Wireless pressure sensor is laser powered

Researchers at Case Western Reserve University (Cleveland, OH) have demonstrated a high-temperature pressure sensor that requires no batteries or wires for communication or power.

Oct 1st, 2004
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Researchers at Case Western Reserve University (Cleveland, OH) have demonstrated a high-temperature pressure sensor that requires no batteries or wires for communication or power. The device, which can measure ambient pressure at temperatures up to 250°C, is designed to combine optical, micromechanical, and electronic elements in a way that maximizes the usefulness of each element while minimizing power consumption. It is hoped that future generations of the technology will be used in industrial and aerospace applications (the work was partially funded by NASA).

Sensing over a wide range of temperatures is difficult because the performance of conventional electronics degrades with temperature, generally failing entirely around 180°C. Although there are more robust technologies based on silicon-on-insulator and piezoresistance, they tend to require relatively high voltages. They either require wires to supply power or batteries, but existing battery technology cannot drive them at these high temperatures. The team at Case Western, therefore, sought to develop a very low-power circuit that could be driven using energy that was remotely generated and ­wirelessly delivered .

The microsystem they designed consists of three main components: a tunnel diode that acts as an oscillator and transmitter, a gallium arsenide photodiode to convert incoming optical power, and a microelectromechanical pressure sensor (see figure). The tunnel diode has the advantage that, when properly biased, its negative resistance can compensate for changes in the resistance of other circuit ­elements as they change with temperature. This bias-in the 100- to 150-mV range for their demonstrator-is small enough to be supplied by the current from the photodiode.


Light from an external laser is turned into current by a photodiode, serving as the only power source for the high-temperature pressure sensor. The oscillating frequency of the tunnel-diode transmitter is controlled by the change in capacitance of the microelectromechanical ambient-pressure sensor, and this is detected by the external receiver.
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Capacitance across it and the inductance of the circuit control the oscillation frequency of the tunnel diode. Because the pressure sensor is essentially a variable capacitor, the transmitted signal is a frequency encoding of its state. Known as a touch sensor, it consists of a silicon diaphragm that is pushed (by ambient atmospheric pressure) toward a silicon wafer coated with an insulator. Although the relationship between capacitance and pressure changes in the different regimes, the sensor works both when the diaphragm and insulator are in contact and when they are not. In particular, in the touching case, the relationship is linear because it is dominated by the contact area.

Inductance is the other important part of the oscillator control, so doctoral student Michael Suster designed the system with an induction loop that doubles as an antenna. The loop is patterned in gold in the same step as the wiring for the circuit. As long as the negative resistance of the tunnel diode is less than the total resistive loss contributed by the sensor-capacitor and induction loop, then the frequency of the circuit stabilizes at the network resonance frequency.

The system operates using only the electricity derived from a 9-mW laser beam, with a conversion efficiency of 28% at room temperature and 9% at 250°C. Although the ­sensor performed well across the temperature range, with a good match of predicted/measured pressure versus ­frequency at room temperature, a temperature-dependent frequency shift was also observed. This, researchers say, could be calibrated out through the introduction of an IR-sensitive temperature sensor to the system.

Another feature of the device’s behavior is that it takes up to 30 minutes to settle down when installed. The variation in output for a given pressure also depends on temperature. At 25°C it is just 80 kHz (around a base frequency of around 23‑MHz), doubling to 170 Hz at 250°C. Suster and his colleagues attribute this to temperature cycling in both the tunnel and photodiodes, and possibly to changes in the output power of the laser beam.

REFERENCE

1. Michael Suster et al., J. Microelectromechanical Systems13(3) 536 (June 2004).

SUNNY BAINS is a scientist and journalist based at Imperial College London; e-mail: sunny@sunnybains.com.

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