Re: Re: Re: Out of Estop.. BUT

Bertho

We seem to be moving this topic toward one big BUT.

It's a bit of a simplification, but Jon's servo amp uses at least three
analog signals, motion command, tach feedback and motor current, to close
the motor loop. Yes the position loop is digital. Sure the motion command
is derived from a digital signal, and the product of the analog signals is
PWM that triggers IGBT's.

Yes all of those signals could be and are being converted to digital
devices on some machines. That's why the latest versions of the top end
machine tools, that I get to service, use pulse coders that run 125k ppr.
One service guy said he's seen them produce as many as 0.5M ppr. They
apply these expensive devices in an attempt to get smooth enough movement
near the axis crossover points on a big circle so that a part doesn't look
like it was made with a large bastard file.

You're right about the screwdrivers and pots. But the production guys I
know would rather put parts with arcs like that into an old machine, have
me do a little adjustment to the analog circuits, and cut a part that is
cleaner than the new, all digital, machine can make.

Analog has it all over digital for both speed and resolution. I remember
as a kid talking to a navy pilot. He explained how he wore contact lenses
with mirrors ground into the edge. The analog signals from light bounced
off those lenses were used to align his weapons to where he was looking. I
can't even imagine the resolution required to hit a 10' target on a five
mile radius. Or the speed required to do it to a 600 mph target from a
platform that's going the same speed.

A few weeks ago a biologist friend told me about someone who is making
robot pets using simple analog circuts. She said the snake was very
lifelike and would curl across your shoulder and around your neck. (it was
set up not to be a constrictor) Try that with embedded linux and EMC, or
Ahha or anyones digital. But for the home shop maybe digital is good enough!

Ray (flame suit in place) Henry

> Date: Sat, 17 Jun 2000 00:42:07 -0400
> From: Bertho Boman <boman@...>
>Subject: Re: Re: Out of Estop.. BUT
>
>Hello Jon,
>This is getting back to what I was suggesting many month ago:
>
>It is kinda silly to go from a digital command in the motion controller to
>an analog output, then to process it in the analog form and adding the

extra tach

>signal and then finally convert it back to a digital PWM signal.
>
>There has been several articles about encoder to tach conversions and
>software/speed problems. If I remember right, in IEEE Industrial Electronics.
>That was several/many years ago that I saw the articles. Today it ought to
>be extremely easy to do with super fast DSPs cheaply available.
>
>Ideally, we should do away with all the analog stuff and just output a

digital

>command, and then let the DSP read the encoder signals and convert them to
>speed and finally calculate the PWM needed. All feedback and filtering
>would be done in software and programmable without screwdrivers and pots.
>Nice and clean!
>Bertho Boman