Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2000-01-07
2001-06-05
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06240776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermal mass flow sensors.
2. Description of the Related Art
Circuits for mass flow sensors have long used a bridge configuration in measuring the mass flow rate of a fluid. The common bridge configurations included a servo constant temperature configuration, such as Hubbard, U.S. Pat. No. 4,843,881, and a constant current configuration. The bridge configurations were included as a part of a feedback network or servo loop.
The servo loop of the constant temperature configuration has required numerous current paths to be driven by a differential amplifier. That differential amplifier was used to drive the current path including an inductive heating and sensing coil for heating the fluid and sensing the mass flow rate of the fluid. Servo loop control of the sensing coil was undesirably influenced by the power requirements for the numerous current paths that did not drive the sensing coil. The power requirements of some of these current paths were at current levels on the order of 50 to 80 mA. Other of these current paths were typically on the order of 0.1 to 0.5 mA. As can thus be appreciated, these current paths typically carried high levels or amounts of current.
The constant current configuration has required even higher levels of current typically on the order of 160 mA. In the constant current bridge, the current paths were more interdependent than the current paths present in the servo loop of a constant temperature configuration. The high levels of current present in both configurations were unsuited to sensing coils used for fast temperature sensing responses.
Further, sensors using bridge configurations have required sensing coils in arms of the bridges to have the same coil parameters in order to achieve optimum sensing performance. Typical coil parameters included the number of coil turns, the electrical resistance of the coils, and the temperature coefficients of change of the coils. Lack of uniformity of these coil parameters has been a significant limitation on the performance of mass flow sensors using bridge configurations. Coils with even slightly differing parameters have significantly reduced the performance of such mass flow sensors.
SUMMARY OF THE INVENTION
A sensing circuit of a thermal mass flow sensor is provided for enhanced precision control of heating and temperature sensing coils used for fast sensing responses. The sensing circuit includes a pair of coil servo loops for achieving servo loop control of the sensing coils in each servo loop. Each servo loop in the sensing circuit includes one leg that is driven by a primary servo amplifier. The current paths in the servo loop include a main current path carrying low levels of current and minor current paths carrying very small levels of current. These small levels of current have insignificant current levels compared to the levels of current in the main current path. The use of a single main current path in the servo loop allows for nearly all of the current from the primary servo amplifier to be used for driving the coils, thereby improving the efficiency of the coil servo loop. A portion of the minor current paths is provided to a secondary servo amplifier. Since a servo amplifier requires little current, the use of a servo amplifier in the minor current paths allows for very small levels of current in such paths.
The heating and sensing coil in the servo loop is coupled to the primary servo amplifier configured as an integration network for measuring, amplifying, and linearizing power provided to the coil. The heating and sensing coil is also coupled to the secondary servo amplifier configured as a voltage follower network for providing a programmable voltage level to the integration network to compensate for any differing coil parameters between the sensing coils. The programmable voltage level is achieved by using a programmable reference resistor. The sensing circuit thus has the flexibility to accommodate coil parameter differences.
Further, the sensing coils of the servo loops are wrapped around a sensing tube of a mass flow sensing apparatus. The mass flow sensing apparatus includes a flow chamber for housing incoming fluid and outgoing fluid, an incoming flow tube for directing the incoming fluid to the sensing tube, an outgoing flow tube for directing outgoing fluid exiting the sensing tube, and a flow bypass element for receiving a portion of the incoming fluid to bypass the sensing tube. The sensing tube of the mass flow sensing apparatus is short and holds a low fluid mass, allowing for small sensing coils, a fast temperature sensing response, and insensitivity of the sensing tube to the position of the heating and sensing coils.
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McMillan Robert D.
McMillan Robert M.
Bracewell & Patterson L.L.P.
Fuller Benjamin R.
McMillan Company
Thompson Jewel V.
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