Geometrical instruments – Indicator of direction of force traversing natural media – Level or plumb – terrestrial gravitation responsive
Reexamination Certificate
1999-11-03
2001-09-04
Bennett, G. Bradley (Department: 2859)
Geometrical instruments
Indicator of direction of force traversing natural media
Level or plumb, terrestrial gravitation responsive
C033S366260
Reexamination Certificate
active
06282804
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the measurement of angular inclination or tilt and, more particularly, to the continuous monitoring of changes in angular position of a spirit medium partially filling an enclosed space.
BACKGROUND OF THE PRIOR ART
The spirit level is a familiar tool in which the position of an air bubble within a curved glass tube partially filled with a liquid indicates whether the tool is level or not. This principle has given rise to many different embodiments such as one disclosed in Scopacasa, U.S. Pat. No. 3,786,472. There, a toroid chamber is used which is half-filled with mercury. Pairs of electrodes are spaced around the periphery of the chamber. Contact between the mercury and variously situated electrodes as the chamber is inclined corresponds to predetermined angles of displacement. Cantarella U.S. Pat. No. 4,167,818 and Kent patent U.S. Pat. No. 4,672,753 both employ arcuate resistance elements which are variably submerged in a conductive liquid as the housing is tilted. The Cantarella patent applies a DC or AC potential between the resistance sectors and uses a bridge connected to circular hoop which is always in contact with the liquid to measure the change in resistance caused by the liquid wetting different parts of the sectors as the housing is tilted. Beitzer patent U.S. Pat. No. 4,536,967 provides a thermal shunt about the glass tube and accurate spacing and symmetry of the electrodes to minimize the effect of changes in fluid temperature on its resistivity.
Donati U.S. Pat. No. 4,937,518 uses methylene diglycol as a conductive liquid which partially fills a chamber formed between two spaced-apart ceramic plates. Each plate bears a pair of imprinted palladium electrodes in the shape of semi-circular segments arranged with their respective cordal edges facing each other and with their curved edges lying on the same imaginary circumference. The liquid forms a resistive electrical connection between the electrodes which varies in accordance with the position of the liquid.
The problem with the above described devices employing fluid electrolytes is that they tend to suffer from both leakage currents and electroplating of their sensing elements. Leakage currents are possible in certain embodiments because of surface adsorption of the electrolyte. Leakage current causes non-linearity, especially a thigh tilt angles. Asymmetry in the exciting AC voltage that is necessary in order to measure the change in resistance occasioned by movement of the sensing elements with respect to the liquid may cause electroplating of the sensing elements, as may accidental exposure to DC voltage. Electroplating of the sensing elements over time brings a battery effect into a tilt sensor and degrades the linearity of angular measurement and scale factor which, in some cases, can be noticeable even after a very short period of operation.
Some prior art devices have employed curved glass tubes in order to achieve a linear response to change in tilt angle. Such devices must be carefully fabricated to provide arcuate sectors which form portions of a perfect circle and are therefore quite costly. Those devices which employ mercury as the conductive liquid tend to be insensitive to small changes in angular rotation because of mercury's tendency to roll erratically and suffer also from the polluting effects of mercury's high vapor pressure.
On the other hand, those devices employing conventional electrolytes in ceramic instead of glass structures suffer, in addition to the battery effect, from surface absorption of the electrolyte by the porous ceramic surface which gives rise to surface leakage currents which degrade the linearity.
SUMMARY OF THE INVENTION
The problems of these prior art devices are alleviated in accordance with the principles of the present invention in an illustrative embodiment in which an arcuate resistive sensor element is imprinted upon a ceramic substrate forming a wall of a ceramic body in which an electrolyte, advantageously silver nitrate dissolved in a mixture of methanol, water, and butanol, is sealed. To avoid the effects of electroplating, an AC exciting voltage is applied between the ends of the sensor element which is bridged by a discharge resistor whose resistance is much lower than the internal resistance of the sensor element so as to discharge any polarization caused by asymmetry of the voltage supply through an external path, other than through the internal electrolyte. A signal output is taken from a midpoint of the sensor element which is always immersed in the electrolyte so that the effect of any leakage current is minimized. Linearity of output with change in the tilt angle achieved with an illustrative embodiment has been measured at better than 99.9% in the tilt angle over the range of −50 to +50 degrees as compared with commercially available devices whose linearity is no better than 99%.
REFERENCES:
patent: 3786472 (1974-01-01), Scopacasa
patent: 4028260 (1977-06-01), Zuest
patent: 4167818 (1979-09-01), Cantarella
patent: 4536967 (1985-08-01), Beitzer
patent: 4641434 (1987-02-01), Engler
patent: 4672753 (1987-06-01), Kent
patent: 4866850 (1989-09-01), Kelly
patent: 4937518 (1990-06-01), Donati
patent: 5031329 (1991-07-01), Smallidge
patent: 5159761 (1992-11-01), Cagan
patent: 5279040 (1994-01-01), Kippelt
patent: 5625955 (1997-05-01), Han
patent: 5852878 (1998-12-01), Seipp, Jr. et al.
patent: 6032376 (2000-03-01), Shijo et al.
patent: 6123866 (2000-09-01), Shijo
The Fredericks Company (catalog r-200-10) “Electrolytic Tilt Sensors” (no date).
Bennett G. Bradley
Nanotron, Inc
Popper Howard R.
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