02-05-2013, 12:32 PM
Recent Advances in MEMS Sensor Technology—Thermo-fluid and
Electro-magnetic Devices
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INTRODUCTION
This is the final of a three-part series on micro-electromechanical systems (MEMS)
sensor technology. In the first part [1], a general introduction to MEMS sensing was
given, including their underlying principles. Biomedical MEMS sensors were described
and the principles of bio-sensing in a typical set of biologically inspired sensors were
presented. In the second part [2], mechanical sensors were discussed with their
applications. Some important issues of MEMS sensors were addressed including
compensation for environmental effects, the Casimir effect, harvesting of energy for selfpowered
sensors, and the subject of sensor selection. The present part of the series treats
MEMS sensing in the thermo-fluid and electro-magnetic domains. Sensors for pressure,
fluid flow, shear stress, viscosity, fluid concentration, humidity, temperature, thermal
shear stress, ac current, microwave power, RF power, magnetoelastic parameters, and
magnetic field measurement are discussed. The considered applications include the
measurement of the wing pressure and intramuscular pressure of insect-like flying robots,
robotic liquid dispensing and drug delivery, respiratory flow sensing, controlling the
methanol to water ratio in fuel cell membranes, monitoring gas in automobile cabins, and
self-sustained current measurement.
THERMO-FLUID SENSORS
MEMS-based sensors for applications involving fluid flow, thermodynamics, and heat
transfer are available. Several types of such sensors are outlined now.
Pressure Measurement
Design and analysis of insect-like flying robots require a knowledge of the pressure
applied to its wings during flight. For this, a pressure sensor with a weight at least ten
times lighter than the wings should be used [3]. For instance, the wing length and weight
of a hawk moth is about 50 mm and 100 mg, respectively. This requires a sensor of a
weight about 10 mg with good performance in flight. A MEMS-based pressure sensor
that measures differential pressure can be employed to evaluate aerodynamic forces of
the flapping wings of an insect-type Ornithopter. The sensing is carried out using a
micro-cantilever with a piezoresistive layer (Fig. 1) on its surface whose resistance varies
when subjected to mechanical stresses. Deflections of the cantilever due to aerodynamic
forces change the resistance of the piezoresistor which is calibrated to measure pressure
fluctuations.
MEMS Pressure Sensor
Treatment of neuromuscular diseases requires assessment of the patients’ muscles.
Common muscle strength assessment techniques include manual muscle testing,
instrumented strength testing, and electromyography. Pressure is developed inside a
muscle as the contracting muscle fibers apply pressure on the interstitial fluid volume.
There is a linear relationship between intramuscular pressure (IMP) and joint torque [4].
By attaching a capacitive MEMS sensor to the muscle tissue the IMP can be measured
using the change in capacitance. The movement of the pressure sensor during
measurement inside tissue has to be minimized. Anchors are designed in the sensor which
grip on to the surrounding muscle tissue during muscle contractions.
Shear Stress and Viscosity Sensors
A surface experiences a shear stress when it is exposed to fluid motion. This is commonly
used in obtaining lift and drag forces on surfaces in studies of aerospace, automotive,
marine and biomedical systems. Shear stress at the fluid–wall interface requires the
measurement of very small parameters and high resolution force sensors. Macro-sensors
exhibit limited resolution due to fluctuating shear stress. MEMS devices are capable of
high resolution measurement particularly suitable for turbulence research and industrial
process control. The shear force is measured using capacitive, piezoresistive and optical
principles. Piezoelectric and piezoresistive sensors are relatively slow in response in
dynamic measurements (e.g., for turbulence). An optical sensor is capable of
measurement in the resolution range of 0.003 Pa to 10 Pa. Capacitive sensors require a
large number of combs (capacitors) and measure shear stress of fluid flow with resolution
0.01 Pa and bandwidth 50 kHz. The sensor composed of a beam element and capacitive
comb drives supported by an in-plane torsional spring [8]. Displacements of the floating
beam due to shear is detected by capacitive sensing of a resonant RLC circuit at subfemtofarad
sensing scale. A micro-pillar shear stress measurement system (Fig. 4) allows
the detection of wall shear stress at high spatial and temporal resolutions [9].
Concentration Sensor
A MEMS-based chemical concentration sensor measures the density and temperature output, for monitoring and controlling the methanol to water ratio. The sensor can be used for other fuels and biofuels such as ethanol, ethylene glycol, butanol, gasoline and diesel fuel. Methanol is a relatively inexpensive fuel, has high specific energy density and is easy to transport and store. Commercializing fuel cell technology still requires advancement of the performance and efficiency of methanol crossover—the transport of methanol through the fuel cell membrane with no reaction or power generation. Optimizing the methanol to water concentration by a sensor can improve the performance of the system. A low-cost MEMS sensor is embedded in the system and works based on the variation in vibration frequency due to fluid density. A vibratory member is damped inside the fuel cell fluid when the density of the fluid is increased. Raising the methanol concentration increases the resonant frequency output [10].
Micro and Nano Gas Sensors
A MEMS metaloxide gas sensor is used to control the access of (combustion) gases from outside a vehicle to the car cabin, and detects odor events created within the car cabin. A human test panel for examining the hedonic impression on a scale from 0 to 5 is employed as a reference [11]. A MEMS metal-oxide-sensor array consisting of three different sensors is calibrated by human-sensory impression data. This system allows the design of an effective ventilation system.
Atoms, molecules and concentration in the gas phase can be identified based on the electric characteristics in the field-induced gas discharge processes. This process requires high-voltage operation and is complicated. The development of sensors at micro scale for high-voltage operation is not practical. An ionization micro-gas sensor with carbon nanotube film (CNTF) electrode and short-gap spacing has been designed that can be operated at a low supply voltage. This system consists a CNTF electrode with a lead film underneath, a top electrode, and a spacer between them.
Humidity Sensors
A humidity sensor that changes in capacitance when it absorbs or desorbs water molecules has been developed. In order to avoid signal drift, a micro-heater provides a super-ambient working temperature in the sensor.
Many industrial machineries require a standard level of humidity for reliable and efficient operation. The polyimide material coated on a micro-cantilever made of silicon (Si), silicon nitride (SiN), or polymers absorbs water vapor and increases the mass of the cantilever beam. The increase in mass changes the capacitance of the sensor on the order of picofarads. A glass cover with sufficient thickness with a thick base material that supports the glass walls allow effective operational performance in harsh environmental conditions [12]. A platinum based heater evaporates absorbed vapor in the polyimide layer and prepares the sensor for the next measurement.
Microwave Power Sensor
Microwave power is measured in wireless systems such as modern personal communication and radar systems. Thermoelectric measurement is a common technique which converts microwave power that is absorbed by a load resistor into heat energy [16]. The thermal energy (heat) is detected by a thermopile. Generated direct current (dc) voltage from this thermal energy (through Seebeck effect) is proportional to the microwave power.
The MEMS microwave power sensors exhibit excellent sensing capability with a low power consumption. In a capacitive membrane MEMS microwave power sensor a fraction of the applied microwave power is transmitted to the capacitive membrane. This microwave power is then converted into heat by a load resistor. Nearby thermopiles detect the resulting temperature increase and convert it into a dc voltage.
RF Power Sensor
MEMS sensors measure the signal power by detecting electrostatic force induced between an RF signal line and a suspended membrane, using the capacitance principle [17]. The required force for maintaining the movable capacitor plate (Fig. 6) in a stationary position is proportional to the square of the root-mean-square (rms) signal voltage and thus to the signal power. Alternatively, measurement of the displacement of the movable capacitor plate gives the rms voltage amplitude of the ac signal.