[0002] CROSS REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of U.S. provisional application

Nos. 61/357,753, filed June 23, 2010 and 61/359,096 filed June 28, 2010, the contents of which are hereby incorporated by reference herein.

[0004] FIELD OF INVENTION

[0005] The present disclosure relates to strain gages and more particularly, to strain gages including shunts for calibration.

[0006] BACKGROUND

[0007] Increasing a device's resistance value using shunts is shown in US

Patent No. 4,172,249 where a resistive foil material is deposited by cemented on a rigid substance, usually a ceramic. A photoetched pattern of resistive lines is formed with shunts so that the total resistance can be increased by opening one or more of the shunts until the desired resistance value is obtained. Mechanical or thermal strains will produce resistance changes rendering the resistor unstable. Therefore, the resistor must be insulated from mechanical or thermal strain so as to maintain the resistance stable. To accomplish this, the resistor is enrobed in a flexible rubber, and then it is encapsulated by an epoxy resin. Any outside force (mechanical or thermal) does not deform the resistor because of its rubber and epoxy protections. Sometimes, to insulate the resistor from outside forces, the resistor is put into a metal case with oil surrounding the resistor. Thus, the resistor is meant to be stable and its resistance should not respond to or be altered by outsides forces.

[0008] A strain gage is a device used to measure the strain of an object subject to a force. The most common type of strain gage consists of a very thin insulating backing which supports a very thin resistive grid. A typical strain gage looks like a postage stamp. When the strain gage is bonded to a structure (e.g., a wing of an airplane) and the structure is deformed the resistance of the strain gage changes its Ohmic value as a function of the strain due to deformation. Thus, a strain gage differs from the known resistive device in that the application of force to the surface where the strain gage is mounted results in a change to the resistive value of the strain gage.

[0009] When manufactured through photoetching of a resistive grid, strain gages have an initial or first resistance value that is set below a desired target resistance value. The final adjustment or calibration of the resistive grid to the target resistance value (e.g., 120 or 350 Ohms) is done by abrading or etching the grid which reduces the thickness of the strain gage. The reduction of the thickness increases the resistance until the value arrives at the desired target resistance value. Adjustment by etching of the resistive surface removes the oxide and during re-oxidation the strain gage changes value with time. Rubbing of the resistive surface in addition to removing the oxide, produces cold working of the metal, hence changing the temperate coefficient of resistance (TCR) and introduces instability, due to changes in resistance value with time. There is, therefore, a need for structures and methods for adjusting strain gage resistance without using rubbing or etching techniques.

[0010] SUMMARY

[0011] A resistive grid is formed on an insulating substrate. A plurality of shunts are associated with the resistive grid, each of the shunts include one or more shunting bars. Severing a shunting bar alters (e.g., increases) the resistance of the resistive grid. Severing of one or more shunting bars can be performed by: chemical etching, electro-chemical etching, a laser beam, metal cutting and particle blasting. Shunting bars can be located on at least one edge of the resistive grid or in a defined internal area within the resistive grid.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings: [0014] Figure 1 shows a strain gage with shunts disposed on multiple edges of a resistive grid;

[0015] Figure 2 shows the strain gage with trimmed shunts;

[0016] Figures 3a and 3b show a portion of a resistive grid with internal shunts formed in a protrusion formed in the resistive grid; and

[0017] Figure 4 shows a portion of a resistive grid with internal shunts formed in the internal area of the resistive grid.

[0018] DETAILED DESCRIPTION

[0019] Before explaining embodiments of the invention in detail, it is understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the host description or illustrated in the drawings.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. The methods and examples provided herein are illustrative only and not intended to be limiting.

[0021] This disclosure provides an innovative structure and method for calibrating a strain gage. Figure 1 shows an exemplary strain gage 100, having multiple shunts 130 providing a variety of trimming or severing options. Figure 2 shows the exemplary strain gage 100 including a number of shunts that have been trimmed to form trimmed shunts 132. The preferred strain gage 100 includes a flexible insulating substrate 110, a resistive grid 112 extending between terminal pads 140 and shunts 130. The resistive grid 112 can be formed with one or more legs 120 in a desired pattern. Shunts 130 include one or more shunting bars 134. The resistive grid has a number of edges 114, 115, 116 and 117. In this embodiment, the shunts 130 can be disposed on one or more edges of the resistive grid.

[0022] During calibration, the resistance of the resistive grid 112 is adjusted by changing the length of the grid to alter the resistance towards the target value. This is accomplished by severing one or more of the shunts 130 to create a discontinuity. This exposes additional portions of the resistive grid 112 to the current path between terminal pads 140. The resistance of the grid is checked after each shunt is severed until the resistance is in the desired range. Severed shunts are identified by reference number 132 in Figure 2. Terminal pads 140 are used for measuring the resistance of resistive grid 120 during the calibration procedure.

[0023] The severing of a shunt 130 can be performed by a variety of techniques including: chemical agents, electro-chemical agents, metal cutting, a laser beam, sand blasting or the like. One or more shunts 130 are opened until the target resistance is within a desired range ( ± a percentage range or ppm).

[0024] It should be noted that the operation of severing a shunt 130, to form a severed shunt 132 is a very quick operation that can take about 0.001 seconds, for example, when using a laser beam.

[0025] It is understood that other shunt configurations are possible. For example, Figures 3a, 3b and 4 show other embodiments where shunts are formed internally within the resistive grid rather than at the edges. Figures 3a, 3b and 4 illustrate a portion of the resistive grid 120a, 120b, 120c that generally correspond to the leg 120 shown in Figures 1 and 2. In these embodiments, the resistive grid is formed with one or more openings 132a, 132b, 132c or discontinuities in the resistive pattern that connect by shunting bars. The openings or voids 132a, 132b, 132c are shown as rectangular and circular respectively. It is understood that a wide variety of geometric shapes could be used without departing from the scope of this disclosure. Shunting bars 134a, 134b, 134c are disposed between and connect the voids 132a, 132b, 132c. It is understood that internal shunts can be formed in any portion of the resistive grid.

[0026] Figures 3a and 3b show a portion of a resistive grid 120a, 120b with internal shunt bars 134a, 134b formed in protrusions 136, 138 formed in the resistive grid. It is understood that the protrusions 136, 138 can be formed in a variety of shapes without departing from the scope of this disclosure. Figure 4 shows a portion of a resistive grid with internal shunt bars 134c formed wholly within a portion of the resistive grid. It is understood that shunt configurations shown in Figures 1-4 can be combined in a single device.

[0027] The structures disclosed herein have a variety of desirable attributes. The shunts as disclosed do not introduce instability or TCR change because the resistive element does not undergo additional etching or cold working. Resistance adjustment by etching of the resistive surface removes the oxide and during re-oxidation the gage changes value with time. The embodiments disclosed herein do not remove surface oxides. Furthermore, both etching and surface abrading is time consuming and very delicate, hence expensive and there is a danger to produce a reject: a strain gage that is open or that has a resistance value that has been adjusted too high. The embodiments disclosed herein address these problems as well.

[0028] Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The invention being thus described in terms of embodiments and examples, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the claims.