Hi Steve,

I first started this project using the MC33974 back in mid November.
I overnighted the eval kit in from DigiKey thinking I would be all
set...

Unfortunately, the MC33974 is very insensitive. It can detect a car
seat (it's real use) but it cannot detect to the fine levels needed
for robot work. With a 10 bit AtoD (it has analog output), the chip
can go to 0.1pF if you are "really" lucky. Most of the details I
stuck here:

http://hot-streamer.com/temp/MC33974EvalKitCD/

Although neat for many uses, it was instantly obvious that the chip
was not going to work for the range of 0.0001pF which such a system
really needs. I gave the kit to a guy at the robot club for his
object grabber where it might be more useful (I think I also have a
bunch of bare chips in my junk box...). I then started looking into
the theremin's circuits. There was a version-I which used all analog
circuits that is still described at:

http://thereminvision.com/

However, the new all digital version is vastly better and really made
this whole system fly:

http://thereminvision.com/version-2/TV-II-index.html

ThereminVision can detect capacitance levels about to 1/10000th what
the Motorola chip can. The Motorola chip does have nine channels and
can do shield referenced antennas which is cool, but the sensitivity
is just not there.

http://hot-streamer.com/temp/MC33974EvalKitCD/CD33794DWBEVM%
20Documentation/MC33794.pdf

The key difference is that the Motorola chip just sets up a 120kHz
capacitance controlled oscillator and measures the frequency delta
directly. A nice solid thing for critical car safety systems, but
too insensitive in our case. However, ThereminVision "heterodynes"
the oscillator signal with a reference signal just like the theremin
instrument to run the sensitivity way up while still remaining
stable. That is the key difference. This is all explained in the
manual under "Theory of Operation" on page 5.

http://thereminvision.com/version-2/ThereminVision-II-manual.pdf

What is really sad is that apparently the Motorola chip was
originally going to heterodyne the signal against a reference and be
very sensitive just like the theremin. I might guess that the analog
designers were just not able to get it going or the car application
changed there plans with million piece sales prospects.
ThereminVision could actually be adjusted to be just as insensitive
as the MC33974 if anyone really wanted too. If they had just gone
all digital and allowed for the full range of sensitivity adjustment,
the Motorola chip might have been perfect for everyone!! But they
didn't... Maybe someday they will... The Motorola package is also
pretty messy for home soldering and such. They added a bunch of
automotive things too like the incandescent dash board light driver
circuits (it does have an error signal if it burns out :-p).

If I have not already told you far more than you wanted to know ;-))
Here are two posts that go over this too:

http://groups.yahoo.com/group/FrontRangeRobotics/message/1678

http://groups.yahoo.com/group/FrontRangeRobotics/message/1711

There has been a long running saga about this at:

http://groups.yahoo.com/group/FrontRangeRobotics/messages/1697

All the relevant posts are "MC33947" or "ThereminVision" reports.

Cheers,

Terry


--- In thereminvision@yahoogroups.com, "bestbobleonard"
<bestbobleonard@y...> wrote:
> On Fri, 2004-04-30 at 16:16, Bob Leonard wrote:
> > The principle of the sensor is based upon the fact
> > that there is a very weak electromagnetic field which
> > surrounds an antenna.
>
> Didn't Motorola already invent this? Sounds just like Motorola's
> e-field
> imaging sensor. It allows you to detect the distance and shape of
> objects that pass through its electromagnetic field. They're used in
> cars for things like turning off side impact airbags when your body
> is
> blocking the inflation area. Here's a robots.net post from last year
> with links to the Motorola specs on the e-field sensor and a
Circuilt
> Cellar article about them.
>
> http://robots.net/article/832.html
>
> From a quick glance at the specs it looks like the main difference
is
> that you need two electrodes to measure size and shape so the
> Theramin
> sensor may be measuring only proximity.
>
> -Steve