BACKGROUND OF THE INVENTION
The invention is an apparatus for determining orientation with respect to acceleration, and more particularly the invention is an apparatus for determining orientation with respect to gravity, and where the orientation can be determined statically or dynamically.
Orientation devices, sometimes referred to as levels, tilt devices or inclinometers, are similar, in that all measure at least one component of an angle of deflection from a true vertical, where the true vertical is defined as a direction that is coextensive with gravity. The angle of deflection is conventionally termed tilt, or angle of tilt. A simple device for determining the direction of gravity is a plumb line. A plumb line is a string or line having a weight attached to one end and the other end attached to a stationary point, such that the hanging weight is free to move. After being positioned, once the weight becomes stationary, the plumb line indicates the true vertical.
If an object is not level then this is another manifestation of being tilted. Unlevelness is how much a plane is deflected from a true horizontal, where the true horizontal is a plane that is perpendicular to true vertical. Many orientation devices measure tilt by measuring levelness. A bubble tube is a means of using a gravitational force to determine true vertical by measuring levelness. The bubble tube is based on the observation that an air bubble in a liquid will seek to escape that liquid, and that under the pull of gravity, the action route will take the air bubble to a position in the liquid of lowest gravitational influence, that is lowest pressure. This action occurs because the pressure drops as the bubble moves upward, enabling the bubble to expand. The expanded bubble continues to proportionately increase in buoyancy, and the process continues until the bubble is either restrained or it reaches the surface. When the bubble is in a horizontal liquid filled tube, where the tube is slightly arced upward, then the position of lowest gravitational influence is at the apex of the arc, and this coincides with true vertical for one planar component. If the bubble is displaced from the apex, then this indicates that the tube is tilted, because the highest point in the tube is displaced. An example of a commonly utilized tool that employs the bubble tube technique is a level. One bubble tube can measure only one axis component at a time, so to determine the tilt on the other axes the level either has two or more bubble tubes offset 90 degrees, or the user has to rotate the level through 90 degrees, taking measurements at each angle. Even then, unless the user knows exactly how to line up the level, the level could be actually rotated slightly, and thereby produce an erroneous level reading in the bubble tube. More recent inventions employing bubble tubes, often called electronic bubble tubes, have a domed chamber instead of a tube and the chamber has a shallow convex curvature. The convex curvature enables the air bubble to move in either the x or the y direction or any combination thereof, as it seeks the point of lowest pressure. However, the additional degree of freedom comes at a substantial price. The forces acting on the air bubble producing movement, are now spread over a much larger angle, instead along just one axis. Therefore, domed chambered bubble tubes are substantially less sensitive to small changes in tilt compared to tubes.
There are several considerations the user must be aware of when using orientation devices that use the bubble tube technique. Firstly, bubble tubes are subject to error because if either the tube or the apparatus seating the tube is rotated, a false orientation reading will be generated. Also, competing interfacial interactions can be substantial. By way of example, the reader is encouraged to recall the way small bubbles cling to the walls of a glass of water, or a glass of champagne. As previously implied, omni-directional bubble tubes offer the advantage of determining the direction of tilt with one measurement, but omni-directional bubble tubes are less sensitive than single axis bubble tubes. An improved technology would be an orientation sensor that had the convenience of measuring tilt through 360 degrees, but without the loss of sensitivity. Another limitation of the prior art is that to amplify small changes in tilt requires using a tube that is has very little arch. However, with a substantially straight tube the operative gravitational forces and the competing interfacial forces are approaching each other in magnitude, and the net result is that the bubble tends to stick and then jump, in an all or none fashion. An improved technique would be one that utilized the heretofore described action of the bubble to move to the point of lower pressure, while at the same time minimizing the interfacial interactions.
Kikuo Shimura U.S. Pat. No. 5,101,570 discloses an Inclination Angle Detector that is a bubble tube that casts a shadow on a circular detector, where the detector is divided into quadrants. The resulting electronic signal is converted/calculated into an angle of tilt or incline, where not only the degree or magnitude of tilt is determined, but also the direction.
Fumio Ohtomo U.S. Pat. No. 5,953,116 discloses a Tilt Detecting Device that is comprised of a bubble tube, light, detector and electronics. His invention is geared for survey equipment, i.e. transit theodolite. Page 11, FIG. 18 shows a light profile of the photo detector of the prior art. By using plates (slits) the overall light is reduced but the background noise light is greatly reduced, permitting accurate determinations.
Franklin U.S. Pat. No. 4,800,542 discloses an orientation device that uses moving mercury to change the capacitance in response to a change in orientation.
Augutin U.S. Pat. No. 5,704,130 discloses an invention developed for Bayer that uses a chamber containing two immiscible media, where one of the media is either a gas, liquid or solid. The preferred invention uses a gas.
SUMMARY OF THE INVENTION
The invention is an apparatus for measuring tilt, where tilt can generally be described as an angular component of a vector, where the vector is equal and opposite the force exerted by gravity and, on occasion, one or more additional forces. The angular component or tilt is three dimensional. The three angular dimensions can be measured and described in a two dimensional format. Two of the dimensions are described in terms of direction and the third dimension in terms of magnitude. The format can use purely scalar units, purely angular units or a blend of units, such as polar notation. The reader is encouraged to preview FIG. 4 for a pictorial explanation of tilt. In the current discussion, for purposes of clarity, the direction of tilt will refer to the orientation of a substantially horizontal plane where the tilt is steepest. The magnitude will refer to the grade of that horizontal plane.
The invention is set up to adhere to this format, which has a visual representation that is easily understood, and is grounded with an historical basis Mechanistically, the invention uses the observed action of a buoyant element immersed in a liquid to seek a position of minimum pressure to determine orientation. Under static conditions, that position is as close to the surface as permitted by the constraints of the invention. The action is not dissimilar to the bubble tube, but with several significant departures that all but eliminate some of the more onerous limitations associated with the prior art.
The invention is an apparatus for determining the orientation with respect to gravity, where the apparatus consists of:
a lower chamber that is filled with a liquid, and an upper chamber that is superimposed over the lower chamber, where the lower chamber and the upper chamber share a light communicating wall, where said light communicating wall is a window;
a light transmitting cable that is transmitting light through the cable, wherein said cable has a source end which is an entrance point for the light, and an exit end that has an emitting light, and where the exit end is fitted with a flotation collar, a light source;
an observing means in the upper chamber for detecting the position of the emitting light, where the observing means can be a visual eye portal, a lens or an instrumental means, such as a photodetector that generates an electronic signal, where the photodetector has a supporting electronic circuit, where the electronic signal is unique to a spatial coordinate;
wherein, the source end of the cable passes through a bottom wall of the lower chamber and is sealed to the bottom wall such that the cable is substantially vertically tethered to the bottom wall at a position that is substantially central and opposite to the window, and wherein the cable is tensioned by the floatation collar submerged in and buoyed by the liquid, such that the cable is substantially straight, and long enough such that the exit end of the cable is in close proximity to the window, where the exit end of the cable is buoyed, therein seeking a lowest pressure point;
wherein the light source delivers light to the source end of the cable, the cable conveys the light to the exit end, and the emitting light projects a point light at the window;
wherein, the upper chamber has a ceiling wall opposite the window and in the upper chamber, and preferably in the ceiling wall, is located the observing means that can detect the exact planar position of the point light, and, optionally, can generate an appropriate electronic signal that describes the position;
whereas, when the apparatus experiences an effective tilt, the lowest pressure point shifts, and the cable adjusts its angle to the new low pressure point, which accordingly causes the point light to move to a new position, and the new position is detected by the observing means.
The invention works as follows. The flotation collar buoys the cable. The cable, which is tethered to the bottom, is flexible, and pulled straight by the buoyant flotation collar. The liquid generates a pressure that increases as the depth increases. The pressure forces the flotation collar upward, because the flotation collar is pushed by the higher pressure to a position of lower pressure. The tether restrains the flotation collar and the exit end of the cable from moving to a point against the window. The exit end is close enough to the window that the light emanating from the cable can be seen through the window. The invention is generally set up so that when the apparatus is orientated with no tilt, the exit end of the cable is located at the center of the window.
When the apparatus is tilted, the flotation collar moves to the new minimum pressure point. The overall pressure is slightly higher because the lower chamber holding the liquid causes the liquid to stack up higher, and the flotation collar shifts to this new position which is the new minimum pressure for that depth. The exit end of the cable, buoyed by the flotation collar always seeks the minimum pressure, and this is coincident with the true vertical.
The observing means enables the user to visually track the position of the point light. The eye portal or lens can be fitted with a grid or a gradient to help the user graphically measure the tilt. The gradient or grid can alternatively be on the window. A lined film laminated to the window would also provide the graphical reference marks. The preferred grid or gradient for visual observation has both scalar and polar coordinates, so that the direction and magnitude of tilt can be easily read or quickly calculated. A preferred initial set up is to have the point light centered at a zero x and zero y when the apparatus is not tilted.
The observing means can also be the instrumental means, as for instance a digital camera, a photographic camera, a video camera, a stereo microscope or a photodetector. A video camera feed to a computer would have a presentation suitable for easy viewing and calculating.
The geometry of the instant apparatus has a number of advantages over the prior art. By using a tethered flotation collar, even small changes in tilt can be determined by extending the length of the cable from the flotation collar to the tether. A long cable would amplify the movement of the point light. Small angles will generate a larger arc, the further the plane of measurement is from the origin. For instance if the cable is 10 cm and the resultant is 1 cm, the magnitude of the tilt is 5.7 degrees. More examples are given in Table 1. The dimensions of the instant invention can be easily modified to accommodate whatever range of tilt is needed. Furthermore, a false tether can be created by immobilizing the cable above where the cable is actually tethered, and this would have the effect of using a shorter cable. For instance, if a 10 cm cable was immobilized 5 cm above the tether this would double the range of measurement. If more precision is required the false tether could be moved down.
|TABLE 1 |
|Length of Cable ||Length of Resultant ||Magnitude of Tilt in |
|(cm) ||(cm) ||Degrees |
|10 ||1 ||5.7 |
|9 ||1 ||6.3 |
|8 ||1 ||7.1 |
|7 ||1 ||8.1 |
|6 ||1 ||9.5 |
|5 ||1 ||11.3 |
|4 ||1 ||14.0 |
|3 ||1 ||18.3 |
|2 ||1 ||26.3 |
|1 ||1 ||45.0 |
Another advantage of the invention is that, unlike bubble tubes, there is no gas, and the buoyant element never contacts anything but the liquid, therein eliminating problems associated with gas—liquid interfacial interactions.
The light source for the preferred invention is a light emitting diode, commonly known as an LED. The LED is selected so that it has good transmission properties through the cable, and is easily seen by the observing means. Generally, the preferred frequency is in the visible spectrum. If the observing means is a photodetector, then the photodetector and LED are selected so as to optimize the response of the photodetector.
The preferred cable for the invention is a fiber optic cable, and preferably a bundled fiber optic cable. A fiber optical cable having good transmission characteristic in the visible range is preferred.
The preferred flotation collar for most applications is a hollow sphere, wherein the cable passes through the center of the sphere and protrudes, slightly, on the opposing side. The cable is sealed to the sphere with an air tight seal, where said seal is symmetrical and smooth. The sphere is suitably constructed to have a precise shape and to retain its shape, even under slight hydraulic pressure. The sphere is uniform in all directions and has a minimum surface area for a given volume.
The preferred liquid is pure water, and preferably degassed, distilled, deionized water. The water can, optionally, contain anti-freeze compounds, such as ethylene glycol, propylene glycol and alcohols. Certain metal salts can also be added to lower the freezing point. Salts have the benefit of also increasing the solution density. The water can optionally contain surfactants to lower the surface tension. The surfactants can be either ionic or nonionic. Fluorinated surfactants are excellent for lowering the surface tension. Other liquids that are commonly employed in tilt corrective instruments, like compasses, where the compass floats in a reservoir of the liquid, use high boiling organic compounds that are fluids. High boiling organic liquids have generally not been found to be required for the instant invention, albeit could be also employed if circumstances justified their use.
The upper chamber is preferably completely dark, except for the point light source. In a variation of the invention, two chambers are cylindrical in shape and have a nested configuration, wherein the upper chamber has a side wall that extends down to a base, where the side wall of the upper chamber encase the lower chamber. In essence the lower chamber is nested inside the upper chamber. The nested configuration creates an inter-wall space that has a Dewar insulating effect, and in general protects the lower chamber from the environment.
The preferred observing means of the invention is the photodetector, where the photodetector has an array of sensors which are mounted in the ceiling of the upper chamber. There are preferably a set of a pair of sensors mounted in a plane parallel to the window, and positioned such that one pair, designated an x pair, are orthogonal to other pair, designated the y pair. The sensors are located equidistant from each other and near the perimeter of the window. The x pair is part of a circuit containing a balanced Wheatstone bridge circuit, and the circuit generates an x axis signal. The y pair are part of a similar balanced Wheatstone bridge circuit and the similar circuit generates a y axis signal. The sensors are the photoconductive type, wherein their resistance is a function of luminance.
The upper chamber side of the window has a treated surface, such that the treated surface diffuses the emitted light, therein making light emanating from the window isotropic. The sensors are sufficiently offset from the window that, even when the point light is in an extreme peripheral position of the window, all the sensors can see the point light. The photodetector senses the position as follows. The x pair of sensors are tuned such that when the orientation device is aligned with the true vertical, the exit end of the cable, and therein the point light, is in the center of the window, and the x axis signal, which is a voltage, is zero. The signal is zero because both sensors in the x pair are seeing the same intensity of light, as they are equidistant from the point light. The same is true for the y axis signal. When the orientation device is tilted, for the sake of the discussion, such that the direction of tilt is 225 degrees and the magnitude of tilt is 5.37 degrees (or 5 degrees and 22 minutes), then the exit end of the cable, and therein the is point light will move to a new position in the window that compensates for this azimuth. The Wheatstone bridge is now out-of-balance and generates a commensurate signal. In this hypothetical case the signal is 2 volts for the x axis and 2 volts for the y axis signal also. This means that the new position of the point light is in the first quadrant, 1 cm along the x axis and 1 cm along the y axis. Using the Pythagorean theorem we calculate that the resultant is 1.41 cm long, and from the arctangent we calculate the angle is 45 degrees. The direction is opposite the direction of tilt, or 180 degrees out of phase. The phase shift can be compensated for by adding 180 degrees, making the direction 225 degrees (45+180), or preferably, by setting up the Wheatstone bridge so that the signal compensates for the phase shift. With the latter set up, the Wheatstone bridge signal for the hypothetical case would be −2 volt for the x axis and -2 volts for the y axis. Therein, unambiguously, indicating that the direction of tilt is in the third quadrant, and is 225 degrees. The magnitude is calculated by taking the arctangent of the resultant divided by the window—tether length, or 5.37 degrees tan−1 (1.41/15.00).
It is anticipated that it may be preferable to set the circuit up so that the signal is offset, or so that the sign of the signal be reversed. It is also anticipated the signal can be analog or digital. The foregoing hypothetical case was designed merely to be illustrative that the detector can generate a signal that correlates to spatial coordinates.
The sensors of the photodetector can be photoconductive cells, photodiodes, or phototransistors. Photoconductive cells are preferred sensors for Wheatstone balance bridge circuits.
It is anticipated that under some circumstances it may be desirous to change the response to tilt, and the fluid mechanics of the invention will favor another shaped flotation collar, or a liquid of different density or viscosity. For instance a larger sphere weighted to have no net increase in buoyancy would move slower through the liquid because of increased fluid drag. Expanding on that idea, the sphere could be symmetrically fitted with fins that would create even more drag. A flotation collar having a mushroom shape might be more sensitive to small changes in pressure and could have a faster response. A liquid having a lower viscosity would tend to have a faster response than a higher viscosity liquid. A liquid having a higher density would have a faster response, if the viscosity were substantially the same. For example, contrast pure water versus water loaded with metal salts. The water with metal salts could have a much higher density, and the same flotation collar would be much more buoyant.
It is further anticipated that the apparatus is maintained with some temperature control, to improve reproducibility, and to ensure that the orientation device is not damaged. Under some severe weather circumstances the liquid could expand or contract enough to damage the apparatus. For instance if pressure increased within the chamber the effect of gravity would be diminished, and if pressures became high enough the flotation collar would collapse. When the invention is used outdoors it is fitted with a protecting housing such that there is some temperature control and protection from the elements. It is anticipated that the instant invention could be fitted with pressure release plugs or diaphragms. The preferred invention has a machined recess covered with a flexible diaphragm to allow for pressure changes in the liquid. The machined recess acts as a small expansion chamber.
It is anticipated that the invention can be used under static conditions and under dynamic conditions. In general, static conditions are those where it is experiencing negligible acceleration other than gravity, and dynamic conditions refer to those wherein other forces are significant. An example of the latter, is when the orientation device is experiencing is significant angular velocity. Under these conditions, the forces can be resolved, as gravity is constant.
It is anticipated that the invention can be fitted with a timing mechanism such changes in orientation can be correlated to a finite time period.