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BACKGROUND OF INVENTION
This invention relates generally to level measurement, and more
particularly to an improved inclination gauge providing both a digital and
an analog read-out.
As used in mechanics, level is a term referring to any direction that is at
right angles to the force of gravity. Because of the earth's curvature,
this direction is not precisely horizontal, but the deviation from the
horizontal is negligible for short distances. The tool kit of any
competent bricklayer or carpenter invariably includes a spirit level.
However, the use of such levels is by no means limited to the field of
construction, for this level is an essential component of many delicate
physical, astronomical and engineering instruments.
Operation of the spirit level depends on the simple principle that an air
bubble seeks the highest point in a container enclosing the liquid in
which the bubble is formed. In a standard spirit level, the glass tube
housing the liquid is either slightly curved with its convex side upward,
or the tube is ground with a curved inner core.
The glass tube is supported on a rigid base or bar, and the bubble therein
always comes to equilibrium at the same point whenever the bar has the
same slope with respect to the horizontal plane. By providing the spirit
level tube with a graduated scale, the level may be made a sensitive gauge
for measuring changes in the angle of inclination.
From the practical standpoint, the value of a spirit level in providing an
accurate indication of inclination depends on the ability of the user to
discern the precise position of the air bubble with respect to the scale
indicia on the tube. While the condition of the user's eyesight is a
factor in this regard, as well as the angle at which he views the bubble,
for many ordinary level applications, the fact that the readings taken by
the user are somewhat inexact may not be a serious drawback; but in other
situations, even a slight error cannot be tolerated.
But whether or not the user of the spirit level is capable of reading the
level without difficulty, the fact is that the user, in order to obtain a
correct reading, must exercise care, and this consumes time--which, in a
procedure requiring frequent level readings, adds materially to the cost
of the operation.
While the invention will be described in connection with an inclination
gauge which supplants or supplements conventional spirit levels of the
type used in the construction industry, it is to be understood that the
gauge in accordance with the invention is also useful as a surveyor's
level attachable to a telescope to determine differences in elevation, and
in other instruments which entail the measurement of inclination.
In applicants' above-identified copending application, there is disclosed
an inclination gauge having a digital readout whereby the observer,
regardless of the condition of his eyesight, as long as he is able to read
numbers, is given an accurate indication of inclination. This inclination
also includes a spirit level components whereby the observer is given both
a "coarse" spirit level analog reading and a concurrent "fine" electronic
level digital reading.
The inclination gauge disclosed in this copending application includes a
bar that is placeable against a surface to determine the extent to which
this surface is inclined with respect to the horizontal or vertical axis.
Mounted on the bar is a gravity-sensing potentiometer which is connected
in a balancing circuit to produce an analog output voltage which attains a
null value only when the bar is placed against a vertical or horizontal
surface, and which has an amplitude and polarity or phase that depends on
the extent and direction of deviation when the surface is sloped.
The analog voltage derived from the balancing circuit is applied to an
analog-to-digital converter which output is fed through a display actuator
to a digital display to present a reading indicating inclination in terms
of angular degree or inches-per-foot. By appropriately setting the
conversion factor of the digital converter a reading may be obtained in
mils or radians.
Also mounted on the bar are conventional spirit level tubes, one arranged
for horizontal and the other for vertical gauging, so that the user of the
gauge can observe the displacement of the air bubble in each tube from a
null position and thereby obtain an analog reading of the inclination, the
analog and digital readings being concurrently presented on the same bar.
The practical advantage of an inclination gauge which affords both an
analog and a digital readout is that the user in many situations need not
check the "fine" digital reading, where it becomes immediately apparent
from the "coarse" analog reading that the surface being gauged is grossly
out of line. On the other hand, the fact that the analog and digital
readings are concurrently available on the gauge for comparison gives the
user an immediate check of any defect in the operation of the gauge.
The potentiometer, which is sensitive to inclination with respect to either
the horizontal or vertical plane is formed by a circular cell partially
filled with a pool of semi-conductive liquid, and an electrode assembly
disposed within said cell and constituted by a disc-shaped center
electrode and four electrode segments concentrically-arranged in
quadrature relation about the center electrode. The pool partially
immerses the center electrode and the lowermost two of said electrode
segments when the bar is placed against a surface to be gauged, the
distribution of liquid between the immersed electrode segments depending
on inclination, the other two electrode segments being connected
respectively to the immersed electrode segments.
Because of the quadrature relationship of the electrode segments,
regardless of where the bar is placed, a pair of segments will always be
immersed; and because of the connection between the two pairs, whichever
pair is immersed will be operatively connected to the balancing circuit.
In an actual working embodiment of a gravity-sensing potentiometer of the
type disclosed in the copending application, the circular cell includes a
pair of opposing walls in parallel relation, and the central disc
electrode and the electrode segments surrounding the disc element are
fabricated from sheet metal or foil, these elements being secured to one
wall of the cell.
Calibration tests conducted with potentiometers of this type indicate a
margin of error of about 0.03 degrees. While this error factor in most
instances is too small to be of practical significance, when greater
precision is required, it is not acceptable.
We have found that this error arises mainly because of surface tension
effects which cause the liquid to attain a slightly different level with
respect to the electrodes of the assembly than is dictated by the existing
inclination. Though one can somewhat reduce surface tension effects by the
selection of a liquid having a relatively low surface tension, the liquid
chosen for this purpose may lack the proper conductivity and other
characteristics desirable in the gravity-sensing potentiometer.
Another factor which comes into play with regard to the accuracy of the
gravity-sensing potentiometer is transverse tilt. In this regard, let us
first consider a situation in which the inclination bar carrying the
potentiometer is placed on a surface to be gauged which is inclined in its
longitudinal axis with respect to the horizontal plane, the surface having
a transverse axis which extends in the horizontal direction. In this
instance, the parallel walls of the potentiometer cell will be positioned
vertically, and the surface of the liquid in the cell will be horizontal
and at right angles to the cell walls.
Now let us consider a second situation in which the surface to be gauged is
again inclined in its longitudinal axis to the same degree as in the first
situation, but in the transverse axis is now tilted or inclined with
respect to the horizontal direction. In this second case, the parallel
walls of the potentiometer will be tilted with respect to the vertical and
the horizontal surface of the liquid in the cell will strike a different
level on one wall than on the opposing wall.
Since the electrodes which sense inclination are disposed against one of
the walls, the resulting reading will contain an error component; for it
will not only indicate the inclination of the longitudinal axis of the
surface being measured relative to the horizontal plane, but it will also
reflect the transverse tilt.
SUMMARY OF INVENTION
Accordingly, the main object of this invention is to provide a
gravity-sensing potentiometer for an electronic inclination gauge which
minimizes surface tension effects and is insensitive to transverse tilt,
whereby precise readings are obtained of inclination under all conditions.
More particularly, an object of this invention is to provide a
potentiometer of the above type in which the electrode assembly within the
cell is physically displaced from the front and rear walls thereof.
Also an object of the invention is to provide an accurate and reliable
inclination gauge whose digital display is selectively capable of
precisely indicating inclination of a surface being gauged relative to the
horizontal or vertical, regardless of the tilt of the surface.
Briefly stated, these objects are attained in an inclination gauge in
accordance with the invention that includes a bar that is placeable
against a surface to determine the extent to which this surface is
inclined with respect to the horizontal or vertical axis and to provide a
readout of the deviation from the norm in both analog and in digital
terms.
Mounted on the bar is a gravity-sensing potentiometer which is connected in
a balancing circuit to produce an analog output voltage which attains a
null value only when the bar is placed against a vertical or horizontal
surface, and which has an amplitude and polarity or phase that depends on
the extent and direction of deviation when the surface is sloped.
The analog voltage derived from the balancing circuit is applied to an
analog-to-digital converter which output is fed through a display actuator
to a digital display to present a reading indicating inclination in terms
of angular degree or inches-per-foot. By appropriately setting the
conversion factor of the digital converter, a reading may be obtained in
mils or radians.
The gravity-sensing potentiometer is formed by a cell having parallel front
and rear walls, the cell being partially filled with a pool of
semi-conductive liquid. Disposed within the cell in an intermediate plane
parallel to the walls thereof is an electrode assembly constituted by an
annular central electrode and four electrode segments concentrically
arranged in quadrature relation about the central electrode. The central
electrode and the electrode segments are defined by shaped wires
preferably having a circular cross section and a polished surface.
The pool partially immerses the central electrode and the lowermost two of
the electrode segments when the inclination bar is placed against a
surface to be gauged, the distribution of liquid between the immersed
electrode segments depending on inclination. The other two electrode
segments are connected respectively to the immersed electrode segments so
that the gauge is operable to gauge deviations from either the horizontal
or the vertical.
Because the surfaces of the electrode assembly are displaced from the walls
of the cell, surface tension effects are limited to those arising between
the liquid and the polished electrode wires, the surface tension effects
on the walls of the cell having no significant effect on the reading. And
because the electrode assembly is symmetrically disposed with respect to
the opposing cell walls, errors due to transverse tilting of the cell are
thereby minimized.
OUTLINE OF DRAWINGS
For a better understanding of the invention as well as other objects and
further features thereof, reference is made to the following detailed
description to be read in conjunction with the accompanying drawings,
wherein:
FIG. 1 is a perspective view of a dual reading inclination gauge that
includes a gravity-sensing potentiometer in accordance with the invention;
FIG. 2 is a schematic diagram of the gravity-sensing potentiometer and the
electronic circuit associated therewith;
FIG. 3 is a section taken through the potentiometer shown in FIG. 2 in the
plane indicated by line 3--3 therein;
FIG. 4 shows the potentiometer in one condition of tilt; and
FIG. 5 shows the potentiometer in the reverse condition of tilt.
DESCRIPTION OF INVENTION
The Basic Structure
Referring now to FIG. 1, there is shown an inclination gauge in accordance
with the invention which provides concurrent readings in digital and
analog terms with respect to inclination of a bar 10 relative to a
horizontal or a vertical axis.
Bar 10 has an I-beam formation constituted by parallel upper and lower
planar flanges 11 and 12 and an interconnecting web 13. Mounted within web
13 on opposite sides of an elongated opening 14 are standard spirit level
tubes 15 and 16. Tube 15 is arranged transversely relative to the
longitudinal axis of the bar, whereby when the bar is placed against a
vertical surface, the tube then extends horizontally and the bubble
therein is at its midpoint or null position; but when the surface is
inclined relative to the vertical axis, the bubble is displaced from the
null to an extent and direction which represents an analog of the degree
of displacement. In practice, beam 10 may be a wooden bar, a channel or a
rectangular bar beam.
Spirit level tube 16 is arranged along the longitudinal axis of bar 10 so
that when the bar is placed on a horizontal surface, its bubble is at
null; and when the surface is inclined, the bubble is displaced
accordingly. Thus spirit level tubes 15 and 16 operate in the conventional
manner.
Also mounted on web 13 of the bar is a gravity-sensing potentiometer cell
17 which operates in conjunction with a balancing circuit to provide an
analog voltage whose magnitude and sense reflect the deviation of the bar
from either the horizontal or vertical norm. By "sense" is meant the
polarity of the analog voltage in the case of d-c excitation of the
potentiometer balancing circuit, and the phase of the voltage in the case
of a-c excitation thereof.
Thus an inclination toward the right produces an analog voltage in one
polarity or phase whose magnitude depends on the extent of deviation from
the vertical or horizontal axis, and an inclination toward the left to the
same extent produces a voltage of the same magnitude but of opposite
polarity or phase.
The analog voltage from the potentiometer balancing circuit is processed to
change its amplitude for range calibration. The analog voltage is then
converted into a corresponding digital value which is applied to the
actuator of a digital display 18 to provide a readout in digital terms.
The number is preceded by a - or + symbol indicating the direction of
inclination. The display may be of the LED or LCD type used in pocket
calculators, wherein each station of the display is formed by multiple
segments which, when selectively actuated, define the digits 0 to 9.
Thus the inclination gauge has an electronic and a spirit level section
that concurrently present analog and digital readings of inclination with
respect to the horizontal or vertical axis. The user of the gauge is given
a "coarse" analog reading from which he can quickly determine the degree
of deviation from the norm, and a "fine" digital reading from which he can
determine the precise deviation.
The Potentiometer
Referring now to FIG. 2, there is shown an electronic inclination gauge
section which includes gravity-sensing potentiometer 17. This
potentiometer is in the form of a sealed circular cell partially filled
with a semi-conductive liquid 19 such as alcohol. Housed within the cell
is an electrode assembly constituted by an annular central electrode E,
and concentrically-disposed about central electrode E are four arcuate
electrode segments, A, B, C and D. Electrode segments A to D are
symmetrically arranged in quadrature so that, as shown in FIG. 2, when the
bar of the gauge lies in a horizontal plane, electrodes A and B are
partially immersed and liquid 19 is equally distributed with respect to
these electrodes.
The surface of liquid 19 is always parallel to the horizontal axis
regardless of how the bar is oriented. Hence when the bar is placed on a
surface S.sub.1 which is inclined with respect to the horizontal axis X,
the surface of the liquid remains parallel to this axis; but the
orientation of segments A and B reflects the inclination. The distribution
of liquid 19 is now unequal, such that more of electrode segment A is
immersed than segment B.
Associated with the potentiometer is a balancing circuit which is formed by
a pair of resistors 20 and 21 serially-connected between electrodes A, C
and electrodes B, D to form a bridge across whose input diagonals is
connected a d-c source 22, the output being yielded at terminals 23
connected to the output diagonals.
When the inclination gauge is placed against a perfectly horizontal or
vertical surface, than electrode E, which behaves in a manner equivalent
to a slider, is effectively at the midpoint of the potentiometer formed by
the resistance path between electrode pair A and B or whichever pair is
then operative with respect to the liquid pool, the resistances on either
side of the slider being equal. By providing matching resistors 20 and 21
, the bridge is then in balance and a null output is obtained. In
practice, however, because of slight geometric asymmetries in the
electrodes, small disparities in the electrode resistances may be
encountered. Resistor 21 is therefore made adjustable to exactly null the
bridge for a gauge placement in line with the horizontal or vertical axis.
The analog voltage at output diagonals 23 of the bridge for any given
inclination has a magnitude and polarity in accordance therewith. That is
to say, the amplitude of the voltage is proportional to the angle of slope
and the polarity thereof is indicative of the direction of the slope.
This amplitude is adjusted by a range-setting circuit 24, which may take
the form of an operational amplifier so that a given increment in
amplitude represents a change of one angular degree, or whatever other
span is desired. The voltage from range-setting circuit 24 is applied to
an analog-to-digital converter 25 of any suitable design to produce a
corresponding digital value which is applied to a display actuator 26 for
controlling display 18.
Display actuator 26 is provided with a switch-operated selector 27 so that
the display gives a numerical reading in angular degrees or in inches per
foot, whichever is preferred by the user. Or the display may be in radians
or mils.
All of the circuits associated with the gravity-sensing potentiometer can
be produced in integrated circuit form; hence, in practice, an integrated
circuit chip of small size may be used to minimize the space requirements
of this electronic gauge section on the bar.
Because of the quadrature relationship of the segment electrodes,
regardless of where the bar is placed, two of the electrodes will always
be immersed. In order, therefore, to provide a gravity-sensing
potentiometer which is operative for both horizontal and vertical
inclinations, electrode C, as shown in FIG. 2, is connected to electrode
A, and electrode D to electrode B.
The Electrode Assembly
As shown in FIG. 3, the cell of potentiometer 17 includes opposing front
and rear walls 17F and 17R in parallel relation, the cell being partially
filled with liquid 19.
The electrode assembly is constituted by central electrode E and electrode
segments A, B, C, and D, all of which are formed by shaped wires which lie
in a plane intermediate to and parallel with the front and rear walls 17F
and 17R of the cell. Thus the assembly is symmetrically disposed with
respect to the walls.
In practice, these wires may take the form of polished stainless steel wire
of the quality used to fabricate needles, the wire being bent or shaped to
assume the desired electrode configuration.
Because the electrodes are free of burrs or other roughened areas and are
preferably highly polished, surface tension effects are minimized. In any
case, since the electrodes are separated from the walls of the cell,
whatever surface tension effects are experienced are limited to the wires
themselves. By minimizing surface tension, the accuracy of the sensing
potentiometer is significantly improved.
When the potentiometer is transversely tilted with respect to the
horizontal, as shown in FIGS. 4 and 5, the level of the liquid 19 is
different with respect to the front and rear walls of the cell. However,
the critical level of the liquid which determines the output of the
electrode assembly is at the midpoint between the two walls; and since the
electrode assembly lies at this midpoint, the potentiometer is
substantially insensitive to transverse tilt, since the midpoint level
remains substantially the same regardless of the degree of tilt.
This would not be the case if the electrodes were mounted on either the
front and rear walls of the cell; for then, as will be evident in FIGS. 4
and 5, the level, as seen in the cell, would depend on the transverse tilt
as well as on the inclination of the surface in the longitudinal direction
and would therefore include an error component.
While there has been shown and described a preferred embodiment of an
improved electronic inclination gauge in accordance with the invention, it
will be appreciated that many changes and modifications may be made
therein without, however, departing from the essential spirit thereof.
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