Robotarm v3.0

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august 10th, 2002.

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access to my Lego pages since creation.

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Copyright 1996, 2000, Denis Cousineau


The ultimate Lego robot arm!

A quick survey of the robot arm located on the Mindstorms site show that all the models have in common a very important default: their weight. They are all heavy and cumbersome.  One disadvantage of this is the speed: in order to have enough strength, a lot of gear reduction must be used, increasing even more the weight. As a consequence, these robots are slow moving, and prone to breaks.

We propose here the lightest robot arm ever made, the simplest too, since it requires only 240 pieces exactly. It is also an arm that has a very wide range, reaching as far as 18 inches (50 cm) but also good for close range since it can touch its base. It can also turn more than 360 degree, but at some point, the hoses and wires inside the base will get too stuck... Finally, speed is high, going from fully retracted to fully extended in less than 3 seconds.

This arm necessitates 4 controls (1 for rotating the base, 2 for the arm and forearm, and one pneumatic for the hand - not visible in the picture). It can be troublesome for the RCX, but using multiplexors (either electronic, see TFM pages, or mechanical - see my pages below), the RCX has the ability to control this arm.

I give below some snapshots to help you build this arm.  If you appreciate the design, please let me know. If you have improvement (to make it lighter, faster, or whatever), let me know too.

Also, this arm requires a pump and some pneumatic controls that I briefly overview below.

You can see:

Making this Robot Arm
Controlling pneumatics

Making this Robot Arm:

For your convenience, I added the LDRAW files for each pieces. That way, you can peruse in more details the assembly.

  1. The base (1_base.dat)

    The base simply hold the turntable.  It has to be rigid, and somehow heavy is a good idea.  On the back, there's an axle connected to an endless worm gear.  Connect this axle to the motor of your choice.

  2. The arm (2_arm.dat)

    The arm is perfectly symetrical both left-right, and back-forth. Two motors are embedded in this part, that will be responsible for the movement of the arm and the forearm. You will need to connect the wires and pass them through the base.  If you need more space, the last axle on top of the turntable can be removed easily, and the blue beams can be lifted forward.

  3. The forearm (3_forarm.dat)

    The forearm is simple.

  4. The levers and the sliders (6a_dented.dat, 6b_dented.dat, 5_slider.dat)

    The levers are dented, so that the motors on the arm will push on them, or pull them.  The sliders are simply a slippery place where the levers can move freely without friction, and who can change their orientation as the arm extend or contract.  The levers, when contracting, acts like muscles, they simply for the arm to get closer to the other section, bending it along the axle.


  5. The hand (4_hand.dat)

    (funny, it should have been black!)  The only part missing from the hand is a large piston connecting from the top axles to the connectors inside the two triangular parts. Instead of these triangles, you may use right-angle pieces, but I had no more... The tires are meant for a better grip, and to compensate the weight of the piston, so that the hand is always correctly oriented toward the floor.

    hand_1.gif (6103 bytes)

  6. The full assembly (0_FullArm.dat)

    You need to connect the motors located on the arm, put hoses to the piston on the hand, and install a motor at the base to make it turn, and Voila!


Controlling pneumatics

In order to control pneumatics, we need to be able to turn on or off a Lego switch, and we also need to supply pressure into the hoses.  To accomplish all this, I build a control center, as see below.

pneumatic_station.jpg (48799 bytes)

This control center has, from left to right:

A Lego switch controller, taken directly from Soh's pages (see link on my home page), except that I placed it vertically for reasons of space. 
A pump.  I used the exact same design as for the Lego switch above, that is, the same motor and gear reduction, but instead of being connected to a switch with a lever, it is connected to a pump.  There is space for a second pump (if I had a second pump) on the other side so that they could work together, 180 degree apart.
Since I didn't want the pump to function all the time, I install a pressure tank (see my own pages on that topic in Physics of Lego).  However, I needed a pressure detector that will start or stop the pump when pressure is too low or too high.  I borrowed the following pressure switch from the Hempel design pages.

pneumatic_station_switch.jpg (48024 bytes) pneumatic_station_pump.jpg (60072 bytes) pneumatic_station_pressure.jpg (43629 bytes)

Connections: the pressure switch is connected to a power source that is always on (not the RCX), such as a battery pack with the On button pushed in. The other extremity of the pressure switch is connected to the pump.  The Lego switch can be connected to the RCX. However, because the three motors on the arm and the pressure switch totalized 4 outputs (only three are available on the RCX, I used a serial multiplexor of my invention (described below) to control either the base or the hand with one output.