DESCRIPTION

TECHNICAL FIELD

The invention relates to an electronic sensor with a sensor circuit based on a sense element and a reference element . The invention further relates to a corresponding method .

BACKGROUND

Many sensors use a detector whose resistance may change depending on a parameter measured, e . g . , a thermistor for temperature measurement or a resistor whose resistance changes with pressure . In such sensors the sensor circuit determining the temperature ( or pressure ) measures the "detector/sensor" resistance with respect to a reference resistor . Slightly more formally and generally speaking, the resistance or a related electrical property of a sense element is measured and compared with ( or set in relation to during measurement ) a corresponding electrical property of a reference element , to derive a measurement value for a physical property, in particular an ambient physical property, on which said electrical property of said sense element measurably depends ( and varies with) .

However, in the speci fic introductory example the resistance of the reference resistor may dri ft/change with usage , more speci fically with repeated or long exposure to current and/or voltage . I f the reference resistor dri fts with usage , this leads to a systematic dri ft with time of the physical parameter sensed . Again, this problem statement can be slightly generali zed in that an electrical property of a reference element on which the measurement principle is based may dri ft in time , therefore leading to a measurement error . SUMMARY

An obj ective underlying the present invention is to provide an electrical sensor of the kind described above whose measurement or output is largely unaf fected by a dri ft of electrical properties of electrical components or elements . Preferably, such a sensor shall be easy and cost-ef ficient to reali ze and to operate . Furthermore , a corresponding method of operating an electronic sensor shall be provided .

According to claim 1 the device-related obj ective is met by an electronic sensor with a sensor circuit based on a sense element and a reference element , further comprising a control unit , wherein in a measurement mode an electrical property of the sense element is measured and compared to an electrical property of the reference element to derive a measurement value for a physical property on which said electrical property of said sense element measurably depends , wherein said sensor further comprises a reference calibration element with a known electrical property, which, in a calibration mode , can be connected to said reference element , and wherein said control unit is configured to compare said electrical property of said reference element to said electrical property of said reference calibration element , to calibrate the measurement and to correct said measurement value for a dri ft in time of said electrical property of said reference element .

Hence , rather than minimi zing the stress on the reference element to minimi ze the dri ft or shi ft in time , or rather than tolerating a related measurement error, a second reference element , called reference calibration element , is used to calibrate the first reference element at least from time to time and therefore to correct the measurement value for the dri ft-related ef fects . Hence , while the dri ft or degradation of the first reference-element is not prevented per se , the accuracy of the sensor is not compromised over the li fetime of the part . One may conclude that the measurement is drift-invariant due to the automatic correction .

Preferably, the control unit is configured such that in measurement mode said reference calibration element is not exposed to an electric current and/or an electric voltage.

This exploits the feature that it usually takes time for the electrical (or other) stress to shift the properties of a part. With the secondary (or tertiary) copies of the reference element, i.e., the reference calibration element (s) , spending significantly lower times under stress, the drift on these elements is minimized. Thus, the reference calibration element (s) can be used to reliably calibrate the original reference using this/these additional element (s) .

In a particularly useful application said physical property to be measured is a temperature. Hence, the sensor is a temperature sensor.

Expediently, said sense element is a sense impedance, said reference element is a reference impedance, and said reference calibration element is a reference calibration impedance. In general, each impedance may comprise resistive, capacitive and/or inductive parts.

However, in an advantageously simple case there are only resistive parts. That is, said sense impedance is a sense resistor, said reference impedance is a reference resistor, and said reference calibration impedance is a reference calibration resistance. Consequently, said electrical property of said sense element preferably is a resistance, said electrical property of said reference element is a resistance, and said electrical property of said reference calibration element is a resistance.

In the already mentioned case of a temperature sensor said sense element preferably is a thermistor. In a preferred embodiment , in measurement mode , said sense element and said reference element are arranged in series , hence being in a voltage divider arrangement . Similarly, in calibration mode , said reference element and said reference calibration element are preferably also arranged in series .

Furthermore , said control unit is advantageously configured to initiate said calibration mode at pre-determined or dynamically adj usted time intervals and preferably exits said calibration mode after calibration . In a preferred simple case calibration is initiated at regular intervals .

Furthermore , said control unit is preferably configured to apply the correction for the dri ft in real-time ( so-called online correction) .

There are countless applications of the described concept in di f ferent fields , like automotive or computers or smartphones . However, in a notably advantageous application, there is a portable device , in particular a wearable device , or a biomedical device , with a sensor of the kind described herein, in a particular a temperature sensor .

In terms of method, the present invention provides a method of operating an electronic sensor, in particular a sensor of the kind described herein, comprising the steps of :

• measuring an electrical property of a sense element and comparing it to an electrical property of a reference element to derive a measurement value for a physical property on which said electrical property of said sense element measurably depends ,

• comparing and calibrating said electrical property of said reference element at least once or from time to time to an electrical property of a reference calibration element , to derive and quanti fy a dri ft in time for said electrical property of said reference element ,

• correcting said measurement value for said dri ft . Advantageously, said reference calibration element is exposed to an electrical current and/or an electrical voltage only during the comparing and calibrating step, but not at the actual measurement step .

What has been said with respect to the device may analogously be applied to the method, and therefore need not be repeated here . Device embodiments and details have a counterpart in the method . Again, no explication is required here in view of the above description .

BRIEF DESCRIPTION OF DRAWINGS

Further below, exemplary embodiments of the invention are discussed with reference to the accompanying drawings .

FIG . 1 shows in a schematical manner an electrical circuit of a known sensor, in particular a temperature sensor .

FIG . 2 shows in a schematical manner an electrical circuit of a sensor according to the invention, in particular a temperature sensor .

DETAILED DESCRIPTION

FIG . 1 illustrates in a schematical manner core elements of a temperature measurement device , also known as a temperature sensor 2 , according to the prior art . As will be discussed further below, the temperature sensor is 2 exemplary for other types of sensors as well . The temperature measurement provided by the temperature sensor 2 is based on the well- known principle of a resistive voltage divider circuit 4 . Again, the voltage divider circuit 4 is exemplary for a more general sensor circuit . The actual sensor probe is reali zed by an electrical resistor, called sense resistor 6 , whose electrical resistance has a relatively strong dependency on temperature , at least in the proj ected or intended temperature interval of operation, also called working temperature range . That is , relatively small temperature changes lead to relatively large changes in resistance which can be accurately measured with the help of the voltage divider circuit 4 . To this end, the sense resistor 6 is wired or connected in series with a resistor of known resistance , called reference resistor 8 , to form a voltage divider . Preferably, the resistance of the reference resistor 8 has a much weaker dependence on temperature than the resistance of the sense resistor 6 and can therefore be regarded fixed or constant in time , at least in the working temperature range , during operation of the temperature sensor 2 .

Alternatively or additionally, the reference resistor 8 may be kept at essentially a constant temperature during operation of the temperature sensor 2 , for example by virtue of thermal insulation and/or by spatial separation from the sense resistor 6 or the sensing region .

To measure the resistance or a related electrical property of the sense resistor 6 , a known voltage or electrical potential di f ference is applied across the divider, i . e . , across the series connection of the sense resistor 6 and the reference resistor 8 . For example , in the simplest case a DC (= direct current ) input voltage is applied and/or measured at electrical contacts kO and k2 in the schematic circuit according to FIG . 1 . This leads to distributing the input voltage among the components of the divider, a phenomenon also known as a voltage division, according to Ohm' s law . By measuring the voltage across the sense resistor 6 and comparing or relating it to the voltage across the reference resistor 8 ( or alternatively to the input voltage across the divider ) , the resistance or a related electrical property of the sense resistor 6 can be derived . For example , the voltage across the sense resistor 6 can tapped at electrical contacts kO and kl , and the voltage across the reference resistor 8 can be tapped at electrical contacts kl and k2 . Based on the known resistance of the reference resistor 8 and on the known or given temperature-resistance characteristic of the sense resistor 6 , the temperature prevailing at the location of the sense resistor 6 can be calculated or derived . Typically, the assessment or derivation of the ambient temperature involves calculating ratios of at least some of the above-mentioned voltages to derive the resistance or a related quantity for the sense resistor 6 and then looking up or applying said temperature-resistance characteristic to derive the temperature at the location of the sense resistor 6. This assessment is usually done or executed in an associated control unit or analysis unit which can be an analog and/or a digital circuit . In the case of a digital circuit the measured analog voltages are converted into related digital quantities by the help of an AD (= analogdigital ) converter and then further processed in the digital regime or domain . In a preferred embodiment , a suitable digital assessment can be provided by a multi-purpose computer or controller running or executing a dedicated software .

Instead of the electrical resistance a related quantity like the electrical conductivity (= reciprocal of resistance ) or similar can be used in the calculations .

One problem related to a temperature senor 2 according to FIG . 1 is that certain physical properties , in particular the resistance of the sense resistor 6 and/or the reference resistor 8 , may slowly change or dri ft over time . While in typical embodiments the sense resistor 6 may be designed or chosen such that the dri ft of its properties over time may be negligible , this is generally not the case for the reference resistor 8 . Rather, the resistance of the reference resistor 8 may dri ft over time in a noticeable manner during the li fetime of the temperature sensor 2 . In particular, it has been found that the resistance of the reference resistor 8 may dri ft or change essentially proportional with usage or with the intensity of electrical stress applied . In other words , the magnitude of the total dri ft usually relates to repeated and/or long exposures to electrical current and/or voltage . For example , the higher the accumulated or integrated electrical current and/or voltage applied is , the higher the total dri ft usually is .

One possible solution to deal with this problem would be to minimi ze the exposure of the reference resistor 8 to electrical current and/or voltage by reducing the amount of current and/or current density and/or voltage . Additionally or alternatively, one may modi fy the circuit design and/or the mode of operation to reduce the time for which the reference resistor 8 may be exposed to such currents and/or voltages . However, such solutions impose restrictions on the circuit and/or the mode of operation and may not be suited for implementation in practical context , in particular with already existing circuits . Furthermore , i f there is a demand for continuous monitoring, it may not be possible to restrict or shorten the exposure time .

Another possible solution would be to use a reference resistor 8 with a low enough dri ft for the design speci fication of the sensor circuit . For example , one may choose a high-quality reference resistor 8 with an empirically proven small or low dri ft . Apart from the fact that such an electronic part might be expensive or even di f ficult to get at all , this does not prevent that in certain exceptional cases the dri ft might be higher than indicated in the part speci fication .

Finally, one might simply tolerate the dri ft of the reference resistor 8 . However, this might not be allowed for certain applications . Even i f the dri ft is tolerable to some extent , one would prefer to have at least an estimate of the magnitude of the ef fect .

To overcome all these problems , the present invention suggests comparing and calibrating the reference resistor 8 at least from time to time against another resistor of known resistance , called reference calibration resistor 10 , and to compensate or correct the measured or derived temperature values for the dri ft of the reference resistor' s 8 resistance. As explained below in more detail, the reference calibration resistor 10 is advantageously used only sparingly, in particular only during relatively short (in comparison to the total working period or operation time of the temperature sensor 2) periods of calibration and is otherwise preferably not exposed to electrical current and/or voltage, such that its degradation in time or drift in resistance is negligible.

This concept is illustrated schematically in FIG. 2 which builds on FIG. 1 and extends the sensor circuit known therefrom. Basically, there is a junction 12 in the electrical connection line 14 between the sense resistor 6 and the reference resistor 8 at which an electrical branch line 16 is connected (or branches off, depending on view) . The branch line 16 comprises the reference calibration resistor 10. The connection line 14 comprises electrical switches SI and S2, SI being arranged between the sense resistor 6 and the junction 12, and S2 being arranged between the junction 12 and the reference resistor 8. Furthermore, there is a switch S3 in the branch line 16 between the junction 12 and the reference calibration resistor 10. In a simple schematic embodiment all the three switches SI, S2, S3 are simple on-off switches: The two terminals of the respective switch are either connected together or disconnected from each other.

If, in a first configuration, the switches SI and S2 are closed while switch S3 is open, reference calibration resistor 10 is disconnected from the circuit and the sensor circuit of FIG. 1 is replicated. Then, everything already said above regarding this circuit also holds true in this case. Hence, there is no need to repeat it here.

If on the other hand, in a second configuration, switches SI and S3 are closed while switch S2 is open, the reference resistor 8 is disconnected from the circuit while reference calibration resistor 10 takes its place. This means that a measurement circuit with a voltage divider similar to the original one is reali zed in which reference calibration resistor 10 takes the part of the former reference resistor 8 . This way, reference calibration resistor 10 may act as a replacement or backup for the reference resistor 8 in an alternative voltage divider circuit of a temperature sensor 2 , in particular i f both their resistances are the same or at least have a comparable magnitude .

In a third configuration switches S2 and S3 are closed while switch S I is open, thereby disconnecting or decoupling the sense resistor 6 from the measurement circuit . Instead, reference resistor 8 and reference calibration resistor 10 are now connected in series in a voltage divider arrangement . In this configuration it is possible to compare the reference resistor 8 to the reference calibration resistor 10 by measuring the corresponding voltage drops , analogously as described above for the measurement circuit in the first configuration . In particular the assessment may involve measuring and/or calculating a resistance ratio for the reference resistor 8 versus the reference calibration resistor 10 . Given the known resistance value of the reference calibration resistor 10 one can derive the present resistance value of the reference resistor 8 which may have dri fted from its original value at time zero . This information in turn can be used, after switching back to the first configuration, to accurately derive the temperature of the sense resistor 6 , for example from a resistance ratio for the sense resistor 6 versus the reference resistor 8 .

In other words , in the third configuration of the circuit the reference calibration resistor 10 may be used from time to time to calibrate the reference resistor 8 - which in the first configuration is used as a reference for the sense resistor 6 during actual temperature measurement . Hence , the third configuration corresponds to a calibration mode while the first configuration corresponds to the actual temperature measurement mode . In the second configuration the reference calibration resistor 10 may be used as a backup or reserve reference resistor for the temperature measurement , for example i f it is found that the reference resistor 8 has degraded or dri fted beyond a point where correction is useful .

Preferably, the reference calibration resistor 10 used to calibrate the reference resistor 8 is used only sparingly, when, and only when the circuit is deliberately brought into calibration mode ( and soon afterward exits this mode ) . Only then, the reference calibration resistor 10 is exposed to electrical voltage and/or current . Therefore , during li fetime of the temperature sensor 2 the dri ft of the reference calibration resistor 10 is negligible . Hence , the relative shi ft in the reference resistor 8 with respect to the reference calibration resistor 10 may be determined and used to determine the absolute extent of dri ft in the reference resistor 8 . In the subsequent temperature measurement a suitable dri ft correction may be applied, yielding a very accurate dri ft invariant measurement .

Calibration mode may be entered every once in a while , or at given time intervals , in particular periodically . Preferably, the period or interval length between calibration cycles may be dynamically adj usted, in particular with respect to the degree of dri ft of the reference resistor 8 determined before . For example , i f the calibration history indicates that the dri ft of the reference resistor 8 has been relatively low throughout , the interval length may be chosen large in comparison to the case where increasing or accelerating dri ft per unit time interval has been detected .

As already discussed in connection with FIG . 1 there is preferably a control unit 20 ( indicated here in FIG . 2 in a purely schematical manner ) or analysis unit associated with the measurement circuit of FIG . 2 which receives the raw measurement data or measured values , in particular measured voltages , and determines or derives the prevailing temperature ( or any other senor output ) from these data . Preferably, the control unit 20 also controls the switches SI , S2 , S3 and therefore the switching between measurement mode and calibration mode at arbitrary (e.g. user-initiated) , or predetermined or dynamically assigned intervals. To this end, said switches SI, S2, S3 are preferably electronically controlled switches. The control unit 20 preferably also does the necessary calculations for assessing the drift of the reference resistor 8 and/or for correcting the sensor output for the drift related effects. In other words, the control unit 20 implements suitable routines for the calibration of the temperature sensor 2 and for the related correction of measured temperature values. This correction is preferably done in real-time during measurement operation, i.e. in measurement mode. As mentioned before, the control unit 20 and the associated measurement, calibration and/or correction routines may be implemented in either hardware, software, or a combination thereof.

In one exemplary embodiment the reference resistor 8 or the temperature sensor 2 as a whole has a planned or expected lifetime of, e.g., 10 years. The resistance ratio of the reference resistor 8 to the reference calibration resistor 10 (or the inverse value) is measured at an initial time t=0 and stored in a corresponding control unit 20. The initial time t=0 may for example be triggered when the temperature sensor 2 leaves the factory or is taken into operation. Then, calibration may be scheduled for, e.g., every 10% of the part lifetime, in this case at the end of each year. Hence, every year said resistance ratio is measured and compared to the initial value at time t=0. Based on the change in said ratio since time zero, the drift in the reference resistor 8 can be corrected for by processing the real-time measuring data obtained in temperature measurement mode.

To summarize: The exemplary reference resistor 8 of the temperature sensor 2 described above drifts slowly during the life of the part due to usage, in particular due to the passage of current and/or the exposure to voltage. If not corrected for, this results in increased measurements errors (for example 5%) over the lifetime of the part. If, however, accuracy requirement necessitate a smaller error (for example 0 , 5% ) , the circuit arrangement according to the present invention, using the described sel f-calibrating technique , facilitates reaching the requirement with only moderate additional hardware ef forts . In fact , most of the evaluation and calibration may be executed in an easy-to— implement and even retro- f ittable software routine of a control unit 20 which typically is already present in any case .

A person skilled in the art will readily recogni ze a number of generali zations of the above-described concept :

Firstly, there may be more than one reference calibration resistor 10 . Rather, there may be several of reference calibration resistors , for example being arranged in parallel branch lines with corresponding switches , such that preferably for any given time during calibration operation a single one among them may be activated . Hence , one calibration can be checked against another .

Furthermore , while hitherto a temperature sensor 2 based on a temperature-dependent sense resistor 6 and a corresponding reference resistor 8 has been described, the sense resistor 6 or more generally an electrical sense element may alternatively comprise an electrical resistance which varies in a measurable way with a physical property other than temperature , for example pressure ( e . g . of a surrounding gas or atmosphere or applied by a contact element ) , or luminosity of incident light . Hence , in general the sensor 1 may be a pressure sensor, or a luminosity sensor or any other sensor, based on a resistive sense element whose resistance varies with the physical or chemical quantity of interest . In such cases , the sense resistor 6 or FIG . 2 is replaced by the according sense element , but the rest of the circuit and the description basically remains unaltered ( one might then call the measurement circuit an impedance divider circuit ) . Generally, the sense element may also be a semiconductor element . Finally, the described concept can be extended even one step further such that instead of sense resistor 6 , a reference resistor 8 , and a reference calibration resistor 10 a sense element 24 , a reference element 26 , and a reference calibration element 28 respectively are used, each of them being in general a complex impedance which may comprise at least one of or several of resistive , capacitive , and/or inductive properties . Generally, instead of a DC (= direct current ) voltage or current source , an AC (= alternating current ) source may be used .

All the indicated generali zation can be combined with each other in arbitrary combinations .

The sensor probe , in particular the sense resistor 6 , may be spatially separated from the rest of the circuit , including the reference resistor 8 and the reference calibration 10 resistor, and/or from the control unit 20 . The reference resistor 8 and the reference calibration resistor 10 and optionally the control unit 20 or parts thereof may be arranged within a common housing 30 , indicated in FIG . 2 by dashed lines .

The control unit 20 may be distributed over several devices , for example , there may be one hardware-close control unit providing low-level functions like signal converting and the like , and a control unit providing high-level calculations , assessments , and the like , preferably implemented as software routines .

Particularly suited applications may comprise sensors , in particular resistor-based sensors , notably temperature sensors , for example in the field including but not limited to smartphones , computers , automotive , and/or bio-medical applications .

In a speci fically advantageous application the senor 1 is a biosensor intended to be attached to or implemented into or wearable by an animal or human being . For example , the sensor may be a body temperature sensor, for example within or on a wearable device .

LIST OF REFERENCE SIGNS

1 sensor

2 temperature sensor

4 voltage divider circuit

6 sense resistor

8 reference resistor

10 reference calibration resistor

12 j unction

14 connection line

16 branch line

20 control unit

22 sense element

24 reference element

26 reference calibration element

30 housing k0 , kl , k2 electrical contact

SI , S2 , S3 electrical switch