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Henry's Arm
09-16-2008, 01:20 AM
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<!-- / icon and title --><!-- message -->I've been deeply interested in robotic arms lately and have been researching and design a 6 axis one from scratch.

I have purchased a Labvolt Armdroid that uses 6 stepper motors for 5 DOF + gripper and it's not bad but more of a toy than a tool (other words adult toy) so want to build my own.

I'd like to see if anyone is interested out there to help.

My expertise and capabilities:
-math and programming, I will write most code in Matlab
-electronics layout (can mill my own boards and design the circuit)
-3d CAD drawing (Solidworks)
-CAM machining, almost anything 3 axis we can draw I can program with CAMworks to machine in about 5-10mins

Weaknesses that would like help:
-motor control (stepper voltages, currents, drives, etc)
-gearing
-material selection

Goals:
-FREE full open source 3d model is solidworks that can be machined/built easily from the assembly file
-free code to drive it
-almost every part will be able to be made easily on a 3 axis CNC machine with minimum tooling or bought from master.com
-mostly made of 1/4" AL plate to reduce cost
-easy to assemble/take apart
-anything that can be controlled in software rather than hardware will be
-2 to 4 ft reach at full extension
-6 to 10 lbs at full extension
-6 DOF

Intentions:
-minimize use of gears to reduce weight
-use stepper motors (cheaper and less gear requirements) and microstep them, 400 steps/rev, prob NEMA 23 - 34
-accuracy .001-.002" repeatable
-absolute encoder on all axes (stepper motor for easier control but need absolute encoder to get solid repeatable accuracy)
-code to translate the spherical coordinates to XYZ moves
-speed to be around 200-360 degrees/sec
-perhaps similar to CRS Robotics F3 ($60,000 robot) but bigger
-material cost under $2000 (realisticly it's not going to be super cheap, motors, drives and encoders aren't super cheap)

Rather than just building it and leaving everyone else on their own, I am building a detailed model for each part. Every plate, every hole, every screw will be added into the 3d solidworks model. You can freely move each axis/part to see how the set screw holds the motor bracket in place and view the range of motion, calculate the weight, etc.

I did some prelim drawing and made up a quick wrist design. I will post the solidworks assembly file to show the quality of the drawing. You can hide each plate and machine them individually.

Later on I will even include CAMWorks CAM data so you can just select the machine you want to it will pop out the set up sheets and G-Code to build your own robot.

End results:
-robot arm can pick up a tool and carve out a sculpture in 3d with many undercuts
-robot arm can hold things/weld/etc
-robot arm can pick up a rum bottle, pour it into a glass and shake you a martini http://www.cnczone.com/forums/images/smilies/smile.gif
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Henry's Arm
09-16-2008, 01:20 AM
My first steps are power and motor selection.

Let's assume the arm is going to do 4ft and hold 20bs (say 10 + 10 pounds of arm) so I need 80 ft-lbs at the base.

I want 360 degree/sec so around 60rpm.

Power = torque * rpm = 80 ft-lbs *2*pi*60rpm /33000 so close to 1 HP (745W) at the base!

That's a lot of juice.

Most stepper motors are rated with a voltage and current. My understanding is that the voltage doesn't mean anything because you can drive it at much higher voltage as long as you meet the current requirements.

Anyway, let's look at something like the NEMA 23 PH266M which can be bought for around $20 on ebay and has 400steps/rev.

It's rated at 6V and 1.2A so does that mean it's only a 6W motor?

henry99
09-16-2008, 07:03 AM
Whoops meant to post in the robotics forum. You can move it there if it's more appropriate...