Strain gauge apparatus and means for treating temperature dependency

Strain gauge characteristic stabilization apparatus comprises a parallel connected combination of strain sensitive resistance (i.e., R.sub.eq), temperature conditioning means proximate R.sub.eq, and circuit means between R.sub.eq and the temperature conditioning means. The circuit means senses deviations in R.sub.eq from a predetermined value and causes the temperature conditioning means to control the temperature of R.sub.eq so as to reduce the deviations substantially to zero.

The present invention relates to strain gauge apparatus and more 
particularly to piezoresistive pressure sensors and the treatment of 
temperature effects thereon. 
Strain sensitive apparatus of the semiconductor piezoresistive type is well 
known in the art. See for instance (a) Journal of Applied Physics, October 
1961, Vol. 32 No. 10, Pages 2008-2019 (b) U.S. Pat. No. 3,049,685 (c) U.S. 
Pat. No. 3,456,226 and (d) U.S. Pat. No. 3,641,812. Such devices commonly 
comprise a flexible silicon diaphragm in or on which a plurality of 
piezoresistive strain gauge elements are essentially integrally formed. 
The diaphragm commonly comprises crystalline silicon of one conductivity 
type with the strain gauge piezoresistors formed of opposite conductivity 
type by diffusion or other appropriate process whereby a p-n junction is 
achieved. With the diaphragm rigidly supported at its periphery, a 
transverse force applied centrally to one face causes flexure strain of 
the diaphragm and attendant changes in resistance of the piezoresistors. 
Usually at least two of the piezoresistors are arranged relative to each 
other on the diaphragm so that the force induced diaphragm movement causes 
one piezoresistor to increase in resistance while the other piezoresistor 
decreases in resistance. Thus when the force is due to pressure, one 
resistor varies directly with pressure and the other resistor varies 
inversely with pressure. 
However, the values of these piezoresistors are dependent not only on 
pressure but also on temperature. More particularly, the resistance of 
each of these piezoresistors increases with temperature and thus a 
temperature change, if not properly dealt with, can infuse errors into 
measurement of pressure with such devices. This problem of piezoresistance 
dependency on temperature has been recognized and treated by various forms 
of signal conditioning and temperature compensating circuitry. See for 
instance U.S. Pat. Nos. 3,956,927; 3,841,150; 3,836,796; and 3,457,493. In 
accordance with the present invention there is provided simplified 
apparatus for stabilizing the operating characteristics of such devices 
and making same more immune to temperature changes in their environment.

Referring now to FIGS. 1a and 1b, therein is represented a piezoresistive 
pressure sensor assembly employed with the present invention. In 
accordance with the descriptions hereinabove, a diaphragm 11 of n-type 
material is secured at its periphery to a tube 13. Diffused into diaphragm 
11 are four piezoresistors R.sub.r1, R.sub.r2, R.sub.t1, and R.sub.t2, 
each of p-type material. A resistor R.sub.H, also of p-type material and 
diffused into the n-type material, is located around the chip periphery. 
Metallized pads 15 permit wire connections to be made to said resistors. 
The n-type material is connected to ground reference potential. As force 
or pressure P increases, the diaphragm flexes slightly and causes the 
resistances of said four piezoresistors to change. More particularly, the 
resistances of R.sub.r1 and R.sub.r2 increase with increasing pressure and 
the resistances of R.sub.t1 and R.sub.t2 decrease with increasing 
pressure. Also, and as pointed out hereinabove, the resistances of all 
four piezoresistors increase with increasing temperature. 
Referring now to FIG. 2, the circuitry therein illustrated comprises a 
closed loop system wherein a signal indicative of the temperature of all 
four FIG. 1 piezoresistors is derived from a pair of said piezoresistors 
and is used to control the heat output of a heat generator which in turn 
controls the temperature of said piezoresistors. More particularly, FIG. 1 
piezoresistors R.sub.r2 and R.sub.t2 are connected in parallel and their 
equivalent composite resistance R.sub.eq (R.sub.eq =R.sub.r2 R.sub.t2 
/(R.sub.r2 +R.sub.t2)) is connected between the output and the inverting 
input of operational amplifier 17. The non-inverting input of op-amp 17 is 
tied to ground. Also connected between the op-amp 17 output and inverting 
input is the series connection of resistances R.sub.a, R.sub.b, and 
R.sub.c. The junction between R.sub.a and R.sub.b is connected to voltage 
source +V.sub.dc and the junction between R.sub.b and R.sub.c is fed to 
the input of integrator 19. The output of integrator 19 is connected to 
the input of power amplifier 21 which in turn drives the FIG. 1 resistor 
R.sub.H causing same to output heat and control the piezoresistor 
temperature. In operation, the FIG. 2 circuit applies voltage to heater 
R.sub.H and heats the piezoresistance device until piezoresistors R.sub.r2 
and R.sub.t2 increase their values to a level such that R.sub.eq =(R.sub.a 
/R.sub.b)R.sub.c. Any deviation from or error in this condition is 
integrated until "nulled out". That is, heater R.sub.H warms R.sub.r2 and 
R.sub.t2 to a temperature at which R.sub.eq =(R.sub.a /R.sub.b)R.sub.c and 
at said temperature the integrator input goes to zero and the integrator 
output becomes constant. At a constant pressure, this temperature, once 
achieved, is maintained since any deviation of the integrator input from 
zero causes appropriate compensating change in the integrator output, the 
heat output, and the temperature of the piezoresistors. 
Although the FIG. 2 system will maintain R.sub.eq equal to R.sub.a R.sub.c 
/R.sub.b, the FIG. 2 system does not maintain a perfectly constant 
temperature as the pressure P varies. That is, the parallel combination of 
R.sub.r2 and R.sub.t2 (i.e., R.sub.eq) is still somewhat a function of 
pressure. However, R.sub.eq is considerably more insensitive to pressure 
changes than either R.sub.r2 or R.sub.t2 alone and the system maintains a 
piezoresistor temperature which is sufficiently constant to reduce 
temperature dependency enough to enable use of R.sub.r1 and R.sub.t1 in 
several pressure measuring applications. Furthermore, as indicated in FIG. 
3, which except for the addition of R.sub..beta. is the same as FIG. 2, 
dependency of R.sub.eq upon pressure can be still further reduced by 
connecting a resistance R.sub..beta. in series with either R.sub.r2 or 
R.sub.t2, whichever has the larger magnitude of pressure sensitivity, such 
that R.sub.eq, now redefined to incorporate the added series resistance 
R.sub..beta., is substantially independent of pressure. In FIG. 3, when 
R.sub.t2 is the more pressure sensitive, the dashed line connections apply 
and R.sub.eq is redefined as R.sub.r2 R.sub.t.SIGMA. /(R.sub.r2 
+R.sub.t.SIGMA.) where R.sub.t.SIGMA. =R.sub.t2 +R.sub..beta.. When 
R.sub.r2 is the more pressure sensitive, the dotted line connections apply 
and R.sub.eq is redefined as R.sub.t2 R.sub.r.SIGMA. /(R.sub.t2 
+R.sub.r.SIGMA.) where R.sub.r.SIGMA. =R.sub.r2 +R.sub..beta.. 
It will be apparent to those skilled in the art that temperature 
conditioning means other than heater resistor R.sub.H may be used. For 
instance a surrounding oven, or heaters not unitary with the chip, could 
be employed. Also, a temperature conditioning means capable of extracting 
heat as well as generating/adding heat could be employed. 
Thus, while various embodiments of the present invention have been shown 
and/or described, it is apparent that changes and modifications may be 
made therein without departing from the invention in its broader aspects. 
The aim of the appended claims, therefore, is to cover all such changes 
and modifications as fall within the true spirit and scope of the 
invention.