Monday, 13 March 2017

Another Robot Arm!

I recently 3D printed a new bigger and better all singing all dancing Robot arm!  I got the plans off the most excellent thingiverse site and set the printing going.  17 hours later I was rewarded with the parts which for the most part fitted perfectly.

daGHIZmo's Robot Arm Mk 2

Credit should go to daGHIZmo for the mechanical design which is excellent.  I found the robot a little hard to put together but I'm not the best at this sort of thing...it frustrates me sometimes!

Here is the robot in it's full glory:

The 3D Printed Arm!
In order to control the robot we will need to drive several high current servo motors.  I tried to use my original servo driver shield designed in this post:

http://langster1980.blogspot.co.uk/2015/04/mearm.html 

I found that the current required by the motors was not nearly available.  When I designed that shield it was only meant to drive low power servos and not the high torque stuff required here.  I also didn't have a great deal of experience in driving servo motors...I hadn't done it for a very long time and truth be told I don't think I have ever done it properly.  So in order to use this robot I'm going to have to design a better circuit.

The main issue I'm seeing is that the servo motors are not working because the current draw is too high.  I suspect that the 5 Volt regulated arduino supply cannot source the instantaneous current needed to make the servos start moving - once inertia has been overcome the motors are driven quite well...Solution - More POWER!

If we provide an alternative high current 5 volt supply to the servo motors they will work as intended and it will still be possible to power the arduino and everything else required.  I also would like to apply some filtering to the servo motor drive connections as I suspect the DC motors used inside the servos are quite poor due to motor brush noise.

Here is the circuit diagram for the Servo Section:

Servo Connections and Filtering
Connectors for Distance sensors and Bluetooth Module and Control Potentiometers

Arduino Mega Connections
 Now to lay out the PCB.  Hopefully this will all fit!
The Top Layer with dimensions
The bottom layer with dimensions
I am a little worried that the external 5 Vdc track to the servo motors is not thick enough to carry the current so I may augment that track with extra wire soldered on.

To make sure every component fits on the board - here is the 3D render:
3D model - Top Layer

ISO of the 3D model
I have ordered the boards from Elecrow - It costs £10.40 including shipping which I think is excellent.  As soon as it arrives I'll populate the board and probably write up another post showing the robot arm in action.

That is all for now - Langster!