Thermoelectric system and method

Abstract

A method for detecting a fault in a thermoelectric device (102), the method comprising: applying a voltage across the thermoelectric device (102); ceasing to apply the voltage to the thermoelectric device (102) after a predefined period of time; measuring a Seebeck voltage V.sub.s across the thermoelectric device (102); comparing V.sub.s to a first threshold voltage V.sub.T; and creating a record of a fault if V.sub.s is below V.sub.T.

Claims

1. A method for detecting a fault in a thermoelectric device, the method comprising: applying a voltage across the thermoelectric device for a predetermined period of time; measuring a Seebeck voltage V.sub.S across the thermoelectric device; comparing V.sub.S to a first threshold voltage V.sub.T; and creating a record of a fault if V.sub.S is below V.sub.T; using a temperature measuring system, wherein a length of the predefined period of time depends upon a measurement of the temperature measuring system; driving the thermoelectric device with a pulse width modulation signal, the pulse width modulation signal comprising on periods and off periods, a length of the on periods and the off periods being determined by a temperature determined with the temperature measuring system; and measuring V.sub.S during an off period of the pulse width modulation signal.

2. The method of claim 1, wherein creating a record of a fault if V.sub.S is below V.sub.T comprises incrementing a first counter.

3. The method of claim 2, further comprising decrementing the first counter if V.sub.S is above V.sub.T.

4. The method of claim 2, wherein the method further comprises: comparing the first counter to a threshold counter number; and causing the thermoelectric device to enter a fault mode if the first counter is greater than the threshold counter number.

5. The method of claim 1, the method further comprising: increasing the length of the off periods in the pulse width modulation signal such that they are greater than a threshold length of time before measuring V.sub.S.

6. The method of claim 1, wherein the thermoelectric device comprises a first side and a second side, the method further comprising: measuring T.sub.1, a temperature of the first side of the thermoelectric device, using the temperature measuring system; determining T, a difference in temperature between the first side and the second side of the thermoelectric device, from the Seebeck voltage; calculating T.sub.2, where T.sub.2=T.sub.1+T; comparing T.sub.2 to a threshold temperature T.sub.T; and creating a record of a fault if T.sub.2 is above T.sub.T.

7. A method for monitoring a thermoelectric device, the method comprising: repeatedly performing a method for detecting a fault in a thermoelectric device according to claim 1.

8. A vehicle comprising a thermoelectric device and a control unit, the control unit being arranged to carry out the method of claim 1.

9. A seat for use in a vehicle, the seat comprising a thermoelectric device and a control unit, the control unit being arranged to carry out the method of claim 1.

10. A method for detecting a fault in a thermoelectric device, wherein the thermoelectric device comprises a first side and a second side, the method comprising: driving the thermoelectric device with a pulse width modulation signal, the pulse width modulation signal comprising on periods and off periods, a length of the on periods and the off periods being determined by a temperature determined with a temperature measuring system; measuring a Seebeck voltage V.sub.S across the thermoelectric device during an off period of the pulse width modulation signal, measuring T.sub.1, a temperature of the first side of the thermoelectric device, using the temperature measuring system; determining T, a difference in temperature between the first side and the second side of the thermoelectric device, from the Seebeck voltage; calculating T.sub.2, where T.sub.2=T.sub.1+T; comparing T.sub.2 to a threshold temperature T.sub.T; and creating a record of a fault if T.sub.2 is above T.sub.T.

11. A thermoelectric system, comprising: a thermoelectric device; temperature measuring system; and a control unit, the control unit being arranged to: apply a voltage across the thermoelectric device for a predefined period of time; measure a Seebeck voltage V.sub.S across the thermoelectric device; compare V.sub.S to a first threshold voltage V.sub.T; and create a record of a fault if V.sub.S is below V.sub.T; wherein a length of the predefined period of time depends upon a measurement of the temperature measuring system; and wherein creating a record of a fault if V.sub.S is below V.sub.T comprises incrementing a first counter, wherein the control unit is further arranged to: compare the first counter to a threshold counter number; and cause the thermoelectric device to enter a fault mode if the first counter is greater than the threshold counter number.

12. The thermoelectric system of claim 11, wherein the control unit is further arranged to decrement the first counter if V.sub.S is above V.sub.T.

13. The thermoelectric system of claim 11, wherein the control unit is arranged to: drive the thermoelectric device with a pulse width modulation signal, the pulse width modulation signal comprising on periods and off periods, a length of the on periods and the off periods being determined by a temperature determined with the temperature measuring system; measure V.sub.S during an off period of the pulse width modulation signal; and increase the length of the off periods in the pulse width modulation signal such that they are greater than a threshold length of time before measuring V.sub.S.

14. The thermoelectric system of claim 11, wherein the thermoelectric system further comprises a temperature measuring system and the thermoelectric device comprises a first side and a second side, where the control unit is further arranged to: measure T.sub.1, a temperature of the first side of the thermoelectric device, using the temperature measuring system; determine T, a difference in temperature between the first side and the second side of the thermoelectric device, from the Seebeck voltage; calculate T.sub.2, where T.sub.2=T.sub.1+T; compare T.sub.2 to a threshold temperature T.sub.T; and create a record of a fault if T.sub.2 is above T.sub.T.

15. A vehicle comprising a thermoelectric system according to claim 11.

16. A seat for use in a vehicle, the seat comprising a thermoelectric system according to claim 11.

17. A thermoelectric system, comprising: a thermoelectric device; a temperature measuring system comprising a first side and a second side; and a control unit, the control unit being arranged to: drive the thermoelectric device with a pulse width modulation signal, the pulse width modulation signal comprising on periods and off periods, a length of the on periods and the off periods being determined by a temperature determined with the temperature measuring system; measure a Seebeck voltage V.sub.S across the thermoelectric device during an off period of the pulse width modulation signal, measure T.sub.1, a temperature of the first side of the thermoelectric device, using the temperature measuring system; determine T, a difference in temperature between the first side and the second side of the thermoelectric device, from V.sub.S; calculate T.sub.2, where T.sub.2=T.sub.1+T; compare T.sub.2 to a threshold temperature T.sub.T; and create a record of a fault if T.sub.2 is above T.sub.T.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are further described hereinafter, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a block diagram of a thermoelectric system;

(3) FIG. 2 is a flow chart showing a first method for detecting a fault; and

(4) FIG. 3 is a flow chart showing a second method for detecting a fault.

(5) FIG. 4 illustrates a vehicle with a thermoelectric system according to embodiments of the present invention.

DETAILED DESCRIPTION

(6) FIG. 1 shows a thermoelectric system 101 according to an embodiment of the invention. The thermoelectric system 101 is intended for use in the seat of a vehicle, in order to provide seat heating or cooling, and so provide more comfort for a user. The thermoelectric system 101 comprises a thermoelectric device 102, which comprises a first side and a second side.

(7) The thermoelectric device 102 is controlled by a control unit 103, which can apply a voltage across the thermoelectric device 102. In use, when the control unit puts a voltage across the thermoelectric device 102, this creates a heat flux between the first side and the second side of the thermoelectric device 102 due to the Peltier effect. Depending upon the polarity of the voltage, the heat flow can be from the first side to the second side, or from the second side to the first side.

(8) Therefore, if a voltage is applied to the thermoelectric device 102 such that the heat flows from the first side to the second side, then the temperature of the first side of the thermoelectric device 102 will drop. Since the first side of the thermoelectric device faces into the seat, this has the effect of cooling the seat. Conversely, if the voltage across the thermoelectric device 102 is reversed, then the heat flows from the second side to the first side, increasing the temperature of the first side of the thermoelectric device 102. The thermoelectric device 102 will then tend to warm the seat in the vehicle.

(9) As such, the thermoelectric system 101 can be used to heat or cool the vehicle seat as required.

(10) The thermoelectric system 101 further comprises a fan 104, which directs an air flow across the second side of the thermoelectric device 102, which helps to maintain the second side of the thermoelectric device 102 at a temperature close to the air temperature in the vehicle.

(11) The thermoelectric system 101 also comprises a first temperature sensor 105 and a second temperature sensor 106. The first temperature sensor 105 is attached to the first side of the thermoelectric device 102, such that the first temperature sensor 105 can measure T.sub.1, the temperature of the first side of the thermoelectric device 102, and report T.sub.1 to the control unit 103. The second temperature sensor 106 is located within the seat, and measures T.sub.A, the ambient temperature in the seat.

(12) The control unit 103 is also provided with a dashboard display output 107 and a control input 108. The control unit 103 provides signals to a display on the dashboard of the vehicle through the dash board display output 107. As such the control unit can notify the user of the temperature of the seat, and notify the user if there is a fault, through the dashboard display.

(13) In use, the control unit 103 receives a signal indicating a target temperature through the control input 108. The target temperature may be set directly by a user, or it may be set by an automatic temperature control system in response to settings entered by the user. The target temperature may also depend upon the status of the vehicle. For example, if the vehicle is a convertible, the target temperature may automatically increase when the vehicle's roof is down, in order to keep the user warm.

(14) Once the control unit 103 has a target temperature, it compares the target temperature with the ambient temperature T.sub.A. If T.sub.A is below the target temperature, then the control unit 103 operates the thermoelectric device 102 so as to increase the temperature of the seat. If T.sub.A is above the target temperature, then the control unit 103 operates the thermoelectric device 102 so as to decrease the temperature of the seat.

(15) If T.sub.A is much larger or much smaller than the target temperature, then the control unit is configured to apply a continuous voltage to the thermoelectric device 102. However, if the temperature of the seat needs only a slight adjustment, then the control unit is configure to provide a voltage to the thermoelectric device 102 according to a Pulse Width Modulation (PWM) signal. The PWM signal comprises on periods, in which a voltage is applied to the thermoelectric device 102, and off periods, in which no voltage is applied to the thermoelectric device. The durations of the on periods and the off periods are determined by the change in temperature required in the seat. If a large change is required then the on periods will be longer and the off periods shorter. For a smaller change, the off periods are longer and the on periods are shorter. As such, as T.sub.A approaches the target temperature, the control unit 103 is configured to vary the PWM signal so as to decrease the on periods and increase the off periods. Once T.sub.A reaches the target temperature, the control unit 103 then provides a PWM signal which is suitable for maintaining T.sub.A at the target temperature. The proportion of the on periods to the off periods is sometimes referred to as the duty cycle of the PWM signal and may be expressed as a percentage, e.g. 100% being fully on. Lastly, the thermoelectric system 101 comprises a voltmeter 109. The voltmeter is attached to the thermoelectric device 102 such that the voltmeter can measure a voltage across the thermoelectric device 102 and report the measured voltage to the control unit 103.

(16) FIG. 2 is a flow chart showing a method for detecting a fault according to one embodiment of the invention, which may be carried out by the control unit 103. If the thermoelectric device develops a fault, then the fault may prevent the thermoelectric device from generating a heat flux when a voltage is applied. As such, the thermoelectric device may be unable to heat or cool the seat. The method shown in FIG. 2 tests for such a fault while the thermoelectric system 101 is in a heating mode, in which the thermoelectric device 102 is used to heat the seat.

(17) At S1 the control unit 103 checks to see if the thermoelectric system 101 is turned on, and operating. If it is not, then the control unit 103 returns to a standby mode at S2. The control unit 103 is configured to perform this check periodically, such that once the system is turned on the control unit 103 will be able to carry out the rest of the method.

(18) If the thermoelectric system 101 is turned on, then the control unit 103 proceeds to S3, in which it checks the PWM signal being supplied to the thermoelectric device 102, which is referred to as the TED PWM in the diagram. In particular, the control unit checks to see if the PWM signal is on less than y % of the time. The threshold figure y can be configured according to the needs of the system, but a typical value is 80%. In order to perform a test according to the method, the PWM must comprise a minimum off time, so if the PWM is on more than y % of the time the control unit 103 proceeds to S4, in which it reduces the on periods of the PWM signal to less than y %.

(19) In an alternative method, a control unit according to the invention may check the duration of the off periods according to the PWM signal, and increase the duration of the off periods if they are below a threshold. The duration of the off periods can be increased without decreasing the percentage on time by also increasing the duration of the on periods.

(20) In step S5, the control unit increments a counter N.sub.OC1 to indicate that the PWM signal has been changed, before ultimately returning to S1 and proceeding with the test once more. The thermoelectric system 101 may have been damaged in such a way so as to prevent the percentage on time from staying below y %. As such, at S6 the control unit 103 compares N.sub.OC1 with a threshold A. If N.sub.OC1 reaches A, then the control unit 103 logs a fault. Otherwise the method continues.

(21) Once the percentage on time is below y, the control unit 103 proceeds to S7. In step S7 and S8 the control unit 103 decrements N.sub.OC1 by one.

(22) At S9 the control unit 103 measures V.sub.S, the Seebeck voltage across the thermoelectric device 102, during an off period in the PWM signal. At S10 the control unit 103 compares V.sub.S to a threshold voltage z.

(23) If V.sub.S is below z, then the control unit 103 increments a counter N.sub.OC2 at S11. If V.sub.S is above z, then the control unit 103 checks that N.sub.OC2 is equal to zero at S12. If N.sub.OC2 is not equal to zero then the control unit 103 decrements the counter N.sub.OC2 at S13.

(24) If the thermoelectric device 102 is functioning properly and in a heating mode, then the first side of the thermoelectric device 102 should heat up during each on period, while the second side of the thermoelectric device 102 cools. As such, during the off period heat will tend to flow back from the first side to the second side, creating a Seebeck voltage V.sub.S across the thermoelectric device 102. If the thermoelectric system is broken, then the difference in temperature between the first side and the second side may be reduced or even non-existent. As such, V.sub.S will drop.

(25) If N.sub.OC2 is incremented, then the control unit 103 compares N.sub.OC2 to a threshold B at S14. If N.sub.OC2 reaches B, then the control unit 103 logs a fault. If no fault is logged, then the control unit 103 returns the PWM signal to pre-test conditions at S15 or S16 and returns to S1. The control unit 103 may be configured to wait a predefined period of time before returning to S1, in order that the testing does not interfere with the ordinary operation of the thermoelectric system 101.

(26) The control unit 103 can be configured such that the threshold voltage z depends upon the percentage on time of the PWM signal. Therefore the control unit 103 can take account of a smaller temperature change cause by a reduced on time.

(27) Although the method above has been described as being used when the thermoelectric system 101 is in a heating mode, the same method can be used when the thermoelectric system is in a cooling mode, with an appropriate adjustment to value of z, since the voltage polarity will be reversed.

(28) FIG. 3 is a flow chart showing a method for detecting a fault according to another embodiment of the invention, which may be carried out by the control unit 103. If the thermoelectric device develops a fault, then the fault may cause the second side of the thermoelectric device to become too hot. However it is not straightforward to measure the temperature of the second side of the thermoelectric device 102, since a sensor in this location would impede the air flow to the thermoelectric device 102 and could therefore cause overheating in and of itself. The method shown in FIG. 3 tests for overheating without the need for a further temperature sensor.

(29) At S101 the control unit 103 checks to see if the thermoelectric system 101 is turned on, and operating. If it is not, then the control unit 103 returns to a standby mode at S102. The control unit 103 is configured to perform this check periodically, such that once the system is turned on the control unit 103 will be able to carry out the rest of the method.

(30) If the thermoelectric system 101 is turned on, then the control unit 103 proceeds to S103, in which it checks T.sub.1, the temperature of the first side of the thermoelectric device 102, which is measured by the first temperature sensor.

(31) In S104, S105, S106 and S107 the control unit 103 checks which mode the thermoelectric system 101 is in. If the system is in a cooling mode, in which it reduces the temperature of the seat, then the method continues. Otherwise another appropriate control strategy is chosen.

(32) At S108 the control unit 103 checks the PWM signal being supplied to the thermoelectric device 102. In particular, the control unit checks to see if the PWM signal is on less than y % of the time. Again, the threshold figure y can be configured according to the needs of the system, but a typical value is 80%. In order to perform a test according to the method, the PWM must comprise a minimum off time, so if the PWM is on more than y % of the time the control unit 103 proceeds to S109, in which it reduces the on periods of the PWM signal to less than y %.

(33) As in the previous method, in S110 and S111 the control unit 103 increments a counter N.sub.OT1 and compares that counter to a threshold A, such that if the thermoelectric system is damaged such that the PWM on time cannot be reduced to less than y, a fault is logged. Once the percentage on time is below y, the control unit 103 proceeds to S112. In S112 and S113 the control unit 103 decrements N.sub.OT1 by one.

(34) At S114 the control unit 103 measures V.sub.S, the Seebeck voltage across the thermoelectric device 102, during an off period in the PWM signal. From V.sub.S, the control unit can calculate T, the difference in temperature between the first side and the second side of the thermoelectric device 102.

(35) At S115 the control unit 103 sums T.sub.1 and T to get T.sub.2, the temperature of the second side of the thermoelectric device 102.

(36) At S116 the control unit 103 compares T.sub.2 to a threshold temperature T.sub.3.

(37) If T.sub.2 is below T.sub.3, then the control unit proceeds to S117, in which the control unit 103 runs further diagnostics according to the method shown in FIG. 2 to check for an open circuit (OC) fault before returning to S101. Advantageously, the OC fault check can be carried out using the same measurement of V.sub.S as the use in S114 of FIG. 3, reducing the amount of disruption to the operation of the thermoelectric system.

(38) If T.sub.2 is above T.sub.3, this indicates that the second side of the thermoelectric device 102 is too hot, which may damage the thermoelectric device 102 or some other component. The control device 103 therefore increments a counter N.sub.OT2 at S118 and enters a control strategy to bring T.sub.2 within acceptable limits at S119. The control strategy may comprise increasing the speed of the fan 104, changing the PWM signal, or temporarily shutting down the thermoelectric device 102. Once the control strategy has been implemented, the control unit 103 recalculates T.sub.2 and checks to see if T.sub.2 is now below T.sub.3 at S115 and S116 again.

(39) Each time N.sub.OT2 is incremented, the control unit 103 checks to see if N.sub.OT2 has reached a threshold value x at S120. When N.sub.OT2 reaches x, a fault is logged.

(40) The use of counters in the above methods helps to ensure that a temporary fluctuation in temperature, for example due to a change in the ambient temperatures around the vehicle, is not misdiagnosed as a fault.

(41) When a fault is logged, the control unit 103 may be configured to display the fault to the user via the dashboard display output 107. Alternatively the fault may simply be stored for later retrieval by an engineer. Also, when a fault is logged the control unit 103 may be configured to cease or reduce the operation of the thermoelectric system, in order to reduce the potential for further damage.

(42) In the embodiments above the thermoelectric system 101 is described for use in a vehicle, and in particular for use in a seat. However it may have various other uses. FIG. 4 shows a car 201 which comprises a seat 202 and a steering wheel 203. Both the seat 202 and the steering wheel 203 comprise a thermoelectric system 101 according to the invention. Hence both the seat and the steering wheel can be heated or cooled according to the user's needs. A thermoelectric system 101 can also be used in motorbike handles and seats, and in under floor heating for vehicles such as car 201. A thermoelectric system 101 could also be used in applications other than in vehicles, wherever heating or cooling is desirable.

(43) Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(44) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.