Christopher P. Townsend

e-Knee: evolution of the electronic knee prosthesis. Telemetry technology development.

Our purpose was to develop a fully instrumented tibial component of a total knee prosthesis (Fig. 1) containing multichannel transducers (load cells), a microtransmitter, and an antenna. Three complex technologies were incorporated into a prosthesis capable of measuring in vivo tibiofemoral compressive forces to transmit “real-time” data. The geometry was modified to accommodate the transducer and electronic technologies (Fig. 2). Fig. 1: Photograph of instrumented prototype and polyethylene insert. Fig. 2: Schematic of the e-Knee, a total knee prosthesis containing transducers (load cells), a transmitter, and an antenna. As a result of the development of a fully instrumented implant, direct load measurements were recorded intraoperatively during the course of a total knee replacement. Long-term collection of in vivo measurements of loads on a total knee prosthesis will allow future improvements in knee-replacement design, rehabilitation, and assistive devices such as lower-extremity braces, orthotics, and shoes. This report describes the history and preliminary results of the use of a total knee prosthesis with a telemetric implant, the e-Knee. Until now no devices or methods for measuring loads in vivo in the knee have been available. Measurement of tibiofemoral forces is directly relevant to the design and technique of implantation of knee prostheses. The wear and deformation of the articulating surface (polyethylene), the distribution of stress to the implant, the mechanical behavior of the bone-implant interface, and the load-carrying characteristics of the remaining bone are intimately related to the joint load. Until now, investigators have relied principally on theoretical data from mathematical modeling, biomechanical testing of cadavers, and clinical gait analysis to predict the physical forces on the knee (Fig. 3). Fig. 3: Theoretical model. Predictions based on theoretical data have revealed that tibiofemoral forces range from three to six times body weight during walking, ascending and descending stairs, and isokinetic exercise. While the information gleaned from …

Energy harvesting wireless sensors and networked timing synchronization for aircraft structural health monitoring

Energy harvesting, combined with wireless sensors, could greatly improve our ability to monitor and maintain critical structures. This paper reports on the development of an integrated structural health monitoring and reporting (SHMR) system for use on Navy aircraft. Our goal was to develop and test a versatile, fully programmable SHMR system, designed to synchronize and record data from a range of wireless and hard wired sensor networks. Wireless sensors included strain gauges, accelerometers, and thermocouples. Hard-wired sensors included gyroscopes, accelerometers, and magnetometers. Data from an embedded Global Positioning System (GPS) provided position, velocity, and precise timing information.

Wireless inspection of structures aided by robots

The inspection of structures by humans is often hampered by safety and accessibility concerns. One method of reducing human inspection activities is to use remotely located sensors, such as strain gages and accelerometers. Running cables to power the sensors and transmit data can be expensive and inconvenient. This paper describes a development effort in which a robot is used to power and interrogate remotely placed sensors. The sensors are powered by a noncontact inductive system, which eliminates the need for batteries or interconnecting lead wires. The data are sent by a wireless connection back to a central data logger and processor. The power demands of telemetering data are decreased by close proximity of robot. The system utilizes existing microminiature, multichannel, wireless programmable Addressable Sensing Modules (ASMs) to sample data from a wide variety of sensors. Demonstration style robots are built and tested with ASMs in simple tabletop design, and a more robust task specific I-beam crawler robot for structural application.

Strain Energy Harvesting for Wireless Sensor Networks

Our goal was to demonstrate a robust strain energy harvesting system for powering an embedded wireless sensor network without batteries. A composite material specimen was laminated with unidirectional aligned piezoelectric fibers (PZT5A, 250 um, overall 13x10x.38 mm). The fibers were embedded within a resin matrix for damage tolerance (Advanced Cerametrics, Lambertville, NJ). A foil strain gauge (Micro-Measurements, Raleigh, NC) was bonded to the piezoelectric fiber and shunt calibrated. The specimen was loaded in three point cyclic bending (75 to 300 με peak) using an electrodynamic actuator operating at 60,120, and 180 Hz. Strain energy was stored by rectifying piezoelectric fiber output into a capacitor bank. When the capacitor voltage reached a preset threshold, charge was transferred to an integrated, embeddable wireless sensor node (StrainLink, MicroStrain, Inc., Williston, VT). Nodes include: 16 bit A/D converter w/programmable gain and filter, 5 single ended or 3 differential sensor inputs, microcontroller w/16 bit address, on-board EEPROM, and 418 MHz FSK RF transmitter. Transmission range was 1/3 mile (LOS, 1/4 wavelength antennas, 12 milliamps at +3 VDC). The RF receiver included EEPROM, XML output, and Ethernet connectivity. Received data from network nodes are parsed according to their individual addresses. The times required to accumulate sufficient charge to accomplish data transmission was evaluated. For peak strains of 150 με, the time to transmit was 30 to 160 seconds (for 180 to 60 Hz tests).

Remotely powered multichannel microprocessor-based telemetry systems for smart implantable devices and smart structures

The development of improved implantable devices and materials require knowledge of their in vivo behavior. However, little is known about the actual loads borne by implanted devices in vivo. Direct load measurement would provide extremely valuable information, for the improvement of device designs, and for the rapid rehabilitation of individuals in which devices have been implanted. Multichannel telemetry systems, combined with strain gauges, can provide this information.

Frequency Agile Wireless Sensor Networks

Our goal was to demonstrate a wireless communications system capable of simultaneous, high speed data communications from a variety of sensors. We have previously reported on the design and application of 2 KHz data logging transceiver nodes, however, only one node may stream data at a time, since all nodes on the network use the same communications frequency. To overcome these limitations, second generation data logging transceivers were developed with software programmable radio frequency (RF) communications. Each node contains on-board memory (2 Mbytes), sensor excitation, instrumentation amplifiers with programmable gains & offsets, multiplexer, 16 bit A/D converter, microcontroller, and frequency agile, bi-directional, frequency shift keyed (FSK) RF serial data link. These systems are capable of continuous data transmission from 26 distinct nodes (902-928 MHz band, 75 kbaud). The system was demonstrated in a compelling structural monitoring application. The National Parks Service requested a means for continual monitoring and recording of sensor data from the Liberty Bell during a move to a new location (Philadelphia, October 2003). Three distinct, frequency agile, wireless sensing nodes were used to detect visible crack shear/opening micromotions, triaxial accelerations, and hairline crack tip strains. The wireless sensors proved to be useful in protecting the Liberty Bell.

Telemetered sensors for dynamic activity and structural performance monitoring

The development of improved structures requires knowledge of their dynamic behavior. Minimally intrusive wireless systems, capable of monitoring vibration and impact, are needed in order to provide this knowledge. Our objective was to design, build, and test a high speed data collection and wireless data communications system, including microsensors, and capable of being embedded or externally worn. Our previous transmitter designs were small and could be used to transmit multichannel digital data, but they were not capable of fast data transmission rates. The addition of a remotely triggered datalogger allowed us to overcome the limitations of our earlier designs. A bi-directional RF communications link was used to trigger a sample to be logged (from 30 meters), as well as to request data to be transmitted to the host PC for data acquisition/analysis. Sweep rates of 2000 Hz were successfully demonstrated from a triad of MEMs accelerometers. The remote datalogger and transceiver and accelerometer package measured 12 mm by 24 mm by 6 mm thick; these were mounted to the feet of thoroughbred horses to study their impact levels. These small, fast, wireless data recording systems can be used to monitor rotating/ vibrating machinery and civil/automotive/aerospace structures.

Scaleable, wireless web-enabled sensor networks

Our goal was to develop a long life, low cost, scalable wireless sensing network, which collects and distributes data from a wide variety of sensors over the internet. Time division multiple access was employed with RF transmitter nodes (each w/unique16 bit address) to communicate digital data to a single receiver (range 1/3 mile). One thousand five channel nodes can communicate to one receiver (30 minute update). Current draw (sleep) is 20 microamps, allowing 5 year battery life w/one 3.6 volt Li-Ion AA size battery. The network nodes include sensor excitation (AC or DC), multiplexer, instrumentation amplifier, 16 bit A/D converter, microprocessor, and RF link. They are compatible with thermocouples, strain gauges, load/torque transducers, inductive/capacitive sensors. The receiver (418 MHz) includes a single board computer (SBC) with Ethernet capability, internet file transfer protocols (XML/HTML), and data storage. The receiver detects data from specific nodes, performs error checking, records the data. The web server interrogates the SBC (from Microsofts Internet Explorer or Netscapes Navigator) to distribute data. This system can collect data from thousands of remote sensors on a smart structure, and be shared by an unlimited number of users.

Microminiature temperature-compensated magnetoelastic strain gauge

Our objective was to demonstrate a microminiature magnetoelastic strain gauge that provides both strain and temperature signals without additional sensors. Iron based magnetoelastic materials were embedded within superelastic nickel/titanium (NiTi) tubing. NiTi stress was transferred to the ferrite, causing a permeability change sensed by a tiny coil. The coil/bridge was excited (70 KHz AC), synchronously demodulated, and amplified to produce a voltage output proportional to coil/ferrite impedance. A DC voltage was also applied and separately conditioned to provide an output proportional to coil resistance; this signal was used to provide thermal compensation. Controlled strains were applied and 6 Hz cyclic outputs recorded simultaneously from the magnetoelastic strain gauge and conventional foil strain gauges. The magnetoelastic strain gauge tracked the foil gauge with minimal hysteresis and good linearity over 600 microstrain; repeatability was approximately 1.5 microstrain. The magnetoelastic strain gauges gauge factor was computed from delta inductance/original inductance under static strain conditions. Temperatures of 25-140 deg C resulted in an uncompensated shift of 15 microstrain/deg C, and compensated shift of 1.0 microstrain/deg C. A sensitive micro-magnetoelastic strain gauge was demonstrated using the same sensor to detect stress and temperature with no moving parts, high gauge factor, and good thermal stability.