WO2017158373A1

WO2017158373A1 - Temperature measurement

Info

Publication number: WO2017158373A1
Application number: PCT/GB2017/050745
Authority: WO
Inventors: Peter John Miller
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2016-03-18
Filing date: 2017-03-17
Publication date: 2017-09-21
Also published as: GB2541482A; GB201700144D0; GB2547978A; GB201700143D0; GB2548452B; GB2547978B; GB2541482B; GB2548452A; GB201604660D0

Abstract

A temperature measurement system is disclosed. The system comprises a current source; a first multiplexer having an input channel, connected to the current source, and a plurality of output channels;a second multiplexer having an output channel, connected to a processor, and a plurality of input channels; and a plurality of temperature sensors. Each sensor is connected to an output channel of the first multiplexer so that the sensor can receive current from the current source via the first multiplexer and to an input channel of the second multiplexer so that a voltage across the sensor can be monitored by the processor via the second multiplexer. The system further comprises two fixed value resistors. Each fixed value resistor is connected to an output channel of the first multiplexer so that the fixed value resistor can receive current from the current source via the first multiplexer and to an input channel of the second multiplexer so that a voltage across the fixed value resistor can be measured by the processor via the second multiplexer.

Description

TEMPERATURE MEASUREMENT

Field of the Invention

The present invention relates to temperature measurement systems and methods for measuring a plurality of temperatures. In particular, but not exclusively, the present invention relates to temperature measurement systems for use in battery management systems and methods for measuring a plurality of temperatures in battery packs.

Background Many applications require the measurement of temperatures. Often the temperature measurement is for a critical, for example a safety critical, function and it is therefore important to know that the correct measurement has been taken and that the measurement is accurate. One such application is the measurement of cell temperature within battery packs. The cell temperature in a battery pack would typically be considered as being safety relevant.

Known methods of measuring temperatures include temperature dependent resistors, such as thermistors or resistance temperature detectors (RTD), and semi-conductor junctions, such as diodes and transistors. Thermistors can have a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). In both temperature dependent resistors and semi- conductor junctions, if a current is passed through the device it will produce a voltage across the device that depends upon the temperature of the device. In the case of a semiconductor it is also known that by using 2 or 3 different currents a more accurate temperature may be calculated, which is largely independent of the actual sensor used. When multiple temperatures need measuring, it is known to multiplex the outputs of the sensors into a small number of measurement channels. A typical system would involve a current source that is passed through a current multiplexer to deliver a source current to multiple temperature detectors such as thermistors or RTDs. The output voltages of the temperature detectors are passed through a voltage multiplexer to create a single signal that is passed to an analogue to digital converter, which creates a digital signal to pass to a processor. The multiplexers have a fixed number of channels available and a single temperature detector will typically be connected to each channel. While such systems work well there is no built in self-test capability so the system cannot confirm that the correct temperature, or indeed any temperature, is being measured. Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide an improved system and method for measuring temperature. Summary of Invention

According to a first aspect of the invention, there is provided a temperature measurement system comprising: a current source; a first multiplexer having an input channel, connected to the current source, and a plurality of output channels; a second multiplexer having an output channel, connected to a processor, and a plurality of input channels; and a plurality of temperature sensors, each sensor being connected to an output channel of the first multiplexer so that the sensor can receive current from the current source via the first multiplexer and to an input channel of the second multiplexer so that a voltage across the sensor can be monitored by the processor via the second multiplexer; wherein the system further comprises two fixed value resistors, each fixed value resistor being connected to an output channel of the first multiplexer so that the fixed value resistor can receive current from the current source via the first multiplexer and to an input channel of the second multiplexer so that a voltage across the fixed value resistor can be measured by the processor via the second multiplexer. The system may comprise an analogue to digital converter to convert measured voltages into digital signals for processing by the processor.

Thus the temperature measurement system of the present invention includes two fixed value resistors connected to the multiplexers in the same way as the temperature sensors. Because the resistance of, and hence the voltage dropped across, the fixed value resistors does not change with temperature, the processor can check that the temperature measurement system is operating correctly by confirming that the expected values are being read for the two fixed value resistors. Examples of faults that can be detected include a multiplexer address line being stuck at a value (0 or 1), faults in the current source, faults in any analogue to digital converter included in the circuit (for example between the second multiplexer and the processor) and faults within the multiplexers. The temperature sensors may for example be thermistors, resistance temperature detectors

(RTDs), diodes or transistors. The system may comprise an analogue to digital converter to convert measured voltages into digital signals for processing by the processor. One analogue to digital converter may be provided per input channel of the second multiplexer but preferably a single analogue to digital converter is provided between the output of the second multiplexer and the processor. The processor may, for example, be a microprocessor or an application specific integrated circuit (ASIC).

Preferably the two fixed value resistors are connected to output channels that are the complement of each other on the first multiplexer and input channels that are the complement of each other on the second multiplexer. For example, if the multiplexer has four address lines, one of the temperature sensors may be connected to channel 1010 on each multiplexer and the other temperature sensor to 0101 on each multiplexer. Of course, other complimentary channel pairs, such as 1000 and 0001 , could be used and different complementary pairs could be used on each multiplexer. By connecting the temperature sensors to channels that are the complement of each other, errors in any of the address lines will be detected.

Preferably the two fixed value resistors have different values to each other. Preferably the values are selected to give voltages either side (i.e. one above and one below) of the range of voltages generated by the temperature sensors over the temperature range of interest. For example, one of the fixed value resistors may have a value such that the voltage across the fixed value resistor is not less than 75%, preferably 90%, and more preferably 99%, of the minimum voltage generated by the temperature sensors over the temperature range of interest. The voltage across that fixed value resistor is preferably also not more than 105%, preferably 100%, and more preferably

99.95%, of the minimum voltage generated by the temperature sensors over the temperature range of interest. The other of the fixed value resistors may have a value such that the voltage across the fixed value resistor is not more than 125%, preferably 1 10%, and more preferably 101 %, of the maximum voltage generated by the temperature sensors over the temperature range of interest. The voltage across that fixed value resistor is preferably also not less than 95%, preferably 100%, and more preferably 100.05%, of the maximum voltage generated by the temperature sensors over the temperature range of interest. In that way the expected measurement range is bracketed by the two fixed value resistors. Preferably the processor is configured to use measured voltages across the two fixed value resistors in calculating the temperature measured by a temperature sensor. For example the processor may be configured to use the measured voltages across the two fixed value resistors to account for imperfections in the current source, multiplexers or any analogue to digital converter in the system.

Preferably the processor is configured to interpolate between the measured voltages across the two fixed value resistors to calculate the resistance of the temperature sensor based on the known resistance of the fixed value resistors. The interpolation may be carried out directly in a single step or indirectly, for example by using the measured voltages across the two fixed value resistors to calculate coefficients in a linear relationship between voltage and resistance and subsequently using the calculated coefficients to calculate the resistance of the temperature sensor. It will be appreciated that such a calculation may be applied when, for example, a thermistor or RTD is used as the temperature sensor. The processor may be configured to measure voltages across a temperature sensor and the two fixed resistors at different currents and to calculate the temperature based on the currents and the measured voltages. For example, the voltages may be measured at two different currents, at three different currents or even at more different currents. It may be that the processor is configured to measure voltages across a temperature sensor and the two fixed resistors at two different currents and to calculate the temperature based on the ratio of the two currents and the difference in the measured voltages across the temperature sensor at the two different currents. Preferably the processor is configured to determine the ratio of the currents based on the measured voltages across the two fixed value resistors at the two different currents and to use the ratio thus calculated in the calculation of the temperature. It may be that the processor is configured to measure voltages across a temperature sensor and the two fixed resistors at three different currents and to calculate the temperature based on ratios of the currents and the difference in the measured voltages across the temperature sensor at the different currents. Such calculations may be applied when, for example, a transistor or a diode is used as the temperature sensor. For example, the temperature sensor may be a semiconductor temperature sensor and the temperature may be calculated based on the base-emitter voltage of the semiconductor temperature sensor at the different currents. Using two or three different currents may be advantageous in that the measured voltages across the fixed value resistors are used to account for any inaccuracies in the current supplied to the temperature sensors, for example due to inaccuracy in the current source, resistance in the circuit to the sensor, or current leakages in the multiplexers.

The system of the invention may be particularly advantageous for use in a battery management system. A battery pack typically contains a number of cells arranged in series and the cell temperature of each cell may be monitored for safety reasons. That monitoring is typically carried out by a battery management system that forms part of the battery pack. A particularly advantageous arrangement includes 14 lithium ion cells arranged in series, giving a cell pack voltage of around 50V. If the temperature of each cell is measured by a temperature sensor in a system of the invention comprising multiplexers with 4 address lines, each multiplexer has 16 channels, which match with the 14 temperature sensors and 2 fixed value resistors. In that way the system makes efficient use of the multiplexers to provide robust and accurate temperature measurement of the cell temperatures.

Thus according to a second aspect of the invention there is provided a battery management system comprising a temperature measurement system according to the first aspect of the invention.

According to a third aspect of the invention there is provided a method of measuring a plurality of temperatures, the method comprising supplying current from a current source, via a first multiplexer, to a plurality of temperature sensors; and measuring, via a second multiplexer, a voltage across each of the plurality of temperature sensors; wherein the method further comprises supplying current from the current source, via the first multiplexer, to two fixed value resistors; measuring, via the second multiplexer, a voltage dropped across each of the fixed value resistors; and determining the temperature at each of the temperature sensors based on the voltage measured across the temperature sensor and the voltages measured across the fixed value resistors.

Preferably the method comprises confirming that the voltages measured across the fixed value resistors fall within an expected range. In that way, the correct functioning of the measurement system may be confirmed. Preferably that confirmation is carried out prior to determining the temperature at each of the temperature sensors. For example, the method may include confirming that the voltages measured across the fixed value resistors fall within 10%, preferably 1 % of their expected values. If they do, the method may include using those voltages in the calculation of the temperatures at each of the temperature sensors. If they do not, the method may include entering an error state. In that way a highly deviant value is treated as indicating a fatal error condition (such as failure of the multiplexer), while a value that deviates by only a small amount may be attributed to correctable errors (such as small current leakages or small errors in the current supply) that can be accounted for by the measurement method.

Preferably the temperature at a temperature sensor is determined by relating, for example by interpolation, the voltage across the sensor to the voltages across the two fixed value resistors and thus determining the resistance of the sensor based on the known resistances of the two fixed value resistors. Such a method may be advantageous in that the calculation does not used the value of the current supplied by the current source and thus accounts for any errors due to differences between the theoretical and actual values of the current delivered from the current source.

Preferably the temperature at a temperature sensor is determined by supplying multiple, for example two or three, different currents from the current source and measuring the voltages across the sensor and the fixed value resistors at each of the different currents and determining the temperature based on, for example the difference in, the voltages across the sensor measured at the different currents and, for example ratios of, the different currents. Preferably ratios of the different currents are determined based on the voltages measured across the fixed value resistors at the different currents. That may provide the advantage that the calculation does not use the theoretical value of the supplied current and thus accounts for any errors between the theoretical current and the actual current supplied to the sensors and the fixed value resistors.

It will be appreciated that features described in relation to one aspect of the invention may be equally applicable in another aspect of the invention. For example, features described in relation to the temperature measurement system of the invention, may be equally applicable to the method of the invention, and vice versa. Some features may not be applicable to, and may be excluded from, particular aspects of the invention. Description of the Drawings

Embodiments of the present invention will now be described, by way of example, and not in any limitative sense, with reference to the accompanying drawings, of which: Figure 1 is schematic diagram of a temperature measurement system according to an embodiment of the invention.

Detailed Description

In figure 1 a temperature measurement system 1 comprises a current source 2. The current source 2 is connected to the input channel 5 of a first multiplexer M^ . The first multiplexer Iv^ has four output channels 6a, 6b, 6c and 6d with addresses 00, 01 , 10 and 1 1 respectively. Channels 6a and 6d are connected to temperature sensors T₂ and T ₇ while channels 6b and 6c are connected to fixed value resistors R₂ and F^ .The temperature sensors and T₂ are thermistors. The temperature sensors and T₂ and the fixed value resistors F^ and R₂ are also connected to the input channels 7a, 7b, 7c and 7d of a second multiplexer M₂. The multiplexer M₂ outputs the signals from the temperature sensors and T₂ and the fixed value resistors R^ and R₂ on output channel 8 to a processor 3 via an analogue to digital converter ADC. The processor 3 can control the address lines 4 of the multiplexers Iv^ and M₂. In use the current source generates a current which is supplied to the temperature sensors and T₂ and the fixed value resistors R^ and R₂ via the multiplexer M^. The processor 3 measures the voltages dropped across each of the temperature sensors and T₂ and the fixed value resistors RT and R₂ via the multiplexer M₂ and the analogue to digital converter ADC. The temperatures at the temperature sensors and T₂, which may for example be located on individual cells in a battery pack, are measured by measuring the resistance of the thermistors. That can be done by comparing the voltages measured across the fixed value resistors with the voltages measured across the thermistors. In a perfect system the resistance of the fixed value resistor R^ is related to the voltage \Λ measured across that resistor R^ and the current I produced by the current source by the relationship R^ = \Λ/Ι. However, in practice there may be errors in the current source or elsewhere in the system that may alter the value of the current delivered to the temperature sensors and T₂ and the fixed value resistors R^ and R₂ from the theoretical value I supplied by the source or may result in offset errors in the measured voltages. The system can account for such problems by treating the relationship between the resistance R^ and the voltage \Λ as a linear relationship with unknown coefficients m and c: R^ = ηΆΛ+α Because the temperature sensors and T₂ and the fixed value resistors R^ and R₂ are connected in the same way to the same current source 2 and processor 3 via the same multiplexers Iv^ and M₂, the coefficients m and c will be equal for the temperature sensors and T₂ and the fixed value resistors R^ and R₂. The values of the voltages \Λ and V₂ across the fixed value resistors R^ and R₂ respectively can thus be used to solve for the unknown coefficients m and c. Thus m =

If a voltage of V_Ti is then measured across temperature sensor the true resistance of temperature sensor can then be calculated as R_Ti = mV_T1+c. A similar relationship can also be used for the temperature sensor T₂. Note that the relationships for m, c and R_Ti do not use the theoretical value of the current supplied by the current source and thus are not susceptible to errors in that value.

Of course the temperature sensors could also be another type of temperature sensor, such as RTDs or diodes or transistors. In the case of, for example diodes or transistors, voltage measurements may be taken at multiple, for example two or three, different currents in order to calculate the temperature. For example, voltage measurements may be taken at two different currents I , and l₂. The temperature T_n at a diode or transistor can be approximated by the expression T_n = (V_n2-V_n )k/ln(l₂/li), where V_ni is the voltage across diode or transistor n when current I , is supplied, V_n2 is the voltage across diode or transistor n when current l₂ is supplied and k is a constant. In this case, the ratio l₂/ can be calculated from the voltages measured across the fixed value resistors R^ and R₂ when the current is I , and l₂. Assuming that the error is in the current supplied to the resistors and not in the measurement of the voltage across the resistors, if the voltage measured across resistor R, when current l_j is supplied is Vy, and Vy = ml_jR, + c, where m and c are unknown constants, then four equations can be created for the two resistors R^ and R₂ at the two currents I , and l₂ and solved to eliminate m, c, R^ and R₂ and give: /l₂ = (V -V₁₂)/(V₁₂- V₂₂). Thus the ratio l₂/ of the actual current supplied to the resistors can be determined from the measured voltages across the resistors R^ and R₂ without needing to use the theoretical current values supplied from the current source. Thus any errors in the current source or leakages in the multiplexers are accounted for in the calculation of the temperature. The embodiment of the invention thus uses the voltages measured across the fixed value resistors RT and R₂ to correct for correctable errors in the measurement circuit and thus to improve the accuracy of the temperature measurement. However, the system 1 also uses the voltages to check that the system is working as expected. For example, if the second multiplexer M₂ has failed such that the first bit is stuck at 0, then the voltages will always be measured on channels 7a or 7b, even if channels 7c or 7d were intended. The system 1 detects that the voltage obtained on, allegedly, channel 7c does not match within a predetermined tolerance (say 1 %) the value that would be expected for resistor R^ for the current supplied by the current source 2. Since the deviation exceeds the allowable tolerance the system 1 does not use the voltage to calculate temperatures from the temperature sensors and T₂, but instead enters an error mode and issues an alert. The tolerance is chosen so that anticipated normal variations in the performance of the current source or multiplexer current leakages (i.e. correctable errors) do not trigger the error mode and are instead compensated in the calculation method, while fatal errors, such as failure of a multiplexer or M₂ to correctly select the instructed channel will trigger the error mode. In that way a robust and accurate measurement system is provided. It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, the multiplexers could include more channels and thus more temperature sensors could be connected to the system along with the two fixed value resistors. In some embodiments a mixture of different types of temperature sensors could be used. The processor 3 could be the battery management system of a battery pack, for example for an electric vehicle, and the temperature sensors ΤΊ and T₂ (and others if present) could be mounted on individual cells within the battery pack. In a particularly preferred embodiment, the multiplexers have four address lines, giving 16 channels, 14 of which are connected to temperature sensors attached to lithium ion cells in series in a battery pack of around 50V and two of which are connected to fixed value resistors.

Claims

A temperature measurement system comprising:

a current source;

a first multiplexer having an input channel, connected to the current source, and a plurality of output channels;

a second multiplexer having an output channel, connected to a processor, and a plurality of input channels; and

a plurality of temperature sensors, each sensor being connected to an output channel of the first multiplexer so that the sensor can receive current from the current source via the first multiplexer and to an input channel of the second multiplexer so that a voltage across the sensor can be monitored by the processor via the second multiplexer;

wherein the system further comprises two fixed value resistors, each fixed value resistor being connected to an output channel of the first multiplexer so that the fixed value resistor can receive current from the current source via the first multiplexer and to an input channel of the second multiplexer so that a voltage across the fixed value resistor can be measured by the processor via the second multiplexer.

A temperature measurement system according to claim 1 , wherein the processor is configured to confirm that expected values are being read for the two fixed value resistors.

A temperature measurement system according to claim 1 or claim 2, wherein the two fixed value resistors are connected to output channels that are the complement of each other on the first multiplexer and input channels that are the complement of each other on the second multiplexer.

A temperature measurement system according to any preceding claim, wherein the two fixed value resistors have different values to each other.

A temperature measurement system according to claim 4 wherein the values are selected to give voltages either side of a range of voltages generated by the temperature sensors over a temperature range of interest.

A temperature measurement system according to any preceding claim, wherein the processor is configured to use measured voltages across the two fixed value resistors in calculating the temperature measured by a temperature sensor.

A temperature measurement system according to claim 6, wherein the processor is configured to interpolate between measured voltages across the two fixed value resistors to calculate the resistance of the temperature sensor based on the resistance of the fixed value resistors.

8. A temperature measurement system according to claim 6, wherein the processor is configured to measure voltages across a temperature sensor and the two fixed resistors at multiple different currents and to calculate the temperature based on the different currents and the measured voltages across the temperature sensor at the different currents.

9. A temperature measurement system according to claim 8, wherein the processor is configured to calculate the temperature using at least one ratio of the different currents and to determine the ratio based on the measured voltages across the two fixed value resistors at the different currents and to use the ratio thus determined in the calculation of the temperature.

10. A temperature measurement system according to any preceding claim, wherein the system is for use in a battery management system.

1 1 . A battery management system comprising a temperature measurement system according to any preceding claim.

12. A method of measuring a plurality of temperatures, the method comprising supplying current from a current source, via a first multiplexer, to a plurality of temperature sensors; and measuring, via a second multiplexer, a voltage across each of the plurality of temperature sensors; wherein the method further comprises supplying current from the current source, via the first multiplexer, to two fixed value resistors; measuring, via the second multiplexer, a voltage dropped across each of the fixed value resistors; and determining the temperature at each of the temperature sensors based on the voltage measured across the temperature sensor and the voltages measured across the fixed value resistors.

13. A method according to claim 12, wherein the method comprises confirming that the voltages measured across the fixed value resistors fall within an expected range.

14. A method according to claim 13, wherein if the voltages measured across the fixed value resistors fall within the expected range the method includes using the voltages measured across the fixed value resistors in the calculation of the temperatures at each of the temperature sensors.

15. A method according to claim 14, wherein the temperature at a temperature sensor is

determined by relating the voltage across the sensor to the voltages across the two fixed value resistors and thus determining the resistance of the sensor based on the resistances of the two fixed value resistors.

16. A method according to claim 14, wherein the temperature at a temperature sensor is determined by supplying multiple different currents from the current source and measuring the voltages across the sensor and the fixed value resistors at each of the different currents and determining the temperature based on the voltages across the sensor measured at the two different currents and the different currents.

17. A method according to claim 16, wherein the temperature is determined using at least one ratio of the different currents and the ratio of the different currents is determined based on the voltages measured across the fixed value resistors at the different currents.

18. A temperature measurement system substantially as herein described with reference to the accompanying drawings.

19. A method of measuring a plurality of temperatures substantially as herein described with

reference to the accompanying drawings.