Re:EHP External Hardware Proposal

Bertho Boman wrote: -
snip

> -----Original Message-----
> cost of the Servo-to-Go interface board.
>
> As it is now, the digital output command is converted to an
> analog signal inside the PC and then the signal is feed to an
> external servo amp which also typically is expensive. If the
> output signal was left as digital and feed to an external micro-P
> that would be part of the Servo amp the internal A/D would
> not be needed. The shaft encoder for each channel would also be
> connected to the same board and signals cleaned up and then
> communicated back to the PC. I think a bi-directional printer port
> or a high speed serial port easily could handle the updates.
> The changes to EMC would be minimal, it would write the digital
> commands to the printer port instead of the I-O card and it
> would read the printer port for position data. A standard PC would
> be fine without modifications.
>
> Note, I am not proposing to use external motion calculations,
> EMC would operate as normal. Basically, what I suggest is to get
> an interface with a servo amp on one board for each channel.
>
> The board (one per axis) that I envision is actually a DSP
> programmed to do several things:
>
> 1. Interface with the PC for communication of data and status
> (a simple protocol is needed for the boards not to fight or they
> could be daisy chained)
>
> 2. Interface with the encoder, limit switches and home switch.
>
> 3. PWM generation. Since a DSP typically has a built in A/D
> it can sense the motor current and generate the PWM internally. A
> medium power output stage could be included on board with an
> option to feed the PWM control signal to an external high power
> stage.
>
> 4. A software tachometer calculated from the encoder signals.
> This is not new and works well. That will remove the need to add
> a mechanical tachometer to each servo channel with its cost
> and mechanical mounting problems.
>
> 5. The DSP software would replace the analog filter and gain
> pots and make a much more reliable system with the gain and filter
> constants accurately set in software.
>
> 6. We could even add one more PWM channel and the DSP could
> be programmed to run steppers, with or without encoder feedback.
>
> Summary: Basically one DSP chip with all its I-O pins can do
> all the interfacing to the PC, the encoders and via a FET driver
> chip the power output FETs. That would make a really high
> performance and low cost servo or stepper system.
>
> What do you guys think? My DSP programming experience is
> zero but I am not really suggesting something new or drastic. It
> really makes sense to combine the I-O functions with the PWM
> generation on one board and then just duplicate it for each
> channel as needed.
>
> Bertho Boman
> Vinland Corporation
>

Carlos Wrote: -

>
> Message: 7
> Date: Tue, 1 Feb 2000 11:35:39 -0500
> From: "Carlos Guillermo" <cnk@...>
> Subject: RE: NT-Linux Comments
>
> Bertho -
>
> This is the approach that I've been starting to favor as
> well. I think with
> the low cost DSP's out there, we should be able to move many of the
> functions that an I/O card handles right onto the servo amp. Texas
> Instruments is coming out with a motion control-oriented DSP, the
> TMS320LC2402 at:
>
> http://www.ti.com/sc/docs/apps/digital/index.htm TI DSPs
> http://www.ti.com/sc/docs/products/dsp/tms320lc2402.html TMS320LC2402
>
> that they sell for $2.95 in higher quantities that has tons
> of PWM channels,
> A/D's, I/O pins, and a hardware quadrature encoder/counter
> port. This thing
> would be overkill for what you're talking about, but at that price who
> cares?
>

Snip
Both Bertho and Carlos echo my own thinking on this issue. My concern is
that sooner or later, as the need to increase cutting speeds and feeds
arises, EMC will run out of puff in EMCMOT where the cycle time will need to
be reduced. This is just where all the intensive computation takes place for
trajectory interpolation and servo PID. Looking at the code, it seems fairly
simple to insert a digital interface after the interpolation routines and to
do the PID loops (position, velocity and current) in a DSP. As Carlos
pointed out, the TMS320c2402 is designed just for this type of arrangement.
Incidently the '2402 is only in preliminary silicon at present. However the
TMS320F240 should be just the ticket.
Analog Devices produce a similar part ADMC401 that I am presently
bread-boarding to try it out. I prefer Analog's programming environment to
the TI style but that is just personal preference.
The prices quoted to me in Australia for single quantities are as follows: -

TMS320F240 AUD$32.93 Equiv US$21.08
TMS320F2402 AUD$23.31 $14.92
ADMC401 AUD$44.55 $28.51
ADMC331 AUD$28.70 $18.36
ADMC201 AUD$31.43 $20.12 (this part is a
motion back end and hooks into a standard DSP. it contains all the PWM,
encoder and current loop control)

Using one of these chips for each axis with opto-isolation (they will drive
optos directly) and either an IGBT or FET power stage can, I believe, end up
with a relatively cheap driver for BLDC, and AC servos. Steppers should be
fine too.

Incidently the TMS320F series chips have in-built flash memory which should
be sufficient for the PID algorithms thus reducing the chip count even
further.

One of the problems with these DSPs is in the manufacture. They are all Quad
flat-packs which means surface mount construction techniques. However, I
made up one of Hans' hot air guns and it works a treat for this type of
construction - even with my rotten eyesight.

Analog Devices have just announced the ADMCF32 which will come out in a 28
pin DIP. I haven't looked closely at this chip yet.

What does all this mean? I believe that these types of chips offer the
opportunity to accomplish all that you desire at not much more in cost than
an MCU and with a lot less programming! A bonus is 16 bit resolution
internally, 12 bit resolution on the ADCs (if you need it); 16 bit
resolution on the PWMs ; plenty of digital IO for alarms, limits and homes,
fast ports to the EMC PC and >1 MHz on quad encoder input with position and
velocity resolution.

Anyway, I am going down this track. I'll keep you posted.
Regards
John Craddock