I've never settled on a good way to attach my polorgraphSD to my drawing board. Too often you'll find it dangling by its stepper cables, like this:

My natural impulse is to put it up on top of the board, but the cables come out the bottom, so I'd have to turn the polargraphSD on its side. Also, my servo extension cord isn't long enough to allow the polargraphSD to be perched on top (I could buy or make a longer one).

I've been meaning to make a mount that fits along the side of the drawing board, low enough to be out of the way of the counterweights, but then I'd have to deal with the stepper cables coming out the front and needing to be longer. Not a big thing, but it's so much easier to leave the poor thing dangling. 😉

I'm not sure what I'd suggest you do, because you can't anticipate everyone's desired orientation, but with the present arrangement there doesn't seem to be a convenient, default one.

I know what you mean. I designed the case and the cables to be hung up on a hook, with everything hanging out the bottom (so there was no twisting on the cable connectors), but I'll admit that's not what my own machines are like (more like dangling, or precariously balanced), and when I _have_ set up machines like that, the routing is never very convenient, even though it might look neat by the end.

The first machines were mounted on the top edge of the board (http://www.flickr.com/photos/euphy/6785955898/in/set-72157626497662024) but that's a bit bespoke. Mounting stuff on walls is always a bit fraught (paintwork etc).

I wonder if I could hang it off one of the motor brackets or something like that. That's not a bad idea. Is it?

Changing the orientation of the LCD is not out of the question either.

cheers!!
sn

Yes, Kelticfox, up top is fine. You'll still have to deal with routing the power cord so it doesn't interfere with the weights (not a big hassle), and, assuming you're using a servo, the servo cable (is it long enough, and is it out of the way of the weights?).

Each position has its advantages and complications, so do whatever works for you. You can always alter it later if your situation changes.

Be sure to play around with speeds before you settle at a slow one. I've done the same drawing with fast and slow settings, and have gotten sharper, better drawings done at the 'fast' end of the scale. I don't know why -- it's certainly counterintuitive. Maybe I'm setting acceleration unwisely for slow drawings? Maybe when the gondola is traveling slowly there's more time/opportunity for wobble? Maybe beaded cord slack is taken up more quickly when using fast speeds? I dunno.

So, what I'm saying is, test your assumptions. These machines are simple, but they can defy expectations sometimes. Have fun!

Kong is right, though I reckon deceleration speed might even have more to do with the quality of lines, than actual top speed. When there's a fast, sudden move to a corner, I think the momentum of the gondola carries the pen tip deeper into its "proper" place. Whereas a dead slow, painstaking move will end up stopped a small distance away from the intended corner.

I haven't ever done any kind of proper testing of that, and that might just be a naive conception. I also think the initial max speeds (and particularly acceleration) are very conservative. They were low by default so that your machine doesn't pull itself to pieces when you have first built it, and give it a test, and I just never changed them. My common speed is 2000 for max speed and acceleration.

When drawing with felt-tip pens, on fairly absorbant paper, its quite easy to move the pen so fast the line fades out, but that's really the only negative aspect of fast moves that I've noticed. There is, in principle a hard limit to how fast the arduino can practically spit out steps though, opinions vary about this, but I've not gone higher than 4000 odd. Of course there are practical limits that depend on motors, current, voltage etc too.

sn

I don't think I know much more about it than you do, ha! But aye, I assume I almost never hit the max speed either, and I have taken to bumping the acceleration right up, 4000+.
I do think that acceleration needs to be tuned to the friction and weight of the gondola too though - I don't reckon it's a very fine piece of tuning, but I think it's the particular deceleration (and nothing else) that leads to judders at the ends of lines.

The acceleration number is acceleration in steps per second, per second. Which I thought I understood, but now my brain isn't working so I can't remember how to work out how long it'll take to reach top speed. Perhaps I must load a new hamster, this one is tired.

sn

Er yes that sounds right to me I think, the speed is steps per second, and is zero while the motor is stopped. And the acceleration is a statement of how rapidly that speed is changing.

I think your description is good, except that the calculation is based on milli or microseconds rather than seconds (although the value uses full seconds as the time base because that makes sense to us). Essentially the core of the movement engine is a loop in which current speed, max speed, acceleration and steps to go is constantly evaluated. The loop itself runs at a fairly high frequency (at least a few kHz - thousand times a second), and each run through decides to step or not step the motor based whether a step is due to make it move at a certain speed.

The acceleration calculator is constantly changing that particular speed over the course of any given second.

For a fixed loop frequency of 4000 Hz, running at 2000Hz will mean every alternate loop does not result in a step. You can see where the hard limit creeps in here - once the motor is running at 4000Hz, every single run through the loop results in a step, and there's nowhere to put the extra steps to make it go faster.

But, I think that means that with an acceleration of 3000 and a max speed of 3000 it would take only one second to reach top speed. That intuitively sounds too fast - in real life, accelerations have always seemed more protracted, and top speeds rare, but perhaps I just run with lower numbers in reality than I think I do!

sn

Arduino is attractive for a couple of reasons, but simplicity is the main one. No OS or resource sharing to worry about, but of course that goes both ways - doing complex, memory-hungry stuff is complex, or impossible in arduino.

It wouldn't end up that much cheaper because you'd still need an addon board with motor drivers and probably some power handling hardware, but that would be a condensed version of a polarshield I guess. Could use a I2C/SPI touchscreen for standalone work. But it probably would be a little cheaper if pi supplies stay reliable.

sn

I've moved the 2.54mm pin headers further out from the 3.5 pin headers, so there's room to actually fit both now. Not much else has changed in v1.4, except that there's no longer a 3.3v voltage regulator because there is no longer an XBee socket on board. Nobody seemed to use it, and it increases the cost and build time by a wee bit.

On the other other hand, I have added space for a third stepper driver. This will be unpopulated until I figure out what to do with it, but I have a few schemes that might benefit from an extra motor.

The revision is mainly to get the new screens fitting inside the board footprint again.

sn

The plot thickened slightly with the new screens:
The 2.2" touch screen supply dried up.
I didn't really want to move to the 2.4" screens because the different resolution means maintaining and testing on two code bases, but I didn't have a choice.
So I started making plans for the revision to the board to move the connector, and then my supplier announced a new 2.2" touchscreen, with the orientation changed, and _just_ perfect, so I put the shield redesign on hold and ordered a pile of those 2.2" screens.
But then when I got them I found that the item description had been misdescribed and it was a screen only, not a touchscreen! Boo!
So they are currently on the way back to be exchanged for 2.4" touchscreens, and we're back to where we started.
I've got a handful of 2.4" touchscreens for now, but don't have the v1.4 polarshields yet.
The v1.4 shields will have no xbee port, but will have space for a third stepper driver. I have half a dozen v1.3 polarshields leftover, and am still using that stock for vitamin kits, along with a the 2.4" screens. I hope to get some preliminary v1.4 boards back from the makers around next weekend, but I wouldn't expect to have anything to release until the weekend after that.

sn