Patent Publication Number: US-6668641-B2

Title: Apparatus and method for thermal dissipation in a thermal mass flow sensor

Description:
RELATED APPLICATIONS 
     Patent applications entitled, “Method and Apparatus For Thermal Isolation Of A Thermal Mass Flow Sensor”, having inventors, Jesse Ambrosina and Isao Suzuki, (Attorney Docket No. MKS-92) and “Apparatus and Method For Thermal Management of A Mass Flow Controller”, having inventors, Jesse Ambrosina, Isao Suzuki, and Ali Shajii, (Attorney Docket No. MKS-94), assigned to the same assignee as this application and filed on even date herewith are hereby incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to mass flow measuring systems and, more particularly, to mass flow sensor housings that substantially eliminate thermal gradients that might otherwise be imposed upon a mass flow sensor. 
     BACKGROUND OF THE INVENTION 
     Capillary tube thermal mass flow sensors exploit the fact that heat transfer to a fluid flowing in a laminar tube from the tube walls is a function of mass flow rate of the fluid, the difference between the fluid temperature and the wall temperature, and the specific heat of the fluid. Mass flow controllers employ a variety of mass flow sensor configurations. For example, one type of construction involves a stainless steel flow tube with two or more resistive elements in thermally conductive contact with the sensor tube. The resistive elements are typically composed of a material having a high temperature coefficient of resistance. Each of the elements can act as a heater, a detector, or both. One or more of the elements is energized with electrical current to supply heat to the fluid stream through the tube. If the heaters are supplied with constant current, the rate of fluid mass flow through the tube can be derived from temperature differences in the elements. Fluid mass flow rates can also be derived by varying the current through the heaters to maintain a constant temperature profile. 
     Such thermal mass flow sensors may be attached to a mass flow controller, with fluid from the controller&#39;s bypass tube feeding the capillary tube (also referred to herein as the sensor tube). Since mass flow measurements are dependent upon the temperature effects of the fluid upon the resistive elements, any external differential temperature imparted to the resistive elements could produce errors in the measurement of a mass flow rate. Unfortunately, thermal mass flow sensors are frequently operated in environments where an external thermal gradient might be imposed upon them. For example, a thermal mass flow sensor may be operated in close proximity to a valve coil that dissipates significant power as it operates. Heat generated from operation of the valve coil may be communicated through a conductive thermal path, such as that provided by a mass flow controller housing, to the mass flow sensor. The heat thus-communicated may impose a thermal gradient upon the mass flow sensor housing which could, in turn, superimpose the external thermal gradient upon the sensor&#39;s resistive elements, thus leading to errors in mass flow measurements. 
     A mass flow sensor that substantially eliminates externally imposed thermal gradients would therefore be highly desirable. 
     SUMMARY OF THE INVENTION 
     A mass flow controller in accordance with the principles of the present invention includes a sensor assembly, a valve assembly, and a mass flow controller housing to which the valve and sensor assemblies are attached. A thermally conductive element conducts heat from the valve assembly away from the sensor assembly, thereby reducing uncontrolled contributions of heat to the sensor assembly. In an illustrative embodiment, the mass flow controller includes a thermally conductive enclosure that substantially envelops the sensor and valve assemblies. The thermally conductive element substantially surrounds and makes thermally conductive contact with the valve assembly while, at the same time, making substantial conductive thermal contact with the enclosure. 
     In an embodiment that does not include the enclosure, the thermally conductive element may include structure on one or more surfaces that accelerate the dissipation of thermal energy from within the valve assembly. Such structure may include fins located on one or more exterior surfaces of the thermally conductive element that do not face the sensor assembly. The thermally conductive element may be composed of a high thermal conductivity material, such as aluminum. In an illustrative embodiment, the thermally conductive element is integral to the valve assembly. That is, in this illustrative embodiment, the exterior wall of the valve assembly is formed to conduct thermal energy away from the mass flow controller sensor assembly. 
     These and other advantages of the present disclosure will become more apparent to those of ordinary skill in the art after having read the following detailed descriptions of the preferred embodiments, which are illustrated in the attached drawing figures. For convenience of illustration, elements within the Figures may not be drawn to scale. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a sectional drawing of a mass flow controller in accordance with the principles of the present invention that employs heatsink that provides a thermally conductive path between the mass flow controller valve assembly and a thermally conductive mass flow controller enclosure; 
     FIG. 2 is a partial conceptual block diagram and partial sectional drawing of a mass flow rate sensor assembly in accordance with the principles of the present invention; 
     FIG. 3 is a perspective view of the exterior of a mass flow rate controller in accordance with the principles of the present invention; and 
     FIG. 4 is a perspective view of the exterior of a mass flow controller in accordance with the principles of the present invention, which includes a thermally conductive enclosure. 
    
    
     DETAILED DESCRIPTION OF DISCLOSURE 
     A mass flow controller in accordance with the principles of the present invention includes a mass flow sensor assembly and a valve assembly. In an illustrative embodiment, the mass flow sensor assembly is attached to the mass flow controller housing and includes a heat sink that is attached to the valve assembly housing and provides a low thermal impedance path to the surrounding atmosphere in order to dissipate heat generated through operation of the valve within the valve assembly. 
     The sectional view of FIG. 1 illustrates the major components of a mass flow controller in accordance with the principles of the present invention. The mass flow controller  100  includes a thermal mass flow sensor assembly  102  and a valve assembly  104 . The valve assembly  104  is connected to the mass flow controller housing  108  to control the rate of flow of gas in response to control signals generated by a mass flow sensor circuitry described generally in the discussion related to FIG.  2 . The mass flow controller  100  includes an inlet  106  for receiving a flow of gases to be metered. The process gas enters the mass flow controller though the inlet  106  and travels through the valved opening  110  to a bypass channel  112 . The valve  114  operates under control of the mass flow sensor and related circuitry to admit a precisely measured quantity of process gas into the inlet port  106 , through the controller, and out the outlet port  116  for a processing application, such as may be employed in integrated circuit manufacturing. The bypass channel  112  is connected to the inlet port  106  to receive and carry the stream of gas. 
     A laminar flow element  118  rests within the channel  112  and provides a pressure drop across the thermal mass flow sensor assembly  102  and drives a portion of the gas through the sensor tube  120  of the thermal mass flow sensor  102 . The thermal mass flow sensor assembly  102  includes circuitry that senses the rate of flow of gas through the controller  100  and controls operation of the valve assembly  114 . The thermal mass flow sensor assembly  102  is attached to a wall  122  of the mass flow controller that forms a boundary of the bypass channel  112 . Input  124  and output  126  apertures in the wall  122  provide access to the mass flow sensor assembly  102  for a gas travelling through the mass flow controller  100 . In this illustrative embodiment the mass flow sensor assembly  102  includes a baseplate  128  for attachment to the wall  122 . The baseplate  128  may be attached to the wall and to the remainder of the thermal mass flow sensor assembly  102  using threaded hole and mating bolt combinations, for example. Input  130  and output  132  legs of the sensor tube  120  extend through respective input  134  and output  136  apertures of the baseplate  128  and, through apertures  124  and  126 , enter the mass flow controller wall  122 . 
     The mass flow sensor assembly includes top  138  and bottom  140  sections that, when joined, form a thermal clamp  141  that holds both ends of the sensor tube active area (that is, the area defined by the extremes of resistive elements in thermal contact with the sensor tube) at substantially the same temperature. The thermal clamp also forms a chamber  142  around the active area of the sensor tube  120 . That is, the segment of the mass flow sensor tube  102  within the chamber  142  has, in thermal communication with it, two or more resistive elements  144 ,  146 , each of which may act as a heater, a detector, or both. One or more of the elements is energized with electrical current to supply heat to the fluid as it streams through the tube  120 . 
     The thermal clamp  141 , which is typically fabricated from a material characterized by a high thermal conductivity relative to the thermal conductivity of the sensor tube, makes good thermally conductive contact with the portion of the sensor tube just downstream from the resistive element  144  and with the portion of the sensor tube just upstream from the resistive element  146 . The thermal clamp thereby encloses and protects the resistive elements  144  and  146  and the sensor tube  120 . Additionally, the thermal clamp  141  thermally “anchors” those portions of the sensor tube with which it makes contact at, or near, the ambient temperature. In order to eliminate even minute errors due to temperature differentials, the sensor tube may be moved within the thermal clamp to insure that any difference between the resistance of the two coils is due to fluid flow through the sensor tube; not to temperature gradients imposed upon the coils from the environment. 
     A heat sink  150  is attached in good thermal communication with the housing  152  of the valve assembly  104 . Power dissipated by a solenoid in operation of the valve  114  may generate a significant amount of heat that might travel through the mass flow controller housing  108  and superimpose an external thermal gradient upon the sensor assembly  102 . Such an external thermal gradient superimposed on the sensor resistive elements  144  and  146  would create an error in the determination of mass flow through the sensor tube. That is, since the thermal mass flow sensor assembly  102  relies upon the tendency of the fluid flowing through the sensor tube  120  to establish a thermal gradient to measure the mass of fluid flowing through the tube  120 , an externally imposed thermal gradient would render a false reading. In this illustrative embodiment, the heat sink  150  substantially envelopes the valve assembly housing  152  and conducts heat away from the valve assembly housing  152 . 
     In an embodiment in which the heat sink  150  is open to the atmosphere, features such as fins  151  may be added to the heat sink to increase the transport of heat away from the valve assembly housing  152  and away from the sensor assembly  102 . That is, fins may be added to one or more sides of the heat sink  150  not facing the sensor assembly  102  in order to increase convective and radiant heat transport away from both the sensor assembly  102  and the valve assembly  104 . Alternatively, in an illustrative embodiment described in the discussion related to FIGS. 3 and 4, the heat sink  150  extends to make thermally conductive contact with a controller housing  164  that encloses the thermal mass flow sensor  102  and valve  104  assemblies. In such an embodiment, a smooth-surfaced heat sink  150  provides greater surface contact with the controller housing and thereby provides a more substantial thermally conductive path for dissipation heat from the valve assembly  104 . A mass flow controller in accordance with the principles of the present invention such as presented in the illustrative embodiment of FIG. 3 employs both a heat sink  150  and thermal ground  148  to substantially eliminate thermal gradients that might otherwise impair operation of the thermal mass flow sensor assembly  102 . 
     FIG. 2 illustrates in greater detail an embodiment of a mass flow sensor in accordance with the principles of the present invention. The bypass tube  112 , laminar flow element  118 , upstream resistive element  146  and downstream resistive element  144  are as previously described. The arrangement of the thermal clamp  141 , including top  138  and bottom  140  portions, and its thermally conductive communication with the sensor tube  120  is illustrated in greater detail here. The broken lines of the sensor tube  120  indicate that the tube is substantially surrounded and in conductive thermal contact with the thermal clamp  141  around its entire circumference at each end of its operational segment. The operational segment of the sensor tube is defined for illustrative purposes as that segment of the sensor tube disposed between the upstream  154  and downstream  156  legs of the thermal clamp. 
     One end of the bypass tube  112  defines an input port  119  and the other end of the bypass tube  112  defines an output port  121  so that fluid may flow from the input port  119  to the output port  121  in a downstream direction indicated by arrows  123 . The laminar flow element  118  is disposed within the bypass tube  112  for restricting the flow of fluid through the bypass tube  112 . An upstream end of the sensor tube  120  couples to the bypass tube between the input port and the laminar flow element. A downstream end of the sensor tube  120  couples to the bypass tube  112  between the laminar flow element  118  and the output port  121 , a fixed proportion of the total mass of fluid flowing from the input port  119  to the output port  121  flows through the sensor tube  120 . The sensor tube  120  may be of capillary dimensions and is fabricated from a material, such as steel, that is characterized by a relatively high thermal conductivity in comparison to the thermal conductivity of the fluid. 
     Each of the resistive elements  144  and  146  includes a thermally sensitive resistive conductor that is wound around a respective portion of the sensor tube  120 , each successive turn of the conductors being placed close to the previous turn. Each of the resistive elements extends along respective portions of the sensor tube  120  along an axis defined by the operational segment of the sensor tube  120 . Downstream resistive element  144  is disposed downstream of the resistive element  146 . The elements abut one another or are separated by a small gap for manufacturing convenience and are electrically connected at the center of the tube. Each resistive element  144 ,  146  provides an electrical resistance that varies as a function of its temperature. The temperature of each resistive element  144 ,  146  varies as a function of the electrical current flowing through its resistive conductor and the mass flow rate within the sensor tube  120 . In this way, each of the resistive elements  144 ,  146  operates as both a heater and a sensor. That is, the element acts as a heater that generates heat as a function of the current through the element and, at the same time, the element acts as a sensor, allowing the temperature of the element to be measured as a function of its electrical resistance. The mass flow sensor  102  may employ any of a variety of electronic circuits, typically in a Wheatstone bridge arrangement, to apply energy to the resistive elements  146  and  144 , to measure the temperature dependent resistance changes in each element  144 ,  146  and, thereby, the mass flow rate of fluid passing through the tube  120 . Circuits employed for this purpose are disclosed, for example, in U.S. Pat. No. 5,461,913, issued to Hinkle et al and U.S. Pat. No. 5,410,912 issued to Suzuki, both of which are hereby incorporated by reference in their entirety. 
     In operation, fluid flows from the input port  119  to the output port  121  and a portion of the fluid flows through the restrictive laminar flow element  118 . The remaining fluid flows through the sensor tube  120 . The circuit (not shown) causes an electrical current to flow through the resistive elements  144  and  146  so that the resistive elements  144  and  146  generate and apply heat to the sensor tube  120  and, thereby, to the fluid flowing through the sensor tube  120 . Because the upstream resistive element  146  transfers heat to the fluid before the fluid reaches the portion of the sensor tube  120  enclosed by the downstream resistive element  144 , the fluid conducts more heat away from the upstream resistive element  146  than it does from the downstream resistive element  144 . The difference in the amount of heat conducted away from the two resistive elements is proportional to the mass flow rate of fluid within the sensor tube  120  and, by extension, the total mass flow rate through the mass flow rate controller  100  from the input port  119  through the output port  121 . The circuit measures this difference by sensing the respective electrical resistances and generates an output signal that is representative of the mass flow rate through the sensor tube  120 . 
     The baseplate  122  may be integral to the thermal mass flow sensor assembly  102  or it may be attached to the assembly  102  through any of a variety of attachment means, such as threaded through-holes and bolts, for example. The baseplate  122  is configured to provide a thermal path between the sensor assembly  102  and the remainder of the mass flow controller  100  and to thereby maintain the average temperature of the sensor assembly  102  and the remainder of the mass flow controller at substantially the same average temperature. 
     The perspective view of FIG. 3 provides an external view of the sensor assembly and valve assembly as previously described. The valve assembly includes a heatsink  150  that is formed to provide a substantial thermal path between the valve assembly  104 , the heatsink  150  and an enclosure  164  to be described in the discussion related to FIG.  4 . The inlet  106  and outlet  116  ports are as previously described. The sensor assembly  102  includes top  138  and bottom  140  thermal clamp components attached to the wall of the controller assembly through the baseplate  128 . The baseplate  128  is held in position with bolts  160 . Mass flow controller electronics (not shown) are attached to the controller with a metal flange  162 . Exterior surfaces of the heatsink  150  make conductive contact with the interior surfaces of a controller enclosure. In this illustrative embodiment, at least three exterior surfaces of the heatsink make thermally conductive contact with the thermally conductive enclosure; which three exterior surfaces of the heatsink will be apparent from examination of FIG.  4 . 
     The perspective view of FIG. 4 illustrates a view of a mass flow controller assembly in accordance with the principles of the present invention in which a thermally conductive enclosure  164  envelopes the valve assembly  104  and thermal mass flow sensor assembly  102 , as previously described. A heatsink that makes conductive thermal contact with the valve assembly may also make thermal contact with the enclosure  164 . The enclosure may also incorporate features, such as fins, to accelerate heat exchange between the enclosure&#39;s interior and exterior. 
     While there have been illustrated and described particular embodiments of the present disclosure, it will be appreciated that numerous changes and modifications will occur to those skilled in the art. Accordingly, it is intended that the appended claims cover all those changes and modifications which fall within the true spirit and scope of the present disclosure.