WO1999046559A1

WO1999046559A1 - Orientation/attitude sensor

Info

Publication number: WO1999046559A1
Application number: PCT/US1999/004938
Authority: WO
Inventors: Bonne Ulrich; David Kubisiak; Ernest A. Satren
Original assignee: Honeywell Inc.
Priority date: 1998-03-12
Filing date: 1999-03-05
Publication date: 1999-09-16

Abstract

An orientation sensor includes a chamber holding a gas at a predetermined pressure. The chamber includes flow sensors and based on the pressure of the gas in the chamber and the temperature difference across the flow sensors, orientation of an object the sensor is attached to can be determined.

Description

ORIENTATION/ATTITUDE SENSOR

BACKGROUND OF THE INVENTION Present orientation sensors have slow response, require complex electronics, but are readily available. Many of such provide an electrical signal, indicative of an angular displacement of a body with respect to the horizontal, for subsequent use by servo-type mechanisms to take corrective action that realigns the body to the true horizontal if desired. Some available position sensors utilize a pendulum in conjunction with other components to sense and indicate angular position, such as that shown and described in U.S. Pat. No. 4,163,325 granted on Aug. 7, 1979 to D. Hughes for Verticality Sensors. Other position sensors utilize a source of radiation and associated optical elements, such as that shown and described in U.S. Pat. No. 4,159,422 granted on June 26, 1979 to S. Okubo for Temperature Stable Displacement Sensor With Fine Resolution. However, such position sensors are cumbersome and bulky and could prove to be unusable to environments where weight, space, response time and similar criteria are essential or critical.

Some available position or tilt sensors utilize a glass vial or tubular container within which an air or gas bubble is disposed in an electrolytic solution and to which suitable electrodes are attached. As the position of the tube in such fluid level devices moves with respect to the horizontal so does the relative position of the bubble and electrolyte with respect to the electrodes. A current passed between electrodes and through the electrolyte provides an indication of the angular position of the container. However, devices such as those shown in: U.S. Pat. No. 2,977,559 granted on Mar. 28, 1961 to A. M. Rosenberg et al for Low Resistance Electrolytic Tilt Device; in U.S. Pat. No. 3,114,209 granted on Dec. 17, 1963 to F. B. Foody et al for £eve/ Sensor, and in U.S. Pat. No. 3,299,523 granted on Jan. 24, 1967 to L. N. Lea for Levels fail to effectively isolate the electrolyte in the container from the effects ambient temperature and its changes.

It is the change in electrical resistance of the fluid within the container of these fluid levels that is sensed and utilized as the basic indication of change in angular position for these devices. However, the electrical resistance of the fluids utilized for such sensors is also dependent upon the temperature of the fluid. Thus, changes in resistance of the fluid due to changes in temperature will provide a false indication of -2- position if not accounted for. Correcting for such unwanted resistance changes can be cumbersome, expensive, time consuming and render sensors subject to temperature changes unusable for applications where thermal isolation and stability are required. Still other available sensors seek to provide some degree of isolation from changes in ambient temperature by utilizing insulating blocks for the fluid container as shown and described in U.S. Pat. No. 2,713,727 granted to L. L. Balsam on July 26, 1955 for Linear Bubble Level Signal Device; or by placing the fluid container in a container within a container as shown and described in U.S. Pat. No. 4,312,131 granted on Jan. 26, 1982 to P. J. Scriffiignans et al. fox Accurate Level Sensor. But, such sensors also do not provide static and dynamic thermal environments for the fluid wherein the fluid temperature remains isothermal (i.e. no temperature gradients within the level).

Available tilt sensors do not readily provide an accurate reading or are too complex or bulky to be efficiently used. Further, they have slow response time. Therefore, it would be desirable to have an orientation sensor that is accurate, simple, small in size as well as having a quick response time.

SUMMARY OF THE INVENTION An orientation sensor has a chamber with a gas at a predetermined pressure. The orientation sensor includes a thin film heating element placed in the chamber and a pair of thin film heat sensor elements suspended in air and disposed on opposite sides of the heating element in the chamber. The heating element creates a small natural convection flow in which the sensor elements sense a change in temperature over them. An apparatus is used to determine orientation based on the direction and magnitude of the prevailing acceleration vector and the temperature difference across the central heating element. The thin film heating element operates at a controlled temperature above ambient temperature under both flow and no-flow conditions in the chamber such that the ambient temperature influences are eliminated. Additional factors in determining the orientation sensor signal include pressure in the chamber and the type of fluid in the chamber. -3-

BRIEF DESCRIPTION OF THE DRAWINGS Fig. la shows a side view of the orientation sensor of the present invention. Fig. lb shows a side view of the cross section of the flow sensor used in the orientation sensor. Fig. 2 shows a top view of the flow sensor used in the orientation sensor.

Fig. 3 shows a graph of the sensor signal of the flow sensor vs. tilt of the orientation sensor.

Fig. 4 shows two side view configurations of the orientation sensor using a liquid placed in the chamber of the orientation sensor. Fig. 5a shows a heater control circuit.

Fig. 5b shows an amplifier circuit.

DETAILED DESCRIPTION OF THE PRESENT INVENTION The present invention is an orientation sensor 1 using two flow sensors 2 as taught in U.S. Pat No. 4,478,076 to sense orientation. The detail of the structure and operation of the mass air flow sensor will not be discussed here but more information can be found in the aforementioned patent. Fig. la shows a side view of the present invention 1 mounted on a mounting 3 where the sensors 2 are to be placed. The sensors 2 are placed in a small chamber 4 on top of the mounting 3. A gas pressurized at a specific pressure exists in the chamber 4 for the gas to flow and the flow sensors 2 to be able to sense the gas flow. In the present invention, Nitrogen is used. However, the invention is not limited to the use of this gas alone, but other gases within a certain density may be used such as Argon for example. Further, the gas in the chamber is pressurized at 500 psi. However, this pressure is used as an example only and other pressures can be used as long as the pressure is high enough to satisfy the signal-to- noise ratio needs of the tilt sensor application. The signal-to-noise ratio increases approximately as p is squared. The higher pressure is desirable since the effect of the convection flow is larger at higher pressures and thus easier to detect the flow. Fig. lb shows the side view of one of the flow sensors 5. Fig. 2 shows the top view of the flow sensor 5. The sensor 5 includes two sensor elements 6 and a heater element 8. The heater element 8 creates a small natural convection flow in which the sensor elements 6 sense a change in temperature over -4-

them. The temperature difference is processed as in U.S. Pat. No. 4,478,076. A correlation function of the factors involved in determining the orientation is then used to determine tilt wherein the function is:

A=arcsin[G/G_A(p,T)] where G is the output of the flow sensor 5, G_A is a correction factor for a specific flow sensor 5 dependent on pressure and temperature, p is the pressure of the gas in the chamber 4, T is the absolute temperature the sensor 1 is operating at and A would be the tilt angle which is relative to some reference such as the horizontal orientation for example when there is no acceleration in any horizontal direction. As described above, the sensor elements 6 will experience a temperature differential which will cause a resistance differential which in turn creates a voltage output. The output of the sensor 1 generated by sensor elements 6 is the voltage difference that is sensed between the sensor elements 6. With this function, the output signal (G) of the sensor elements 6 can be output and graphed based on the temperature and the pressure to calculate the tilt angle which is shown in Fig. 3 so that the orientation can be determined.

As stated before, the orientation sensor 1 uses two or three flow sensors 5 placed at right angles relative to one another in order to sense tilt (and acceleration) around the prevailing acceleration vector which predominantly would be the lg gravity vector. The use of two orthogonal flow sensors is to determine tilt around the orthogonal axes. For example, one flow sensor would be placed in the direction of sensing flow around the x- axis and one flow sensor would be placed in the direction of sensing flow around the y- axis direction. Therefore, if the sensor 1 is tilted around the x-axis, a flow would be created at a right angle to the x-axis sensor so that the tilt is sensed for this axis. However, the y-axis sensor would not sense anything since there is no flow perpendicular to that direction. An equivalent operation would occur for the y-axis sensor sensing tilts around the y-axis. The flow sensors 5 can be placed on top of one another or next to one another or any other configuration as long as the flow sensors 5 are at right angles with one another. As stated before, the sensors 2 are sealed into the chamber 4 with a pressurized gas. The present invention can be used with a pressurized gas or an unpressurrized liquid. Fig. 4 shows configurations of the orientation sensor 1 of the present invention -5- using liquid in the chamber 4 instead of gas. Fig. 4a shows a first configuration of the orientation sensor with the use of liquid. Liquid is placed in the chamber 4 and a second chamber 10 with liquid is connected to the first chamber 4 and a frit seal 12 exists between the two chambers 4, 10. A plug 14 is placed at the other end of the second chamber 10 merely to keep the liquid in the second chamber 10. As can be seen in Fig.

4a, a bubble 16 exists in the second chamber 10. However, the bubble 16 is not used for determining orientation, but is used to relieve stresses induced by the thermal coefficient of expansion mismatches that may exist in the orientation sensor 1. The flow sensors 2 would operate the same as above by now sensing the flow of the liquid instead of flow of the gas.

Another embodiment of the present invention with the chamber 4 filled with a liquid is shown in Fig. 4b. This embodiment uses a flexible cap 30 that seals the chamber 4 filled with liquid. The flexibility of the cap 30 allows the stresses induced by the thermal coefficient of expansion mismatches that may exist in the orientation sensor 1 to be relieved and thus, all the components of a frit seal 12 and a second chamber 10 are no longer required. This embodiment is a much simpler configuration, but either embodiment is functional. The description above and shown in Fig. 4 are for illustration purposes only and are not limited to these specific components, materials or sizes. Other components, materials, and sizes can be used as long as they operate according to the purpose of the present invention.

A heater control circuit and a sensor amplifier circuit are connected to the sensors 2 and are shown in Fig. 5. The two circuits can be connected to the sensors 2 outside of the chamber 4 or they can be connected on the sensor chip inside the chamber 4. Either configuration is sufficient and other configurations are possible as long as the connection exists. The heater control circuit, shown in Fig. 5a maintains the heater element 8 at approximately a constant temperature above the ambient temperature. The heater control circuit is based on the proper choice of the resistance value, R^, in the heater control circuit shown in Fig. 5. The proper choice of R^ is achieved by finding the proper resistance to optimally compensate for changes in ambient temperature, the pressure inside the orientation sensor chamber 4, the effective thermal conductivity and specific heat, the natural convection driven flow, the heating (R_H), sensing and reference -6- elements (R_R and Rς), or effect by other electronic circuit elements so that the changes in the sensor output due to changes in ambient temperature are minimized.

The amplifier circuit, shown in Fig. 5b, amplifies the voltage signal for a better reading. Heater control circuits and amplifier circuits are well known in this area of technology and any circuit to control heat or amplify signals can be used and will not be discussed in any further detail here. As a result, the components involved in the present invention are much simpler to the previous sensor implementations. Also, the orientation sensor 1 of the present invention has a faster response time in detecting orientation since it is only dependent on sensing flow of a low mass gas, rather then having to cope with the inertia of a proof mass, and not dependent on multiple other factors such as the sensors described in the background of the invention.

The invention has been described herein in detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized materials and components as are required. However, it is to be understood that the invention can be carried out by specifically different materials and components, and that various modifications, both as to the processing details and operating procedures, can be accomplished without departing from the scope of the invention itself.

Claims

-7-CLAIMS

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 1. An orientation sensor, comprising: a chamber filled with a high density fluid; a thin film heating element placed in the chamber; a pair of thin film heat sensing elements disposed on opposite sides of the heating element in the chamber; means for controlling and maintaining the thin film heating element temperature at a temperature above ambient temperature, under both flow and no-flow conditions in the chamber, at various ambient temperatures; and means for determining orientation based on the flow of the fluid across the heat sensing elements.

2. The orientation sensor of claim 1 wherein the fluid is a gas at an elevated pressure.

3. The orientation sensor of claim 1 wherein the orientation is determined by the flow of the fluid across the heat sensing elements, pressure the orientation sensor is operating at, and heating element temperature rise.

4. The orientation sensor of claim 1 wherein means for controlling and maintaining the heating element controls the heating element temperature at other temperatures than one constant temperature above ambient temperature so that the ambient temperature dependence of the orientation sensor is largely eliminated.

5. The orientation sensor of claim 1 wherein the fluid is a liquid.

6. The orientation sensor of claim 5, further comprising: a second chamber connected to the first chamber containing a fluid with an air bubble wherein the air bubble is used to reduce stress due to mismatches in the -o- coefficient of thermal expansion in the orientation sensor between the liquid and the chamber.

7. The orientation sensor of claim 6, further comprising: a porous seal between the first chamber and the second chamber to prevent the air bubble from going into the first chamber.

8. The orientation sensor of claim 7 wherein the seal is a frit seal.

9. An orientation sensor, comprising: a chamber filled with a high density fluid; a thin film heating element; a pair of thin film heat sensing elements; a semiconductor body supporting the heating element and sensing elements with at least a major portion of the heating element and sensing elements out of thermal contact with the body and with the sensing elements disposed on opposite sides of the heating element; means for controlling and maintaining the thin film heating element temperature at a temperature above ambient, under both flow and no-flow conditions in the chamber, at various ambient temperatures; and means for determining orientation based on the flow of the fluid across the heat sensing elements.

10. The orientation sensor of claim 9 wherein the fluid within the chamber is a gas at a high pressure.

11. The orientation sensor of claim 9 wherein the orientation is determined by the flow of the fluid across the heat sensing elements, pressure the orientation sensor is operating at, and heating element temperature rise.

12. The orientation sensor of claim 9 wherein means for controlling and maintaining the heating element controls the heating element temperature at other temperatures than -9- one constant temperature above ambient temperature so that the ambient temperature dependence of the orientation sensor is largely eliminated.

13. The orientation sensor of claim 9 wherein the fluid is a liquid.

14. The orientation sensor of claim 13 , further comprising : a second chamber connected to the first chamber containing a fluid with an air bubble wherein the air bubble is used to reduce stress due to mismatches in the coefficient of thermal expansion in the orientation sensor between the liquid and the chamber.

15. The orientation sensor of claim 14, further comprising: a seal between the first chamber and the second chamber to prevent the air bubble from going into the first chamber.

16. The orientation sensor of claim 15 wherein the seal is a frit seal.

17. An orientation sensor, comprising: a chamber filled with a fluid; at least one flow sensor sealed within the chamber; and means for sensing orientation based on a convection flow of the fluid over the flow sensor.

18. The orientation sensor of claim 17 wherein the fluid is a gas.

19. The orientation sensor of claim 17 wherein the orientation is determined by the flow of the fluid across the heat sensing elements, pressure the orientation sensor is operating at, and heating element temperature rise.

20. The orientation sensor of claim 17 wherein means for controlling and maintaining the heating element controls the heating element temperature at other -10- temperatures than one constant temperature above ambient temperature so that the ambient temperature dependence of the orientation sensor is largely eliminated.

21. The orientation sensor of claim 17 wherein the fluid is a liquid.

22. The orientation sensor of claim 17 wherein the orientation sensor comprises a plurality of flow sensors.

23. The orientation sensor of claim 22 wherein the flow sensors are orthogonal to each other in differing axes so that the orientation around the axes can be measured.

24. The orientation sensor of claim 5, further comprising: a seal made of a flexible material to reduce stress due to mismatches in the coefficient of thermal expansion in the orientation sensor between the liquid and the chamber.

25. The orientation sensor of claim 13 , further comprising: a seal made of a flexible material to reduce stress due to mismatches in the coefficient of thermal expansion in the orientation sensor between the liquid and the chamber.