Low cost servo controller

Hi gang !

Overworked, too tired and can't sleep, - so I just have to do
something. I use too come with all the silly questions, - so why
should I try to be a "smart ass" - and try too preach a bit ?
Well, forgive me.

Steppers versus servos:
------------------

Steppers is easy to program and interface. I 4bit micro controller
can do a terrific job ! That is why it is so popular.
Servo system is something else. It is just a system that has a
feedback that will follow and track what you ask it to do. It has
nothing to do with motors. It can be mechanical, hydraulic,
pneumatic, etc.

But in your context, it will control a motor, but the EMC and
ServoToGo board could also be used to send a rocket to the moon. It
is very complicated. For control of what you want, there is more
like two systems: Position servo and velocity servo - combined.
That is why you need the tacho feedback - put in simple terms.

The servo amp that Jon Elson has designed, is - I am sure - better
than most commercial systems, but he has added a lot of nice
features that you don't need, - if you would like to have something
cheaper. All honor to Jon. I would like to test it myself, but I
can't afford it right now, and I am also working on a design of my
own ( simpler and cheaper - but if I could just wander over to his
workshop - I would probably chose one of his )

I have rebuilt and fixed servo systems using motors of several
hundred horsepower. Can anyone tell me where I might find steppers
in that range ?
The point is, - if you need more power, then you will hit the wall
with steppers.

Someone mentioned "Sercos ? " - I believe. These have modules for
stepper interface, but is used to overcome the power limit of
steppers. Steppers poses other problems, - it is easy to get
problems with harmonics in some ranges of step rates. You loose
steps. This is problems you could easy get in a "diy" situation.

I believe encoder feedback is a difficult matter. If the encoder
"steps" don't match the resolution of a step motor, you may have hit
a lot of problems. It would be more useful in a micro stepping
system, - but that is a complicated drive system, and very
expensive. The nice thing about Servo control, - is that you can
use it with almost any system size, and technology. Like with
hydraulic systems.

With the ServoToGo, - you don't need encoder feedback. You could
use the ADC feedback. Just need another module inserted. EMC could
do that, also you could easy use a fuzzy control scheme, adaptive
control or what ever in an easy way. Just insmod another module. Or
if you need control systems for a Hexapod or what ever.

Actually - you can think of EMC as two computer systems. The
realtime systems, and the Linux system. It is a modular system.
It's like having a dedicated controller and a second PC for
controlling it - all in one packed, and you could even separate the
two. If you overload the Linux part with too many application, - it
may be the cause of hanging the system. Otherwise it is probably
more stable than most platforms, and I don't think you can
outperform all the possible options you have at your disposal. It is
just getting better and better. Most of us just have not been using
it enough to really understand what it is capable of doing. If you
give your self the same time as you invested in Windows and dos, you
will soon find out that. I think you all are in a very exiting
situation: At this time, you can gain insights that will be harder
later on. Okay, it will be easier to install later on, but you may
lose a lot of valuable knowledge. ( this is what I believe - maybe
I am wrong )

The servo subject is very complicated, - because there is a lot of
different options. Like a constant torque vs. a constant velocity.
All have different applications. To give an example: let say you
would like to use a tap to make treads. Use a constant torque servo
amp, and you are done - no more broken taps. ( There is other ways
to do it, but it cost you the same kind of money - and it is not an
easy DIY kind of thing)

Here is a little idea:

Someone said they thought the Stg board was to costly, - because
they would like to use EMC on different machines. Okay, why not
make a nice connector board. Cables with plugs. Use the computer
on a cart, - and roll it to where you need it. A menu, to select
the machine, and you are done ? Instead of a computer on each
machine, just hook it up to the one you need. The Amps and power
supply could be on the same "box" - just leave the motors and
encoders on machine . What about it ?

Some have mentioned Pico servo system, dedicated chips, even a DSP
system. Well, the major problem is: Synchronization ! Thise is
just as bad as a stepper system loosing steps, - and a lot more
difficult system to design. In EMC - you have all these problems
taken care of, plus if you add more axis - that is also taken care
of. I would like to see any DSP system have that modularity.

By the way - a servo amp can be very simple !? You can make one
for a couple of dollars !! The thing is - the size of the motors
you need. A small DC motor can be run with a amp for less than $10
! You can also get a complete amp for several hp on a single chip
! ( this has specs. that outperform most of today's servo amps, -
but it is not cheap ! ) There is by the way also some hitech DSP
chips available that is capable of doing more than you can imagine.

So, what it boils down to:

There is so many systems available - the big question is - what do
you need ? Motor sizes and such. If this question could be
answered, then it would be a lot easier to solve what everyone
seems to look for: a low cost solution to what you would need.

I am died tired, - and may have said a lot of things I can't
deliver. I just have to many things to take care of, - but I think
you ask some wrong questions. That's all. The main question should
be what you need it for, and look for solutions to those needs.

Here is an idea:

FORM:
---------------------------------------------------------------
Your name: | Your application | # axis | size | what you
would like | price range
---------------------------------------------------------------
|
| | | |

---------------------------------------------------------------

Fill out a simple form like this, just a single line to this list.
Then we would now how many is looking for the same info, and we
could do a search on what is available, or what is most practical to
use for a design draft.

Comments ? ( Please change the form, if you feel this is not
enough )

This way I think it could lead the way to do a group effort to take
part in a system design and setup. The only thing I can do, is to
come with some
comments, - I have no time to be responsible to lead any activity
like this.

//ARNE