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Steve Lutes
11-10-1997, 01:28 AM
On 10/24/97 I posted the following messge to the list

I'm designing a device to measure phase between two low-frequency sine
waves using a phase-locked loop. The idea here is to measure the phase
angle between an angular velocity stimulus and a torque response. The
reason I'm
using a phase-locked loop is that a PLL works well in noise.

I find this device useful to me. I'd like to know if such a device would
useful to anyone else.

My questions are:
(1) Would you or anyone you know use a phase measurement
device in your work?
(2) If so, what would you use it for and how?
(3) What features would you like such a device to have?

I will post a summary of replies


Thank you for your time and answers.


The summary of replies is----------------------------------


******************** Reply 1 ****************************************

Hello Steve,

we use the phase of the head and of the hoof motion to detect
lameness in horses. Now we do kinematic motion analysis
(motion analysis corporation) this works only efficiently on a
treadmill. If you use a accelerometer in combination with your
phasedetector (PLL) it will work also outside. I think this will be
a very useful device in lameness detection.

Good luck
Christian Peham

************************************************** ******
* Christian Peham *
* email: Christian.Peham@vu-wien.ac.at *
* Clinic for Orthopaedics in Ungulates *
* Locomotion Research Group *
* University of Veterinary Medicine Vienna *
* Phone: +43-1-250 77/5506; Fax: +43-1-250 77/5590 *
* Josef Baumanngasse 1; A-1210 Wien *
* http://www.vu-wien.ac.at/i111 *
************************************************** ******

******************** Reply 2 ****************************************

Hi Steve,

I know that Solartron used to make such a device, based upon the
phase-locked loop, in the 1960's and 1970's. It was called a Digital
Transfer Function Analyser (Digital TFA) and worked over a wide range of
frequencies from around 0.001 Hz (or lower?) to several kHz. It was an
excellent precision device and, as you say, was extremely tolerant of noise.
They also provided a Mechanical Reference Synchronizer unit so that a
sinusoidal motion in a mechanical system could be "locked onto" the output
of the TFA.

In the past, I published some work on skin using such a device:

Finlay,J.B. (1970) Dynamic mechanical testing of human skin in vivo. J.
Biomechanics 3(6): 557-568.

Finlay,J.B. (1978) Thixotropy in human skin. J. Biomechanics 11(6/7):
333-342.

For most folk, Pseudo Random Noise Generators (Binary and then Multi-level)
were the next step in TF analysis, as they permitted small signals to be
injected onto the "working" signal of an existing system - and so permitted
extraction of the Impulse Response via Cross Correlation.

Hope these comments are of interest.

Best wishes,

Bryan Finlay, PhD, PEng

Director, Biomedical Engineering
LifeTECH Corporation
133 The West Mall, Unit 6
Toronto, Ontario
Canada, M9C 1C2

http://www.lifetech.on.ca

******************** Reply 3 ****************************************

>(1) Would you or anyone you know use a phase measurement
> device in your work?

You bet! The workhorse instrument of phase detection is
the digital lock-in amplifier.

>(2) If so, what would you use it for and how?

The generic use is to detect a signal that is known to
be phase-coherent with a reference. Our specific use
is to measure tiny forces (10^-15 Newtons) against
tiny noise backgrounds (10^-16 Newtons/rtHz).

>(3) What features would you like such a device to have?

Digital lock-in amplifiers typically come with almost
every conceivable feature. For example, they cover a frequency
range from milliHertz (15 minutes per cycle!) to 100 kiloHertz.
We use the ones made by Stanford Research Instruments, but
there are other vendors just as good.

For biomechanics purposes, it is particularly convenient that
a noisy external reference can be supplied (generated, e.g.,
from a force plate signal), which the lockin will clean-up
via an internal phase lock loop. Thus both signal and reference
can be noisy in both amplitude and phase -- and a good digital
lockin won't be bothered.

You can do pretty much *anything* if you have:

(1) a scope
(2) two general-putpose voltage amplifiers
(3) two programmable pass-band filters
(one for input, one for output)
(4) a digital lock-in amplifier.

We caution against designing special-purpose phase-look loops.
They have their place, and we have built them (using
analog PLL chips available for the purpose), but they
are tricky to analyze and adjust. In most cases you can
get a better result, faster and with less hassle, with a
good digital lock-in.

Borrow a good digital lockin, e.g. from a Physics Dept.
on-campus, and give it a try!



******************** Reply 4 ****************************************

1- Yes our Lab, Nonlinear Dynamics Group, Virginia Tech, would be
interested in such a device.
2- Various measurements of phase-lag as it relates to nonlinear
vibrations.
3- A well calibrated device where the device will either add NO phase,
of its own, to the response being measured or add a well defined phase
to the response.

Eihab Abdel-Raman, PhD
Research Associate
Nonlinear Dynmaics Group
Virginia Tech
Eihab Abdel-Rahman


Steve Lutes
Center for Balance Disorders Rm. NA315
Dept of Oto
Baylor College of Medicine
One Baylor Plaza
Houston, TX 77030
Voice: 713-798-6336
Fax: 713-798-8658
e-mail: slutes@bcm.tmc.edu

Check out GeometryID, a package for
linear or nonlinear system identification at:
http://www.bcm.tmc.edu/cfbd/geometryID/

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