Tuesday, December 19, 2006

 

First cross-hatch

Well, I've got about +/-2 mm play in my pencil holder.

The vertical dimension is just at 20 mm and the spacing of the cross-hatch lines is a bit over 8 mm. It's pretty easy to see the slewing at the corners. I definitely need a more rigid pencil holder.

Here is a slightly different cross-hatching pattern where I locked the pencil down a bit more solidly. Much less pencil slewing, though the line quality leaves something to be desired.

You can access the firmware code here.

Comments:
Hi Forrest. It's been a while since I've commented on your stuff. This looks really good, though I agree that the line quality is a little rough.

Is this line using your pseudo-stepper logic? Are the line 'pulses' caused by your feedback that tries to correct the motor placement, or something else? Any ideas?
 
Am I reading the code right, that you couldn't get PWM to work correctly on its own, so you used a manual high-frequency loop to control pwm?
 
***Am I reading the code right, that you couldn't get PWM to work correctly on its own...***

I don't do PWM any more after I discovered that there is a bottom limit to how slow you can make a motor run using it. Instead I do what you'd call pseudo-stepping where I just turn the motor on and off for short intervals (between 5 and 100 msec which isn't high frequency) for each gearmotor and then adjust the pulse rate using the relevant shaft encoder feedback.

***Are the line 'pulses' caused by your feedback that tries to correct the motor placement, or something else?***

As best as I can see watching the lines being put down the "pulses" are caused by the pencil point chattering as it is pushed rather than pulled over the paper. That's why it only happens on the up stroke rather than the downstroke.
 
Poking around, I came across this site and thought you guys might be interested:

Fab@Home

Looks to be a bit higher-priced then what RepRap is aimed at but there does seem to some overlap.
 
We know about 'em.
 
>> As best as I can see watching the lines being put down the "pulses" are caused by the pencil point chattering as it is pushed rather than pulled over the paper. That's why it only happens on the up stroke rather than the downstroke. <<

How difficult would it be to use an ultra fine tip ball point or felt tip pen, instead of a pencil? Or do you think such a tool would have as much chatter as the pencil?
 
I bought a fine tip ballpoint pen today to find out. Unfortunately, I have the controller board taken apart installing a new dual darlington so that I can run the z-axis and the Mk II polymer pump so I can't answer the question just now.
 
A nice simple pen holder would be useful. I'll let you know what I come up with
 
Post a Comment

Links to this post:

Create a Link



<< Home

This page is powered by Blogger. Isn't yours?

Subscribe to
Posts [Atom]