Abstract:
A resistance is measured without physical contact/connection to the resistance. A resistive element to be measured is provided, physically connected as part of a passive electrical circuit. Without physically contacting the resistive element, an electromagnetic field is used to produce an excitation in the passive electrical circuit. The resistance of the resistive element is determined based on an effect of the excitation on the electromagnetic field.

Description:
FIELD 
       [0001]    The present work relates generally to measuring electrical parameters and, more particularly, to contactless measurement of resistance. 
       BACKGROUND 
       [0002]    There are numerous circumstances where contactless measurement of a resistance, from a remote measurement position across a gap of material (or air) from the resistance, is desirable. One example is measuring a thermistor resistance buried in a completely sealed device (e.g., a Li-ion battery pack). Contactless measurement (also referred to herein as non-contact measurement) of this resistance can provide information (correlated to temperature) needed for operational qualification and protection of the remote system. Numerous other applications employ thermistors that are not practically accessible through contacts, e.g., fluid temperature measurements in automotive applications, and rotating machinery such as motor windings. 
         [0003]    Another example is resistance measurement for configuration control. If a resistor buried within a product is measured by an external contactless system, the measurement result can provide an indication of product configuration. For example, if a resistor embedded in a high volume product has resistance indicative of unique information such as lot code, authentication, expiration date, or other parametric information (such number of battery cells, container size, lot calibration information), then measurement of that resistance identifies the parameter(s). 
         [0004]    There are other situations where contact measurements of resistance are possible, but problematic. One example is measuring the resistance of medical fluids. For instance, the resistance of a blood/reagent mixture (e.g., blood/glucose mixture) can be correlated to useful information (e.g., blood/glucose level) about the blood. The resistance measurement is conventionally performed by depositing the mixture on a test strip that is then inserted into a meter via a connector. The meter uses current sources and voltage measurements to determine the resistance of the blood/reagent mixture. However, after enough test cycles, the connector is prone to contamination and eventual failure. Significant material and process costs are required in order to make the connector interface between test strip and meter connector interface sufficiently robust to ameliorate the contamination/failure problem. Cost-effective contactless measurement of the blood/reagent mixture resistance could beneficially eliminate the need for the connector interface. 
         [0005]    Although there are known techniques for making contactless voltage measurements, there is not currently available a technique to implement contactless resistance measurements, much less to do so in a simple and cost-effective manner. 
         [0006]    It is desirable in view of the foregoing to provide for simple and cost-effective contactless measurement of resistance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  diagrammatically illustrates circuitry used for non-contact resistance measurement according to example embodiments of the present work. 
           [0008]      FIG. 2  diagrammatically illustrates an arrangement that supports non-contact measurement of resistance according to example embodiments of the present work. 
           [0009]      FIG. 3  diagrammatically illustrates an arrangement that supports test data collection according to example embodiments of the present work. 
           [0010]      FIG. 4  illustrates test data collection operations according to example embodiments of the present work. 
           [0011]      FIG. 5  illustrates graphically a database produced by test data collection operations according to example embodiments of the present work. 
           [0012]      FIG. 6  illustrates operations for obtaining a calibration relationship according to example embodiments of the present work. 
           [0013]      FIG. 7  illustrates non-contact resistance measurement operations according to example embodiments of the present work 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Example embodiments of the present work exploit capabilities of the LDC1000 Inductance-to-Digital Converter. This conventional measurement device is commercially available from Texas Instruments Incorporated, and its corresponding Datasheet is submitted herewith and incorporated herein by reference. The LDC1000 is designed for measurement of eddy current losses to support positional and proximity measurements. The present work recognizes that the LDC1000 may be used to obtain accurate, contactless measurements of a remote resistance as described in detail below. 
         [0015]    Referencing  FIG. 1 , and as described in the Datasheet, the LDC1000 makes a measurement of a “virtual resistance” Rs(d) that is associated with eddy current losses in a metal target (not shown in  FIG. 1 ) located at a distance apart from the LDC1000. The LDC1000 controls the operation of voltage source Vs, thereby producing an electromagnetic field E as a function of inductor  15  and capacitor  16 . The field E generates eddy currents in the aforementioned metal target, and the LDC1000 measures energy injected back into the inductor/capacitor arrangement  15 / 16  due to changes in the field E caused by the eddy currents. 
         [0016]    Example embodiments of the present work utilize an inductor  17  and (optionally) a tuned capacitor  18  connected in parallel with the remote resistive element  11  that is to be measured without contact, to form a passive electrical circuit  19 . So, for example, the entire passive circuit  19  would be embedded in a high volume product instead of just the resistance  11 . In some embodiments, inductor  17  is the same construction as (i.e., identical to) inductor  15 , and capacitor  18  is the same as capacitor  16 . The LDC1000 may be used in the same manner as described in the Datasheet, but with the aforementioned metal target replaced by the passive circuit  19  as shown in  FIG. 1 . In conventional use of the LDC1000, a “PROXIMITY” reading is produced by the LDC1000&#39;s graphical user interface (GUI) companion software, and this reading corresponds to the aforementioned eddy current losses in the metal target. According to example embodiments of the present work, the PROXIMITY reading produced by the GUI software correlates to the resistance of the resistive element  11 . 
         [0017]      FIG. 2  diagrammatically illustrates an arrangement that supports contactless remote measurement of resistance according to example embodiments of the present work. The arrangement of  FIG. 2  maintains a predetermined positional relationship between the LDC1000 and the passive circuit  19  (embedded in an enclosing material in some embodiments). As shown, some embodiments use a variable spacer  20  (e.g., a polycarbonate material) that maintains the LDC1000 and the passive circuit  19  separated by a selected spacer distance (see also d in  FIG. 1 ), with the winding axes of the inductors  15  and  17  maintained in substantially coaxial alignment with one another, as shown generally by axis  21 . Numerous suitable techniques and structures are conventionally available for use in effecting and maintaining the aforementioned predetermined positional relationship, and their application for such purposes is well within ordinary skill in the art. The LDC1000 is connected to a suitable computer  24  via a suitable connector  22  and cable  23  assembly. In some embodiments, the computer  24  is a desktop or laptop personal computer, and the cable/connector assembly  22 / 23  is a USB assembly. 
         [0018]    The LDC1000 is capable of providing its PROXIMITY reading to its companion GUI software on the computer  24 . The variable spacer  20  permits collection of PROXIMITY readings for a plurality of known separation distances (see d in  FIG. 1 ) between the LDC1000 inductor  15  and the inductor  17  of the passive circuit  19 . For a given distance and unknown resistance  11 , the PROXIMITY reading can be evaluated relative to corresponding test data, namely, PROXIMITY readings taken at the same distance for a plurality of different known resistances. By this evaluation, the resistance of the resistive element  11  may be determined (or interpolated or inferred). 
         [0019]    In some embodiments, the aforementioned test data is collected by providing a passive circuit having inductor  17  (and optionally capacitor  18 ) connected in parallel with a variable resistance. With the LDC1000 and the passive circuit separated by a selected distance, PROXIMITY readings are taken for a plurality of different known resistances. The process may then be repeated for each of a plurality of different separation distances. 
         [0020]      FIG. 3  diagrammatically illustrates an arrangement that supports test data collection according to example embodiments of the present work. In some embodiments, the arrangement of  FIG. 3  is the same as  FIG. 2 , except that a variable resistor  35  (e.g., a potentiometer in some embodiments) replaces unknown resistance  11  in passive circuit  19  (see  FIGS. 1 and 2 ). The variable resistor  35  is arranged in parallel with inductor  17  (and capacitor  18  in some embodiments) to construct a “test” passive circuit for test data collection. This “test” passive circuit is designated generally by  19 ′ in  FIG. 3 . The variable resistor  35  can provide a plurality of known resistance values for use in test data collection. As shown in  FIG. 3 , the variable resistor  35  is removably connected into the passive circuit  19 ′ by suitably disconnectable jumper wires  39 , and is also removably connected to an ohm meter  38  by similar disconnectable jumper wires. This permits the variable resistor  35  to be removed from the “test” passive circuit  19 ′, then connected to the ohm meter  38 , then set to a desired resistance using the ohm meter  38 , then disconnected from the ohm meter  38 , and then reconnected into the passive circuit  19 ′ for a test measurement. 
         [0021]    For calibration purposes, some embodiments provide a “calibration” passive circuit that is constructed the same as the passive circuit  19  in the actual product whose resistance will be measured (e.g., configured as in  FIG. 1  and embedded in a high volume product), but containing one of the aforementioned “known” resistance values (e.g., 10 K ohms +/−1%) in place of the unknown resistance  11  of  FIG. 1 . This “calibration” passive circuit is disposed to be nearly co-located with the passive circuit  19  (see  FIGS. 1 and 2 ) that is provided in the actual product and whose resistance  11  is unknown. Measurements of the known resistance of the calibration circuit, which known resistance is also measured during the aforementioned test data collection, provides an opportunity to determine a calibration relationship to be applied between measurements of the unknown resistance and measurements of known resistances taken during test data collection. In this manner there may be effected reductions in the impacts of factors such as variability in coupling, nearby metal objects and, more generally, differences between operating conditions in effect during test data collection and operating conditions in effect during unknown resistance measurement. Additional description of determining the calibration relationship appears further below. 
         [0022]      FIG. 4  illustrates test data collection operations described above according to example embodiments of the present work. With the spacer distance set at  41 , and the resistance (variable resistor) set at  42 , a PROXIMITY reading is taken at  43 , and then recorded at  44  together with the spacer distance and resistance. As shown at  45 , the operations at  42 - 44  are repeated for a plurality of different (known) resistances. As shown at  46 , the operations at  41 - 45  are repeated for a plurality of spacer distances. 
         [0023]    After test data collection is completed, the information that has been recorded at  44  provides, for each of a plurality of spacer distances, a plurality of resistances and their respectively corresponding PROXIMITY readings. This information may be used to construct a suitable database (DB) from which a resistance value may be obtained for a given combination of a PROXIMITY reading and a spacer distance. One example of such a database is shown graphically at  50  in  FIG. 5 , where each of the PROXIMITY versus resistance curves  51  corresponds to a respective one of the noted spacer distances. The leftmost curve corresponds to the largest noted spacer distance (146 mils in the  FIG. 5  example), and the spacer distances decrease from left to right, with the rightmost curve corresponding to the smallest noted spacer distance (38 mils in the  FIG. 5  example). For a given PROXIMITY reading taken at a given spacer distance, the information provided by the curves  51  may be used to determine the unknown resistance of the resistive element  11 . 
         [0024]      FIG. 6  illustrates operations described above for obtaining a calibration relationship according to example embodiments of the present work. At  61 , a known resistance is set within a “calibration” passive circuit that is otherwise constructed (e.g., embedded in a product, etc.) the same as the passive circuit  19  containing the unknown resistance  11  (see also  FIGS. 1 and 2 ). The spacer distance is set at  62 . A PROXIMITY reading is taken at  63 , under operating conditions that approximate, as nearly as is practical, those in which PROXIMITY readings for the unknown resistance are taken. The PROXIMITY reading is used at  64 , together with the spacer distance, to obtain the corresponding resistance from the database produced by the operations of  FIG. 4 . At  65 , the known resistance, as set at  61 , and the database resistance obtained at  64  are used (e.g., compared) to establish a calibration relationship for resistances measured at the current spacer distance, as set at  62 . As shown at  66 , the operations at  62 - 65  may be repeated to obtain calibration relationships for a plurality of spacer distances. 
         [0025]      FIG. 7  illustrates operations described above for non-contact resistance measurement according to example embodiments of the present work. After setting the spacer distance at  71 , a PROXIMITY reading is taken at  72 . At  73 , the PROXIMITY reading and spacer distance are used to obtain a resistance from the database produced by the operations of  FIG. 4 . In some embodiments, the obtained resistance is then calibrated at  74 , using the calibration relationship produced by the operations of  FIG. 6 , to produce a calibrated resistance. Some embodiments omit calibration, as shown by broken line. As shown at  75 , some embodiments repeat the operations at  71 - 74  for a plurality of spacer distances. At  76 , the desired resistance determination is made. In various embodiments, the determination at  76  is one of: simply a single database resistance value obtained using a singe PROXIMITY reading taken at a single spacer distance; a single calibrated resistance value determined by calibrating a singe database resistance value obtained using a single PROXIMITY reading taken at a single spacer distance; a result of combining (e.g., averaging) a plurality of database resistance values respectively obtained using a plurality of PROXIMITY readings taken respectively at a plurality of spacer distances; and a result of combining (e.g., averaging) a plurality of calibrated resistance values determined respectively by calibrating a plurality of database resistance values respectively obtained using a plurality of PROXIMITY readings taken respectively at a plurality of spacer distances. 
         [0026]    In some embodiments that measure thermistor resistances, for each spacer distance used, each of the PROXIMITY readings described above relative to  FIGS. 4-7  is taken at a plurality of different temperatures, thereby providing additional information indicative of relationships between PROXIMITY reading, thermistor resistance and temperature. 
         [0027]    The LDC1000 is further capable of providing an inductance measurement reading to the GUI software on computer  24  (see  FIG. 2 ), together with its aforementioned PROXIMITY reading. Some embodiments use the inductance measurement to aid in normalizing the aforementioned resistance determinations as a function of spacer distance. 
         [0028]    As is evident from the foregoing, the present work provides simple, integrated, low cost non-contact resistance measurement. Example embodiments can be implemented in harsh environments. Other advantages include simple operation; low cost measurement device (e.g., LDC1000); elimination of existing connectors in various products; and enablement of new use-cases without requiring addition of connectors. 
         [0029]    Although example embodiments of the present work have been described above in detail, this does not limit the scope of the work, which can be practiced in a variety of embodiments.