Introduction: Hydraulic Judo Robots - Exciting STEM Project for Kids

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Two robots face off: The goal is to push your opponent out of the square, flip them over, or push them off of the table!

You can find the lesson plan, 1-page project sheet, and more project ideas at

More footage from actual student-built JudoBots:

I call them JudoBots because of the way they seem to throw and grapple with each other during combat.

Although designed for children in grades 4-6, I think just about anyone can have fun with this. Even parents can't resist trying it out!

The bots are assembled from four components built separate from one another: the base, the stand, the arm, and the hydraulic system. The hydraulic system uses plastic syringes as pistons and water as the hydraulic fluid.

Step 1: Materials, Tools, and Design Criteria


The Engineering Creation Kit has everything you need for this project and many more from STEM Inventions.

Design Criteria

  • The base of the JudoBot must fit within a 10x10" square (okay if the arm extends beyond that)
  • This prevents students from building gigantic, sprawling bases that are impossible to flip
  • For younger kids (grades 5 and under) I recommend using a 10x10" square.
  • Use a maximum of 40 craft sticks in total (or adjust as you see fit)
  • This prevents students from building super-heavy robots that are difficult to move

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Step 2: The Base

Start with a square base and a cube with holes (picture 1)

Build the base (picture 2). There's no "best" base design, but there are key criteria that will contribute to a successful one:

  1. Wide: A wider base is harder to flip over.
  2. Symmetrical: An unevenly weighted base is more likely to fall over to one side.
  3. Strong: A rigid base is better at resisting being moved.
  4. Supported cube with holes (picture 3): Without support, that cube will bend or break off of the base.
  5. Resists moving (picture 4): Add the rubber bumpers to the parts of the base that come into contact with the ground. Use as many as you're allowed, and make sure to glue them on (even if they're already adhesive). These give your robot traction, and prevent it from being pushed around too easily.

Step 3: The Stand

Create a column with 2 cubes with holes (picture 1). Make sure the holes are aligned.

Attach pair of beams that are half of a craft stick long (picture 2). At the top, make sure the holes in the holes are aligned as shown.

Lastly, create the stand's hydraulic connector by creating another half-stick beam at the bottom of the stand (picture 3).

Step 4: The Arm and the Wedge

The length of the arm and the design of the wedge are up to you. There's no "best" design since each judobot will have different sets of advantages and disadvantages, depending on the opponent's design.

One important technique: Make sure to use craft stick-and-cube building technique to create a rigid arm. Flexible arms don't work well. You will also need 1 cube with holes on one end.

There are a few things to consider when choosing the length of the arm.

  • Longer arms can reach farther, but they will lose mechanical advantage. To explain: The hydraulic system has a limited amount of force. The farther the tip of the arm is from the hydraulics, the more work will be required to move it. Therefore, extremely long arms may not have enough force to lift or push opposing robots.
  • Short arms can muster more mechanical advantage, but at the cost of not having as much reach.

A few notes regarding the wedge:

  • The wedge design is, in my opinion, the most interesting part of the project. You can design a sleek and nimble wedge that slips under the opponents base with precision to tip them over, or you can create a wide fork-shaped device that is designed to catch any part of the robot and simply push it out of the arena.
  • In the example pictured, the wedge is designed to slide sideways under the base to flip the opponent. It also has a full-length craft stick set perpendicularly at the tip, which can also be used to push against the opponent's stand in close quarters.

I'd like to emphasize that the example pictured is definitely not the best design. The only way to find that is to battle other robots, and redesign your arm and wedge to compete against your specific opponents.

Step 5: Assembling the Pieces

Glue a dowel into the base cube (picture 1). Make sure to choose a straight dowel without defects.

Place a washer onto the dowel (picture 2). This helps to reduce friction between the base and the stand.

Place the stand onto the dowel (picture 3), cut the dowel to size, and wrap masking tape around the end to prevent the stand from sliding off (picture 4).

Lastly, thread the scrap of dowel through the topmost cubes of the stand and the arm (picture 5). Cut to size and wrap tape around the ends.

Step 6: Make a Hydraulic System

This part can be a bit tricky for students. Although the process is fairly simple, it isn't easy to commit to memory by watching it done once or twice. You may want to outline these steps on a whiteboard:

  1. Connect tubing to one syringe
  2. Fill completely with water
  3. Point the tip of the syringe up and push on the plunger. This expels all of the air and fills the tubing with water
  4. Refill halfway and set aside
  5. Submerge the tip of the second syringe and repeatedly pump the plunger to expel air. Fill halfway
  6. Connect the syringes and try it out. If the total amount of water in either syringe exceeds the 10ml mark then there is too much water in the system. There should be little to no air bubbles, too.
  7. Attach one of the syringe toppers (or glue on a cube with holes).

For extra fun, use food dye to color the water

Step 7: Attach the Hydraulics to the Arm

Place the robot on a flat table. Empty the syringe that has the topper, and tape it to the stand with about a 1/2" gap between the topper and the arm (picture 1).

Tie the topper to the arm with a cable tie (picture 2).

Test it out! (picture 3 and 4). The arm should have a range of motion similar to what's shown here.

Step 8: Attach the Hydraulics to the Stand

Glue a cube to the base about 3-4" from the stand connector (picture 1).

This next step is tricky. Read carefully. Students may need assistance with this step:

  1. Point the arm straight forward.
  2. Fill the syringe with the topper with 5 ml (halfway) of water.
  3. Position the topper so it's right next to the connector (picture 2)
  4. Glue and tape the syringe to the cube.
  5. Cable tie the topper to the cube (picture 3).

Test it out! (picture 4 and 5). By making sure the arm is pointing straight forward when the syringe is halfway full (5 ml) of water, it should have an equal range of left-right motion. In this example, when the syringe is full of water (10 ml), the arm goes to the right. When it's empty (0 ml), it goes to the left.

Step 9: Official Judobot Match Rules

The arena consists of two 12" squares spaced apart by 3" drawn onto a tabletop (depicted in the video). Ideally there should be about 6" between the sides of the squares and the edge of the table.

  • Alternatively, do not place the squares near a table edge. Instead, a match can be won by pushing the opponent more than 50% outside of the square.

Rules of a match:

  1. Judobots begin by squarely facing each other with the front of the base touching the edge of the square and the arm completely raised up.
  2. A referee counts down from three, and the match begins.
  3. There are three ways to win a match: flip your opponent, push your opponent off of the table (or push more than 50% of the base outside of the square; referee makes the call), or if your opponent needs to stop to repair something.
  4. A stalemate occurs when the bots are both active but unable to reach one another.
  5. A draw occurs if both bots are either flipped or have fallen off within 3 seconds of each other.
  6. Players cannot touch the judobot with their hands, push or pull on the tubing, or otherwise try to manipulate the robot in any way other than using the hydraulic controls.

The most common repair is hydraulic failure, which is when the the plunger is yanked out of the syringe casing. Repairs are not as straightforward as filling the lines, so emphasize the importance of operating the JudoBots with precision. If you see a student recklessly pulling on the syringe in the heat of battle, call for a time out. It's better to spend a few seconds reminding your student to slow down than it is to spend a minute or two fixing the line.

The first time students battle it will look like two poorly programmed machines bumbling into each other. This is normal - it just takes some practice before getting the hang of the controls and basic tactics.

Step 10: Presenting This Information to Your Class

I break this project up into two parts: 1. Construction and assembly, and 2. Attaching the hydraulics and battling

  1. Prepare two working JudoBots and an extra hydraulic system before class.
  2. Demonstrate how to operate one JudoBot in front of your class. Tell them right away about the importance of operating the JudoBot with precision, and show them what happens when a syringe is yanked on too hard.
  3. Let each student have a chance to operate a JudoBot, though not in combat. This allows the students to experientially grasp what hydraulics are, which is necessary because you will be referring to that concept a lot.
  4. Afterwards, have the students focus on you as you disassemble a JudoBot into its 4 components: base, stand, arm and hydraulic system. Show students the key features of each part, such as the materials used for the pivot column and how it operates. Do not explain how to create or attach the hydraulic system yet.
  5. Once you feel confident that the students comprehend how to construct each part, allow them to access the materials and begin.
  6. At the beginning of the second part of class, show the students how to create a hydraulic system and outline each step on the board. You may want to show them a second or third time. Demonstrate how to attach each hydraulic system to the JudoBot
  7. Finally, remind your students to operate their bots with precision once again! Never yank unrestrainedly on the controls.
  8. Once the battling begins, continue to allow your students to redesign and build. Oftentimes the very first design includes some flaws that need to be worked out.

Step 11: Further Thoughts

This project is relatively new in my curriculum lineup, so there are lots of unexplored ideas. What design aspects could be improved? How could this be motorized? What new element could make the battles more interesting and decisive? Could this idea be turned into a toy that uses easily interchangeable parts which allow children to customize their bots and battle with their friends? If you try this out, post some photos and share your insight. Or if you don't, let me know what you think of it anyway :) Thanks for reading through this Instructable - now go forth and create an unforgettable experience for the kids in your life with awesome engineering projects!

Same materials, different project

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