Flex sensors are passive resistive devices that can be used to detect bending or flexing. The flex sensor shown in this article is a bi-directional flex sensor that decreases its resistance in proportion to the amount it is bent in either direction. The sensor we are building is about 3/8" wide by 5" long. You can easily make a sensor wider and longer depending upon your application:
Flex sensors may be used in robotics to determine joint movement or placement. They may also be used like whiskers for wall detection. The sensors we are making are also pressure sensitive so they can also be used as bumper switches for wall detection or pressure switches on robotic grippers. For bio-metrics, the sensor can be placed on a moving joint of athletic equipment to provide an electrical indication of movement or placement. A few of the sensors can be incorporated onto a glove to make virtual reality glove.
Step 1: Components:
The materials needed for the construction of the bi-direction flex sensor is shown in figure 1 and listed below. The size of the materials listed is only a guideline to the sensor we are constructing in this article. These types of sensors can be manufactured to larger widths and lengths.
1. Copper foil laminate 1/4" x 4.5" (see text)
2. Acetate 1/4" x 4.5" x .010 thick
3. Heat shrink tubing 3/8" dia x 5"
4. Resistive material 5/16" x 5" (see text)
Copper foil laminate is used in the electronics industry to make flexible circuits. It is thin copper cladding on a plastic material substrate like acetate. The material we are using is single sided copper. Copper on one side and the substrate (plastic) on the other, The copper cladding material is cut into two pieces 1/4" wide x 4.5" long strips. The material is easily cut with a scissors. Solder about 6" of wire to one end of each strip. You may fine it easier to solder the wire to the strip if you tin the bottom 3/8" of each strip. Solder each wire to one corner side of the strip. It doesnt matter which side you choose, just make sure you solder both strips on the same side, see figure 2.
There are a variety of resistive materials available; cloth, plastic and paper. The common element of all the appropriate materials is that the material is somewhat conductive or resistive. The degree to which the material is resistive will determine the scale at which your flex sensor operates. For the example here I am constructing here, I using conductive black plastic poly bags conductive bags used in the electrical industry. These bags are used to store components that are static sensitive. The bags are made from single layer of carbon-loaded polyethylene and its conductivity does not depend on humidity. I cut the bags into the 3/8 " wide by 5" long strips.