CNC USB motion controller

[amplexus]

Kroko
here are some comments by mariss about usb

#17 08-18-2005, 08:09 PM
Mariss Freimanis
Gold Member Join Date: Mar 2003
Location: United States
Posts: 2,053


It's hopeless to even try. Here's why:

Step pulses have to be timed to sub-microsecond accuracy.

Don't believe it? Try this: 20,000 pulses per second have 50 microseconds of time between pulses (1/20,000 = 0.00005 seconds). 19,999 step pulses per second have 50.0025 microseconds between the pulses. The difference is 2.5 nano-seconds (2.5 billionths of a second). Small time-scales indeed.

USB can have 1,000 microseconds of latency (delay), sometimes much less, sometimes much more. How is this exquisitely sensitive step pulse timing going to be preserved then? Answer: It can't.

Neither you nor your printer much cares if data is delayed anywhere from 1 uS to 1,000 uS. Your step motor drives will care a lot though and they will not run.

Mariss


Amplexus Ender

[pminmo]

:-), I don't know that I've ever disagreed with Mariss, but a portion of that post is incorrect in the context of the above post. MACH3 and EMC both frequently use kernel and thread times that can be 40us to 50 us and easily have 10us jitter, not sub microsecond accuracy.

But I suspect the post is also out of context, it would make more sense if it was in respose to a suggestion of using USB without intellegence to handle the motion control, so maybe I'm not really disagreeing with him.

While 1ms latency is a USB issue, it's also kind of a myopic look at how to use USB. If you consider that you can stream large amounts of data via USB in some of the more exotic modes of USB, the latency essentially isn't a huge issue. Read up on isochronous transfers in USB.

[harryn]

Hi, if you don't mind, I would like to post my targeted setup as an example and see if this controller would be viable for my use.

Target speed - approx 5 - 6 in / second, so call it 150 mm / sec. (Edit - thanks Kroko for the math correction - guess I was tired)

Target resolution - approx 0.1mm linear motion per stepper motor step.

Microsteps - assuming 10 micro steps per step, 200 steps per rotation, so 2,000 micro steps per revolution.

The router is belt drive. Each belt is driven by a 100mm pitch diameter timing pulley, with a 2:1 reduction from the stepper motor.

Using my math, each time the timing pulley makes one revolution, the belt will move

Dia x Pi = 100mm x 3.14 = 314mm.

Each revolution of the stepper motor will move the belt 1/2 of this, so 314/2=157mm.

Each microstep of the controller will move the belt 1/2000th of this, so

157mm / 2000 micro steps = 0.0785mm, which is close enough for me to 0.1mm.

I think I am ok so far, but please feel free to check my math.

As far as speed, I am not as certain on this calculation, but here goes:

25 KHz pulse rate x 0.0785 mm per pulse = 1900 mm/ sec. (edit - Thanks Kroko for catching my typo / math error)

This seems to be well within my accuracy and speed needs unless I am missing something.

[amplexus]

The post is indeed somewhat out of context, i posted it to bring up latency issues in usb.
usb3 may make all of this moot. I will do some reading on isochronous transfers if I find a bit of extra time. Is there an advantage to usb over parallel ports other than the fact that it is ubiquitous?
Amplexus Ender

[pminmo]

Is there an advantage to usb over parallel ports other than the fact that it is ubiquitous?
Amplexus Ender

Parallel ports aren't on most new machines.

[harryn]

The post is indeed somewhat out of context, i posted it to bring up latency issues in usb.
usb3 may make all of this moot. I will do some reading on isochronous transfers if I find a bit of extra time. Is there an advantage to usb over parallel ports other than the fact that it is ubiquitous?
Amplexus Ender

Hi, I posted on this thread with my question because as a potential customer for a new product, I am attempting to make sure that it would work for my application. Clearly there is a separate discussion going on regarding its applicability and capability that I have also of course read.

As far as USB vs parallel, I guess the bottom line is that none of my existing computers have a parallel port (all laptops) so that means I would be buying a dedicated desktop just to run a DIY hobby router. If this controller can help allow me to use my existing laptop by just plugging in an external controller to the USB, then that is pretty high value to me. Doing the design work inside and just carrying it to the garage as needed is quite appealing. I know that this whole Laptop + USB driven area is pretty dicey, so I don't go into this lightly.

[amplexus]

I think the pcmia parallel port cards will work perhaps better than usb, also free or dirt cheap desktops are easy to find.
Amplexus Ender

[pminmo]

Hi, I posted on this thread with my question because as a potential customer for a new product, I am attempting to make sure that it would work for my application.

I think it's appropriate for Kroko to answer your question.

[Kroko]

...
USB can have 1,000 microseconds of latency (delay), sometimes much less, sometimes much more. How is this exquisitely sensitive step pulse timing going to be preserved then? Answer: It can't...

You are completely correct if we assume that step/dir are going through USB. In my case computer sends commands (similar to g-code but highly optimized) to controller and controller has a buffer for 20 commands.

Most PCMCIA cards don't work becouse they are not true paraller port cards but USB->parallel converters. There is one card thar works from Trans Digital. Problem is that it requires PC card slot and most modern laptops have only Express slot. I got answer from them and they claim that it is impossible to create such a card for Express slot.

[Kroko]

Hi, if you don't mind, I would like to post my targeted setup as an example and see if this controller would be viable for my use.

I quickly checked your math. I found two mistakes. 6 in/sec is 150 mm/sec and not 15. And at the end 25 KHz pulse rate x 0.0785 mm/pulse = 1900 mm/sec.
I guess my controller will work fine for you.

In my foam cutting machine with 8 microsteps I use M8 screw for spindle and I need 2560 steps for 1mm of travel (that is 0.000390625 mm/step). Maximum teoretical speed is around 10mm/sec. In reality I get 5.5mm/sec without losing steps. I start losing steps becouse of motors and not controller.

With 16mm/4mm spindle and 10 microsteps maximum teoretical speed is around 50mm/sec.

For faster speed I recomend servo motors and not steppers. It is not only limitation of controller. Stepper motors are not designed to work at high speeds.

[harryn]

Hi - Thanks for the math check - I guess I was more tired than I thought last night. I edited my original post with the corrections.

So, the idea is that I can just take this board and your software, and it will work on most modern laptops with Windows Vista Business 32 or 64 bit through a standard USB 2 port and it should work?

Thanks

Harry

[Kroko]

I didn't try 64 bit Vista. There is no reason not to work but I'll try and report. I just need to find one.

Andrej