What is a Fabric Antenna Sensor?
Note: This is a past research project of mine, and has been continued in the ExCITe Center after I left for UC Berkeley. For an updated account of the current progress, please check the ExCITe Center description.
The Brief Explanation:Fabric antenna sensors are a new creation that we have made in the ExCITe Center to measure deformation. Silver-coated yarn is knitted into an antenna pattern using Shima Seiki industrial knitting machines. When the knitted antenna is stretched, the change in size, shape, and tension in the fabric cause changes in antenna properties, namely a change in its central frequency. Therefore this technology can be applied as a sensor for stretch in fabric. This has a huge number of potential applications, but the current application we are focusing on is for our Smart Fabric Bellyband: a band of fabric worn around the stomach of a pregnant woman to comfortably measure uterine contractions.
The Thorough Explanation:
Knitting is a process of using one or more long strands of yarn to create a series of loops resulting in a fabric. Because of its loop structure (compared to say weaving, which is simply a large number of perpendicular threads overlaying each other), knitting can be used to create interesting structural and visual designs. At Drexel University's ExCITe Center we have several industrial knitting machines donated from Shima Seiki, which act as essentially 3d printers for fabrics. We can create a design using Shima Seiki CAD software, export this program to the machines, and within minutes have a knitted fabric with our intended pattern.
Knitting Silver-Coated Yarn
Silver is a metal and is therefore electrically conductive. We use woolen yarn that has been coated with silver as our antenna material.
Knitting Silver-Coated Yarn into Fabric Antennas
Using wool for nonconductive materials and the silver-coated yarn for conductive material, we can design and create RFID antennas.
First the antenna is configured in HFSS (High Frequency Structure Simulator) to determine its desired central frequency, radiation patterns, and return losses.
These dimensions are then configured into the Shima Seiki CAD software, and edited to determine the stitch, speed, and tension for every loop in the pattern.
The CAD program is then exported and constructed using the knitting machines. Our result is a fully-functional antenna.
Knitting Silver-Coated Yarn into Fabric Antenna Sensors
Because our medium for these antennas is fabric we have a lot of cool properties to take advantage of. Knits are very elastic and can be stretched easily (think of the cuffs on your hoodie or a pair of stockings). This stretching causes a number of changes in the antenna. First, it changes the tension in the fabric, causing the silver yarn loops to press tightly together. This increase in connection of the loops leads to a decrease in resistance across the antenna, providing us with some interesting potential applications for sensing deformation as a function of resistance change.
Additionally, the antenna size and shape changes, affecting its radiation properties. Specifically, as the antenna is stretched its central frequency shifts downwards. This gives us the potential to measure deformation wirelessly.
Why Study Fabric Antenna Sensors?
As I mentioned earlier, this flexible deformation sensor has a huge number of potential applications, particularly in wearable technology. Currently we are working towards implementing this device into a "Smart Fabric Bellyband." This band would be worn by a pregnant mother as a device to measure uterine activity and contractions. Current devices are cumbersome, tethered, restrict moment, and are uncomfortable to wear for long periods of time.
We want to replace these devices with our Bellyband. The fabric antenna sensors will pick up deformation changes due to contractions and can be monitored cheaply, wirelessly, and more accurately than the strain-based technology used today. Further work will incorporate EKG sensors for monitoring fetal activity.