@o_lampe said in Duet Polargraph:
So the accuracy lies in the details, like a constant angle at the motor pulley, but a varying angle at the effector. That's why my build was off the track for so much.
To make the triangles work, the line from the anchor point to the effector centre really needs to be as close as possible to being in a straight line. Otherwise you're going to get an error. The motor standoff that the belt runs around is only 5.8mm in diameter. I chose the point at 45° as the pivot point, which is roughly at the point the belt touches when the gondola is at the centre of the area, ie X0 Y0. As the gondola moves to the limits, the anchor point does move, but only about 0.8mm in the 'significant' direction. The 'less significant' direction error is around 1.5mm, but has less effect on the calculations, as it is the long side of the triangle.
Close up of the 'C' anchor (top left motor). The arrow points to the pivot point on the 5.8mm standoff:
75a01843-73e4-4590-8000-0ce4d5824071-image.png
I think it would be easy to 'do the math' to account for the anchor point movement, and it's possible that other firmwares already do that. I haven't looked at the Makelangelo code (it uses it's own branch of Marlin). Putting the motor centre as the initial anchor point, with the radius of the motor pulley, and the direction of rotation, it could be calculated and adjusted for.
Where you set X0 Y0, and making sure you accurately home to that point, so the anchor points are accurate, is also important.
OTOH, I was happy with my belt tensioning system, using no weights but a BLDC motor in torque mode. Also good for horizontal use cases.
That's cool. Maybe four BLDC motors in torque mode would be a better way to spool the cable/belt if you built something with a motor at each corner.
In your case, I'd add rollers for the belt guides, as they might add friction and disturb smooth motion. The fish hooks look pretty flimsy, too as do the wings of the effector. But that's the original Makelangelo design.
Adding rollers would make the diameter of the pivot point bigger, and increase the error. The back of the belt is sliding on the standoff, which is printed in PLA, so without much weight on the gondola, I think it's fine.
The effector wings are printed flat, so are actually pretty strong. The hooks are also printed with the layer lines along the hook, so should be okay. I did break one putting the belt in, but it had warped off the bed anyway, so was too tight. That did break across a layer line. Generally pretty happy with the printed parts, nothing I'd change.
Ian
