Catapult built from LEGO Mindstorms EV3/NXT (Part 2 - Base)

In this second video lesson on catapults we improve the stability of the base of the robot. An important feature of the new base is that it is not bending. Use the same principle in all of your constructions to achieve better, stronger robots.

  • #66
  • 04 Jan 2015
  • 7:09

Improving the base

In the previous video lesson of the Catapult series - Catapult build from LEGO Mindstorms EV3/NXT (part 1) we built the initial construction with its Base, Frame and Lever. In this lesson the catapult is still loaded manually but before proceeding to automatic loading we should improve the base. This is done by making its surface larger which depends on the number of LEGO parts that you have.

Rotating the whole robot

I have also added a large gear wheel from the NXT constructor which allows the catapult to change direction freely.

All episodes from the series:

How to use the Ultrasonic Sensor with the Catapult built from EV3/NXT (Part 5)

Catapult build from LEGO Mindstorms EV3/NXT (Part 4 - EV3 clutch and loading)

Catapult build from LEGO Mindstorms EV3/NXT (Part 3 - Automatic loading)

Catapult build from LEGO Mindstorms EV3/NXT (Part 2 - Base)

Catapult build from LEGO Mindstorms EV3/NXT (Part 1)


Building instructions:

  • #material("Building instructions for the Base of Catapult Robot from Mindstorms kit","Catapultv2_base.pdf","pro")

Instructions for building the robot are available in video lesson:

The rotation frame requiers an NXT large double gear wheel that is not available at the EV3 set. You can use the instructions below to construct a similar mechanism from Mindstorms EV3 set.

  • #material("Rotation frame constructed from EV3 set","CatapultChange.pdf","pro")

Building instructions for the Base of Catapult Robot from Mindstorms kit

Rotation frame constructed from EV3 set


In the first episode of the series I showed you how to construct your own catapult, the base, the frame, the nest that holds the brick and the lever that fires the small Lego parts. The catapult uses only gravity forces but it was not very stable. In this video I will show you how to improve the stability of the base and in the same time achieve this free turning of the catapult.

Now let's think about the problem for a moment. With our construction from the first video you can fire small Lego parts and, as you can see, the whole construction isn't very stable when firing. It moves and it might fall. Probably if the brick is heavier it might even fall. A good way to enhance the base of the robot is to use these frames available in the EV3 version of the robot and in some of the newer NXT versions. So, we can find different ways to attach these frames and increase the surface of the base. For example, I've prepared this construction which is very simple. If I attach some of the frames like this, I will have a larger frame. Now we can connect the catapult to this larger base and probably we will improve the stability of the robot. But the problem is not only to improve the stability of the robot but also to make it turn freely at a random degree. So, we can use a part available only in the NXT version. If you have an EV3 version, you will have to go with a smaller base. But if you have an NXT version, you can use this gear wheel. Attach it to the bottom of the robot

Here it is. So, as a last step, if we find a way to attach the whole catapult to this wheel, we'll be able to turn it freely. I've prepared the pins and now we just add the construction on top. Of course, we can think of ways to attach it.

Like this. These are only examples. You can find different ways.

And the whole process of attaching the robot and enhancing the base would look like this.

Attach these parts on one side and then on the other side.

Here's the attachment. The whole catapult is attached to the gear wheel and the wheel is attached to the enhanced larger base. Now the robot will be much more stable and it will be much more difficult for the catapult to fall. Now we have the base and it's quite stable. We can fire various small not very heavy Lego parts. We can turn the robot

but there's one more thing we should improve in the base. I'll detach the catapult from the base. As you can see, we wanted to achieve something very stable. But all the frames in the base are connected only in one direction. This means the base will bend and when it bends it's not very stable. I have forgotten to add a part here. You will have this in the construction. And the whole base will be very unstable. One of the ways to fix this - and you should always fix this in each of your constructions - is to add some parts that will prevent the bending. We can add the parts in the following way.

Thus, we'll make the whole construction much more stable, without bending, and it will be suitable for our catapult. As you can see, now the base is not that fragile. Now we'll add the catapult.

Attach it on one side and then on the other and now we have our very stable construction.

In the next video we'll continue with adding a motor and a sensor, so the whole process of loading the catapult and firing becomes completely automatic.