Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
  • G01B7/16—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge

Definitions

• the invention relates to thin film strain gages.
• Thin film strain sensors are particularly attractive in the gas turbine engine environment since they do not adversely effect the gas flow over the surface of a component and do not require adhesive or cements for bonding purposes.
• thin film strain gages are deposited directly onto the surface of a component by rf sputtering on other known thin film deposition technology and as a result are in direct communication with the surface being deformed.
• the piezo-resistive response or gage factor (g), of a strain gage is the finite resistance change of the sensing element when subjected to a strain and can result from (a) changes in dimension of the active strain element and/or (b) changes in the resistivity (p) of the active strain element.
• the active strain elements used in a high temperature static strain gage must exhibit a relatively low temperature co-efficient of resistance (TCR) and drift rate (DR) so that the thermally induced apparent strain is negligible compared to the actual mechanical applied strain.
• ITO indium-tin oxide
• Pt a metal, e.g. Pt as a thin film resistor placed in series with the active ITO strain element.
• the invention comprises a self-compensated strain gage sensor having an automatically determined TCR including a TCR of essentially zero.
• the sensor comprises a wide band semiconductor deposited on a substrate.
• a metal is deposited on the substrate and in electrical communication with the semi-conductor functioning as serial resistors, the length, width and thicknesses of the semi-conductor and the metal are selected based on their resistivities at selected working and reference temperatures and the TCR automatically determined.
• the semiconductors can be selected from the group consisting of silicon carbide, aluminum nitride, zinc oxide, gallium mtride, indium nitride, scandium nitride, titanium nitride, chromium mtride, zirconium nitirde, boron carbide, diamond, titanium carbide, tantalum carbide, zirconium carbide, gallium phosphide, aluminum gallium nitride, zinc oxide doped with alumina, cadmium telleride, cadmium selenide, cadmium sulfide, mercury cadium telleride, zinc selenide, zinc telleride, magnesium telleride, tin oxide, indium oxide, manganates-manganese oxides with iron oxides, iron oxide-zinc- chromium oxide, iron oxide-magnesium-chromium oxide, ruthenium oxide, lithium doped nickel oxide, tantalum nitride, indium-tin oxide-gall
• the metal resistors can be selected from the group consisting of platinum, rhodium, palladium, gold, chromium, rhenium, irridium, tungsten, molybdenum, nickel, cobalt, aluminum, copper, tantalum, alloys of platinum and rhodium and combinations thereof.
• a particularly preferred semi-conductor is indium tin oxide and a particularly preferred metal is platinum.
• Fig. 1 is an illustration of a sensor design
• Fig. 2 is a simulated circuit of the design of Fig. 1
• Fig. 3 is an illustration of an alternative sensor deisgn
• Fig. 4 is a graph of the resistance (signal) of the sensor of Fig. 1 changing with temperature.
• the TCR of a self-compensated strain sensor is first modeled.
• the following approach is used to model the TCR of an ITO sensor with Pt self-compensation circuitry:
• TCRCOMP (RcoMP,f- RCOMP.O)/ (RCOMP.O * AT) (1)
• RcoMP.f is compensated sensor resistance at a specific temperature
• R CO M P.O is compensated sensor resistance at a reference temperature
• AT is the temperature difference
• TCRCOMP ((Rpt,f + R ⁇ o,f) - (Rpt,o + RITO . O)) / ((Rpt,o + RITO . O) * AT) (4)
• the resistance R is related to resistivity (p) which is a constant at a specific temperature
• ⁇ pixo P ⁇ o,f - prro.o (8)
• a 1TO L ⁇ T0 / (wrro * trro) (1 ) ppt,f. PPt.o .
• PiTO . f. Prro.o are resistivities of Pt and ITO at a working and reference temperatures. In equation (6), all resistivities and AT are constants, ⁇ pp t > 0 and Aprro ⁇
• a different length (L), width (w) and thickness (t) of ITO and Pt can be designed to let the TCR of self- compensated ITO-Pt sensor film be zero.
• the TCR of self-compensated sensor can be related to TCR ofPt and lTO.
• TCRCOMP ((Rpt,f + R ⁇ o,f) - (Rpt,o + RITO . O)) / ((Rpt,o + RITO.O) * AT) (4)
• TCRCOMP ⁇ [(Rpt,f - Rp-,o)/(Rpt,o*R ⁇ o,o* ⁇ T)]+[(R ⁇ o,f
• TCRCOMP (TCR Pt *Rp t ,o + TCR ⁇ TO *R 1 ⁇ o,o)/(Rpt . o + RITO.O) (11)
• a self-compensated ITO sensor was fabricated by sputtering Pt and ITO films that were subsequently patterned.
• a sensor design embodying the invention is shown in Fig. 1.
• a sensor is shown generally at 10 comprises a wide band semiconductor e.g. ITO, 12 and a metal, e.g. Pt, compensation circuit 14, deposited on a substrate S.
• a metal e.g. Pt, compensation circuit 14
• Pt bond pads 16a, 16b, 16c and 16c there are four Pt bond pads 16a, 16b, 16c and 16c.
• This self-compensated sensor 10 can be simulated as a circuit composed of resistors as shown in Fig. 2.
• Chi can measure the resistance of whole sensor
• Ch2 is used to measure resistance of the Pt
• Ch4 is for ITO part
• Ch3 and Ch5 are for contact resistance between Pt and ITO.
• ITO films Indium tin oxide (ITO) films were developed by rf reactive sputtering at low temperature using an MRC model 822 sputtering system.
• Aluminum oxide constant strain beams were cut from rectangular plates (Coors Ceramics - 99.9% pure) using a laser cutting technique.
• the constsant strain beams were then sputter-coated with 4 ⁇ m of high purity alumina prior to the deposition of the ITO strain gages.
• a 2 ⁇ m thick layer of positive photoresist was spin-coated onto the ITO film coating.
• the ITO films were etched in concentrated hydrochloric acid to delineate the final device structure. Sputtered platinum films (1.1 ⁇ m thick) were used to form ohmic contacts to the active ITO strain elements.
• a sensor 20 is shown and comprises a wide band semiconductor 22 and a metal compensating circuit 24 on a substrate S.
• the G(-) of the semiconductor is maximized and the G(+) of the metal is minimized.
• metal bond pads 26a and 26b are also shown.
• a monitor (not shown) connects to the bond pads 26.
• Strain measurements were made using a cantilever bending fixutre fabricated out of a machinable zirconium phosphate ceramic.
• a solid alumina rod was connected between an alumina constant strain beam and a linear variable differential transducer (LNDT) to measure deflection of the strain beam.
• LNDT linear variable differential transducer
• Corresponding resistance changes were monitored using a four wire method with a 6 and Vi digit Hewlett Packard multimeter and a Keithley constant current source.
• the high accuracy LNDT, multimeter and constant current source were interfaced to an I/O board and an IBM PC employing an IEEE 488 interface. Lab Windows software was used for data acquisition.
• Broad band semiconductors over a defined temperature range may exhibit a single TCR or two or more TCRs. It will be understood that if two linear TCRs are exhibited in two distinct temperature ranges within the defined temperature range that a sensor will be fabricated based on each distinct temperature range in order that the strain can be measured over the defined temperature range.
• T > 800° a linear response with a TCR of -210 ppm/°C has been observed and that T > 800°C, a TCR of -2170 ppm/°C has been observed. More recently, an ITO with a single TCR of -300 to -1,500 ppm/°C has been measured.
• Example in the example a four- wire method was used connecting to the bond pads 16. This method is well known to one skilled in the art.
• the sensor was fabricated and tested as outlined in the sections above. Four cycles of heating and cooling were measured, the results are shown below and in Fig. 4. After the first heating, the resistance changing with temperature is almost identical in four cycles, thus it shows the reproducibility is good.
• TCRCOMP ( ⁇ Pt * A Pt + ⁇ p ⁇ T0 * Arro) / ((ppt,o * A Pt + p 1TO ,o * A IT0 ) * AT) (6)
• TCRCOMP (TCRpt*Rpt,o+TCR IT o*R ⁇ o,o)/(Rpt,o+R ⁇ o,o)
• TCRCOMP -23.5 (ppm/°C)
• This sensor was thermally cycled to 1200 °C.
• the experimental data shows that the TCR of self- compensated gage was almost zero ( 0 ppm/°C ⁇ 20 ppm/C) over the temperature range
• the dimensions of the platinum resistor 14 were (0.6mm x 500mm x 0.8 ⁇ m thick) and the dimensions of the ITO sensor 12 placed in series with the platinum resistor were (5mm x 60 mm x 4.4 ⁇ m thick). These dimensions correspond to the (width x length x thickness) of each resistor and the results in the table were obtained for these particular dimensions.
• the room temperature resistances can be read from Fig. 4, approximately 240 ohms for the ITO resistor and 160 ohms for the platinum resistor.
• self-compensating resistors can be used in any electrical device which requires control of a TCR as a function of temperature, i.e. thermistors, temperature sensors, RTD's, etc.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)

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• 2000
  • 2000-04-27 WO PCT/US2000/011334 patent/WO2002035178A1/en not_active Application Discontinuation
  • 2000-04-27 CN CN 00808433 patent/CN1384914A/zh active Pending
  • 2000-04-27 AU AU44962/00A patent/AU4496200A/en not_active Abandoned
  • 2000-04-27 EP EP00926433A patent/EP1247068A1/de not_active Withdrawn
  • 2000-04-27 CA CA002391164A patent/CA2391164A1/en not_active Abandoned
  • 2000-04-27 JP JP2002538116A patent/JP2004512515A/ja active Pending

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