A rotating shaft in a wheel, a motor and a turbine uses sensors to measure torque, vibration, and acceleration for better control and maintenance of a machine. Such data is currently acquired in a laboratory and utilized for tuning the machine. For adaptive control and failure prognosis in future, the data need to be collected by the sensors on the fly. Connecting the moving sensors by wire and a brush to a microcontroller leaves reliability issues due to mechanical wear and electrical noise. A wireless data link with small form factor and low maintenance cost is needed. Bluetooth consumes large power and requires external parts such as a battery and a crystal. RFID loses connectivity when the sensors move into the shadow of a radio wave from a transponder. Continuous sensing on all sides is needed as the direction to be sensed is not determined.Our proposal is illustrated in Fig. 11.7.1. A rotating shaft with sensors attached (Sx's) is wrapped by a 2D waveguide sheet through which data and power is transmitted by electromagnetic waves. There is no shadow by the wrapping, which allows continuous sensing and measurement on all sides. The 2D waveguide sheet consists of two conductor layers with a dielectric layer in between. One of them has mesh windows through which the electromagnetic waves exude as evanescent waves [1]. A coupler, which is half the wave-length, couples with the evanescent waves. When the couplers are placed near the sheet, a link for data and power is formed. Note that data can be transmitted with power by amplitude modulation. Energy is barely lost from other windows. The sheet is bendable with a negligibly small characteristic change. Edges are terminated to keep waves from reflection or transmission. When the sheet is rolled to form a cylinder, the waves propagate in just the same fashion as in the sheet. To reduce the coupler size 2.4GHz ISM band is used for power delivery. Multiple transponders (Tx's) are placed in line on the edge to cover a wider area of the sheet. It also contributes to the reduction of size, and hence cost, of the power amplifier, as the output power of each transponder can be reduced. They generate a banding pattern of power levels. The transponders' output power should be increased such that a sufficient amount of power is received at the lowest position in the banding pattern. A beamforming technique, such as a retrodirective transponder array [2], saves this wasted of power. In the transponder a pilot signal from a sensor is compared with a clock signal from a reference (Ref) and its retrodirective wave is generated as an output. This approach, however, requires very high accuracy in both reference clock distribution and frequency matching between the pilot signal and the clock signal. Should there exist a 5% phase error among 4 transponders for instance, transmission efficiency degrades by 10%. Phase calibration in the transponders is not enough due to discrepancy in the frequency of crystal oscillators used in the sensor and the reference clock.A reference scheme ...
