Scara kinematics

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7 years 4 months ago #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:
Code:
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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7 years 4 months ago #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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7 years 4 months ago #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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7 years 4 months ago #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) ....
The following user(s) said Thank You: DPFlex

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7 years 4 months ago #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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7 years 4 months ago #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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