Re: HP plotter servo

Graham,

Here's the parts cost for the power section of the G320:

2 IR2104S half-bridge driver $0.81 2@$1.62
4 IRF540 MOSFETs $0.46 4@$1.84
1 0.01 Ohm 3W non-inductive resistor $0.31
1 100nF/100V ceramic monolithic cap $0.27
1 100uF/100V aluminum 'lytic cap $0.38
6 0805 chip resistors $0.002 6@$0.012
2 SOT-23 diodes $0.05 2@$0.10
2 1206 chip caps $0.06 2@$0.12
1 0805 chip cap $0.04

Total power section parts cost: $4.69
Total power section assembly cost: $2.81

That gets you a 20A, 80VDC rated power output. A power section that
actually comes apart at 33A and 118VDC.

The G320 sells for $114. With with our exorbitant profit margin, (3X
parts and assembly cost) a zero amp, zero volt output G320 would
still cost $91.53. At that price, it couldn't turn the head of a flea
(no power section). My point is the majority cost of the drive is not
in the power section. It is in everything else that is required to
make a drive go; pcb, low-level circuitry, connectors, trimpots,
optos, enclosure, etc. Also include phone support, repair and
replacement.

Not included is development and design maintenance. The design time
and effort is a killer. I started a design of a brand new drive last
December 15. I have spent some part of nearly every working since
then. I have an engineering project book nearly as thick as a
Mitchner novel. The drive is just coming to life and if I'm lucky (no
setbacks), it may be marketable by the very end of this year. The
engineering diaries on the G201/210s and the G320/340s are far fatter.

Good and "on the cheap" is the holy grail and I see that as my
perpetual goal. It can also be an oxymoron if you are not very
careful. By that I mean it's a fine balance between designing
something that costs too much or designing something that's too
fragile to use.

For me "cheap" means a drive shouldn't cost more than the motor it
drives and "good" means the drive gets the best out of the motor it
drives. If I satisfy both requirements I feel I have done well. My
satisfaction as a designer is to conjure up a design that balances
without tipping in either direction.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Graham Stabler" <eexgs@n...>
wrote:

>
> > As I understand a Gecko 320 would probably work, but that is over
> > $100 each. I hope to build my entire machine for under $100.
>
> I realize it does not fit with the cheap approach although the
> reason I ask is that the inductance of the motors can make a
> difference to the control I was once told. Also anyone who already
> has a set of geckos would find it quite a cheap way to build such a
> machine. I am such a person.
>
> Here is the link to the microchip app note:
>
> http://ww1.microchip.com/downloads/en/AppNotes/00532c.pdf
>
> It would be great if this code could be modified to produce a
> general purpose step/direction controller for small servo control

on

> the cheap.
>
> Graham