# Kinematics... again:/

15 May 2017 12:00 - 15 May 2017 12:35 #93221 by bkt
Replied by bkt on topic Kinematics... again:/
As you all understood, as well as speaking poor English are also inattentive.... but watch your video I see this end effector arrangement .... the red lines are not the blue ones ... (my sketch):

I'm in right?

To better explain what I see and what I understand:

And I belive Andy suggest you the right Z direction and offset ...
Normally in scarakins tooltip is these (I hope the sketch it's understandable):

so in your case the toltip axis is 90° respect normal case ... (you have only rotary toltip, not linear also, you have only J0 linear).

regards
giorgio
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Last edit: 15 May 2017 12:35 by bkt.
The following user(s) said Thank You: Boogie

15 May 2017 17:01 #93227 by Boogie
Replied by Boogie on topic Kinematics... again:/
Thank you Giorgio for input. I can see similarities between robokins and scara. Robokins is, as you said Giorgio, scara arm rotated 90 deg around Y axis, with linear joint 0 added to slide along X axis. But joint 4description in robokins.c file says:

joint[4] = End effector rotates around horizontal axis, that is
perpendicular to previous rotation axis and is parallel to Y axis.
This module assumes there is no offset between joint[3] and joint[4] axes.

...perpendicular to previous rotation axis (which was parallel to cartesian X axis) and parallel to cartesian Y axis. What you have drawn is joint 4 parallel to cartesian Z axis (as long as previous joints keep tool tip normal to cartesian XY plane).

I'm confused:/

15 May 2017 20:39 #93239 by andypugh
Replied by andypugh on topic Kinematics... again:/
To an extent you don't need to understand robokins, you need to understand the mechanics of your own robot.

16 May 2017 09:23 - 16 May 2017 16:11 #93253 by bkt
Replied by bkt on topic Kinematics... again:/
for better see your 90° in kins ... see these:

I'm not a little genius/magician like AndyPugh, but I hope to understand and explain your problem.
I have the sincere admiration for the work done by Andy (I add this to not be misunderstood).

Regards
giorgio
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Last edit: 16 May 2017 16:11 by bkt.
The following user(s) said Thank You: tommylight

17 May 2017 13:35 #93299 by Boogie
Replied by Boogie on topic Kinematics... again:/
Guys the robot arm i have in mind is the same as in the video i attached in my first post. The only difference is i don't need joint 4. In the video Joint 4 clearly is not parallel to Y axis (as described in robokins.c) but it is vertical (parallel to Z axis).
See attached screenshot.

I want to mount standard milling spindle instead of joint 4 stepper motor (which was used to rotate welding head in original design). And here comes the D3 parameter vs tool lenght offsets.

So it will look like this:

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17 May 2017 16:49 #93305 by andypugh
Replied by andypugh on topic Kinematics... again:/
Do you intend to do 4-axis work?

LinuxCNC will automatically add the tool-Z to the Z-word in G-code. You probably don't want this. One way round this is to put the tool length in the tool table as a W-offset. Then feed the W-length into the kinematics from HAL to use in your calculations. This is _probably_ optional if you intend to keep the tool vertical, but if you intend to rotate about the A-axis then I think you need to do this.

17 May 2017 20:43 #93312 by bkt
Replied by bkt on topic Kinematics... again:/
in your sketch "offset?" label is distance from centerline of joint3 and centerline of joint4 ... your label "D3" you can use as tool lenght (so it seems you have inverted the label ... offset instead D3 and vice-versa ... I hope I undestand well).

regards
giorgio

19 May 2017 08:22 #93376 by Boogie
Replied by Boogie on topic Kinematics... again:/
Andy, I'm not thinking about 4-axis right now. It is too complitacted now and 4th axis will make it even more complicated. I can't agree with you Giorgio about joint 3 and joint 4: look on the robokins.c:

joint[3] = End effector rotates around horizontal axis, that is
parallel to previous rotation axis and also X axis.
A value of zero means that the tooltip (if offset from the axis)
is pointing in the same direction as the centerline of the outer arm.

joint[4] = End effector rotates around horizontal axis, that is
perpendicular to previous rotation axis and is parallel to Y axis.
This module assumes there is no offset between joint[3] and joint[4] axes.

This model assumes THERE IS NO OFFSET between joint3 and joint4 - so this is the situation a drew here:

Joint3 rotation axis and joint4 rotation axis has common point and those axes are prependicular to each other and at the same time joint3 is parallel to Cartesian X and joint4 is parallel to Cartesian Y.

So if the assumption is there is no offset1, and that D3 is the distance from joint3&joint4rotational axes then it will look like this:

Guys am I reading robokins description in the right way?

And this is not exactly what we see in the video, right? If I put spindle in place of that unused stepper motor (let's call it spindle platform) we can see in the video, Offset1 is not important AS LONG AS joints 1 thru 3 keep spindle platform PARALLEL to X-Y plane. In fact if i do not use A-axis (rotational around X-axis) that kinematics keeps spindle platform at constant angle in respect to X-Y plane. Am i clear?

Andy have you got INI and HAL files axamples for the kinematics you wrote for Viesturs? I can't contact him (no respond:()
Have a good day guys!
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19 May 2017 11:53 - 19 May 2017 12:09 #93379 by bkt
Replied by bkt on topic Kinematics... again:/
As i see, Andy modify scarakins for your use .... so in robokins there are still pin names that report scarakins ... I've draw a sketch with scarakins axis structure .... than I draw an other sketch with scarakin joint/axis structure ant your joint/axis structures ... as you can see joint4 in scarakins is exactly 90 ° compared to your joint4 .... as i can see from my sketch and your last image with label name on your post 17 May 2017 15:35 #932.

any how (using your label name on your post 17 May 2017 15:35 #9329) you have 2 offset ... the plane of joint3 is not the same of joint4 ... joint4 plane is 90° referred to joint3 plane + have 2 offset from centerline of each joint .... I'm quite sure that Andy make these calc on robokins (not remember now with precision, but i'm quite sure).

In your of "19 May 2017 10:22" your offset2 is only tool leght I think ..... or you install a some sort of tool after offset2 lenght?

After these .... you can able to work with joint0-1-2-3 only? these question for know if you have solve the configuration problem before furter exploring. For try you can eliminate every refer to join4 into robokins and try, or make joint4 equal to axis without cos calculation.... (for "cancel" you can use /* world...*/ way, so not delete it).

p.s. I'm reading better your mail .... if your plane where install the joint4 (is an axis or only a spindle?) why not use a parallelogram to keep the weight in place?

regards
giorgio
Last edit: 19 May 2017 12:09 by bkt.

19 May 2017 12:58 #93382 by Boogie
Replied by Boogie on topic Kinematics... again:/
Giorgio, i'm not the guy Andy modified kinematics for:) I'm the guy who wants to use it almost in the same way. Or even in simpler way as i don't need 4th axis. If i understand you right here:

... if your plane where install the joint4 (is an axis or only a spindle?) why not use a parallelogram to keep the weight in place?

the answer is: parallelogram is mechanically complicated and more constrained in comparision with robokins mechatronics. And not suitable for future modifications if any needed.

Yes, you are right: in my case i don't want joint4. I want only a spindle in this place. And what i want from kinematics is to keep that spindle perpendicular to the X-Y plane.