Re: [CAD_CAM_EDM_DRO] [EE]: USB stepper drive system

Lawrence Glaister wrote:

The usb bus

> can
> be thought of as a fast 12mb/sec serial port (about 1/2 of this is actually
> usable). I am starting to think along the lines of a usb board that
> contains
> some number of rate generators (like Marriss's DDS system or even pic based
> software DDS) to generate the stepper drive signals smoothly and the same
> number of encoder interfaces (either in hardware counters or possibly pic
> based software interfaces). The interface would use an isosyncronous read
> port and a write port to read/write the command/status signals every ms. A
> four channel card would cover most applications although once design is a
> little further along, it may be possible to put 6 axis on a card and
> produce
> them in various states of population. USB support is getting pretty good
> under linux, with quite a few example drivers. Between the lists, there is
> a
> lot of talent available... where to we need to be in a year from now? Would
> USB make sense in a shop environment for getting machine control
> information
> in/out of the computer?

The big problem, which I don't have an answer for, is whether the USB can handle
a read burst (encoders and limit switches) and a write burst (rate gen or servo
velocity DACs) within the time constraints of a real time control loop. At a rate
well below 1 KHz, the answer is certainly yes. There is a 1 mS 'timer' on the USB
which synchronizes events. I don't know if you have to abide by that clock or not.
(In other words, can a single device send results MORE often than 1000 times
a second?)

EMC is capable of doing servo loop rates well above 1 KHz, which is not particularly
necessary for servo velocity drive on a Bridgeport-size machine, but might be
VERY important to an assembly robot or pick-and-place machine.

>
> Would DAC outputs for servos make sense?

Sure.

> Would +-50ma outputs make sense
> for
> driving hydraulic servo valves?

I wonder how many people are using this kind of drive anymore? Even the giant
K&T bed mills are often converted over to AC flux-vector drives, or anything else
to get away from the horrible hydraulics!

> How much would the board cost to produce?

Hard to say. I'm using 2-sided boards only for the encoder counter and
DAC boards that I'm making for use on the parallel port. I'm not sure you
can get away with that on a USB design.

Jon