Quick thoughts: DT Ints with encoders

[HenrikOlsson]

Hi,
Yes, it look OK to me, the quadrature decoding seems to be correct but I'm sure someone around here knows of a faster/slicker way of doing it. (I'm not saying your way is slow).

Are you sending directly to a VB program for testing or are you sending to a terminal program or something like that? If a terminal program I'd put Pause 100 or whatever in the mainloop and add a CR to the end of the "message". If you're sending to a VB program, you could send the high and low byte of the counter and put them together in the PC.

If the encoders have Index-channel you could use that to zero the counter, at startup you move each joint of the arm thru "center" and it automatically zeros.

/Henrik.

[kevlar129bp]

Hi Henrik,

The thought behind not using the index channel is:
After firmly attaching the base...
You will probe anywhere in 3d space, then click the "zero" button on the stylus.
At that point, all encoder rotations will be based from that point.
Logging 3d points will be performed by clicking the "log point" button on the stylus.
I'm hoping to have a routine to log points as fast as serially possible while holding the "log point" button down continuously.
The way I see it, this will be all variant on:
1) "slave" pic interrupt speed to acquire an accurate count
2) "slave" pic to "master" pic serial speed
3) "master" pic to pc serial speed

All of the above will hinge on the user's stylus "drag speed" across the intended surface.

Check out this link...maybe it will explain it a bit better than I can



Let me know whatcha think.

Thanks,
Chris

[HenrikOlsson]

Yes, but don't you need to know the absolute angle between segments in order to calculate the stylus position?

You can still use your zero button for relative measurements but shouldn't it be more like an offset from the absolute zero position?

If you wire a third signal between the master PIC and all slaves you can use that signal capture the count at all six joints simultanously", then the master "pings" each slave for the value. Perhaps that was what you meant, just thought I'd mention it if it wasn't.

This is a cool project!

[kevlar129bp]

Hey Henrik,

You are thinking correctly on the absolute angle needed.
That's where the pulse counting comes in...
On the arm I've got designed in SolidWorks now has the following:

500 line encoders
2000 counts in quadrature
10.8:1 gearing
21600 quad counts per 360 degrees

Upon "zero button" press, we'll get degrees "moved" like this:

\\\\ Snip of VB Code ////
Const DPP As Double = 1.66666666666667E-02 <<< 360 / 21600
Dim DegreesNow As Double
Dim CountsNow As Integer
DegreesNow = CountsNow * DPP

If I'm thinking correctly, that will give us the degrees "traveled" by each joint.
If I am reading your question correctly, are you referring to the angles from "zero planes"? (dead vertical and dead horizontal)

Let me know if my thinking is correct?

Hard for me to keep it all straight!

I'm will also be doing the CNC machining for the arm components as well...

Too much for 1 guy to think about!

Let me know what you think.

Thanks again,
Chris

[HenrikOlsson]

Hi Chris,
Let's take the "wrist" joint for example, or any joint for that matter, it doesn't make any difference. It and the stylus is pointing straight down and the length from the pivot point to the tip is 100mm, you press zero at this point, the counter is 0.

Now you rotate that single joint 20° (1200 counts), the tip has moved 34.2mm "away" and 6.03mm "up", you collect that point. Now move it another 20°, this time the same amount of rotation in the joint (20°) moved the tip 30.08mm "away" and 17.36mm up.

Now concider that you from the beginning had pressed zero at the "20° position", the counter is reset to 0. Now you rotate the joint 20° (and ends up the previous 40° position) the counter says 1200 counts which is of course correct but where in 3D-space is the tip?

Is it 34.2mm "away" and 6.03 "up" from your "zero" like before or is it 30.08mm "away" and 17.36 "up"? Obviosuly the later is correct but how would you calculate that when you don't know that your "zero" was actually 20° from dead straight vertical - all you got was 1200 counts.

As far as I can see it will impossible to calculate the position of the stylus without having some kind of absolute reference to start from, like all joints at 90° to each other or something. Therefor my sugestion to use the index-channel. But since you have reduction that may not be as easy as I first thought.

Does that make sense or is it me who's missing something?

/Henrik.

[kevlar129bp]

Ahhh, ya got me on that one Henrik! You are absolutely correct. It looks like it's back to the math. I think it's going to have to be based on the Denavit Hartenberg Representation. Crazy math, I must say. I'm actually chatting on another forum with a guy from Carnegie Mellon University, National Robotics Engineering Center. He told me one of his co-workers built a six axis arm and worked out the kinematics for it too. Oh boy, I hope I can garner some info from him! In the mean time, if you want to tinker with it, or have any thoughts on it, it would be a huge help. I hope to have something very soon, as I think this project would be beneficial to so many!

Thanks Henrik,

Keep me posted, and I will do the same.

Chris