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(How Big is Max the Megapod?)
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==How Big is Max the Megapod?==
 
==How Big is Max the Megapod?==
Max stands 13" tall when in a normal standing position. The diameter from the tip of one leg to the tip of the opposite leg is 22 inches when in normal standing position.
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Max stands 13" tall when in a normal standing position. The diameter from the tip of one leg to the tip of the opposite leg is 22 inches when in normal standing position. It is exactly twice the size in every direction as Vorpal The Hexapod (eight times the volume). Vorpal The Hexapod looks very tiny standing side by side with Max The Megapod.
  
 
==What does the Megapod Weigh?==
 
==What does the Megapod Weigh?==

Revision as of 20:49, 23 August 2018

Will the Megapod be Open Source?

All of our projects are open source, and so Max the Megapod will be as well!

When will the files be posted?

We are working to post STL files by September 22 (NYC Maker Faire) along with draft build instructions. The official final version 1.0 files and instructions are targeted for October 1.

How Big is Max the Megapod?

Max stands 13" tall when in a normal standing position. The diameter from the tip of one leg to the tip of the opposite leg is 22 inches when in normal standing position. It is exactly twice the size in every direction as Vorpal The Hexapod (eight times the volume). Vorpal The Hexapod looks very tiny standing side by side with Max The Megapod.

What does the Megapod Weigh?

Max weighs about 9 pounds with a typical battery.

What type of battery does it use?

Max uses a five-cell NIMH battery with a nominal voltage of 6.0 volts to drive its motors, and it uses a standard 9v battery to drive the electronics (Arduino, Bluetooth module, etc).

Our standard configuration uses a standard size Tamiya female connector on the NIMH battery. We recommend 5000 mAh or more, and we recommend a minimum wire gauge on the battery of 18 gauge.

A good example of a compatible battery is the Venom DRIVE series 5000 mAh battery. For more information see [the Venom DRIVE 5000 web page]. These are available on Amazon.com in the USA for about $40. We have tested this battery and it will give you over 60 to 70 minutes of runtime on a full charge.

The battery specs should confirm that it can output 5 amps continuously and can handle spikes to 10amps on an intermittent basis (5 seconds maximum).

Many hobby and RC suppliers have cable converters to go from Tamiya connectors to other types (cross, XT60, etc.) so it is possible to use batteries with other kinds of connector. We are currently considering whether to offer other battery connectors on the robot, such as XT60, bullet, t-connector, etc.

Will other Battery Voltages Work

No, this project specifically requires a 6.0 volt rechargeable battery for the servos, and a 5 cell (5s in RC parlance) NIMH has the right attributes. Lower voltages will not work, we've tried 4.8 and there's not enough torque for the motors at that voltage. Higher than 6.0 volts nominal voltage will cause premature wear on the servos, or even destroy them instantly.

For example, you can't use LIPO batteries because a 2 cell configuration outputs too high a voltage (close to 8 volts when fully charged), and 1 cell is way too low voltage.

It may be possible to use NICAD batteries but we don't like those due to the use of highly toxic cadmium metal. NICAD batteries also generally are inferior to NIMH and cost just about the same so there's no real reason to use them.

Why did you use two batteries in the Megapod instead of one like in the smaller hexapod?

This is rather technical so stop reading now if you don't care to geek out on the details!

If we powered the Nano and Bluetooth modules using the 6v battery in the Megapod, momentary voltage sags would cause the Bluetooth connection to constantly drop out. We were able to get away with one battery on the smaller hexapod due to the use of LIPO batteries there that run at a higher voltage. The downside there was the need for a BEC to regulate the power down to the range the servos required. But for the Megapod version, the current requirements of all those large servos would have required a very expensive voltage regulator (BEC) if we attempted to use the same strategy. As the robot gets larger and larger, it starts to pay to split up the electrical system between motors and control logic circuits running at different voltages.

What were the main changes between the original Hexapod and the Megapod, besides size?

A lot of people think "wow this was an easy mod, you could just scale up the STL models by 200% and maybe upgrade some of the electronics and call it a day!". That's far from true! To make this work we had to make literally hundreds of changes to the STL files and did a complete redesign of the electronics. The only thing that did not significantly change was the software (the Scratch extensions are unchanged; the Arduino code only has a handful of adjustments). The Megapod represents hundreds of hours of work, testing, and rework.

Some of the most important changes are:

  • There are now real bearings for the leg hinges. The small hexapod could get away with little plastic nubs as bearings, because it's so lightweight. But the Megapod needed real bearings there to support the weight in a stable way. We're using commonly available 608 skate bearings in the spirit of open source design.
  • The largest parts are now split up so they can be printed on smaller printers. If we did not do this, the project would require a 12" cube (300mm) build volume for the largest parts, but by splitting it up you can print on as little as 8 inches cube (200mm). This split up required dozens of changes to the STL models. The split parts are assembled using screws.
  • Leg hinges are reinforced. Due to the way the layers run on the leg hinges, there is a great deal of stress right where the servo horn is that runs perpendicular to the layer boundaries. For the small robot, in most cases this did not cause a problem, and even if a crack did develop a few drops of super glue normally fixed it right up. With the Megapod, there's a lot more weight and stress. A new part has been added that clamps those high stress layers together to ensure reliable operation.
  • Legs were redesigned to hold standard size servos, and it was no longer possible to have the servos just snap in place, there's too much stress on them to be held in with a tiny bit of plastic. So servos are now screwed into the base and legs.
  • The electrical system has been extensively reworked:
    • Power for the servos does not flow through the servo controller. There is a power distribution harness that provides power to the servos. The open source servo controller could not handle the up to 8 amps required, it would constantly burn out. The servo controller now only handles logic and servo signal generation for the servos, not power distribution.
    • As mentioned earlier, there are now two batteries, one small 9v for logic power (with a BEC used to regulate to 5v logic voltage) and a much larger 6.0V NIMH battery pack for servo power. This also required the on/off switch to be a DPST switch instead of a SPST switch.
    • Voltage displays for both batteries are standard equipment. In this way you can see how your batteries are doing at all times. If the servo battery drops below about 5.3 it's time to swap it out, and likewise if the logic battery falls under about 7.2V.

What type of servos does the Megapod use?

We're using genuine Tower Pro MG958 servos. We tested many different servos but found the MG958 has the right balance of torque, speed, and is economically priced. As with the small hexapod project, beware of low priced counterfeit servos on sites like banggood and aliexpress.

How does the build time compare to the small hexapod?

The Megapod takes a little longer to build due to the need to install bearings and assemble some of the larger parts from several pieces (which where a single piece in the smaller hexapod). I would estimate these things add about 30 minutes to the build. IF you've built Vorpal the Hexapod before, you should be able to complete Max The Megapod in about 2 to 2.5 hours.

If you source your own parts, the build time will be much longer, however. For our original hexapod, self-sourcers typically required at least an extra hour due to the need to solder up the electrical parts, flash nanos, configure bluetooth modules, etc. These are all things that are not necessary when building from our kits. Self sourcing Makers will find the Megapod electrical system takes significantly more time to build than the original Vorpal Hexapod (about three times as long).