AVR-Chat

On Thursday 16 December 2004 01:51, Mike Murphree wrote:
> Bernd Felsche said:
> > I'm considering all sorts of low-power options for providing an
> > analogue indication (moving needle or bar). Typical ammeters draw
> > current and the quantity I want to indicate isn't directly available as
> > a current... it's number-crunched. Scale will be semi-log.

> > At first, I looked at small stepper motors but they're simply
> > over-kill.  During my search, I've stumbled across radio-control
> > servos... which are quite popular in robotics and the driving
> > technology is certainly well known in AVR circles.

> At the risk of showing my age, does no one remember moving coil meters
> anymore?

Sure do! 

> See: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/galvan.html#c1

> > The question is; has anybody used these servos for instrumentation?

> > The accuracy doesn't need to be high; it only needs to provide an
> > at-a-glance feel for the operator. The indicator only needs to be
> > "accurate" at one point, which could be calibrated at "boot time". I
> > expect the AVR controlling the servo to have a table of position vs
> > value... with linear interpolation for points inbetween.

> You should be able to create any sort of scale you want with the AVR
> driving it.  They typically require very little current.

The indicator is for a "power balance" display in a solar car.  It
will be subjected to vibration as well as high ambient temperatures.
IIRC, the moving coil meters don't typically like that sort of
thing.

I'm not sure of all the options available to drive one to display
the quantities (PWM would depend on the mechanical response) ... and
finding one with a highly-visible needle is another task altogether.

Response time isn't that critical. As long as the full scale can be
traversed in about half a second or less, that's fast enough. I
intend to only show a moving average over one or two seconds anyway,
otherwise the display gets "too busy", causing the driver to make
many adjustments over time which aren't really necessary.

-- 
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