Firmware for delta printer with telescoping arms

Hi everyone!

I'm a student as part of a group working to create a portable, delta printer that prints concrete. We're looking for any insights into the controls for a delta printer with telescoping arms, that is actuated along 3 axis (similar to the hangprinter design in that these are cables, except the hang printer actuates along the z axis with vertical cables that we don't have)
Was looking around but couldn't find a similar design

Thank you so much!
Can refer to the illustration attached for more clarity!
(PS I'm a newb to this site)

This concept is (mathematically) similar to the gus simpson, there are gcode processors and i believe a version of smoothie that supports these kinematics at the moment.

The kinematics are essentially solving the equation of a sphere for each tower/actuator.

(x1-x)^2+(y1-y)^2+(z1-z)^2=r^2

r is what you're looking for, that's the length of cable that's unwound from the spool. x1, y1 and z1 are the (x, y, z) coordinates of the spools and (x, y, z) are the x, y and z you want for the effector. Nice and simple kinematics, just solve it for each tower and you're sorted.

You think, you can control a stiff and stuborn concrete "extruder" by pulling on ropes? Sure the z-height isn't critical for you, if you only want to fill a construction with concrete. But how do you keep the nozzle vertical?

@Trakyan - Thanks for the insight! The gus simpson looks like a good place to start. Its got the same physics and we can probably tinker around to adjust for the difference in the ways we calculate 'length' of arm

@o_lampe -
We're using really high torque motors that we're further gearing up in order to control something that massive. That said, flexibility of the tube has been an issue which has limited our print area to a size smaller than that of the maximum print area (to reduce the maximum deflection the tube experiences). Would love to hear if you or others have faced similar issues!
Keeping the nozzle vertical has been challenging - we're using ujoints at our connections to make the nozzle rig act as a type of gimbal. That said, we have no idea if it'll work

Here is another concrete printer: [www.duet3d.com]. It uses an additional "axis" to keep the concrete extruder facing the direction of travel.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

O_lampe has a good point. As a motion platform you only need the three towers, but with a heavy concrete tube I can't imagine any reasonably sized effector staying stable unless you either add a fourth actuator like the hang printer, or somehow make the arms rigid so they don't have the compliance (i.e. go slack under "compression") that allows the effector to tilt. I think the hang printer like solution is much simpler/easier to implement and most probably cheaper. Another option I guess is to position the tube somewhere where it's less likely to cause the effector to tilt, like above the build area rather than off to the side.

If you do want to go the route of making the arms rigid, have a look at rigid chain actuators. They're rigid actuators that can be retracted much more neatly than using something like a rack and pinion, where you always have the full length of your arm somewhere (not so back when its in your build area, but not all that great when it's swinging around outside of it when that arm is retracted).

If the effector is always at the same height than the rollers of the rope-motors, they would build a stable triangle. Then add a second roller below or above the first and the effector can't tilt anymore. ( so 6 ropes with three motors ) That would limit the movement to XY-plane.
But the picture shows a different approach.

Edited 1 time(s). Last edit at 03/10/2018 03:56AM by o_lampe.

Yes that would technically be stable, but I imagine the weight of a concrete filled tube hanging off to one side would have little trouble tilting the effector.

Edited 1 time(s). Last edit at 03/10/2018 08:33AM by Trakyan.