Family of Micromachined Wall Hot-Wire Sensors on Polyimide Foil

Abstract

Micromachined wall hot-wire sensors composed of a highly sensitive, nickel, thin-film resistor spanning an airfilled cavity in a mechanically flexible substrate are presented. Cavity design and sensor materials are optimized to reduce thermal losses, thus enabling measurement of high-frequency fluctuations in fluid flows. Successfully realized sensors featuring wire widths of 2 and 5 m, wire lengths from 400 to 2000 m, and various cavity dimensions were characterized in wind-tunnel experiments. Static sensor calibration, cutoff frequency determination using a sine sweep, and recording of angular characteristics were conducted at wall shear stresses of up to 1 N=m 2 , obtained at 20 m=s freestream velocity in an open wind tunnel on a flat plate with fully developed turbulent flow. An overheat ratio of 1.8 was used for hours without thermal failure of the sensors, and a maximum cutoff frequency in still air of 73 kHz was obtained. The highest average sensitivity of 0:196 V=N=m 2 was recorded for a sensor of 5-m wire width and a length-to-width ratio of 400 in a wall shear stress range from 0 to 1 N=m 2 with a power consumption of less than 30 mW. Nomenclature a W = overheat ratio of the hot wire d C = depth of the cavity, m k = yaw factor l C = length of the cavity, m l W = length of the hot wire, m P = electrical power consumption, W R W = electrical resistance of the hot wire in operation, R 0 = electrical resistance of the hot wire at room temperature, S = sensitivity of the sensor to changes in wall shear stress, V=N=m 2 T W = temperature of the heated wire, K T 0 = ambient fluid temperature, K t F = thickness of the polyimide foil, m t W = thickness of the hot wire, m U = anemometer output voltage with flow in constanttemperature operation, V U 0 = anemometer output voltage without flow in constanttemperature operation, V U 0 = anemometer output voltage with flow normal to the wire length in constant-temperature operation, V v = flow velocity at the hot wire, m=s v eff = flow velocity contributing to cooling of the hot wire, m=s v n = component of flow velocity normal to the hot-wire length, m=s v 1 = freestream velocity, m=s w C = width of the cavity, m w W = width of the hot wire, m = yaw angle, deg = wall shear stress, N=m 2

Flexible hot-film anemometer arrays for flow measurements on curved structures

“…The sensor arrays presented here rely on the same principle as [8], but substantial improvements have been achieved. The major drawbacks of the existing sensors are improved.…”

“…Unfortunately, the commercially available arrays are not optimized regarding the heat losses into the substrate or their dynamic behavior. To address this issue, a flexible sensor has been presented, on which the thermal transport mechanisms have been simulated and later been improved [8]. The result is a flexible hot-film sensor, being fabricated by dry-etching the substrate under the sensing element.…”

“…For aerodynamic measurements, a different sensor concept has been presented, namely a wall shear stress sensor [8]. The working principle is based on heat transfer from a heated film into the fluid, similar to a hot-wire anemometer [9].…”

Flexible smart sensing skin for “Fly-by-Feel” morphing aircraft

“…By now, plenty of research has proved that wall shear stress (WSS) measurement along the leading edge is a realistic way to detect the separation line/point in real-time. Basically, WSS measurement can be divided into two major schemes: hot-wire/film based shear-stress sensor [108,109] and floating-element based sensor [110][111][112]. The structure of Hotwire/film sensors is relatively simple, employing heated components on the sensor surface to indirectly measure the variation of shear stress, with fast response (around dozens of kilohertz) and high sensitivity (<1 Pa) [108,109].…”

“…Basically, WSS measurement can be divided into two major schemes: hot-wire/film based shear-stress sensor [108,109] and floating-element based sensor [110][111][112]. The structure of Hotwire/film sensors is relatively simple, employing heated components on the sensor surface to indirectly measure the variation of shear stress, with fast response (around dozens of kilohertz) and high sensitivity (<1 Pa) [108,109]. Oppositely, floating-element based sensors directly sense the shear-stress force with the floating element.…”