Use a circular knitting machine to knit a stretch sensor with regular and conductive yarns in five minutes! The values of the sensor range from roughly 2.5 Mega Ohm when relaxed, to 1 Kilo Ohm when fully stretched.
The stretch sensing is actually due to the structure of the conductive yarn which is made up of lots of short steel fibers mixed with polyester. Even without knitting the yarn into a structure you can use it as a stretch sensor by simply pulling it taught or relaxing it. But the yarn is not very strong and easy to tear. The knit structure allows you to accumulate more yarn and thus more resistance in less length and also by combining the conductive yarn with regular yarn you can gauge the sensitivity of the sensor by choosing a thicker or thinner yarn - thicker yarn gets more in the way of the conductive yarn making extra contact through the knitted loop structure. Plus knitting creates the stretchy structure giving you some natural tangible feedback.
Circular knitting machines
Circular knitting machines range from 20-250$ in price and differ in diameter, number of needles and spacing of needles. Spool or wire knitters have only four needles and resemble manual knitting dolls, while the knitting machines resemble manual knitting wheels or looms. See these posts on HOW TO GET WHAT YOU WANT for a more detailed summaries of circular knitting machines:
and circular knitting looms:
Conductive yarn is hard to come by, but hopefully soon online shops like www.plugandwear.com will offer such yarns in various thicknesses, conductivity and at a good price. The conductive yarn used in this tutorial is called Nm 50/2 and is made up of 80% Polyurethane and 20% Inox steel fiber and is manufactured by the Austrian company Schoeller (www.schoeller-wool.com) but they only produce and distribute it in industrial quantities of 300KG. See the following post for more information:
Circular knit stretch sensor post:
The following Instructable will cover the knitting of stretch sensors using conductive and regular yarn with the following models of circular and spool knitting machines:
- Play Go Knit Knit (17 Euro from Amazon Germany)
- Singer Spool Knitter (19$ from Amazon USA)
Video of the full process of knitting and testing a circular knit stretch sensor using the Play Go Knit Knit circular knitting machine (6 minutes)
Video of the full process of knitting and testing a circular knit stretch sensor using the Singer Spool Knitter (5 minutes)
Video demonstration of both wide (circular knitting machine) and narrow (spool knitter) stretch sensors