Abstract:
Embodiments of apparatuses, articles, methods, and systems for measuring sensor drift characteristics are generally described herein. Other embodiments may be described and claimed.

Description:
FIELD  
       [0001]     Embodiments of the present invention relate generally to the field of sensors, and more particularly to sensor drift characteristic testing.  
       BACKGROUND  
       [0002]     Sensors, in general, are devices used to detect physical stimuli such as heat, particulates, chemicals, gasses, etc. and output a signal that may be used to quantify the stimuli. Over time, a sensor&#39;s response to stimuli may change, which may in turn affect the accuracy of any quantification based on the output signal.  
         [0003]     In order to account for this change in sensor response, sensor manufacturers typically provide specifications having a relatively wide range of operating parameters so that, even after the change, the sensor will most likely still be in the range. However, knowing that a sensor is operating within this operating parameter range may not be sufficient for a user of the sensor who may have calibrated the sensor based on a pre-change response curve. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:  
         [0005]      FIG. 1  illustrates a sensor testing facility in accordance with an embodiment of the present embodiment;  
         [0006]      FIG. 2  illustrates a sensor circuit in accordance with an embodiment of the present invention;  
         [0007]      FIG. 3   a  illustrates a graph representing a test environment as a function of time, in accordance with an embodiment of the present invention;  
         [0008]      FIG. 3   b  illustrates a graph representing a sensor output signal curve as a function of time corresponding to the test environment described by  FIG. 3   a  in accordance with an embodiment of the present invention;  
         [0009]      FIG. 4  illustrates operational phases of a sensor analysis iteration in accordance with an embodiment of the present invention;  
         [0010]      FIG. 5  illustrates a number of output signal curves recorded over a number of testing iterations in accordance with an embodiment of the present invention;  
         [0011]      FIG. 6  illustrates a graph of a sensitivity drift curve in accordance with an embodiment of the present invention;  
         [0012]      FIG. 7  illustrates a graph of a settling drift curve in accordance with an embodiment of the present invention;  
         [0013]      FIG. 8  illustrates a graph of an output drift curve in accordance with an embodiment of the present invention; and  
         [0014]      FIG. 9  illustrates a testing facility having a sensor platform in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]     Illustrative embodiments of the present invention may include analysis of sensor drift characteristics as a sensor performance metric.  
         [0016]     Various aspects of the illustrative embodiments will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that alternate embodiments may be practiced with only some of the described aspects. For purposes of explanation, specific devices and configurations are set forth in order to provide a thorough understanding of the illustrative embodiments. However, it will be apparent to one skilled in the art that alternate embodiments may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative embodiments.  
         [0017]     Further, various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present invention; however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation.  
         [0018]     The phrase “in one embodiment” is used repeatedly. The phrase generally does not refer to the same embodiment; however, it may. The terms “comprising,” “having,” and “including” are synonymous, unless the context dictates otherwise.  
         [0019]     In providing some clarifying context to language which may be used in connection with various embodiments, the phrase “A/B” means “A or B.” The phrase “A and/or B” means “(A), (B), or (A and B).” The phrase “at least one of A, B, and C” means “(A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C).” The phrase “(A)B” means “(B) or (A and B),” that is, A is optional.  
         [0020]      FIG. 1  illustrates a sensor testing facility  100  in accordance with an embodiment of the present invention. The sensor testing facility  100  may include a chamber  104  to provide an environment having controllable levels of a physical stimulus. The levels of the physical stimulus may be controlled by an environmental conditioner  108  coupled to the chamber  104 . The sensor testing facility  100  may also have a data acquisition device  112  to couple to a sensor  116  disposed within the chamber  104  and to record a signal output from the sensor  116  based on the stimulus. The output signal from the sensor  116  may be conveyed to the data acquisition device  112  via a communication link  120 . In some embodiments, the sensor  116  may be coupled to a sensor platform to facilitate the operating state of the sensor  116  and/or the communications to and/or from the data acquisition device  112 .  
         [0021]     In some embodiments, the environmental conditioner  108  may vary the stimulus in a predetermined manner over time. The output may be recorded and analyzed to determine acceptability of the sensor  116  based at least in part on measured drift characteristics.  
         [0022]     In some embodiments, drift characteristics such as settling drift, sensitivity drift, and/or output drift may be focused on as an indication of the potential field performance of a sensor. The analysis of these particular drift characteristics prior to field implementation may facilitate a more reliable identification of underperforming sensors. Analysis of a sensor&#39;s drift characteristics may be described in further detail below in accordance with embodiments of the present invention.  
         [0023]     In various embodiments, the data acquisition device  112  may include one or more processor(s)  120  and data storage  124 . The data storage  124  may include various combinations of volatile, non-volatile, removable and/or non-removable memory structures. In some embodiments, the data storage  124  may include one or more structure(s) integrated within the processor(s)  120 , e.g., a processor-level cache. The data storage  124  may provide storage of historical data of the output signal as well as programming instructions for execution by the processor(s)  120  to enable data acquisition device  112  to perform various operations, some of which may be described herein.  
         [0024]     In some embodiments, the data acquisition device  112  may be a general purpose computing device, e.g., a laptop computing device, a desktop computing device, and the like. In some embodiments, the data acquisition device  112  may be a specific-computing device designed to perform a limited number of functions directed to a particular application.  
         [0025]     In an embodiment, the sensor  116 , which may be a metal-oxide semiconductor (MOS) sensor, may respond to stimuli such as volatile organic compounds (VOCs) and may therefore be referred to as a VOC sensor. However, in other embodiments, other types of sensors may be evaluated in a manner described by the embodiments of the present invention. Other types of sensors may be particularly adapted to detect, e.g., temperature, pressure, fluid flow, various chemicals (either specific chemicals, e.g., carbon dioxide, carbon monoxide and/or radon or classes of chemicals, e.g., VOCs, etc.), particulates, and so forth.  
         [0026]      FIG. 2  illustrates a schematic of the sensor  116 , in accordance with an embodiment of the present invention. The sensor  116  may include a heater resistor  204 , a sensor resistor  208  and a load resistor  212  coupled to each other as shown. A heater voltage V H  may be applied to the terminals of the heater resistor  204  to provide an operating temperature for the sensor resistor  208 . A sensor voltage V S  may be applied over the series of the sensor resistor  208  and the load resistor  212 . The heater voltage V H  and the sensor voltage V S  may be provided by the same power supply circuit. The sensor resistor  208  may be a variable resistor having a resistance that changes proportional to the amount of a stimulus, e.g., VOC, present. An output voltage V out  may be measured across the load resistor  212 , having a resistance R L , and the resistance R S  of the sensor resistor  208  may then be calculated based on the V S . The resistance R S  may be computed through the following equation:  
               R   s     =       R   L     ⁢         (       V   S     -     V   out       )       V   out       .               Eq   .           ⁢   1               
         [0027]      FIGS. 3   a - 3   b  respectively illustrate a graph representing the varying VOC levels within the chamber  104  over time and a graph of an output resistance of the sensor resistor  208  disposed within the chamber  104  as they gradually respond to a change in the VOC level in accordance with an embodiment of the present invention. Furthermore,  FIG. 4  illustrates phases of a sensor analysis operation in accordance with an embodiment of the present invention. Operational phases may be noted by reference numerals in parentheses.  
         [0028]     At an initial time T 0  the environmental conditioner  108  may begin to purge the chamber  104  of VOCs until time T 1  ( 404 ), which may result in a locally low VOC level  304 . As can be seen, the sensor resistance R S  may be inversely proportional to the VOC levels. Therefore, from time T 0  to time T 1 , as the VOC level decreases to a relative low point, e.g., low VOC level  304 , the sensor resistance R S  may increase to a relative high point, e.g., high sensor resistance  308 . In some embodiments the purge of the VOCs may be done gradually enough such that the sensor resistance R S  does not exhibit any response delay.  
         [0029]     In some embodiments, the chamber  104  may be kept at approximately the low VOC level for a period to allow the output of the sensor  116  to increase to a stable value. After the time period, the environmental conditioner  108  may introduce a sufficient quantity of VOCs to result in a targeted high-VOC level  312  within the chamber  104 , e.g., 1 part per million (ppm) of isobutylene ( 408 ). Likewise, the sensor resistance R S  may show a corresponding drop to a low sensor resistance  316 .  
         [0030]     The response curve  320  shown in  FIG. 3   b  may be recorded and analyzed for attributes, such as settling time ( 412 ) and sensitivity values ( 416 ), which may be used in determination of drift characteristics.  
         [0031]     In some embodiments, the settling time of the response curve  320  may be determined by measuring a settling time  324  between an occurrence of an upper value  328  of the resistance and a lower value  332  of the resistance. In an embodiment, the upper value  328  and lower value  332  used for the calculation of the settling time  324  may be values inside of the high resistance  308  and low resistance  316 . For example, in an embodiment the upper value  328  may be when the sensor resistance R S  has dropped approximately 10% of the total resistance change from the high resistance  308  and the lower value  332  may be when the resistance has dropped approximately 90% of the total resistance change from the high resistance  308 . Determination of the rise and fall times based at least in part on this reduced range may substantially eliminate marginal effects of the extreme values. The upper value  328  and the lower value  332  may be calculated upon determination of the high resistance  308  and the low resistance  316 . In an embodiment, this measured settling time  324  may be qualified against a settling threshold value of an acceptance criteria. For example, the acceptance criteria may require the measured settling time  324 &lt;settling threshold value. The measured settling time  324  may additionally/alternatively be used as a data point for a subsequent analysis of settling drift.  
         [0032]     In an embodiment, the rate of injection of VOC in chamber  108  may be faster than an expected settling time of a sensor in order to more accurately determine the settling time.  
         [0033]     In some embodiments, the sensitivity of the response curve  320  may be determined through analysis of a ratio of the high resistance  308  to the low resistance  316 . Taking the ratio of these values may account for the variability among sensors&#39; absolute measurements. For example, different sensors may provide different output levels for the same amount of stimuli. These differences may not be very significant, as the output of the sensors will typically be calibrated to a control amount of stimuli present. Therefore, a sensitivity ratio may be a more reliable metric for determining sensitivity of sensors. Similar to the settling time, in an embodiment, the sensitivity ratio may be qualified against a sensitivity threshold value of the acceptance criteria. For example, the acceptance criteria may require the measured sensitivity ratio&gt;sensitivity threshold value. The sensitivity ratio may additionally/alternatively be used as a data point for a subsequent analysis of sensitivity drift.  
         [0034]     In various embodiments, qualification of the sensitivity ratio and/or the settling time  324  may be done at a beginning of an analysis period, at the end of the analysis period, and/or through one or more points during an analysis period.  
         [0035]     In an embodiment, the response curve  320  may represent one iteration of an analysis that may be repeated a number of times. For example, in one embodiment, a response curve of the sensor  116  may be taken weekly for four consecutive weeks. In various embodiments, response curves may be taken at any frequency over any time period.  
         [0036]      FIG. 5  illustrates a graph having four response curves overlaid with one another in accordance with an embodiment of the present invention. In this embodiment, curve  502  may represent an analysis performed in week 1; curve  504  may represent an analysis performed in week 2; curve  506  may represent an analysis performed in week 3; and curve  508  may represent an analysis performed in week 4.  
         [0037]     Curve  502  may have a high resistance  510 , an upper level  512 , a lower level  514 , a low resistance  516  and, through analysis of the previous values, a settling time  518  and a sensitivity ratio  520 .  
         [0038]     Curve  504  may have a high resistance  522 , an upper level  524 , a lower level  526 , a low resistance  528  and, through analysis of the previous values, a settling time  530  and a sensitivity ratio  532 .  
         [0039]     Curve  506  may have a high resistance  534 , an upper level  536 , a lower level  538 , a low resistance  540  and, through analysis of the previous values, a settling time  542  and a sensitivity ratio  544 .  
         [0040]     Curve  508  may have a high resistance  546 , an upper level  548 , a lower level  550 , a low resistance  552  and, through analysis of the previous values, a settling time  554  and a sensitivity ratio  556 .  
         [0041]     As illustrated in  FIG. 5 , curves  502 ,  504 ,  506 , and  508  have been normalized such that the high resistances  510 ,  522 ,  534 , and  546 , respectively, are at the same point. This may be done to more clearly illustrate drift characteristics of the subsequent VOC cycling.  
         [0042]      FIG. 6  illustrates a graph of a sensitivity drift curve  600 , in accordance with an embodiment of the present invention. In this embodiment, the plotting of the sensitivity ratios  520 ,  532 ,  544 , and  556  taken at respective data capture periods  604 ,  608 ,  612  and  616 , e.g., weeks 1, 2, 3 and 4, respectively, may result in the sensitivity drift curve  600 . It may be noted that the sensitivity drift curve  600  may decrease in a non-linear manner with the steeper part of the curve  600  being located towards the first interval  604 .  
         [0043]      FIG. 7  illustrates a graph of a settling drift curve  700  in accordance with an embodiment of the present embodiment. In this embodiment, the plotting of the settling times  518 ,  530 ,  542  and  554  taken at respective data capture periods  604 ,  608 ,  612  and  616  may result in the settling drift curve  700 . Similar to the sensitivity drift curve  600 , the settling drift curve  700  may be a non-linear function that is steeper towards the first interval  604 ; however, the settling drift curve may be increasing as opposed to decreasing.  
         [0044]     While the above data is retrieved from the sensor  116  over periods where the chamber  104  is cycled through a high and low VOC concentration; other embodiments, may additionally/alternatively focus on other data. For example,  FIG. 8  illustrates a graph of an output drift curve  800  in accordance with an embodiment of the present invention. In this embodiment resistance R S  measurements  804 ,  808 ,  812 , and  816  may be recorded at data capture periods  820 ,  824 ,  828 , and  832 , respectively. In an embodiment, periods  820 ,  824 ,  828 , and  832  may all occur when the VOC level in the chamber  104  is approximately the same. For example, these periods  820 ,  824 ,  828 , and  832  may occur at a time when VOC levels are at a relatively low level between the periods  604 ,  608 ,  612  and  616  discussed above. Examination of this data may facilitate a determination of whether the sensor  116 &#39;s output measurements are drifting at the end of the analysis period. While  FIG. 8  illustrates that the measurements are drifting upwards over time, other embodiments may have sensor output drift downward.  
         [0045]     The drift curves  600 ,  700 , and/or  800  may prove to be valuable indicators of the performance of sensors in the field. For example, analysis of the drift curves  600 ,  700  and/or  800  may reveal that the sensor  116 , at four weeks, is still experiencing substantial drift in one or more areas. If the sensor  116  was still experiencing substantial drift when it is calibrated for field deployment, the accuracy of the calibration may only last a limited time. On the other hand, analysis of the drift curves  600 ,  700 , and/or  800  may reveal that the sensor  116  has already experienced the majority of its drift. Therefore, a calibration may be performed and the sensor  116  may be implemented in the field with a certain amount of assurance that drift will not compromise the data received from the sensor  116 .  
         [0046]     In an embodiment, after an analysis period, e.g., after interval  616 , one or more of the drift curves  600 ,  700  and  800  may be compared to predetermined acceptance criteria to determine acceptability of the sensor. In various embodiments, the acceptance criteria may be an absolute value (e.g., the relevant drift curve must have a slope no greater than X); and/or a relative value (e.g., the relevant drift curve must have a slope within the Xth percentile of like sensors). As mentioned above, the acceptance criteria may also include other, non-drift, performance metrics, e.g., an acceptable sensor may be required to have a certain max/min ratio.  
         [0047]     In various embodiments, sensors that do not conform to the acceptance criteria may be dealt with in a variety of ways. In some embodiments, a non-conforming sensor may be subjected to an extended analysis period to determine if the particular drift characteristic may level-off. In some embodiments, a non-conforming sensor, after one or more analysis periods, may be rejected. Additionally, levels and/or types of non-conforming sensors may be handled in accordance with the factors of a particular embodiment.  
         [0048]     While each of the drift curves  600 ,  700  and  800  are based on the same number of measurements taken over substantially the same time period, in other embodiments the number of measurements and/or time period may be varied for one or more of the drift curves  600 ,  700  and/or  800 .  
         [0049]      FIG. 9  illustrates a testing facility  900  in accordance with an embodiment of the present invention. In this embodiment, the testing facility  900  may include a chamber  904 , an environmental conditioner  908  and a data acquisition device  912 , which may be similar to, and substantially interchangeable with, like-named elements described with reference to  FIG. 1 . In this embodiment, the testing facility  900  may include a sensor platform  916 , which may include one or more boards  920  adapted to receive up to a plurality of sensors  924 . In some embodiments, the boards  920  may include sockets to receive the sensors  924  in a removable manner.  
         [0050]     The boards  920  may have circuitry to communicatively couple the data acquisition device  912  to each of the sensors  924  coupled to the board(s)  920  via a communication link  928 . In this embodiment, the data acquisition device  912  may sequentially select the particular sensor (or socket) through a selection mechanism  932 , e.g., a multiplexer/demultiplexor, for recordation of the output signal.  
         [0051]     In an embodiment having sensors  924  with a heater resistor and sensor resistor, similar to sensor  116  in the embodiment depicted by  FIG. 2 , the sensor platform  916  may apply a continuous voltage to the heater resistor to effectuate exhibition of the drift characteristics and selectively apply a voltage (and an additional load resistance) to the sensor resistor for recordation of the output signal.  
         [0052]     In an embodiment, the sensor platform  916  may include a signal converter  936  coupled to the selection mechanism  932 . The signal converter  936  may include, e.g., an analog-to-digital converter (ADC), and may be embedded into one of the boards  920 . The signal converter  936  may facilitate the conversion of the voltage levels output from a particular sensor into a digital value for recordation and/or further processing by the data acquisition device  912 . In various embodiments, the signal converter  936  may be located in the data acquisition device  912  or in a separate device.  
         [0053]     In various embodiments, the communication link  928  may provide a conduit for communicating data (including requests for data) from the data acquisition device  912  to the sensor platform  916  and from the sensor platform  916  to the data acquisition device  912 . For example, in an embodiment there may be, e.g., up to eight boards  920 , each adapted to receive up to, e.g., 100 sensors  924 . In this embodiment, the communication link  928  may include a parallel port cable or a universal serial bus (USB) to supply a 10-bit address to select a particular sensor. Seven bits may be used for sensor selection while three additional bits may select a specific board. If, e.g., a 12-bit address were to be used, either the number of sensors  924  per board or the number of boards  920  may be higher, or any combination thereof. Furthermore, in accordance with some embodiments, the communication link  928  may additionally/alternatively include connections to supply the signal converter  936  with a clock signal and/or a data-out connection. In various embodiments, the sensor platform  916  may provide relative acceptance criteria for a particular sensor by comparing the output signal to an average of the plurality of sensors  924 . These acceptance criteria may account for the probabilities that the majority of the sensors  924  will be in compliance with performance requirements, with only a relatively small number of non-compliant sensors. In some embodiments, relative acceptance criteria may be additionally qualified by other predetermined criteria. For example, use of an average of the plurality of sensors  924  as relative acceptance criteria may be contingent on the average being greater than a certain value. This may account for the possibility of a bad batch of sensors.  
         [0054]     In some embodiments, the sensor platform  916  may provide for batch processing of a large number of sensors  924  which may facilitate homogeneous testing conditions as well as allowing for the simultaneous testing of the sensors  924  in a relatively compact chamber  904 .  
         [0055]     Furthermore, in various embodiments, the sensor platform  916  may be utilized to facilitate the calibrating of the plurality of sensors  924 . Calibration of the sensors  924  may be controlled by the data acquisition device  912 , e.g., by the computation of calibration coefficients based, at least in part, on data captured through the drift analysis. An operator may therefore determine compliance of the sensors  924  and/or calibrate the sensors  924  prior to placement of the sensors in monitors or other field implementing device.  
         [0056]     Referring again to  FIG. 1 , in some embodiments the output signal may be processed by the data acquisition device  112  in a manner to emulate an actual sensor implementation. This may facilitate, for example, a drift analysis being performed with reference to values that may be seen in an actual sensor implementation.  
         [0057]     Although the present invention has been described in terms of the above-illustrated embodiments, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the art will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This description is intended to be regarded as illustrative instead of restrictive on embodiments of the present invention.