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Read Rope / Electronic Nerve

This is an open-source hardware project for a device that outputs a certain voltage depending on where the rope/wire is bent.

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This is an open-source hardware project for a device that outputs a certain voltage depending on where the rope/wire is bent. https://github.com/adct-the-experimenter/read-rope

Hackaday Design Contest:
This project addresses 2 challenges.

One challenge it addresses is providing a good modular control interface for differently disabled people. With this concept, very small bends in lightweight wire caused by movement of fingers, arms, or legs can be turned into useful changing input for an electronic device to use.

Another challenge that this can address is transferring physical phenomenon from the real world into virtual reality, augmented reality, or mixed reality. Movement of fingers, arms, or legs can be turned into an electrical signal that is turned into digital information.

How It Works:

A wire segment is in parallel with a resistor which shorts the resistor. If there is a bend in the wire, then the wire segment shorting the resistor is broken which makes the resistor part of the circuit and in series with another resistor connected to the voltage source. The resistor connected to voltage source is called Rtrack.

Multiple other resistors are connected in parallel to a wire segment whose connection is broken with a bend. The sum of these resistors is referred to as Req. In the read-rope-configs-series.py simulation, Req is added in series. This is the best configuration because it gives output voltage values very different and far from each other. In the read-rope-configs-parallel.py simulation, Req is added in parallel. This is the not a good configuration because it gives output voltage values very similar and close to each other.

The resistor is always connected to the wire and the wire segment shorting the resistor is what is switched. Think of the wire segment as being connected to a SPST switch that is normally closed. The physical bend is like opening the connection of the switch with a press.

See the diagrams/schematics/figures for read rope here.

read-rope-sim-results.png

Results of python simulation.

Portable Network Graphics (PNG) - 56.82 kB - 08/08/2019 at 23:42

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Adobe Portable Document Format - 523.60 kB - 07/23/2019 at 03:31

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Portable Network Graphics (PNG) - 16.28 kB - 07/14/2019 at 01:17

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  • 1 × Wire
  • 2 × Resistor Any resistor. At least 2 for 1 bend. Amount of resistors is number of bends plus 1 for project.
  • 1 × Normally-closed SPST pushbutton switch

  • Viablity of Through-hole Prototype vs Surface-Mount Flexible PCB Prototype

    Pablo Antonio Camacho Jr.08/05/2019 at 23:45 0 comments

    I tried to solder together through-hole resistors with surface-mount SPST NC push-button switches.

    It failed. I managed to get all the resistors and switches soldered together, but the solder joints broke with little bit of force from pushing the read rope into the vinyl tubing.

    This experience showed me that read rope will require an expensive prototype of a flexible pcb with only surface-mount components to accomplish the objective of creation of a thin device that outputs voltage depending on location of lightweight bends.

    A through-hole prototype for this is not viable because small through-hole switches are relatively big and have a higher actuator force requirement than surface-mount SPST NC switches. This would not work well with the requirements of the read rope of being small and requiring little force from a bend to activate the switch.

    A through-hole prototype for read rope is also not viable because the through-hole resistors are relatively big for the intended application and bring more noise than surface-mount chip resistors.

  • Considerations for Switch Components and Tubing

    Pablo Antonio Camacho Jr.07/30/2019 at 21:01 0 comments

    I managed to solder together SMT tactile switches in parallel with resistors.

    Unfortunately, a solder joint breaks when trying to push the read rope into 0.170 inch inside diameter vinyl tubing.

    From this experience, I have learned more considerations to make to turn this into a final product.

    Switch

    • The actuator force, force applied to switch to change its state(Closed to Open), needs to be light enough so that bending the tubing at the location of the switch will push the button.
    • The switch needs to be small enough to have a resistor soldered underneath or next to it and still fit in tubing. Unfortunately, this leaves only SMT tactile switches available.

    Tubing

    • The tubing that will contain the read rope wires and switches and resistors will need to be very flexible. It also needs to contain a long wire segment that leads back to ground.
    • It also must be thick enough to push the tactile switch button if bent at a location where there is a switch.
    • Vinyl works well enough; however, more appropriate options for material need to be explored.


    Flexible PCB

    • Since SMT tactile switches need to be used, flexible printed circuit board will need to be considered for the version of the product that will be manufactured.
    • The flexible PCB will need to be thin enough to fit into tubing.
    • Fortunately, using SMT tactile switches and chip resistors will make read rope using flexible pcb to be very small due to size of the chip resistors and SMT tactile switches. 

  • Considerations for Precision of Input and Output

    Pablo Antonio Camacho Jr.07/23/2019 at 02:36 0 comments

    One important consideration to take when developing read rope into a product is the precision of the output voltage as determined by precision of input.

    1. Precision of Output Voltage. How well can output voltage be read.
      1. The resistors used will determine the ratio of resistance between output terminal and ground to total resistance in the circuit which determines the output voltage.
        1. The tolerance of these resistors needs to be considered for when output voltage will be fed into an analog-to-digital converter which expects a precise value for a certain bend. 1% tolerance in resistors is ideal.
        2. The difference between resistors will determine the difference between output voltages at certain bends. Ideally the resistors should be on the same order to avoid 1 bend at lower end not adding noticeable amount to output voltage.
    2. Precision of Input. Where the bends can be made.
      1. Ideally for a very precise small read rope, the resistors will need to be placed very close to each other for bends to be interpreted that are very close to each other.
      2. Size of SPST switch and resistors will determine placement of resistors and size of bends.
      3. Ideally, for a large rope where only a few bends need to be read, resistors can be placed far apart from each other and connected through long wire. 

  • How to Develop the Project Further - Different Directional Bends

    Pablo Antonio Camacho Jr.07/23/2019 at 02:19 0 comments

    I realized that the project will make a rope that can only register multiple bends in one direction ( down ) with 1 output.

    However, this is a good thing since it is one thing it can do well and its simplicity allows it to be used in a modular way.

    For example, multiple read ropes in different orientations - different down directions for each rope - can be joined together to form a rope that can register multiple bends in multiple different directions. 

    This rope will have an output for each direction since it is multiple 1 output read ropes joined together.

  • Challenge of Finding Small NC SPST pushbutton switches

    Pablo Antonio Camacho Jr.07/23/2019 at 02:09 0 comments

    One challenge that will need to be addressed when developing this project into a product will be that finding appropriate pushbuttons.

    Requirement for Push Button

    • The push button needs to be able to small enough to fit inside small tubing that has a diameter of less than 0.25 inches. This is so that it can be portable and easily bendable.
    • It needs to be normally-closed SPST switch of which there are few options available. It must be this type in order for the resistor to get shorted which is how the project works.
    • It must have an acceptable life of at least 100k cycles for a durable consumer product.

  • 4 Bend Prototype Result and Feedback

    Pablo Antonio Camacho Jr.07/21/2019 at 23:49 0 comments

    I made a 4-bend prototype with resistors 3 Mega, 2 Mega,  1 Mega, 750k ohms. 

    I put the wire and connected resistors in vinyl tubing of 1/4 inch outside diameter , 0.170 inch inside diameter.

    It worked!

    Unfortunately, it did not work in a consistent manner because the wire connections would also bend and get out of place.

    The wire connection is not reliable; therefore, I will use very small normally-closed momentary push button switches since those achieve the same function as the 2 wires touching.

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