Extensometer
United States Patent 1 1 Randolph, Jr.  
 EXTENSOMETER George J. J. Randolph, Jr., 1505 177th Ave. NE, Bellevue. Wash. 98008 Filed: Sept. 24, 1973 Appl. No.: 400,186  
 lnventor:  
 1.1.8. Cl. 73/885 R; 338/6 Int. Cl. G0lb 7/16 Field of Search 73/885 R, 141 A;  
  References Cited UNITED STATES PATENTS 3.082.621 3/1963 Soderholm 73/88 R 3,599,479 5/1969 Kutsay 73/885 R FORElGN PATENTS OR APPLICATIONS 274,.454 5/1971 U.S.S.R 338/6 .lll  
 [4 1 Apr. 22, 1975 220,590 4/1969 U.S.S.R 73/141 A Primary E.\&#39;aminerJames .l. Gill Attorney, Agent, or Firmvl(olisch, Hartwell, Dickinson &amp; Stuart ABSTRACT 4 Claims, 7 Drawing Figures EXTENSOMETER BACKGROUNDAND SUMMARY OF THE INVENTION This invention pertains to an extensometer, and more particularly, to such a device characterized by high degrees of both &#39;precision performance and ruggedness.  
  An extensometer is a device which is used for monitoring displacements, such as dimensional changes which occur in some body or structure of interest. There are numerous applications where not only is there required a relatively high degree of precision in the information obtained from an extensometer, but there is also required in the extensometer sufficient ruggedness to withstand adverse handling and/or environmental conditions during installation and use. Both of these features are, of course, usually desired in a device which costs as little as possible, and which can easily be installed and operated by relatively unskilled personnel.  
  A general object of the present invention is to provide a practical, easily installed, easily used extensometer which offers all of these advantages.  
  According to a preferred embodiment of the invention, the proposed extensometer comprises an elongated, generally rectangular bar made of a suitable sturdy material, such as steel. The medial portion of the bar is provided with a pair of elongated, longitudinally spaced (e.g., spaced along the length of the bar), transversely overlapping slots which extend into the bar from opposite sides thereof. These slots produce, in a medial portion of the bar, a flexure portion having a relatively low longitudinal (relative to the bars longitudinal axis) spring rate, and result in a structure where the opposite end portions of the bar (with axial loading thereof) an move longitudinally relative to one another, with inherent and accompanying shear-bending occurring in the medial portion of the bar.  
  Also provided in the medial portion of the bar, between and generally parallel with the slots, is an elongated bore in which is mounted an elongated electromechanical deflection sensor. In particular, a sensoris used which responds to bending along its length. The opposite ends of this sensor are anchored in the bore at points therein located adjacent the bases of the slots. With such an arrangement, the ends of the sensor may be thought of as being essentially coupled to the opposite end portions of the bar, whereby with longitudinal relative movement between such end portions, the sensor deflects proportionally.  
  In such an arrangement the proposed bar is, of course, quite rugged and can withstand fairly abusive handling. The deflection sensor, on the other hand, may be a relatively delicate precision instrument which, when mounted in the bore mentioned, is protected against abuse by the surrounding material in the bar. Slotting of the bar as outlined enables ready telegraphing through the bar of bending into the sensor, whereby the precision performance characteristics of the sensor may be fully utilized.  
  Another important advantage attending the construction outlined is that the completed .rxtensometer can easily be mounted in place for use through bolting, welding, or otherwise anchoring the opposite end portions of the bar to whatever external body is to be monitored. Obviously, such attaching can be accomplished by persons having relatively low skill levels. Further, and as will become evident from the description below, very littleprecision is required in the way or position in which the extensometer is mounted for use, and thus it can be installed relatively quickly and easily.  
  These and other objects and advantages attained by the invention will become more fully apparent as the description which follows is read in conjunction with the accompanying drawings.  
 DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation of an installation incorporating an extensometer which is constructed in accordance with the present invention;  
  FIGS. 2 and 3 are views taken generally along the lines 22, 33, respectively, in FIG. lFIG. 2 being on about the same scale as FIG. 1, and FIG. 3 being on a much larger scale;  
  FIG. 4 is a cross-sectional view taken generally along the line 4-4 in FIG. 3, and on a larger scale than FIG.  
  FIG. 5 is a view similar to and on about the same scale as FIG. 3, but showing a modified form of an extensometer;  
  FIG. 6 is a circuit diagram showing how electrical deflection-sensitive elements included in the extensometer of FIGS. l4 may be connected for use; and  
  FIG. 7 is similar to FIG. 6, but relates to the modified extensometer shown in FIG. 5.  
 DETAILED DESCRIPTION OF THE INVENTION Turning now to the drawings, and referring first to FIGS. 1 and 2, indicated generally at 10 is an extensometer constructed in accordance with the present invention. Extensometer 10 is intended to monitor displacements between a pair of spacedapart points, and in FIGS. 1 and 2, is shown attached for use on a support leg, partially shown at 12, of a water storage tank (not illustrated). In this installation, the extensometer monitors extensions and contractions of leg 12 which result from changes in the amount of water stored in the associated tank. As will be explained, the extensometer produces an electrical signal which is directly related to such extensions and contractions, and which can be interpreted to give a direct indication, for example, of the quantity of stored water.  
  It should be kept in mind, that while a particular type of installation is disclosed herein for extensometer 10, this is for illustration purposes only. In other words, the extensometer of the invention has numerous applications which will be apparent to those skilled in the art.  
  Extensometer 10 includes an elongated, generally planar, rectilinear bar, or body member, 14 formed preferably of a suitable steel. The longitudinal axis of this bar is vertical in FIG. 1. The central portion of the bar contains a pair of longitudinally spaced, transversely overlapping, substantially parallel elongated slots 16, 18 which extend inwardly from opposite sides of the bar. With such construction, the bar may be thought of as containing three portions, including a pair of end portions 14a, 14b joined through a medial portion 140. End portions 14a, 14b are also referred to herein as interface portions, and medial portion is also called a flexure portion. The end-portions in the bar each have about the same longitudinal spring rate e.g., a spring rate measured parallel to the bars longitudinal axis. In other words, each will contract or extend by about the same amount with the same compressive or tensive force carried in the end portion. However, because of the presence of slots l6, 18, the medial por tion in the bar exhibits a lower longitudinal spring rate than that of the end portions. This spring rate is also one herein that is measured parallel to the bars longitudinal axis. In particular, the medial portion permits appreciable relative longitudinal movement to occur between the end portions with axial loading of the bar. Compression in the bar, of course, causes the end portions to move toward one another, whereas tension causes the reverse.  
  It is considered herein that the term longitudinal axis&#34; as applied to portions 14a, 14b, 140 in bar 14 refers to an axis coinciding with the bars longitudinal axis. This is simply a definitional device believed to be useful for explanatory purposes.  
  In the installation of FIGS. 1 and 2, bar 14 is anchored to leg 12 through bars 20, 22 which are welded to the opposite ends of bar 14 and also to vertically spaced points on leg 12. As can be seen in FIG. 1, bar 14 is disposed with its longitudinal axis substantially paralleling that of leg 12.  
  Considering FIGS. 3 and 4 along with FIG. 1, provided in the medial portion of bar 10, between and substantially parallel to slots l6, 18, is an elongated bore, or recess, 24. Mounted through suitable adhesive deposits 26 in this bore is an elongated electromechanical deflection-sensitive device, or sensor, 28. Sensor 28 includes an elongated bendable carrier 30 which includes a pair of end parts 30a that are joined through a central stem, including a portion 30b having an outer conical surface of revolution, a portion 300 having an outer cylindrical surface of revolution, and a portion 30d having a flattened, somewhat planar transverse cross section which lies in a vertical plane 32 which is the plane of bar 14. It is through end parts 30a that adhesive deposits 26 anchor the sensor in place.  
  Secured on opposite sides of the outside surface of carrier portion 30b are two elongated piezoresistive elements 34. These elements are held in place through ceramic bonding deposits 36, with the longitudinal axes of the elements lying substantially in plane 32. Electrical conductors (not shown) are connected to the opposite ends of each element 34, these conductors leading away from the extensometer in a cable 38 (see FIG. 1) which fits within an elongated groove 40 that extends as shownalong one side of end portion 14b in bar 14.  
  It should be understood that while a particular deflection sensor is illustrated and described herein, other styles of deflection sensors may be used. The particular sensor disclosed is one which has been found to be particularly suitable, since it exhibits a high degree of precision performance. For a further description of the construction and operation of a sensor like sensor 28, reference is made to my co-pending United States application for Letters, patent, entitled Dual-Mount Electromechanical Deflection Sensor,&#34; filed Sept. 24, 1973, Ser. No. 4O0,l84.  
  Explaining how extensometer l performs, with axial loading in bar14, end portions 14a, 14b move toward or away from one another, depending upon whether such loading results from compression or tension, respectively. Such action results in shear-bending in bar portion 140, and hence in bending of carrier 30 along its longitudinal axis in plane 32. Such bending, then, results in related deflections occurring in elements 34,  
 which deflections produce related electrical resistance changes in the resistances of these elements.  
  FIG. 6 illustrates atypical circuit which may be used outside the extensometer to sense such resistance changes. This circuit is, basically, a Wheatstone bridge circuit, one side of which includes elements 34 connected in series, and the other side of which includes a pair of external balancing resistors, such as resistors 42. The junctions between elements 34 and between resistors 42 are connected to terminals 44, 46, respectively, which terminals may be connected to a suitable meter, such as a DC voltmeter. The two junctions between elements 34 and resistors 42 in the circuit are connected to terminals 48, 50, which may be connected to a suitable source of DC voltage.  
  With extensions and contractions in leg 12 resulting from changes in the amount of water stored in the tank, proportional changes occur in the resistances of elements 34. These changes can easily be observed by a meter, such as that described above, connected to terminals 44, 46 and can readily be interpreted to give a direct indication of the amount of water stored at any given time in the tank. Also, it will be obvious that such resistance changes, and the associated meter readings generated, can be interpreted as direct indications of the exact displacements occurring in leg 12 between the two points of attachment between it and bar 14.  
  The sensitivity of extensometer 10 can be adjusted in several ways. For example, the position of sensor 28 may be changed in bore 24, both by shifting the sensor axially in the bore, as well as by rotating the sensor about its axis. The latter type of adjustment produces a more pronounced change in sensitivity than the former. Maximum sensitivity is obtained with sensor 28 oriented with elements 34 disposed in plane 32, and with the opposite ends of the sensor located essentially directly adjacent the bases of slots l6, 18. In FIGS. 1-4, the sensor is shown in a position to produce maximum sensitivity. Further, by mounting the extensometer with its longitudinal axis at some angle (other than a right angle) to the axis along which displacments are to be observed (such as the longitudinal axis of leg 12), sensitivity can be reduced.  
  It is believed evident, now, how the several important advantages discussed earlier are obtained in extensometer l0. Sensor 28 may be, and herein is, a relatively delicate precision instrument which, when mounted as described, is afforded ample protection by the surrounding material in bar 14. Bar 14, on the other hand, is extremely rugged, and can easily withstand rough handling. With the bar slotted as described, a sufficiently low longitudinal spring rate is obtainable in the center portion of the bar whereby adequately readable deflections can be telegraphed into sensor 28. The depth of slotting is, of course, a matter of choice. Generally speaking, the deeper or longer the slots, the lower the longitudinal spring rate produced in the medial portion.  
  It is obvious that the bar can easily be attached for use in any one of a number of relatively simple ways. Further, while there is some affect on sensitivity depending upon the orientation of the longitudinal axis of the extensometer, exact alignment is not required with the axis along which displacements are to be monitored. Thus, it is not necessary to spend much time p0- sitioning the extensometer prior to securing it in place.  
  There are some applications, of course, where it will be anticipated that an extensometer will be subjected not only purely to axial tension and compression, but also to longitudinal bending in its plane. Where such is likely to occur, and it is desired to be able to distinguish between these respective actions (since both will affect the piezoresistive elements), a modified extensometer, such as that shown generally at 52 in FIG. 5, may be used. This extensometer includes an elongated bar 53 which, in side view, looks the same as previously described bar 14. Bar 53 differs from bar 14 in that it is thick enough in its plane to accommodate, in the portion thereof disposed between the central slots, a pair of side-by-side disposed bores, such as bores 54, 56. Bores 54, 56, like previously mentioned bore 24, essentially parallel the slots.  
 Provided in bores 54, 56 are deflection sensors 58,  
 60, respectively, which, herein, are identical in construction to previously described sensor 28. Sensors 58, 60 are adhesively anchored in bores 54, 56, with the opposite ends of the sensors adjacent the bases of the slots in bar 53. However, and as can be seen in FIG. 5, sensors 58, 60 occupy reversed positions in the bores. More specifically, the piezoresistive elements 59 in sensor 58 are disposed toward the right end of bore 54 in FIG. 5, whereas the piezoresistive elements 61 in sensor 60 are disposed toward the left end of bore 56in the figure. Offset grooves 62, 64 are provided adjacent an opposite set of ends of bores 54, 56,. respectively, to receive cables like previously mentioned cable 38.  
  With an extensometer constructed as shown in FIG. 5, the piezoresistive elements therein may be connected, essentially, in two different waysin one of which the extensometer will respond primarily solely to axial loading, and in the other of which it will respond principally to longitudinal bending in the plane 66 of bar 53.  
  The first-mentioned kind of connection is shown in the circuit of FIG. 7. This connection recognizes that with axial loading in bar 53, one of elements 59 and one of elements 61 will each experience axial tension (along its own longitudinal axis), while the other two elements will each experience axial compression. In particular, and with axial compression in bar 53, it will be the upper elements 59, 61 (the ones visible in FIG. 5) which will experience axial tension, with the lower elements experiencing axial compression. Axial tension in bar 53 produces the reverse situation.  
  To make extensometer 52 sensitive, essentially, only to axial loading, the two sets of elements 59, 61 which simultaneously experience an axial force in the same direction are connected in parallel, with these two parallel sets connected in series. FIG. 7 shows this arrangement, with the subscripts t&#34; and c used to indicate which elements are in tension and compression, respectively, under the situation of axial compression in bar 53. With axial tension in bar 53, it would be the upper elements 59, 61 in FIG. 7 which would have the subscript c, and the lower elements which would have the subscript t. This series arrangement is connected in parallel with series-connected balancing resistors 68, 70. Terminals 72, 74, 76, 78 are provided in the circuit of FIG. 7, which terminals correspond to terminals 44, 46, 48, 50, respectively, in FIG. 6.  
  If it were desired, instead, to make extensometer 52 sensitive primarily to bending in plane 66, this can be done simply by reversing either the electrical positions of elements 59,, 59, in FIG. 6, or the electrical positions of elements 61,, 61,. in the figure.  
  While a preferred embodiment, and a modification, of the invention have been described herein, it is appreciated that other variations and modifications may be made without departing from the spirit of the invention.  
 It is claimed and desired to secure by letters patent:  
 1. An extensometer comprising an elongated bar including, intermediate its ends, a  
 pair of longitudinally spaced, transversely overlapping slots which extend into the bar from opposite sides thereof,  
 means located adjacent the ends of said bar accommodating attachment thereto at spaced-apart points to an external body,  
 means in said bar between said slots defining an elongated recess having one end adjacent the base of one of the slots and its opposite end adjacent the base of the other slot, and  
 an elongated electromechanical deflection-sensitive device mounted in said recess with one of its ends anchored therein adjacent one end of the recess and its other end anchored therein adjacent the other end of the recess.  
  2. In an extensometer, an elongated unitary body member comprising a pair of spaced-apart interface portions adapted for attachment to an external body and each characterized by a relatively high longitudinal spring rate, such being measured along a line paralleling the body members longitudinal axis, and flexure portion joining said interface portions defined by at least a pair of transversely overlapping slots which are spaced along said axis and which extend inwardly from opposite sides of said member and including, between said slots, a recess for receiving a deflection-sensitive device, said flexure portion having a lower longitudinal spring rate, also measured parallel to said axis, than that of said interface portions, said flexure portion accommodating, with shear stressing therein, longitudinal relative movement between said interface portions with accompanying shear-bending in the flexure portion, and said recess accommodating coupling of an electromechanical deflection-sensitive device to said interface portions through a pair of spaced-apart points in the recess located adjacent the bases of said slots in such a manner that the device will be deflected by an amount proportional to the relative movement along the body members longitudinal axis which occurs between said interface portions.  
 3. An elongated extensometer comprising a pair of spaced-apart interface portions adapted for attachment to an external body, each characterized by a relatively high spring rate as measured along the extensometers longitudinal axis,  
 a flexure portion integral with and disposed between said interface portions defined by at least a pair of transversely overlapping slots which are spaced along said axis and which extend inwardly from opposite sides of said body and including between said slots a recess for receiving a deflectionsensitive device, said flexure portion having a lower spring rate measured along said axis than that of said interface portions,  
 said flexure portion accommod-ating,with shearstressing therein, longitudinal relative movement 7 between said interface portions with accompanying shear-bending in the flexure portion and an electromechanical deflection-sensitive device mounted in said recess in the flexure portion and coupled to said interface portions through spacedapart points in said recess which-are adjacent the bases of said slots so as to be proportionally deflected by such relative movement between said interface portions.  
 4. An elongated extensometer comprising a pair of spaced-apart interface portions adapted for attachment to an external body, each characterized by a relatively high spring rate as measured along the extensometers longitudinal axis,  
 &#39; a flexure portion integral with and disposed between said interface portions defined by at least a pair of a transversely overlapping slots which are spaced along said axis and which extend inwardly from opposite sides of said body and including between said slots a recess for receiving a deflectionsensitive device, said flexure portion having a lower spring rate measured along said axis than that of said interface portions, 1  
 said flexure portion accommodating, with shearstressing therein, longitudinal relative movement between said interface portions with accompanying shear-blending in the flexure portion and an electromechanical deflection-sensitive device mounted in said recess in the flexure portion and coupled to said interface portions through spacedapart points in said recess so as to be proportionally deflected by such relative movement between said interface portions.