MPSCARA G28: Intermediate position outside machine limits

oliof

Thanks, I will try that tonight!

Just one note: The distal arm can move clockwise beyond 0 if the area is cleared (i.e. when it's not too close to the endstop setup) -- will I lose this reachable area when I set B0:140 (for accurate values instead of 140)?

dc42

@oliof said in MPSCARA G28: Intermediate position outside machine limits:

Thanks, I will try that tonight!

Just one note: The distal arm can move clockwise beyond 0 if the area is cleared (i.e. when it's not too close to the endstop setup) -- will I lose this reachable area when I set B0:140 (for accurate values instead of 140)?

Yes you will lose that area, however using it means doing an arm mode switch, which you don't want to do in the middle of a print. If it's a problem then I expect I can find a workaround.

oliof

Not a problem, just want to make sure I understand the implications. Thanks again!

oliof

It moves! Well, somehow. But as the moves are repeatable, I probably just need to understand what I'm doing wrong. Since I don't yet, no questions right now, just a fun image about what should be some straight lines ...

dc42

Try sending this command immediately after homing:

G1 S2 X0 Y0

That should centre both arms, so that they are in line. What XY coordinates does DWC show, or M114 return? It should be X200 Y0.

Likewise, if you send:

G1 S2 X0 Y90

then the distal arm should be 90deg anticlockwise from the proximal arm, and the position should read X100 Y100.

oliof

I looked up some more info from the Marlin Configuration.h included with the model and updated my config.g accordingly (updated version attached).

I also added the G1 S2 X0 Y0 to the homeall.g and the proximal arm moves a couple degrees further than "absolute north", and the distal arm keeps it's relative position in space parallel to the end stop.

Rough sketch (not to scale, angles not representative):

M114 returns

X:109.070 Y:0.000 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count 0 0 0 Machine 109.070 0.000 0.000

X109.07 is almost right as I corrected the arm lengths and set the X offset to 90 in accordance to the Marlin Configuration.h from the thingiverse project. I have added it here for completeness' sake. At the end of the proximal arm moving, there is some blocking in the mechanics, possibly because the distal arm moves in the wrong direction and crashes into the proximal arm.

That hypothesis is supported by the fact that if I run G1 S2 X0 Y90 instead of G1 S2 X0 Y0, the distal arm moves clockwise until it crashes into the proximal arm which gets in the way as it travels to it's new position, and if I run G1 S2 X0 Y-90, the distal arm keeps position equidistant to the proximal joint:

0_1551520109306_MPSCARA G1 S2 X0 Y-90 after homing.jpg

Here, M114 returns

X:8.388 Y:-100.660 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count 0 -2581 0 Machine 8.388 -100.660 0.000

It's worth noting that the end of the distal arm is pretty much in line with the z axis and the proximal joint after this move.

This probably means the motor for the distal arm moves in the wrong direction after homing? Can I just set M569 P1 S0 at the end of homeall.g? And then I probably need to calibrate the motor steps ...

files:

dc42

In your SCARA machine, when the proximal motor turns but the distal motor doesn't, does the distal arm stay pointing in the same absolute direction, or does its angle relative to the proximal arm stay the same?

oliof

The distal arm stays pointing in the same direction. In the MPSCARA design, the motors for proximal and distal arm are stacked on top of each other, and the distal arm is driven via a 2-belt stack:

       |
       Z
       |
PM...{[PJ]---PA---[DJ]}
DM....[DG]˙˙˙˙˙˙˙{[DJ]---DA---}
       |
       Z
       |

PM and DM are the proximal and distal motors
PA and DA are proximal and distal arms
gear belts are denoted with dots (fullstop for the belts from the motors, high dots for the belt from the distal gear to the distal joint).
PJ and DJ are the proximal and distal joint
DG is a gearwheel that transmits from the motor gear to the joint gear, moving indepentently. It has one belt connecting it to the motor, and another belt connecting to the distal joint.
PJ and DG are centered on the Z axis.

oliof

Did some further thinking and now I think I got the configuration right. The Marlin configuration mentions the distal motor (Y) goes backwards, which makes sense considering it's flipped 180 degrees ...

So I changed the kinematics for the distal arm from B0:108 to B-108:0, and inverted the moves for distal arm in homeall.g

After this, the G1 S2 ... moves as requested by @dc42 behave as expected.

G1 S2 X0 Y0;
M114;
X:199.070 Y:0.000 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count 0 0 0 Machine 199.070 0.000 0.000
G1 S2 X0 Y90;
M114;
X:98.388 Y:100.660 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count 0 2581 0 Machine 98.388 100.660 0.000

dc42

The distal arm stays pointing in the same direction.

In that case you need to use the crosstalk factors in the M669 command, because by default the firmware assumes that the distal motor drives the distal joint angle directly. The value I think you need is C-1:0:0.

Edit: but if the machine is now behaving correctly (after you changed the distal joint range), then I am confused.

oliof

Should I do that in addition to, or in place of, my changes?

dc42

Please check that when you send G91 followed by G1 S2 moves:

+X moves the proximal arm anticlockwise
+Y moves the distal arm anticlockwise

(viewed from above).

oliof

Viewed from above, without the crosstalk setting but with the flipped Y motor changes, +X and +Y do indeed move the arms anticlockwise.

Without the flipped Y motor, the crosstalk setting does not alleviate the movement confusion I had before.

I can now draw a nice 180 degrees arc by going from X-90 to X+90 (the little line ar Y-90 happened when I accidentally moved the arm noting the coordinates; any deviation is due to mechanical imprecision and angled distortion of the view):

dc42

That's good. So try this test again:

@dc42 said in MPSCARA G28: Intermediate position outside machine limits:

Try sending this command immediately after homing:

G1 S2 X0 Y0

That should centre both arms, so that they are in line. What XY coordinates does DWC show, or M114 return? It should be X200 Y0.

Likewise, if you send:

G1 S2 X0 Y90

then the distal arm should be 90deg anticlockwise from the proximal arm, and the position should read X100 Y100.

oliof

G1 S2 X0 Y0
M114
X:199.070 Y:0.000 Z:4.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0  Count 0 0 10240 Machine 199.070 0.000 4.000
G1 S2 X0 Y90
M114
X:98.388 Y:100.660 Z:4.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0  Count 0 2581 10240 Machine 98.388 100.660 4.000

The values are not exactly X200/Y0, X100/Y100 since I edited the arm lengths according to the Marlin configuration that's been made available by the original designer. The proximal arm length is 98.41, and the distal arm length is 10.6, so the values match.

dc42

Looks good! Is movement working correctly now?

oliof

I can now issue moves, but I don't get straight lines, only arcs.

I have modified my kinematics with some initial offsets so I stop crashing the distal arm into the proximal arm ... it seems like the A... and B... limits are not used to limit motion (I also entered an erroneous G1 S2 X6060 once, and the arm promptly tried to do 6060 degrees of counterclock movement, which was stopped by the robot's base of course). The current kinematics definition is

M669 K4 P98.41 D100.66 A-108:108 B-108:0 X160 Y-150; setup SCARA kinematics

here are the results from trying to reach the four corners of my currrently defined bed:


G0 X0 Y0
M114
X:-15.497 Y:14.529 Z:4.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count -1071 -329 10240 Machine -15.497 14.529 4.000
G0 X0 Y300
MM114
X:-15.497 Y:285.471 Z:4.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count 1404 -329 10240 Machine -15.497 285.471 4.000
G0 X40 Y300
G0 X40 Y0
M114
X:-1.540 Y:268.845 Z:4.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count 1223 -329 10240 Machine -1.540 268.845 4.000
G0 X0 Y0
M114
X:-15.497 Y:14.529 Z:4.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0 Count -1071 -329 10240 Machine -15.497 14.529 4.000

Further tests show that if I "re-center" the printer with G1 S2 X0 Y0, I reach different locations depending on whether I did a move before re-centering or not, or whether I combine X and Y moves.

I am trying to plot this so I can show rather than tell, but it's a bit difficult. See the attached photo for some nice arcs that depend on whether I send a G0 Xn Ym, or G0 Xn and G0 Ym separately

oliof

here are some more tests:

G28; includes G1 S1 X0Y0 -- marked A in the picture
M114
X:39.070 Y:150.000 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0  Count 0 0 0 Machine 39.070 150.000 0.000
G0 Y0 ; marked B in the picture
M114
X:-1.807 Y:30.801 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0  Count -895 -329 0 Machine -1.807 30.801 0.000
G0 Y300; marked C in the picture
M114
X:-16.267 Y:286.288 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0  Count 1413 -329 0 Machine -16.267 286.288 0.000
G0 Y150 ; distal arm crashed due to blockage of end stop trigger block on distal arm -- circled in the picture
M114
X:-16.267 Y:150.000 Z:0.000 E0:0.0 E1:0.0 E2:0.0 E3:0.0 E4:0.0 E5:0.0 E6:0.0 E7:0.0 E8:0.0  Count 1273 -2511 0 Machine -16.267 150.000 0.000

dc42

Did you add the C parameter to the M669 command, that I said I thought you needed in my earlier response?

oliof

I must have inadvertedly removed that while I was trying to find sensible offsets for X and Y ...

C-1:0:0 brings me one step closer, but is not perfect yet. There seems to be some elliptical effect for X moves still that gets more pronounced the more the arm is away from the middle of the bed (i.e., the position after G1 S2 X0 Y0). Y moves are still very much arcs, but now the positions from absolute G0 moves match what DWC reports.

This is mostly visible in longer moves (more than 2-5mm), so maybe it's irrelevant for my use case where individual moves will most certainly be shorter. I tried to play with the S and T parameters, but I don't know what reasonable values would look like.

Another observation is that only G0 ... moves work, whereas G1 ... moves fail with "intermediate position outside machine limits" -- I guess this is because G1 is special with SCARA kinematics ... G1 S2 ... still works to set proximal/distal arm angles. I guess the error message is for the usual case of people trying to move beyond axis limits, and not for people that try to do angle-based moves while in a distance-based coodinate system ...

Also, final observation: If you set a value at the second or third factor of M669 C, your Z motor will keep going even if you hit the Z endstop. Don't do that (-: