Coupled polar kinematics
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@joergs5 said in Coupled polar kinematics:
make sure when calcuation of the segments occur, that there are no big angle changes.
I think this is the bit I need to find in the code. Since it is segmenting the path, I should be able to identify when something is wrapping around, i.e. in a segmented path, there shouldn't be a segment where angle changes by nearly 360 degrees. (But there could be a 180 degree change - e.g. from [0.99,0.01] to [-0.99,-0.01] is a 2mm long segment that features a 180 degree rotation).
Thanks for the pointers.
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@achrn
I think you have to change the CartesianToMotorSteps motorPos values when the described situation arises. The solution is, to set motorPos[1] to a value less (or more) one full rotation value. For analysis, the M114 Count values can be used, which are the motorPos values.
The minimum and maximum values of motorPos should probably be limited to avoid wire or filament break (wires of heated bed e. g.). -
@joergs5 said in Coupled polar kinematics:
The minimum and maximum values of motorPos should probably be limited to avoid wire or filament break (wires of heated bed e. g.).
No wires - the radial and angular position are of the effector, and bed is stationary. There's also no material fed across the movement interface - it's effectively a pen plotter. Thus it is a genuinely continuous rotation bed.
(The first limit is motor steps counter stored in a 32 bit variable, which gives me 'only' about 26 hours of continuous one-way rotation. I don't anticipate exceding that limit, however dc42 advises that G92 will reset the steps counter, so some gcode can overcome that 'limit'.)
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@achrn said in Coupled polar kinematics:
the radial and angular position are of the effector, and bed is stationary.
How about linking an encoder to the rotary axis - eg. via a 1:1 timing belt drive - and using closed loop control for the radial position, with that encoder as the feedback?
It should cause the radial motor to track and inherently compensate the angular motion?
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@achrn said in Coupled polar kinematics:
No wires - the radial and angular position are of the effector, and bed is stationary.
So there will be no problem to implement the proposed change to the code.
One could use brushes (or pogo pins) to implement continuous heated roting plates if needed.
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@achrn I think the reason is the jump of the atan2 function at 0 from pi (180 degrees) to -pi if x < 0, please see https://en.m.wikipedia.org/wiki/Atan2. x=0 is not defined for atan2.
I would propose to store an absolute counter (how many full rotations are made) and caching the motorPos of last move, so instead of the big change, a small one with one rotation more/less can be made. The absolute counter is not important for your indefinite rotation plate, but for those who have to rewind. The 180 degree positions can be calculated from the M92 values, because M92*180 (and *(360+180), *(-360+180) etc.) are the 180 degree positions.
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@rjenkinsgb said in Coupled polar kinematics:
How about linking an encoder to the rotary axis - eg. via a 1:1 timing belt drive - and using closed loop control for the radial position, with that encoder as the feedback?
It should cause the radial motor to track and inherently compensate the angular motion?
I can see how it would work (in principle), but I fear it would be more complicated than getting the code to determine when it is jumping and doesn't need the implied radial position adjustment. Also, I'm endeavouring to avoid wires across the rotating interface, and the encoder tracking radial position is the 'otehr side' of that interface, so would need to signal across it. At the moment I'm using stall detection to home the radial axis and it's purely coaxial mechanical elements across the rotating interface.
@joergs5 said in Coupled polar kinematics:
I think the reason is the jump of the atan2 function at 0 from pi (180 degrees) to -pi if x < 0, please see https://en.m.wikipedia.org/wiki/Atan2. x=0 is not defined for atan2.
Yes, I'm sure that's the cause of the jump (as the mechanism moves through that location the stepper driving the radial position chirps / screams / jumps - I think it basically just gets hit with an instantaneous demand to turn one whole revolution because it 'thinks' the angular position has turned one whole revolution). I need to look at the code more closely, or more accurately, look at more of the code (than just the two functions CartesianToMotorSteps and MotorStepsToCartesian) to work it out.
Thanks all.
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@achrn said in Coupled polar kinematics:
Also, I'm endeavouring to avoid wires across the rotating interface, and the encoder tracking radial position is the 'otehr side' of that interface
?? The encoder would be fixed, probably near the rotation drive motor - it's to measure the rotary position, not radial, even though it's to add in to the radial drive position to counter the rotational influence.
Another alternative setup would be to use a differential (ie. like a vehicle axle type) to combine the rotation with the radial feed.
That's done with machine tools (horizontal borers) to control the radial position of the tool slide on a rotary facing heat, as that otherwise has the same problem.
Other complications & hardware vs software are a different matter and are all down to you, I'm just throwing in ideas..
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@rjenkinsgb said in Coupled polar kinematics:
@achrn said in Coupled polar kinematics:
Also, I'm endeavouring to avoid wires across the rotating interface, and the encoder tracking radial position is the 'otehr side' of that interface
?? The encoder would be fixed, probably near the rotation drive motor - it's to measure the rotary position, not radial, even though it's to add in to the radial drive position to counter the rotational influence.
Sorry, yes you're right. I had it swapped round in my mind.
I also hadn't thought about a purely mechanical solution - that has some attraction. Thanks.
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@achrn said in Coupled polar kinematics:
With radial position controlled by a shaft coaxial to the turntable, the 'motorPos' X axis (radial distance) depends upon whether that position is reached turning clockwise or anticlockwise. Mostly this works out OK, but my code (up the thread) doesn't work when you cross the line where the atan2 function swaps (i.e. +/- 180 degrees).
To me, the obvious solution is to disable continuous rotation when the crosstalk factor is nonzero. However, that means that you won't be able to print in a small sector of the bed.
