Pull Sensor Research
Picture: http://files.andymark.com/am-2674_assembled.jpg
Datasheet: String Pot
##Pull Sensor Requirements
Attacking
- The Bow must recognize when the hero nocks an arrow, and how far back the hero draws the arrow
- The system should determine the intensity (the distance of the IR beam) of each attack by the strength of each draw
##Pull Sensor Summary
| Hardware | Size (in) | Supply Voltage (V) | Max Power Rating (W) | Price |
|---|---|---|---|---|
| String Potentiometer | 0.59 | Any | 2 | $40.00 |
| Optical Incremental Encoder | 1.1 | 5 | 0.13 | $28.00 |
######Option 1 - String Potentiometer
The string potentiometer is used in FRC robotics that can be built and assembled
through the manufacturer or yourself. It uses a strong nylon string connected to
the potentiometer and a coil spring to bring it back when tension is not applied.
3-D parts to house all the parts could either be ordered or printed on your own
that the files have already been created.
The primary option is the first choice because the hardware is practically built for us and
we could immediately go to testing when hooked up to a 5V source along with the
10K resistor. Even though it is a little pricy, a big issue with the E-Bow is placement
and packaging. Having a housing already built could make mounting a lot easier.
Another thing to consider with costs is the mechanics of the bow. Some other designs
may consist of having pulleys, but with this design decision, that cost is all included.
We don’t want something that will create too much tension since it is a handmade
wooden bow. It’s used in robotics with great reviews, so durability doesn’t appear
to be an option from other people. If a string does break, then another string can be
replaced by taking it apart and adding a new one.
######Option 2 - Optical Incremental Encoder
An incremental optical encoder measures how many times a photodiode peaks with
rotation. The mechanism is a disk that consists of windows for light to be detected every
time light passes through the slots. The output of the encoder is square wave every time
light is detected it outputs a digital 1 (5 VDC) and when no light is detected
a digital 0 (0 VDC). The specific model I have picked out has 8 pulses per revolution.
The incremental encoder will tell you what direction the shaft is turning and how
fast.
This option could cause an issue because encoders could have issues with
other light sources and the environment for the bow could be in other lighting conditions.
Writing code for the encoder could also be more complicated than measuring a voltage
drop. Also, with the incremental encoder it counts versus the string pot where it
measures values for all 360 degrees of rotation.
##Pull Sensor Software Flowchart
