spacer Mechanical Design

When designing the hardware for the walker my goals were ...

  • keep it simple
  • keep it light
  • keep it rigid
  • keep it cheap.

A lot of designs I've seen use beautiful metal brackets to hold servos. When I started designing my robot I considered using something similar but the cost just kept sticking in my craw. The weight also quickly adds up if you use too much metal. In the end I went with a simpler approach. Not as flexible perhaps as I do using a bit of "hot melt glue" from place to place but nothing a sharp razor knife can't fix.

My main building material for the robot body is 1/8" thick MDF. The stuff I have (local Home Depot) weighs in at about 0.22 g/sq.cm. Altogether I've used about 400 sq.cm for a quadruped contributing about 88g to the overall weight.

Each leg consists of three servos. I call these HipZ, HipY and KneeY. In my coordinate system the Z axis is vertical, the X axis is right/left and the Y axis is towards and away from me.

Movement around the HipZ joint allows the leg to swing in the horizontal plane of the robot. The other two joints are responsible for the extension of the leg.

Hipz and Hipy servos in leg
HipZ (foreground) and the HipY servos.

Bamboo axils help to support the servos.
Bamboo pivots opposite the servo shafts help support the leg.

I started building by joining the HipZ and HipY servos with a bit of hot glue as seen in the photo above. The two servos are twisted by 90 degrees. The side of the HipY servo is coplanar with the bottom of the HipZ servo which allows me to hot glue a piece of MDF across the two servos for additional support. This piece of MDF has a hole cut in it that is coaxial with the shaft of the HipZ servo. A short piece of a bamboo skewer is glued into the hole. These two pivots will be used to mount the leg to the robot body.

Mounted leg seen from side
Servos HipZ (top right), HipY (top centre) and KneeY (lower left).

The horns that come with the servos fit pretty well and I had trouble making anything that would fit as snugly. In the end I mounted the horns into recesses cut into the piece of MDF that would join the HipY and KneeY servos. I also hot glued small pieces of MDF with pivots onto the bottoms of these servos. This allows adding a supporting piece of MDF opposite the servo shafts. Too much flex is a constant danger when building these legs.

Finally a small piece of MDF is slipped over the KneeY servo to create a foot. This piece will change as I plan to add tactile and optical sensors to each foot. For now it's good enough.

The parts were designed in CorelDraw and exported as DXF files. I converted these into g-code and cut the parts on my home built CNC. The bit I use is a 1/8" round bottom, carbide, tile cutting bur. MDF is harsh to cut and dulls steel cutters very quickly. The round bottom of the cutter means that the recesses for the servo horns don't have clean sharp sides. A bit of hand work is required to clean out these recesses to accept the horns.

Weight Budget

So far my best guess for the final robot's weight is about 500g. Of course this depends on what exactly I will mount on the head. Probably an IR range sensor. Perhaps a small video camera. Maybe I'll invent something else. :-)

Part
Qty
Weight
Total
MDF Parts 400 sq.cm 0.22 g/sq.cm   88g
Leg Servos 12 10g 120g
Leg PCB 4 8g   32g
Brain PCB 1 21g   21g
Li Ion Battery (3.7V 2400mAH) 3 46g 138g
Bolt plus 3 nuts 1 8g     8g
Additional 2 servos for Head 2 10   20g
      427g

 

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