A two-axis inclinometer having a central electrically conductive member inserted within and spaced from the interior walls of a non-electrically conductive vessel. A plurality of four plates are mounted symmetrically on the exterior surface of the vessel. A dielectric fluid is provided in the gap between the center member and the interior wall of the vessel so as to provide a differential change in the capacitance between opposite plate members as the inclinometer is tilted about an axis perpendicular to its longitudinal axis.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to inclinometers in general and in particular 
to a two-axis inclinometer comprising four capacitor plates separated from 
a common capacitor plate by a gap partially filled with a dielectric 
fluid. 
2. Description of the Prior Art 
Inclinometers provide accurate measurements of rotation about one or more 
horizontal axes. The primary output is typically an analog voltage which 
is proportional to the tilt position of the inclinometer relative to the 
horizontal axes. Some types of inclinometers comprise a pendulum. Others 
comprise a capacitor separated by a dielectric liquid. Typically, prior 
known inclinometers are single-axis devices, i.e. duplicate sensing units 
are required in order to obtain an input about two orthogonal axes. 
SUMMARY OF THE INVENTION 
The present invention comprises a two-axis inclinometer. In a preferred 
embodiment there is provided a pair of concentric cylindrical members. The 
interior member comprises an electrically conductive material. The 
exterior member comprises a non-electrically conductive material. The 
interior member is spaced from the exterior member by a gap in which there 
is provided a dielectric fluid. Symmetrically placed on the outside wall 
of the exterior member are four capacitor plates. 
As the inclinometer is tilted about an axis perpendicular to its 
longitudinal axis, the dielectric fluid in the gap between one of the 
plates rises while the dielectric fluid in the gap between the opposite 
plate and the common member falls. The resulting change in the capacitance 
between each of the plates and the common member is used for providing an 
output signal which corresponds to the differential of the capacitance 
between each of the plates and the common member. A second output signal 
is generated which corresponds to the differential capacitance between the 
second pair of plates and the common member as the inclinometer is tilted 
about the orthogonal axis. A principal advantage of the present invention 
is that a minimum amount of apparatus is required to obtain information 
concerning tilts about two orthogonal axes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is provided in accordance with the present 
invention an inclinometer apparatus designated generally as 1. In the 
apparatus 1 there is provided a hollow cylindrical non-electrically 
conductive vessel 2. Inserted in the vessel 2 and concentric therewith 
there is provided an electrically conductive cylindrical center member 3. 
The member 3 is spaced from the interior wall of the vessel 2 by means of 
a gap 4. In the gap 4 there is provided a dielectric fluid 5. Mounted to 
the exterior wall of the vessel 2 there are provided four electrically 
conductive plate members 6, 7, 8 and 9. The interior member 3 is kept 
spaced from the interior walls of the vessel 2 and the dielectric fluid 
located therebetween is contained within the vessel 2 by means of an 
O-ring 10. 
Referring to FIG. 2, the center member 3 is coupled to ground as shown by 
an electrical line 15. Plates 6, 7, 8 and 9 are coupled to electrical 
wires 16, 17, 18, 19, respectively. 
Referring to FIG. 3, there is provided an electrical circuit for generating 
a signal corresponding to the differential capacitance of each of the 
pairs of orthogonal plates 6-9. As shown in FIG. 3, there is provided a 
pair of current sources 20 and 21 which are coupled to a source of 
potential B+and to a pair of switches S1 and S2. A capacitor C1 comprising 
the center member 3 and the exterior plate 9 is coupled between the switch 
S1 and ground. A capacitor C2 comprising the center member 3 and the 
exterior plate 7 is coupled between S2 and ground. Coupled in parallel 
with the capacitor C1 there is provided a switch S3. Coupled in parallel 
with the capacitor C2 there is provided a switch S4. Switches S1, S2, S3 
and S4 are opened and closed on a repetitive basis, as will be further 
described below by a clock-driven switch driver 23. Switches S1 and S2 are 
further coupled to an amplifier 24 by means of an RC low pass filter 
circuit R2,C4 and R1,C3, respectively. The output of the amplifier 24 is 
designated Sy. An identical circuit, not shown, is provided for generating 
an output signal Sx as a function of the differential change in 
capacitance between the member 3 and the plates 6 and 8. That is, plates 6 
and 8 are coupled to switches S2' and S1' which correspond to switches S1 
and S2, respectively. 
Typically, the size of the gap between the member 3 and the interior wall 
of the vessel 2 is approximately 50 mils and the volume of the dielectric 
fluid inserted therein in such that when the apparatus 1 is tilted at an 
angle of 45.degree. relative to its longitudinal axis, the fluid extends 
approximately from the bottom of one of the exterior plates to the top of 
the opposite exterior plate. For example, referring to FIG. 2, there is 
shown a pair of orthogonal axes x and y. When the apparatus 1 is tilted 
45.degree. relative to the x axis, the dielectric fluid 5 in the gap 4 
moves to the bottom of one of the plates 6 and 8 and to the top of the 
other of the plates 6 and 8. Similarly, when the apparatus 1 is tilted 
relative to the y axis, the fluid 5 moves to the bottom of one of the 
plates 7 and 9 and to the top of the other plate 7 and 9. 
The change in capacitance resulting from the change in the amount of 
dielectric fluid located between the plates and the center member 3 is 
processed in the circuit of FIG. 3 so as to provide an output signal 
corresponding to the differential capacitance between the orthogonal pairs 
of plates. Thus, an output signal Sy from the amplifier 24 corresponds to 
a change in the differential capacitance between the plates 7 and 9 while 
a corresponding output signal Sx from the companion circuit (not shown) 
corresponds to a change in the differential capacitance between the plates 
6 and 8 as the apparatus 1 is tilted relative to the y and x axes, 
respectively. 
Sensors or transducers often utilize a balanced pair of capacitors that 
vary differentially. Sometimes a very necessary characteristic of the 
circuit sensing this differential variation is that it be as insensitive 
as possible to equal variations (common mode variations) of the two 
capacitors. 
Accordingly, the capacitor C1 and C2 and the corresponding capacitors in 
the companion circuit (not shown) are repetitively cross-coupled to the 
current sources 20 and 21 so as to compensate for differences in the 
current sources 20 and 21 as may occur due to temperature and processing 
variations in the transistors used therein. Similarly, capacitors C1 and 
C2 are discharged to ground or other constant potential on a periodic 
basis by means of switches S3 and S4 and thereafter low pass filtered so 
as to provide, in conditions of no motion, DC inputs to the amplifier 24; 
the inputs to the amplifier 24 being a voltage proportional to the inverse 
of the capacitance measured between the plates and the center member 3. 
The operation of the switches S1-S4 is provided by a clock in the switch 
driver 23. 
While an embodiment of the present invention is described above, it is 
contemplated that various modifications may be made thereto without 
departing from the spirit and scope of the present invention. For example, 
in an alternative embodiment the plates 6, 7, 8 and 9 can be mounted in 
the interior of the cylinder 3 and held spaced therefrom by suitable 
insulating members. Electrical connections thereto may be provided by 
means of insulated feedthroughs. In this embodiment the need for a 
separate vessel to contain the dielectric fluid is not required. 
Accordingly, it is intended that the embodiment described be considered 
only as an illustration of the present invention and that the scope 
thereof should not be limited thereto but be determined by reference to 
the claims hereinafter provided.