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Robot Arm
Building: Expert
Program: Intermediate

Building Instructions
















The two brackets built in this step are used to attach the arm motor to the turntable base.  You will need to attach one bracket first, then attach the arm motor, then attach the second bracket, as shown below.






Four wires are needed in this step:
  1. Use the shortest wire to connect the touch sensor to port 1 on the NXT.

  2. Use one of the longest wires to connect the claw motor to port C on the NXT.

  3. Use one of the medium-length wires to connect the turntable motor to port A on the NXT.

  4. Use one of the medium-length wires to connect the arm lifting motor to port B on the NXT.

Robot Arm Programming

Use the program Arm Control for the Robot Arm.  This program allows you to control the robot arm using the Left, Right, and Enter buttons on the NXT brick and the touch sensor button.


Using The Robot Arm
  • Pressing the orange Enter button on the NXT brick will toggle the controls between "Rotate" and "Lift" modes.

  • In "Rotate" mode, the gray left and right arrow buttons on the NXT brick will rotate the arm left and right as long as you hold the button down.

  • In "Lift" mode, the left and right buttons will lift the arm up and down as long as you hold the button down.

  • Pressing the touch sensor button will make the claw switch between grabbing and releasing.

The turntable and lifting arm motors both use gears to "gear down" the motion (decrease speed and increase force).  You will notice that this keeps you from being able to turn the motors by hand.

To work around this problem, you can use the handle on the side of the arm lifting motor to adjust the arm's up/down position by hand when the program is not running.  (When the program is running the NXT has the electric brakes on the motors and will not allow them to be turned by hand).

This design does not include a similar lever on the turntable motor.  Can you figure out a way to add one?


  • Practice using the controls to pick up and move objects.   Here is an example challenge.  Place the red and blue balls on tire stands in different locations but both at the correct distance from the arm so that the claw will reach it, as shown below.  Now try to use the robot arm to swap the positions of the two balls.  What is smallest number of button presses that you think this can be done in?

  • Think of some mechanical improvements you might make to this design.  For example:

    • Come up with a way to keep the wires out of the way.

    • Add a lever to the turntable motor (similar to the one on the lifting arm motor) so that you can turn it by hand to adjust its position when the program is not running.

    • The claw is designed to be as simple as possible to keep it light weight.  However, it would be nice if both sides of it opened and closed.  Can you come up with a simple way to do this?  See the Claw Car project for one idea (that might be too heavy).

    • Gears are used on the turntable and lifting arm motors to reduce the speed and increase the force.  The gear ratios used are adequate, but the program still needs to used reduced power to keep the motion from being too fast.  It would be better if the gear ratios were even lower.  For an expert building challenge, can you figure out a way to do this?

  • The Arm Control program allows you to control the arm using the buttons.  Try making a program of your own that controls the arm automatically by itself with a pre-programmed sequence of moves.  Here's a big challenge: Can you write a program that will swap the balls as shown in the challenge above?  Francesc X. Alvarez was the first to send me video proof of a working solution.  Here is a YouTube video of his solution.

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