The following information describes demonstration of a wireless self-powered pressure sensor.
Piezoelectric materials, vibrating machinery and rectifying circuitry was kindly made available by Advanced Cerametrics.
The goal of the demonstration is to show feasibility of creating vibration-powered sensors that do not use batteries for data acquisition.
The following image show the experimental setup of the demo (click on the image to see a high-resolution version):
In this specific example the energy was provided by a vibrating bimorph cantilever element excited by a small electric motor. The speed of the motor was controlled by a potentiometer. During experiments the motor speed was set to the minimal possible excitation level.
Vibrating bimorph provided about 40V peak-to-peak voltage that was rectified by a bridge circuit and stored in a capacitor bank of about 1000uF. Ceramic capacitors were used by ACI to minimize leakage currents. The max voltage on the capacitors was limited to 6V by a Zener diode. This voltage was converted to a 3.3V level by a linear regulator from TI. The regulator used in the circuit featured very low quiescent current (~3uA).
In the present incarnation the energy conversion circuitry has low energy efficiency, but still provides enough power to the sensor node.
The 3.3V voltage was used to power a WISAN node. Most of the time the sensor node spent in deep sleep (ultra-low power consumption mode) consuming only about 9uW of energy (or about 3uA current consumption). Only the real-time clock circuitry was active on the processor core, keeping precise time until the next sample of data. The data were sampled at 30Hz sampling rate.
The pressure sensor (see the following Figure) was of resistive type, made from carbon-infused conductive foam that changes resistance with applied pressure. The sensor was powered on immediately before a sample of data was taken and powered off immediately after. Duration of each sample (sample-and-hold plus conversion) was about 64us to minimize sensor noise.
The sampled sensor data were stored in the WISAN’s node memory until accumulation of 100 data points at which time a single radio transmission by an IEEE802.15.4 MAC compatible software transmitted a packet of data to the receiver. The receiver node displayed pressure data on the LCD and transmitted the same data packet to the host PC via a USB interface.
The following figure shows the front end of the PC viewer application.
The following video shows the system at work.
Both clips are encoded by Indeo codec and should display without problems by most Media Player applications.
Energy harvesting conclusions
In collaboration with ACI we have succesfully demonstrated an operational demo of a self-powered pressure sensor based on piezoelectric energy harvesting. The vibrating bimorph produced power which was sufficient to provide continuous sampling of the pressure sensor data at 30Hz and deliver acquired data via an IEEE802.15.4 link to the remote destination. The efficiency of the energy harvesting circuit can be further improved and give more energy for data acquisition and/or storage or allowing less vibration levels to power the system.