A lot has happened in the past week or so. We've gone from our first printed part, to two iterations of V0.2 (V0.2 and V0.21) of our working draw string mechanism/encoder.

V0.2

GIF of mechanism

V0.21 is an improvement in clearances so the 3d print needs less hand mods, and an improvement to the spindle mechanism. We also changed the spring from a 12 foot tape measure to a 6 foot tape measure to reduce resistance on the bar. V0.21 was printed but not shown.

We're also in the midst of V0.3, which will be laser-cuttable for much faster prototype iteration (~5 hour print to ~ 3 minute laser cut). V0.3 will be very helpful in getting as many units out there as possible to OpenBarbell's various contributors.

Now that the encoder is up and running, we've been able to plug it into an Arduino and take some data. The following data points were grabbed from the serial monitor at 57600 baud. We didn't even hook it up to the interrupt pins on the board, just straight digital in, so we have plenty of optimization to do before we hope to read bar velocity.

This may not look like much, but it's some of the first bits read from the OpenBarbell project. You can see the string being pulled out, slowing down, reversing direction, and retracting back into the spindle a total of three times. This data will change a lot, as currently we're polling pretty quickly, but in the future we will be getting about half a million to a million looks at the encoder every second, giving us the ability to track both the rising and falling edge of the encoder effectively doubling our resolution.

Here is the customary optimistic goals for the upcoming week:

• Improve encoder code
• Laser cut V0.3
• Measure reasonably accurate encoder displacement
• Spit out a velocity measurement

Number 4 above will the trickiest of them all, because in order to know if we're even close to an accurate velocity, we'll need something to compare it to. We know of one Tendo unit around here, so we may need to pay it a visit.