Abstract:
A thermal mass flow sensor is disclosed that includes a housing ( 16 ) having a first sensor region and a second sensor region, a first thin film temperature sensor ( 39 ) formed at the first sensor region and a second thin film temperature sensor ( 58 ) formed at the second sensor region. A heating element ( 40 ) is arranged to heat the first temperature sensor ( 39 ) and a controller ( 46 ) is operably connected to the first temperature sensor ( 39 ), the second temperature sensor ( 58 ) and the heating element ( 40 ), and controls a power level to the heating element ( 40 ) to maintaining a temperature difference between the first temperature sensor ( 39 ) and the second temperature sensor ( 58 ). A thin film temperature sensor and a method of using the thermal mass flow sensor are also disclosed.

Description:
FIELD OF THE INVENTION 
   The present invention is directed toward an improved thermal mass flow sensor, and, more specifically, toward a low power, thermal mass flow sensor that provides a rapid and substantially linear output in response to flow changes. 
   BACKGROUND OF THE INVENTION 
   Thermal mass flow sensors operate by maintaining a temperature difference between two elements mounted in a mass flow passageway. A heating element is generally provided to heat one of the elements, and the temperatures of the elements are monitored. A mass flow, such as a mass of air, moving through the passageway and over the two elements cools the heated element. Large mass flows cool the heated element to a greater extent than do small mass flows. The amount of power required to maintain a given temperature difference therefore provides an indication of the mass flow. 
   Known thermal mass flow sensors suffer from several shortcomings. For example, with many designs, there is a non-linear relationship between the flow rate and the power required to maintain a temperature difference. Moreover, some sensors require significant power to operate, and many have slow response times. And, while a particular sensor may function adequately in a given environment, it is difficult to replicate the characteristics of that sensor and to make multiple sensors having the same output characteristics. Thus, recalibration is required each time a sensor is replaced. It is therefore desirable to provide a thermal mass flow sensor that addresses these and other shortcomings. 
   SUMMARY OF THE INVENTION 
   These and other shortcomings of prior temperature sensors are addressed by the present invention, which comprises, in a first aspect, a thermal mass flow sensor that includes a housing having a first sensor region and a second sensor region. A first thin film temperature sensor is formed at the first sensor region, and a second thin film temperature sensor is formed at the second sensor region. A heating element is arranged to heat the first temperature sensor, and a controller is operably connected to the first temperature sensor, the second temperature sensor and the heating element in order to control a power level to the heating element to maintain a temperature difference between the first temperature sensor and the second temperature sensor. 
   Another aspect of the invention comprises a flow sensing element for use in a thermal mass flow sensor that includes a substrate, a planar resistive thermal device (RTD) supported by the substrate that has a periphery, and a planar heating element comprising a strip of resistive material disposed along the periphery of the RTD. 
   A further aspect of the invention comprises a method of measuring mass flow that involves providing a first planar temperature sensor and a second planar temperature sensor and mounting a planar heating element along the first planar temperature sensor. Next, the first planar temperature sensor is mounted in a mass flow passageway defining a mass flow direction at a first angle to the mass flow direction, and the second planar temperature sensor is mounted in the mass flow passageway at a second angle to the mass flow direction. A predetermined temperature difference is maintained between the first temperature sensor and the second temperature sensor, and a mass flow is determined from the amount of power required to maintain the predetermined temperature difference. 
   An additional aspect of the invention comprises a method of forming a flow sensing element for use in a thermal mass flow sensor that involves providing a substrate, depositing a thin film of platinum on the substrate, forming a thin film strip of TaN on the platinum, electrically connecting the platinum film to a controller, and electrically connecting the strip of TaN to the controller. 
   A further aspect of the invention comprises a thermal mass flow sensor that includes a housing comprising a first support region and a second support region, a first substrate at the first support region and a second substrate at the second support region. A first planar temperature sensor is formed on the first substrate, a second planar temperature sensor is formed on the second substrate, and a heating element is arranged to heat the first temperature sensor. A controller is operably connected to the first temperature sensor, the second temperature sensor and the heating element and controls a power level to the heating element to maintain a temperature difference between the first temperature sensor and the second temperature sensor. 
   Another aspect of the invention comprises a flow sensing element for use in a thermal mass flow sensor that includes a substrate, a thin film RTD formed on the substrate and having a periphery, and a thin film heating element formed along the periphery of the RTD. 
   An additional aspect of the invention comprises a method of measuring mass flow that involves providing a first thin film RTD and a second thin film RTD and forming a thin film heating element around the first thin film RTD. The first thin film RTD is mounted in a mass flow passageway defining a mass flow direction at a first angle to the mass flow direction, and the second thin film RTD is mounted in the mass flow passageway at a second angle to the mass flow direction. A predetermined temperature difference is maintained between the first RTD, and second RTD and a mass flow is determined from an amount of power required to maintain the predetermined temperature difference. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other benefits of the present invention will be better understood after a reading of the following detailed description together with the following drawings wherein: 
       FIG. 1  is a perspective view of a mass flow sensor including a housing supporting a mass flow sensing element and a temperature sensing element according to an embodiment of the present invention; 
       FIG. 2  is a perspective view of the housing of  FIG. 1  with the sensing elements removed; 
       FIG. 3  is a perspective view of the mass flow sensing element of  FIG. 1 ; 
       FIG. 4  is a front elevational view of the temperature sensing element of  FIG. 1 ; 
       FIG. 5  is a side elevational view of the mass flow sensor of  FIG. 1 ; 
       FIG. 6  is a top plan view of the mass flow sensor of  FIG. 1 ; 
       FIG. 7  is a sectional elevational view taken along line  7 — 7  of  FIG. 5 ; 
       FIG. 8  is a graph showing the relationship between the power level required to maintain a temperature difference between two temperature sensors and the angle of one of the sensors to the mass flow direction; 
       FIG. 9  is a graph showing the relationship between flow rate and power; 
       FIG. 10  is a graph showing the response time of the temperature sensor of  FIG. 1 ; and 
       FIG. 11  is a schematic diagram showing a controller for controlling the mass flow sensor of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings, wherein the showings are for purposes of illustrating preferred embodiments of the invention only, and not for the purpose of limiting same,  FIG. 1  illustrates a mass flow sensor  10  comprising a mounting plate  12 , a hollow mast  14  extending through (or integrally formed with) mounting plate  12 , and a housing  16  attached to hollow mast  14  by a suitable chemical adhesive or by laser welding, for example. Housing  16 , also illustrated in  FIG. 2 , includes a top wall  18 , a front wall  20 , a rear wall  22 , and first and second sidewalls  24 ,  26 . Each of the first and second walls  24 ,  26  includes a recessed mounting portion  28  with an opening  30  into the interior of housing  16  for receiving sensing elements described hereafter. Front wall  20  also includes a portion  29  of reduced thickness that functions as a thermal choke to reduce heat transfer between first sidewall  24  and second sidewall  26 . As will be appreciated from  FIG. 6 , front wall  20  and rear wall  22  comprise arcs of a circle, while first and second sidewalls  24 ,  26  comprise chords of that circle. 
   A first sensing element  32 , illustrated in  FIG. 3 , comprises a carrier  34  formed from an iron-chromium alloy (FeCrAlloy). Carrier  34  includes a portion  36  having a reduced thickness which functions as a thermal choke, and a central portion  37 . Thermal choke  36  is optional, and carrier  34  may alternately be formed with a planar surface and without the thermal choke. A substrate  38  is formed on the central portion  37  of carrier  34 , such as by brazing, for example, from a material such as aluminum oxide to provide electrical insulation between the carrier and the elements described hereafter. Substrate  38  is preferably about 0.010 inches thick. A resistive temperature detector  39  (hereinafter “RTD”) is formed in a central location on substrate  38  from a material having a high thermal coefficient of resistance such as platinum. The carrier and the substrate material should be chosen to have similar coefficients of thermal expansion to avoid possible damage to the flow sensing element  32  from different rates of expansion and contraction as the sensor is heated and cooled during use. A planar heating element  40  is formed around the periphery  42  of the RTD. The heating element  40  may be formed, for example, from a material such as tantalum nitride (TaN) or Nichrome having a nominal resistance of about 20 ohms. The RTD  39  and the heating element  40  are preferably formed as thin films by a suitable process such as chemical vapor deposition, sputtering, etc. 
   A first pair of leads  44  connects the RTD  39  to a controller  46  via gold pads  45  connected to the RTD  39 , illustrated for example, in  FIG. 6 , and a second pair of leads  48  connects the heating element  40  to the controller  46  via additional gold pads  45 . As illustrated in  FIG. 7 , temperature sensing element  32  is mounted in housing  16  such that RTD  39  and heating element  40  face the interior of housing  16  while the rear side  50  of heating element  40  faces outwardly as illustrated in  FIG. 2  and is exposed to the ambient atmosphere surrounding flow sensor  10 . 
   A second sensing element  52 , best seen in  FIGS. 4 and 5 , is mounted in recessed mounting portion  28  of second sidewall  26 , and comprises a substrate  54  having a portion  56  of reduced thickness forming a thermal choke  56 , and a second RTD  58  formed on substrate  54 . Leads  60  connect the second RTD  58  to controller  46 . Second sensing element  52  is mounted in the recessed mounting portion  28  of second sidewall  26  with a rear face  62  facing outwardly from housing  16 . 
   Referring now to  FIG. 6 , sensing element  10  is shown divided by a center line  64  running between front wall  20  and rear wall  22 . First sensing element  32  and second sensing element  52  are preferably mounted at equal angles α from this center line, and are separated from one another by a second angle β equal to 2α. Flow sensing element  10  is preferably mounted in a mass flow passage way with center line  64  aligned with the direction of mass flow and front wall  20  facing in the direction of the mass flow. This arrangement exposes first sensing element  32  and second sensing element  52  to substantially equal amounts of mass flow, assuming the sensor is mounted at a location in the mass flow passageway where airflow is substantially laminar. Because the relationship of the first and second sensing elements to the housing is fixed by the geometry of the housing, and the housing can be accurately attached to the mast, the present arrangement can readily and accurately be replicated to produce a multiple sensors having very similar response characteristics. 
   The basic operation of the mass flow sensor  10  will now be described. The mass flow sensor  10  is mounted in a mass flow passageway so that centerline  64  is aligned with the direction of mass flow and front wall  20  faces into the mass flow. Controller  46  monitors the temperature detected by first RTD  39  and second RTD  58  and provides power to heating element  40  sufficient to maintain a temperature difference ΔT, such as 100° F., between first RTD  39  and second RTD  58 . Mass flow past the heated RTD  39  cools the heated RTD  39 , and the amount of cooling is proportional to the amount of mass flow. Consequently, the amount of power required to maintain a temperature difference is related to the mass flow. 
   Two countervailing factors influence the selection of the temperature difference ΔT: the temperature difference should be made as high as possible to minimize errors and to increase the sensitivity of the thermal mass flow sensor and 2) the temperature difference should be made as low as possible to minimize power consumption and overheating of the RTD. The present applicant has found that a temperature difference of 50 to 500 degrees could be used and that a temperature difference of about 100 degrees F. provides good sensitivity while consuming an acceptable amount of power. 
   The power required to maintain the temperature difference is proportional to the mass air flow as shown by the formula:
 
 P =( C   0 ( T )+ C   1 ( T )·( Q   M ) N )·Δ T  
 
where P equals power (watts) Q M  equals the mass flow rate in pounds per minute, C 0 (T) and C 1 (T) are coefficients related to the header geometry and the thermophysical properties of the mass flow and the flow sensing element materials, n is a coefficient related to the laminar/turbulent regime of the flow, ΔT is the temperature difference between the first and second RTD&#39;s  39 ,  58 , and T is the flow temperature in degrees F.
 
   The graph of  FIG. 8  illustrates the relationship between the power required to maintain a 100° F. temperature difference between the first RTD  39  and second RTD  58  at a mass flow rate of 50 pounds per minute. As will be clear from this graph, maximum cooling of the RTD&#39;s occurs, and therefore maximum power is required to maintain the temperature difference, when angle alpha is equal to about 15°. Power consumption is also high at angles α ranging from 10° to 20°, and reasonably elevated at angles α between 6° and 35°. Setting angle α equal to approximately 15°, therefore, provides the greatest sensitivity for flow sensing element  10 , while significant benefits are still obtained at angles α between 10° and 20° and, to some extent, at angles between about 6° and 35°. 
     FIG. 9  illustrates the substantially linear relationship between flow rate and power consumption provided by the flow sensing device of this embodiment of the present invention over a range of flow rates from 0 to 70 pounds per minute. Control circuitry associated with the mass flow sensor of the present invention can therefore be significantly simplified and does not need to adjust for non-linear changes in power consumption. Greater accuracy can be obtained by noting that a first linear relationship exists between power consumption and flow rate over a mass flow range of 0 to about 15 pounds per minute and that a second linear relationship exists over a mass flow range of about 15 pounds per minute to 70 pounds per minute. Calculations based on two linear relationships are still significantly easier to perform than calculations based on the non-linear power to flow rate relationships found in conventional mass flow sensors. 
     FIG. 10  illustrates the rapid response time of first temperature sensing element  32 . As illustrated in this figure, at a flow rate of 70 pounds per minute, a 100 degree F. temperature increase is detected by RTD  39  in less than 5 seconds. Known sensors do not exhibit this sensitivity, and thus flow rates may change substantially before being detected by a conventional mass flow sensor. The high sensitivity thus allows changes in mass flow rate to be reliably detected. 
     FIG. 11  illustrates schematically the elements of controller  46 . Controller  46  includes digital transducer hardware  70  and digital transducer software  72 . The resistance of second sensing element  52  is measured via line  74  by a circuit  76  which circuit  76  outputs a voltage on line  78  to a first multiplexer  80  while the resistance of first RTD  39  is measured by a second circuit  82  via a line  84  and second circuit  82  outputs a voltage on line  86  to first multiplexer  80 . First multiplexer  80  is connected to a second multiplexer  88  via a line  90 , second multiplexer  88  is connected to an A/D converter  92  which outputs a voltage to circuit element  94  on line  96 . Circuit element  94  produces an output on line  98  indicative of the temperature of second RTD  58 , and line  98  is connected to a first comparator  100 . A second line  102  connects A/D converter  92  to a circuit element  104  which converts the voltage on line  102  to an indication of the temperature of first RTD  39  on a line  106 , and line  106  is connected to a second input of comparator  100 . Comparator  100  outputs a temperature difference ΔT between the sensed temperature of RTD  58  and the sensed temperature of RTD  39 , on line  108  which in turn is connected to a second comparator  110 , a predetermined temperature difference ΔT is input into a second input of second comparator  110 , and second comparator  110  outputs a signal on line  112  indicative of the error between the set temperature difference and the existing temperature difference, and this error signal is sent to a proportional integration loop  114 . PI loop  114  outputs on line  116  a signal representing the current that should be supplied to heater  40  in order to maintain the desired temperature difference, the signal is received by a third circuit  118  which outputs on line  120  a voltage level necessary for maintaining the required current. This signal is received by a digital analog converter  122  which outputs a signal indicative of the required control voltage on line  124  to a constant current driver  126  connected to heating element  40 . 
   Two signals are fed back from constant current driver  126  to first multiplexer  88  along a first line  128  and a second line  130 . The signal on line  128  represents the voltage being applied to heater  40 , while the signal on line  130  represents the current being supplied to heater  40 . These values are converted to digital values by A/D converter  92  and fed to a circuit element  132  which calculates heater power consumption and sends a signal indicative of heater power consumption to circuit element  134  which in turn calculates the mass flow rate passed flow sensing element  10  from these values. 
   It should be recognized that additional variations of the above-described implementations may be reached without departing from the spirit and scope of the present invention.