DUAL MEASUREMENT DISPLACEMENT SENSING TECHNIQUE

Abstract

A method for determining a length of a span of electrically conductive material, comprising a first voltage measurement across the entire span, and a second voltage measurement across a constant-length segment of the span. The dual measurements allow the calculation of the span length in a manner that is robust to many disturbances including ambient temperature, material temperature, and material stress and fatigue.

Claims

1. A method for estimating a length of an electrically conductive material, comprising the steps of: a. providing a span of electrically conductive material, b. applying a voltage across the entire said span of electrically conductive material from a base of said span to an opposing base thereof, c. determining a first voltage which is equal to the applied voltage across said span of electrically conductive material, said first voltage determined by use of a voltage sensor or by predetermined knowledge of said applied voltage; and d. measuring a second voltage at a fixed probe location within said span of conductive material, said fixed probe location being a predetermined distance from the base of said span of conductive material, whereby the length of said span of electrically conductive material is determined by computing the ratio of said first voltage to said second voltage and scaling that ratio by the distance from said base to said fixed probe.

2. The method of claim 1, wherein said span of electrically conductive material is comprised of a shape memory alloy.

3. The method of claim 1, wherein said applied voltage is varied to heat said electrically conductive material or to allow it to cool, causing it to contract or extend.

4. The method of claim 1, further comprising using said length computation in a feedback control system.

5. A method for estimating a length of an electrically conductive material, comprising the steps of: a. providing a span of electrically conductive material, b. applying a predetermined first voltage across said span of electrically conductive material from a first base of said span to an opposing second base thereof, and c. measuring a second voltage at a selected probe location within said span of electrically conductive material, said selected probe location being a predetermined distance from the base of said span of conductive material, whereby the length of said span of conductive material is determined by computing the ratio of said first voltage to said second voltage and scaling that ratio by the distance of the fixed probe location from the base.

6. An apparatus for estimating a length of an electrically conductive material: a span of electrically conductive material supported between a first base and an opposing second base thereof, a source of electrical voltage attached at said based of said span for applying a predetermined first voltage to said span; and a voltage probe having a sensor for measuring voltage and movable to a selected probe location within said span, said selected probe location being a predetermined distance from the base of said span of conductive material, for determining a second voltage, whereby the length of said span of conductive material is determined by computing the ratio of said first voltage to said second voltage and scaling that ratio by the distance of the fixed probe location from the first base.

7. The apparatus of claim 6, wherein said span of electrically conductive material is a shape memory alloy.

8. The apparatus of claim 6, further comprising means for varying said applied first voltage over a predetermined period of time, wherein said electrically conductive material is heated and cooled, causing said electrically conductive material to contract or extend.

9. The apparatus of claim 6, further comprising a feedback control system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGS. 1A and 1B illustrate a schematic of the measuring technique according to the present invention applied to a length of an electrically conductive material.

[0017] FIGS. 2A-2D illustrate the results of an experiment validating the performance of the method of the present invention when applied to shape memory alloy wire.

[0018] FIG. 3 illustrates an embodiment of the present invention applied with a single shape memory alloy wire used as an actuator on a printed circuit board.

[0019] FIG. 4 illustrates the application of the present invention with a shape memory alloy in a feedback control system.

DRAWINGS—REFERENCE NUMERALS

[0020] 1. span of conductive material [0021] 2. applied voltage [0022] 3. first voltage [0023] 4. second voltage [0024] 5. fixed probe location [0025] 6. printed circuit board [0026] 7. first electrical connection [0027] 8. coil-spring [0028] 9. pivoting beam [0029] 10. second electrical connection [0030] 11. third electrical connection [0031] 12. adjustable voltage supply [0032] 13. feedback controller [0033] 14. reference position

DETAILED DESCRIPTION—EMBODIMENT 1—FIGS. 1-2

[0034] In reference to the drawings, in which like parts have like reference numerals, FIG. 1A illustrates a span of electrically conductive material 1 attached at respective opposing first base and second base, with an applied voltage 2 across the span from a source denoted V.sup.+ to sink at electrical ground denoted GND. The end of the material near the source is free to move, while the end near ground is fixed. A first voltage 3 is determined by measurement with a voltage sensor connected to the end of the span of material 1 such that it moves with the end of the span of material 1, or alternatively by a predetermined knowledge of the magnitude of applied voltage 2. A second voltage 4 is measured at fixed probe location 5 within the span of the conductive material 1.

[0035] Looking now to FIG. 1B, the mathematical terms used to calculate the total wire length are denoted. The span of material 1 has total length L, which may vary with strain. A measurement of the input voltage 2, (denoted V.sub.2), is made at the source voltage level (denoted V.sup.+). A second measurement of voltage V.sub.0 is made at the bottom of the span of material 1 which is generally the sink ground voltage level (denoted GND). Along the span of the material, a sliding-contact probe is positioned a fixed distance 5 (denoted l.sub.s) from the material 1 end at a selected probe location. At the fixed probe location 5, a second voltage measurement 4 (denoted V.sub.1) is made. The voltage difference (V.sub.2−GND) will vary in a complex manner as the span of material 1 extends and contracts, heats and cools, fatigues, and is tensioned. The physical distance between the points 3 and 4 changes as length L varies. The voltage difference (V.sub.2−GND) will also change, however the distance between GND) and 4 will remain a constant distance l.sub.s.

[0036] Looking now to FIGS. 2A-2D, the results of an experiment using the present invention are shown. The span of material in this case is a shape memory alloy wire which contracts when electrically heated, and expands when cooled in ambient air. FIG. 2A shows the electrical input to the wire. The span of material is electrically heated by varying the electrical current in a saw-tooth manner, with period of 40 seconds. This input is repeated three times. Because the material is a shape memory alloy, this varying current heats the wire and causes it to contract and extend. FIG. 2B shows the tensile stress applied to the material which also causes it to strain. During each of the three electrical repetitions, the tensile load applied to the wire is regulated to one of three different magnitudes. FIG. 2C shows the electrical resistance of the entire wire. It is seen in FIG. 2C that electrical resistance alone is not a robust sensor of position. The relationship between electrical resistance and position is complicated by changing wire temperature, load, hysteresis, and ambient conditions. FIG. 2D shows the wire length predicted using the method of the present invention. It is seen that none of temperature, load, hysteresis, nor ambient conditions adversely affect the estimate of length using the present invention.

Operation

[0037] Consider that the electrical resistivity of the material is constant along the entire length so long as the entire length is subject to similar electrical current, ambient temperature, and stress—all of which are reasonable assumptions for a wire, cable, or ribbon. Then, the relationship between the voltage measurements (3,4), the total length L of the span of material 1, and the fixed probe location 5 whose distance from GND is denoted l.sub.s, is:

[00001]$L=l_s*\frac{V_2-\mathrm{GND}}{V_1-\mathrm{GND}}$

[0038] If measurement at V.sub.0 is taken at electrical ground, the relationship between material length and the measurements can be simplified, when V.sub.0=0, then

[00002]$L=l_s*\frac{V_2}{V_1}$

[0039] In many cases, the supply voltage V.sub.2=V.sup.+ is known without direct measurement, such as when a battery is connected to the material. In such cases, it is apparent that the calculation can be simplified farther; when V.sub.2=V.sup.+ then

[00003]$L=l_s*\frac{V^{+}}{V_1}$

[0040] It is practical that the sliding contact at midpoint 4 be formed from a spring-loaded probe, at a bend in the material, or in other practical manners that ensure continual contact with the material in all heating and loading conditions.

[0041] It should be noted that this technique is most accurate when the electrical noise is small. Noise is also reduced when V.sub.1 is as large as possible with respect to V.sub.2— thus in the application of this technique l.sub.s should be as large as possible with respect to L. The invention assumes the state of the wire (its temperature, stress, strain, and crystal state) is the same in the constant-length segment as it is in the entire span of the material—this assumption is also most valid when the measurement l.sub.s is as large as possible with respect to L

DETAILED DESCRIPTION—EMBODIMENT 2—FIG. 3

[0042] Looking now to FIG. 3, an alternate embodiment of the present invention is shown, sensing the position of a span of material 1 which is formed from shape memory, and which actuates a rotary actuator on a printed circuit board. A substrate printed circuit board 6 contains an electrical trace from source of applied voltage 2, to the first electrical-mechanical connection 7 where an electrically conductive coil-spring 8 is fastened. Coil-spring 8 is fastened to one end of an electrically conductive pivoting beam 9 which rotates about a central pin. At the opposite end of pivoting beam 9, the span of material 1 is fastened at connection point 10. The path from applied voltage 2 to second electrical-mechanical connection 10—passing through first electrical-mechanical connection 7, through conductive coil-spring 8, through conductive pivoting beam 9—creates an electrical path with minimal electrical resistance. Span of conductive material 1 extends from second electrical-mechanical connection 10 to third electrical-mechanical connection 11, touching the fixed probe location 5 at an intermediate location. Third electrical-mechanical connection 11 is intended to be electrical ground. Second voltage 4 is measured at the fixed probe location 5.

[0043] FIG. 4 illustrates an application of the present invention as used in a feedback control loop. A feedback controller 13 such as a computer accepts predetermined reference input 14 as well as first voltage 3 and second voltage 4. The controller provides a signal to adjustable voltage supply 12, which subsequently changes the electrical voltage across the span of material 1.

Operation

[0044] Thus, by applying known voltage 2 and grounding at third electrical-mechanical connection point 11, the span of material 1 will heat and contract. While the voltage is applied 2, the second voltage measurement 4 taken at fixed probe location 5 can be used to calculate the length of the conductive material 1, and subsequently to calculate the angle of pivoting beam 9. The calculated angle can then be used by the feedback controller 13 to compute the error between the true length and reference length 14 of the pivoting beam 9, and subsequently adjusts the voltage supply 12 which changes the applied voltage 2, changing the temperature of the material 1, causing it to change length, thereby reducing the error.

ADVANTAGES

[0045] The present invention allows sensing of position with minimal cost and complexity, and without complex models which can introduce errors. The technique is simple, only requiring knowledge of the applied voltage and one additional voltage measurement at a stationary point along the length of the material, and can easily be incorporated in practical devices. By making the dual measurements, the length of the material can be calculated directly and accurately, requiring no assumptions about ambient conditions or loads except that they are constant along the entire span length.

[0046] The present invention has particular benefits for shape memory alloy materials. It is not affected by hysteresis or by twinning or R-phase transformation or effects that may cause other methods to fail. This technique can be employed for an SMA material used as a sensor alone, used as an actuator and a sensor, or used in a feedback control system. A feedback control system can use the sensed position to control electrical current into the SMA material and drive it to a desired position or otherwise control the state of the actuator.

[0047] The invention accordingly provides an apparatus and a method for length sensing of a material using few electrical measurements. First, a voltage is applied across the length of material, the source attached in such a way to ensure the voltage spans the entire length of material even if it stretches or strains. Often this voltage is controlled or known or measured, and is considered the first measurement of voltage. A second measurement of voltage is then made across a known fixed-length section of the material within the span of the applied voltage. By multiplying the ratio of first voltage to second voltage by the known fixed length span, the entire length is calculated.

[0048] The present invention accordingly provides the apparatus and method for reliable and robust length sensing of a material in various embodiments. The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification with regard to illustrative, non-limiting embodiments. The invention accordingly is not to be construed as limited to the particular forms disclosed as these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention described in the following claims.