Abstract:
An apparatus and method for determining the concentration of alcohol present in a gaseous mixture wherein an electronic device employing a fuel cell determines the concentration of alcohol present in the gaseous mixture is disclosed. In particular, the device may be used to determine a person&#39;s sobriety by determining the alcohol concentration in a breath sample. To determine the alcohol concentration, the device must be able to accurately determine the volume of breath in the breath sample. The device controls the volume of the breath sample by measuring the pressure of the breath flow through the device and, in response to the pressure, electronically controlling a valve diverting a portion of the breath flow into the fuel cell. One application of the invention is a sobriety interlock for a machine where the machine remains disabled unless the operator is first determined to have an alcohol concentration below a predetermined level.

Description:
This is a continuation of application Ser. No. 09/136,837 filed Aug. 20, 1998 which parent application became U.S. Pat. No. 6,026,674 as of date Feb. 22, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to devices and methods for determining the concentration of alcohol in a mixture of gases. More particularly, the invention relates to a device and method for determining the concentration of alcohol in a breath sample for application in sobriety detection systems and sobriety interlock systems for vehicles and other machines. 
     Various techniques have been employed for calculating a person&#39;s blood alcohol concentration by measuring breath samples. In a first method, the alcohol content in a breath sample is measured using a semiconductor sensor commonly referred to as a Tagucci cell. Although this method provides a low cost device, instruments incorporating this method have proved to have poor accuracy. 
     A second method employs an infrared absorption technique for determining the blood alcohol concentration. This method has proven to have very high accuracy levels. However, the sensor systems incorporating this technique are cost prohibitive for many applications. 
     A third method employs a fuel cell together with an electronic circuit. This method is described in U.S. Pat. No. 4,487,055. Although this method allows for more accuracy than the first method, the systems employing this method have continued to be relatively expensive. One reason for the high cost associated with the fuel cell techniques is that the method requires that the breath sample be of a determinable volume. Historically, this has been accomplished through the use of positive displacement components such as piston-cylinder or diaphragm mechanisms. The incorporation of such components within an electronic device necessarily increases the costs associated with the device. 
     The current invention departs from the use of the expensive positive displacement mechanisms while retaining the accuracy provided by the fuel cell. The current invention employs a valve controlled by a computing device such as a microprocessor to regulate the total volume of gas passing through the fuel cell. 
     SUMMARY OF THE INVENTION 
     According to the invention, a sobriety detection system obtaining relatively good accuracy at an acceptable cost is disclosed. In one embodiment, the alcohol concentration in a person&#39;s breath is determined by passing a breath sample of determinable volume through a fuel cell. The invention obtains a breath sample of determinable volume by electronically controlling a valve permitting breath flow into the fuel cell. A valve controller limits the duration of time the valve is open based upon the pressure of the breath flow. The fuel cell produces a voltage output proportional to the total volume of alcohol in the breath sample. A computing device determines the total volume of alcohol in the sample from the fuel cell voltage output. The computing device adjusts the valve-open time as a function of pressure to obtain a constant-volume breath sample. Therefore, the alcohol concentration is known directly from the fuel cell voltage by applying the appropriate scale factor. 
     In another embodiment, the invention is employed in a sobriety interlock device to prevent operation of a machine such as a vehicle, unless the operator&#39;s breath alcohol concentration is first determined to be below a predefined level. In this embodiment, the machine operator breathes into the interlock device incorporating the invention. The device then determines whether the alcohol concentration in the breath sample is below the acceptable level. If the concentration is acceptable, the interlock permits enablement of the machine ignition or power supply. If the concentration is above the permissible level, the interlock prohibits ignition or operation of the machine. 
     When used in an interlock device, the invention may incorporate additional elements and features to prevent an intoxicated person from defeating the interlock by introducing a sample from a source other than the operator&#39;s breath. The device may include a temperature sensor which will prohibit enablement of the machine unless the breath flow is within an acceptable range around human body temperature. The device may also incorporate logic within the computing device to prohibit enablement of the machine unless a pressure oscillation within the breath flow is detected. In the proper operation, the operator generates the pressure oscillation by humming while breathing into the device. The device may also enforce minimum and maximum blow-pressure limits to prevent the use of artificial samples and to detect the use of alcohol-removing filter media, such as activated charcoal, by detecting the resultant low pressure caused by pressure drop across the filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic section of a sobriety detection device of this invention with circuitry shown diagrammatically. 
     FIG. 2 is a flow diagram illustrating the method of this invention. 
     FIG. 3 is a flow diagram illustrating various outputs that may be generated in response to a “pass” test result. 
     FIG. 4 is a flow diagram illustrating various outputs that may be generated in response to a “failure” test result. 
     FIG. 5 is a flow diagram illustrating alternative features that may be included within the invention to prevent an intoxicated person from defeating the device to obtain a “pass” test result. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a sobriety detection system  10  embodying the invention includes a breath induction tube  12  for receiving the breath of the person being tested. The breath induction tube  12  encloses a breath flow channel  34  and includes an inlet  30  and an exit  26 . A pressure sensor  16  is connected to the breath induction tube  12  for measurement of the pressure in the breath flow channel  34 . Preferably, the pressure sensor  16  is of a resistance-bridge type. The pressure sensor  16  produces an electrical signal proportional to the pressure detected. That signal is input to a computing device  18 . Preferably, the computing device  18  is a microprocessor, but it will be appreciated that other devices capable of performing mathematical calculations and generating output signals may be employed. A temperature sensor  32  is also incorporated into the system to measure the temperature within the breath flow channel  34 . The temperature sensor  32 , which is preferably of a thermistor type, is electrically connected to computing device  18 . 
     Also connected to the breath induction tube  12  is a fuel cell  24  which is of a type well-known in the art. A valve  20  is positioned between the breath induction tube  12  and the fuel cell  24 . When the valve  20  is closed, all of the breath flow exits the breath induction tube  12  through the exit  26 . When the valve  20  is open, a portion of the breath flow passes through the valve into the fuel cell  24  while the remainder of the breath flow exits the breath induction tube  12  through the exit  26 . 
     The valve  20  can be of any type suitable for low volume gas flow applications, but preferably is of a needle type with a tapered rubber tip. In the preferred embodiment, when the valve is closed, the tip of the valve  20  seats against a valve seat circumscribing an orifice of approximately 0.015 inches. 
     Operation of the valve  20  is controlled by a valve controller  22 . The valve controller  22  is capable of opening and closing the valve  20  in response to signals from the computing device  18 . Preferably, the valve controller  22  is a solenoid electrically connected to the computing device  18 . 
     FIG. 2 discloses the functional operation of the sobriety detection system  10 . As shown at step  50  in FIG. 2, a person&#39;s breath is introduced into the breath channel  34 . At step  52 , the pressure within the breath flow channel  34  is measured using the pressure sensor  16 . 
     Next, the computing device  18  determines the flow rate of breath through the breath induction tube  12  based on the pressure within the breath channel  34 . As depicted at step  60 , in the preferred embodiment, the computing device  18  determines an instantaneous flow rate by using a table look-up routine based on empirical data generated for the particular configuration of the device. At the relevant pressure levels, the flow rate through the breath induction tube  12  is approximately proportional to the square root of pressure. At step  70 , the computing device  18  calculates the volume of breath that has passed through the breath induction tube  12  by numerically integrating the flow rate over time. 
     In the preferred embodiment, the valve  20  remains closed for a delay period sufficient for a predefined amount of breath to pass through the breath flow channel  34 . Preferably, the valve  20  should remain closed for a time sufficient for at least 1.5 liters of breath to flow through the breath flow channel  34  after the breath flow begins. 
     Referring to FIG. 2, the decision block  72  requires a determination of whether the requisite amount of breath has passed through the breath induction tube  12  prior to opening the valve  20 . If the requisite amount of breath has not passed through the breath induction tube  12 , the valve  20  remains closed. But if a sufficient amount of breath has passed through the breath induction tube  12 , the computing device  18  transmits a valve open signal to the valve controller  22  to open the valve  20  and to permit a breath sample to flow into the fuel cell  24 , as shown at step  74 . 
     Referring to FIG. 1, the fuel cell  24  is electrically connected to the computing device  18 . As is well known in the art, the fuel cell  24  produces an electrical signal proportional to the volume of alcohol present in the breath sample. This signal is input to the computing device  18 . Referring again to FIG. 2, at step  75 , the computing device  18  determines the pressure within the breath flow channel  34  from the output of the pressure sensor  16 . 
     At step  76 , the computing device  18  executes a table look-up routine based on the flow pressure measured by the pressure sensor  16  to determine an instantaneous flow rate passing into the fuel cell  24 . The breath sample flow rate values within the table look-up routine are based upon empirical data generated for the geometric configuration of the particular device. In the preferred embodiment, the values within the look-up table were developed from experimentation wherein the period of time required to obtain a constant fuel cell output from a flow at a constant pressure was measured. At the relevant pressures, the experimentation confirmed that the breath sample flow rate is approximately proportional to the square root of the pressure. At lower pressures, the actual flow rate departs somewhat from the pressure square root curve because of the presence of a laminar boundary layer. The table look-up routine in the preferred embodiment incorporates adjustments determined from experimentation to correct for the laminar flow. 
     As shown at step  78  in FIG. 2, the computing device  18  integrates the instantaneous readings of breath sample flow rate over time to calculate the total volume of the breath sample. Decision block  80  requires a determination of whether the breath sample volume has exceeded a predetermined amount. In the preferred embodiment, that predetermined amount is approximately 2 milliliters. If the total volume of the breath sample exceeds the predetermined amount, the computing device  18  transmits a valve close signal to the valve controller  22  to close the valve  20 , as represented by step  82 . 
     At step  84  in FIG. 2, after the valve  20  is closed, the computing device  18  calculates the alcohol concentration by dividing the fuel cell signal, representing the total amount of alcohol contained in the breath sample, by the known sample volume calculated from the integration of the instantaneous breath sample flow rate over time. As will be appreciated by those of ordinary skill in the art, the alcohol concentration of “deep-lung” air is approximately proportional to the person&#39;s blood alcohol concentration. Thus, the computing device  18  may calculate the person&#39;s blood alcohol concentration from the alcohol concentration in the breath sample. 
     Based on either the blood alcohol concentration or the alcohol concentration in the breath sample, the computing device  18  may generate various outputs depending on the application for the sobriety detection system  10 . Because of the approximately-proportional relationship between blood alcohol concentration and the alcohol concentration in the breath sample, it is readily understood that outputs dependent upon predetermined values can be based on either blood alcohol concentration and breath sample alcohol concentration by applying a scale factor to the predetermined values. 
     As depicted by decision block  86  of FIG. 2, the output may vary depending on whether the alcohol concentration is less than a predetermined level. If the alcohol concentration is less than the predetermined level, the computing device  18  may generate one or more outputs consistent with a “pass” of a sobriety test at step  90 . If the alcohol concentration is equal to or more than the predetermined level, the computing device  18  may generate one or more outputs consistent with a “fail” of the sobriety test at step  100 . As shown in FIG. 1, the computing device  18  may be connected to various output devices including a display  38 , a printer  42 , a machine enablement circuit  40 , and a memory device  44  through leads  36 . 
     Referring now to FIG. 3, if the blood alcohol concentration is below the predetermined level, the “pass” outputs generated by the computing device  18  may include illumination of an indicator on the display  38  that the sobriety test resulted in a “pass,” indication of the numeric value for blood alcohol concentration or another message on the display  38  as shown at step  92 , a printout from the printer  42  indicating that the sobriety test resulted in a “pass,” and/or a printout of the numeric value for blood alcohol concentration or another printout from the printer  42  as shown at step  96 . Likewise, the results of the sobriety test or other information may be stored in a memory device  44  for retrieval at a later time, as shown at step  94 . 
     In a machine interlock device, the computing device  18  may produce an output signal through one of the leads  36  (as shown in FIG. 1) to a machine enablement circuit  40  to permit enablement of the starting system, such as an ignition system, of the machine only if the blood alcohol concentration level is less than or equal to a predetermined amount, as shown at step  98  of FIG.  2 . 
     It is easily appreciated that the interlock device may have application in a wide variety of devices from automobiles and other motorized vehicles, to power equipment and tools, and to industrial or commercial machinery. In fact, the interlock device may be used for relatively simple machines such as electronic locks on doors or mechanical devices. As used herein, the term machine applies to any mechanical device that may rendered inoperative either on signal from the interlock device or unless a signal is received from the interlock device. 
     As shown in FIG. 4, if the blood alcohol concentration is greater than or equal to the predetermined level, the “fail” outputs generated by the computing device  18  may include illumination of an indicator on the display  38  that the sobriety test resulted in a “fail,” indication of the numeric value for blood alcohol concentration or another message on the display  38 , as shown by step  102 , a printout from the printer  42  indicating that the sobriety test resulted in a “fail,” and/or a printout of the numeric value for blood alcohol concentration or another printout from the printer  42 , as depicted by step  106 . Likewise, the results of the sobriety test or other information may be stored in a memory device  44  for later retrieval at step  104 . The computing device  18  may also generate a signal to disable the operating system of a machine, as shown in step  108 . 
     When the sobriety detection system  10  is used in a machine interlock device or any other device where the person using the device may be unsupervised, there is a need to prevent the person being tested from defeating the interlock by introducing air from a source other than the person&#39;s breath. FIG. 5 shows an alternative embodiment of the testing process, including features to prevent a person from improperly obtaining a “pass” result. Steps  50 ,  52 , and  60  relate to initiation of the testing process as discussed above in connection with FIG.  2 . As shown by decision block  62  in FIG. 5, the sobriety detection system  10  may be configured such that if the pressure in the breath flow tube is below a threshold level, the computing device  18  will cause the test to be aborted. Preferably, the threshold pressure level for performing the sobriety test is approximately 6 inches of water. 
     In addition or in the alternative, the sobriety detection system  10  may be designed to require the person being tested to hum or make another vocal sound while breathing into the breath induction tube  12 . As will be understood by those having ordinary skill in the art, the vocal sound generates a slight pressure oscillation within the breath induction tube  12 . As shown by decision block  64  in FIG. 5, the computing device  18  may be configured to abort the test if voice or vocal sound is not detected in a pressure oscillation at the pressure sensor  16 . 
     Referring again to FIG. 1, the sobriety detection system  10  includes the temperature sensor  32  connected to a temperature probe  28  located within the breath flow channel  34 . The temperature sensor  32  measures the temperature of the flow within the breath flow channel  34  and transmits an electrical signal to the computing device  18  based on the measured temperature. As shown by decision block  66  in FIG. 5, the computing device  18  may be configured to abort the test if the measured temperature does not fall within limits approximating human body temperature. Steps  70  and  72  are described above in connection with the basic operation of the invention in regard to FIG.  2 . 
     It is understood that the actual implementation of the abort test step  68  in FIG. 5 may cause the sobriety detection system  10  to shut down or generate outputs consistent with a “failure” of the test. As applied in a machine interlock device, an aborted test at step  68  will prevent enablement of the machine or disable the machine, depending on the configuration of the machine. 
     In an additional embodiment, the sobriety detection system  10  may include a heating element. Those skilled in the art will appreciate that the accuracy of the fuel cell  24  is reduced at temperatures below room temperature. Therefore, the heating element may be used to heat the fuel cell  24  when the ambient air temperature is below room temperature. 
     While the above description constitutes a preferred embodiment of the apparatus and method of the invention, it is to be understood that the invention is not limited thereby and that in light of the present disclosure of the invention, various other alternative embodiments will be apparent to persons skilled in the art. The scope of the present invention is to be limited only by the appended claims.