Compensated pressure transducer employing digital processing techniques

#The semiconductor pressure transducer has acheived widespread use in a variety of applications involving the medical, aerospace and automotive industries. The high output obtainable from semiconductor transducers employing piezoresistive sensing elements have specified their use in a wide range of applications and structures. These factors together with constant improvement in integrated circuit technology, have allowed the design of ultraminiature transducers which are capable of operating in very diverse environments.

#It is, of course, known that a major problem in regard to these devices is the problem of compensating the transducer for changes both in temperature and pressure. As is known, the gage factor and resistivity of the sensor varies with temperature. In order to construct a transducer whose output and zero unbalance are essentially independent of temperature, the temperature coefficient of gage factor and resistance should be maintained as low as possible. In practice, it is extremely difficult if not virtually impossible to maintain a gage factor independent of temperature. Hence, the prior art transducers employ compensating techniques which are integrally associated with the sensor configuration. A usual method for output compensation utilizes the increase of bridge resistance to offset the decrease of gage factor with temperature. Hence, in such applications, it was important that the temperature coefficient of resistance over the entire range of interest be positive and somewhat larger than the decrease of gage factor with temperature.

#In order to compensate for such effects, many circuits and techniques were devised to add external resistors to the bridge circuit in order to control the output of the bridge according to temperature. Typical techniques employed a plurality of resistive elements which, if properly arranged, would provide a temperature compensated transducer. See, for example a patent entitled TEMPERATURE COMPENSATED SEMICONDUCTOR STRAIN GAGE UNIT, U.S. Pat. No. 3,245,252 issued on Apr. 12, 1966 to David J. First, Anthony D. Kurtz and Jean-Pierre A. Pugnaire. In any event, typical techniques to compensate strain gage transducers may require the addition of eleven or more external resistors to provide temperature compensation.

#The sole FIGURE is a block diagram partially in circuit form of a transducer structure employing digital techniques and operating according to the invention.

#Before proceeding with a description of the block diagram, it is to be noted that the functions to be described can be implemented by the use of a microprocessor in conjunction with a suitable program. In this manner, the discussion of the FIGURE will be described in general terms in order to define the specific requirements for implementing the structure by the use of a microprocessor unit and a software program. It is, of course, understood that the apparatus depicted is completely capable of being implemented by using conventional integrated circuit components in lieu of a generalized computer system.

#Referring to the FIGURE, reference numeral 10 depicts a Wheatstone bridge array comprising resistors 11-14. Resistors 11-14 may be piezoresistive semiconductor elements which are deposited or diffused on a suitable diaphragm such as silicon and which exhibit a change in resistance according to an applied pressure or force on the diaphragm.

#As is conventional in such bridge arrays, two of the resistors are positioned so that they are subjected to a tensile force, while the other two are subjected to a compressive force. This aspect of operation in piezoresistive bridge configurations is well known in the art and suitable examples of such bridge circuitry as well as operation of the same can be had by referring to U.S. Pat. No. 3,654,579 entitled ELECTROMECHANICAL TRANSDUCERS AND HOUSINGS issued on Apr. 4, 1972 to the Assignee herein.