LIQUID BEARING TWO DEGREE OF FREEDOM INERTIAL SENSOR APPARATUS

Abstract: Liquid bearing two degree of freedom inertial sensor apparatus is disclosed. A single liquid bearing sensor is con­ figured in a two degree of freedom rate gyro mode, or two of the liquid bearing sensors are made pendulous along their spin axes and configured in a two degree of freedom rate gyro and accelerometer mode.

Description: Inertial sensor apparatus is used in guidance systems for air or space craft, or missiles or projectiles or other like vehicles. The apparatus may include sensors such as gyroscopes (gyros) for sensing the rate of change of attitude (rate) of the vehicle along its axes and/or accelerometers for sensing the acceleration of the vehicle along said axes. In some such apparatus a mass is suspended by a bearing arrangement and is driven to spin or rotate about a spin axis, and the rate or acceleration is sensed along an input axis or axes. Torquing and signal generating devices are arranged about the output and input axis or axes, respectively, of the sensor to provide signals corresponding to the sensed condition, i.e. rate of acceleration. Such inertial sensor apparatus may have a hydrodynamic bearing arrangement, wherein a gas is usually used as the working fluid. This implies tight fluid gaps, close tolerances, large bearing sizes and relatively high rotational speed of the sensor mass to achieve high angular momentum and high environmental (performance) acceleration capability. Tactical sensor apparatus, i.e. sensor apparatus used, for example, in systems to guide missiles fired from tanks or aircraft or other like vehicles usually requires a small, rugged sensor, but does not necessarily require the aforenoted high angular momentum and thus permits reduced rotational speed. The present invention takes advantage of the aforenoted by replacing a working gas with a working liquid so that relatively open gaps, loose tolerances and smaller bearing sizes, with an associated decrease in the rotational speed of the mass, are acceptable. The liquid, by way of buoyancy and increased viscosity, significantly enhances the environmental (performance) capability of the sensor apparatus. Sensor apparatus is thus provided having cost effective characteristics and improved reaction time to achieve the decreased rotational speed, as will now be understood. In this connection it is noted that in some inertial sensor apparatus using a liquid as the working fluid, a neutrally buoyant mass or an external pump for the liquid is required, adding to the complexity of the apparatus. In the present invention the requirement for the neutral buoyancy or an external pump is avoided by arranging the components thereof to provide an internal pumping action. U.S. 3,025,708 discloses inertial sensor apparatus comprising a mass suspended in a case by a bearing arrangement which includes a shaft and a ball supported by the shaft with a gap between the facing surfaces of the ball and mass; a sensor comprising said shaft rotatably supported in said case; means for rotating said mass in one direction about a first axis (SA₁); sensing means arranged in a two degree of freedom configuration for sensing rates of change of attitude along a second (IA₁) and third (IA₂) axes, said first, second and third axes being mutually perpendicular and said sensing means being arranged to provide signals corresponding to said sensed rates of change of attitude. U.S. 3,877,317 discloses a gyroscope having a spherical bearing which can have a liquid medium in the gap for use in high acceleration conditions. SUMMARY OF THE INVENTION The present invention provides inertial sensor apparatus as disclosed in U.S. 3,025,708, characterised in that said gap is filled with a liquid, said ball is initially eccentrically disposed within said mass, said means for rotating said mass about said first axis (SA₁) is arranged to provide a pumping action for the liquid in said gap whereby the liquid is pumped around the ball of said bearing arrangement with said ball thereupon tending to be centred in said gap, and means are provided for rotating said shaft of said bearing arrangement in a direction counter to the direction of rotation of said mass, whereby the differential velocity across the gap is increased. Preferred features of the invention are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional view of inertial sensor apparatus according to the invention. Figure 2 is a vector diagram showing the relationship between the spin axis of the sensor apparatus and the two input axes thereof. Figure 3 is a diagrammatic representation illustrating the internal liquid pumping feature of the invention. Figure 4 is a diagrammatic representation showing an embodiment of the invention wherein the apparatus functions in a dual, gyro and accelerometer, mode. DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1, a mass designated by the numeral 2 is suspended by a liquid spherical hydrodynamic bearing arrangement. The bearing arrangement includes a ball 4 disposed in mass 2. The ball and mass are arranged to provide a gap 6. Gap 6 is filled with a working liquid such as, for purposes of example, a low viscosity oil or some other such suitable liquid, as is well known in the art. Mass 2 is driven by a motor designated generally by the numeral 8 to spin or rotate about a spin axis. The arrangement includes torquers designated generally by the numeral 10 and pick-offs designated generally by the numeral 12. Torquers 10 torque the sensor about its output axes and pick-offs 12 function as signal generators to provide appropriate sensor signals, as is well known in the art. The relationship between the aforenoted axes will be next described with specific reference to Figure 2. It is noted that the gimballing arrangement of the apparatus (not otherwise shown) is such that torquers 10 and pick-offs 12 may be in a dual axis (two degree of freedom) arrangement, with only one of the axes being shown in Figure 1. The relationship between the spin axis and the two input axes will best be understood with reference to Figure 2, wherein three mutually perpendicular axes are shown, i.e. in put Axis IA₁; input Axis IA₂; and spin axis SA. Mass 2 rotates about spin axis SA and rate is sensed along axes IA₁ and IA₂. Hence the sensor apparatus has two degrees of freedom. That is to say, one degree of freedom about axis IA₁ and another degree of freedom about axis IA₂. It will be understood that in the arrangement described there is actually a third degree of freedom about primary suspension axis SA. As shown in Figure 1, ball 4 is supported by a shaft 14. As shown in Figure 1, in order to enhance environmental capabilities, shaft 14 is journalled in case 16 via suitable bearings 18. Shaft 14 is driven by a motor 20 in a direction counter to the direction of rotation mass 2. This feature of the invention increases the differential angular velocity across gap 6, thereby increasing the acceleration capability of the described apparatus. Sensor case 16 supports a temperature compensating bellows 17, the same being well known in the art. As heretofore noted, a feature of the invention is that a liquid hydrodynamic bearing is provided without the need for an external pump. This feature is best illustrated and described with reference to Figure 3. Thus as shown in Figure 3, mass 2 rotates about spin axis SA (Figure 2) at an angular velocity ω . Ball 4 and mass 2 form a gap 6, said gap being filled with a suitable liquid as aforenoted. Due to the force of gravity, ball 4 is initially eccentrically disposed within gap 6, with the gap being greater on one side of the ball than on the other side. The hydrodynamic pressure on the one side of the ball is designated as p₁ and said pressure on the other side of the ball is designated as p₂, with p₂ being greater than p, (p₂ > p₁). Thus as mass 2 rotates an internal pumping action of the liquid around the ball occurs. The tendency is for the ball to thereupon become centered in gap 6. With further reference to Figure 1, a pendulosity feature may be incorporated into the invention, as is required when the apparatus is used in a dual gyro and accelerometer mode. Thus, as shown in the Figure, one side of mass 2 is unbalanced relative to the other side of the mass via weights 22. It is noted that weights 22 may be replaced by grooves or the like to provide the aforenoted unbalanced condition. As will now be understood, this feature of the invention renders mass 2 pendulous along spin axis SA (Figure 2). Another feature of the invention is that two pendulous, two degree of freedom liquid bearing inertial sensors may be packaged with suitable processing electronics to provide two gyro (rate) axes outputs and two accelerometer axes outputs simultaneously. This dual sensor, two degree of freedom configuration is illustrated with reference to Figure 4. Thus, a first sensor is designated generally by the numeral 24 and a second sensor is designated generally by the numeral 26. Sensor 24 and 26 are structurally as described with reference to Figure 1, 2 and 3. Sensor 24 includes a mass, such as mass 2 shown in Figure 1, which is driven to rotate in one direction about a spin axis SA₁ as shown by the arrow. Sensor 26 has a like mass which is driven to rotate in an opposite direction about a spin axis SA₂ as shown by the arrow. Sensors 24 and 26 include torquers and pick-offs such as 10 and 12 described with reference to Figure 1 which provide appropriate signals T₁, S₁ and T₂, S₂. The torquer (T₁, T₂) and pick-off (S₁, S₂) signals from sensors 22 and 24 are applied to conventional processing electronics designated generally by the numeral 28. Processing electronics 28 provides an acceleration output which is the summation of signals T₁ and T₂ and provides a rate output which is the difference between signals T₁ and T₂. The output scale factors and polarities are appropriately addressed, as is well known in the art. It will be understood that Figure 4 illustrates a single axis output, but in the arrangement shown and described dual axis (two degree of freedom) gyro and accelerometer outputs may be provided in accordance with the foregoing description of the invention, as will now be understood. There has thus been described two degree of freedom sensor apparatus utilizing a liquid spherical hydrodynamic bearing. The apparatus is smaller than single degree of freedom apparatus now known in the art and provides two degrees of freedom at comparable performance and at reduced cost. Dual sensor implementation, i.e. gyro and accelerometer implementation, may be achieved by packaging two of the aforenoted two degree of freedom sensors with appropriate processing electronics.

Claims: Inertial sensor apparatus comprising: a mass (2) suspended in a case (16) by a bearing arrangement (18) which includes a shaft (14) and a ball supported by the shaft with a gap (6) between the facing surfaces of the ball and mass; a sensor (24) comprising said shaft rotatably supported in said case; means (8) for rotating said mass in one direction about a first axis (SA₁); sensing means (10, 12) arranged in a two degree of freedom configuration for sensing rates of change of attitude along second (IA₁) and third (IA₂) axes, said first, second and third axes being mutually perpendicular and said sensing means (10, 12) being arranged to provide signals corresponding to said sensed rates of change of attitude, characterized in that said gap (6) is filled with a liquid, said ball (4) is initially eccentrically disposed within said mass (2), said means (8) for rotating said mass about said first axis (SA₁) is arranged to provide a pumping action for the liquid in said gap whereby the liquid is pumped around the ball of said bearing arrangement (18) with said ball (4) thereupon tending to be centred in said gap, and means (20) are provided for rotating said shaft (14) of said bearing arrangement in a direction counter to the direction of rotation of said mass (2), whereby the differential velocity across the gap (6) is increased. Inertial sensor apparatus as claimed in claim 1, further characterized by: a further sensor (26); a further case; a further mass; a further bearing arrangement for suspending the further mass in the further case; the further bearing arrangement including a shaft and a ball supported by the shaft and initially eccentrically disposed within the further mass for forming a further gap therebetween; a liquid for filling the gap; means for rotating the further mass about a fourth axis (SA₂); further sensing means arranged in a two degree of freedom configuration for sensing a rate of change of attitude along a fifth axis and for sensing a rate of change of attitude along a sixth axis; said fourth, fifth and sixth axes being mutually perpendicular; and said further sensing means providing signals corresponding to said sensed rates of change of attitude; said further sensor (26) comprising the shaft of the further bearing arrangement rotatably supported in the first case; the means for rotating the further mass about the fourth axis rotating said mass in one direction for providing a pumping action for the liquid in said further gap, whereby said liquid is pumped around the ball of the further bearing arrangement, with said ball thereupon tending to be centered in said further gap; and means for rotating the shaft of the further bearing arrangement in a direction counter to the direction of rotation of the further mass, whereby the differential velocity across the further gap is increased. Inertial sensor apparatus as claimed in claim 2, further characterized by: means (22) for unbalancing one side of said first-mentioned mass (2) relative to the other side, whereby said first-mentioned mass (2) is rendered pendulous along the first axis (SA₁); and means (like 22) for unbalancing one side of said further mass (like 2) relative to the other side, whereby said further mass is rendered pendulous along the fourth axis. Inertial sensor apparatus as claimed in claim 3, further characterized by: the means for rotating the further mass being arranged to rotate said further mass in a direction opposite to the direction of rotation of the first-mentioned mass. Inertial sensor apparatus as claimed in claim 4, further characterized by: means (28) connected to said sensors (24, 26) and responsive to the signals (T₁, S₁, T₂, S₂) from each of said sensing means (24, 26) thereof corresponding to the sensed rates of change of attitude for providing signals (T₁ - T₂) corresponding to the rates of change of attitude along first and second vehicle axes, and for providing signals (T₁ + T₂) corresponding to accelerations along said first and second vehicle axes. Inertial sensor apraratus as claimed in claim 5, wherein said means (28) connected to said sensors (24, 26) and responsive to the signals from said sensing means (24, 26) thereof is responsive to the sum (T₁ + T₂) between the signals from the respective sensing means (24, 26) for providing said acceleration signals; and means responsive to the difference (T₁ - T₂) of the signals from the respective sensing means (24, 26) for providing said rate of change of attitude signals.

Inventors: FICKEN, WILLIAM H., GONG, FRANK, GONSKA, JOSEPHINE
Applicants: ALLIED-SIGNAL INC. (A DELAWARE CORPORATION)
Application year: 1986
Main classification: G01C19
IPC codes: G01C19, G01P9