Semiconductor Strain Gage Applications to Skin Friction Sensors for High-Speed Aerospace Testing

Herb Chelner1 & Robert A. Mueller2
Micron Instruments, Simi Valley, CA

Ryan J. Meritt3
Ahmic Aerospace LLC, Dayton, OH 45440
www.AhmicAerospace.com

Abstract

Semiconductor Strain Gages (SSGs) are used in a variety of force and temperature measurement systems when the requirements include any or all of high sensitivity, fast response time, high impedance, small physical size, and long-term mean time between failures (MTBF). An ideal example is the application of SSGs to meet the rigorous requirements for skin friction sensors used for high-speed aerospace testing. This paper explains how Ahmic leveraged the benefits of SSGs to create a state-of-the-art sensor for high-speed aerospace testing.

I.                   Introduction

A.      Problem Statement

Accurate knowledge of skin friction is critical in assessing the energy efficiency and controllability of any aerodynamic system. At the most fundamental level, drag must be overcome to achieve forward motion and maneuverability; how effectively this is accomplished defines the economics of the flight performance. In many cases, the skin friction drag contribution (or area-integrated total surface shear stress) can account for more than half the total aerodynamic drag acting upon the system. Since skin friction increases on the order of velocity squared, considerations grow ever more important in the realm of high-speed flow. For high-speed and high-enthalpy test environments including scramjet vehicles, propulsion systems, re-entry flight articles, and material testing, skin friction can have a first-order effect on overall performance. 

Further complicating matters, in high-enthalpy flow fields, extreme pressures, temperatures, and vibrations increase the difficulty in quantifying even the most basic aero-thermodynamic properties [1]. Unfortunately at the present time, there is a paucity of experimental data from which to develop a truly credible foundation for analytical and computational fluid dynamics (CFD) modeling of high speed, hypersonic systems. Experimental wall shear is an important measurement needed to anchor, validate, and verify CFD methods and their turbulence, heat transfer, and shear stress sub-models. Over the past few decades, research in sensor designs has extended the capability of measuring skin friction by various mechanical, electromagnetic, and optical techniques [2]. However, at the present date, reliable commercial-grade instruments are not available with the capability of making accurate shear force measurements in high-speed, high-enthalpy ground and flight applications, although prototype designs have been built and tested with very favorable results [3]. As aerospace technology develops, advanced instrumentation needs have to be effectively addressed in order to close this growing capabilities gap.

B.     Skin Friction Sensor Technical Background

Methods for determining skin friction drag can be categorized into two fundamental measurement techniques – indirect and direct.  They are distinguished by their approach and the physical quantity, which they measure.  Indirect methods require properties of the boundary layer to be well defined and known a priori [4]. Shear at the wall ( ) is subsequently inferred as a function of other flow field measurements through analytical correlation or analogy. A prime example is the use of Reynolds Analogy to infer skin friction from a measurement of surface heat flux. Although indirect techniques have been shown to work in many common, well-understood flow environments, they are not considered reliable in complex flow fields. 

Figure  1. Generic Direct-Measurement Skin Friction Sensor

Figure 1 . Generic Direct-Measurement Skin Friction Sensor

Alternatively, direct methods do not require any foreknowledge, but instead directly measure the tangential frictional forces imparted by the moving flow [5]. To illustrate this concept, an idealized schematic of a generic direct measuring sensor is shown in Figure 1 . The floating element, or sensing head, typically circular in cross-section, is flush mounted and parallel to the surrounding wall. A very small gap surrounds the element while a support system enables the element to “float” in the wall and move minutely in the flow direction. The displacement of the element or strain imposed on the support system can be measured and is directly proportional to the wall shear acting upon its sensing face. Through calibration, the direction and magnitude of the wall shear stress may be determined. 

Under most circumstances, each flow environment and application requires a unique skin friction sensor design. The wide varieties of designs differ in their approach, measuring mode, and mechanism function, each with its own advantages and disadvantages. On both practical and philosophical grounds, direct methods present the most accurate and sound approach to measure skin friction in complex, poorly understood flows.

Photographs of various skin friction sensors developed at Ahmic Aerospace are shown in Figure 2. Ahmic has conducted research in world-class facilities with top community partners including the NASA 8-Ft HTT, AEDC arc jets, von Karman facilities (VKF), Tunnel 9, AFRL scramjet research cells, CUBRC LENS facilities, Orbital ATK GASL facilities, and a variety of academic tunnels.

a - Head Center = 0.50 in., Actively Cooled Design b- Head Center = 0.25 in, Dual-Axis Design

a)       Head 𝜙 = 0.50 in., Actively Cooled Design

b)       Head 𝜙 = 0.25 in., Dual-Axis Design

Figure 2 . Skin Friction Sensors Developed at Ahmic Aerospace

C.     Strain Gages Technical Background

Strain gages convert mechanical motion into an electronic signal. A change in resistance is proportional to the strain experienced by the sensor. If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the gage factor (GF), is given by:

GF = (ΔR / R) / ε

Equation 1

Ideally, a strain gage would change resistance only due to the deformations of the surface to which the sensor is attached. However, in real applications, temperature, material properties, the adhesive that bonds the gage to the material surface, and the stability of the material metal all affect the measured resistance. The Temperature Coefficient of Resistance (TCR) and the Thermal Coefficient of Gage Factor (TCGF) are important measures of the thermal effects in the silicon matrix of the strain gage inhibiting the flow of electrons.

Wheatstone Bridges are important electrical circuits when using semiconductor strain gages. A Wheatstone bridge is an electrical circuit used to measure an electrical resistance by balancing legs of a bridge circuit [6], one leg of which includes the unknown component. Wheatstone Bridges are used in one-quarter configurations where there is one gage and three precision resistors; half-bridge configurations with two gages, and full-bridge configurations with four gages. The voltage output of a full-bridge is twice that of a half-bridge, and 4 times that of a quarter bridge. And, as discussed below, the use of a half-bridge or full-bridge configuration is required for temperature compensation. Information on Wheatstone Bridges is widely available on the Internet.

D. Micron Instruments’ Semiconductor Strain Gages

Semiconductor strain gages make use of the piezo-resistive effect exhibited by certain semiconductor materials such as silicon and germanium in order to obtain greater sensitivity and higher-level output. Semiconductor gages can be produced to have either positive or negative changes when strained. They can be made physically small while still maintaining a high nominal resistance. Semiconductor strain gage bridges may have 100 times the sensitivity of bridges employing metal films, but are temperature-sensitive and therefore require temperature compensation.

Micron Instruments’ Semiconductor Strain Gages are micro machined from a solid single grown crystal of "P" doped Silicon. This results in a two terminal resistive device that has a minimum of molecular slippages or dislocations permitting repeatable use to high strain levels.

E. Micron Instruments’ Semiconductor Strain Gages vs. Metal-Foil Gages

Perhaps the most popular strain gage is the foil-type gage, produced by photo-etching techniques, and using similar metals to the wire types (alloys of copper-nickel, nickel-chromium, nickel-iron, platinum-tungsten, etc. Foil gages are considerably less expensive than semiconductor gages, and can be useful in less demanding applications.

However, Micron Instruments’ Semiconductor Strain Gages offer significant benefits that make them the obvious choice for a diversity of applications, including medical, industrial, robotic, precision instruments, infrastructure, aerospace, and defense applications.

Below are some of the most compelling benefits of Micron Instruments’ Semiconductor Strain Gages.

(i) Size

Many applications, e.g., implantable medical sensors, offer extremely small space for gage or bridge placement. Micron Instrument’s Semiconductor Strain Gages typically require less than 2% of the area needed for a metal-foil gage, and are as small as .018” long (with an active area of .011”) and .0004” thick.

(ii) Sensitivity and Signal Output

The Gage Factor (GF) for metal-foil gages is typically 1 to 4. In contrast, Micron Instruments’ Semiconductor Strain Gages have a GF as high as 200 – roughly two orders of magnitude! A consequence of having a low GF is the need to place a metal-foil gage in very high strain regions to achieve a sufficiently strong output signal. Micron Instruments’ Semiconductor Strain Gages can operate effectively as low as 50 μ-strain, and as high as 3,000 μ-strain. There is even a “crash gage” available that operates up to 9,000 μ-strain. Micron Instruments’ Semiconductor Strain Gages will also operate for an infinite number of cycles, provided that operating strain is kept within limits, as discussed below. Where metal-foil gages have a typical full-scale sensor output at 500 μ-strain of 2mV/V, Micron Instruments’ Semiconductor Strain Gages deliver a typical full-scale output of 20mV/V when temperature compensated.

(iii) Life Cycles

Metal-foil gages usually fail from fatigue after 10 thousand to ten million cycles. Micron Instruments’ Semiconductor Strain Gages will operate for an infinite number of cycles provided that operating strain is kept under 3000 μ-strain for the gage, and the maximum full-scale strain is kept under the one μ-strain precision elastic limit for the material the gage is being bonded to.

(iv) Precision Temperature Compensation and Gage Matching

Semiconductor Strain Gages have large temperature coefficients of resistance (TCR) making single gage strain measurements difficult unless used at a constant temperature. Micron Instruments’ Semiconductor Strain Gages are predominantly used in half-bridge and full-bridge configurations, which compensate for temperature and deliver highly accurate strain output. Micron Instruments uses advanced instrumentation for precision measurement of gage slope and intercept. This temperature characterization is then used to carefully match gage sets for use in half or full bridges.

(v) Resistance

Metal-foil gages typically offer an impedance range of 120Ω to 5,000Ω. This can be limiting for wireless sensing applications, especially passive wireless sensors that require over-the-air power. High impedance gages reduce the required power at a fixed voltage, enabling stronger RF signals at greater transmission range.

Micron Instruments is driving the high end of strain gage impedance, currently as high as 30,000Ω, and expected to reach 100,000Ω in the near future. This makes Micron Instruments’ Semiconductor Strain Gages an ideal choice for wireless sensing applications.

II.                Design Approach

A.     Program Strategy

Transducer based skin friction sensors have primarily utilized foil strain gages as the primary sensing element. Semiconductors have been used in past research but with limited success. To further develop skin friction sensor technology, Ahmic is working with Micron to apply their advanced semiconductor strain gages as an alternative transducer solution. As described in detail above, semiconductors have a significantly larger gage factor, are much smaller in size, stronger, exhibit less hysteresis, are less susceptible to electrical noise, are more accurate, and have a longer life cycle.

The challenges to adapting semiconductor technology so far have been primarily the signal-to-noise ratio. Wall shear is an extremely small force that correlates to a small strain output and thus electrical reading. New sensor designs continue to improve output but it remains on the order of magnitude as other error sources such as pressure, temperature, vibration, and in some cases electromagnetic interference (EMI). Years of development and testing foil gage systems have led to the control of these error sources within reasonable levels. New manufacturing strategies are currently being explored and tested with Micron to control these same errors sources with semiconductors.

B.     Sensor Validation

To validate the successful integration of semiconductors, each skin friction sensor will undergo a rigorous series of calibrations designed to systematically evaluate individual aspects of its functionality. NIST-traceable static and dynamic load calibrations will be taken to demonstrate linear and repeatable output. Sensors will be cycled through a range of temperatures, pressures, and EMI frequencies to document the response and error source contribution. Vibratory excitation will be applied to determine primary resonance modes. Impact load testing will determine the sensor’s response time performance.

Lastly, the skin friction sensors will be tested in Ahmic’s supersonic wind tunnel. Run entries will be conducted under well-characterized, nominal Mach 2.2 unheated flow conditions. Both analytical and numerical wall shear estimations will be used to compare and justify the experimental sensor results.

III.             Company Profiles

A.     Ahmic Aerospace

Ahmic Aerospace occupies 2,200 square feet of office and laboratory space. Their facilities are equipped with the state-of-the-art resources necessary for the design, development, characterization, testing and evaluation of measurement instrumentation. Sensor design work and research is supported through SolidWorks, ANSYS, and other commercial software packages to facilitate CAD, FEA, and CFD requirements. Proprietary software and tools also play a key role in tailoring sensor development quickly to customer requirements. Ahmic strongly believes in investing in internal research and development efforts to continually advance their instrumentation technology. Their laboratory contains all necessary support equipment for NIST-traceable sensor calibration and testing, including high-speed DAQ systems, load cell test stands, vacuum/pressure chamber, environmental test chamber, high-temperature ovens, shaker table, and miniature EMI chamber. Their calibration procedures follow the ANSI/ASME standard on test uncertainty. Ahmic utilizes NI LabVIEW, SignalExpress, and Matlab software to control laboratory instrumentation and signal processing. Their facilities enable them to move quickly from concept to advanced prototype and low-rate production.

Ahmic collaborates with numerous leaders in academia, government, and private research across a broad range of world-class wind tunnel and water channel facilities. Ahmic maintains a DCAA approved time keeping, estimating and accounting system. Ahmic is also an ITAR Certified Manufacturer. More information can be found through their website, www.AhmicAerospace.com.  

B.     Micron Instruments

Micron Instruments has been supplying high-quality semiconductor strain gages, pressure transducers and temperature sensors for more than thirty years. 

Their standard semiconductor strain gages are made from "P" doped bulk silicon; they have no P/N junction. The silicon is etched to shape eliminating potential molecular dislocation or cracks thereby optimizing performance. These strain gages have been used in the manufacture of many different types of sensors such as load cells, torque meters, pressure transducers, accelerometers, and flow sensors.

Professional installation of strain gages on customer products along with the option of temperature compensation and calibration is available. Micron’s staff of experienced engineers can also design custom sensors and special strain gages to meet your unique needs.

Micron's standard line of pressure transducers has corrosive resistant Titanium diaphragms and is available in a wide assortment of pressure ranges. Typical applications are for OEM, process control and liquid level measurement. In addition to our standard line of Titanium transducers, Micron has experience in providing sensors made of 17-4PH CRES, 316SS, Inconel, Tantalum, Aluminium, Beryllium Copper, Invar Plastics and others. 

Dual pressure/temperature transducers, pressure transmitters with 4-20 mA outputs and dual pressure/temperature transmitters are available in all of the physical configurations of their standard pressure transducers. The dual sensors incorporate silicon temperature sensors manufactured by Micron, which are more accurate and easier to use than thermistors. These temperature sensors typically produce a 100 mV change with 5 VDC excitation between 30°F and 130°F when used as a single element in a full bridge configuration. They can be set to produce 1000 mV over the same temperature range for more resolution if required.

Micron’s customer base includes aerospace companies, rocket motor manufacturers, robotics and industrial control companies, sensor manufacturers, medical device manufacturers, major U.S. defense contractors, and sub-contractors for all of these industry categories. Micron has a diverse experience in severe environment applications to include measurement and control devices for spacecraft, satellites, and military aircraft; embedded bond stress, bore strain, and temperature sensors for tactical and strategic (e.g., Minuteman III) rocket motors; and missile environmental monitoring. More information can be found through their website, www.MicronInstruments.com.

 
 

 REFERENCES

[1]
T. A. Heppenheimer, Facing the Heat Barrier: A History of Hypersonics, NASA History Office, 2011. 
[2]
J. A. Schetz, “Direct Measurement of Skin Friction in Complex Flows,” AIAA, no. 2010-44, 2010
[3]
R. J. Meritt, “Skin Friction Sensor Design Methodology and Validation for High-Speed, High-Enthalpy Flow Applications,” PhD Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA, 2013.
[4]
W. Nitsche, C. Haberland and R. Thunker, “Comparative Investigations of the Friction Drag Measuring Techniques in Experimental Aerodynamics,” International Council of The Aeronautical Sciences, 1984.
[5]
K. G. Winter, “An Outline of the Techniques Available for the Measurement of Skin Friction in Turbulent Boundary Layers,” Progress in Aerospace Sci., vol. 18, no. 1-57, 1977.
[6]
“Wheatstone Bridge,” [Online]. Available: https://en.wikipedia.org/wiki/Wheatstone_bridge.
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