Digital level indicator

A level indicator providing a digital display of gravity-related attitude comprises a capacitive gravimeter pickup assembly the complementary capacitor pair of which are alternately switched, on an equal-time basis, into an R.C. oscillator circuit. Concurrently, each oscillator pulse train is switched, respectively, into the up- and down-mode of reversible counting means to yield a net oscillator frequency difference directly, digitally indicative of the degree of departure from a gravity-referenced level condition.

BACKGROUND 
Previously known gravimeters, or accelerometers, utilizing capacitive 
pickups have been of the type which generally employ driving means 
responsive to a change in capacitive coupling to provide a null-restoring 
force to a pendulously-supported mass which includes one plate of a 
variable capacitor. The amount of feed-back current, as determined by the 
capacitor plate spacings due to movement of the mass, needed to restore 
the mass to a null position provides an indication of the momentary degree 
of variation from a level attitude in the device. 
In most instances the recovery of a gravity-referenced null is the prime 
concern in such devices, the indication of degree of mislevel being for 
the most part axillary to that function of the device. Thus, such 
"restoring drive" devices have, at best, been capable of indicating a 
maximum momentary deviation from gravity null, yet have not provided a 
persistent signal of varying mislevel in any associated instrument. 
Further, it has been a consistent disadvantage of such mislevel-correcting 
devices that a significant amount of power is required, particularly in 
the restoring drive circuitry, in order to enable these instruments to 
complete their intended functions. 
Other types of devices, more closely akin in function to the "spirit 
level", include a pendulously-supported core member which effects an 
inductive coupling in appropriate circuitry to generate an analog signal 
proportional to the degree of deviation of the instrument from a 
gravity-referenced level attitude. Again the power consumed in the 
operation of such an inductive pickup device, as well as the extensive 
circuitry required and unavoidable translation errors occasioned in 
analog-to-digital conversion, have further rendered these devices 
particularly unsuited to use in compact level-indicating instruments. 
The present invention avoids the earlier disadvantages generally found in 
previously available instruments and provides a rugged, compact device 
which requires little power for operation and which provides, to a high 
degree of accuracy, a direct digital reading of conditions of 
gravity-related attitudes over a range of plus or minus 200 seconds of 
arc. 
SUMMARY 
The present invention utilizes a capacitive pickup in a level-indicating 
device comprising a pair of capacitors which share a common, 
pendulously-supported plate and, as a result, individually exhibit greater 
and lesser degrees of capacitive coupling as the gravity-urged movements 
of the suspended plate between the two fixed, cooperating plates of the 
respective capacitor devices change the spacings between these elements. 
By appropriate clocked switching means, first one and then the other of 
these variable capacitors are alternately placed in an oscillator circuit 
which generates a pulse train at a frequency determined by the then 
spacing between the common plate and that fixed plate in operative 
connection with the circuit. In this manner there is effectively produced 
a pair of oscillators which share substantially all major components, 
thereby ensuring a high degree of comparative stability in frequencies. 
A reversible counter in the device receives the pulse trains generated by 
the oscillator circuit and accumulates a pulse count in whichever of its 
up or down counting modes is enabled. As the switching means effects the 
alternating between capacitors in the oscillator circuit, it 
simultaneously switches the counter between up-count and down-count 
conditions, thus enabling an increasing count at the first oscillator 
frequency followed by a decreasing count at the other oscillator 
frequency. 
At the conclusion of each such counting cycle, the remaining net count, 
indicative of the difference in plate spacings, thus the pendulum 
displacement in the capacitive pickup, is latched and strobed over to a 
display, or other utilization means, where any deviation of the device 
from a level attitude may be digitally shown.

DESCRIPTION 
An embodiment of the level sensor, or capacitive pickup device, employed in 
the present invention is shown in FIG. 1 and comprises a body 10 having a 
central cavity in which a pendulum assembly 15 is suspended by strap 
members 14 for gravity-responsive movement with any change in the tilt 
attitude of body 10. Pendulum assembly 15 may be more clearly seen in FIG. 
2 as comprising a pair of pendulum support members 16 in each of which is 
affixed one end of the shaft extensions of pendulum member 18 which serves 
as the common capacitor plate of the pickup. Supports 16 are slotted at 17 
allowing tiebolts 19 to effect a firm engagement of the shaft ends of 
pendulum member 18. 
The upper extensions of suspension straps 14 are affixed to body 10 by 
clamp bar 12, while the lower extensions of those straps are laterally 
displaced and affixed to pendulum support members 16 by lower clamp bars 
13. Thus constructed, pendulum assembly 15, and, in particular, common 
capacitor plate member 18, are free to move within the cavity of body 10, 
as seen in FIG. 2 with any tilt or displacement of body 10 from a 
gravity-horizontal attitude. 
Mounted within the cavity of body 10 by means of spring rings 22 are plates 
21 of insulating material, such as glass or the like, which carry, in the 
form of coated metal films, fixed capacitor plates 23, 24. In the 
described embodiment, such metal films having an area of about 650 
mm.sup.2 are separated from common plate 18 by about 125 .mu.m when the 
device is in a level condition, yielding a capacitive value in each pickup 
capacitor amounting to about 50 pF. Extensions of the metal film coatings 
on plates 21 to the accessible exterior of the inter-plate space, as at 
tongues 25, provide contacts for circuit leads 26, 27. The ground contact 
for the circuitry associated with the sensor is effected through metal 
suspension straps 14, of Be-Cu for example, to conductive body 10. The 
metal film coatings are preferably limited in size to extend to not less 
than about 3 mm from body 10 in order to minimize stray capacitance which 
could adversely effect the linearity of the intended response in the 
pickup. End covers 28, of which one only is shown in FIG. 2, complete the 
level sensor, forming a rugged, dustproof assembly. 
The capacitive pickup device described above will be seen to provide a 
means whereby a common capacitor plate 18 may move, under the influence of 
gravity, between two cooperating capacitor plates 23, 24 to provide 
varying capacitive couplings which may be employed as an indication of the 
displacement of the pickup assembly from a horizontal attitude. The 
physical stability of this sensor is enhanced by such structural features 
as, for example, the angled disposition of suspension straps 14 which, in 
particular, discourages radial movement of suspended plate member 18. 
Further, the close spacing, for example about 95 .mu.m, between the 
periphery of plate 18 and the walls of its cavity as formed by body 10 and 
plates 21 simply and effectively provides both air dampening and 
mechanical caging to eliminate vibratory oscillation of the pendulum and 
guard against damage from sudden jars or jolts. 
In FIG. 3 the capacitive pickup assembly is simply shown schematically as 
common plate 18 and cooperating capacitor plates 23, 24 which are placed 
in circuit with the remainder of the electronics by means of conductors 
26, 27. 
A square wave oscillator 31 is constructed, for example, from Schmitt 
Trigger elements, such as the MM74C14 device supplied by National 
Semiconductor Corporation, Santa Clara, Calif., connected in circuit with 
plates 23, 24 of the capacitive pickup through low capacitance bilateral 
switches 33, 34, such as the CD4016 device supplied by RCA Corporation. By 
means of additional circuitry described below one each of switches 33, 34 
is closed alternately with the other during counting cycles, thus causing 
oscillator 31 to generate pulse trains varying in frequency, for example 
between about 15 to 25 KHz, according to the capacitive coupling between 
pendulously-suspended common plate 18 and the respective ones of capacitor 
plates 23, 24. The pulse train output from oscillator 31 is conducted to 
the clock input terminal, C, of up-down counter 36 which may, for example, 
be a device supplied by Hughes Aircraft Company, Newport Beach, Calif., 
under the designation HCTR4010. 
A second square-wave oscillator 32 arranged to provide a pulse train of 
constant frequency of about 4 KHz serves as a master clock in the level 
indicator device. Conducted through a conventional divider to switching 
control device 35, which may be, for example, a four-stage shift register 
such as the CD4015 device supplied by RCA Corporation, the lower frequency 
pulse train, at about 1 Hz, proceeds to switches 33, 34 along parallel 
conductors, including inverter 37, and effects the simultaneous respective 
opening and closing of switches 33, 34. Thus, on the fixed time basis 
established by control device 35, capacitors 23, 24 are alternately placed 
in the circuit of oscillator 31 to provide, whenever the sensor is out of 
level, the pair of varied frequency pulse trains as earlier described. It 
will be appreciated, of course, that a level condition of the sensor will 
result in a pair of alternating pulse trains of equal frequency. 
Synchronized with the switching between capacitor plates 23, 24, one train 
of switching pulses is directed to the up-down control terminal, U/D, of 
counter 36. As a result, each of the two pulse trains from oscillator 31 
are applied to the counter to alternately effect an up-counting at the 
first frequency and a reversed down-counting at the other pulse frequency. 
During each counting cycle, therefore, there is effected a net count which 
is directly related to the degree of deviation of the pickup device from a 
true gravity horizontal attitude. 
Further synchronized with the up-down count switching sequence through the 
operation of shift register device 35 and logic elements comprising NOR 
gate 38 and inverters 37, 39 is a series of pulses directed to input 
terminal, L, of counter 36 to control the latching function which updates 
and holds the net up-down count through the following counting cycle. A 
strobing pulse, either generated within the circuit or originating as a 
command from external electronics, not shown, acts to relay the up-down 
count indicative of the extent of mislevel in the device as digital data 
to conventional drivers and visual display elements, all generally 
represented at 41. Appropriate factoring, depending upon the actual count 
frequency range, is applied in the display circuitry to achieve a final 
readout of the actual deviation from true horizontal directly in 
conventional units, such as seconds of arc. 
Subsequent to transfer of the digital data from counter 36 to display 41, a 
further output synchronized by control device 35 is directed to reset 
function input terminal, RST, of counter 36 to clear the counter prior to 
commencement of each following up-down counting cycle. 
In addition to the actual display of the degree of mislevel at 41, the 
level indicator of the present invention may be employed to provide 
digital data to an associated greater instrument represented at 51. In 
this manner the present device may be employed to inject such deviation 
data as will enable the greater utilizing instrument to effect appropriate 
correction in overall attitude or in display of function. For example, 
incorporation of the present level indicator as a generator of digital 
data indicative of deviations from true horizontal attitude in a surveying 
instrument provides a means whereby the digital data output of the instant 
device may be incorporated directly into measured elevation data to 
provide a true reading of elevations measured in the surveying system.