Feb. 15, 1955
`
`G. A. HEAD ET AL
`
`2,702,186
`
`ACCELEROMETER
`
`.
`
`Filed July 7, 1947
`
`A/G 2
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`
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`IPR2017-01058
`Garmin EX1017 Page 1
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`United States Patent Office
`
`2,702,186
`Patented Feb. 15, 1955
`
`1
`
`2,702,186
`ACCELEROMETER
`George A. Head and Edward L. Norton, Summit, N. J.,
`assignors to Bell Telephone Laboratories, Incorporated,
`New York, N. Y., a corporation of New York
`Application July 7, 1947, Serial No. 759,452
`-
`5 Claims. (CI. 264—1)
`
`2
`slightly less than the width of gap 12. Under an ac
`celeration of structure 11 in-the direction a parallel to
`the faces of poles N, S, block 15 moves relatively to
`magnet 10 in a direction opposite to a. This relative mo
`tion is opposed by springs 16, 17 and is damped by eddy
`currents generated in the copper mass as it cuts the lines
`of force.
`Springs 16 and 17 are formed as wings bent from a
`thin Phosphor bronze sheet 18 at lines 18–1, 18–2. In
`asmuch as the springs are identical, only spring 16 will
`be particularly described. As shown in the drawing,
`Fig. 2, spring 16 is divided into three legs by slots 19 and
`20 which extend from the end of this wing of sheet 18
`nearly to the line at which the wing is bent. Center leg
`This invention relates to an improved accelerometer
`A is made twice the width of each outer leg B1 or B2,
`adapted especially to the measurement of constant or
`wherefore the bending stiffness of leg A is equal to the
`slowly varying accelerations, as of a body falling under
`combined stiffness of legs B1 and B2. Spring 17 is iden
`gravity.
`tically like spring 16. For the sake of lightness, the por
`The general object of the invention is to provide an ac
`tion of sheet 18 between the lines at which the wings are
`celerometer for measuring accelerations which are steady
`formed is pierced by aperture 21, which serves also to
`or vary slowly with time.
`-
`make accessible the potentiometer presently to be de
`A feature of the invention in one embodiment thereof
`scribed. To give stiffness to this weakened central por
`is the inclusion of a circular potentiometer on which a
`tion of sheet 18, originally wider than the combined width
`brush is rotated proportionately to the rectilinear dis
`of the spring legs and slots, the overhanging width is, on
`placement of an acceleration responsive element from
`each side at right angles to the planes of the springs, bent
`25
`a rest position. This displacement is opposed by a resilient
`down into a lip 22.
`linkage between the responsive element and a fixed ele
`The springs 16 and 17 are affixed as follows to mag
`ment, of such design that the relative motion of those.
`net 10 and to block 15: Brass mooring plates 24 and 25
`elements is wholly rectilinear. To provide such a link
`are suitably fastened by screws 28, 29, through brass
`age is another object of the invention, as is also the pro
`plates 30, 31 on opposite sides (vertical in the figures)
`30
`vision of means for converting the straight line motion
`of block 15. On magnet 10 are provided similar moor
`of the responsive element into rotary motion of the po
`ings generally indicated as 32, 32’, 33, 33' for the outer
`tentiometer brush. The purpose of this linkage is to
`legs B1 and B2 of springs 16 and 17, in such fashion that
`render the accelerometer sensitive to acceleration in one
`the ends of all three legs of each spring are clamped in
`direction alone.
`a line at right angles to their length and to the opposing
`35
`The circular potentiometer shunts a source of con
`faces of poles N, S.
`stant voltage, and the voltage between the brush and the
`The operation of this spring suspension is evident from
`electrical midpoint of the potentiometer is proportional to
`Fig. 3. Here block 15 is shown displaced to the right
`the angle of rotation of the brush from the radius of that
`in response to an acceleration of magnet 10 in the di
`mid-point. Calculations or experiments permit the cali
`rection of the arrow a.
`It will be obvious that, inas
`bration of a meter reading the brush voltage in terms of
`much as the length is the same for all the legs of springs
`acceleration, either directly in suitable units, for example,
`16 and 17 and the stiffness of leg A equals the com
`cm./sec.2, or as multiples or fractions of gravity.
`|
`bined stiffness of legs B1 and B2, the motion of block 15
`Instead of providing an electrical circuit to show the
`is wholly in a straight line without rise or descent. The
`displacement of the responsive element as a voltage, one
`lines 18–1 and 18–2 at which the legs are bent down from
`may use optical means of any known kind to record this
`the original Phosphor bronze sheet, move in the same di
`displacement as the motion of a pointer over a gradu
`rection but half as much as does block 15; lip 22 moves
`ated scale, either of which may be carried by the respon
`parallel to its original level but descends slightly there
`sive element while the other is mounted on the fixed
`from. It will be noted that the deformation of leg A is
`the mirror image of that of legs B1 and B2, for which rea
`element.
`-
`Another object of the invention is therefore to provide
`son the whole resilient linkage connecting block 15 and
`means for measuring an acceleration as an electrical
`magnet 10 will herein be termed a “conjugate leaf spring
`quantity of magnitude proportional to the acceleration
`suspension.” Except for convenience in manufacture,
`and of sign depending on the sense thereof.
`springs 16 and 17 could be connected at their upper ends
`A further object of the invention is to provide in an
`by any suitable member.
`accelerometer the union of compactness and small weight
`As a result of the strictly rectilinear motion of mass 15
`due to this type of spring suspension, the accelerometer
`with high sensitivity.
`-
`-
`The invention itself will be understood from the fol
`is sensitive to acceleration in only one coordinate and
`lowing description of a preferred embodiment thereof,
`is not subject to “cross-talk” from accelerations in the
`read with reference to the accompanying drawing, in
`coordinates at right angles thereto.
`The relative deflection of block 15 may be indicated
`which:
`-
`Fig. 1 is a perspective view of the assembled accel
`by the motion of pointer P carried by block 15, over
`scale D, fixed on magnet 10, Block 15 may suitably be
`erometer;
`F Fig. 2 is an exploded view of the instrument shown in
`of aluminum instead of copper.
`-
`When it is not required
`to represent the deflection by a voltage as next described,
`ig. 1;
`-
`Fig. 3 is a front elevation of the accelerometer show
`and the indication by pointer P is sufficient, the entire elec
`ing its response to an acceleration directed toward the
`trical system later explained may be omitted.
`‘left, and
`The rectilinear motion of block 15 is utilized to ro
`Fig. 4 is a diagram of an electrical circuit suitable for
`tate. brush 35 on circular potentiometer 36, shown in
`measuring the response of the instrument.
`Fig. 2, where casing 37 is partly broken away. Brush
`In all figures, like numerals and letters designate like
`35 is carried with suitable insulation on shaft 38 which
`continues through casing 37 to end in pulley 39. Re
`parts.
`-
`Referring to Figs. 1 and 2, numeral 10 designates a
`ferring particularly to Fig. 2, there is found enwrapping
`permanent magnet, suitably of “Alnico V,” in the general
`pulley 39 a nylon thread 40 stretched between the arms
`form of a C, supported in any convenient way by the
`of a stiff yoke 41 formed of a Phosphor bronze wire 42
`structure 11 of which the acceleration is to be measured.
`threaded, as indicated in the figure, through holes in
`The faces of poles N, S of magnet 10 are separated by an
`block 15 and suitably bent to form yoke 41, the arms of
`air-gap 12 within which copper block 15 moves at right
`which lie in a plane parallel to block 15 and extend up
`angles to the magnetic lines of force when structure 11
`ward at right angles to the permitted motion thereof.
`experiences an acceleration. Block 15, the acceleration
`It is clear that sidewise displacement of block 15, such
`responsive element, is a square flat sheet of thickness
`as indicated in Fig. 3, entrains yoke 41 and with it
`
`10
`
`15
`
`20
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`40
`
`50
`
`60
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`65
`
`70
`
`80
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`IPR2017-01058
`Garmin EX1017 Page 2
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`2,702,130
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`'
`
`Ct
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`10
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`20
`
`4
`side of the ce_nter, the radius r of pulley 39 is given by
`the ‘equation
`.,
`
`__X .735
`0.312 cm.
`?
`The choice of resonant frequency is determined by
`consideration of the requirements of the apparatus (not
`a part of the invention) with which the accelerometer
`cooperates; for a particular system this consideration
`lead to the choice of fo as 16 cycles per second. This
`resonant frequency once chosen
`'
`
`where S is the stiffness of the _suspension and m is the
`effective mass of the moving parts. The force F on the
`mass m corresponding to an acceleration a is F=mu;
`the displacement X under this force is
`F
`X-S
`
`wherefore
`
`X=
`
`(2Tfo)i
`Thesmass m is chosen large enough for the torque
`T=mar on pulley 39 to be greater than enough to over-
`come static friction of the potentiometer for the least
`acceleration to be measured. For the type of poten-
`tiometer selected,
`In is appropriately 50 grams. This
`determines the stiffness leading to the value above stated
`for the spring suspension, which is then designed in ac-
`cordance with well-known principles. At the same time
`the 50 gr-am mass (most of which is the copper block
`15, 3.5 cm. square by 0.2 cm.
`thick)
`is small enough
`to permit adequate damping by eddy currents.
`It is known to the art to design accelerometers on the
`principle of measuring the force required to check the
`excursion of an inertia elementdisplaced relatively to a
`body undergoing acceleration.
`‘So far as known to us,
`however, no accelerometer of the prior art has taken
`advantage of the increased sensitivity consequent on the
`conversion of the straight line motion of the inertia ele- '
`ment into motion of rotation as is done in the present
`invention through yoke 41,
`thread 40 and pulley 39.
`Thereby with multiplied sensitivity, the rotation of brush
`35 measures the defiection of springs 16, 17 and there-
`with the force acting on the inertia element 15 relative
`to the fixed element 10.
`Parallel motion spring suspen-
`sions are known but we are not aware of any such as
`herein termed a “conjugate leaf spring suspension" where
`a U-shaped spring,
`the arms.of which are each subdi-
`vided into E’s as shown, so links the fixed and the inertia
`' elements that
`the relative motion thereof is exclusively
`in one straight line.
`What is claimed is:
`,
`I. An accelerometer including a magnet having opposite
`poles separated by an air-gap, a non-magnetic metallic
`mass, a connection between the magnet and the mass
`whereby the mass is supported in the air-gap movably
`relative to the magnet, the connection comprising a resili-
`ent member formed into a pair of E springs connected in
`spaced parallel relationship and individually secured at
`their ends to the magnet by the outer legs and to the
`mass by the central legs of the E5, and means for indi-
`cating the relative motion of the mass.
`2. In an accelerometer,
`the combination of a magnet
`having opposite poles separated by an air-gap, a non-
`magnetic metallic mass, and a resilient connection be-
`tween the magnet and the mass whereby the mass is .
`movably supported in the air-gap, said connection com-
`prising a resilient member formed into a pair of E springs
`connected in spaced parallel relationship and individual-
`ly secured at their ends to the magnet by the outer legs
`of the E's and to the mass by the central legs of the E's,
`a circular potentiometer supported from the magnet and
`traversed by a brush carried on a shaft supported co-
`axially with the potentiometer, the shaft being perpendic-
`ular to the motion of the mass and terminating at one
`end in a pulley, a yoke carried by the mass and provided
`with arms lying in a plane perpendicular to the shaft,
`and a cord interconnecting the arms and enwrapping
`the pulley.
`.
`V
`. 3. An accelerometer comprising a first element adapt-
`
`3
`throughthread 40 proportionally rotates pulley 39. As
`shown principally in Fig.2 but indicated also in Fig.
`l
`potentiometer 36 may be supported by bracket.45 mount-
`ed in any convenient fashion on the upper -surface of
`magnet 10 (a surface at right angles to the face of pole
`N) which positions -the axis of the potentiometer at right
`angles both to block 15 and parallel
`to the planes of
`springs 16, 17.-
`The necessary electrical connections to the winding of
`—potentiometer 36 and to brush 35 are made by soldered
`connections at external binding posts indicated generally
`by numeral 46 in Figs. 1 and 2. These connections are V
`better explained‘ in reference to Fig. 4, where an electrical
`-circuit
`is ‘schematically shown.
`Fig. 4 shows a diagram of a current supply.circuit for
`potentiometer 36 which avoids the use of an actual
`grounded midpoint connection to the potentiometer
`winding itself. Battery 50 is shunted by potentiometer
`36 and also by a high resistance auxiliary potentiometer
`51 03 which a grounded tap 52 may be manually ad-
`juste .
`Meter 53 is connected between ground and.
`brush 35.
`In assembly, brush 35 is positioned on the
`physical mid-radius of potentiometer 36 when’ block 15
`is in its undisturbed position, and by adjustment of tap
`52 the point of the winding on this radius of potentiom-
`eter 36 is made the electrical. mid-point and caused to
`stand at ground potential. This adjustment is made by
`‘ setting tap 52 at such a point on potentiometer 51 that
`meter 53 reads zero voltage between brush 35 and
`ground. Thereafter, displacement in either direction of ‘
`block 15 from its neutral position produces rotation of
`shaft 38 and brush 35 rotates through an angle propor-
`tional to the rectilinear motion of block 15.
`*
`Where several accelerometers of the type described are
`C.CI
`to be used in parallel from the same voltage source such ‘
`as battery So, obviously potentiometer 51 is to be omitted
`and a physical grounding of the mid-point of potentiom-
`eter 36 is required.
`It will be understood that the show-
`ing of Fig. 4 is illustrative only, numerous other circuits
`for the same purpose being permissible. Meter 53 may,
`of course, be replaced by a direct current amplifier and
`meter of conventional design.
`Potentiometer 36, with its case and fittings is suitably
`an “Autofiight Microtorque Potentiometer" manufactured
`by G. M. Giannini Company, Pasadena, California.
`An actual embodiment of the invention weighed 21
`ounces complete except for battery and meter.
`In this
`particular instrument, designed to measure accelerations
`up to -7.5 times that of gravity,
`the dimensions were
`(refer to Fig. 3) 3.75 inches vertically by 2.25 inches
`normal to the plane of the figure and 1.63 inches in the
`direction of motion of block 15. Potentiometer 36 was
`of 2,000 ohms resistance with a carrying capacity of 2
`watts; these data limit the voltage of battery 50 to some
`60 volts.
`-
`The conjugate-leaf sgring suspension was stamped from
`ronze about 0.3 millimeter thick.
`"a sheet of Phosphor
`The effective mass of the moving parts (copper block,
`springs, yoke and participating elements in the poten-
`tiometer) was 50 grams; the stiffness of'the spring sus-
`pension 5x 105 dynes/cm. Consequently,
`the resonant
`frequency of the block and suspension was
`
`[5Cl
`
`40
`
`{:0
`
`60
`
`100
`fieycles per second.
`
`Overshoot and unwanted oscillations are avoided by eddy
`currents generated in the copper block as it cuts the lines
`of magnetic force. The pole faces of the magnet were
`2 square cm. in area, the air-gap 0.28 centimeter wide.
`For the resonant frequency (fo=l6 about), the displace-
`ment of block 15 required to measure an acceleration
`of 7.5 times gravity is
`-
`
`X
`
`_7.5x 980
`— (2rfo)’
`
`=0.735 cm.
`
`Thevrequired radius of pulley 39 is equal to this dis-
`placement divided by the desired angle of brush rotation
`either side of the neutral radius.
`In the “autoflight"
`potentiometer chosen for this purpose the electrical line-
`arity of the winding is satisfactory throughout the 270
`degrees of brush rotation available. Accordingly, for
`X=0.735 cm..- to rotate the brush 135 degrees (0) either
`
`-1 c,-
`
`b0
`
`IPRZO17-01058
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`Garmin EX1017 Page 3
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`IPR2017-01058
`Garmin EX1017 Page 3
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`azonise
`ed to be secured to a body of which the acceleration is
`be measured, a second element adapted to move rela
`to be measured, a second element adapted to move rela
`tively to the first element responsively to the accelera
`tively to the first element responsively to the accelera
`tion, a connection between the said first and said second
`tion, a resilient third element connecting the first and
`elements comprising a resilient member formed into a
`second elements and opposing the relative motion there
`pair of E springs connected in spaced parallel relation
`of, said third element consisting of a conjugate leaf spring
`ship and individually secured at their ends to the said
`suspension comprising a pair of E springs of each of
`first element by the outer legs and to said second ele
`which the three legs are of equal length, the outer legs
`ment by the central legs of the E's, and means for indi
`are affixed at their ends to one of the first and second
`cating the relative motion of said second element,
`elements and the central leg is affixed to the other of the
`first and second elements, the two springs being thus
`References Cited in the file of this patent
`affixed in spaced parallel planes at right angles to the
`UNITED STATES PATENTS
`direction of relative motion of the second element and
`being connected remotely from the ends of the legs by
`Wimperis -------------- Jan. 24, 1911
`a stiff member in a plane perpendicular to the parallel
`Tsujita ---------------- Jan. 5, 1932
`planes and to the planes of the E's, and means for in
`Wilson --------------- Mar. 19, 1940
`dicating the relative motion of the first and second ele
`Bonell ---------------- Aug. 13, 1940
`Chappell -------------- Apr. 20, 1943
`ments.
`4. * :*. º in #: # which *. :
`OTHER REFERENCES
`of each E spring have together the same bending
`20
`#: as the central leg thereof.
`“The Measurement of Riding Qualities,” in SAE Jour
`3. An accelerometer comprising a first element adapted
`nal, June 1928, pages 636 and 637.
`to be secured to a body of which the acceleration is to
`
`982,336
`1,840,001
`2,193,910
`2,210,970
`2,317,028
`
`15
`
`IPR2017-01058
`Garmin EX1017 Page 4
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