[Part 5] Low Precision Fabrication

Hardware fabrication is an integral part of any mechatronics project. Unfortunately, due to the time constraints (I'll be graduating, therefore I will lose my access to the campus machine shop in three weeks), I have to work fast. There won't be any luxury of trying out a prototype of any sort. Be that as it may, I still had to set myself enough time to minimize errors from hasty work. A very rough schedule I followed was this.

1st Week : High tolerance fabrication
1. Make the base plate and structural members for the navigation pole.
2. Cut the navigation pole and make appropriate features for handle grips and electronics.
3. Complete the battery pack assemblies.

2nd Week : Low tolerance fabrication
1. Modify the Aluminum casing to fit my 4-disc tumbler, potentiometers, a display.
2. Make shear pin slots at the navigation pole. 
3. Groove out some parts for the purpose of embedding electric wires.


I made my very first cuts on my 1'x2' Aluminum plate. You can check the CAD design from my very first post on UPC in this blog. I didn't have access to a fancy water-jet cutter like the MIT Segway teams did. So as far as cutting goes, everything was a brute-force saw cutting. I have a decent skill with saws so getting a straight cut is no problem. But no matter how straight I cut, I could never get a smooth edge like EDM  or Waterjet would give. So I had to belt sand every cut surface to obtain a desired surface finish.


 This picture shows three cut-surfaces which I cut and then belt-sanded. No longer jagged edge by saw blade here. This angular edge is simply for aesthetics.


Now that I completed shaping the plate, it's time to make a slot for the navigation pole. The pole is simply a 0.75''x1.5'' rectangular pipe. The pipe geometry is chosen so that all the wires connecting controls and display can be embedded inside the navigation pole. In the picture, I'm using a 0.75'' mill bit to make a 0.5'' length slot in the middle of plate. 


This is a structural support (triangles) of the navigation pole. I designed my navigation pole in such a way that the it is stiff and not allowed to move or tilt. This is different from the commercial Segways where a user must tilt its pole to give it a directional navigation. Navigation on my UPC will be done by a potentiometer knob which will be placed next to the display module. Note that all the cuts were made using a saw and surface-finished with a belt sander later.


The structural triangles for the navigation pole will be fixed to the plate using six 8-32 screws (three for each). About ten threads from each screw get engaged with the plate so this will stand up the torque exerted by the navigation pole. This is important if one thinks that the pole length is almost 4 ft. Any slight force at the handle will translate into a significant amount of torque at the base. The long rectangular pipe is to house battery packs and charging interface. It will also mount back lights (red LEDs). 


I made eight linear slots in which docking screws from the battery packs will be engaged (two slots for each pack). The large slot in the middle will be used to gather all the battery wires and charging wirings. On the other side (not shown), there are two holes, each for a battery charge indicator light and a standard three prong female charging cable connector. This is identical to the generic desktop power connectors. Since the battery is going to be inside of this pipe, it also provides protection to the batteries from potential external shocks.  One important thing to mention here is that the battery pack works as a structural member as well by preventing the base plate from bending. 


This shows how each battery pack is engaged to the slot. I'm using four 4-40 screws. I had to cut away a whole section of a side so that the battery pack can sit nicely inside the pipe. This will not compromise the structure stiffness of the pipe according to a quick FEM using NX 7.5. Since I bought plastic battery casings for AA cells, making a battery pack was really easy. I just had to fill the batteries in, solder 14 AWG wires at each terminal and electrically insulate the whole pack with duct tape.

Put a wire on the terminal. Solder the connection and make sure that the solder spread out evenly through out the wire. I'm using a standard DC color code scheme (red for positive, black for negative).

Once soldering is done, put a piece of shrink wrap to protect the bare wires.

After shrink wrapping, make sure that a solid electrical contact has been established with a multimeter.


Over time, the wires vibrate and this may cause the terminal leads to fail. To prevent this, I had to fix the wires  to the pack using cable ties. This will also alleviate extra tension through the wire.


This is a finished battery pack (which houses small battery packs). I'm using four packs so 40 AAs batteries. Therefore this guy is rated at 12V 10Ah. This provides a pretty good energy to mass (or volume) ratio compared to other alternatives. Only two packs get protected by sitting inside of the pipe. The other two packs exposed will be covered (therefore protected) by other sturdier members, such as motors and gear boxes which you will see later.

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