A slight contact of a conductive paper over an open interrupter of a power-led circuit is enough to let it act as a resistor and substantially close the circuit and leave a little current flow towards the led.
The current flow will then vary if you move anything and the larger and stronger is the contact, the more current will flow trough the circuit.
We review here various possible solutions to generate solid and stable connections between our papers and a conductor.
Connection Techniques
Nude Wires or conductive yarns and adhesives and/or staples
Copper Tape Adhesives and soldering or custom connectors
Printed conductive inks and custom connectors
Conductive glues or pastes
Custom Connectors and mechanical clamping
Connexion Design
Influence of the resistive sample size
When a resistive material has a Votage sent through 2 points over the material surface, we can consider that the current will flow all along the material through any possible path from the highest to the lowest electrical potential.
This leads to a nice looking drawing with current flowing along ellipsoid path so as to cover the whole space and eve start backwards if the siz of the sample is sufficient.
Influence of the connectors contact size
Let us now consider a band of resistive paper sized W * L and thickness e.
Let us start using 2 connectors band along each sides of the length L and distoant of L.
It is known that in this case, if the paper has a uniform resistivity rô (in Ohm.m) then the Resistance measured between the 2 connectors is:
R = rô * L / ( e * W )
with em W and L in meter.
Thus if you double the length, you double the Resistance, and if you double de Width (or the thickness), you will divide the resistance by 2.
Testing the theory with PLK-Papers
We produced pads of various width and lengths to check the validity of the equation mentionned above. We also tested various connection techniques, but each time the connection takes the whole width of the sample.
Minimalist connectors with Copper Tape adhesive
As connectors are made of metal, it is good to minimise their size. However the quality of the contact between the paper and the connector must remain sufficiant and stable.
Each of the connection techniques mentioned above provides a different connection quality inducing various resistance loss orders. When using Copper Tape adhesive we searched for the minimal size to tape to provide sufficiant contact quality and durability.
Testing with PLK-Papers
We reproduced the same testing as mentioned above, but this time the connectors are smaller than the band's width but always centered along the band's with.
This time the theoretical equation is less in accordance the the results, especially with sensors or smaller length because the electric filed lines are extended when compared to the previous and theoreticaldesign.
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The previous considerations directly shows how the location and size of the connectors influence the resistance output of a component design. Let us apply this fact to a few sensor design:
Influence of the sensor type
Force Sensing Resistors and Flexion sensors
Position sensors and Arrays
Influence of the sensor design and connectors location
Force Sensing Resistors
Flexion sensors
1D Position sensors
Force sensors Arrays