WO2001038839A1

WO2001038839A1 - Apparatus for performing a temperature measurement

Info

Publication number: WO2001038839A1
Application number: PCT/US2000/031303
Authority: WO
Inventors: David G. Bellemore; David R. Fournier; Michael A. Davis
Original assignee: Cidra Corporation
Priority date: 1999-11-23
Filing date: 2000-11-15
Publication date: 2001-05-31
Also published as: AU1609301A

Abstract

A temperature measurement circuit includes a switched sensing circuit and measures alternately a characteristic of the switched sensing circuit that depends on a thermistor and on a reference resistor, using common electrical components, including a multi-pole switch. The resistance of the thermistor varies with temperature over a known temperature range, and the resistance of the reference resistor remains substantially fixed over the range of temperatures being measured. The temperature measurement circuit also includes a detection circuit. The switched sensing circuit responds to a temperature to be measured, and also responds to a detection circuit sitich control signal. It provides switched sensing circuit signals containing information about the characteristic of the sensing circuit with the thermistor and the reference resistor, in turn, switched into the circuit. The detection circuit provides the detection circuit switch control signal, and responds to the switched sensing circuit signals. It provides a detection circuit signal containing information about the temperature to be measured. In one embodiment, the common set of electrical components includes a capacitor connected in series with a multi-pole switch in the switched sensing circuit and a comparator in the detection circuit. In another embodiment, the switched sensing circuit is a (switched) resistance-tunable oscillating circuit.

Description

APPARATUS FOR PERFORMING A TEMPERATURE MEASUREMENT

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to and co-filed with patent applications identified by Express mail Nos . EL 381 225 795 US (WFVA/CiDRA Ref. Nos. 712-2.77/CC-0219) and EL 381 225 804 US (WFVA/CiDRA Ref. Nos. 712-2.76/CC- 0218 ) , both hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to the field of thermometry. More particularly, the present invention pertains to an apparatus for performing a temperature measurement in a way that avoids some environmental and aging effects, making possible a high precision, high accuracy temperature measurement .

2. Description of Related Art According to the prior art, in performing a highly accurate (yielding a correct value) and precise (high repeatability) temperature measurement, a temperature probe is commonly used. Many of these probes are based on a thermocouple, thermistor or resistance temperature device (RTD) . In each of these probes, temperature is correlated with a change in the value of a parameter, such as resistance or voltage, that changes with temperature in a predictable and reproducible manner. In a thermocouple the parameter is voltage, and in both a thermistor and an RTD, the parameter is resistance.

For any such parameter, a probe needs an interrogation unit. Such a unit is required to determine the value of the parameter (such as resistance or voltage) with a desired accuracy, and either provide the value of the parameter or provide the correlated temperature value. A probe based on a thermistor, compared to a thermocouple or RTD, is a relatively low cost and highly accurate probe, and can be easily encapsulated (isolated from the environment) so as to provide long term stability. Recent data show that many kinds of thermistors offer better resolution than a typical RTD, and possess stability of better than 0.01°C for over 100 months at 70 °C.

The design of an interrogation unit for determining the value of resistance of a thermistor is commonly based on either a Wheatstone Bridge circuit or a resistance-mode operational amplifier circuit. A design based on either of these circuits can provide relatively accurate measurements. However, extreme care must be taken to ensure that ambient temperature effects, and effects caused by other environmental factors generally or by aging of the components of the measurement system, are kept to a minimum so as to obtain high, reproducible accuracy. For a typical 100 kW thermistor, accuracy of 0.01°C requires a measurement of as little as 0.005% of the base value of the resistance of the thermistor. This type of accuracy is difficult to achieve with standard resistance measurement techniques, because the value of the parameter is often changed in the attempt to measure it accurately, especially when the value is small. For example, in the case of a thermistor whose resistance is changing by only a small fraction, more current through the resistance would provide a more discernible value of the resistance but would also cause self-heating, fouling the measurement.

What is needed is an apparatus that performs a temperature measurement in a way that avoids, at least to some extent, the effects associated with environmental factors or with aging, effects which interfere with interrogating parameters in a temperature probe. SUMMARY OF THE INVENTION

The present invention provides a temperature measurement circuit based on measuring an operating characteristic of a switched sensing circuit that depends alternately on the resistance of a thermistor, exposed to the temperature being measured, and on the resistance of a reference resistor. The measurement is made using the same electrical components, including a mult1-pole switch.

In particular, the temperature measurement circuit comprises the switched sensing circuit and a detection circuit .

The overall sensing circuit has a common set of electrical components and two different electrical components, the thermistor and the reference resistor. The switched sensing circuit responds to a temperature to be measured, and further responds to a switch control signal provided by the detection circuit . It provides switched sensing circuit signals containing information about the operating characteristic of the switched sensing circuit switched to use each of the two different electrical components in turn. The detection circuit provides the detection circuit switch control signal to the switched sensing circuit, and responds to the switched sensing circuit signals. It provides a detection circuit signal containing information about the temperature to be measured.

The resistance of the thermistor varies in a known way with temperature over a known temperature range . The resistance of the reference resistor does not vary appreciably with temperature over the known temperature range . The thermistor and the reference resistor are each connected in parallel to the multi-pole switch.

In one embodiment, the switched sensing circuit s a switched charge or discharge circuit, and the common set of electrical components includes a capacitor connected m series with a multi-pole switch m the switched sensing circuit, and a comparator in the detection circuit; and the operating characteristic is the time to charge or discharge the common capacitor.

The detection circuit includes a voltage source for providing a voltage source signal . The detection circuit includes a comparator that responds to the switched sensing circuit signals, and further responds to the voltage source signal . The comparator provides a high or low comparator signal depending on whether the voltage of the switched charge or discharge circuit signal is higher or lower than the voltage of the voltage source signal .

The detection circuit also has a clock for providing a clock signal, and a counter that responds to the clock signal, and further responds to a start/stop signal, for providing a counter signal containing information about the operating characteristic of the switched sensing circuit to be detected, namely, the time for the capacitor to discharge with either the thermistor or the reference resistor switched into the switched sensing circuit . The detection circuit includes a circuit controller that responds to the comparator signal, and further responds to the counter signal, for providing the start/stop signal to the counter, for providing the detection circuit switch control signal to the switched charge or discharge circuit, and for providing the detection circuit signal containing information about the temperature to be measured.

In another embodiment, the switched sensing circuit is a switched resistance-controlled oscillating circuit, where a switch causes a thermistor, exposed to the temperature to be measured, to be included in the circuit, and, in turn, a fixed reference resistor. The detection circuit in this embodiment detects, as the operating characteristic, the oscillating frequency of the resistance-controlled oscillating circuit with the thermistor and, in turn, the fixed resistor switched into the circuit. In another aspect of the invention for measuring a temperature, what is provided is: a sensing circuit, responsive to the temperature being measured, for providing a time-varying voltage depending on a sensing characteristic that varies with temperature in a known way, and, in turn, a time-varying voltage depending on a substantially fixed reference characteristic; and a detection circuit, responsive to both the time-varying voltage depending on the sensing characteristic, as well as the time-varying voltage depending on the substantially fixed reference characteristic, for providing a value corresponding to the sensing characteristic and thus also corresponding to the temperature being measured, and, in turn, a value corresponding to the substantially fixed reference characteristic. In a particular embodiment of this other aspect of the invention, the sensing circuit is a discharge circuit having a capacitor that discharges at a rate that depends on a resistance in the discharge circuit; the sensing characteristic is the resistance of a thermistor; and the substantially fixed reference characteristic is a substantially fixed reference resistance.

In one aspect of this particular embodiment, the invention further comprises a switch, for providing a switch voltage that is the time-varying voltage depending on the thermistor resistance, and in turn, the time-varying voltage depending on the reference resistance. Also, the capacitor is responsive to the switch voltage at one terminal and to a set voltage at the other terminal, and the value corresponding to the thermistor resistance represents a first length of time, and the value corresponding to the substantially fixed reference resistance represents a second length of time.

In another particular embodiment of this other aspect of the invention, the sensing circuit is an oscillating circuit having a resonance frequency that depends on a resistance in the oscillating circuit; the sensing characteristic is the resistance of a thermistor; the substantially fixed reference characteristic is a substantially fixed reference resistance; and the value corresponding to the thermistor resistance represents a first oscillating frequency, and the value corresponding to the substantially fixed reference resistance represents a second oscillating frequency.

In all embodiments, the known value of the fixed reference resistance is used to refine, or make more precise and accurate, the value of the operating characteristic of the switched sensing circuit with the thermistor switched into the circuit, thus providing a high accuracy (to the extent that the value of the reference resistor is accurately known) and high precision measurement of the thermistor resistance, which can then be correlated to the temperature being measured, i.e. the temperature to which the thermistor is exposed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the invention will become apparent from a consideration of the subsequent detailed description presented in connection with accompanying drawings, in which:

Figure 1 is a block diagram of a temperature measurement circuit according to the present invention;

Figure 2 is a set of graphs indicating the principles for performing a temperature measurement according to the present invention;

Figure 3 is a schematic circuit diagram of the temperature measurement circuit shown in Figure 1; and

Figure 4 is a block diagram of another embodiment of a temperature measurement circuit according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Figure 1 Figure 1 shows a temperature measurement circuit generally indicated as 10 for performing a temperature measurement according to the present invention. The temperature measurement 10 includes a discharge circuit 21, as one embodiment of a switched sensing circuit, and a detection circuit 25. The temperature measurement circuit shown and described herein can be used as part of an overall system shown and described m the aforementioned cross-referenced patent applications.

The Discharge Circuit 21 The discharge circuit 21 has a capacitor C_D having a first terminal Pi connected to a voltage source Vi, and having a second terminal Pi connected to a multI -pole switch 22 for being switched to a thermistor R_τ, a voltage source V₃ or a reference resistor R_R.

In operation, the detection circuit 25 provides a switch control signal to switch the multi-pole switch 22 for either charging or discharging the capacitor C_D through either the thermistor R_τ or the reference resistor R_R by connection to a voltage source V₃.

For example, when the discharge circuit 21 is used to measure the time for the capacitor C_D to discharge, the detection circuit 25 switches the multi-pole switch 22 to connect the voltage source V₃ to the terminal P₂ of the capacitor C_D so as to charge the capacitor C_D. Next, the detection circuit 25 switches the multi-pole switch 22 to connect the terminal P₂ of the capacitor C_D to the reference resistor R_R so the capacitor C_D discharges, since the voltage V₂ is less than V_x . The detection circuit 25 measures the time for the capacitor C_D to discharge to some level (at or above V₂) , as described below. The process is repeated for the thermistor R_τ. First, the detection circuit 25 switches the multi-pole switch 22 to connect to the terminal P₂ of the capacitor C_D to the voltage source V₃ to charge the capacitor C_D . Next, the detection circuit 25 switches the multi-pole switch 22 to connect the terminal P₂ of the capacitor C_D to the thermistor R_τ so the capacitor C_D discharges since the voltage V₂ is less than Vi . The detection circuit 25 again measures the time for the capacitor C_D to discharge to some level, as described below. It is this time to discharge, with the thermistor switched into the discharge circuit, that is the operating characteristic used to determine the thermistor resistance, which is then correlated to the temperature being measured, i.e. the temperature to which the thermistor is exposed. The switching of the multi -pole switch 22 permits the capacitor C_D to discharge alternately through the reference resistor R_R and the thermistor R_τ and the same other electrical components, including the multi-pole switch 22 itself, for measurement of the resistance R_R and alternately the resistance R_τ. By knowing the time t_τ for discharging below the threshold for the thermistor R_T the time t_R for discharging below the threshold for the reference resistor R_R, and knowing the resistance value the reference resistor R_R, one can determine the resistance value of the resistance of the thermistor R_τ, according to equation (2) below.

When the capacitor C_D is discharged through either the thermistor R_τ or reference resistor R_R, so as to have a third voltage source V₂ as the final voltage of the second terminal Pi, while it is in the process of discharging, it has either voltage V_R(t) or voltage V_τ(t) at the second terminal P₂.

As the capacitor C_D discharges, the voltage V_s on the terminal P₂ of the capacitor, which is either the voltage V_τ or V_R depending on whether the thermistor R_τ or reference resistor R_R is switched into the discharge circuit 21, decreases over time, (to the voltage source V₂ which may be either ground potential or some other voltage lower than V₃) , because current flows through the mult1 -pole switch 22 to the other terminal Pi of the capacitor C_D, removing the charge on the capacitor C_D.

The Detection Circuit 25

In Figure 1, the detection circuit 25 has a detection voltage source V_D that applies a voltage V_D to one input of a comparator 27. The voltage V_s at the second terminal P₂ of the capacitor C_D is applied to the other input of the comparator 27. The comparator 27 responds to these two voltage signals, and produces high and low comparator signals to the circuit controller 26, i.e. e.g. it produces a high voltage as long as the voltage across the capacitor is detected as higher than the detection voltage V_D, and produces a lower voltage otherwise. The high and low comparator signals indicates whether the voltage V_D is greater than or less than the voltage V_s shown m Figure 2 as a detection pulse 13 (see Figure 2) .

The circuit controller 26 responds to a comparator signal from the comparator 27, for providing a start or stop signal to the counter 28 to start and stop the same. In operation, the circuit controller 26 responds to one comparator signal when the voltage V_D is greater than the voltage V_s, for providing a start signal to the counter 28 to start the count of the counter 28. The circuit controller 26 responds to another comparator signal when the voltage V_D is less than the voltage V_s, for providing a stop signal to the counter 28 to stop the count of the counter 28. The circuit controller 26 follows this procedure for the reference resistor R_R and the thermistor R_τ. The counter 28 provides an estimate, in clock cycles, of the duration of the detection circuit voltage pulse 13, 14 (see Fig 2) . In providing the estimate, the counter 28 is pulsed by a clock 29 that is a high frequency clock. The circuit controller 26 provides the switch control signal to switch the multl -pole switch 22 at some predetermined time after the counter is done counting. The counter 28 may also be configured to throw the multi -pole switch at predetermined constant -length intervals, of appropriate length so that the counter would always finish counting during an interval.

The circuit controller 26 provides, to another circuit that is not shown in Figure 2, a circuit control signal from the detection circuit 25 to indicate the time to charge/discharge. The circuit control signal contains information about the values determined by the counter 28, which correspond to the times t_τ and t_R of equation 1. In the preferred embodiment, the circuit controller 26 has a separate microprocessor (CPU) that uses the counter values and the known value R_R of the fixed reference resistor to provide a value R_τ for the thermistor according to equation 1 below, and then uses a description of how R_τ varies with temperature to determine the temperature being measured. The circuit controller 26 can be any of a number of different devices, including a micro-controller, programmable gate arrays, or different combinations of different discrete components.

Figure 2

Figure 2 has a series of graphs that show the fundamental relationships upon which a temperature measurement is made according to the present invention.

Figure 2 shows a discharge voltage 11 that decreases with time, a detection voltage 13 that is essentially a square wave pulse, and a clock voltage 16, each indicated at two temperatures: Ti and T₂>Tι. The principles of operation are based on using a thermistor, usually one having resistance R_τ that decreases with increasing temperature, exposed to two different temperatures, T_x and T₂ > T_x . (Instead of a thermistor, any other resistance-based sensor can also be used.) The voltage decay curves 11, 12 associated with the discharging of the capacitor of the discharge circuit at T_x and T₂ are shown along with the expected voltage responses 13, 14 at Ti and T₂, from the detection circuit 25 (Figure 1) using a fixed voltage threshold level 15. The voltage responses 13, 14 of the detection circuit 25 is a voltage pulse having a width proportional to the time required for the capacitor to discharge below the threshold 15, a time that is longer for a higher value of the thermistor resistance, and so longer for a lower temperature.

In the preferred embodiment, the thermistor having resistance R_τ is the usual sort of thermistor, which has a negative coefficient of resistivity (i.e. has a resistance that decreases with increasing temperature) . However, a thermistor with a positive coefficient of resistivity can also be used. What is essential is only that the thermistor resistance vary with temperature in a known way.

Figure 2 also shows a fixed frequency clock signal 16, provided as part of a pulse width measurement technique used to determine an estimate of the width of the voltage pulse (voltage response) 13, 14 of the detection circuit. The number of clock cycles that elapse during the time interval corresponding to the width of the voltage response 13, 14 indicates the time elapsed for the discharge of the capacitor, through the thermistor, below the threshold 15. A high frequency clock should be used to eliminate the digitization error associated with the finite clock pulse width.

Besides digitization, error is also caused by variations in the capacitance of the capacitor used in the discharge circuit, and by slight variations in the threshold value used in the detection circuit, both of which are caused by changes in environmental conditions, such as ambient temperature, or by aging of the elements of the respective circuits used to provide the capacitance and threshold voltage.

To eliminate these sources of error, in the preferred embodiment, as described above and in embodiments described below, an apparatus is provided that measures the value of a reference parameter in addition to the value of the thermistor resistance; a high precision fixed resistor is used as a reference resistor and measured using the exact same circui try (threshold detection and pulse width measurement) as is used for measuring the resistance of the thermistor. Such a reference measurement provides the basis for a temperature measurement m which the interfering effects of the prior art are essentially canceled out, at least to the extent that the reference resistor is not appreciably affected by environmental changes or aging. A simple ratio between the thermistor pulse width to the reference pulse width (as provided by equation 2 below) can then be used to eliminate common mode effects and thereby obtain an accurate measurement of the resistance of the thermistor. Once the resistance of the thermistor is measured, it is readily converted into a value for the temperature sensed by the thermistor, using the known correlation of thermistor resistance with temperature.

In Figure 2, the voltage decay 11, 12 associated with the discharging of the capacitor of the discharge circuit at either i or T₂ , i.e. the voltage provided by the thermistor to the input of the comparator m the detection circuit, is given by

V_τ(t) = V_{l T}exp(-t / τ_τ) , (1)

m which exp ( ) denotes raising the quantity m the parenthesis to the power e (approximately 2.718), where V_τ(t) denotes the voltage after time t seen by the comparator (i.e. the voltage difference V_s - V_D) because of the capacitor discharging through the resistor, where V_lT denotes the initial voltage, and where t_τ is a time constant having a value of RC_D, where R_τ is the resistance of the thermistor (which depends on temperature) , and C_D is the capacitance of the capacitor m the discharge circuit. An equation of the same form gives the voltage as a function of time m case of the capacitor discharging through the reference resistor R_R. The use of the comparator in the circuit ensures that the final voltage using the thermistor can be set equal to the final voltage using the reference resistor. In addition, the same voltage source is used to charge or discharge the circuit with either the fixed resistor or the thermistor switched into the circuit, which allows also setting the initial voltage using the thermistor to the same value as the initial voltage using the fixed resistor. Thus, all applied voltages are the same for the circuit with the thermistor as for the circuit with the fixed resistor.

Knowing the time t_τ for discharging below the threshold for the thermistor, the time t_R for discharging below the threshold for the reference resistor, and knowing the value R_R of the reference resistor, allows an evaluation of the thermistor resistance R_τ, according to the equation,

Figure 3 The temperature measuring circuit of Figure 1 can also be used to determine temperature not by discharging the capacitor C, but by charging it, since the time constant for charging it is the same as for discharging it, i.e. the time constant is RC in both cases, where R is alternately the thermistor resistance and the fixed reference resistance, i.e. R is alternately R_τ and R_R.

Figure 3 shows a schematic diagram of the temperature measurement circuit in Figure 1 that is used by timing the charging of a capacitor. In Figure 3, the temperature measurement circuit is shown as incorporating all of the features of the preferred embodiment, including the use of a common capacitor C4 (corresponding to C_D of Figure 1), i.e. one capacitor for both a thermistor R2 (corresponding to R_τ of Figure 1) and a fixed reference resistor Rl (corresponding to R_R of Figure 1) , as well as the use of a common detection circuit 26-29, with part of the circuit controller 26, namely the CPU, and the counter 28 and clock 29 shown in block diagram form.

The circuit of Figure 3 is based on generating a digital square wave oscillation in a D flip-flop, U4B, and accurately measuring the period of oscillation. The oscillation of the circuit of Figure 3 is sustained by charging C4 through the reference resistor, Rl , discharging C4 to ground through an analog switch, U3 , (for a set time period based on the R3'C6 time constant), charging C4 through the thermistor, R2 , and then discharging C4 to ground through the same analog switch, U3.

The C4 charge path is selected by the output of the D flip-flop, U4A.Q. The flip-flop turns on one of two analog switches that connects C4 to either Rl or R2 in series with the output of the inverter U1A, which provides the current that flows through the resistors and onto C4 while it is charging .

The square wave includes two distinct 'ON' times, an ON time for an interval depending on the value of the reference resistor Rl , and an ON time for an interval depending on the value of the thermistor R2 ; each ON interval also depends on the capacitance of the same capacitor C4. The two different ON time intervals are separated by an OFF time interval, so that the square wave is, in succession, a thermistor ON time, an OFF time, a reference resistor ON time, the same OFF time, and so on .

The reference resistor ON time interval has a length depending on the time needed to charge the capacitor C4 , through the reference resistor Rl , from zero volts to a trip voltage VT+ of approximately 3.8V, the voltage on the capacitor used as input to a digital Schmidtt trigger input inverter U1B .

The thermistor ON time interval has a length depending on the time needed to charge the capacitor C4 , through the thermistor R2 , from zero volts to the same trip voltage VT+ of approximately 3.8V. Using the same capacitor C4 and the same voltage thresholds for determining both time intervals eliminates all differences between the time intervals except those resulting from different values of the two resistances, that of the reference resistor and that of the thermistor.

As explained above in connection with equation (2), the value of the thermistor resistance R2 is determined from the ratio of the thermistor ON time to the reference resistor ON time, multiplied by the value of the reference resistor Rl .

The ON times of the oscillation can be measured by a number of techniques but a simple method is to clock-enable a counter that keeps track of the number of high frequency (typically 6 MHz) clock pulses that occur during the ON time. The two OFF times have a length depending on the time constant R₃C₆, associated with the reset resistor R3 and capacitor C6, for charging the capacitor C6 from zero volts to the trip voltage VT+ of U1D . C6 is shorted to ground through analog switch, U3 , during the ON time periods. The output of the inverter, U1C, provides the current that charges C6 through R3.

The OFF time period performs two functions. First, it sustains the circuit oscillation; and second, it allow a microprocessor to read the count value from the period counter and then reset the counter for the next ON time.

This technique of measuring temperature using a thermistor ensures that changes in the capacitance of C4 , or that changes in the threshold voltage VT+ and other circuit parameters due to ambient temperature and humidity, will not affect the measurement of the thermistor resistance.

A circuit according to the present invention should meet several guidelines. The capacitor C4 should have very low leakage current, and also a low temperature coefficient. The analog switches U3 should have very low resistance, very low leakage current, and also very low input capacitance. The inverter U1A should have very low bias current, and should have very low input capacitance.

It is central to any embodiment of the present invention, including both the discharge circuit embodiment described above and the resistance-tunable oscillating circuit described below, that exactly the same components be used with the thermistor R_τ switched into a circuit according to the present invention, as for the reference resistor R_R. Without such a common-component circuit, the desired high accuracy and high precision cannot be achieved. Accordingly, the switching m the present invention provides, for the timed phase of the circuit operation (i.e. the discharging phase when it is the time for discharging that is being measured to determine the temperature-dependent resistance) , exactly the same circuit except for either the thermistor (or other resistance-based sensing means) and the fixed, reference resistor; the switching does not result m other components, such as diodes, being included m the timed phase of the circuit operation (as e.g. m U.S. Pat. No. 4,488,823 to Baker, at Figure 5, where diode D9 is used with sensing resistor 51 and diode D8 is used with fixed resistor R14) . To switch both a resistor and a diode into such a temperature measuring circuit, m place of another resistor and diode, allows the possibility of variation caused by differences m the operating characteristics of the different diodes. While such a difference might cause errors that are acceptable m some situations, the errors caused by most violations of the principle of common components will prevent achieving the high accuracy and high precision aimed at by the present invention. Thus, the switching mechanism m the embodiment of the present invention, i.e. switch 22 (Figure 1) implemented as an ADG513 (Figure 3), switches into the circuit only the resistors R_R and, m turn, the thermistor R_τ, and not also other components. One coincidental advantage of using a switching mechanism 22 as in the discharge circuit 10 of the embodiment is that the discharge circuit can be easily adapted to measure either the time to charge the capacitor C_D or the time to discharge it. Essentially, all that is required is to provide different set voltages V_i; V₂, V₃ , and V_D.

In addition, it is crucial to achieving the high accuracy and precision that the voltage sources V_{l t} V₂, V₃, and V_D be unaffected by the operation of the discharge circuit, so that the voltages each provides is the same whether the thermistor or the reference resistor is switched into the discharge circuit. Thus, for example, in the discharge circuit embodiment (Figure 2) , none of the voltages V_x, V₂, V₃, and V_D is provided as part of a voltage divider component of the apparatus, susceptible to variation depending on changing current flows in the branches of the voltage divider. (The use of a voltage divider as part of a circuit intended to measure temperature is also shown in U.S. Pat. No. 4,488,823 to Baker, where in Figure 5, a voltage reference K_ref is shown as provided at a point in a voltage-divider branch including R₃ and R₄, so that the voltage at the pickoff for K_ref, intended to be determined by V_ΞS and the ratio of R₃ to R₄ , is in fact affected by current flow through other branches besides the voltage divider, such as through the branch including resistor R_i3 or resistor R_i2. Thus, the Baker uses uncommon components besides a thermistor and reference resistor.)

A circuit essentially the same as provided by Figure 3 has permitted temperature measurements with a precision of as little as 0.002°C, i.e. that vary by less than that amount in successive measurements of the same temperature.

Figure 4 Figure 4 shows another embodiment of the present invention, having a thermistor resistance, and in turn a fixed resistance, are converted into a time varying signal by including a thermistor and m turn a fixed resistor as part of a sensing circuit that is a resistance-tuneable oscillating circuit 81, such as an operational amplif er-based tuneable RC oscillator circuit. In such an implementation, the temperature and therefore the resistance of the thermistor or fixed resistor cause the oscillator frequency to change. A detection circuit 82 for detecting resonance frequency is then used to determine the oscillator resonance frequency with either the thermistor or fixed resistor. The thermistor resistance can then be determined from the fixed resistance and the ratio f_T/f of thermistor and fixed resistor frequencies, based on an equation similar to equation 2, namely,

R_τ = F(^-)R_R , (3)

J R

where F is some function depending only on the ratio of the measured frequencies, which could be an inverse relationship or other non- linear relationship. Once R_τ is thus determined, the temperature being measured can be inferred from the known way m which the thermistor resistance varies with temperature (m the same way as in the preferred embodiment) .

In any embodiment of the present invention, it is possible that the detection circuit provides not a final, refined (made more accurate and precise) value of the thermistor resistance R_τ, but instead both of the time values, t_τ and t_R, m equation 2 (or equation 3) for use by other components (not shown) . The other components would then determine a precise and accurate value of R_τ using equation 2 or the more general equation 3, depending on the embodiment.

The Scope of the Invention It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. For example, any characteristic of a circuit component can be used as the basis for a measurement according to the present invention, not only resistance, provided that the relationship between the characteristic and temperature is known, and provided that there is another component having a characteristic that is analogous, but independent of temperature. Numerous other modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention, and the appended claims are intended to cover such modifications and arrangements.

Claims

What is claimed is:

1. A temperature measurement circuit, comprising: a) a switched sensing circuit having a common set of electrical components and two different electrical components, responsive to a temperature to be measured, and further responsive to a detection circuit switch control signal, for providing switched sensing circuit signals containing information about an operating characteristic of the switched sensing circuit using the common set of electrical components switched respectively through each of the two different electrical components; and b) a detection circuit, for providing the detection circuit switch control signal, and responsive to the switched sensing circuit signals, for providing a detection circuit signal containing information about the temperature to be measured.

2. A temperature measurement circuit according to claim 1, wherein the common set of electrical components includes a capacitor connected in series with a multi-pole switch; and wherein the operating characteristic is a time to charge or discharge the capacitor.

3. A temperature measurement circuit according to claim 1, wherein the two different electrical components are a thermistor and a reference resistor; wherein the resistance of the thermistor varies with temperature over a known temperature range; and wherein the resistance of the reference resistor does not vary substantially with temperature over a range of temperature including the temperatures to be measured.

4. A temperature measurement circuit according to claim 1, wherein the common set of electrical components includes a capacitor connected m series with a multi-pole switch; and wherein the two different electrical components are a thermistor and a reference resistor, each connected in parallel to the multi-pole switch.

5. A temperature measurement circuit according to claim 4, wherein the detection circuit includes a voltage source for providing a voltage source signal; and wherein the detection circuit includes a comparator responsive to the switched sensing circuit signals, and further responsive to the voltage source signal, for providing a high or low comparator signal depending on whether voltage of the switched sensing circuit signals is higher or lower than the voltage of the voltage source signal .

6. A temperature measurement circuit according to claim 5, wherein the detection circuit further comprises a clock for providing a clock signal; and wherein the detection circuit further comprises a counter responsive to the clock signal, and further responsive to a start/stop signal, for providing a counter signal containing information about the number of pulses of the clock signal.

7. A temperature measurement circuit according to claim 6, wherein the detection circuit includes a circuit controller responsive to the high or low comparator signal, and further responsive to the counter signal, for providing the start/stop signal to the counter, for providing the detection circuit switch control signal to the switched sensing circuit for switching the multi-pole switch, and for providing the detection circuit signal containing information about the temperature to be measured.

8. A temperature measurement circuit according to claim 1, wherein the switched sensing circuit includes a resistance-tunable oscillating circuit having a thermistor and a reference resistance and a switch for switching either the thermistor and the reference resistance into the resistance- tunable oscillating circuit, and wherein the operating characteristic is the frequency of the oscillating circuit; and wherein the detection circuit detects the resonance frequency of the switched charge or discharge circuit, for providing the detection circuit signal as a measure of the frequency of oscillation of the resistance-tunable oscillating circuit alternately when the thermistor is in the resistance- tunable oscillating circuit and when the reference resistor is in the resistance-tunable oscillating circuit.

9. An apparatus for measuring a temperature, comprising: a) a sensing circuit, responsive to the temperature being measured, for providing a time-varying voltage depending on a sensing characteristic that varies with temperature in a known way, and, in turn, a time-varying voltage depending on a substantially fixed reference characteristic; and b) a detection circuit, responsive to both the time-varying voltage depending on the sensing characteristic, as well as the time-varying voltage depending on the substantially fixed reference characteristic, for providing a value corresponding to the sensing characteristic and thus also corresponding to the temperature being measured, and, in turn, a value corresponding to the substantially fixed reference characteristic.

10. An apparatus according to claim 9, wherein the sensing circuit is a discharge circuit having a capacitor that discharges at a rate that depends on a resistance in the discharge circuit; wherein the sensing characteristic is the resistance of a thermistor; and wherein the substantially fixed reference characteristic is a substantially fixed reference resistance.

11. An apparatus according to claim 10, further comprising a switch, for providing a switch voltage that is the time- varying voltage depending on the thermistor resistance, and m turn, the time-varying voltage depending on the reference resistance; wherein the capacitor is responsive to the switch voltage at one terminal and to a set voltage at the other terminal; and wherein the value corresponding to the thermistor resistance represents a first length of time, and the value corresponding to the substantially fixed reference resistance represents a second length of time.

12. An apparatus according to claim 11, wherem the detection circuit comprises: a) a comparator, responsive to the switch voltage and to a threshold voltage, for providing a trigger signal indicating when the switch voltage decays below the threshold voltage; b) a clock, for providing clock pulses; c) a counter, responsive to the clock pulses, and to a start and a stop signal, for providing a counter signal indicating the number of clock pulses received between receiving the start signal and receiving the stop signal; and d) a circuit controller, for providing a switch control signal for controlling the switch and for providing the start and stop signals for controlling the counter, responsive to the number of clock pulses received between receiving the start signal and receiving the stop signal, and for providing as output the value representing the first length of time and corresponding to the temperature being measured, and, m turn, the value representing the second length of time and corresponding to the substantially fixed reference.

13. An apparatus according to claim 9, wherein the sensing circuit is an oscillating circuit having a resonance frequency that depends on a resistance in the oscillating circuit; wherein the sensing characteristic is the resistance of a thermistor; wherein the substantially fixed reference characteristic is a substantially fixed reference resistance; and wherein the value corresponding to the thermistor resistance represents a first oscillating frequency, and the value corresponding to the substantially fixed reference resistance represents a second oscillating frequency.

AMENDED CLAIMS

[received by the International Bureau on 21 March 2001 (21.03.01); original claims 2 and 4-13 cancelled; original claims 1 and 3 amended (2 pages)]

1. A temperature measurement apparatus, comprising: a switched sensing circuit for sensing a temperature to be measured based on charging or discharging a capacitor through a temperatur -dependent sense resistance, the switched sensing circuit having a common set of electrical components including the capacitor and a multi-pole Bwitch to which the capacitor is connected in series, and also including a substantially temperature-independent reference resistor, wherein the reference resistor and the sense resistance are connected to different poles of the switch, the switched sensing circuit switchable via the switch so as to alter the charging or discharging time of the capacitor by including as an impedance to the charging or discharging of the capacitor either the reference resistor or the sense resistance, the switched sensing circuit for responding to the temperature to be measured via the sense resistance and for providing switched sensing circuit signals containing information about the time to charge or discharge the capacitor alternately through the temperature-dependent sense resistance and the temperature-independent reference resistor; and a detection circuit, including a comparator responding to the switched sensing circuit signals and providing a comparator signal indicating when a comparison of the capacitor voltage to a reference voltage changes, and a controller responding to the comparator signal and to a counter signal and providing a detection circuit output signal containing information about the temperature to be measured taking into account the time for the capacitor to charge or discharge through the reference resistor, wherein the controller provides a detection circuit switch control signal for controlling when the switched sensing circuit switches from charging or discharging the capacitor through either the reference resistor or the sense resistance to charging or discharging the capacitor through the other, the providing of the detection circuit switch control signal being based on the comparator signal, and also wherein the counter signal that is provided by a counter that is reset by the controller using a start/stop signal and conveys information about the time elapsed since the controller last provided the detection circuit switch control signal ; whereby, in the sensing circuit, except for the sensing resistance and the reference resistor, all of the operative components are the same when the capacitor charges or discharges, so that a temperature measurement provided by the temperatur -measuring apparatus is unaffected by aging of any component except for the reference resistor.

3. A temperature measurement circuit according to claim 1, wherein the temperature-dependent sense resistance is a thermistor.