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Deborah King
02-16-1995, 03:54 AM
Several weeks ago I requested information on ultra sound measuring systems.
Despite accidently neglecting to mention our specific application, I received
many replies which I have attached below. Thanks to everyone who responded.

Deborah King
Biomechanics Lab
Penn State University

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I just finished a sabattical at the universityof Bern where I used a
Toshiba Tosbee ultrasound unit to document and survey the abdominal wall.
It is a very versatile unit with good calibration routines for distance
measurements. Don't know the cost over here though. Incidentally do you
know of Peter Cavanaughs e-mail address, thanks, Stu McGill

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The is a German company marketing an ultra-sound based
motion analysis system if that is what you are looking for.
The address is as follows:

Zebris Medizintechnik
Grabenstr. 17
88316 Isny (Allgaeu)
Germany

phone: (01149 for Germany) 7562 - 4221
sorry, I don`t have a fax number.

You should contact Mr. Brunner.

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While the SAC device is quite accurate, beware of systems that use speakers
as sources. Even with fairly sophicticated electronics and signal
processing, wave jumping occurs, which can throw the systems off by 1 or
more wavelengths of the sound that is being used. This is the case with an
Israili system (forget the name), that is a hybrid IR/sonic device.

Also beware of ambient noise suceptability, and use in drafty areas where
the spead of sound may fluctuate between the speed measurment apparatus and
the sensor array.

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There is a system called Signal/RTS. This system runs on a DOS PC and can
handle a sample rate of up to 250KHz.

For more information please contact:

Engineering Design
43 Newton Street
Belmont, MA 02178
Tel: 617-484-3520
Fax: 617-484-6559
E-mail: 70700.701@compuserve.com

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We have made extensive use of SAC digitizers (the older
GP8-3D) over the past five years to record the posture of
automobile drivers in laboratory vehicle mockups. We have
written our own software, the latest in LabVIEW, to communicate
with the digitizer and handle data acquisition, calibration,
transformation, etc.

We have found that we can repeatedly digitize a point within
the target volume with a standard deviation of about 1 mm.
Measurements of known lengths (e.g., 200 mm) within the volume
are also routinely accurate to within 1 mm. Since we are
recording the location of palpated body landmarks, this
accuracy and precision are considerably better than our
ability to locate those landmarks.

Important limitations:
1. Throughput. Because the system relies on sound travel
times to determine location, sequential emitters cannot be
fired closer together in time than the maximum amount of time
it could take for the sound to reach all of the microphones.
In practice, the minimum time spacing is usually longer,
because of ...
2. Echo. When we have used the system in smaller
laboratories, echo from the walls and other hard surfaces has
caused erroneous readings, particularly when using high
sequential sampling rates. We have instituted software checks
to eliminate potentially spurious data arising from echo
effects and have used acoustic baffles to reduce echo.
3. Sequential measurement. The principle of measurement is such
that multiple body landmarks are recorded sequentially rather
than simultaneously, as is the case with video or film-based
systems. For quasi static or relatively slow motions, this is
not a problem, but for faster motions it could be.
4. Measurement volume. The spacing of the microphone array
determines both the measurement volume and the accuracy of
measurement within that volume. We use square arrays of about
600 mm on a side. Slant ranges to three microphones are
required to locate the emitter, if the emitter can be assumed
to be on one side of the microphone array. This spacing gives a
measurement volume of about 1 m^3. If larger spacing is used,
the volume goes up but the accuracy goes down. We have used
two arrays side-by-side to expand the sampling volume, and
switched data acquisition between them as needed.

Ulrich Raschke and others at the University of Michigan Center
for Ergonomics have developed an ultrasonic system that allows
for a much larger digitization envelope, and includes
algorithms for using the redundant information from multiple,
arbitrarily positioned microphones to reduce error. He'll
probably respond to your post. If not, you can contact him at

ulrich@caen.engin.umich.edu.

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I worked with an SAC GP-8 3D sonic digitizer at Columbia Unviersity
for three years. It had numerous problems but some advantages as well.
If given the choice, I would not use an sonic digitizer for human or
animal work however.

First, as you may know, active markers emit a spark which creates an
acoustic signal. Recievers mounted in a fixed arrangement detect
the acoustic signal, and the time between generation and reception
provides a measure of distance. The sparking of the emitters was
a little distressing to subjects, especially at first. In any case,
you can not place the emitter directly on the surface of the skin
or the emitter will not spark properly.

Second, we had lots of problems with reflections. The signal from
the emitters would bounce off of the table, the subject, the light,
and who knows what else. We found that we had to completely cover
the walls and ceiling of our 6x6x8 ft room with acoustical foam to
dampen the reflections, and even then we had substantial problems.
On some days we had to throw out up to half of all trials because
they included a reflection.

Next, the sampling rate was quite slow, 100 Hz maximum divided by the
number of emitters in use. That meant we could only use 5 emitters
at a time if we wanted to have any chance of capturing the frequency
range of human movement. Moreover, we achieved this sampling rate
only after purchasing a speeded up version of the ROM chip from SAC
and doing substantial reprogramming of the interface between the
digitizer and the computer. This speed is more dependent on the
digitizer than on the computer. Recall that sound moves relatively
slowly.

Next, the calibration process was difficult, though it is relatively
permanent. We found it nearly impossible to square the receivers
accurately enough to insure that the entire workspace was
measured in the same manner. As a result, we ended up doing an
extensive regression analysis on data collected systematically
throughout the workspace and then adjusting all subsequent data based
on those results. On the other hand, once we finished with this
process, the system was very stable over a two year period and reliable
to within .3mm.

Next, we used the system for measuring arm movements. We had it running
for 10 seconds at a time, for up to 500 trials within a session and
probably ran up to 200 sessions over the two year period. We found that
emitters burned out periodically and we had to be careful that we always
had extras on hand. They were not particularly cheap either (about $40?),
and some were defective to begin with. Also, replacing emitters during
an experimental session was sometimes a real hassle.

Finally, on the down-side, we found the device driver to be very badly
written. I can't give you the details on that because my colleague,
Ted Wright, did the work of re-writing it. I can say it was very
inefficiently written, used lots of goto loops. Also, the digitizer
was designed so that the computer could not query it in any way to
discover its current state. As a result, the driver had to be written
to reset the device periodically, so that we could insure it was in
a known state. We had some lengthy discussions with Science Accessories
about this and suggested some changes for subsequent versions. Perhaps
they've cleaned up that side of things.

On the up side, it was GREAT to have a device that digitized automatically,
and with which you could generate on-line feedback in real time. The
GP-8 3D system is about least expensive means I know to do that. Despite
these problems, we did use it to collect a substantial amount of very
useful kinematic data.

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we have a GP-12 in the lab and a collegue of mine is working with
it. It seem that the system is very unreliable and unaccurate. I don't
know if this due to the instrument or to the fact that it is not optimally
configured by us. For sure forget to use with success as is out of the
box.
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Try to contact this company:
Logitech, Inc.
6505 Kaiser Drive
Fremont, CA 94555
Logitech faxback phone # 1-800-245-0000
sale information 1-800-231-7717
Technical support hotline 1-510-795-8100
General Fax (510) 792-8901
Developer relations 1-510-713-5338

Their products include 3D ultrosonic head tracker and 3D mouse.

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I would like to know what you want to measure? What freqency do you want?
What type of scanning (A-scan, B-scan or C-scan). We have two companies
in Denmark making ultrasound equipment (B&K Medical and Cortex Technology).
I am by myself working with ultrasound (High freqency 20MHz) and
Ultrasound Microscopy (up to 1GHz). My Ph.D. thesis is biomechanics of
wall mechanics by ultrasound (elasticity ).
I will be in USA from Feb 16 to March 4. Do you know SPIE Medical
Imaging95 in San Diego. I will be there.

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I have used several sonic digitizers from Science Accesories Corporation.
They are quite reliable and easy to install for laboratory work. My only
concerns are:
1) occasional obstructions (sonic shadowing),
2) variations due to temperature and humidity,
3) jitter due to background noise, and
4) variations due to air currents caused by large rapid motions.

You may wish to investigate electromagnetic trackers (Polhemus, Bird) if
your workspace does not include large metallic objects.

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Thanks again to everyone who replied. Your responses were very helpful.

Deborah