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vorpal

Max The Megapod

by vorpal Sep 21, 2018
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how much does it weigh approximately?

With a typical battery, about 9 pounds (about 4 kg).

The smaller robot is about 1.5 pounds, for comparison.

so why mg996 servos don´t work well?

Specifications:
MG996 11 Kg-cm stall torque
MG958 20 Kg-cm stall torque

If you do the calculations, the 4kg robot needs to be lifted at a 15cm leg length so each knee servo provides 11/15 = 0.7 kg of lift, but that's at stall. You normally would only want to use about half that to avoid constantly stressing the servos, but let's push it and say we expect each knee to lift 0.5 kg. Then: 6 legs x 0.5 kg of lift per leg = 3.0 kg. But the robot has 4kg mass. We're asking each servo to lift 4/6 = 0.67 kg but that basically puts every single knee servo in stall during a lift.

This shows that the MG996 doesn't have enough torque to really make the robot work. It sort-of works but the servos struggle constantly, especially in moves that require the robot to come up off the ground, or "scamper mode" which moves the legs very fast.

You can use MG996, and it will kind of work, but the servos will die very quickly because they're being overstressed, after just a few hours of use, and it will constantly struggle.

I did test with MG996. The robot strains and struggles to get off the ground. Scamper mode can't move the legs fast enough so the robot sinks near the ground, totally stressing the knees. Some moves require the robot to lift up using only 3 legs, those don't work at all.

With the suggested MG958 servos, the robot pops up off the ground strongly, almost jumping off the ground. Scamper is fast and smooth. Three leg lifts struggle just a tiny bit but they work. In short, it works great with these servos.

To make matters worse, however, if you buy MG996 servos from places like aliexpress or banggood, it is very likely you will get fake servos not really made by tower pro, and those fake servos will not even give you the 11 kg-cm of torque of a real MG996. So, there's a really good chance if you buy cheapo fake servos that it won't work at all. It's quite hard to tell a fake MG996 from a real one.

I know a lot of people want to build this robot super cheaply by using lesser servos. Trust me, I did the calculations and I also confirmed this during days and days of testing. The MG958 is solid and gives a really nicely working robot at a decent cost. The MG996 is not an acceptable replacement.

This is a big project and it really impresses people who see it in person. If you want to do it, please do it right! Walking robots this big usually cost $1500 to $2000, this one can be built for roughly $400.

For a walking robot, skimping on the motors is fatal, I've learned that over the years, trust me.

Ok, I finally undestood the problem. You seem to be very experienced whit this stuff and I really apreciate your time to answer this kind of questions. Thank you!!

You are welcome, and thanks for the kind words! I have been building robots for many years, it is true. I taught an honors robotics class at the high school level where I stressed to the kids the need to learn how to do the actual computations to pick things like motors. I also coached competition robotics for ten years. I've been building electronic circuits for nearly 50 years, I got my first soldering iron when I was 10.

Hii, is it also possible to build it with Raspberry Pi instead of an Arduino? If so, what raspberry would be required? Like, is a Raspberry Pi Zero fine enough for example (powerfull and small enough?).

I haven't ported the software to the pi. The pi is easily powerful enough to work of course, since it's far more powerful than the arduino nanos we use right now.

I would say there should be enough room for a pi inside the robot.

Comments deleted.

Hi
Where can i get the servo's? They are out of stock everywhere!
Do you have an alternative for the servo's?

Yes unfortunately I started getting so many megapod orders that I had to give priority to Maker kits and pulled the individual servos off the shelf. The banggood link referenced by ongelofeloos below might be real tower pro servos if you want to take a chance on them. Otherwise I will be back in stock estimated November 5.

Regarding alternatives, the MG958 is the only low cost servo we found that works well. Please don't try using MG995 servos, they kind of work but you won't be happy with them. The robot will struggle to stand up during certain maneuvers and will be very low to the ground in scamper mode. They just don' have enough torque. because they'll be strained past what they can handle they won't last very long either.

Basically, you need a servo that outputs 20 kg-cm torque at 6 volts. The MG995 is only rated 15 kg-cm even if its genuine, even less if it's a counterfeit.

Hi
I have found an other servo motor with the same amount of torque, but it's rotating the wrong way.
Do you have a solution for that? Perhaps in the software?

Hello, yes some vendors sell servos with nonstandard directions without even telling you in the description.

In the robot code there is this line (you can search for SERVOMIN and find it pretty quickly):

int p = map(position,0,180,SERVOMIN,SERVOMAX);

Just swap the 0 and the 180 to make it like this:

int p = map(position,180,0,SERVOMIN,SERVOMAX);

That will swap the direction of every servo move.

hi. its a toy? max can do anythin that can do the litle?

I know it's not just a toy it's super cool I meant that if I could do everything the little one did, it's super customizable I do not have a 3D printer but I love customizing it and I put horns on it hahahahaha are amazing

seriously in my country there is only a 3d printing service here no libraries is a 3rd world country Sericio who speaks to you in the capital of the country and can not take me for something I gave them a lot of research and is very expensive the impression so that they give an idea a dollar is worth 6000 guaranies the minimum salary is 2200000 I only tell you this so that they understand I appreciate them very much and I love their robot they are the best

You may have access to a 3d printer and not even know it! Many libraries now have 3d printers for public use. Makerspaces are popping up all over the place, and some schools let parents use their 3d printers. Check around, you might be surprised!

Yes, Max is 100% compatible with the original Vorpal Hexapod. All the same dance moves and other movement modes. Same exact gamepad and same controls. Scratch programmable in exactly the same way. There is actually no difference at all in the software between the two robots.

Is it a toy? Well it can be. We have already ported our "Joust" game and actually played it at the south jersey Makerfest a few weeks ago. We're porting capture the flag and our other activities as well. See this youtube video for a couple of rounds of joust:
https://www.youtube.com/watch?v=ethJloC0yGA

But is it JUST a toy? No! Like our smaller robot, it's scratch programmable. The small robot is being used by hundreds of schools worldwide to teach students about robotics, programming, 3d modelling, and more. Because it's open source, you get the source 3d models and all the source code to mod and hack to your heart's content.

hi you can speak spanish?

Unfortunately no. Only english and a bit of german.

MPORTANT UPDATE: The Leg Hinge part previously posted appears to have been the wrong version. I have updated it to MM-V1r2c-Leg-Hinge. If you printed anything previous to that version you will find it is very difficult to get the leg hinges to lock into place. I'm not sure what happened but I think I just grabbed the wrong file previously when I uploaded the models.

Very sorry for the error. It is possible to use angle cutters to carefully nibble away a little plastic around the top of the U shape and get the old ones to work if you don't want to reprint them. I will create a short video showing the procedure to make the old ones work and will post a link here tomorrow when I'm back in my office.

Anyway, make sure you're printing version V1r2c or later.

Thank you for this thing! Thumbs up to you!

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Why not use standard servos? not the tiny ones

Hello,

Max does use standard size servos, mg958. Mini servos wouldn't work for Max, too heavy.

Our smaller hexapod project uses mini servos, mg90s

ЖЖЖ Внук будет в восторге...ЖЖЖ

Да, дети действительно наслаждаются этим роботом!

Very cool that you're getting some love from Thingiverse! Well deserved!

Thank you! The folks at Thingiverse rock!

Comments deleted.

Posted today: A new cap variation for Halloween (see photo). This megapod-sized pumpkin head requires a 10" cube printer. The face parts can be glued on. The mouth is very thin in two places allowing it to be wrapped around the pumpkin's curvature.

Ooh. Any chance the Halloween pumpkin head could be made for the smaller Vorpal Hexapod too? I just bought the smaller one ...because budget and noob level.

Some new items released today:

Lashes Eyes
Angry Eyes
Joust Lance and Joust Rider (The lance requires two M3x25 or #6-32 1 inch screws to attach the tip to the base)

I will be at the South Jersey Makerfest on October 20 with two megapods and we will be holding the first ever Mega-Joust competitions! Should be wild.

There was a minor update to the Cap-1, Cap-2, and Cap-3 parts that makes the magnet holes 3mm shallower. This makes it easier to install the magnets. There is no need to reprint if you've already printed them, you would just need to dribble a few mm of hot glue into each pocket before installing the magnets.

Released Today:

Caddy-Servo-Driver & Caddy-Nano Are two structures to protect the servo driver and nano against shorts inside the base. These slip on the same way as in the small hexapod. Unlike the small hexapod, these are meant to just live inside the base, they are not screwed down. They can be covered by the covers (see below) to make things look neat and hide all the wires.

Caddy-Bars are used with the two above files, they slip into the other structures the same way as the smaller hexapod.

Base-Cover-1, Base-Cover-2, Base-Cover-3, these are three flat parts to hide all the wires and make the base look nice when you open the cap to change the batteries.

Other changes today:

Base-2 has a very slightly larger switch opening. It was possible to get the switch in place but it was just a bit too tight. No need to reprint if you've already printed Base-2. If you have trouble getting the switch in, just heat up the opening for a few seconds with a lighter (be careful not to scorch your work, keep the flame moving, it only takes a few seconds). Alternatively slightly trim with a razor but be careful please.

We are super close to being final for V1 now. I am leaving the "work in progress" marker up there because I'm still working on build instructions, they are not final. It takes a lot of diagram-making, photos, etc. to get the instructions finished. But that is also getting close.

Is the electronics caddy the same as in the original vorpal hexapod?

No, we will be posting the new caddy hopefully Monday. We wanted to get the big parts posted first so people who pre-ordered parts could get started on the long prints.

UPDATE: I have uploaded some revisions to the original files, these are all marked with version number V1r2a (there is no "a" on the older files that have not been revised).

Highlights:

  • All four BASE files have been slightly modified. The main improvements are that there is better switch placement, we found during the NYC Maker Faire that it was actually possible for a leg to move in such a way that it turned off the robot with the prior placement. If you've already printed the base, you can avoid this problem by simply installing the switch with "1" on the bottom and "0" on the top, this stops the accidental turn-off situation. No need to reprint.

  • The BASE BOTTOM part now has a separate compartment for the 9v battery. This is cleaner than the prior design which had you stuffing both batteries into the same compartment. Again, there is no compelling reason to reprint if you've already printed it, this is a minor improvement.

  • The BASE BOTTOM and other BASE parts now have a little nub that ensures they cannot be assembled in the wrong orientation. There's a hole in the BASE BOTTOM that fits over the nub. If you mis-align during assembly they won't fit until you rotate the BASE BOTTOM to match up with the nub. Again, if you have already printed the old version there is no compelling reason to reprint for this improvement, just be sure you carefully match the diagrams in the build instructions when you align the BASE BOTTOM and other BASE parts.

  • The LEG HINGE part now has a little wall over the horn pocket into which the servo horn slides. This serves to make it impossible for the horn to slip out of its pocket. We found during the Maker Faire that under stress, after a a bunch of hours of use, the servo horns could occcassionally slip free of the pocket causing a leg to collapse. We fixed it at the Faire by using a thin strip of duct tape to keep the horn secured. If you've already printed the old design, you can either reprint the hinges or you can use the duct tape solution.

I’ve ordered a couple of your kits, but am a little confused on which batteries to order. Can you provide a link or a brand name that is working well for you?

Thank you :-)

There is a link to a particular battery on our Megapod FAQ here to one that really works well for us in demos and at Maker Faires. See here:

http://vorpalrobotics.com/wiki/index.php/Max_The_Megapod_FAQ#What_type_of_battery_does_it_use.3F

(Sorry to point you to yet another page, but we will keep that FAQ up to date and someone may read this comment a year from now so this way everyone gets the most current info.)

Hello Muthinor,

Our preliminary documentation on how to build the electrical system is here:
http://vorpalrobotics.com/wiki/index.php/Max_The_Megapod_Battery/Switch_Construction

We run using two voltages: a 9v battery through a 5v regulator (BEC) is used for all logic circuits (nano, servo driver, bluetooth) and a 5 cell (6V nominal) NIMH pack (4000 mAh minimum) drives the servos. Because the battery pack matches the voltage the servos want to use, there is no need for a regulator.

A 9v battery of course cannot drive 12 full sized servos. At most you can get 1 amp if it's a high end rechargeable 9v. Each servo may draw an amp or more under load. At most you can drive a couple of full sized servos under very light load with a 9v battery.

You may want to read up on power systems before further experimentation. Not matching the right power source with your projects can very quickly fry electronics or even start a fire in some cases. Short of that, you can drastically reduce battery life by not using the right power for the project. The voltage is important but so is the current. A battery of a certain size can only safely output a certain amount of current. Having voltage too high or too low can cause issues. Too high will start burning up circuit parts, too low simply won't work.

Using an Arduino's internal voltage regulator cannot drive large loads. Increasing the voltage from 9V to 12V won't significantly increase the amount of power the arduino can source to external electronics. The arduino on-board voltage regulator is not designed to drive servos under load.

hi there!
i've been trying to make 12 servos work with a 9v 1.5A power plugged to an expansion shield+arduino nano but the servos never seemed to work.. adding a 12v would fry the arduino, and without it, servos would just vibrate.. how does the 5 cell battery make them work?
or how's the connection made? thanks!

This robot is twice the size and six times the weight, there is no way the little mini servos could work here. This uses full sized servos which are high torque mg958. It uses an entirely different electrical system which can deliver four times the current to power the larger servos.

So unfortunately, no,you can't use the parts in the smaller version hexapod kit to build the megapod.

I got all the parts from the kickstarter. I haven't built the original yet. If I decide to do this do all the same parts from the original kit fit?