Elastic stretch - sensor

The stretch sensor can be used embedded on a danser or a performer, or integrated into a mechanism. The sensitive module is a thin elastic string that reacts to being stretched and therefor to being extended into a curve or deformed by a type of material or body.

Depending on the situation, it can behave like a bend sensor, or a short proximity sensor (for very short distances), or even as a twisting/torsion sensor. Its diverse possibilities in regard to placement on the body or in devices, make its applications very varied.

Stretch sensor characteristics

Full sensor : sensitive module + electronic card with settings

We offer two variations : one with a 10 cm sensitive elastic string, and one with a 20 cm elastic string.

The three pin white connector on this sensor lets one plug it onto an analog sensor interface such as our 8 Analog to midi" interface, "2 analog to Midi" interface, or our "Mini-HF" wireless board.

Settings

Our sensor is paired with electronics to denoise and adjust the measurement. Two potentiometers let one adapt this sensors to various uses that could require different sensitivities :

• Potentiometer 1 (closer to the sensitive module) : adjusts the rest level , or « zero level », which is the returned value when the sensitive module is flat and not stretched. This value is supposed to be set close to 0.
• Potentiometer 2 (closer to the 3 pin connector) : adjusts the sensitivity, making the sensor more or less reactive to being stretched.

Fastenings

The sensitive module ends on a little metallic hook on each side which can be sown, stuck, taped or glued easily.

Constraints

The sensitive module is fragile : one mustn't submit it to abrasive friction, and shouldn't use it against a sharp object. It's best not to stretch it further than 1 and ½ times its initial length.

When integrating it into a costume or a device, be careful to plan a limit beyond which the sensor can not be stretched (under any circumstances).

If the sensor get stretched too hard and too far, the sensitive string can get torn or break out from the hooks. If it's out of the hooks it can sometimes be repaired, but the hooks are often damaged and more fragile afterwards.

Reactiveness

The stretch sensor reacts very quickly when being stretched but needs some time to get back to its rest level as the elastic relaxes : it's reactiveness is asymmetrical.

It can be used as a bend sensor on the elbow or knees (but not the neck or the foot's instep), however it behaves differently from our standard bend sensor, which reacts no matter which direction it is bent in.

Advantages and applications

• Versatile
• Not sensitive to twisting or torsion
• Easy to attach
• Adapts to very different movements as an embedded sensor : around the waist (in a belt, around the chest or stomach to detect breathing for example), from one shoulder to another, along the rib cage or the spine, …
• It has no preferential curving axis.

Associated programming and data processing

The returned data values increase when the elastic is stretched out. When loosened, the elastic relaxes back into it's original length but, before the values decrease there is quick spike.

This Max patcher analyses the data in order to clean out this spike. It can be downloaded in the « Téléchargement » tab found towards the top of this description page.

In this example the sensor is plugged to a « 4 analog to Midi » interface board.

Spare modules

In case the sensitive module breaks, spare elastic modules are sold separately. They are made of the sensitive elastic string, the fastening hooks, the limit wire and a two way connecter to be plugged to the sensor's electronic card.

They are directly usable with the little card without any extra soldering (current electronic cards should have the appropriate 2 pin connectors for the sensor module).