CA1071661A - Leaf spring weighing scale with optical detector - Google Patents

Abstract

LEAF SPRING WEIGHING SCALE WITH OPTICAL DETECTOR

Abstract of the Disclosure A weighing scale having a tare structure carried by a pair of vertically spaced parallel leaf springs includes an optical detector having an apparatus for generating a moire fringe pattern and moving the pattern with scale tare deflection. One of a pair of ruled parallel grating sets is fixed while the other is mounted to the tare structure in movable, parallel alignment with the fixed set. A beam of light projects through both grating sets to project the pattern on a photodetector array carried by the scale frame.
Upon placement of a load on the scale, deflection of the tare structure results in linear movement of successive fringes across the photodetectors and the sequential generation of weight indicative signals. The photodetectors are secured to a rotatable base for adjustment of the effective spacings between each photodetector with respect to fringe pattern image.

Description

7~
sack~ound of the Invention 1. Related Application and Field_of th _Invention The invention relates generally to mass measuring and more particularly to a weighing system employing an optical readout apparatus for the detection of scale deflection.
In a copending application Serial No . 269, Q18 - ;
filed Decembe~ 31, 1976,entitled~ Weighing System with a Moire Optoelectroni~ Transducer, and having the same assignee, a moire weighing system is shown not unlike the inventive subject weighing system. In this co-pending system, elaborate lengths were taken to insure the parallelism of the grating sets over the entire weighing range, i~e. an expensive, high tolerance, rail-type carrier was designed to movably-support one of the gratings.
The present inventive moire weighing system eliminates the need for utilizing an expensive structural parallel I
guide to insure a proper grating relationship.
The present inventive weighing system accomplishes this parallelism between the grating sets by employing a scale having a tare structure ~eaturing an inherent parallel movement. The weighing system of this invention fix the movable grating to this parallel moving tare, so it too will experience a parallel movement.
This invention comprises a tare having a pair of vertically spaced parallel leaf springs.
These leaf springs are caused to deflect, in parallel when a load is placed on the pan of the scale. The movable grating, which is affixed to these movable lea~ springs, - ~ - 2 - ~

: : ,.--:- : . : . . .. . . . :.
.... ... , . .. : - . ..... . .. , :, ,.
. : : . . .. . . : : . . :
.: .. : . .. , ,. , : -:- :: : . - . , -6'1 deflects with a translational movement that maintains its pa~allelism throughout the weighing ranye with respect to its complementary fixed grating (the fixed grating is immovably anchored to the frame of the scale).
In addition to the reduction o~ parts, the invention also features a secure fixing of both gratings to their associated meT~ers. This contrasts sharply with the apparatus of the aforementioned application Serial No. 269,018, wherein the movable grating is universally connected to the weighing pan.
Thus, it will be seen, that the subject invention provided for a reduc~ion in parts, a reduction in cost, and a - simplification in assembly and construction.

2. Brief Description of the Prior Art In United States Patent No. 3,861,480 to Zuckex et al, granted January 1975 a high speed automatic mail processing system for sorting and imprinting postage on large quantities of mixed mail has been described. The system employed an optical readout apparatus for detecting the deflection of the scale tare when supporting a piece of mail for the determination of the postage to be affixed thereon. This prior apparatus included a bank of multiple photodetectors at the end of a light path. A shutter, carried by the tare, was operable to gradually block the light path to successive photodetectors of the bank. A signal indicative of the number of photodetectors remaining in the light path, hence tare d~flection, was produced and utilized to set the postage to be imprinted on tne piece of mail.
Several disadvantagés accompanied the use of this prior optical readaut apparatus. For example, nuTnerous . ,( - - . :. :
. : . .. ...
'-. . '' . :: .. :
. ., . . : : ;
, .~ .
. ., : . ,., :
... ......
: :.': :
. .

~07~
photodetectors were required to provide weiyhk indicative signals over a small weiyht load range with the amour~t of photodetectors increasing with increased scale capacity; further, the particular transition point wherein each photodetector switched, for exmaple, from conduction to nonconduction states had to be individually set for successive postage weight increments.
These transition or switching points were highly critical and tolerance requirements rendered final adjustments difficult and time consuming.
Ano-ther disadvantage rela-ted to the relatively large projection path, e.g. 18 inches to 24 inches (45-60 cm.), which was required in order to obtain the necessary optical magnification. As a result of such a large projection path and the critical switching points, accuracy of readouts was highly sensitive to vibrations and other environmental conditions.
A similar optical detection system for weight determina-tion is illustrated in United States Patent 3,528,517 to Or~al J. Martin, granted September 15, 1970 wherein a coded chart is moved by a load responsive lever with coded markings of the chart being projected upon a bank of photocells. Among the disadvantages of this system was that numerous photocells were required for weight determination with the transition point of each photocell being critical.
In United States Patent No. 3,826,318 to Baumgartner, granted July, 1974 a movable and a stationary bar grid in optical alignment were employed. The movable grid functioned as a shutter to sequentially block a light beam through the grids.

. ~, .

sb/~

, .
., ' ' ' ' . ' . ' ` , " . . `:
: . . . :
, . ,' : -.. . . .

6~
Electrical signals were genera-ted from the sequential light pulses and were processed to provide a weight indicative signal.
A significant drawback of the shutter-grid optical system was that minute displacement of less than the spacings be-tween successive bars of the movable grids was practically impossible to detect.
The utilization of optical control systems employing moire fringe patterns have been suggested as measuremen-t and control devices for machine tools. Typically, measuring devices such as those illustrated in United States Patents Nos. 2,886,717 to David T.N. Williamson et al, granted May 12, 1959, 2,886,71B
to Alexander Turnbull Shepherd et al, granted May 12, 1959, 2,861,3~5 to Rolf Edmund Spencer, granted November 25, 1958 and

3,154,688 to Alexander Turnbull Shepherd, granted October 27, 1964, have been designed for utilization in tools such as milling machines in order to measure the movement of the work table of the machine with respect to the framework.
A further position detecting system utilizing moire fringe patterns is disclosed in United States Patent No.
3,755,682 to Russell, granted August, 1973, which required at least eight photoelectric cells with the output of the photo-cells being selected in cyclic order by cam pulses to produce a composite output signal, which was compared to a reference signal. With the introduction of numerous photocells, added complexity, costs and potential malfunction were incurred.
United States Patents Nos, 3,713,139 to Norman R.
Sanford et al, granted January 23, 1973 and 3,708,681 to Richard J. Ivers, granted Janaury 2, 1973 illustrate typical moire optical apparatus for determining displacement and are sb/J ~

~: .. -. , :, . .
. . ~ - .. :-., . : . - . .... .
. ~ . . ., , : , .: , - : .
',~,:'~''" , :' .'' , ~7~

indicakive of one of the drawbacks of prior moire displacement measuring apparatus. A plurality of strip photodetectors were employed, each having a length approxima~ing the width of the index grating. In order to synchronize the photodetector spacing with the fringe spacing, the skew angle between the gra-tings was adjusted to vary the fringe spacings. Unfortunately, minute adjustments in the skew angle resulted in substantial changes in the fringe spacing, and dif~icult precise skew angle : adjustment was necessary to properly coordinate the fringe patterns with the photodetector spacings.
According to the present invention, there is provided a weighing scale having a frame, a displaceable tare structure carried by the frame, the tare structure being yieldably displaceable in accordance with the mass of a load being weighed and including a pair of vertically spaced and substantially horizontally extending arms. An optical detector system is caxried by the frame for determining the extent of tare structure displacement, the optical detector system including - means for providing a light beam. First and second optical : 20 grating sets are provided, each of which has an optical grating, - the first optical grating set being mounted on the frame and the second grating set being mounted to move with the tare structure and relative to the first grating set. Means is provided for rotatably adjustably mounting the first optical grating set on the rame for positional adjustment relative to the second grating set. The two optical gratings are position~d at an angle with respect to one another and modulate -the light beam so as to generate a pattern of moire optical interference.
~ fringe lines. Movement of the second grat.ing set with the ; . - 6 -. . .

', 3L~7~

tare structure relative to the first yrating set produces transverse movement of the fringe lines. Photod~tector means is stationarily mounted on the frame for detecting the movement of the fringe lines produced by the displacement of the tare structure.
More specifically, the present invention relates to a leaf spring weighing scale having an optical detector for sensing tare deflection by generating a moire fxinge pattern and moving the pattern as an amplified direct function of the deflection. The fringe pattern is generated by modulation of a light beam which projects through a pair of ruled parallel grating sets. Deflection of one of the grating sets as a result of tare deflection produces sequential movement of successive moire fringes or bands across an array of photo-detector pairs. A fixed grating set is mounted in one leg of a U-shaped transducer, while the light source is mounted in the other. The movable grating set is mounted to the tare structure ~ for relative parallel movement in a vertical plane between the ; legs of the transducerO
This parallel movement of the movable grating, is the inherent result of its securement to a paix of vertically spaced leaf springs. The leaf springs~ which form part o the tare, are constructed to provide a translational deflection tparallelism) for the load and the movable grating. The translational movement furnishes the necessary linearity for proper and accurate weight measurement.
Coarse adjustment of the synchronization between the photodetector pairs and the fringe pattern is obtained - 6a -sb/~o .. .. , . ::
- : "' .. ,. ,, ' ' .. ., ~, :
' by adjustment of the skew angle between the grating sets while fine adjustment is achieved by changing the effective photodetector spacings relative to ~ringe patter~ movement.
- A load of a given weight produces a given deflection of the tare and the linear movement of a given number of ringes past a reference, i.e. a photodetector, thereby providing a given number of sequential pulses suitable for proeessing and subsequent utilization.
From the foregoing compendium it will be appreciated that it is an object of the present invention to provide a seale of the general eharacter deseribed, having an optieal detector which, however, is not subject to the disadvantages of the prior systems.
Another object of the present invention is to provide a scale of the general character described having an optical detector which is economical and simple in construction.
A further object of the present invention is to provide a seale of the general eharaeter described having an optical deteetor whieh may be easily adjusted and calibrated.
Yee another object of the present invention is to provide a seale of the general character described having an optieal detector wherein a fixed minimal amount of photo-deteetors is required regardless of the seale wei~ht load ` range.
It is a further objeet of the present invention to provide an optical detector of the general eharaeter deseribed which is simple in eonstruction yet suitable for rapid and aeeurate weight measurement of sueeessive pieees of mail in sn aoto=atie mall proeessing apparatus by providing an -~ ~

': ' ' ' .
'.: '. ~ , '' ' ' , "
: . '' ', ' ~ ' ' .
. .

7~
output signal capable of controlling the arnount of postage to be imprinted.
Another object of the present in~ention is to provide a scale of the general character described having an optical detector which detects tare deflection by sensing the movement of an amplified fringe pattern directly projected across a rela~ively short optical path.
A further object of the present invention is to provide a scale of the general character described having an optical detector which provides reinforcement of weight indicative signals to achieve increased reliability.
Other objects of the present invention in part will be obvious and in part will be pointed out hereinafter.
With these ends in view, the invention finds embodiment in certain combinations of elements and arrangements of parts by which the objects aforementioned and certain other objects are attained, all as fully described with reference to the accompanying drawings and the scope of which is more particularly pointed out and indicated in the appended claims.
Brief Description of the Drawings In the accompanying drawings in which is shown one of the various possible exemplary embodiments of the invention:
FIG. 1 is a perspective illus-tration of a mail weighing station at an automatic mail processing system and illustrates a typical environment wherein the scale constructed in accordance with the present invention may be employed;
FIG~ 2 is an auxiliary side elevation view of the scale, illustrating an optical detector ~ith portions o~ the ~ 8 -~,i ,, .
' . .

.

.

.

~97~6~ -scale deleted for clarity;
FIG. 3 is a sectional view through the optical detector with portions of ~he scale deleted for clarity, the same being taken substantially along the line 3-~3 of FIG. 2 and depicts a light source, a condenser, a pair of grating sets and a photodetector;
FIG. 4 is a fragmen~ary sectional vie~ through the optical detector, the same being taken substantially along the line 4--4 of FIG. 3 and indicates the positions of a movable grating set at various deflections of the tare structure;
and FIG. 5 is a greatly enlarged auxiliary view taken substantially along line 5--5 of FIG. 3 showing the moire fringe pattern as projected on an array of photodetectors mounted to a rotatable disc in the transducer.
~ escription of the Preferred Embodiment Referring now in detail to the drawings wherein a typical exemplary embodiment of the invention is illustrated, the reference numeral 10 denotes a leaf spring scale positioned at a.weighing station 12 of an automated mail processlng system such as that described in United States Letters Patent No. 3,877,531 to Storace et al, issued April 15th, 1975.
In FIG. l an edgewise oriented envelope 14 is depicted moving along a mail feed path of the system about to be transported to the weighing station 12 by a pair of feed : rollers 16 and a deflection guide baffle 18, all as fully descrlbed in the aforementioned patent.

.

-~ . ~ ' ' _ g _ :

6~

It should be appreciated that the scale 10 is designed .
to provide substantially linear deflection as a result of different weights carried in a tray 20. The tray 20 includes a side wall 22 slightly tilted from a vertical plane and an envelope supporting trough 24 which projects from its lower edge toward the envelope feed path. When an envelope is deposited on the scale 10 it remains substantially upright with its bottom edge cradlèd in the trough 24 while its rear face leans against the tray side wall 22.
The tray 20 is afEixed to a tare structure 26 by conventional means such as a pair of spaced brackets 27 while the tare structure is secured to the distal edges of a pair of horizontally oriented vertically spaced broad leaf springs 28, 30. Each leaf spring is cantilever supported at its opposite edge to an inclined mounting frame 32 as is more fully described in both the aforementioned Patent No.
3,877j531 and Ln Paten~ No. 3,807,517 to Freeman issued April 30th, .
1974. A leaf spring scale of such design provides substantially .
linear vertical deflection in response to various loads carried by the tray 20 with only negligible deflection in the horizontal plane over a predetermined working load range. .
Of substantial significance is the extended lateral width of the springs 28, 30 which prevent lateral deflections .
of the tare structure 26. .
An optical detector 34 is positioned within the scale 10 between the leaf springs 28, 30, such that vertical tare structure displacement as a result of a load being positioned on the tray 20 will be detected~ As heretofore mentioned, the movement of a moire frlnge pattern for the detection -. . .
- . ~ . , : :
.. . . : :
.

.. .
- :

6~

and measurement of load weight is utilized in the present inventionO For this purpose, a pair of ruled parallel grating sets on transparent substrates is provided with one of the sets being fixed and the other set moving with the tare structure 26 in a plane parallel to the fixed set. By way of example, the gratings of each set may comprise Ronchi rulings spaced two thousandths of an inch apart, however spacings between the rulings of each grating set may vary and need not be equal as will be later described.
The optical detector 34 includes a U-shaped transducer 36 which is fixed to a spacer block 38 extending from the mounting frame 32. A pair of spaced parallel legs 40, 42 of the transducer 36 project toward the tare structure 26.
Each of the legs includes a transverse bore 44 which extends in a horizontal plane.
A fixed grating set 46 formed on a planar face of a transparent, e.g. glass, substrate 48 is mounted on the forward end of a sleeve 50 seated in the leg 40. In order to adjust the distance between the grating sets and the spacings between the fringes of the moire interference pattern, the sleeve 50 is movable within the bore and manipulated by a ring 52 seated within an enlarged end of the bore 44.
The ring 52 is clamped after the grating set has been positioned.
A movable grating set 54 formed on a planar surface of a transparent substrate 56 is mounted in a vertical position within a supporting channel 58. The channel 5~ is secured to the tare structure 26 at its opposite ends by conventional means such as brackets. The movable grating set 54 is positioned between the legs 40, 42 o~ the transducer 36 and adapted , . , :
. .
" ' 1~71f~61 for vertical movement over the weight load range of the scale 10 and limited horizontal movement with respect to the transducer. The planar face o~ the movable grating substrate is abutted against a flat inner ~lange wall of the supporting channel 58 and tightened in such position by a plurality of clamping screws 60 which apply pressure against an abutment shim 62.
In order to provide a light beam for modulation by the grating sets, the bore 44 through the leg 42 is enlarged adjacent its end distant from the leg A0 and a suitable lamp 64 is mounted therein. A condensing lens 66 is positioned between the lamp 44 and the movable grating set 54 to project a light beam perpendicular to and through ~oth grating sets 54, 46. Because the gratings of the fixed grating set 46 are positioned at a slight skew angle with resp2ct to the gratings of the movable grating set 54, the interference between the two grating sets modulates the light beam to have a plurality of cyclic parallel moire fringes.
The placement of a load upon the tray 20 causes the tare structure 26, hence the movable grating set 5~, to move downwardly. 8ecause of the extended latera' width of the springs 28, 30, lateral deflection of the movable grating set 54 during this downward movement is prevented. Thus, the grating sets 46, 54 are maintained in precisely parallel spaced relationship permitting the optical projection of the moire pattern with high resolution.
The downward movement of the grating set 54 results in horizontal movement of successive substantially verticall~
oriented moire fringes. The fringe movement is detected .:

:.. . . .. . ..
,., . . , : . , . .: -,: , . . . :. .. .

.. : : ' :' .. . - . - : : . . .. . ~ ~ .
,: . . .: -, . . ~ .
. . . .

~iL()7~

and counted in accordance with the present invention to provide a signal indicative of the weight of the load, e.g.
the envelope 14, deposited upon the tray 20. Such signal, in an application such as the weighing station 12 of an automated mail processing system may be processed and utilized to determine and control the amount of postage to be imprinted upon the envelope 14 at a subsequent station.
Movement of the moire interference pattern is sensed by a plurality of photodetectors 68, 70, 72, 74. Although four photodetectors have ~een illustrated, satisfactory results have been obtained with only two photodetectors which sense both the magnitude and direction of the movement of tha moire fringe pattern. The photodetectors 68, 70, 72, 74 are mounted within a nonconducting disc 76, each equidistantly spaced from one another and positioned along ar axis coincident with a diagonal of the disc. The disc 76 is positioned within the inner bore of the sleeve 50 in abutment against a peripheral stop and secured in its desired position by a threaded collar 78. lt should be appreciated that each of the photodetectors is optimally placed 90 degrees apart from each adjacent photodetector with respect to the moire interference pattern which moves across the photodetector array.
The photodetector 68, 72 are arranged to provide a single output signal while the photodetectors ~0, 74 are similarly arranged to provide a single output signal. Each interconnected pair of photodetectors is physically dispo~ed 90 degrees from each other, so that the combined output signal of each pair is in phase quatrature with respect - i3 -: ~ .

.
, .:: . :
.

.
~ - ~, ' , ' , .
::
: :' :.' ' ~4:171~

to the other pair and serves as a reinforcement signal.
It has been ~ound that in order to accurately synchronize the photodetector spacings with the moire fringe spacings, extensive and precise adjustment of the skew angle between the respective grating sets was necessary. This was due primarily to the functional relationship between skew angle and fringe spacing, and often small angular changes resulted in disproportionate fringe spacing changes.
ID accordance with ~he present invention, a fine adjustment to synchronize and coordinate photodetector spacing with fringe spacing is achieved by altering the effective distance between photodetectors with respect to the fringe pattern movement by rotating the disc 62 and locking the disc by tighteninq the collar 64. In order to utilize this technique each photodetector is required to be of a maximum dimension not greater than the thickness of a fringe band.
Thus, rotation of the disc 62 such that the a~is of the photodetectors is not perpendicular to fringe pattern movement will not result in inaccurate weight readings.
With reference now to FIG. 5, the fine adjustment of photodetector spacing with respect to fringe spacing will be described in more detail. Assuming, for example, that the spaces between successive moire fringes are too large for the photodetector spacings, fine adjustment compensa-tion for larger fringe pattern spacing is achieved by rotating the disc 62 in a counterclockwise direction thereby increasing the efPective distance between the photodetectors with respect to fringe pattern movement. The output signals of the photo-detector pairs are subsequently processed utilizing circuitry '`I
. .
- : : ,. . :
.
., -. ' ~ . , ~ : . , : .: , ',' . ' ,,, . .' ,. :, ' :.
.- . ., :
,.. . ..
", .. . . ..
, . : ., , . ~ .

10~166~

well kno~n in the art, for example, that shown in United States Patent 2,886,718 or 3,713,139 and counted to provide a signal suitable for optical display and indicative of the weight of the load placed on the scale platform.
As more fully described in United states Patent No.
3,807,517, the tare structure 26 deElects from a zero load position along a deflection curve to a maximum or full load position. With reference to FIG. 4, wherein the deflection curve is illustrated in dotted lines and with the zero load position of the grating set 54 shown in solid lines, it will be appreciated that the full load position is shown in dashed lines and the mid-range load position is shown in dot and dash lines. Ideally, the tare angle is selected to place the zero and maximum scale positions at equal distances from the horizontal mid-range positions of the leaf springs 28, 30.
~ It should be appreciated that horizontal movement - of the tare structure 26, hence the grating set 54, inherent in the twin leaf spring scale lO, must not affect the operation of the optical detector 34. The horizontal gratings o~ the gcating set 5~ each extend a distance substdntially longer than the maximum transverse light sensitive area o~ the photo-conductor array. Thus, the limited hori~ontal movement of the grating set 54 does not present any discontinuity of gratings and both the appearance and detection of the moving moire pattern remain unaffected.
As mentioned previously, the rulings of the grating sets need not both be the same number per inch. If the gratings are superimposed in parallel relationship with .

. .
.. .. . . .
' ' ', : ~

: , : . :
,, ~7~L66~

unequal numbers per inch, an interference beat pattern willresult wherein the number of frinyes per inch is the difference between the unequal number o~ fringes of each grating.
Movement of such a beat pattern is detected in the same manner as previously described.
: Thus, it will be seen that there is provided a weight measuring system having an optical detector which achieves the various objects of the invention and which is well suited to meet the conditions of practical use.
Having thus described the invention, there is claimed as new and desired to be secured by Letters Patent:

' . ~ :
.

.

. ' , . ...... , ., ~' . ' ' .
~ - 16 - ~
.,, 1 .
.. . ..

. .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A weighing scale having a frame, a displaceable tare structure carried by said frame, said tare structure being yieldably displaceable in accordance with the mass of a load being weighed and including a pair of vertically spaced and substantially horizontally extending arms;
an optical detector system carried by said frame for determining the extent of tare structure displacement;
said optical detector system including means for providing a light beam;
first and second optical grating sets, each of said grating sets having an optical grating, said first optical grating set being mounted on said frame and said second grating set being mounted to move with said tare structure and relative to said first grating set;
means for rotatably adjustably mounting said first optical grating set on said frame for positional adjustment relative to said second grating set, said two optical gratings being positioned at an angle with respect to one another and modulate said light beam so as to generate a pattern of moire optical interference fringe lines, movement of said second grating set with said tare structure relative to said first grating set producing transverse movement of said fringe lines;
and photodetector means stationarily mounted on said frame for detecting said movement of said fringe lines produced by the displacement of said tare structure.