controller; the temperature of the control logic of each controller; the voltage and/or temperature of DC link tracks; ADC calibration voltages; analogue inputs; supply voltage (Vcc) monitors (for monitoring programmable and fixed supply voltages); speed feedback measurements (such as sin-cos encoder data; resolver measurements); other digital inputs and motor PTC inputs. These are merely examples of parameters which can be monitored, some or all of them may not be used in practice and/or other additional safety parameters may be monitored. Fault detection may be performed by comparing one or more of these parameters with selected threshold values or selected ranges or by comparing sampled values of a parameter over a time interval to measure its transient behaviour, for example the rate of change of one or more of these parameters may be compared with a selected threshold level or a selected range; the thresholds and ranges may be selected based on stored (e.g. predetermined) values or may be determined on the fly based on other parameters.

(84) The external safety monitor 2026 is operable to detect a fault condition based on monitoring of one or more of these safety parameters and to control the switching unit 2024 in response. For example, in the event that the external safety monitor detects a fault it can control the switching unit 2024 to decouple the failsafe 2020 of the second motor controller 2002 from the health indicator 2018 of the motor controller 2002 and/or to decouple the failsafe 2020 of the first motor controller 2002 from the health indicator 2018 of the second motor controller 2002. In response to disconnection of the health indicator 2018, 2018 from the failsafe 2020, 2020 the controllers will shut down. For the configuration where 2018 is connected to 2020 and 2018 is connected to 2020 then both controllers will be shut down as a result of failure of one controller.

(85) FIG. 14 shows another example of the motor controller 2002 of FIG. 12 (in which like reference numerals indicate like elements) in use with a second similar motor controller 2002. To assist understanding of the example of FIG. 14, elements which are not directly relevant to the example are shown only in broken lines and connections which are not directly relevant have been omitted.

(86) The local current monitor 2010 of motor controller 2002 is electrically coupled in series between the positive battery terminal B+ of a battery 2200 and the power provider 16 of motor controller 2002. The remote current sense module of the second motor controller 2002 is electrically coupled in series between the power provider 2016 of the first motor controller 2002 and the negative battery terminal B of the battery 2200. The local current monitor 2010 of motor controller 2002 is electrically coupled in series between the positive battery terminal B+ of a battery 2200 and the power provider 16 of motor controller 2002. The remote current sense module of the first motor controller 2002 is electrically coupled in series between the power provider 2016 of the motor controller 2002 and the negative battery terminal B of the battery 2200. In this way the remote current monitor 2014 of controller 2002 is coupled to monitor the sense of the current drawn by the power provider 2016 of motor controller 2002 and the remote current monitor 2014 of controller 2002 is coupled to monitor the sense of the current drawn by the power provider 2016 of motor controller 2002. Thus, each controller 2002, 2002 is arranged to monitor the sense of the current drawn or generated by the other motor controller.

(87) In some examples according to FIG. 14, to provide a paired controller interlock the failsafe 2020 of the motor controller 2002 is coupled to the health indicator 2018 of the second motor controller 2002. The external fail safe 2020 of the second motor controller 2002 is coupled to the health indicator 2018 of the motor controller 2002.

(88) Although they are shown as being connected as series current measurement devices the current monitor 2010, 2010, 2014, 2014 may be provided by inductive or Hall Effect current transducers which need not be conductively coupled to the power supply lines and/or maybe provided by other current sensing devices such as current sensing transistors. In some examples the current monitors may also be configured to monitor current magnitude as well as or instead of current direction.

(89) FIG. 15 uses the same reference numerals as FIG. 13 and FIG. 14 to indicate like elements and shows an example which uses components of FIG. 13 and FIG. 14 in combination. In FIG. 15 the switching unit 2024 and the external safety monitor 2026 are shown in broken lines to indicate that the failsafe 2020 of the controller 2002 may be coupled to the health indicator 2018 of controller 2002 without the presence of a switching unit; likewise, the failsafe 2020 of the controller 2002 may be coupled to the health indicator 2018 of controller 2002 without the presence of a switching unit. In some cases the failsafe input on controller 2002 and 2002 can be supplied directly by the external safety monitor 2026 with the health signal generated by controllers 2002 and 2002 being monitored by the external safety monitor 2026.

(90) In particular, the current monitor 2014 may be coupled to a current sensor arranged to provide a monitoring coupling in an electric motor controller (such as that described with reference to FIGS. 1 to 4) comprising at least one power supply input coupling and a monitoring coupling adapted for monitoring an operational parameter of another controller, wherein the monitoring coupling and the power supply input coupling are disposed in positions selected so that, when two controllers are arranged back-to-back the DC power supply (e.g. battery) terminal of one controller may be coupled to a DC power supply through the monitoring coupling of an adjacent controller. For example, if the controller is carried on a circuit board the monitoring coupling may be provided as a hole through the board with a current transducer arranged around/adjacent the hole. Thus the battery current to one board can be provided by a DC supply coupling that passes through the current transducer on an adjacent board, thereby providing the current monitor 2014 described herein.

(91) FIG. 16 shows a vehicle 3018 having an electric motor 3006 coupled by a drive shaft 3010 to a differential 3008. The axle 3014 couples the differential 8 to the wheels 3012, 3012 so that the vehicle's drive line comprises the drive shaft 3010, differential 3008, axles 3014 and the wheels 3012, 3012. Although only two wheels are shown in FIG. 16, this is merely illustrative and the vehicle may have more or fewer wheels.

(92) The vehicle 3018 comprises a battery 3002 coupled by an inverter 3004 to the motor 3006. The motor controller 3004 is coupled to a controller 3020. Controller 3020 is coupled to sensor 3024 and to a driver control interface 3022.

(93) In operation the inverter 3004 converts the DC supply from the battery 3002 to alternating current (AC) power supply for the motor 3006. The motor 3006 is operable, in response to power from the inverter to apply torque to the drive line 3008, 3010, 3014 to drive the wheels 3012, 3012. The sensor 3024 is configured to sense the angular acceleration or torque output provided by the motor 3006 and is coupled to provide a signal to the controller 3020 indicating the sensed speed output of the motor 3006.

(94) The driver user interface 3022 comprises an accelerator or torque demand input user interface for controlling the speed of the vehicle. The driver user interface 3022 is coupled to provide a torque demand signal to the controller 3020. The controller 3020 is configured to provide a control signal to the inverter 3004 to control the power supply to the motor based on the torque demand form the driver control interface 3022 and the motor's output sensed by the motor speed sensor 3024.

(95) The controller 3020 will now be described in greater detail with reference to FIG. 17. Controller 3020 has a torque demand input 3030 for receiving a control signal from a driver user interface 3022 (in FIG. 16) and a sensor input 3032 for receiving a sensor signal from a motor speed sensor 3024 (also in FIG. 1). Torque demand input 3030 is coupled to a first input of a difference determiner 3042 and to a sign checker 3043. Sensor input 3032 is coupled by a filter 3034 to a gain stage 3036. The output of the gain stage 3036 is coupled to a second input of the difference determiner 3042. The output of the difference determiner 3042 is coupled to the sign checker 3043. The output of the sign checker 3043 is coupled to a control signal output 3044. A reference signal provider 3038 is coupled to a limiter 3040 applied to the output of gain stage 3036.

(96) The controller 3020 receives a torque demand signal from the driver user interface 3022 at torque demand input 3030 and a measurement signal from the motor speed sensor 3024 at sensor input 3032. The rate of change of angular speed of the motor is used to determine a torque measurement signal /t.

(97) The filter 3034 filters the torque measurement signal received from the torque sensor to select or emphasise particular signal frequency components and to attenuate others. The inventors in the present case have appreciated that parts of an electric vehicle produce electrical noise signals having known or predictable frequency characteristics. For example, a vehicle may include a noise source at or around 50 Hz so in some examples filter 3034 includes a notch filter adapted to attenuate signals in a frequency band around 50 Hz. In some possibilities a low pass filter with a 50 Hz cut off may be used, this has the advantage of smoothing out sampling noise. In addition, the inventors have realised that the vehicle drive line and the vehicle itself provide an oscillatory system having certain characteristic (e.g. resonant) frequencies. In the example of FIG. 16 the transfer function/pass-band of the filter 3034 is selected based on the characteristic frequency of the vehicle and the vehicle drive line. This characteristic frequency depends upon the torsional stiffness of the components of the drive line, the resilience of the tires, the stiffness of the vehicle suspension, the mass of the vehicle and the moment's of inertia of the rotating components in the vehicle drive line amongst other factors. The filter 3034 may be provided by an analogue filter having an LCR network having band-pass and/or notch features or by digital electronics.

(98) The reference signal provider 3038 provides a reference signal to the limiter 3040. Limiter 3040 is operable to limit the signal provided to the difference determiner such that it does not exceed (e.g. is limited by) the reference signal. In the example of FIG. 16 the reference signal is selected to be 10% of the maximum possible torque demand value. This has the advantage that motor speed oscillations can be damped without intentional changes in torque demand being damped to such an extent that the system does not respond sufficiently quickly to user controls. The inventors in the present case have found that 10% is a safe value for the application. Experiments showed that only small corrections to torque output were required to mitigate the oscillations. As the algorithm is trying to resist changes in speed, removing the 10% limit could result in the vehicle becoming unresponsive. In practice, the gain would be set such that the limit is rarely usedonly small corrections would be necessary. Having the limit in place gives confidence that oscillation damping will never be able to completely override the driver's demand. It may be useful to allow different values to be configured.

(99) The sign checker 3043 compares the sign of the torque demand signal provided at the torque demand input 3030 with the modified torque demand to be provided to the torque demand output 3044 and, in the event that the torque demand input is of opposite sign to the modified torque demand the modified torque demand is set to zero. In this example the sign checker 3043 is implemented using a software algorithm based on the following pseudo-code:

(100) TABLE-US-00001 if( torque_demand > 0 ) { torque_demand = torque_demand correction_value; if( torque_demand < 0 ) { torque_demand = 0; } } else if ( torque_demand < 0 ) { torque_demand = torque_demand correction_value; if( torque_demand > 0 ) { torque_demand = 0; } }

(101) In operation the controller 3020 receives a torque demand signal from the driver control interface 3022 (FIG. 16) and the sensor 3024 (FIG. 16) provides a sensed torque signal to the controller 3020 based on the torque provided by the electric motor 3006 (FIG. 16). The filter 3034 selects components of the sensed torque signal which may be characteristic of unwanted motor speed oscillations and attenuates noise components according to the transfer function of the filter. The output signal from the filter 3034 is provided to the gain stage 3036. The gain stage 3036 applies a scaling to the filter output to provide a subtraction signal to be subtracted from the torque demand inpute.g. it sets the gain of the feedback loop. Typically the scaling applied by the gain stage is tuned by driving the vehicle and trying different gains to see what effect they have on the drivetrain. Oscillations normally manifest as an audible warble from the motor or drivetrain. The scaling value would be set as low as possible where the oscillations are not noticeable. Using a low value minimises the algorithms ability to counter the demanded torque which would reduce the response of the vehicle. The subtraction signal output from the gain stage 3036 is provided to limiter 3040. In the event that the magnitude of the output of the gain stage exceeds the reference signal then the limiter clips the output of the gain stage so that it's magnitude does not exceed (e.g. is limited by) the reference signal provided by the reference signal provider 3038. The difference determiner subtracts the scaled filtered torque measurement signal from the torque demand signal received from the driver control interface 3022 (FIG. 1). The output of the difference determiner provides a modified torque demand output to be used as the input for a controller controlling the motor 3006.

(102) In the example of FIG. 16 the wheels 3012, 3012, the axle 3014, the differential and the drive shaft make up the drive line. Of course this is merely an example of a drive line and, in practice a drive line may comprise additional components or some of the components shown in FIG. 16 may be omitted for example in direct drive systems or in systems which do not include a differential. In some examples a 3004 wheel drive system maybe used. In the example of FIG. 16 the sensor 3024 is shown disposed adjacent the motor however in practice the sensor may also be disposed on the drive shaft or some other aspect of the drive line and/or it may be an electronic sensor which determines the torque output based on the current drawn by the motor 3006. FIG. 16 shows a single motor driving two wheels through a differential, however other topologies may be used, for example each wheel may be driven by a separate motor. In this case each motor would be controlled separately according to the algorithm outlined above.

(103) The controller 3020 may be implemented by analogue electronics using passive and/or active analogue components. In some examples the controller 3020 may comprise or be provided by digital electronics such as DSP circuitry, application specific integrated circuits, FPGA or other digital electronics. The controller may also comprise or be implemented by a programmable processor configured using program instructions to implement features of the methods and/or apparatus described herein.

(104) It is intended that one or more features of any of the examples described above and/or defined in the appended claims may be omitted and/or combined with one or more features of any of the other examples. The methods described herein may be implemented in hardware middleware or software or any combination thereof. In addition, examples of the invention comprise computer readable storage media and computer program products operable to program a processor to perform any of the methods described herein and in particular to configure a processor to perform one or more of the functions carried out by the control means 2012, the failsafe 2020, or the health indicator 2018 as described hereinabove.