WO2016015154A1

WO2016015154A1 - Calibration of sensors for use with portable electronic devices

Info

Publication number: WO2016015154A1
Application number: PCT/CA2015/050718
Authority: WO
Inventors: Christian Leth Petersen
Original assignee: Lionsgate Technologies, Inc.
Priority date: 2014-08-01
Filing date: 2015-07-30
Publication date: 2016-02-04

Abstract

The present disclosure provides a method of calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics that includes connecting a sensor to the electronic device, receiving calibration data comprising a known calibration perturbation and a calibration voltage ratio for the sensor, applying a balancing voltage ratio across first and second arms of a resistive bridge of the unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at a central node is zero, applying the calibration voltage ratio across the first and second arms and detecting a measured calibration response signal at a central node, and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

Description

CALIBRATION OF SENSORS FOR USE WITH PORTABLE ELECTRONIC DEVICES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional Patent Application No.

62/032, 144 filed August 1 , 2014 and entitled CALIBRATION OF SENSORS FOR USE WITH PORTABLE ELECTRONIC DEVICES. For purposes of the United States, this application claims the benefit under 35 U.S.C. §1 19 of United States provisional patent application No. 62/032, 144 filed August 1 , 2014 and entitled CALIBRATION OF SENSORS FOR USE WITH PORTABLE ELECTRONIC DEVICES, which is hereby incorporated herein by reference for all purposes. FIELD OF THE INVENTION

[0002] The present disclosure relates generally to sensor calibration. More particularly, the present disclosure relates to systems and methods for calibrating sensors coupled to audio interfaces of electronic devices.

BACKGROUND OF THE INVENTION [0003] Conventional pulse oximeters, thermometers, blood pressure measurement devices, spirometers, perineometers, ECGs, EEGs, glucose monitors, and other devices for measuring physiological parameters are typically standalone units. Standalone electronic devices for measuring physiological parameters usually contain a power source, a microcontroller, local storage, and a custom display mechanism along with the basic circuit needed to perform the sensing. This makes for relatively complex systems, costly to manufacture, and with many points of potential failure. They are therefore limited in their functionality, difficult to upgrade, and/or relatively expensive.

[0004] International Patent Application Publications No. WO 2012/155245 and No.

WO 2013/170378, which are hereby incorporated by reference herein in their entireties, disclose systems and methods for operating external sensors connected to the audio port of an electronic device such as a smartphone or the like. For certain sensor and device combinations thereof the native audio hardware system of an electronic device may not provide sufficient power, signal conditioning, and/or channels. [0005] The inventors have determined a need for improved systems and methods for operating external sensors using portable electronic devices such as smartphones and the like.

SUMMARY [0006] One aspect of the present disclosure provides a method of calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics that includes connecting a sensor to an analog system interface of the electronic device, the sensor comprising a first arm with a first passive element, a second arm with a second passive element, and a central node between the first and second arms, receiving calibration data comprising a known calibration perturbation and a calibration voltage ratio for the sensor, the calibration voltage ratio comprising a ratio of voltages applied across the arms of the bridge that resulted in the response signal measured at the central node being zero when the known calibration perturbation was applied to the sensor, applying a balancing voltage ratio across the first and second arms of the

unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at the central node is zero, applying the calibration voltage ratio across the first and second arms of the resistive bridge and detecting a measured calibration response signal at the central node, and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

[0007] In another aspect of the present disclosure, the method includes using known properties of said passive sensor elements to determine an absolute reading of the unperturbed sensor, and adding the absolute reading to the calibrated sensor response, such that the total signal is representative of the absolute sensor disturbance.

[0008] In another aspect of the present disclosure, receiving the calibration data includes reading a code from the sensor.

[0009] In another aspect of the present disclosure, receiving the calibration data includes accessing a remote computing device where the calibration data is stored.

[0010] Another aspect of the present disclosure provides a computer program product for calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, the computer program product including a non-transitory computer readable medium storing instructions executable by a processor of the electronic device to cause the processor to carry out a method that includes detecting connection of a sensor to an audio interface of the electronic device, the sensor comprising a first arm with a first passive element, a second arm with a second passive element, and a central node between the first and second arms, receiving calibration data comprising a known calibration perturbation and a calibration voltage ratio for the sensor, the calibration voltage ratio comprising a ratio of voltages applied across the arms of the bridge that resulted in the response signal measured at the central node being zero when the known calibration perturbation was applied to the sensor, applying a balancing voltage ratio across the first and second arms of the unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at the central node is zero, applying the calibration voltage ratio across the first and second arms of the resistive bridge and detecting a measured calibration response signal at the central node, and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

[0011] Another aspect of the present disclosure provides a method of calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, the method including connecting a sensor to a calibration interface, the sensor comprising a first arm with a first passive element, a second arm with a second passive element, and a central node between the first and second arms, applying a known calibration perturbation to the sensor, recording a calibration voltage ratio applied to the first and second arms of the sensor, the calibration voltage ratio selected such that the response signal measured at the central node is zero, connecting the sensor to an audio interface of the electronic device, applying a balancing voltage ratio across the first and second arms of the unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at the central node is zero, applying the calibration voltage ratio across the first and second arms of the resistive bridge and detecting a measured calibration response signal at the central node, and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

[0012] Another aspect of the present disclosure provides a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, wherein the sensor is encoded with calibration data.

[0013] Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS [0014] Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0015] Figure 1 schematically illustrates an example temperature sensor.

[0016] Figure 2 schematically illustrates an example pressure sensor.

[0017] Figure 3 is a flowchart illustrating an example calibration method according to one embodiment.

[0018] Figures 4 to 7 schematically illustrate an example sensor at various stages of the calibration method of Figure 3.

[0019] Figure 8 is a flowchart illustrating an example calibration method implemented at a portable electronic device according to one embodiment. DETAILED DESCRIPTION

[0020] Generally, the present disclosure provides apparatuses, methods and systems for overcoming the limitations of typical low-cost mobile devices, and enabling them to effectively operate external sensors. The solutions disclosed herein have several different aspects including both hardware and software embodiments.

[0021] International Patent Application Publications No. WO 2012/155245 and No.

WO 2013/170378, which are hereby incorporated by reference herein, describe methods and systems for monitoring human physiological measures with various external sensors, including temperature and pressure sensors. Such sensors may have a "bridge"-type architecture with a pair of arms, each having one or more passive elements, connected to outputs (e.g. speaker contacts of an audio interface), and a central node between the pair of arms connected to an input (e.g. a microphone contact) of an audio interface.

[0022] Such bridged sensors typically have an intrinsic response that is relative to a static reference (whether known or unknown in value), and hence independent of the external bridge driving signal amplitudes and input channel gains. Conventional driving circuitry use a fixed known output voltage to power the bridge and a known input gain to convert this intrinsic relative sensor response to an absolute sensor reading.

[0023] In some cases, for example when using consumer grade audio components of a mobile phone to interface a sensor bridge, the input and output gains are unknown, and may vary greatly between devices. The inventors have previously disclosed methods that rely on bridge balancing to overcome unknown gain factors. However, for continuous mode (real time) operation, bridge balancing can be challenging, as it requires feedback and is subject to dynamic performance of a balancing algorithm. Furthermore, bridge balancing will determine only the relative relation between the reference impedance in the sensor and the sensing impedance. If the reference impedance is unknown, such as the static arm in a pressure sensor bridge, then an additional step is needed in order to arrive at an absolute sensor reading.

[0024] To overcome these challenges and limitations, the present specification provides methods and system that can calibrate an arbitrary bridge-type sensor. Certain embodiments provide a calibration method that provides a one-time two-point balancing process. As discussed further below, in some implementations, the first part of the balancing process may be performed by a calibration system operated by a sensor manufacturer, and the second part of the balancing process may be performed by a system running on a computing device operated by a user.

[0025] Although embodiments of the present disclosure include a sensor coupled to the audio interface of a portable electronic device, it is understood that the disclosed calibration method may be utilized in any system comprising separate sensor and an electronic balancing processor that are combined after the sensor has been partially calibrated without the electronic balancing processor present.

[0026] Figure 1 shows an example temperature sensor 100 connected to two speaker contacts SPK and a microphone contact MIC of an electronic device 10. A temperature sensitive circuit element 1 10 is connected between a first of the speaker contacts and the microphone contact MIC, and a reference circuit element 120 is connected between a second of the speaker contacts SPK and the microphone contact MIC. A central node 105 is located between the elements 1 10 and 120, and provides a response signal RES (also referred to as an error signal) to the microphone contact M IC. In some

embodiments, the temperature sensor 100 may include only a single thermistor as the temperature sensitive circuit element 1 10 and a single reference resistor as the reference circuit element 120. Such an arrangement would advantageously minimize the cost of the temperature sensor 100. However, it is to be understood that additional components may be included in other embodiments, such as for example, one or more additional resistors or thermistors connected in series or parallel. In some embodiments, temperature sensor 100 may also comprise a ground connection (not shown) at the central node 105. Example methods and systems for controlling such temperature sensors are disclosed in International Patent Application Publication No. WO 2013/170378.

[0027] Figure 2 shows an example pressure sensor 200 connected to two speaker contacts SPK, a microphone contact M IC and a ground contact of an electronic device 10. Pressure sensor 200 may, for example, comprise a piezoresistive bridge configured to measure any of gauge, differential and/or absolute pressure. Pressure sensor 200 may, for example comprise a commercially available piezoelectric pressure sensor (such as, for example, a MPX2010 Series Pressure Sensor from Freescale Semiconductor, Inc. , or a 2SMPP MEMS Gauge Pressure Sensor from Omron Electronic Components LLC). Such pressure sensors are specified to be provided with DC supply voltage, but may be operated by providing them with harmonic driving signals D and D₂ and measuring a response signal RES through the audio interface as described below. In the illustrated example, the piezoelectric pressure sensor 200 comprises a first pair of elements 210 and 215 connected to receive driving signal D and a second pair of elements 220 and 225 connected received driving signal D₂. A central node 205 is located between the elements 210 and 220, and provides a response signal RES (also referred to as an error signal) to the microphone contact MIC. In some embodiments, systems used to process the signal from the pressure sensor comprise an AC auto-balancing bridge that continuously adjusts the amplitude and phase relationship of the two audio output channels in order to minimize the bridge response signal at the microphone input, and a dual-phase locking-amplifier that does phase-locked detection of the response signal itself. Example methods and systems for controlling such pressure sensors are disclosed in International Patent Application Publication No. WO 2013/170378.

[0028] A bridge-type sensor may be considered as consisting of two impedances Z (a variable impedance that varies with a parameter to be measured) and Z_ref (a fixed reference impedance). Wth reference to Figure 1 and 2, impedance Z may correspond to element 110 of Figure 1 or elements 210 and 215 of Figure 2, and impedance Z_ref may correspond to element 120 of Figure 1 or elements 220 and 225 of Figure 2. The arms of the bridge are driven by voltages V and V_ref. (Di and D₂ in Figures 1 and 2). The error signal (RES in Figures 1 and 2) at the center of the bridge is then:

[0029] v_err = g (^ - ^^V ),

[0030] where g is a factor that depends on the input gain and stray input impedances specific to the device to which the sensor is connected. The bridge can be balanced by choosing V and V_ref such that the error signal is zero, i.e.

[0032] Perturbations in sensor impedance are generally small compared to the total impedance, (e.g. ΔΖ « Z). If the bridge is balanced at Z, the error signal from a sensor perturbation ΔΖ is then

[0034] In other words, the error signal is proportional to the change in impedance in the first order. A non-zero error signal can be generated by deliberately shifting the balance of the physically unperturbed bridge with a voltage perturbation AV:

[0035] ν_εΜΑν) = ₉ ( - ^^ν ) = ₉φ.

[0036] This equivalence may be used to calibrate the sensor, as discussed further below.

[0037] Figure 3 is a flowchart of an example sensor calibration method 300 according to one embodiment. A bridge-type sensor is connected to a calibration interface at 302. At 304, a known calibration perturbation (P_ca/, which causes corresponding sensor impedance Zcai as shown in Figure 4) is applied to the sensor. The voltages across the arms of the bridge are then adjusted to V_cai and V_ref to balance the sensor to make the error signal zero (V_err=0), as follows: [0038] (— - = 0 or— =— ,

\Zcal ^zrreeff / J ^z ^rreeff V ^v _Treeff

[0039] and the ratio of voltages that results in the balance

referred to as the

"calibration voltage ratio") is recorded at 306. In some embodiments, the steps of method 300 performed at 302, 304 and 306 may all occur at a factory or the like, and the results may be provided for use in calibration at an electronic device, as discussed below with reference to Figure 8.

[0040] Continuing with method 300 of Figure 3, at 308 the sensor is connected to the audio interface of an electronic device. The audio interface has an unknown input gain, and may also have unknown stray input impedances. The sensor is subjected to an ambient perturbation (P₀, which causes corresponding sensor impedance Z₀ as shown in Figure 5), which may be the ambient temperature in the case of a temperature sensor, or ambient (e.g. atmospheric) pressure in the case of a pressure sensor. The voltages across the arms of the bridge are then adjusted to V₀ and V_ref to balance the sensor to make the error signal zero (V_err =0), as follows:

[0041] - ii£i = o or

\Z₀ Zre J ^zref V_ref

[0042] and the ratio of voltages that results in the balance (Vt V_refl referred to as the

"balancing voltage ratio") is recorded at 310. At 312, the voltage calibration ratio

is applied across the arms of the bridge, wherein V_cai = V₀ + AV_cai, as shown in Figure 6. This voltage perturbation {A\/_cai) will generate the same magnitude error signal (V_err) as the calibration perturbation:

[0043] \ \V_err \z(W_cal) \ \ = \ \V_err \_v(Z_cal - Zo) \ \,

[0044] where Z₀ is the unperturbed sensor balance on the device. This yields:

[0045] AV_cal = V_ref - ²_) = V_ref

¹ \^zref ^zrefJ ¹ ( \^^vref-^ * ref-X J

[0046] This voltage perturbation corresponds to a balance ratio of

[0047] ^{V +AV}cal ₌∑caL_^

Vref V_ref

[0048] Thus, applying the same voltage ratio at 312 that generated the calibration balance at 306, then the device error signal should correspond to the calibration perturbation.

At 314 the calibration is completed by adjusting the gain factor to make the sensor output equal to the perturbation:

[0049] P(AV) = G_cal x V_err \_Z(AV) + P₀, [0050] where P₀ is the physical perturbation at the balance impedance Z₀ and

[0051] G_cal = P_cal/V_err \_z(W_cal).

[0052] In the case of a pressure sensor (e.g. for a blood pressure monitor), the pressure readings are relative to the ambient pressure, i.e. P₀ = 0, and the device calibration is thus complete. In the case of a temperature sensor, P₀ corresponds to the ambient temperature, which can be determined for example based on the balance reading Vo/V^_f, provided that Z_mf and the temperature coefficient of the sensing element are both known, which is typically the case for a thermistor bridge.

[0053] Once calibration is complete, the sensor can be operated normally (for example, as disclosed in International Patent Application Publication No. WO 2013/170378) by adjusting one or both of the driving voltages to minimize the error signal (X in Figure 7).

[0054] Figure 8 is a flowchart of an example sensor calibration method 300A according to another embodiment. The steps at 308-314 are the same as described above and will not be described again to avoid repetition. Method 300A differs from method 300 of Figure 3 in that at 301 an electronic device receives a calibration perturbation and calibration voltage ratio for a sensor (also referred to as "calibration data"). In the illustrated example, calibration data is shown as being received at 301 before the sensor is connected at 308, it is to be understood that the order of these steps could be reversed, or they could occur substantially simultaneously. In some embodiments, receiving calibration data may comprise reading an electronic code on the sensor. For example, a manufacturer could encode a sensor with the calibration data, for example by encoding the calibration data on an embedded a chip within the sensor circuit that may be queried by, for example RFID, or by embedding the calibration data within machine readable code stored in a memory of the sensor that may be accessible to the electronic device when coupled to the sensor. In some embodiments, receiving calibration data may comprise accessing a remote computing device, such as a server or the like, where the calibration data is stored. For example, a manufacturer may include a QR or other machine readable code with a sensor, and save the calibration data to a URL that may be accessed using the QR or other machine readable code.

[0055] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the understanding. For example, specific details are not provided as to whether the embodiments described herein are implemented as a software routine, hardware circuit, firmware, or a combination thereof.

[0056] Embodiments of the disclosure can be represented as a computer program product stored in a machine-readable medium (also referred to as a computer-readable medium, a processor-readable medium, or a computer usable medium having a computer- readable program code embodied therein). The machine-readable medium can be any suitable tangible, non-transitory medium, including magnetic, optical, or electrical storage medium including a diskette, compact disk read only memory (CD-ROM), memory device (volatile or non-volatile), or similar storage mechanism. The machine-readable medium can contain various sets of instructions, code sequences, configuration information, or other data, which, when executed, cause a processor to perform steps in a method according to an embodiment of the disclosure. Those of ordinary skill in the art will appreciate that other instructions and operations necessary to implement the described implementations can also be stored on the machine-readable medium. The instructions stored on the machine- readable medium can be executed by a processor or other suitable processing device, and can interface with circuitry to perform the described tasks.

[0057] The above-described embodiments are intended to be examples only.

Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art. The scope of the claims should not be limited by the particular embodiments set forth herein, but should be construed in a manner consistent with the specification as a whole.

Claims

WHAT IS CLAIMED IS:

1. A method of calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, the method comprising: connecting a sensor to an analog system interface of the electronic device, the sensor comprising a first arm with a first passive element, a second arm with a second passive element, and a central node between the first and second arms; receiving calibration data comprising a known calibration perturbation and a calibration voltage ratio for the sensor, the calibration voltage ratio comprising a ratio of voltages applied across the arms of the bridge that resulted in the response signal measured at the central node being zero when the known calibration perturbation was applied to the sensor; applying a balancing voltage ratio across the first and second arms of the unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at the central node is zero; applying the calibration voltage ratio across the first and second arms of the resistive bridge and detecting a measured calibration response signal at the central node; and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

2. The method of claim 1 further comprising: using known properties of said passive sensor elements to determine an absolute reading of the unperturbed sensor; and adding the absolute reading to the calibrated sensor response, such that the total signal is representative of the absolute sensor disturbance.

3. The method of claim 1 wherein receiving the calibration data comprises reading a code from the sensor.

4. The method of claim 1 wherein receiving the calibration data comprises accessing a remote computing device where the calibration data is stored.

5. A computer program product for calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, the computer program product comprising a non-transitory computer readable medium storing instructions executable by a processor of the electronic device to cause the processor to carry out a method comprising: detecting connection of a sensor to an audio interface of the electronic device, the sensor comprising a first arm with a first passive element, a second arm with a second passive element, and a central node between the first and second arms; receiving calibration data comprising a known calibration perturbation and a calibration voltage ratio for the sensor, the calibration voltage ratio comprising a ratio of voltages applied across the arms of the bridge that resulted in the response signal measured at the central node being zero when the known calibration perturbation was applied to the sensor; applying a balancing voltage ratio across the first and second arms of the unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at the central node is zero; applying the calibration voltage ratio across the first and second arms of the resistive bridge and detecting a measured calibration response signal at the central node; and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

6. A method of calibrating a sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, the method comprising: connecting a sensor to a calibration interface, the sensor comprising a first arm with a first passive element, a second arm with a second passive element, and a central node between the first and second arms; applying a known calibration perturbation to the sensor; recording a calibration voltage ratio applied to the first and second arms of the sensor, the calibration voltage ratio selected such that the response signal measured at the central node is zero; connecting the sensor to an audio interface of the electronic device; applying a balancing voltage ratio across the first and second arms of the unperturbed sensor, the balancing voltage ratio selected such that a response signal measured at the central node is zero; applying the calibration voltage ratio across the first and second arms of the resistive bridge and detecting a measured calibration response signal at the central node; and adjusting the input and output characteristics of the analog sensor interface based on a difference between the calibration voltage ratio and the balancing voltage ratio such that the measured calibration response signal is representative of the known calibration perturbation.

7. A sensor for use with an electronic device having an analog sensor interface with unknown input and output characteristics, wherein the sensor is encoded with calibration data.