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Step 3


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Building North Paw V1.5

Step 3: Final Assembly

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In this, the final step, you’ll stuff the motor array into the armature, put the electronics into the acrylic enclosure, position the controller and battery onto the exterior of the armature, and hook them all on up together.

Supplies (incl. in kit / made by you already)

  • Completed North Paw V1.5 board
  • Laser cut acrylic enclosure (4 pieces)
  • LiPo Battery or 3xAAA battery enclosure
  • Pager motor array on Veltex backing
  • Ultra-chic velvet North Paw anklet

Tools (not incl.)

  • Pliers
  • A second pair of pliers (highly recommended)
  • — or —
  • A vise (insted of the two pairs of pliers)
  • 3 x AAA batteries (if you have 3xAAA holder)

We’re going to start by putting the electronics into the acrylic enclosure. First, make sure that all of the cut-out bits are actually out. It’s very difficult to see if each piece is out, the only way to be sure is to actually poke something (for instance your finger nail) through each cut out hole. There are ten holes in total, 2 large ones in the small piece and 4 small holes in each of the larger pieces. If you don’t get them all out, it can be very difficult to assemble the pieces later (because the “holes” are not holes!) so it’s worth getting it right now.
Put together two of the pieces and mount the board into them as shown. This step is fairly easy.
Now finish the enclosure! It seems like this step should be easy as well, but my experience is that it’s sometimes difficult to get everything all aligned at once. Starting from the photo above, grab the smaller piece. Lift the board up a so it’s at an angle above the bottom piece, and insert the board through the hole in the small piece. Now lower the smaller piece with the board in it into the small holes in the bottom piece. Finally, grab the top piece and line all the lugs up, it should snap closed. Sometimes it takes a little bit of force – but my experience is that usually if it’s not fitting it’s because some of the lugs are not lined up. Just keep moving things around, making sure the board is through it’s holes – the most common failure mode is for the board to fall out of one of it’s holes, and then the little pieces will make a V instead of being at right angles, which will make it impossible to put the top piece on. Anyway, it can be done.
Use the clear packing tape (or scotch tape as a poor substitute) to hold the battery wires in place, otherwise they are bit long and will catch. ALSO, not pictured, I highly recommend putting some tape around the acrylic enclosure, to hold it together. The strength of the little snaps varies widely, and the probability that one of the four corners in your enclosure has two weak snaps is fairly high.
The larger of the velcro pieces goes on the back side of the electronics. Peal off the backing and stick it in the middle.
The smaller of the pieces goes on the back of the LiPo battery, or equivalently on the back of the 3xAAA battery enclosure. If you have the 3xAAA enclosure, make sure you put the velcro on the side that doesn’t open, otherwise you won’t be able to get at your batteries to charge/replace them!
Now prep the anklet and the strip of pager motors.
Take note of the position of the slit cut into the armature. Unzip the anklet and insert the pager motor array. The thickest part of the ribbon cable should be towards the end of the anklet with the slit. Make sure the pager motors face the soft velvet fabric – that’s the side of the anklet that is held against your skin and we want the motors right on you! As the zipper is slightly shorter than the whole anklet, you may need to pinch from the outside and wiggle the last motor up to the edge.
Once the motor array is snug, fold over the left-over length of ribbon cable and pull it through the slit. Now affix the controller to the anklet such that the edge is 0.5 inches from the slit.

This part is a important and seems to be a little tricky to explain, so here goes: the controller is fairly stupid. All it know is what the compass chip tells it, it doesn’t know anything about where the motors are relative to itself. So, if it thinks it’s pointing North, it will turn on motor number 1, regardless of where motor number one is. If it think it’s pointing South, it will turn on number 5, and so on. The orientation of the motors and the controller relative to each other is vital, as you’ll see later on during the calibration phase.

Place the crimp with the notch up and move it to the best position for it plug into the controller. If you look closely you can see the teeth that will be grabbing onto the strands of the ribbon cable and making electrical connections. Make sure these all line up with the strands properly. It’s very important to get the orientation of the ribbon cable & ribbon snap correct, as it is very difficult to remove if you put it on wrong. It should be as pictured, with the red wire so that it will be in the middle of the board, and the ribbon snap so that the cable will come up from below and the plastic notch is up.
The easiest way to crimp is to hold both sides with pairs of needle nose pliers and squeeze them at the same time. It may take a fair amount of force and the crimp may not snap perfectly; that’s ok. If it’s holding, it’s in well enough and you can squeeze laterally to get the ends to snap. Another way to do this is to set the crimp and cable just right in a vise and turn it shut. When you’re done, snip the extra ribbon cable. Also, if you break off one or both of the plastic lugs, do not despair. My experience is that the ribbon snap will hold closed without them, just keep pressing the halves together. You will however have to be more careful when trying to remove the ribbon snap from the motor jack.
Plug the ribbon snap into the controller, and plug the battery into the other side. Congrats, you should be done building! Slowly turn the North Paw in a circle, and make sure that all eight motors are connected. Put it on and see how it feels!
Put it on and see how it feels!
The final step in this process is the calibration phase. Grab a normal compass, or go somewhere that you know exactly where North is. Put your North Paw on, and turn around slowly. You’ll notice that the motors will sometimes point in different directions. This is because in step 3 you spaced the motors out evenly, but actually this would only work if your ankle was perfectly cylindrical. It’s not, so you now need to move the motors around until they all point to the same North. It’s best to do this one motor at a time. Find a motor that points the same way as your compass does, and then turn until your North Paw changes to another motor. Feel and remember the amount at which that motor is pointing away from North. Take off your North Paw, unzip the motors, and move that motor over by the amount you felt. Put it back on, and repeat, this step will only work after a bit of trial and error. While you do this, be careful not to bend you solder joints too much as they can break.

Looking at the diagram to the left might give you a better spatial understanding of why the motors need to be spaced differently depending on each individual ankle’s unique shape. Note the length of the arcs between the yellow dots, that’s the space that should be between the motors those dots represent. You should also consider that spacing the motors in this way means that if you change ankles, or wear the North Paw turned from how you calibrated it, then it will no longer be perfectly calibrated. Keep that in mind!

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North Paw V1.5, for help contact eric@sensebridge.net

First published May, 2010


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