WO2014049338A2

WO2014049338A2 - Motor safety control

Info

Publication number: WO2014049338A2
Application number: PCT/GB2013/052482
Authority: WO
Inventors: Ken Thompson
Original assignee: Sevcon Limited
Priority date: 2012-09-28
Filing date: 2013-09-23
Publication date: 2014-04-03
Also published as: GB2506423B; WO2014049338A3; GB2506423A; GB201217430D0

Abstract

Monitoring for malfunction of an electric motor of an electric vehicle is described. In response to malfunction, a motor controller may be shut-down to inhibit unsafe operation of the vehicle. One example provides an electric motor control apparatus configured to selectively enable or inhibit operation of an electric motor based on characteristics of a time varying signal, these characteristics may include the duty cycle, and/or the frequency content of the time varying signal. The apparatus may enable or inhibit operation of the electric motor based on the amplitude of the time varying signal in a selected frequency band.

Description

Motor Safety Control

The present invention relates to a method and apparatus for the control of electric motors, and more particularly to safety control for electric motors. Still more particularly embodiments of the invention relate to methods and apparatus for safety control systems of electric motors for use in electric vehicles. There is an increasing demand for electric motors in a range of applications, and particularly to supplement or replace internal combustion engines in vehicles. The use of electric motors in this context has a number of advantages. For example a number of motors can be used in a single vehicle, either coupled together to combine their torque output, or operated separately to provide independent drives to different wheels. The safe operation of the vehicle in such cases may be dependent on the proper operation of all of the motors, and proper operation of other vehicle components such as batteries .

The present disclosure relates to the monitoring of vehicle functions, in particular battery and motor operation, to identify any malfunction, such as a battery running over- temperature. In response to such malfunction, the motor controller may be shut-down to inhibit unsafe operation of the vehicle. One way to achieve this is to use a "dead-man's switch" which enables operation of a motor only in the event that a selected signal is received indicating that the vehicle is "healthy", e.g. that no malfunction has been detected. The use of time varying signals for this purpose has the advantage that short circuits, or latching, are unlikely to produce a time varying signal. Thus the likelihood of operating the vehicle in an unsafe condition is reduced. In this context, aspects of the disclosure provide methods and apparatus for validating a time varying signal to control operation of an electric motor. One example of the disclosure provides an electric motor control apparatus configured to selectively enable or inhibit operation of an electric motor based on characteristics of a time varying signal, as an example, these characteristics may include the duty cycle, and/or the frequency content of the time varying signal. The apparatus may be operable to enable or inhibit operation of the electric motor based on the amplitude of the time varying signal in a selected frequency band. The apparatus may comprise, (or consist solely of) analogue components. In an aspect there is provide an electric motor control apparatus configured to provide an output for selectively enabling or inhibiting operation of an electric motor, the apparatus comprising: a signal coupling for receiving a time varying signal; a validator configured to provide an output to enable or inhibit operation of an electric motor based on at least one of: the duty cycle, and the frequency content of the time varying signal.

In an embodiment the validator is configured to determine the frequency content based on the amplitude of the time varying signal in a selected frequency band.

In an embodiment that apparatus comprises a filter arranged to filter the time varying signal, in some examples the filter comprises a low pass filter. The validator may be configured to enable or inhibit operation of the electric motor based on at least one condition selected from the list comprising: the minimum negative filtered signal being within a first selected range; the maximum positive filtered signal being within a second selected range; and the amplitude of filtered signal being within a third selected range.

The validator may comprise at least one comparer arranged to compare the signal with a reference. The comparer may comprise at least one window comparer arranged to indicate whether the signal is greater than a minimum threshold and less than a maximum threshold. For example, the at least one window comparer may comprise a first window comparer, and a second window comparer having threshold levels different from the first window comparer and in which the validator is configured to enable or inhibit operation of the electric motor based on the output of both the first window comparer and the second window comparer. The output of one of the at least one window comparers may be coupled to a pulse detector and the validator is configured to enable or inhibit operation of the electric motor based on the output of the pulse detector.

In an aspect there is provided a method of controlling an electric motor comprising: receiving a time varying control signal; filtering the control signal to provide a filtered signal; providing an output for enabling or inhibiting operation of the electric motor based on at least one of: the amplitude of the filtered signal, and the duty cycle of the control signal. Filtering may comprise, or consist solely of, low pass filtering.

Being based on the amplitude of filtered signal may comprise at least one of: a minimum negative filtered signal being within a first selected range; and a maximum positive filtered signal being within a second selected range. Being based on the duty cycle of the time varying control signal may comprise being based on duty cycle of the filtered signal. Aspects of the disclosure provide an electric motor controller comprising an inverter for providing power to an electric motor, and an electric motor control apparatus according to any one described or claimed herein arranged to inhibit or enable operation of the electric motor by selectively enabling the provision of power from the inverter to the electric motor .

Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of an electric motor configuration;

Figure 2 shows a schematic view of a validator;

Figure 3A, Figure 3B, Figure 3C and Figure 3D show examples of signal versus time plots at a point in the apparatus of Figure 2;

Figure 4 shows one example of an apparatus according to Figure 2; and

Figure 5 shows an arrangement of motor controllers.

Figure 1 shows a validator 1 coupled to selectively enable or inhibit operation of an electric motor 3 based on validation of a time varying control signal. The time varying control signal may be produced by a vehicle safety monitor (not shown in Figure 1) . Examples of such control signals are pulse trains, e.g. square waves, whose amplitude may be set by the positive and negative DC power supply voltages (rail voltages) . In Figure 1 the validator 1 is coupled to control a power provider 5.

The power provider 5 is coupled to receive DC electric power from a battery 7 and to provide an AC power supply to the motor 3. The power provider 5 includes an inverter circuit and timing control elements (not shown) for controlling the provision of the AC power supply to the motor 3. The validator 1 is configured to receive the time varying control signal, and to provide an output to the power provider 5 based on the amplitude and/or the duty cycle of the time varying control signal. The power provider 5 is configured such that the motor 3 is only operable if the signal from the validator 1 indicates that the control signal has a frequency and duty cycle within selected ranges, e.g using a logical TRUE signal. In an electric vehicle, there may be rapidly varying currents of high magnitude, and cross-talk between cables may generate relatively high amplitude time varying noise signals. In this context, the validator of the present disclosure may provide a robust test for the presence of a genuine safety signal because noise signals are unlikely to provide signals having the correct amplitude/ frequency, and still less likely to produce a signal having the correct duty cycle.

In operation, the validator 1 filters the control signal to provide a filtered signal in a selected frequency band (e.g. provided by the pass band of a filter) . A low pass filter may be used for this purpose to preserve any DC offsets associated with the duty cycle of the control signal.

The validator 1 may then determine whether the minimum negative filtered signal (e.g. a signal trough) is within a first selected range, and whether the maximum positive filtered signal (e.g. peak) is within a second selected range. The validator may also determine whether the duty cycle is within a selected range of duty cycles. Based on the determined characteristics of the control signal, the validator 1 is configured to selectively enable or inhibit operation of the motor 3, for example by switching the power provider 5 on or off.

Figure 1 is a schematic functional block diagram, and although the validator 1 is illustrated in Figure 1 as a separate functional unit, it need not be provided separately. For example, the power provider 5 may include the validator 1. In other examples, the motor 3 may include the validator 1, in which case the validator is operable to enable or inhibit operation of the motor independently of operation of the power provider. For example the validator 1 may be arranged to switch the motor 3 off.

Figure 2 shows one example of a validator 1 such as that illustrated in Figure 1. The validator 1 of Figure 2 comprises an input 10 for receiving a control signal. A low pass filter 12 is coupled to the input 10 to provide the filtered control signal to a first comparer 14, and to a second comparer 16. The output of the first comparer 14 is coupled to an input of a combiner 20. The output of the second comparer 16 is coupled to a pulse detector 18, and the output of the pulse detector is coupled to another input of the combiner 20.

The comparer 14 is configured to provide a logical TRUE output in the event that its input signal is greater than a first (lower) reference signal, and less than a second (upper) reference signal, and a logical FALSE output in the event that it is not. Thus, the comparer 14 defines a range of control signal values that will produce a TRUE output. The second comparer 16 is configured in a similar way to comparer 14, however the first (lower) reference signal and the second (upper) reference signal of the second comparer are selected to lie within the range defined by the first comparer 14. The upper bound of the range defined by the second comparer 16 may be selected to be less than the upper bound of the range defined by the first comparer 14 by at least the resolution of the first comparer 14. Likewise, the lower bounds may also differ by at least the resolution of the first comparer 14.

The pulse detector 18 is configured to provide a logical TRUE output in the event that the input to the pulse detector 18 is a sequence of pulses having at least a selected frequency and pulse duration, and a logical FALSE otherwise.

In the example of Figure 2, the combiner 20 provides the functionality of an AND gate, but is configured to provide a degree of temporal smoothing so that the output of the combiner 20 is only TRUE in the event that the inputs provided from the pulse detector 18, and the first combiner 14, are both TRUE over a selected time interval.

Other configurations of the combiner 20 are possible, for example, it may simply be provided by an AND gate. One or more of the comparers 14, 16, may be provided with a degree of hysteresis to help to avoid a situation in which the output of the combiner oscillates.

The functionality of the elements 12, 14, 16, 18, 20 of Figure 2 may be provided by analogue components, and this has the advantage of providing a system which is reliable and simple to maintain. In some examples the functionality may be provided by digital logic, such as field programmable gate arrays, FPGA, application specific integrated circuits, ASIC, a digital signal processor, DSP, or by software loaded into a programmable processor. Although the filter 12 is described as a low pass filter, in some examples other kinds of filter may be used such as band pass or high pass filters.

Figure 3A, Figure 3B, Figure 3C and Figure 3D illustrate four states of the validator 1 shown in Figure 2. The voltage levels associated with the first and second comparer 14, 16 of Figure 2 are illustrated in Figures 3A, 3B, 3C and 3D. In these Figures :

the lines marked 140, 140', correspond to the lower reference signal and the upper reference signal respectively of the first comparer 14 of Figure 2 ;

the lines marked 160 and 160' correspond to the lower reference signal and the upper reference signal respectively of the second comparer 16 of Figure 2.

The saw-tooth waveform in Figure 3 illustrates the output of the filter 12 in Figure 2, which provides the input to the comparers 14, 16. Figure 3A illustrates a valid' control signal, which: (a) lies within the range 140, 140' of the first comparer 14 and so the first comparer 14 provides a logical TRUE to the combiner 20; (b) periodically lies outside the range 160, 160' of the second comparer 16, so the second comparer 16 produces a train of pulses with a frequency twice that of the filtered control signal. In response to this train of pulses, the pulse detector 18 produces a logical TRUE, so the combiner 20 also produces a logical TRUE. Figure 3B illustrates the case in which the control signal frequency is too high, as a result it is attenuated more strongly by the low pass filter and: (a) always lies within the range of the first comparer 14, so the first comparer 14 provides a logical TRUE to the combiner 20; but (b) it always also lies within the range of the second comparer 16 so the pulse detector 18 produces a logical FALSE, and the combiner 20 also produces a logical FALSE. Figure 3C illustrates the case where the control signal frequency is too low. In this case the first comparer 14 and the second comparer 16 both output a sequence of pulses. In response to this condition the combiner 20 produces a logical FALSE .

Figure 3D illustrates the case where the duty cycle is not within the selected range of duty cycles. In this case the pulse detector 18 receives a series of pulses from the second comparer 16, but the frequency of the pulses is below the threshold frequency of the pulse detector 18. The pulse detector 18 thus provides a logical FALSE output, as does the combiner 20.

Figure 4 shows a validator 100 which is one possible example of an implementation of the validator 1 of Figure 1 and Figure 2. The validator 100 comprises an input 10 coupled to provide a received control signal to a low pass filter 12. The output of the low pass filter 12 is coupled to a first comparer 14, and to a second comparer 16. The output of the first comparer 14 is coupled to a combiner 20. The output of the second comparer 18 is coupled to a pulse detector 18. The output of the pulse detector is coupled to the combiner 20.

The low pass filter comprises a capacitance 50 arranged to couple the input 10, and the inputs of the comparers 14, 16 to a ground or reference voltage.

The first comparer 14 comprises a first comparator 54, and a second comparator 52 configured to provide a window comparator. The non-inverting input of the first comparator 54 is coupled to a first (lower) reference voltage 140 and the inverting input of the first comparator is coupled to the input of the first comparer 14. The non-inverting input of the second comparator 52 is coupled to a second (upper) reference voltage 140' and the inverting input of the second comparator is coupled to the input of the first comparer 14. The second comparer 14 also comprises two comparators 68, 70 coupled between reference voltages 160, 160' to provide a second window comparator in a similar manner.

The pulse detector 18 comprises a reference voltage 67 coupled by series resistances 62, 64 to a capacitance 60 and to the inverting input of the comparator 56. The non-inverting input of the comparator 56 is coupled to a third reference voltage 66. The capacitance 60 is in turn coupled to another (lower) reference voltage, e.g. a ground voltage. The output of the second comparer 16 is coupled between the resistances 62, 64. The output of the comparator 56 is coupled to the combiner 20.

Thus, the reference voltage 67 is arranged to charge the capacitance 60 through the resistances 62, 64. In the event that the output of the second comparer 16 swings ^xhigh' (logical TRUE) the capacitance 60 is charged by the second comparer 16. In the event that the output of the second comparer 16 swings ^xlow' (logical FALSE), the capacitance 60 is discharged through the resistance 64. By selecting the reference voltages 66 and 67, and the time constant (RC) associated with the resistance 64 and the capacitance 60, it is possible to select the minimum frequency and duration of low' pulses required to keep the voltage across the capacitance lower than the reference voltage 66. In this way the output of the comparator 56 will indicate whether the frequency and duration of the ^xlow' pulses is greater than or less than a selected minimum value.

The operation of Figure 4 has been described with reference to the comparison of voltages, and in terms of voltage comparators. However, in some examples current comparators may also be used. In addition, although the comparators are illustrated as operational amplifiers any other means of comparing voltages may also be used. In some examples the comparers 14, 16 of Figure 4 may include diodes arranged to decouple the outputs of the comparators 52, 54, 68, 70.

Figure 5 shows an example arrangement of two electric motor controllers 200, 200' . The first controller 200 comprises a power provider 116, such as an IGBT inverter for providing power to an electric motor (not shown) and a validator 120 coupled to the power provider 116 and configured to selectively enable or inhibit operation of the power provider 116 based on a received time varying control signal input. The validator 120 may be provided by any of the example validators described above with reference to Figures 1, 2, 3 or 4.

A safety monitor 112 is coupled to monitor operation of the power provider 116 to determine that the power provider is operating safely. The safety monitor 112 is also coupled to a health indicator which is arranged to provide a time varying control signal output in the event that the safety monitor 112 indicates that the power provider 116 is operating safely. The second controller 200' is similar and comprises a power provider 116 a validator 120', a health indicator 118' and a safety monitor 112' .

The health indicator 118 of the first controller 200 is coupled to provide a time varying control signal (e.g. a square wave) to the validator 120' of the second controller 200' . The health indicator 118' of the second controller 200' is coupled to provide a time varying control signal (e.g. a square wave) to the validator 120 of the first controller 200' .

Thus, in operation, the first controller 200 only provides power to an electric motor in the event that the validator 120 receives a valid control signal from the health indicator 118' of the second controller. Likewise the second controller 200' only provides power to an electric motor in the event that the validator 120' receives a valid control signal from the health indicator 118 of the first controller 200.

In some examples a switch may be interposed between the health indicator 118 and the validator 120' of the either or both controllers 200, 200' . This may provide an additional safety cut off to enable the controllers to be shut down by an external signal. In one example vehicle a single controller 200 may be used, and the health indicator 118 may be looped back into the validator 120 of that same controller 200. In this example, if the safety monitor 112 detects a malfunction of the controller 200, the control signal produced by the health indicator 118 will be interrupted causing the validator 120 to inhibit operation. In these examples, the loop back between the validator and health indicator may be controlled by an external switch to enable the controller 200 to be shut down by decoupling the health indicator 118 from the validator 120.

Throughout the disclosure reference has been made to signal polarities (TRUE, FALSE, 'high', 'low' ) and to inverting and non-inverting inputs of comparators. As will be appreciated in the context of the present disclosure, these are merely examples and the opposite polarities may also be used with appropriate modification of the arrangement of the comparators. The signal levels indicating logical TRUE and logical FALSE may be any selected levels and should not be taken to imply specific signal polarities or magnitudes.

Claims

Claims :

1. An electric motor control apparatus configured to provide an output for selectively enabling or inhibiting operation of an electric motor, the apparatus comprising

a signal coupling for receiving a time varying signal; a validator comprising a comparer configured to: compare a filtered version of the time varying signal with at least two reference values to provide an indication of at least one of: the duty cycle, and the frequency content of the time varying signal;

wherein the validator is configured to enable or inhibit the operation of the electric motor based on said indication.

2. The apparatus of claim 1 in which the validator is configured to determine the frequency content based on the amplitude of the time varying signal in a selected frequency band.

3. The apparatus of claim 2 comprising a filter arranged to filter the time varying signal.

4. The apparatus of claim 3 in which the filter comprises a low pass filter.

5. The apparatus of claim 4 in which the validator is configured to enable or inhibit operation of the electric motor based on the amplitude of the filtered signal being within a selected range.

6. The apparatus of any preceding claim in which the comparer comprises at least one window comparer arranged to indicate whether the signal is greater than a minimum threshold and less than a maximum threshold.

7. The apparatus of claim 6 in which the at least one window comparer comprises a first window comparer, and a second window comparer having threshold levels different from the first window comparer and in which the validator is configured to enable or inhibit operation of the electric motor based on the output of both the first window comparer and the second window comparer.

8. The apparatus of claim 6 or 7 in which the output of one of the at least one window comparers is coupled to a pulse detector and the validator is configured to enable or inhibit operation of the electric motor based on the output of the pulse detector.

9. A method of controlling an electric motor comprising:

receiving a time varying control signal;

filtering the control signal to provide a filtered signal; comparing the filtered signal with at least two reference valuesto provide an indication of at least one of: the amplitude of the filtered signal, and the duty cycle of the control signal and providing an output for enabling or inhibiting operation of the electric motor basd on the indication .

10. The method of claim 9 in which filtering comprises low pass filtering.

11. The method of claim 9 or 10 in which being based on the amplitude of filtered signal comprises at least one of: a minimum negative filtered signal being within a first selected range; and a maximum positive filtered signal being within a second selected range.

12. The method of any of claims 9 to 11 in which being based on the duty cycle of the time varying control signal comprises being based on duty cycle of the filtered signal.

13. An apparatus configured to perform the method of any of claims 9 to 12.

14. An electric motor controller comprising an inverter for providing power to an electric motor, and an apparatus according to any of claims 1 to 8, or 13, arranged to inhibit or enable operation of the electric motor by selectively enabling the provision of power from the inverter to the electric motor.

15. An electric motor comprising an apparatus according to any of claims 1 to 8, or 13, arranged to selectively inhibit or enable operation of the electric motor.

16. An apparatus comprising the electric motor controller of claim 14, and/or the electric motor of claim 15, and a control signal provider configured to provide a time varying control signal having a selected frequency and duty cycle.

17. An electric vehicle comprising the electric motor controller of claim 14, or the electric motor of claim 15, or the apparatus of claim 16.