Measuring method and apparatus

This invention relates to method and apparatus for improving the accuracy of optical or electrical position measuring apparatus wherein movement of one object relative to another is measured. One known system employs primary and secondary windings each comprising a multiplicity of series-connected conductors arranged on adjacent faces of relatively movable members of insulating material. In another system an optical Moire fringe pattern is moved in correspondence as regards extent and sense with the movement with the object and the number of fringes displaced from a fiducial line are counted. The improvement is applicable to either system and includes placing a series of parallel lines or parallel conductors on the scale or movable object at an angle with respect to the path of movement of the object and placing another set of parallel lines or parallel conductors on the slider or reader followed by providing for controlled movement of the scale transversely of the path of movement of the object to vary the "count" of the lines or conductors on the scale and those on the slider or reader in order to compensate for machine or scale error and render the resulting operational "counts" accurate with respect to the actual movement of the object. Adjustable scale engaging clips are employed to permit the controlled lateral adjustment of the scales.

FIELD OF THE INVENTION 
This invention relates in general to precision measuring apparatus and in 
particular relates to a method and apparatus for improving the accuracy of 
optical or electrical position measuring apparatus. 
DESCRIPTION OF THE PRIOR ART 
It is well known in the prior art to provide measuring means to compute or 
measure the length or degree of linear travel of one object relative to 
another. By way of example only there are known inspection machines for 
checking and inspecting precision machined parts. Those inspection 
machines have scales associated with their various components which 
measure the linear movement of the machine components in the X,Y and Z 
axes to facilitate the inspection procedure. 
Furthermore, it has also long been known that such movement can be measured 
even more accurately electronically. This has primarily been accomplished 
by one of two alternative methods. 
One system commonly used employs transformers in the form of primary and 
secondary windings each comprising a multiplicity of series-connected 
conductors arranged on opposite faces of the movable object or scale and 
the slider which overlies the scale. When the conductors of one winding 
are energized with an alternating voltage, the current in each conductor 
of that winding induces a voltage in the conductor of the other winding 
which happens to be adjacent thereto and these voltages add at the 
terminals of the other winding to give a secondary voltage which varies in 
magnitude according to the relative position of the movable object and the 
slider to thus measure the travel. 
In this system the conductors are carried on the scale and on the slider 
which overlies the scale. These conductors are arranged at 90.degree. to 
the path of travel and a sine wave is created which can be read 
electronically to measure linear movement. 
Another of these methods involves applying a series of parallel lines to 
the scale and also providing an overlying reading head having a plurality 
of lines which are at a different angle than the scale lines with respect 
to the path of movement. When the reading head passes over the scale, an 
optical Moire Fringe pattern is created and the distance of movement can 
be read by reading the pulses that are created by photoelectric cells as 
the lines move across each other. The variance in the electrical energy 
produces a cycle which is broken down into distance and "count" and then 
displayed on a digital readout. 
The effect involved in this system is generally called a Moire Fringe 
effect. By definition, when one family of lines is superimposed over 
another family of lines so that they cross at an angle, a new family of 
curves appear which pass through the intersections of the original lines. 
Optically this produces what is often called a shadow line. In any event, 
the relative linear movement along the machine travel path is then 
electrically read by reflecting these traveling or moving shadow lines. 
Examples of these systems and some of the application of them can be seen 
in Giambiasi U.S. Pat. No. 1,415,627; Turrettini U.S. Pat. No. 2,416,968; 
Williamson U.S. Pat. No. 2,886,717; Shepherd U.S. Pat. No. 2,886,718; 
Williamson U.S. Pat. No. 3,098,186; Ogden U.S. Pat. No. 3,394,248; Burke 
U.S. Pat. No. 3,688,570; Russell U.S. Pat. No. 3,755,682; Grey U.S. Pat. 
No. 3,791,742; Schwebel U.S. Pat. No. 4,074,131; Farrand U.S. Pat. No. 
3,202,948; Farrand U.S. Pat. No. 3,064,218; Farrand U.S. Pat. No. 
3,090,934; Foster U.S. Pat. No. 2,924,978; Foster U.S. Pat. No. 2,915,722; 
Childs U.S. Pat. No. 2,867,783 and Tripp U.S. Pat. No. 2,799,835. 
While all of the measuring systems disclosed in the numerous patents listed 
above are presumably operative for purposes of measuring travel of one 
object relative to another, there is a problem in their practical 
application to various machines such as, for example, the inspection 
machines discussed above. All machines of this nature contain inaccuracies 
either in the scales or in the machine geometry itself thereby leading to 
inaccuracies in their measurements. Since these machines all contain such 
errors it is desirable to be able to compensate for or adjust that error 
out of the machine within reasonable tolerances. 
The prior art in general is not capable of achieving this compensation and, 
therefore, the extreme accuracy often required in machinery of this type 
cannot be readily obtained. The normal arrangement in the first system 
described above between conductors on the scale and conductors on the 
slider is that they both are normally 90.degree. to the longitudinal 
dimension of the scale or, in other words, to the axis of travel of the 
object. In the second or Moire Fringe system, the lines on the reader are 
disposed at an angle different from 90.degree. while the scale lines are 
usually 90.degree. to the path of movement. However, while these 
arrangements produce reasonable accuracy, it does not make it possible to 
compensate for variations or errors in the machine or scale itself. 
BRIEF DESCRIPTION OF THE INVENTION 
It has been discovered, therefore, that if the lines or conductors on both 
the scale and the reader or slider are slanted with respect to the path of 
movement of the scale, that it is possible to operate the optical or 
electrical measuring systems conventionally but it is also possible to 
compensate for error in the scale or machine geometry. 
It has been found that it is advantageous to slant both sets of lines in 
the Moire Fringe system rather than having one slanted and one 90.degree. 
to the path of travel as is conventional. The advantage achieved is that 
by moving the scale sideways or transversely to the path of travel the 
actual effective length of the intersecting point of the angular lines on 
the scale and the angular lines on the slider will be altered. 
The same principle is applicable when conductors are used and the 
conductors are disposed at an angle with respect to the path of the 
movement. This results in a change in the "count" or in the ultimate 
reading because, effectively, movement of the scale laterally results in 
linear movement of the conductors. In other words, it has been discovered 
that the count achieved in a given length of travel can be increased or 
decreased by the lateral movement of the scale. 
It has been discovered that this can be simply and readily accomplished by 
a relatively simple releasable clamping apparatus making positive lateral 
adjustment of the scale possible so that the method of producing improved 
accuracy in the ultimate reading and measurement of the travel of the 
movable object can be easily and quickly accomplished. 
Accordingly, production of an improved method and apparatus for improving 
the accuracy of optically or electrically activated position measuring 
apparatus becomes the principal object of this invention with other 
objects thereof becoming more apparent upon a reading of the following 
brief specification considered and interpreted in view of the accompanying 
drawings.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before referring to the drawings it should be noted that Tripp U.S. Pat. 
No. 2,799,835 clearly discloses the concept of using the position 
measuring transformers and conductors referred to above. 
Similarly the general concept of the Moire Fringe phenomenon is commonly 
known and is also discussed in Giambiasi U.S. Pat. No. 1,415,627. 
Therefore, no great detail with regard to the phenomenon itself will be 
presented herein. 
It should be noted that while this application will often refer to "lines" 
the principles disclosed herein are equally applicable to "conductors". 
Additionally, the various electronic or optical apparatus necessary for 
achieving the utilization of this phenomenon electrically or optically is 
well set forth in the various patents referred to earlier. Therefore, the 
drawings and the description of the drawings in this particular 
application with be in general terms only with it being understood that 
the electronic apparatus necessary to carry out the method of this 
application and adapt the apparatus of this application is well known to 
one with ordinary skill in this art having reference to United States 
patents just referred to. 
Turning then to the drawings, and particularly FIGS. 1 through 3 which 
essentially describe the electrical inductive system referred to in the 
introductory portion of this application, it will be noted that FIG. 1 is 
a schematic view showing the scale 10 and the slider 20. The scale 10 has 
a series of lines 11,11 intended to represent the conductors of this 
system of measurement while slider 20 has a series of broken lines 21,21 
intended to schematically illustrate the conductors of the slider. In FIG. 
1 these are disposed in their usual position or, in other words, at right 
angles to the path of movement designated by the arrow 30. 
FIG. 2 shows how the conductors 11,11 of the scale 10 and 21,21 of slider 
20 have been disposed at an angle with regard to the path of movement 30. 
This figure also shows how movement along the path of machine travel 30 
can be measured as dimension A. 
FIG. 3 shows how, when the scale 10 has been adjusted transversely of the 
path of movement 30 in the direction of the arrow 50 a different reading 
is achieved as designated by dimension B. This type of adjustment makes it 
possible to compensate for errors in the scale or machine geometry and 
will affect the reading engendered by movement of the slider 20 relatively 
of the scale 10 to thereby produce an accurate reading. In this way the 
linear count for a given linear movement is altered by the lateral 
adjustment. The conductors are shown as parallel in FIGS. 2-3 but the 
conductors of the slider may be positioned at an angle different from the 
scale as shown in FIGS. 4-5. 
Turning next then to FIGS. 4 and 5 of the drawings which relates to the 
Moire Fringe system, it will be noted that the scale, shown schematically 
and generally indicated by the numeral 110, has a plurality of parallel 
lines 111,111 which are disposed at 90.degree. with respect to the path of 
movement or travel 130 of the machine. In practice, a common arrangement 
is 1000 lines per inch although this can be varied and is not critical to 
the principles of this invention. 
The reader 120, which overlies the scale, also contains a pattern of 
parallel lines 121,121 which are disposed at an angle with respect to the 
path of movement 130. This is the normal Moire Fringe arrangement and 
movement in the direction 130 gives the linear measurement C. 
It should also be noted here that for purposes of contrast the lines 
121,121 have been shown as broken lines for purposes of illustration only. 
They would, of course, in practice, be solid lines just as the lines 
111,111. 
FIG. 4A shows a modified Moire Fringe arrangement wherein both scale and 
reader lines are disposed at an angle other than 90.degree. to the path of 
movement 130. 
FIG. 5A of the drawings shows the reader lines and the scale lines after 
the scale has been moved in the direction of the arrow 130. At this point 
the "shadow lines" of the Moire Fringe effect appear at the intersection 
of lines 121,121 and 111,111 with this point of intersection being 
indicated by the numeral 140 in FIG. 5. This produces the "count" and is 
translatable into an actual measurement of the distance of linear travel 
indicated by the letter D. 
FIG. 6 is similar to FIGS. 4 and 5 and FIGS. 4A and 5A with FIG. 6 again 
showing the scale and the reader in their starting position and with both 
scale and reader lines at an angle other than 90.degree. to the path of 
machine movement 130. 
FIG. 7, however, shows that the scale 110 has been moved laterally or 
transversely to the direction of the arrow 130 in the direction of arrow 
150 so that the new count is indicated by the letter E. Once that has been 
accomplished and adjustment has been made by the apparatus which will be 
described below, the scale and the machine is then moved in the direction 
of the arrow 130 again causing the shadow lines to appear at the 
intersection of the scale and slider or reader lines. Again this is 
indicated by the numeral 140, but at this time the count will have been 
changed due to the movement of the scale in the direction of the arrow 150 
so that the new count is indicated by the letter E. The count can either 
be increased or decreased depending upon the direction of scale 
adjustment. 
This permits geometry errors in the machine or the scale itself to be 
compensated for and insures reasonably accurate measurement and operation 
of the machine. 
By way of example, with 1000 lines per inch and the scale lines slanted 
10.degree. a lateral movement of 0.010 inches results in a count change of 
0.00176 and a movement of 0.002 inches results in a count change of 
0.00035. The amount of movement would naturally depend on the machine 
geometry error involved and it is believed that the scale can be adjusted 
to read within 0.0001 inches measured in 3 inch intervals so that the 
total error over 50 inches of travel would still be within 0.0001 inches. 
By way of further example, if there were a scale error of 0.0005 inches 
over 50 inches of travel and a machine error of 0.0005 inches the result 
could be an error of 0.001 inches. With the adjustment possible with this 
invention that error would become 0.0001 inches. 
Referring to FIGS. 8 and 9, for example, one apparatus for accomplishing 
this method with the inductive system is illustrated. 
In that illustration the scale 10 is illustrated as being a flat elongate 
member having a plurality of parallel lines 11,11 on its surface. These 
lines schematically represent the conductors. 
The reader 20 has lines 21,21 which also schematically represent 
conductors. 
The scale 10 is held in place on the machine 70 by means of the adjustment 
clips 60,60 which are secured to the machine by means of the screws 61,61. 
In this particular embodiment the clips 60,60 are L-shaped in cross section 
with one leg received in recesses 71,71 of the machine itself. These clips 
also have tapered surfaces 62,62 and the scale has complemental tapered 
edge surfaces 12,12 so that by loosening one clip and tightening the other 
the scale can be cammed in the direction of the arrow 50 to accomplish the 
adjustment step. 
Turning next then to FIGS. 10 and 11, a Moire Fringe system modified in 
accordance with the teachings is illustrated with lines 111 and 121 
representing the usual lines on scale 110 and slider 120. 
The scale 110 is held in place on the movable part of the machine 170 by 
adjustment clips 160,160 which overlie the transverse edges of the scale 
110 and are secured adjustably and releasably to the machine 170 by means 
of screws 161,161. The reader 120 is superimposed over the scale and it 
will be readily apparent that by simply loosening the screws 161,161 the 
scale can be moved laterally or transversely to the line of travel 130 in 
the direction of the arrow 150. The screws can then be retightened and the 
compensating adjustment by the machine will have taken place. Following 
this, of course, normal movement and operation of the machine in the 
direction of the arrow 130 will provide the corrected "count" of FIG. 7 
and ultimately the corrected reading. 
It will be noted that FIGS. 8 and 9 show one mechanical arrangement for 
controlling the transverse adjustment while FIGS. 10 and 11 show another. 
These can be used interchangeably on the two systems. 
While a full and complete description of the invention has been set forth 
in accordance with the dictates of the Patent Statutes, it should be 
understood that modifications can be resorted to without departing from 
the spirit hereof or the scope of the appended claims. 
Thus the measuring apparatus is not intended to be limited to inspection 
machines, which have been referred to herein by way of example, but could 
be employed in any linear system where the extent of movement of an object 
relatively of a reference structure is to be measured.