Scara kinematics

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15 Jun 2019 16:43 #136994 by sliptonic
Scara kinematics was created by sliptonic
I've configured my scara robot so all joints are homing nicely. In Axis, when I jog the Z axis in a negative direction (down), the end effector physically moves down. So far, so good. But it moves 'up' in the 3d window.
In the ini file, traj coordinates are XYZC and Display geometry is the same.

If I switch display geometry to XY-ZC Then the motion on screen is coordinated with the machine but everything else is inverted. Z "up" is below Z 'down' if that makes sense.

I've tried inverted the axis by changing the encoder scale and output scale, but then jogging negative goes up and vice versa. So I don't think my issue is hardware configuration. I think it's related to the kinematics.

in scarakins.c there's a comment:
joint[2] = End effector slides along a vertical axis at the outer end
		of the outer arm.  A value of zero means the end effector
		is at the same height as the center of the outer arm, and
		positive values mean downward movement.

That leads me to believe my joint[2] needs to be negative. But I don't know how to set that.

Am I barking up the wrong tree?

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18 Jun 2019 22:23 #137236 by andypugh
Replied by andypugh on topic Scara kinematics
It sounds to me like the kinematics is set up wrong. positive for down just seems wrong?

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23 Jun 2019 18:39 #137663 by sliptonic
Replied by sliptonic on topic Scara kinematics
I thought so too but that's how the scarakins.c file is now in master. Is this a difference between joint mode and world mode?

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24 Jun 2019 12:31 #137731 by andypugh
Replied by andypugh on topic Scara kinematics

sliptonic wrote: Is this a difference between joint mode and world mode?


It seems that it is:

github.com/LinuxCNC/linuxcnc/blob/master...tics/scarakins.c#L94

z = .... -joint(2) ....

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25 Jun 2019 23:03 #137833 by sliptonic
Replied by sliptonic on topic Scara kinematics
Ok, that worked! I had to invert the Z in the inverse section as well.

github.com/LinuxCNC/linuxcnc/blob/master...ics/scarakins.c#L162

It's not a high priority but scaragui is reversed as well. Any idea where I flip that one?

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25 Jun 2019 23:44 #137839 by sliptonic
Replied by sliptonic on topic Scara kinematics
I'm also finding that while I can jog in joint mode consistently, in world mode, I get following errors at places that should be well inside the usable envelope. These errors happen regardless of the jog speed but happen consistently at certain places. For example, I'll get following errors jogging at 100 mm/min in one part of the table but can easily jog at 6000 mm/min in another. As far as I can tell, it's nothing mechanical since each joint can move at maximum speed in joint mode in the same area.

Advice on how to troubleshoot this would be appreciated.

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25 Jun 2019 23:54 #137840 by andypugh
Replied by andypugh on topic Scara kinematics
Is the problem at the point where the elbow inverts?
Many positions with SCARA can be reached in two ways, and the kins has to avoid flipping between them.

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26 Jun 2019 12:56 #137895 by sliptonic
Replied by sliptonic on topic Scara kinematics
No. This is inside the rectangular area that can be reached without inverting. As far as I can tell, there's nothing special about it..

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27 Jun 2019 19:51 #138026 by andypugh
Replied by andypugh on topic Scara kinematics
Can you try capturing the joint position commands with Halscope when the problem occurs?

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28 Jun 2019 23:23 #138141 by sliptonic
Replied by sliptonic on topic Scara kinematics
Sure but I might need a bit more guidance setting this up. My halscope kung-fu is pretty weak.

Do you mean the a-pos-cmd on the signals tab in halscope? Not sure what that corresponds to as I have joints 0,1,2,3 joint mode and axes XYZ in world mode.

A screenshot or guide how to set up halscope to capture this kind of error would be very helpful.

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