Abstract:
A method and apparatus calibrates an exhaust gas measurement system for a motor vehicle. The method includes the steps of providing a flow of air to the exhaust gas measurement system having a known flow rate, measuring and storing a signal output from the exhaust gas measurement system, repeating the first two steps a predetermined plurality number of times, calculating a calibration curve based on the known flow rates and the stored signal; and using the calibration curve in the exhaust gas measurement system.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to an apparatus and method to calibrate an exhaust flow measurement apparatus, and more particularly, to an arrangement for providing a known flow reference signal to the exhaust flow measurement apparatus and measuring the response of that unit to the reference signal. 
     2. Discussion 
     Several vehicle exhaust flow measurement methods have been developed over time. One method compares an initial concentration of carbon dioxide within a vehicle&#39;s exhaust to a diluted carbon dioxide measurement taken after outside air has been mixed with the exhaust. This method utilizes the dilution ratio of carbon dioxide to calculate the quantity of other exhaust gas constituents such as carbon monoxide. 
     A constant volume sample is coupled to the vehicle exhaust to collect gas samples for testing. It should be appreciated that the constant volume sampler functions properly only when the sample is completely gaseous. To assure that water does not condensate within the constant volume sampler, a large quantity of outside air must be added to the vehicle exhaust. Unfortunately, after a water correction factor is taken into account, the method is accurate to only within 10 percent. 
     Another exhaust flow measurement technique incorporates a smooth approach orifice and a constant volume sampler. The constant volume sampler is plumbed directly in-line with the vehicle exhaust and the smooth approach orifice. Thus, the sum of the gas flow from the vehicle exhaust and the gas flow through the smooth approach orifice equals the flow through the constant volume sampler. Accordingly, if the constant volume sampler flow rate is known and the smooth approach orifice flow rate is known, the vehicle exhaust flow may be calculated. However, only very small pressure drops are present within the smooth approach orifice. As is known, it is very difficult to accurately quantify small changes in pressure and this method is therefore subject to relatively large percentage errors. 
     A preferred method of exhaust flow measurement involves using an ultrasonic measurement device. Ultrasonic exhaust flow measurement units are used to quantify exhaust gas volumes directly from the tailpipe of a vehicle. Each exhaust flow unit has its unique calibration curve relating the output signal measured by the device to the amount of flow passing through it. The calibration curve is provided by the manufacturer of the unit and is built into the program that controls it. 
     As is known, the physical characteristics of any precision measurement device are subject to changes over time. These changes necessitate revising the calibration curve to reflect the changes in the measurement device. In the past, recalibration required sending the entire exhaust flow measurement device back to the manufacturer. This process was time consuming and very expensive. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method and apparatus to calibrate an exhaust flow measurement device. 
     It is another object of the present invention to cost effectively and accurately provide a known flow reference signal for use in calibrating an exhaust flow measurement device. 
     The present invention includes a method and apparatus for calibration of an exhaust gas measurement system for a motor vehicle. The method includies the steps of providing a flow of air to the exhaust gas measurement system having a known flow rate, measuring and storing a signal output from the exhaust gas measurement system, repeating the first two steps a predetermined plurality number of times, calculating a calibration curve based on the known flow rates and the stored signal; and using the calibration curve in the exhaust gas measurement system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of an exemplary vehicle coupled to a portion of an exhaust flow measurement device; 
     FIG. 2 is a fragmentary side view of an exhaust flow measurement device; 
     FIG. 3 is a perspective view of an exhaust flow calibration unit constructed in accordance with the teachings of the present invention; 
     FIG. 4 is another perspective view of the preferred exhaust flow calibration unit; 
     FIG. 5 is a schematic drawing representative of the preferred exhaust flow calibration unit. 
     FIG. 6 is a graphical display operative with the preferred exhaust-flow calibration unit; 
     FIG. 7 is a schematic drawing representative of the preferred exhaust flow calibration unit coupled to an exhaust flow measurement device; 
     FIG. 8 is a schematic drawing showing another embodiment of the exhaust flow calibration unit and exhaust flow measurement device interconnection; 
     FIG. 9 is a schematic drawing showing another embodiment of the exhaust flow calibration unit and exhaust flow measurement device interconnection; and 
     FIG. 10 is a schematic drawing showing another embodiment of the exhaust flow calibration unit and exhaust flow measurement device interconnection. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, an exemplary ultrasonic exhaust flow unit is generally identified at reference numeral  10 . Exhaust flow unit  10  is plumbed in-line with a tailpipe  12  of a vehicle  14 . Exhaust flow unit  10  includes an ultrasonic measurement device  16  having a generally circular hollow cylindrical housing  18 , a first sensor  20  and a second sensor  22  in communication with a timer  23 . Housing  18  is mounted at an angle other than 90° relative to a test chamber  24  coupled to tailpipe  12 . Exhaust flow unit  10  functions by sending a first signal from first sensor  20  to second sensor  22 . The elapsed time for the signal to travel from the first sensor to the second sensor is measured by timer  23 . Noting that the direction of exhaust gas flow corresponds to an arrow  26 , the first signal is assisted by the velocity of the exhaust gas. A second signal is sent from the second sensor  22  to the first sensor  20 . It should be appreciated that this signal is retarded by the exhaust gas flow. The time required for the second signal to travel from second sensor  22  to first sensor  20  is also measured. Because the inner diameter of test chamber  24  is known and the angle at which housing  18  is mounted relative to test chamber  24  is also known, it is possible to calculate the instantaneous gas flow by comparing the difference in elapsed times for first and second signals to travel to their respective sensors. 
     As mentioned earlier, periodic calibration of exhaust flow unit  10  should be conducted to assure optimum accuracy and performance. Referring to FIG. 3, an exhaust flow calibration unit is generally identified at reference numeral  28 . Exhaust flow calibration unit  28 , hereinafter referred to as calibration unit  28 , includes a suction circuit  30 , a central processing unit  32  and a user interface  34 . In general, suction circuit  30  provides an adjustable suction flow rate connectable to exhaust flow unit  10 . Central processing unit  32  collects data representing the flow rates applied from suction circuit  30  and corresponding measured flow rate data supplied from exhaust flow unit  10 . Central processing unit  32  stores and compares the aforementioned data in a manner discussed in greater detail hereinafter. User interface  34  provides an operator with real time access to the data and a method for varying the suction flow rate. Suction circuit  30 , central processing unit  32  and user interface  34  are all conveniently housed in a cabinet  36  which is portable for use in different locations. 
     Referring to FIGS. 3,  4  and  5 , suction circuit  30  includes a blower motor  38  as a primary source of vacuum. Suction circuit  30  also includes a valved outlet  40  connectable to an alternate source of suction such as a constant volume sampler  42 . Regardless of how suction is created, air may be drawn through four laminar flow elements to produce a desired volumetric flow rate. For example, each of a first laminar flow element  44 , a second laminar flow element  46  and a third laminar flow element  48  is preferably rated at a maximum flow rate of 40 cubic feet per minute. A fourth laminar flow element  50  is preferably rated at 100 cubic feet per minute. Each of the laminar flow elements are piped in parallel to provide a regulated known suction flow rate in the range of 0 to 220 cubic feet per minute. Adjustment of the total suction flow rate generated by calibration unit  28  is accomplished by opening and closing valves associated with each laminar flow element. Specifically, a first valve  52  is plumbed in series with first laminar flow element  44 . Correspondingly, the flow rate through second laminar flow element  46  is controlled by a second valve  54 . A third valve  56  limits the flow through third laminar flow element  48 . The flow through fourth laminar flow element  50  may be selectively restricted by a fourth valve  58 . A lower rail  60  interconnects each of the laminar flow elements to the suction source be it either blower motor  38  or constant volume sampler  42 . A first shutoff valve  62  separates blower motor  38  from lower rail  60  when closed. Similarly, a second shut-off valve  64  is positioned between outlet  40  and lower rail  60 . Second shut-off valve  64  is closed when blower motor  38  is used as the suction source. An upper rail  66  connects each of the laminar flow elements to outside air via a filter  68  and an inlet  69 . 
     Suction circuit  30  is constructed using laminar flow elements  44 ,  46 ,  48  and  50  to provide an operator of calibration unit  28  a method to supply an accurate suction flow rate ranging from 0 to 220 cubic feet per minute. It should be appreciated that the 220 cubic feet per minute sum is merely exemplary and that a variety of differently rated laminar flow elements may be interconnected to meet a specific need or purpose. 
     Each of the laminar flow elements includes an upstream pressure transducer  70  and a downstream pressure transducer  72  electrically coupled to central processing unit  32 . In the preferred embodiment, each of the laminar flow elements has a full scale volumetric flow rate equal to a pressure drop of 8 inches of water. The laminar flow elements are designed to linearly correlate pressure drop to volumetric flow rate. For example, a pressure drop of 4 inches of water across laminar flow element  44  equates to a flow rate of 20 cubic feet per minute. Twenty (20) cubic feet per minute was determined by multiplying the maximum rated flow rate of 40 cubic feet per minute by the ratio of a pressure drop of 4 inches of water divided by a full scale pressure drop of 8 inches of water. Central processing unit  32  performs the calculation previously mentioned for each of the laminar flow elements and then sums them to provide a total suction flow rate. 
     User interface  34  accepts data provided from central processing unit  32  and displays it in easily understandable format. User interface  34  includes a monitor  74  having a readable screen  76 . As shown in FIG. 6, screen  76  includes a graphical display  78  having a plurality of analog gages  80  corresponding to each of the laminar flow elements. The total flow rate provided by suction circuit  30  is displayed as Total CFM  82 . Once suction is applied, exhaust flow unit  10  calculates a flow rate as earlier described. This flow rate is displayed on graphical display  78  as a separate data-point entitled Total ACFM  84 . Additional data such as pressure differential across each laminar flow element, atmospheric pressure, and temperature may be displayed. 
     Central processing unit  32  collects a number of data sets pairing Total CFM  82  with Total ACFM  84  and stores them for later mathematical analysis. Preferably, fifteen or more different data sets are collected and stored. Once a known Total CFM  82  is input to obtain a corresponding Total ACFM  84 , a user may direct CPU  32  to measure and record the CFM  82  and ACFM  84  data pair by selecting a TAKE READING field  85 . To properly construct a calibration curve, different Total CFM  82  rates are input to span the useful range of the exhaust flow unit to be calibrated. 
     A calibration curve is calculated based on the charted data pairs. The curve may be generated by a variety of methods including a least square fit polynominal. In the preferred embodiment, a first order polynominal curve fitting technique is used. Once calibrated, the calibration curve is input within the software controlling the exhaust flow unit to correct any error detected in exhaust flow unit  10  measurements. 
     With reference to FIG. 7, a first method of interconnecting calibration unit  28  with exhaust flow unit  10  is shown. In this method, blower motor  38  (FIG. 5) is used as the vacuum source. Exhaust flow unit  10  includes an intake port  88  and an exhaust port  90 . Because blower motor  38  provides suction at a specified rate, exhaust port  90  is simply connected to air inlet  69  to complete the circuit. Thus, fresh air is drawn through intake port  88  and exits at exhaust port  90 . Air continues to travel through air inlet  69  into calibration unit  28 . One or any of the combination of the laminar flow elements previously mentioned are next met. The air passing through each of the laminar flow elements combines as it is drawn through the blower motor. A first cord  92  is adapted to provide 110 volts AC to central processing unit  32  and user interface  34 . A second cord  94  couples blower motor  38  to a three phase,  480  volt source to power the blower motor. 
     FIG. 8 depicts another method of interconnecting calibration unit  28  and exhaust flow unit  10 . Because blower motor  38  has a limited volumetric flow rate, it may be necessary to utilize a different source of suction having greater capacity. Specifically, constant volume sampler  42  is capable of providing a suction flow rate in the range of 500 cubic feet per minute while blower motor  38  is preferably sized to provide a flow rate of 220 feet per minute. However, it should be appreciated that constant volume sample  42  is intolerant to condensation or liquid content of any form. As such, a mix box  96  is plumbed in-line prior to constant volume sampler  42  to add an appropriate amount of air to assure condensation of liquid does not occur within constant volume sampler  42 . Tracing the air path once again, air is drawn into air inlet  69  passing through the designated laminar flow elements of calibration unit  28 , exiting at outlet  40 . Air continues to travel through intake port  88 , ultrasonic measurement device  16  and exhaust port  90  of exhaust flow unit  10 . The air passes through a mix box inlet  97  where it is joined with atmospheric air entering through a screen  98 . The combination of air entering through screen  98  and mix box inlet  97  are drawn through constant volume sampler  42 . 
     FIG. 9 depicts yet another method of interconnecting calibration unit  28  with exhaust flow unit  10 . In this method, a very large laminar flow element  100  is plumbed to communicate with constant volume sampler  42 , mix box  96  and exhaust flow unit  10 . Laminar flow element  100  is too large to be reasonably fitted within a portable housing such as cabinet  36 . Accommodation for such a large laminar flow element may be made by plumbing laminar flow element  100  to intake port  88  of exhaust flow unit  10 . A pair of pressure transducers  102  and a temperature transducer  103  are coupled to laminar flow element  100 . Each pressure transducer  102  electrically communicates with calibration unit  28  as if laminar flow element  100  were mounted within cabinet  36 . 
     With reference to FIG. 10, an alternate embodiment of the exhaust flow unit  10  and calibration unit  28  interconnection is shown. It should be appreciated that the embodiment depicted in FIG. 10 functions substantially similarly to the embodiment previously described with reference to FIG.  8 . The embodiment depicted in FIG. 10 includes constant volume sampler  42  coupled to exhaust flow unit  10  which is in turn coupled to calibration unit  28 . However, it should be noted that instead of a mix box, a mix tee  104  provides an opening  106  for atmospheric air to mix with the exhaust gas prior to entering constant volume sampler  42 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations may be made therein without departing from the spirit and scope of the invention as defined in the following claims.