Phase 2 Robot Design – Claw Robot

Claw Robot

In anticipation to collect loops that were placed in any direction, a double claw robot was built.  This design was later discarded due to concerns with its efficiency and maneuverability, but the robot was such a massive and impressive creation, its design concept is documented here for the record.

The Claw Robot Design was divided into couple different components:

End Effectors: A double claw assembly, cable arm, fold-up basket hopper
Motors: 2 Power Function and 3 NXT motors
Drivetrain: Rear wheel drive, 2 wide flat rubber tires, 2 tank treads, 2 front skis with rollers
Sensors: EOPD, compass, ultrasonic sensor, IR Link
Accessories: Video Camera and counterweight

End Effectors

A four-way claw as envisioned in Moonbots Phase 1 was built to collect loops placed in any orientation. In order to lift the massive arm, a pulley system was used.  This made it easier to raise the arm but too slow to even think of getting more than 4 loops in the alloted 3 minutes.  To speed up the process, two claws were used to grab two loops at oncep. The claws were powered by a single motor driving both claws using chains, and the claw assembly can also rotate by another motor to grab loops when robot was coming in at an angle and to drop the loops into a foldable hopper below.

Double Claw assembly

With such a large robot, the main concern was accuracy. To avoid any instances where the robot could become disoriented, a long arm was used to suspend the claws over the crater, so that the robot wouldn’t need to enter any craters. The arm was attached to the base of a mast which served as the anchor points for the 6:1 pulley, limiter strings, load reduction rubber band, sensor mounting points, and a very convenient handle to carry the robot. To provide more lifting power, the NXT motor was reduced to a 2:1 ratio though a gear train which pulled the line running through the 6:1 pulley assembly. If this pulley design were to be finalized, we had a messy roll of LEGO lines at hand to swap out all temporary lines.

Pulley arm design

Also, if the double claws are placed wide enough to grab both loops, then the hopper to catch the loops will be wider than width of a piece of A4 paper.  Therefore, a foldable collection basket was used to accommodate the width of the double claws.  The basket would drop down as the robot arm unfolded after the robot left base. The basket was anchored to the front of the robot and clipped to the middle of the arm so the entire package when folded is less than the A4 size required.  The tip of the basket also provided a forward mounting location for the EOPD sensor.

Fold out loop basket

With so much weight in front of the robot, it was very front heavy and easily tipped when coming down the ramp or over the ridge, so a folding counter balancing tail was added. By mounting the Power Function battery pack on the tail and a stack of large LEGO wheels that flip out, we managed to achieve a 50/50 front/rear weight bias on the wheels of the robot.

Counter weight with Power Function battery


Two NXT motors were used for the drive train, one NXT motor was for lifting the arm.  Two Power Function Motors were used to control both the opening and closing of the claws and the rotation of the claws, while the collection basket and robot counter tail would unfold by gravity.


Due to the weight of all the end effectors, the drive axles were bending under the weight of the robot.  To create additional support for the axel,  support beams were connected to the outside and inside of the drive wheels hence taking some load off of the axles. Double tank treads were used on the underside to increase traction and power when climbing over the ridge. To make it easier for the robot to turn, small plastic wheels were used for the main robot navigation while skis were mounted in the front to glide over the ridge.  This drive train design pretty much adhere to our Phase 1 concept.

Robot folds up in base


Navigation was provided by a combination of Compass, Ultrasound, EOPD and NXT motor rotation sensors. Rotation sensors provided basic distance information, Compass sensors gave heading information, and Ultrasound and EOPD together were used in wall-following and landmark detection. The last sensors port was used up for the Power Function IR Link to control the Power Function motors.

Design Pros and Cons

Pro: Able to pick up loops at any directions
Con: Too slow and too massive, hard to balance and manuver

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