Wednesday, July 28, 2021
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Popeye the Robotic Crab

A walking LEGO© MINDSTORMS with pneumatically-driven legs

Creating a walking LEGO robotic crab is no easy task, but Carter Payne was up to it. A veteran of Tufts University’s Center for Engineering Educational Outreach’s Summer LEGO Program, Carter was well versed in building with LEGO MINDSTORMS components and programming with ROBOLAB software. After two summers of building ramp climbers, line followers, and mini golf obstacles, this eighth grader was looking for a new challenge.

Close-up of a single leg; detail of cylinder that sweeps the leg forward and backward. A second smaller cylinder (A) extends the foot outward and downward.

“Last year, I had this idea for a cool walking machine. This year, I decided I wanted to build something that had more articulated walking.”

Popeye (so named because the eyes fall off) is a fully automated construction powered by a single RCX. The crab has 6 legs that achieve locomotion by moving 3 legs at a time in a triangle pattern. Over 36 hours of building time went into the creation of the crab with the bulk of time being spent on the design of the legs.


“I tried a lot of different designs for the legs,” notes Carter. The final design was inspired by a lizard he saw on TV. Each leg consists of 2 pneumatic cylinders – a smaller one which extends the leg and a second, larger one that provides the push for the locomotion. LEGO wheel hubs were used to act as feet and provide the right amount of friction and balance. A series of 4 switches controls the flow of air to the 12 pneumatic cylinders in the 6 legs.

Side view of the legs.


“I wanted it to be fully automated.” To achieve automation, Carter used 2 rotation sensors and 2 motors to control a lever that works to physically trigger the switches for his pneumatic leg configuration. The whole pneumatic system is powered by 2 pneumatic cylinders (driven by 4 motors) connected to 3 air tanks. The springs were removed from the 2 pneumatic cylinders to increase the ease and efficiency of pumping. The program, written in ROBOLAB, which controls the automated system, is pretty straightforward. One task continuously runs the motors that drive the pneumatic cylinders that fill the air tanks, while the other task runs the motors (based on the rotation sensor settings) that activate the switches.

ROBOT: Popeye the Crab


CREATOR: Carter Payne



KEY COMPONENTS: 1 LEGO RCX controller, 14 LEGO pneumatic cylinders, 6 LEGO 9-volt motors with gear reduction



Top view showing pneumatic system.

The LEGO RCX is a LEGO brick with an embedded microprocessor (Hitachi H8). The RCX has 3 motor ports, 3 input ports as well as an inferred port. The inputs can read data such as the robot’s temperature, rotation, light or contact via touch sensors. The RCX can use a sensor adapter module developed to accommodate many off-the-shelf sensors that require a 5 volt excitation and returns a 0-5 volt signal to the RCX. Powered by 6 AA batteries, the RCX makes autonomous robotics possible for children as young as five and for hobbyists who want to spend more time building and less time soldering. For more information on the RCX visit Kekoa Proudfoot’s excellent page on the RCX internals (http://graphics.


The lumbering creation is a sight to behold at 14 inches long and 12 inches tall. Carter had originally thought it might be the size of a real crab but his desire to use pneumatics dictated otherwise. Moving only an inch or so a minute, Popeye certainly won’t win any races; but he’s definitely a mechanical LEGO marvel and a credit to the innovation and persistence of his creator.

The mechanism that activates the switches for the pneumatics moves along a gear rack and uses a modified lever to trigger the switches.


LEGO Education

www.legoeducation. com, (866) 349-5346

Center For Engineering Educational Outreach, (617) 627-5888

The bottom of the circuit board.The large square chip is the microcontroller, the
chip below that is a RAM. The large chip in the middle is a bank of flip flops; the small chip above that is a bank of NAND gates. Three identical chips on the left half of the board are motor controllers.
The top of the circuit board, with LEGO block case removed. Two IR LEDs and an IR receiver are on the left, an LCD is in the center, and several capacitors and an adapter socket are on the right. The LCD is covering an LCD controller and a speaker; above and below the LCD are the contacts for the four rubber
keys. Twelve clamps rise from the circuit board to mate with the input and output
ports on the cover.

Words by Merredith Portsmore