bldc output to Aerotech drives

Just a note on the 7I39, Its rather misleading to compare a two channel
3 phase drive with a single channel brush motor drive.

Software generated PWM is not much practical use as a servo drive signal
except for very low end systems Even if you can manage a 50 KHz base
thread, this only gives you about 3 bits of resolution
at a barely tolerable 5 KHz PWM rate

Yes, sorry for the bad comparison. I was just attempting to illustrate why I found this method of control interesting, and I'm really not qualified to make wild suggestions like this (yet I keep on doing it, somehow...).

Anyway, I'm going to experiment with my drives and see how things work.

I dont want to throw too much of wet blanket on all software systems. Thats one of LinuxCNCs strengths. If you want to drive a 3 phase servo motor via a H-bridge from a parallel port and the BLDC component, it _will_ work. Maybe a little bit crunchy but possible.

There may even be ways to improve software PWM generation to at least get better resolution at low PWM duty cycles using a technique similar to the stepgens "reset" mode

Just want to get a check on my math here... I'm working on a system to run LinuxCNC that can run a 4khz servo thread. Am I right in thinking that what I need is a servo thread with max interval no more than 200,000ns, and max jitter as low as possible (<20k)?

So far the motherboards with the lowest latency have all been 1Ghz semi-antiques. I'm wondering if they can do this though... even though their latency is good, I am not sure they have enough CPU speed to process the motion loop code when the servo thread is called before it's time to context switch again.

Can you guys give me any idea of how much processing happens when the servo thread is called and how long it takes, assuming I'm using Linuxcnc for commutation of three axes?

Erik

See section 1.4 about .time and .tmax:

www.linuxcnc.org/docs/2.4/html/hal_basic_hal.html

Reading these parameters will allow you to see where the time goes...

Its known that the Atom MBs will not do more than ~2 KHz servo threads

A couple modern low power MBs that can do 4 KHZ servo threads are:

GIGABYTE GA-E350N
ASUS E45M1-M PRO

Thinking about it... with an appropriate H bridge setup, could bldc be used to control "outrunner" type DC brushless motors like the Turnigy types?

You need to look up how BLDC motors are controlled. It is true that 3 half bridges are needed, but there is also a feedback needed to let the "controller" know when to fire what phase. In almost all of the hobby motors they are sensorless, so they measure the BEMF of the phase not being fired to determine when to fire next sequence. Sensored work the same except they have sensors instead on relying on BEMF, this gives them better ability to run at low speeds.

So the minimum connections for any BLDC is 6 dirve signals (could use some hardware to get complement of A B C) and 3 feedback. So with some creative hardware you can get away with 6 total.

Of course this does not at all address the software side. for a spindle control just use a suitable ESC from the same site as the outrunner. then you need 1 pin to drive it from LinuxCNC. Of course you only have rough estimate of the speed, but in most cases that is prolly fine.

I ordered a GA-E350N.

I'll probably update my mesa hardware to a 5i25+7i77 soon, too.

Erik