GB2294775A

GB2294775A - Measuring apparatus

Info

Publication number: GB2294775A
Application number: GB9521886A
Authority: GB
Inventors: Brian John Cottrell; James Robert Arthur Rowland
Original assignee: Appliance Components Ltd
Current assignee: Schneider Electric Controls UK Ltd
Priority date: 1994-11-07
Filing date: 1995-10-25
Publication date: 1996-05-08
Anticipated expiration: 2015-10-25
Also published as: GB2294775B; GB9521886D0

Abstract

The measuring apparatus is to be used in a programmable thermostat to provide information as to the actual temperature. The thermostat is provided with a microprocessor which controls the measuring apparatus which incorporates a resistive four-arm bridge circuit having fixed precision resistors (R1, R2 and R3) in three arms and a thermistor (RTH) in the remaining arm. To commence an actual temperature measurement, the microprocessor puts a voltage pulse across supply terminals (O, V<+>) of the bridge circuit and switches on two counters. The first counter counts the time for the voltage (VC) to rise to the thermistor voltage (VTH), whilst the second counter counts the time for the voltage (VC) to rise to the reference voltage (VREF). A ratio is formed of the two counts to provide a measure of the voltage (VTH) across the thermistor (RTH). <IMAGE>

Description

MEASURING APPARATUS The present invention relates to measuring apparatus.

The present invention is particularly useful in a programmable thermostat where it can provide both a measure of the actual temperature and a determination of the power level of the battery power supply.

Programmable thermostats, particularly for central heating systems, are becoming increasingly common. Because there is a mass market for such devices it is required that they should be as accurate as possible, but it is also important to have a measurement system which does not require calibration either on production or on site.

According to a first aspect of the present invention there is provided measuring apparatus comprising a four-arm bridge circuit, a first arm of the bridge circuit having a first element arranged to provide a reference characteristic, and a second arm of the bridge circuit having a second element of which a characteristic is to be determined, said measuring apparatus further comprising a timing circuit coupled to power supply terminals of the bridge circuit and arranged, upon the application of power thereto, to produce a timing function having a characteristic which varies with time, and measurement means arranged to establish, in each case, a time at which the characteristic of the timing function substantially coincides with the characteristics provided by said first and second elements whereby the characteristic of said second element is determinable.

Any suitable characteristic of the first and second elements and/or of the timing function may be utilised, but in most circumstances the first element will provide a reference voltage and it will be required to determine the voltage of the second element. In general, therefore, said timing function will have a voltage characteristic which varies with time.

The measurement means establishes a time at which the characteristic of the timing function substantially coincides with the characteristic provided by each of the first and second elements. The time established, in each case, is effectively a ratio of the characteristic of the one element and of that of the timing function and is thus a dimensionless quantity or count independent of the actual time elapsed and representative of the value of the element characteristic.

Any suitable timing function may be used. In this respect it is important that the timing function have only a single characteristic which varies with time, and that this variation be monotonic, that is, in one direction only. This ensures that in establishing a count for the voltage or other characteristic of each of the first and second elements by use of the timing circuit it is ensured that that count is independent of the actual time elapsed as explained above. Furthermore, as the timing function is produced by the power supply at the power supply terminals of the bridge circuit, the counts established are similarly independent of any variations in the supply.

It is always useful when measuring a quantity to use a ratio of the determined quantity and a reference as this removes any variables of the circuit from the measurement.

The use of ratios is a fundamental of bridge circuits.

In a preferred embodiment of the present invention, the measurement means is arranged to form a ratio of the count established in each case for the first and second elements.

It will be appreciated that the count established for each element will be a function, arising out of the measuring circuit, related to the value of the characteristic thereof. However, in forming a ratio of the counts for the two elements, coefficients in the functions are eliminated such that the ratio so formed provides a measure of the characteristic, eg the voltage, of the second element. Furthermore, any variations or irregularities arising in the circuit itself are removed from the determination.

The measuring apparatus of the invention may be used to determine any parameter of an element which varies upon the application of power thereto.

In one embodiment, for use in a programmable thermostat which is battery powered, the battery voltage, or a quantity determined thereby, is arranged to be applied across the second element. In this case, therefore, the measuring apparatus determines the battery voltage and can be used, for example, in conjunction with an indicating circuit arranged to give a warning signal if the battery voltage level falls below a predetermined value.

Additionally, and/or alternatively, the second element may be, or may comprise, an element whose voltage varies with temperature. In this instance, the measuring apparatus, in providing a determination of the voltage across the second element, will provide a measure of the temperature. For example, the second element may comprise a thermistor.

Preferably, the elements in the remaining arms of the four-arm bridge circuit are resistors.

In a preferred embodiment, the measurement means comprises comparison means to indicate when the timing voltage substantially coincides with the voltage provided by each of said first and second elements, and associated timing means to determine the time taken, or count, in each case.

For example, in one embodiment, said measurement means comprises first and second counters each associated with a respective one of said first and second elements and arranged to be switched by the application of power to said timing circuit, and first and second comparators each associated with a respective one of said first and second elements, each said comparator receiving the voltage of the respective element on one input thereto and the timing voltage on a second input thereto, an output of each comparator being applied to switch a respective one of said first and second counters.

In general, each of said counters is arranged to be switched on by the application of power to the timing circuit, to count up when energised, and to be switched off by a change on the output of the respective comparator. Of course, it would equally be possible to cause the counters to count down between the application of power to the timing circuit and a change on the output of the respective comparator. In either circumstance, the first counter holds the count representative of the voltage of the first element, whilst the second counter holds the count representative of the voltage of the second element. These counts can be used in hard logic or in processor means to provide temperature or voltage determinations as required.

An alternative to providing a comparator associated with each of said first and second comparators is to provide only a single comparator and to switch appropriate inputs onto the comparator as required.

Preferably, the timing circuit is connected across the power supply terminals of the bridge circuit. Of course, the timing circuit may be any suitable circuit arranged to produce an appropriate timing function. For example, where the timing function is a voltage which varies with time, the timing circuit may be a ramp generator or a circuit arranged to provide a quadrature of a sine wave.

In a preferred embodiment, the timing circuit comprises a series connection of a resistor and a capacitor, charging of the capacitor on the application of power to the circuit providing the timing voltage. The voltage across the capacitor is preferably taken as the timing voltage and is applied, for example, to the second input of each comparator in the measurement means.

As is made clear above, the measuring apparatus may be used to provide an indication of actual temperature. In a preferred embodiment, the first element, and the elements in the third and fourth arms of the bridge circuit are resistors, preferably fixed precision resistors. The second element is a thermistor. The voltage of the timing circuit is compared in the measurement means with the voltage across the first resistor element, and with the voltage across the thermistor. The ratio determined is a measure of the temperature to which the thermistor is subjected and may be used in a programmable thermostat, for example, when it is wished to compare the actual temperature with a desired temperature.

In an alternative embodiment of measuring apparatus of the invention, the apparatus may be used to measure battery voltage, for example, in a battery powered thermostat, and the first element may be an element providing a fixed reference voltage, for example, a zener diode or a band gap device. In a preferred embodiment, the remaining arms of the bridge circuit each comprise a precision resistor, and the second element is thus a precision resistor. The battery or batteries providing the power provide the power supply for the bridge circuit and hence the voltage across the second element is determined by the battery power level. The fixed reference voltage remains constant such that any changes in the battery voltage level can be determined.

The present invention also extends to a method of determining a characteristic of an element in which said element is arranged in one arm of a four-arm bridge circuit, another arm of the bridge circuit having an element determining a reference characteristic, the method comprising the steps of connecting the bridge circuit to a power supply, producing, by way of a timing circuit, a timing function having a characteristic which varies with time, and establishing, in each case, the time at which the characteristics of the timing function coincide with said reference characteristic and with the characteristic produced by said element.

Preferably, a ratio is formed of the two times or counts established to provide a determination of the characteristic of said element, for example of the voltage across said element in said one arm.

The measuring apparatus and method defined above are preferably used in a programmable thermostat. The present invention also extends to a programmable thermostat comprising measuring apparatus as defined above.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit diagram of measuring apparatus of the invention for measuring temperature, Figure 2 shows comparators for use with the measuring apparatus of Figure 1, and Figure 3 shows an alternative embodiment of measuring apparatus for determining the power level from a battery supply.

The present invention will be described below with reference to a measuring apparatus to be used in a programmable thermostat for a central heating system.

However, this implementation of the present invention is described simply for clarity and it will be appreciated that the measurement technique described may be used in other environments.

Figure 1 shows a circuit diagram of measuring apparatus to be used in a programmable thermostat to provide information as to the actual temperature. In this respect, a programmable thermostat may be provided with processing means, for example a microprocessor, and associated memory, and will act to provide control signals for the central heating signals after having compared a stored desired temperature with a measured actual temperature. It is the measured actual temperature which the measuring apparatus of Figure 1 provides.

The measuring apparatus of Figure 1 incorporates a resistive four-arm bridge circuit. In fact, in the embodiment illustrated the bridge circuit is configured as a Wheatstone bridge. The element in each of three arms of the bridge is a respective fixed precision resistor R1, R2 and R3. The element in the remaining arm is a thermistor RTH. As it is wished to discriminate between temperatures less than 1"C apart, the thermistor RTH is preferably one with close tolerances. The thermistors which are used, for example, in clinical thermometers typically may be used in measuring apparatus as illustrated in Figure 1.

In use, a voltage pulse is applied to the supply terminals 0,V+ of the bridge. A respective voltage will therefore appear at the other points of the bridge. As indicated, at one point there will be a voltage VTH which is determined by the thermistor RTH, and at the other point there will be a voltage VREF determined by resistor R1.

Across the terminals of the bridge there is connected a series connection of a resistor R and a capacitor C.

From this series RC circuit a voltage Vc can be tapped.

It will be appreciated that when a voltage is applied across the supply terminals O,V+ of the bridge the voltages VTH and VREF will appear. In addition, the applied voltage will start to charge the capacitor C by way of the RC circuit, and the familiar rising voltage curve for a capacitor will define the varying voltage vc determined by the time constant of the RC circuit. In time the voltage c across the capacitor C will rise to that of the reference voltage VREF and subsequently to that VTH across the thermistor RTH, or vice versa.

As is indicated in Figure 2, the measuring apparatus also comprises two comparators COMP1 and COMP2. The comparator COMP1 compares the voltage Vc across the capacitor C with the voltage VTH across the thermistor, whilst the comparator COMP2 compares the capacitor voltage Vc with the voltage VREF across the resistor R1. It will be appreciated that initially the output of each comparator will be determined by the fact that its first input, say VTH or VREF, is greater than its second input Vc. However, when the second input voltage Vc becomes greater than the voltage with which it is compared, the output of the respective comparator COMP1 or COMP2 will change. Thus, there is a change of output of the respective comparator COMP1, COMP2 when the timing voltage Vc becomes substantially the same as the voltage VTH, VREF of the respective bridge element RTH, R1.

Where the measuring apparatus of Figures 1 and 2 is used in a programmable thermostat, the microprocessor of the thermostat is arranged to put a voltage pulse across the supply terminals 0, V+ of the bridge circuit of Figure 1 to commence the measurement. At that time the microprocessor will also switch on two counters (not shown). The first counter is arranged to count the time for the voltage Vc across the capacitor C to rise to the thermistor voltage VTH, whilst the second counter is to count the time for the voltage Vc across the capacitor C to rise to the reference voltage VREF across the resistor R1.

Clearly, the application of the voltage pulse to the bridge circuit by the microprocessor initiates the charging of the capacitor. When the voltage vc has risen to a level to switch over the output of one of the comparators COMP1 or COMP2, that switched output is applied to the respective counter to switch it off. Thus, the output of comparator COMP1 is applied to the first counter which hereby holds a count representative of the voltage VTH, whilst the change in output of the second comparator COMP2 is applied to the second counter which thereby holds a count representative of the voltage VREF. It will appreciated that as the time, and hence count, in each case is proportional to the applied voltage, the count is a dimensionless quantity which is independent of the applied voltage, and hence of any variations in the supply.

The microprocessor of the programable thermostat preferably forms a ratio of the two counts held. This produces a number which is independent of any variables of the circuit, but which is a measure of the voltage VTH across the thermistor RTH. Accordingly, the ratio provides a determination of the temperature to which the thermistor is subjected and can be used in the microprocessor as a measure of the actual temperature. The manner, of course, in which the software of the microprocessor uses the ratio obtained can be chosen as is required and is not further described herein.

Whilst the circuit shown in Figure 1 enables actual temperatures to be determined, that shown in Figure 3 enables a check of the voltage level of the battery, for example, of the programmable thermostat. In the embodiment of Figure 3 a zener diode Z provides a fixed reference voltage VREF'. It will be appreciated that the voltage VREF' which is established across the resistor R2 is a truly fixed voltage. The voltage VBATT which is taken from the other point of the bridge is determined by the values of the resistors R1 and R3 and the voltage applied to the bridge.Thus, if the voltage applied to the supply terminals 0,V+ of the bridge circuit is the battery voltage, applying the voltages Vc VREFI and VBATT generated at the points of the bridge to a comparator circuit with associated counters as described above with reference to Figure 2, will produce a determination as to the battery voltage. This can be used by the microprocessor of the programmable thermostat, for example, to provide a warning signal if the battery voltage falls too low.

The particular circuits illustrated in the drawings are only energised when a measurement is being determined.

This means that they are only on for a small time and thus that they have low power consumption. This makes the circuits particularly fitted for use in applications where batteries provide the power supply.

Figures 1 and 3 are shown as separate circuits and it is, of course, possible to provide two such circuits in a single programmable thermostat. However, for this application it is preferred that only a single measuring circuit is provided. In this respect, it will be appreciated that the two bridge arms including the zener diode Z and the resistor R2 can simply be added at the end of the circuit of Figure 1 to provide a measuring apparatus which can be used as required, to provide both actual temperature measurements and battery voltage determinations.

In the embodiments described and illustrated, the fixed reference voltage is provided by a zener diode Z. Of course, any appropriate component or circuit may be used.

Where the voltage levels are low, for example, a band gap device may be utilised to provide the fixed reference voltage.

It will be appreciated that the measuring techniques described above are not restricted in their application to use in programable thermostats for central heating systems.

For example, the measuring apparatus for determining temperature may be used wherever a determination of a temperature differential is required. Furthermore, the applicability of the apparatus described is wider than temperature or battery voltage measurements. Clearly, the circuit and techniques can be utilised whenever it is required to determine a quantity which can be made to have some relationship to voltage or other characteristics of an electrical circuit.

Other variations and modifications may be made to the invention within the scope of this application.

Claims

1. Measuring apparatus comprising a four-arm bridge circuit, a first arm of the bridge circuit having a first element arranged to provide a reference characteristic, and a second arm of the bridge circuit having a second element of which a charatertistic is to be determined, said measuring apparatus further comprising a timing circuit coupled to power supply terminals of the bridge circuit and arranged, upon the application of power thereto, to produce a timing function having a characteristic which varies with time, and measurement means arranged to establish, in each case, a time at which the characteristic of the timing function substantially coincides with the characteristics provided by said first and second elements whereby the characteristic of said second element is determinable.

2. Measuring apparatus as claimed in claim 1, wherein said first element is arranged to provide a reference voltage, it is required to determine the voltage of the second element, and said timing function has a voltage characteristic which varies with time.

3. Measuring apparatus as claimed in claim 1 or claim 2, wherein said timing function has only a single characteristic which varies with time, and said variation is monotonic.

4. Measuring apparatus as claimed in any preceding claim, wherein said measurement means is arranged to establish a count for each of the first and second elements which is effectively a ratio of the characteristic of the respective element and of that of the timing function, and wherein the measurement means is arranged to form a ratio of the count established in each case.

5. Measuring apparatus as claimed in any preceding claim, for use in a programmable thermostat which is battery powered, where the battery voltage, or a quantity determined thereby, is arranged to be applied across the second element such that said measuring apparatus is arranged to determine the battery voltage.

6. Measuring apparatus as claimed in any preceding claim, wherein said second element is, or comprises, an element whose voltage varies with temperature such that said measuring apparatus, in providing a determination of the voltage across the second element, provides a measure of the temperature.

7. Measuring apparatus as claimed in claim 6, wherein said second element is a thermistor.

8. Measuring apparatus as claimed in any preceding claim, wherein the element in each arm of the bridge circuit, except the second element, is a resistor.

9. Measuring apparatus as claimed in any precending claim, wherein said measurement means comprises comparison means to indicate when the timing voltage substantially coincides with the voltage provided by each of the said first and second elements, and associated timing means to determine the time taken, or count, in each case.

10. Measuring apparatus as claimed in claim 9, wherein said measurement means comprises first and second counters each associated with a respective one of said first and second elements and arranged to be switched by the application of power to said timing circuit, and first and second comparators each associated with a respective one of said first and second elements, each said comparator receiving the voltage of the respective element on one input thereto and the timing voltage on a second input thereto, an output of each comparator being applied to switch a respective one of said first and second counters.

11. Measuring apparatus as claimed in claim 10, wherein each of said counters is arranged to be switched on by the application of power to the timing circuit, to count up when energised, and to be switched off by a change on the output of the respective comparator.

12. Measuring apparatus as claimed in claim 11, wherein the first counter holds the count representative of the voltage of the first element, whilst the second counter holds the count representative of the voltage of the second element.

13. Measuring apparatus as claimed in any preceding claim, wherein said timing circuit is connected across the power supply terminals of the bridge circuit, wherein the timing function is a voltage which varies with time, and wherein the timing circuit is a ramp generator or a circuit arranged to provide a quadrature of a sine wave.

14. Measuring apparatus as claimed in claim 13, wherein the timing circuit comprises a series connection of a resistor and a capacitor, charging of the capacitor on the application of power to the circuit providing the timing voltage.

15. Measuring apparatus as claimed in any preceding claim arranged to provide an indication of actual temperature, wherein said first element, and the elements in the third and fourth arms of the bridge circuit are resistors, and the second element is a thermistor, and wherein the voltage of the timing circuit is compared in the measurement means with the voltage across the first resistor element, and with the voltage across the thermistor.

16. Measuring apparatus as claimed in any preceding claim arranged to measure battery voltage, wherein the first element is an element providing a fixed reference voltage, and the remaining arms of the bridge circuit each comprise a precision resistor, and wherein the battery or batteries providing the power provide the power supply for the bridge circuit such that the voltage across the second element is determined by the battery power level.

17. A method of determining a characteristic of an element in which said element is arranged in one arm of a four-arm bridge circuit, another arm of the bridge circuit having an element determining a reference characteristic, the method comprising the steps of connecting the bridge circuit to a power supply, producing, by way of a timing circuit, a timing function having a characteristic which varies with time, and establishing, in each case, the time at which the characteristics of the timing function coincide with said reference characteristic and with the characteristic produced by said element.

18. A method as claimed in clause 17, further comprising the step of forming a ratio of the two times or counts established to provide a determination of the characteristic of said element.

19. Measuring apparatus when arranged to determine an element characteristic by way of a method as claimed in claim 17 or claim 18.

20. Measuring apparatus substantially as hereinbefore described with reference to the accompanying drawings.

21. A method of determining a characteristic of an element substantially as hereinbefore described with reference to the accompanying drawings.