S2 internal Pen Lifter and a couple questions for Phil

[W9GFO]

First, here's the Pen Lifter, it works awesomely well if I do say so myself...

See the blog entry for more details.



Now for some questions;

1. The speaker makes a constant but quiet tick tick tick at about 10 hz. Is that normal?

2. Could you take a look at this code? The goal is to control the S2 using my XBee equipped remote controller. The goal is met, it does control it quite well. The problem however is that I cannot get the S2 to maintain a perfectly steady speed. It is as if every second or two the motors will stutter for just a moment even though they are receiving a steady move value.

3. Related to the above perhaps, if I stop the S2 suddenly and then very gently reverse direction, it will lurch slightly.

If I have not described the issues adequately I can put a video on YouTube.

Rich H

[Martin_H]

That is a really cool mod. I make that to my S1.

[Phil Pilgrim (PhiPi)]

Rich,

Very nice pen lifter! And a beautiful installation! (I'm amazed it all fit.) In response to your questions:

1. No, the ticking is not normal. (At least I've never noticed it.) Does it do it all the time? What about if you don't start the audio cog?

2. The "stuttering" you observe is related to the encoder resolution and the way wheel movements are coordinated. In the motor driver object, the motion of each wheel is monitored in relation to the other one. If one wheel should get ahead, it will not receive PWM pulses until the other wheel has caught up with it. That way, if you grip one of the wheels during a movement to slow it down, the other will slow proportionally. Now, if the motion values going to the two wheels differ by a small number, say, two (e.g. 1000 and 1002), there will be two points along the trajectory where one wheel will appear to get ahead of the other (just due to the encoder math) and will hesitate while the other "catches up". This may be the "stutter" that you notice.

One may also notice a stutter during the drawing routines when drawing arcs. For example, when drawing a full circle, it is necessary to break it into two arcs. Sometimes the correct ending point for the first arc will not have integer coordinates, and you have to round them to integers. If, when the second arc begins, the arc radius is deemed too small for the distance to the ending point, the drawing software will move toward the ending point in a straight line, far enough to make up the extra distance, then draw the arc.

3. The lurching you see upon reversing direction may be related to backlash in the gearbox, but I'm not sure. If you post an example program that produces this behavior, I'll have a look at it.

-Phil

[W9GFO]

Thanks Phil,

I've just discovered that the ticking vanishes completely when I turn off the remote controller and returns after it has turned back on and booted up - and is in sync with the XBee TX Led on the controller.

Looking at the well documented PASM motor code it looks like there should be something that I could do to disable the encoders, or at least the part where one is compared against the other. But since I don't really know PASM I do not know how to accomplish that.

I doubt that the lurching is gear backlash. It only occurs after the S2 is stopped abruptly while moving at faster than half speed. As soon as it begins to move again it appears to be trying to make up some encoder counts.

Rich H

[NikosG]

Hi Phil,
Something about the problem with circles....(from LOGO Theory)
Try this algorithm: REPEAT(36) FORWARD (5 cm or any distance) ROTATE-RIGHT (10 degrees)
When I say Forward I mean both whees must have the same direction and velocity. If the s2 can rotate precisely 10 degrees you 'll have a perfect circle after the 36 steps.
I have tried this method 6 years ago with a homemade robot (with very cheap DC motors with no accuracy) and I had great results.
Unfortunately I don't have a s2 to try this but I believe that you 'll have result.

Nikos Giannakopoulos

[ratronic]

W9GFO - if you want to control the s2's wheel motors directly youself without using the s2 object or the encoders you could use something like this example

Con                                      ''s2 direct wheel motor control                                                                            
  _clkmode = xtal1 + pll16x                                                                                                                         
  _xinfreq = 5_000_000                                                                                                                              
 
  pwm = 16                               ''pwm base io for s2                                                                                       
 
Var                                                                                                                                                 
 
Obj                                                                                                                                                 
 
  pw : "pwmx8"                           ''pwm object                                                                                               
 
Pub main                                                                                                                                            
 
  pw.start(16, %00001100, 20_000)        ''start pwm object on pwm base i/o pin @ 20khz                                                             
  dira[16..17]~~                         ''set s2's motor direction control i/o pins to output                                                      
 
  repeat                                                                                                                                            
              'left      right                                                                                                                      
    move_robot(-75,      255)            ''set s2's left wheel slow reverse, set s2's right wheel maximum forward                                   
 
 
Pub move_robot(left_wheel, right_wheel)  ''set s2's left & right wheel motor's speed from -255 maximum reverse to 255 maximum forward, 0 = stop     
 
  left_wheel := -255 #> left_wheel <# 255                                                                                                           
  right_wheel := -255 #> right_wheel <# 255                                                                                                         
 
  ifnot right_wheel & $80_00_00_00                                                                                                                  
    outa[pwm + 1] := 1                                                                                                                              
    pw.duty(pwm + 3, right_wheel & $ff)                                                                                                             
  else                                                                                                                                              
    outa[pwm + 1] := 0                                                                                                                              
    pw.duty(pwm + 3, (0 - right_wheel) & $ff)                                                                                                       
 
  ifnot left_wheel & $80_00_00_00                                                                                                                   
    outa[pwm] := 1                                                                                                                                  
    pw.duty(pwm + 2, left_wheel & $ff)                                                                                                              
  else                                                                                                                                              
    outa[pwm] := 0                                                                                                                                  
    pw.duty(pwm + 2, (0 - left_wheel) & $ff)

[W9GFO]

Thanks Dave,

Not sure why I didn't go that direction, maybe it's because I was hoping to use as much of the 'native' S2 code as possible. I think using a separate driver object is very sensible.

Rich H

[ratronic]

w9gfo - sorry i posted the wrong file - use the one thats attached in my above post now to get the motors to run the right direction

[W9GFO]

The file you attached worked just great for me.

Thanks,

Rich H

[W9GFO]

I've added a video of the pen lifter to the blog.

Here's a direct link.

http://www.youtube.com/watch?v=FqahLvqI-dk

Rich H

[ratronic]

That's some pretty nice scribblin! What is that controller you are using?

[W9GFO]

Thanks!

The controller is described in this thread.

RIch H

[ratronic]

Very cool! I missed that one.

[erco]

GREAT Scribblin', Rich! I got a bad S2, turns out. I'll have to get mine swapped out before I can Scribble!

[W9GFO]

Thanks!

Here's another;



Rich H

[Whit]

Super cool! I thought I had done some cool scribbling...

[Phil Pilgrim (PhiPi)]

Rich,

That's even better repeatability than I would've guessed possible. Did you use the drawing commands in the S2 object? Did you have to tweak the calibration coefficients? (And what's with "SAINTS"? For Whit's benefit, I hope! Go Seahawks! Not that I give a rip. It's just for editorial balance. )

-Phil

[W9GFO]

It was all done with move_to and arc_to commands. Calibration coefficients? Didn't even know there were any.

"Saints" is for the Interlake Saints Robotics Team.

Rich H

[Phil Pilgrim (PhiPi)]

Calibration coefficients? Didn't even know there were any.

No matter: it doesn't look like you need 'em!

"Saints" is for the Interlake Saints Robotics Team.

Nice save! Um, go Saints! Yeah!

-Phil

[W9GFO]

Here is a larger version. By Larger, I mean the letters are scaled up four times.

http://forums.parallax.com/entry.php...in-big-letters

And here is the code for the "A", any idea why it messed it up? It seems like it lost some encoder counts on the first two arcs.

Pub A

 s2.move_to(420, 0)
 pen_down

 s2.begin_path
 s2.move_to(420, 416)
 s2.arc_to(544, 540, -124)
 s2.move_to(656, 540)
 s2.arc_to(780, 416, -124)
 s2.move_to(780, 0)
 s2.move_to(660, 0)
 s2.move_to(660, 180)
 s2.move_to(540, 180)
 s2.move_to(540, 0)
 s2.move_to(420, 0)
 s2.end_path

 pen_up

 s2.move_to(540, 300)
 pen_down

 s2.begin_path
 s2.move_to(540, 388)
 s2.arc_to(568, 420, -32)
 s2.move_to(628, 420)
 s2.arc_to(660, 388, -32)
 s2.move_to(660, 300)
 s2.move_to(540, 300)
 s2.end_path

 pen_up

Rich H