Each year, hundreds of thousands of hip and knee replacement surgeries are performed in the United States. Most of these prosthetic implants are expected to last for at least 20 years after which revision and replacement surgery will be required. In many cases, however, excessive wear and loosening of artificial joints leads to joint swelling, bone loss, pain and eventually the premature need for replacement surgery. Such cases often prove to be more traumatic and less successful than the initial implant. Long-term monitoring of biomechanical implants for excessive wear and fatigue, which would allow for predictions of implant wear and failure, would significantly reduce patient discomfort and risk. In other situations, the ability to monitor the strain imposed on a mechanical structure and, from the data collected, predict when failure can be expected to occur would be immensely beneficial (e.g., helicopter rotor blades). However, in many cases, such as the biomedical implant scenario, continuous monitoring is not possible using today's strain-monitoring technology. Traditional approaches for strain-rate monitoring involve strain gauges and a dedicated processor for computing and storing the strain-rate statistics. This method requires an embedded battery for powering the processor which not only limits the sensor's operational life but also limits its size.
Description of Technology
The Self-Powered Strain-Rate Monitor utilizes a novel integrated circuit sensor that consumes less than one microwatt of power and interfaces directly with, and draws its operational power from, a piezo transducer. By combining floating-gate transistors with a piezo-electric transducer, the sensor is able to achieve operational limits not possible with other electronic strain-rate sensors. These sensors can be embedded inside structures, vehicles, rotating parts and biomedical implants where they can autonomously compute and store cumulative statistics of the strain rates experienced by the structure. These stored statistics can be remotely retrieved (using standard RFID technology) and used for predicting the onset of mechanical fatigue. Thus, the technology can be used for both preventing mechanical failure and for significantly reducing maintenance costs.
- Self-Powered: The sensor utilizes less than one microwatt of power which is harvested directly from the piezo-electric transducer.
- Automatic Data Collection and Computation: The use of floating gate transistor injection principles allows the sensor to compute the cumulative stress and strain patterns experienced by a structure without having to utilize data converters.
- Small Size and Cost: Integration of the energy source and the data collection/conversion/storage into the circuit/transducer combination greatly reduces the size and cost of the sensor compared to existing approaches.
Applications include long-term autonomous monitoring in civil engineering (bridges, roads) and bio-medical (implants) structures, as well as mechanical monitoring of movable parts in automotive (axles), aeronautical (propellers) and energy (wind turbines) applications.
US 2008/0047355 A1 (filed Aug 24, 2007)
Shantanu Chakrabartty, Nizar Lajnef, Niell Elvin, Amit Gore