Let's Make Robots!

Mobile Robot Arm

Picks up things with a robot arm



I made this CAD in Google Skechup after I had an epiphany. I thought this would be a really cool idea, and would give me some good experience.

Here is how it will go:

 It will use 1/8inch Sintra, simialer to PVC

The Servo brackets will either  be custom made from bigbluesaw.com or pre-made from Lynxmotion.com

The gripper itself will also, either be homemade from aluminum or bought.

On the gripper, the actuator of the gripper itself will either be a regular DC motor with a Motor Controller or a Micro Servo (HS-81) Most likely the latter.


I will be using my custom Atmega8 MCU Board, which has fourteen servo connection ports.






Taken with my webcam so please excuse the crapiness



And I made an AVR Programmer from a kit from adafruit



I am aiming to use all eight servos as  hs-475hb servos because they are cheap and sturdy (~$15).

It will be controlled via Bluetooth from the UART connection on the Atmega board to a nearby laptop with a Bluetooth USB Dongle.

Eventually I will find a suitable wireless webcam, and skin the plastic off and mount it on the bot.


Does anyone have any reccomendations as to the accessories of this robot? (motors, servos...)


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This looks brilliant.  I'm excited to see your progress with it.
It looks great but I suspect where you use two servos in tandum you may have trouble with the servos fighting each other. It may be better to use a single servo that cost up to twice as much but has more torque.
I thought that would have been an issue too, but I think it can be resolved in the code, no?

Since they are mounted back-to-back, they obviously have to turn in opposite directions, so that is the first thing to remember.

Second, put the two servos on the same output from the controller and not to separate outputs. If you use separate outputs, one will get it's "move command" before the other and start ahead of the other.

Yeah, I suspect the aesthetics of having the two base servos mirrored might not be worth the added complexity of reversing one of the servos. Lynxmotion skipped that and just mounted both servos in the same direction. It does look a little odd and asymmetrical, but it's nice to just be able to use a simple Y-splitter cable to control both servos.

Don't forget that using two servos at every joint also adds more dead weight that the lower servos have to lift, which will reduce the amount of payload you can pick up. Again, I guess you have to decide how much you're willing to sacrifice for aesthetics. There's no right answer, just different priorities.

But really, my limitation with the Lynx L6 was the gripper servo -- it just couldn't squeeze very hard at all without stripping gears. I upgraded it to metal gears, and now they don't strip when I try to squeeze hard, but instead the servo just tries to draw so much current that apparently my 2.5 amp power supply can't feed it enough, so the servo jitters, and drops whatever it's carrying.

I would also definitely suggest that you go with a flat gripper like you've drawn, rather than the round 'fingers' that my Lynx L6 came with. It's pretty hard to pick things up reliably with the round one, and there's no good way to mount a pressure sensor to let you control how hard you squeeze things.


I was planning on doing everything you said except for the pressure sensor, I was planning on using a Hall-Effect sensor on one side of the gripper, and a magnet on the other, and when the gripper squeezes it will get closer and closer to zero, and when it there is an impediment (the object it grabs) then the hall effect will get a different reading, showing it has something. Is this very reliable?

Your plan probably wont work. depending on the size and strength of your magnet you may not get a reading with the grippers open beyond a certain point.

When you grip something, unless the object is ferrous then it won't affect the reading.

Ferrous metal used in the gripper construction may have an effect on reading.

I'd stick with force sensing resistors as previously suggested.

I cant believe I didnt think of that mate, cheers. Putting the two on the same output is a good idea.


Might need a servo reverser on one of a pair so that they both move the same direction though, as jka points out, they are back to back.

Might be possible to modify one to turn opposite of the input signal, by reversing a few connections internally.

You are right. Silly me. Putting them on the same output will of course make them turn the same way and then it won't work.

Theoretically, if you opened up one of the servos and swapped the wires for the motor, to make it turn the other way and swapped the outer wires of the potmeter, it might work. Provided, of course, that the motor isn't soldered directly to the PCB in the servo.