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A project log for Choose your own adventure bot

Ultra low cost 3D printed Open Educational Resource Walking Robotic Platform

shane.snipeshane.snipe 09/16/2021 at 11:260 Comments

I want robotics to be accessible.  I want to to be able to hold a make and take class at our makerspace and have the class be able to assemble the robot in an hour.  Well this week I had to admit the present design was not going to get there. 

I worked hard on soldering up the PCBs to the flexes and if the wind was blowing the right way, I might be able to do it. More often then not, I tore a pad of the flex and I had to start all over on that leg.  Or I had a trace break when I was folding the flex and now one of the encoders could not get signal.  I figured it could have been improved through better and better flex design but this was hitting me in two places. 1st, the flex circuits were expensive  and 2nd they were hard to assemble. It was really going against the key tenet of the project which was accessibility.  The final straw came when Doug worked really hard on assembling a bot and despite having everything work as a flex, once he screwed the 3D printed parts on and connected it to the body, he broke a bunch of the pads between the flex and the surface mount header and all that work that ended the parallel path of development.  If the founder of a makerspace can not put my robot together, I know it is time for a redesign!

So we were considering options and he mentioned a ribbon cable. The connectors for ribbon cables are too tall for this project but there are some very low profile FFC connectors! So I was off to redesign the robot to use FFC instead of custom flex circuits.  They are available in any length on Digikey and super cheap on Ali express so that checked the accessibility box. 

If that is all hard to imagine, here are some pictures to help.

Here is the board that goes on the hip. It has a 14 pin header that connects to the main board and then two 10 pin ffc connectors the head off to the femur board and the tibula board. 

I turned the header 90 degrees to make room for the FFC connectors. I am not sure if  I have the angles right on the connectors but I only made 10 sets this time and I will revisit it after I have put some together.

The headers at the top are just for debugging. The through holes J1 and J2 at the bottom are for motor wires. The double row of headers connect to the head board. There is a rectangular hole in the middle of the board to allow the board to sit better on the motor and accommodate the hook that holds the motor strap.  It still will sit proud by the thickness of the motor strap but I think it will be acceptable. Otherwise I will make a 3D printed spacer again.

For the femur and tibula boards,  I was able to make them common with each other.  Both have the ability to connect to the battery and either battery connection will run to the main board through the FFCs and the 14 pin header. Here is the smaller board layout. 

Ignore the headers as they will not be populated. I still have not figured out how to put a connected plated through hole without associating it with a header. If anyone knows, please leave me a note in the comments.

The battery will connect to J3 and J4. Which brings be to the battery management redesign. I really wanted the robot to be able to pop up but even with the 5V boost circuit, the motors did not have enough power to stand up from a splits position. What's the solution? - MORE POWER! - I considered putting back the battery boards I started with that had a adjustable voltage gain but it felt kludgy and I was not sure there would be enough current capacity. So I decide to connect the batteries in serial through a BMS on the head board. This takes the battery management out of the legs and gives me 7.2V.  It also allows the motors to be charged with one  9V adapter that plugs into the main board.  

Tomorrow I will describe the main board and all the changes but last night I ordered new head boards along with populated hip and femur/tibula board. By next Friday I should be cooking with gas! In the meantime I will do some documentation and touch up the mechanics.   

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