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Max The Megapod FAQ

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Will the Megapod be Open Source?

All of our projects are open source, and so Max the Megapod will be as well! For details see: Max The Megapod Creative Commons License Information.

When will the files be posted?

Files are posted right now on Thingiverse. Just search there for "Max The Megapod" and they'll pop right up.

Vorpal-Riding-Max-tilted.jpg

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 pack with a nominal voltage of 6.0 volts to drive its motors, and it uses a standard 9v battery through a 5V regulator to drive the electronics (Arduino, Bluetooth module, Servo driver, etc).

Our standard configuration uses a standard size Tamiya male connector on the NIMH battery pack (there is a matching female Tamiya on the robot). We recommend 4000 mAh or more, and we recommend a minimum wire gauge on the battery of 18 gauge.

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.

Compatible Battery Example

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 about 60 to 70 minutes of continuous runtime on a full charge. That's with the robot constantly moving. At demos like the NYC 2018 Maker Faire where we would fire it up every few minutes for a show, we got typically about three hours of use before needing to change the battery.

Other Battery Notes

  • The battery pack wire thickness is important. Sometimes you'll find a 6.0V battery with 4000 mAh or more capacity but with very thin wires, usually with a JST or just a plain 2-pin Dupont connector. We don't recommend using a battery with wires any thinner than 18 gauge.
  • Having a battery that is too large and heavy is also not recommended as it will stress the servos too much. For example, we have tested with a huge 13,000 mAh pack but the robot struggles a bit under the weight and is not as responsive. We don't recommend going over about 8,000 mAh. Batteries that large get really expensive anyway, you're better off buying two smaller batteries and keep one charging while the other is in use. It only takes about 20 seconds to swap the battery pack.
  • The battery may be configured either as a straight line of cells (lined up parallel), or a "bump pack". We've sized the battery compartment to accept either of these common configurations for 5-cell packs.
  • Make sure you are using a compatible smart charger. Smart chargers cost a little more but they will extend battery life, usually charge faster, and usually have safety cut-off timers built in. Even if using a smart charger, we never trust any electronic device to be 100% reliable so do not charge unattended or overnight and follow all manufacturer recommendations.

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 all 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, the robot struggles and this may lead to servo damage. 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 due to the well known memory effects, 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 10 amps required, it would constantly burn out. The servo controller now only handles logic and servo signal generation for the servos, not power distribution. Servos are now plugged into numbered servo extension cables that come out of a distribution box.
    • 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. In other words, both batteries are turned on and off by a single switch that in effect participates in both sides of the electrical system.
    • With higher current, safety becomes a bigger issue, so we added a 15 amp fuse to the 6V side of the circuit. If a short circuit develops on the 6V side, the fuse will blow, preventing damage to the battery and other safety concerns. A fuse is not really needed on the 9V side of the circuit because a 9V battery really cannot output a lot of current (max 1 amp even for a high end rechargeable). A big NIMH, however, can output 30 or 40 amps if shorted and a fuse is warranted. In the spirit of open source, we used a commonly available automotive blade fuse that you can get just about anywhere.

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.

I know a lot of you will be tempted to substitute the lower priced MG995. It is the same size as the MG958 so it will fit the project. But beware! First of all, 99% of the ones you find on the low priced sites (ebay, banggood, aliexpress, and even most vendors on amazon) are fakes that will fail quickly. But even if you get real ones, Genuine Tower Pro MG995 servos have significantly less torque than Genuine Tower Pro MG958 servos and the robot might somewhat function but it will struggle and you really won't be happy with it. You're spending a lot of time printing these large parts and building this project, do it right! Trust me as someone who's personally built at least a hundred walking robots: skimping on the servos for a walking robot is a really bad idea!

With the MG958 this robot really shines. It leaps up off the ground on power up. It moves quickly and impressively and keeps working hour after hour, day after day. We spent an enormous amount of time researching and testing different servos, and the MG958 was the clear winner for this project.

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). In addition, servos now require screws to hold them in place whereas on the small robot they required no screws, so that adds a little bit of build time.

I would estimate these things add about 60 minutes to the build (I can only estimate because we've only built a handful of these right now). If you've built Vorpal the Hexapod before, you should be able to complete Max The Megapod in about 2.5 to 3 hours. Compared to other hexapod projects, this is an extremely easy build. A competing kit with half the features of Vorpal/Max takes 30 hours of assembly time, for example.

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 (it may take two or more hours just to build the electrical system).

The upshot is, if you build from your own self-sourced parts it's going to take 5 to 6 hours, about twice as long as from the kit. And of course it may take you a couple of hours of research to track down all the parts. If you use the wrong parts, you may also need to rework things. (It seems to be extremely common for self-sourcers to buy the wrong parts or purposely try substituting cheaper or flimsier parts then finding they won't work. Trust me, we tried to find the least expensive parts that will result in a solid robot!)

How long does it take to print the parts?

Longer than the small robot ;)

We have not completed enough timing tests to give a precise figure yet. The gamepad is identical to Vorpal, so no difference for any of those parts. My best guess right now is, three to four times as long as Vorpal The Hexapod (something like 50 to 65 hours total depending on print speed, so plan on a few overnight prints to get the job done).

We will offer two options that include plastic parts:

  1. Just the big parts, parts that require more than a 150mm (6") cube build volume
  2. All the parts for hexapod and gamepad. Note this option does not currently include any accessories other than eyes. Although it would be cool to joust the big hexapods, we're concentrating on the main product right now.

How much filament do I need to print Max The Megapod?

We are currently estimating 2.5 kg of filament if you use our recommended print settings (which are documented in the assembly guide, see link below).

That assumes nothing fails and no reprints of course. When we print one, we make sure we start with 3 full spools of the same color. Of course, it's also fine to mix and match colors! We do recommend making the top of the gamepad the same color as the cap of the robot to keep straight which gamepad goes with which robot.

We recommend you take some time to really dial your printer in before starting the bigger parts.. Make sure your bed surface is good, your nozzle is good (or new, we replace our nozzles every 100 hours of printing and it makes a huge difference in print quality and reliability), the printer is lubricated, etc. It's heartbreaking to kill half a spool of plastic on a large part that fails 80% the way through because of some preventable maintenance issue!

What about gaming and other accessories?

Trust me, I want to joust two Megapods against each other as much as anyone! But we had to concentrate on the main robot at first.

Right now, the only accessory we've published is Megapod sized eye attachments. We will be prioritizing and releasing all the accessories over the next few weeks though, so stay tuned. I'm shooting to have all the accessories available by about mid November.

Will Max The Megapod fit on your Thingiverse-published 4x2 foot Vorpal Field?

Alas, no, Max is way too big for the Vorpal field. He steps right over the 4 inch high walls and can't even take one step. Remember, he's 22 inches in diameter when standing, and the field is only 24 inches wide!

At Maker Faires and other demos we are using a portable dance floor. These are reasonably inexpensive (you can find many models on Amazon and other online stores) and come in 1 foot by 1 foot tiles that can be expanded to any size desired.

In most demos we're using 4 feet by 3 feet of flooring up on a doubled up pair of six-foot folding tables. If we have room and are on a level surface like an indoor floor, we go with 4 feet by 4 feet to give the big guy a little more room. Of course, if the venue floor is already a suitable surface such as tile we can run Max right on the existing floor and we've done that too. Short cropped industrial carpet is ok but sometimes the legs get hung up on a thread so it's not ideal. We're working on "shoes" to put on MAX that are designed for industrial carpet (found in schools, offices, etc.)

Note: if you run Max on a wood floor you'll probably scratch it up, so don't do that. Max weighs six or seven times what Vorpal weighs and plastic can scratch wood quite easily with that much weight behind it.

Max The Megapod Quick Links

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