Scara kinematics
02 Jul 2019 21:53 #138442
by sliptonic
Replied by sliptonic on topic Scara kinematics
Interesting. That helps to clarify a little bit. Playing with it a bit more I can now say that the error happens when joint1 goes from >=90 to < 90 degrees.
My configuration matches the description in scarakins.c. Ie when both joints are extending directly along the X axis away from the pedestal they are at 0 degrees.
This matches the scara sample config as well: /sim/axis/vismach/scara which is really confusing. Jogging the joint in a positive direction results in a counter-clockwise motion (for both joints) That seems exactly backwards to me but it matches the sample config.
I can't tell if the sample has the same following problem at the same point because it won't enter teleop even with all joints homed.
The problem doesn't seem to be related to physical inversion of the elbow but it does make a difference which way the the elbow is oriented. With joint0 positive and joint1 negative, it works and I can cross the -90 degree point without error. Perhaps this is just a limitation of the scarakins and I'm not understanding how it should behave.
But with joint1 positive, it doesn't
My configuration matches the description in scarakins.c. Ie when both joints are extending directly along the X axis away from the pedestal they are at 0 degrees.
This matches the scara sample config as well: /sim/axis/vismach/scara which is really confusing. Jogging the joint in a positive direction results in a counter-clockwise motion (for both joints) That seems exactly backwards to me but it matches the sample config.
I can't tell if the sample has the same following problem at the same point because it won't enter teleop even with all joints homed.The problem doesn't seem to be related to physical inversion of the elbow but it does make a difference which way the the elbow is oriented. With joint0 positive and joint1 negative, it works and I can cross the -90 degree point without error. Perhaps this is just a limitation of the scarakins and I'm not understanding how it should behave.
But with joint1 positive, it doesn't
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02 Jul 2019 23:07 #138457
by sliptonic
Replied by sliptonic on topic Scara kinematics
$ sign is a neat trick! It works, but only the first time. I can home and then $ into teleop but it won't work again after exiting teleop or re-homing. (Strange that my actual config doesn't have this problem. No matter.)
The sample scara config puts the home position for both joints at 0 and that's only place you can enter teleop in the example.
I changed the home position so after homing it ends up in 'bent left' position and it gives the same following error when joint1 passes 90 degrees! So I guess this is just a matter of using it right.
Note to future self and other scara users. Scarakins assumes the arm is 'bent to the right' like this.
Now if I could only figure out where the machine limits and come from.
In the correct orientation and homed, I have machine limits of X:+230 to +400 Y:-390 to +390 I guess that means it assumes the center of the pedestal is at world coordinates X0Y0.
The sample scara config puts the home position for both joints at 0 and that's only place you can enter teleop in the example.
I changed the home position so after homing it ends up in 'bent left' position and it gives the same following error when joint1 passes 90 degrees! So I guess this is just a matter of using it right.
Note to future self and other scara users. Scarakins assumes the arm is 'bent to the right' like this.
Now if I could only figure out where the machine limits and come from.
In the correct orientation and homed, I have machine limits of X:+230 to +400 Y:-390 to +390 I guess that means it assumes the center of the pedestal is at world coordinates X0Y0.
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