Capacitive pressure sensor with intrinsic temperature compensation
Pressure sensors and their methods of use are described. In one embodiment, a pressure sensor includes a probe body and a capacitive sensor disposed at a distal end of the probe body. The capacitive sensor produces a sensing capacitance. The pressure sensor also includes a shunt capacitance. In the described pressure sensor, a change in the sensing capacitance due to dimensional changes associated with a temperature change is offset by a corresponding change in the shunt capacitance.
1. A pressure sensor, comprising:
a probe body;
a capacitive sensor disposed at a distal end of the probe body, the capacitive sensor producing a sensing capacitance;
a shunt capacitance; and
wherein a change in the sensing capacitance resulting from a change in temperature is intrinsically offset by a corresponding change in the shunt capacitance.
2. The pressure sensor according to claim 1 , wherein a capacitive sensor comprises:
a pressure deflectable diaphragm cap having a first capacitive surface electrically coupled to the probe body;
a relatively non-deformable component having a second capacitive surface coupled to the pressure deflectable diaphragm cap, the second capacitive surface being spaced-apart from the first capacitive surface; and
a lead electrically coupled to the second capacitive surface and isolated from the probe body;
wherein the shunt capacitance exists between the lead and the probe body.
3. The pressure sensor according to claim 2 , wherein the pressure deflectable diaphragm cap and the relatively non-deformable component are constructed and arranged in a cooperating relationship wherein a change in capacitance of the capacitive sensor resulting from changes in temperature is offset by a change in the shunt capacitance between the lead and the probe body thereby providing a sensed capacitance between the lead and probe body that is approximately the same over a predetermined temperature range without compensating for sensed temperature change.
4. The pressure sensor according to claim 3 , wherein the predetermined temperature range is approximately −40° C. to approximately 400° C.
5. The pressure sensor according to claim 3 , wherein the second capacitive surface is spaced-apart from the first capacitive surface by a gap, wherein materials and geometric relationship of the pressure deflectable diaphragm cap and the relatively non-deformable component are predetermined such that, as the gap increases with increasing temperature, an increase in the sensed capacitance associated with changes in the modulus of elasticity of the diaphragm with increasing temperature is offset.
6. The pressure sensor according to claim 2 , wherein the relatively non-deformable component comprises a dielectric material having a dielectric constant that changes with changes in temperature.
7. The pressure sensor according to claim 6 , wherein changes in the dielectric constant with changes in temperature results directly in a change in the shunt capacitance offsetting any changes in capacitance of the capacitive sensor.
8. The pressure sensor according to claim 2 , wherein the second capacitive surface is spaced-apart from the first capacitive surface by a gap, wherein the gap increases with increasing temperature thereby decreasing the capacitance of the capacitive sensor.
9. The pressure sensor according to claim 8 , wherein the pressure deflectable diaphragm cap includes diaphragm and a shoulder and wherein the relatively non-deformable component includes a ledge, wherein the shoulder and the ledge abut each other, the relatively non-deformable component having a protrusion extending beyond the ledge, the protrusion comprising the second capacitive surface that is spaced-apart from the first capacitive surface to define the gap, wherein a rate at which the gap increases is a function of a length of the protrusion.
10. The pressure sensor according to claim 9 , wherein the relatively non-deformable component comprises a material having a thermal coefficient of dielectric constant of approximately 180 ppm/° C.
11. The pressure sensor according to claim 10 , wherein the predetermined temperature range is approximately −40° C. to approximately 400° C.
12. The pressure sensor according to claim 11 , wherein the pressure deflectable diaphragm cap includes diaphragm and a shoulder and wherein the relatively non-deformable component includes a ledge, wherein the shoulder and the ledge abut each other, the relatively non-deformable component having a protrusion extending beyond the ledge toward the diaphragm and a length extending opposite the diaphragm, wherein the length is approximately 0.375 inches.
13. The pressure sensor according to claim 12 , wherein the second capacitive surface is spaced-apart from the first capacitive surface by a gap, wherein materials and geometric relationship of the pressure deflectable diaphragm cap and the relatively non-deformable component are predetermined such that as the gap increases with increasing temperature, an increase in the sensed capacitance associated with changes in the modulus of elasticity of the diaphragm with increasing temperature is offset.
14. The pressure sensor according to claim 2 , wherein the pressure deflectable diaphragm cap includes diaphragm and a shoulder and wherein the relatively non-deformable component includes a ledge, wherein the shoulder and the ledge abut each other, the relatively non-deformable component having a protrusion extending beyond the ledge toward the diaphragm and a length extending opposite the diaphragm, wherein the length is approximately 0.375 inches.
15. The pressure sensor according to claim 1 , wherein a decrease in capacitance of the capacitive sensor resulting from changes in temperature is offset by an equal increase in the shunt capacitance.
16. The pressure sensor according to claim 1 , wherein the sensing capacitance is approximately the same over a predetermined temperature range without a need for a temperature measurement to compensate for temperature changes.
17. A method of making a pressure sensor, comprising:
arranging a pressure deflectable diaphragm cap having a first capacitive surface on a probe body;
electrically coupling the first capacitive surface to the probe body;
selecting a material having a desired dielectric constant and forming a non-deformable component from the material;
forming a second capacitive surface on a portion of the non-deformable component;
connecting a lead to the second capacitive surface and positioning the non-deformable component within the probe body wherein a shunt capacitance is defined between the lead and the probe body; and
arranging the non-deformable component relative to the pressure deflectable diaphragm cap by spacing the second capacitive surface away from the first capacitive surface such that the first and second surfaces define a capacitive sensor having a sensing capacitance, wherein a change in the sensing capacitance resulting from a change in temperature is offset by a corresponding change in the shunt capacitance.
18. The method according to claim 17 , further comprising:
forming a ledge on a body of the non-deformable component such that a protrusion of a predetermined length is created;
forming the pressure deflectable diaphragm cap with a diaphragm and a shoulder;
abutting the shoulder against the ledge where the protrusion extends beyond the ledge toward a diaphragm of the pressure deflectable diaphragm cap.
19. The method according to claim 18 , wherein a sensed capacitance between the lead and probe body is approximately the same over a predetermined temperature range without compensating for temperature changes.
20. The method according to claim 19 , further comprising forming the pressure deflectable diaphragm cap from a material and forming the pressure deflectable diaphragm cap with a desired geometry, wherein the second capacitive surface is spaced-apart from the first capacitive surface by a gap, wherein as the gap increases with increasing temperature, an increase in the sensed capacitance associated with changes in a modulus of elasticity of the diaphragm with increasing temperature is offset.