Tuesday, August 30, 2016

Hopping Robot: How I made one

I found this very interesting video, and I will give the author +sauravchakra all the credit for the idea and the demonstration.

However, I would not call this a tutorial--it's a 4+ minute demo of the operation some information on the process. So, I decided to take it to he next step. First, here's all of what I used in the project. Others may add their own ideas, but this is a start, and more than was in the video.

See notes under "Step 5 -Hop!" for planned improvements.


aluminum: 2 pieces 3cmX1cm (I used scrap from a discarded light fixture)  Author says to drill a 2mm hole 5mm from edge. Since I used bigger screws, I used a 1/8" drill bit (~3mm)
batteries:  3 AAA
battery holder: 3XAAA; see Radio Shack (See note under step 5)
carriage bolt: 1 2" bolt--got it a local hardware store
DC Gear Motor: Only spec is 100RPM. It's a 90 degree shaft,
and we can intuit 4.5V from the power used.  See Jameco
I found one on Amazon for less but shipping takes over a month.
metal rod: 1" long, diameter small enought to fit the wheel, below.
I used parts from a printer tear-down.
plastic: you'll need to cut a piece 5" X 1". I had these around from Amazon.
plastic wheel : 1, small enough to fit in plastic and be held by metal rod above; I used parts from printer teardown, author used a bead
PVC pipe:  need 1 piece 1/2" wide.  Tutorial says 7 cm pipe = ~2.75in--I used 3.5in (outside dimension), from scrap.  Find it at a local hardware store or Home Depot
screws/nuts: author calls for tiny metric screws (like M1.5). I tried that, and my fingers are to fat and clumsy to get that done. I used some 1/2" 4-40 machine screws and nuts that I had bought from a local hardware store
slide switch  (SPDT):  This could be SPST, but this is what author used and also what I had, from Adafruit

Tools and Supplies

Coping Saw: to cut plastic, bought at local hardware store
Files: for deburring metal and plastic--I have this set from Harbor Freight
Hack Saw: for cutting metal and PVC, sourced locally
Heat Gun: to soften plastic for bending and for heat shrink;  inexpensive at Harbor Freight
Heat Shrink Tubing: to insulate soldered wires (batteries/motor/switch), from Radio Shack, Adafruit, et al.
Hot Glue Gun: to attach various components; Home Depot has this one
Snips: to trim metal and plastic--sourced locally
Soldering Tools:solder battery/motor/switch connections, from Radio Shack, Adafruit, et al.
Vise: I make great use of my bench vise like this Craftsman
Wire Strippers: for cutting and stripping the motor and battery wires to prepare for soldering--sourced locally


The photo above shows the parts ready for assembly.  Clockwise from upper left:
  1. Slide switch
  2. 2" carriage bolt
  3. DC Gear Motor with aluminum pieces attached to rotating arms
  4. 2 halves of a 1/2" slice of PVC pipe, for the legs
  5. battery holder
  6. piece of ABS plastic, cut to size, bent at the 1/4 with indentation for the wheel, 1/16" holes for the switch leads, and a 1/8" hole for the battery wires
  7. I left the rod and wheel out of the photo. See Step 2.
The process

Step 1 - fabricate the parts
The PVC pipe, the plastic, and the aluminum pieces all needed to be cut out.  
PVC: I found a piece of 3.5" PVC pipe, cut off a half inch slice, then cut that in half
Plastic:  a had some sheets of 1/8" thick ABS, and in fact had plenty of scrap from other projects, so I took a small piece and cut it to size with a coping saw. Then I measured one quarter of the length, drew a line, hit that area with the heat gun, then put it in the vise and bent it to a 90 degree angle. Then I cut out about a 3/4" section, 3/8" high out of the bottom of the bent section.  That's where the wheel will go. I needed a hole for the battery wires and 3 hole for the switch pins, so I drilled those.
Aluminum: I had replaced a failed under-counter fluorescent light fixture and kept the old housing for scrap.  I cut out two 3cm by 1cm pieces (really about 1.25" X .5" ). I tried the small M1.5 screws but my fingers just would not cooperate, so instead of a 2cm hole I drilled a 1/8" hole to accommodate 4-40 machine screws.

legs hot glued to aluminum
Step 2 - attach the legs
one leg with hot glue treads
  1. Drill 1/8" holes in the rotating arms of the motor (make sure you put them on the same side so the metal pieces are oriented in the same direction--i.e., not one horizontal and the other vertical). My drill doesn't do metal too well, so I used a hammer and nail to complete the hole
  2. Fasten the aluminum pieces to the arms with 3/8" 4-40 screws and nuts
  3. Hot glue the PVC pieces, with the convex side facing the rear (opposite from the rotating end).
  4. Add lines of hot glue about every 1/2" along the outside of the PVC for traction.
Step3 - populate the frame
rod, wheel, and slot in frame
wheel hot glued to frame

  1. Put the rod through the wheel and hot glue it to the open area in the frame (making sure that the wheel clears the bottom of the frame and turns freely)
  2. Hot glue the switch to the frame, with the pins through the 3 holes
  3. Hot glue the battery holder to the frame and thread the wires through the 1/8" hole
  4. Hot glue the motor+legs to the frame--check the photos to be sure everything is facing in the right direction
Step4 - wire it up
  1. First connect the red wires and black wires from the motor and batteries to each other.  The legs should turn clockwise. If not, reverse the wires.
  2. With the polarity verified, start trimming and soldering.
  3. After wiring and testing, hot glue the 2" carriage bolt in place for stability

Step 5 - Hop!

It's done and it works, but I need to make some improvements.

  1. I couldn't find my hot glue gun, so I borrowed a low-temp one from my wife.  I'm blaming that for the fact that the legs come off at times, so I'm going to find mine or by a real one, and re-fasten the legs
  2. The open battery holder does not secure the batteries and the tend to fall out.  I have enclosed ones on order from eBay, but ti will take a while
  3. The PVC may be too heavy, because the legs what to stay still while the motor spins.  It's still amusing, but not as designed.  I may try thinner legs (e,g., 3/8" slices of PVC instead of 1/2")