WO2008090448A2

WO2008090448A2 - Rf temperature sensor

Info

Publication number: WO2008090448A2
Application number: PCT/IB2008/000146
Authority: WO
Inventors: Philippe Marquet
Original assignee: Melexis Nv
Priority date: 2007-01-24
Filing date: 2008-01-23
Publication date: 2008-07-31
Also published as: WO2008090448A3; GB0701385D0

Abstract

A wireless temperature measurement system comprises an RF transmitter 100, a temperature sensor 200, an RF receiver 300 and a processor unit 400. The transmitter 100 is operable to transmit an RF signal which is scanned over a range of frequencies. The receiver 300 is similarly operable to detect the transmitted signal and determine its magnitude at a particular detected frequency. The temperature sensor 200 comprises an antenna 201 and a resonant circuit 202 wherein the resonant circuit 202 is adapted such that its resonant frequency varies with temperature. By monitoring the proportion of the RF energy transmitted by the transmitter 100 absorbed by the temperature sensor 200 using receiver 300 over the scanned frequency range, the processor unit 400 is operable to determine the resonant frequency of the resonant circuit 202 and hence determine local temperature of the temperature sensor 200.

Description

RF Temperature Sensor

The present invention relates to an RF (radio frequency) temperature sensor and in particular to a wireless temperature measurement system utilizing such an RF temperature sensor.

In many circumstances, it is desirable to measure the temperature at a locality or inside of an object. On some of those occasions, it is desirable to do so without using a temperature sensor connected via wires to a power source or data transmission means. In such cases, it may be typical to use a conventional temperature sensor comprising a thermistor, a thermocouple, a Surface Acoustic Wave (SAW) resonator or an infrared sensor and an RF transceiver. In order to ensure such devices work and produce a reliable signal, it is however still necessary to provide a power source. In some cases this may be an energy scavenging means and a back up battery. At some times it will still however be desirable for a device to be operable to sense a local temperature which does not require any back up power and/or operate in extreme ambient conditions.

It is therefore an object of the present invention to provide a temperature sensor that alleviates or overcomes the above problems.

According to a first aspect of the present invention there is provided a temperature sensor comprising an antenna and a resonant circuit connected to the antenna, characterised in that the resonant frequency of the resonant circuit varies with temperature. By measuring the absorption of RF signal energy in the vicinity of the sensor as the RF signal is scanned across a range of frequencies, the resonant frequency of the resonant circuit can be determined and hence the temperature of the resonant circuit (and thus its locality) can be calculated. This thus provides a truly passive wireless temperature sensor.

The resonant frequency of the resonant circuit may be adapted to lie in the frequency range adapted to be received by the antenna. The antenna is preferably adapted to receive RF frequencies. Preferably, the resonant circuit is adapted to have a resonant frequency in the range 25-100 MHz when at a temperature within its normal operating range.

The temperature sensor may be packaged within a suitable protective housing.

According to a second aspect of the present invention there is provided a method of measuring temperature using a temperature sensor according to the first aspect of the present invention, the method comprising the steps of: scanning a transmitted RF signal over a range of frequencies; detecting the transmitted signal at a second location; determining the magnitude of the received signal with respect to frequency so as to determine the resonant frequency of the sensor and thereby determine the temperature of the temperature sensor.

The above method enables the use of the sensor of the first aspect of the present invention in the monitoring of temperature. As such, the method of the second aspect of the present invention may incorporate and or all features of the sensor of the first aspect of the present invention as desired or as appropriate. The method may involve the step of comparing the magnitude of demodulated transmitted signals and demodulated received signals to determine the resonant frequency of the resonant circuit. This determination of resonant frequency may be carried out by a suitable algorithm. The determination of temperature from resonant frequency may be carried out by a suitable algorithm or by means of a look up table.

According to a third aspect of the present invention there is provided a temperature measurement system comprising: one or more temperature sensors according to the first aspect of the present invention; one or more transmitter units operable to scan an output RF signal over a range of frequencies; and one or more receiver units for detecting the transmitted RF signals; a controller unit operable to determine the magnitude of the received signal with respect to frequency so as to determine the resonant frequency of the sensor and thereby determine the temperature of the temperature sensor.

The system of the third aspect of the present invention may incorporate any or all of the features described in relation to the sensor of the first aspect or the method of the second aspect of the invention as desired or as appropriate.

If more than one sensor is provided, each sensor may be provided with individual dedicated transmitter units and receiver units. Alternatively, two or more sensors may share a transmitter unit and/or a receiver unit. If sensors are sharing a transmitter, the resonant circuits of each sensor may be adapted to have different non- overlapping ranges of resonant frequencies in the normal operating temperature range. - A - The transmitter and receiver units each preferably comprise antenna means operably controlled by a switch. The transmitter and receiver units each preferably comprise a voltage controlled oscillator and a phase locked loop. The transmitter and receiver units each preferably comprise a demodulator for outputting a demodulated signal to the processing means. The transmitter unit may additionally comprise a modulator unit operable to control the signals output by the antenna. The modulator unit may be controlled by the processor unit.

The processing unit may be operable to compare the magnitude of demodulated signals received from both said transmitter unit and said receiver unit to determine the resonant frequency of the resonant circuit. The processing unit may use a suitable algorithm to carry out this calculation. The algorithm may also enable the processing unit to detect an error in the resonant circuit. The determination of temperature from resonant frequency may be carried out by a suitable algorithm or by means of a look up table.

The processing unit may be operable to output an indication indicative of the determined temperature to a display unit or to other external circuitry.

The system may be implemented to monitor the temperature of components or compartments in vehicles, components in other types of engine or industrial machine or the temperature of rooms, areas or compartments in buildings or containers.

In order that the invention can be more clearly understood it is now described further below with reference to the accompanying drawings the single figure of which is a schematic block diagram of a wireless temperature measurement system according to the third aspect of the present invention incorporating a temperature sensor according to the first aspect of the present invention.

Referring now to figure 1, a wireless temperature measurement system comprises an RF transmitter 100, a temperature sensor 200, an RF receiver 300 and a processor unit 400. The transmitter 100 is operable to transmit an RF signal which is scanned over a range of frequencies. The receiver 300 is similarly operable to detect the transmitted signal and determine its magnitude at each detected frequency. The temperature sensor 200 comprises an antenna 201 and a resonant circuit 202. The resonant circuit 202 is adapted such that its resonant frequency varies with temperature.

In use, the temperature sensor 200 absorbs some of the RF energy transmitted by the transmitter 100. Around the resonant frequency of the resonant circuit 202, the proportion of the RF energy transmitted by the transmitter 100 absorbed by the temperature sensor 200 increases. Accordingly, the proportion of the RF energy received by the receiver 300 drops. By monitoring this behaviour over the scanned frequency range, the processor unit 400 is operable to determine the resonant frequency of the resonant circuit 202. The processor unit 400 is then further able to calculate the local temperature of the temperature sensor 200 from this parameter.

The invention thus provides an unpowered wireless sensor operable to determine the temperature at a location remote from monitoring apparatus. Furthermore, the simplicity of the design of the sensor mitigates against failure of the system and makes it relatively cheap to manufacture. The particular example of a transmitter shown in figure 1 comprises an antenna 101 connected to a switch 102 controlling whether the antenna 101 is operable to transmit RF signals. The switch 102 is connected to a voltage controlled oscillator 103 which is fed by a modulator 106 and a phase locked loop 105. The voltage controlled oscillator 103 additionally outputs to a demodulator 104. The modulator 106 and phase locked loop 105 are driven in response to signals received from the processor 400. In turn the processor receives signals from the demodulator 103.

The particular example of a receiver 300 shown in figure 1 comprises an antenna 301 connected to a switch 302 controlling whether the antenna 301 is operable to receive RF signals. The switch 302 is connected to a voltage controlled oscillator 303 which is fed by a phase locked loop 305. The voltage controlled oscillator 303 additionally outputs to a demodulator 304. The phase locked loop 305 is driven in response to signals received from the processor 400. In turn the processor 400 receives signals from the demodulator 303.

The demodulated signals received by the processor from the transmitter 100 and the receiver 300 allow the processor to determine the proportion of the RF signal emitted by the transmitter 100 that is received by the receiver 300. By recording this value over the range of frequencies output by the transmitter 100, the resonant frequency of the resonant circuit 202 and hence the temperature local to the temperature sensor 200 can be determined. Typically this determination can be made via a direct calculation or by looking up a value in a stored look up table. A suitable look up table might be created during calibaration. The system may be operated at any suitable frequency range but is typically operated in the range 25-100MHz to provide optimum performance and avoid interference with FM radio. The processor may be operable to measure the signal strength directly and determine the likely resonant frequency or may utilise a suitable algorithm to perform this function. The use of such an algorithm can ensure suitable allowances are made for variations in performance of the transmitter 100, sensor 200 and receiver 300 over frequency and for variations in interference in a particular locality to be taken into account. Similarly, the algorithm could provide a means for determining that the sensor is significantly overheated. This might occur due to a misplacement or due to failure of some equipment.

The system may be operable to determine the temperature of more than one sensor 200. In such embodiments, each sensor 200 may be provided with a dedicated local transmitter 100 and receiver 300. Alternatively, a number of sensors 200 may be operable with a single transmitter 100 and receiver 300. In such embodiments, the frequency range of the resonant circuits 202 in each sensor 200 are adapted such that their range of resonant frequencies in normal operation do not overlap.

Such a multiple sensor system may be utilised in automotive applications monitoring the temperature of vehicle components or portions of the vehicle.

Alternatively, the system may be implemented in a building to monitor the temperature of areas within the building and thus monitor the operation of a heating system. It is of course to be understood that the invention is not to be restricted to the details of the above embodiments which have been described by way of example only.

Claims

1. A temperature sensor comprising an antenna and a resonant circuit connected to the antenna, characterised in that the resonant frequency of the resonant circuit varies with temperature.

2. A temperature sensor as claimed in claim 1 wherein the resonant frequency of the resonant circuit lies in the frequency range adapted to be received by the antenna.

3. A temperature sensor as claimed in claim 2 wherein the antenna is adapted to receive RF frequencies.

4. A temperature sensor as claimed in claim 3 wherein the resonant circuit is adapted to have a resonant frequency in the range 25-100 MHz when at a temperature within its normal operating range.

5. A temperature sensor as claimed in any preceding claim wherein the sensor is packaged within a protective housing.

6. A method of measuring temperature using a temperature sensor according to any one of claims 1 to 5, the method comprising the steps of: scanning a transmitted RF signal over a range of frequencies; detecting the transmitted signal at a second location; determining the magnitude of the received signal with respect to frequency so as to determine the resonant frequency of the sensor and thereby determine the temperature of the temperature sensor.

7. A method of measuring temperature as claimed in 6 wherein the method involves the step of comparing the magnitude of demodulated transmitted signals and demodulated received signals to determine the resonant frequency of the resonant circuit.

8. A method of measuring temperature as claimed in 7 wherein the determination of resonant frequency is carried out by a suitable algorithm.

9. A method of measuring temperature as claimed in any one of claims 6 to 8 wherein the determination of temperature from resonant frequency is carried out by a suitable algorithm.

10. A method of measuring temperature as claimed in any one of claims 6 to 8 wherein the determination of temperature from resonant frequency is carried out by means of a look up table.

1 1. A temperature measurement system comprising: one or more temperature sensors according to any one of claims 1 to 5; one or more transmitter units operable to scan an output RF signal over a range of frequencies; and one or more receiver units for detecting the transmitted RF signals; a controller unit operable to determine the magnitude of the received signal with respect to frequency so as to determine the resonant frequency of the sensor and thereby determine the temperature of the temperature sensor.

12. A temperature measurement system as claimed in 1 1 wherein each sensor is provided with individual dedicated transmitter units and receiver units.

13. A temperature measurement system as claimed in 1 1 wherein, two or more sensors share a transmitter unit and/or a receiver unit.

14. A temperature measurement system as claimed in 12 wherein the resonant circuits of each sensor may be adapted to have different non-overlapping ranges of resonant frequencies in the normal operating temperature range.

15. A temperature measurement system as claimed in any one of claims 1 1 to 14 wherein the transmitter and receiver units each comprise antenna means operably controlled by a switch.

16. A temperature measurement system as claimed in any one of claims 1 1 to 15 wherein the transmitter and receiver units each comprise a voltage controlled oscillator and a phase locked loop.

17. A temperature measurement system as claimed in any one of claims 1 1 to 16 wherein the transmitter and receiver units each comprise a demodulator for outputting a demodulated signal to the processing means.

18. A temperature measurement system as claimed in any one of claims 1 1 to 17 wherein the transmitter unit additionally comprises a modulator unit to control the signals output by the antenna.

19. A temperature measurement system as claimed in claim 18 wherein the modulator unit is controlled by the processor unit.

20. A temperature measurement system as claimed in any one of claims 17 to 19 wherein the processing unit is operable to compare the magnitude of demodulated signals received from both said transmitter unit and said receiver unit to determine the resonant frequency of the resonant circuit.

21. A temperature measurement system as claimed in claim 20 wherein the processing unit uses a suitable algorithm to carry out this calculation.

22. A temperature measurement system as claimed in claim 21 wherein the algorithm enables the processing unit to detect an error in the resonant circuit.

23. A temperature measurement system as claimed in any one of claims 1 1 to 22 wherein the determination of temperature from resonant frequency is carried out by a suitable algorithm.

24. A temperature measurement system as claimed in any one of claims 1 1 to 22 wherein the determination of temperature from resonant frequency is carried out by means of a look up table.

25. A temperature measurement system as claimed in any one of claims 1 1 to 24 wherein the processing unit is operable to output an indication indicative of the determined temperature to a display unit or to other external circuitry.

26. A temperature measurement system as claimed in any one of claims 1 1 to 25 wherein the system is implemented to monitor the temperature of components or compartments in vehicles, components in other types of engine or industrial machine or the temperature or rooms, areas or compartments in buildings or containers.

27. A temperature measurement system as claimed in any one of claims 1 1 to 25 wherein the system is implemented to monitor the temperature of components in engines or industrial machines.

28. A temperature measurement system as claimed in any one of claims 1 1 to 25 wherein the system is implemented to monitor the temperature of rooms, areas or compartments in buildings or containers.