As mentioned before, I was able to read the German-language book on
electronic levels well enough to tell that the book wasn't that helpful,
and that the authors did not understand how these levels worked.

However, the book did have two or three references, also in German. So
I googled them down, which led me to an article by a German engineer
(now Diplom Ing) who had in effect combined the Taylvel and Wyler
approaches, yielding a far simpler unit.

The relevant article is "Hochpräzise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Zürich),
Christian Hirt (Universität Hannover), and Hilmar Ingensand (ETH
Zürich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Zürich, 15. * 19. März 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the story.

Some details:

The pendulum hangs on two 50-micron thick (0.002") by 3mm (0.118") wide
bits of metal foil. The figure implies that the foil is made of spring
steel (Federstahl) leafs (Blattfedern) but the text says that they are
made of beryllium-copper foil. Brass shim stock or stainless steel foil
should work, or some feeler gage stock.

Pendulum swing is limited by two nylon screws (Arrierschraube,
Kunststoff).

While a Talyvel-like pendulum is used, the motion of the pendulum
(Pendel) is instead sensed by means of a 3-plate (differential)
capacitor, where the two outer capacitor plates (Kondensatorplatten) are
fixed, and the pendulum mass is the moving third plate. The fixed
plates appear to be made of brass, while the pendulum is aluminum.

Joe Gwinn

On Apr 9, 9:08*am, Joseph Gwinn wrote:
As mentioned before, I was able to read the German-language book on
electronic levels well enough to tell that the book wasn't that helpful,
and that the authors did not understand how these levels worked.

However, the book did have two or three references, also in German. *So
I googled them down, which led me to an article by a German engineer
(now Diplom Ing) who had in effect combined the Taylvel and Wyler
approaches, yielding a far simpler unit.

The relevant article is "Hochpr zise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Z rich),
Christian Hirt (Universit t Hannover), and Hilmar Ingensand (ETH
Z rich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Z rich, 15. 19. M rz 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the story.

Some details: *

The pendulum hangs on two 50-micron thick (0.002") by 3mm (0.118") wide
bits of metal foil. *The figure implies that the foil is made of spring
steel (Federstahl) leafs (Blattfedern) but the text says that they are
made of beryllium-copper foil. *Brass shim stock or stainless steel foil
should work, or some feeler gage stock. *

Pendulum swing is limited by two nylon screws (Arrierschraube,
Kunststoff).

While a Talyvel-like pendulum is used, the motion of the pendulum
(Pendel) is instead sensed by means of a 3-plate (differential)
capacitor, where the two outer capacitor plates (Kondensatorplatten) are
fixed, and the pendulum mass is the moving third plate. *The fixed
plates appear to be made of brass, while the pendulum is aluminum.

Joe Gwinn


Joe:

Damping of the pendulum is accomplished by an eddy current brake. A
powerful permanent magnet is placed below the pendulum in the base.

According to the article this reduces the damping time from about 20
seconds to 2 seconds.

Interesting concept, and buildable by any competent toolmaker.

If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

Thanks for pointing out this article, and the previous discussions.

Wolfgang

In article
,
wrote:

On Apr 9, 9:08*am, Joseph Gwinn wrote:
As mentioned before, I was able to read the German-language book on
electronic levels well enough to tell that the book wasn't that helpful,
and that the authors did not understand how these levels worked.

However, the book did have two or three references, also in German. *So
I googled them down, which led me to an article by a German engineer
(now Diplom Ing) who had in effect combined the Taylvel and Wyler
approaches, yielding a far simpler unit.

The relevant article is "Hochprazise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Zurich),
Christian Hirt (Universitat Hannover), and Hilmar Ingensand (ETH
Zurich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Zurich, 15.-19. Marz 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the story.

Some details: *

The pendulum hangs on two 50-micron thick (0.002") by 3mm (0.118") wide
bits of metal foil. *The figure implies that the foil is made of spring
steel (Federstahl) leafs (Blattfedern) but the text says that they are
made of beryllium-copper foil. *Brass shim stock or stainless steel foil
should work, or some feeler gage stock. *

Pendulum swing is limited by two nylon screws (Arrierschraube,
Kunststoff).

While a Talyvel-like pendulum is used, the motion of the pendulum
(Pendel) is instead sensed by means of a 3-plate (differential)
capacitor, where the two outer capacitor plates (Kondensatorplatten) are
fixed, and the pendulum mass is the moving third plate. *The fixed
plates appear to be made of brass, while the pendulum is aluminum.

Joe Gwinn


Joe:

Damping of the pendulum is accomplished by an eddy current brake. A
powerful permanent magnet is placed below the pendulum in the base.

According to the article this reduces the damping time from about 20
seconds to 2 seconds.

Yes. I didn't have the energy to translate the entire article, given my
schoolboy German.


Interesting concept, and buildable by any competent toolmaker.

Yes. I think that Kahlmann and colleagues were worrying about the same
things as I had been. But they started in 2000, and it was more than a
home project for them.


If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.

See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. The usual implementation of the
two voltage sources is a centertapped transformer. I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. One winding became the primary, the
other two were connected together to make a center-tapped secondary.
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eSdz GJJ2EyATi4_3ODg


Thanks for pointing out this article, and the previous discussions.

Welcome. I don't know why this so drew my attention.

Joe Gwinn

Joseph Gwinn wrote:

In article
,
wrote:

On Apr 9, 9:08*am, Joseph Gwinn wrote:
The relevant article is "Hochprazise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Zurich),
Christian Hirt (Universitat Hannover), and Hilmar Ingensand (ETH
Zurich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Zurich, 15.-19. Marz 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the story.

That was a very interesting try to read.


Some details: *


Joe:

Damping of the pendulum is accomplished by an eddy current brake. A
powerful permanent magnet is placed below the pendulum in the base.

According to the article this reduces the damping time from about 20
seconds to 2 seconds.

Yes. I didn't have the energy to translate the entire article, given my
schoolboy German.

I was looking at the drawing and noticed that magnet. A few winters ago one of the
regulars mentioned how neat it is to drop a neodymium magnet in a length of copper pipe
and watch it slither rather than drop through.

My school boy German was one year when I wasn't paying attention during the first grade.
Dad was stationed in Germany at the time. I should have paid attention.

If you hit a section that you really want to understand (need translation), I suspect an
email to Nick Mueller who posts on u.r.m.e. would get a reply.


Interesting concept, and buildable by any competent toolmaker.

Yes. I think that Kahlmann and colleagues were worrying about the same
things as I had been. But they started in 2000, and it was more than a
home project for them.


If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.



See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. The usual implementation of the
two voltage sources is a centertapped transformer. I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. One winding became the primary, the
other two were connected together to make a center-tapped secondary.
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eSd zGJJ2EyATi4_3ODg

If I'm understanding this correctly, the distance between the plates has a linear effect
on capacitance so if the pendulum leans to one side, using what is essentially two
capacitors in series will cancel since one side will decrease, the other side increase.


Thanks for pointing out this article, and the previous discussions.

Welcome. I don't know why this so drew my attention.


You are doing an excellent job of researching this. Your threads on this has been the
high point of R.C.M. for the last week or so.

Thanks,

Wes

In article ,
Wes wrote:

Joseph Gwinn wrote:

In article
,
wrote:

On Apr 9, 9:08*am, Joseph Gwinn wrote:
The relevant article is "Hochprazise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Zurich),
Christian Hirt (Universitat Hannover), and Hilmar Ingensand (ETH
Zurich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Zurich, 15.-19. Marz 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the
story.

That was a very interesting try to read.


Some details: *


Joe:

Damping of the pendulum is accomplished by an eddy current brake. A
powerful permanent magnet is placed below the pendulum in the base.

According to the article this reduces the damping time from about 20
seconds to 2 seconds.

Yes. I didn't have the energy to translate the entire article, given my
schoolboy German.

I was looking at the drawing and noticed that magnet. A few winters ago one of the
regulars mentioned how neat it is to drop a neodymium magnet in a length of
copper pipe and watch it slither rather than drop through.

Eddy-current dashpots work very well, and no messy oil.

Taylvel uses oil, originally in a little dashpot, lated as drops of oil
between limiter screws and the pendulum.

Wyler uses either air going through small holes in the diaphragm, or
active electronic feedback.


My school boy German was one year when I wasn't paying attention during the
first grade.
Dad was stationed in Germany at the time. I should have paid attention.

I had a few years in High School and then a year in College. And five
years later spent a year in Sweden, 30 years ago. So I can get the gist
from reading.


If you hit a section that you really want to understand (need translation), I
suspect an email to Nick Mueller who posts on u.r.m.e. would get a reply.

That's a thought. What is u.r.m.e. ?

I am borrowing a copy of Kahlmann's thesis. Probably 150 pages of
technical German. We shall see.


Interesting concept, and buildable by any competent toolmaker.

Yes. I think that Kahlmann and colleagues were worrying about the same
things as I had been. But they started in 2000, and it was more than a
home project for them.


If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.



See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. The usual implementation of the
two voltage sources is a centertapped transformer. I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. One winding became the primary, the
other two were connected together to make a center-tapped secondary.
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eSd zGJJ2EyATi4_3ODg

If I'm understanding this correctly, the distance between the plates has a linear effect
on capacitance so if the pendulum leans to one side, using what is essentially two
capacitors in series will cancel since one side will decrease, the other side
increase.

It's arranged so that for small displacements, the change in capacitance
is the same, except that one increases and the other decreases. With a
suitable circuit, one does get linear response.

When I was in Sweden, one thing I did was to design a 3-plate
capacitance sensor for tracking of the movements of small animals in
pharmacological research. I found some of my notes from then, dated
1975, and later notes from 1977 (when I built a sample of such a
sensor).


Thanks for pointing out this article, and the previous discussions.

Welcome. I don't know why this so drew my attention.


You are doing an excellent job of researching this. Your threads on this has
been the high point of R.C.M. for the last week or so.

Thanks. There has to be a way to find some left-right politics in this.
But I guess that the idea of building such a level has tickled more than
just my fancy.


Joe Gwinn

On Apr 10, 12:53*pm, Joseph Gwinn wrote:
In article ,



*Wes wrote:
Joseph Gwinn wrote:

In article
,
wrote:

On Apr 9, 9:08*am, Joseph Gwinn wrote:
The relevant article is "Hochprazise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Zurich),
Christian Hirt (Universitat Hannover), and Hilmar Ingensand (ETH
Zurich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Zurich, 15.-19. Marz 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the
story.

That was a very interesting try to read.

Some details: *

Joe:

Damping of the pendulum is accomplished by an eddy current brake. *A
powerful permanent magnet is placed below the pendulum in the base.

According to the article this reduces the damping time from about 20
seconds to 2 seconds.

Yes. *I didn't have the energy to translate the entire article, given my
schoolboy German.

I was looking at the drawing and noticed that magnet. *A few winters ago one of the
regulars mentioned how neat it is to drop a neodymium * magnet in a length of
copper pipe and watch it slither rather than drop through.

Eddy-current dashpots work very well, and no messy oil.

Taylvel uses oil, originally in a little dashpot, lated as drops of oil
between limiter screws and the pendulum.

Wyler uses either air going through small holes in the diaphragm, or
active electronic feedback.

My school boy German was one year when I wasn't paying attention during the
first grade.
Dad was stationed in Germany at the time. *I should have paid attention. *

I had a few years in High School and then a year in College. *And five
years later spent a year in Sweden, 30 years ago. *So I can get the gist
from reading.

If you hit a section that you really want to understand (need translation), I
suspect an email to Nick Mueller who posts on u.r.m.e. would get a reply.

That's a thought. *What is u.r.m.e. ?

I am borrowing a copy of Kahlmann's thesis. *Probably 150 pages of
technical German. * We shall see.



Interesting concept, and buildable by any competent toolmaker.

Yes. *I think that Kahlmann and colleagues were worrying about the same
things as I had been. *But they started in 2000, and it was more than a
home project for them.

If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. *The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. *It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.

See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. *The usual implementation of the
two voltage sources is a centertapped transformer. *I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. *One winding became the primary, the
other two were connected together to make a center-tapped secondary. *
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eSd zGJJ2EyATi4_3ODg

If I'm understanding this correctly, the distance between the plates has a linear effect
on capacitance so if the pendulum leans to one side, using what is essentially two
capacitors in series will cancel since one side will decrease, the other side
increase.

It's arranged so that for small displacements, the change in capacitance
is the same, except that one increases and the other decreases. *With a
suitable circuit, one does get linear response.

When I was in Sweden, one thing I did was to design a 3-plate
capacitance sensor for tracking of the movements of small animals in
pharmacological research. * I found some of my notes from then, dated
1975, and later notes from 1977 (when I built a sample of such a
sensor). *

Thanks for pointing out this article, and the previous discussions.

Welcome. *I don't know why this so drew my attention.

You are doing an excellent job of researching this. *Your threads on this has
been the high point of R.C.M. for the last week or so.

Thanks. *There has to be a way to find some left-right politics in this.. *
But I guess that the idea of building such a level has tickled more than
just my fancy.

Joe Gwinn

Joe,

One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. Buying commercial sensors is
not warranted because of the cost of the associated electronics.

The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

Wolfgang

On Fri, 10 Apr 2009 13:55:39 -0700 (PDT), wrote:



One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. Buying commercial sensors is
not warranted because of the cost of the associated electronics.

The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

Wolfgang


Solid state accelerometers are smaller. lighter, cheaper, simpler for that
sort of application.

For example:-

Analog Devices ADXL322JCP

I've got some similar ones waiting for a two plane balancer project.

Regards
Mark Rand
RTFM

On Fri, 10 Apr 2009 13:53:06 -0400, Joseph Gwinn wrote:



If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.



See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. The usual implementation of the
two voltage sources is a centertapped transformer. I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. One winding became the primary, the
other two were connected together to make a center-tapped secondary.
The transformer primary was driven with 20 volts at 500 KHz.



I need to ask if they've thrown out all the old Rosemount differential
pressure transducers from work. They used a differential capacitor sensing
head. Ok in its own right, but useless for our purposes due to sensitivity to
static pressure (The silicone oil dielectric used was compressible at a couple
of thousand psi and changed the transducer calibration).


Mark Rand
RTFM

In article
,
wrote:

On Apr 10, 12:53*pm, Joseph Gwinn wrote:
In article ,



*Wes wrote:
Joseph Gwinn wrote:

In article
,
wrote:

[snip]


Interesting concept, and buildable by any competent toolmaker.

Yes. *I think that Kahlmann and colleagues were worrying about the same
things as I had been. *But they started in 2000, and it was more than a
home project for them.

If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. *The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. *It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.

See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. *The usual implementation of the
two voltage sources is a centertapped transformer. *I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. *One winding became the primary, the
other two were connected together to make a center-tapped secondary. *
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eSd zGJJ2EyATi4_3ODg

If I'm understanding this correctly, the distance between the plates has
a linear effect
on capacitance so if the pendulum leans to one side, using what is
essentially two
capacitors in series will cancel since one side will decrease, the other
side
increase.

It's arranged so that for small displacements, the change in capacitance
is the same, except that one increases and the other decreases. *With a
suitable circuit, one does get linear response.

When I was in Sweden, one thing I did was to design a 3-plate
capacitance sensor for tracking of the movements of small animals in
pharmacological research. * I found some of my notes from then, dated
1975, and later notes from 1977 (when I built a sample of such a
sensor). *

Thanks for pointing out this article, and the previous discussions.

Welcome. *I don't know why this so drew my attention.

You are doing an excellent job of researching this. *Your threads on this
has
been the high point of R.C.M. for the last week or so.

Thanks. *There has to be a way to find some left-right politics in this. *
But I guess that the idea of building such a level has tickled more than
just my fancy.

Joe Gwinn

Joe,

One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. Buying commercial sensors is
not warranted because of the cost of the associated electronics.

There is no reason that this cannot be done, and there are variants of
the 3-plate differential capacitor bridge that allow the moving plate to
be grounded, so the moving plate could be the rotating shaft (if round
enough) or the housing.

One also needs a way to sense rotor angular position, so one can tell
where the heavy spots are. A simple display would use the rotor angle
signal to trigger an oscilloscope, which scope would display the housing
motion signal. This yields a stable display of displacement versus
angle, regardless of rotation rate (although the horizontal scale will
change with speed).

How big are these rotors, what are they made of, and how fast do they
spin?

If there are patent numbers on the commercial sensor units, it would be
worthwhile to read those patents, if only for the litany of practical
effects to be considered. And to educate me.


The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

There must be a way ... which way did you say those rotors turn?


Joe Gwinn

On Apr 10, 6:33*pm, Joseph Gwinn wrote:

(No left-right commentary today:-))

There must be a way ... which way did you say those rotors turn?

Joe Gwinn-

Widdershins

In article ,
Mark Rand wrote:

On Fri, 10 Apr 2009 13:55:39 -0700 (PDT), wrote:



One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. Buying commercial sensors is
not warranted because of the cost of the associated electronics.

The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

Wolfgang


Solid state accelerometers are smaller. lighter, cheaper, simpler for that
sort of application.

For example:-

Analog Devices ADXL322JCP

ADI makes very nice MEMS accelerometers, but for turbine rotor
balancing, I would ensure that the bandwidth is sufficient, as turbines
spin pretty fast, especially small turbines.

I have to mention that MEMS accelerometers use differential capacitor
bridges to sense the motion of the silicon proof mass. Silicon is a
very good mechanical material, almost as good as quartz. The
capacitances are in the femtofarads, which makes for a noisy output
signal if wide bandwidth is needed.

So, there is a tradeoff study to be made.

The usual bolt-on accelerometer used on big machines is a proof mass
glued to a piezoelectric crystal, often quartz, with bandwidth in the
tens of kilohertz. Sensitivity is often low. The commercial sensors
are expensive, and home manufacture is difficult. However, I bet they
turn up on evilbay.


I've got some similar ones waiting for a two plane balancer project.

What are you balancing?


Joe Gwinn

On Apr 10, 5:52*pm, Joseph Gwinn wrote:
In article ,
*Mark Rand wrote:



On Fri, 10 Apr 2009 13:55:39 -0700 (PDT), wrote:

One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. *Buying commercial sensors is
not warranted because of the cost of the associated electronics.

The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

Wolfgang

Solid state accelerometers are smaller. lighter, cheaper, simpler for that
sort of application.

For example:-

Analog Devices *ADXL322JCP

ADI makes very nice MEMS accelerometers, but for turbine rotor
balancing, I would ensure that the bandwidth is sufficient, as turbines
spin pretty fast, especially small turbines.

I have to mention that MEMS accelerometers use differential capacitor
bridges to sense the motion of the silicon proof mass. *Silicon is a
very good mechanical material, almost as good as quartz. *The
capacitances are in the femtofarads, which makes for a noisy output
signal if wide bandwidth is needed.

So, there is a tradeoff study to be made.

The usual bolt-on accelerometer used on big machines is a proof mass
glued to a piezoelectric crystal, often quartz, with bandwidth in the
tens of kilohertz. *Sensitivity is often low. *The commercial sensors
are expensive, and home manufacture is difficult. *However, I bet they
turn up on evilbay.

I've got some similar ones waiting for a two plane balancer project.

What are you balancing?

Joe Gwinn


Joe et all,

There was a discussion on this topic a while ago here where I promised
to post some pics of the wee beastie....

Basically it is a functioning 1/16" scale turbo generator for live
steam locomotives, with the prototype for my own. The alternator
rotor is 3/8" dia. x 3/8" long, with permanent magnet. The turbine
rotor is 5/8" diameter x 3/16" wide with buckets around the perimeter
as per Stumpf design. Turbine is overhung design with the ball
bearings on each side of the generator rotor. Rotational speed is
about 56,000 rpm loaded. We've run it on steam and it easily lights
up 4 flashlight bulbs at 80 psig steam pressure. It'll also burn them
all out if the pressure is increased due to malfunctioning steam
pressure regulator but this problem has been resolved:-))

I've searched the 'net and there is much information on dynamic
balancing; but because of the small size most of the methodology is
difficult to implement. I did jury-rig a crude machine to measure
frequency, amplitude, and rpm. I used an inductive pick-up (old
magnetic headphone) with the steel diaphragm glued to the vibrating
part. This produced a very good sine wave output signal, displayed
on an oscilloscope, but...

My son rigged up a LED timing light that would illuminate an ink mark
on the rotor, but...

What we observed was that the timing mark would rotate from its
original position. In fact I was able to move this timing mark over
360 degrees by increasing the turbine speed from about 5,000 rpm (as
low as it would run) to over 60,000 rpm where the old bearings would
howl (I have new ones with high temp light grease, not yet installed).

This moving of the timing mark threw me; I suspect I know why it
moves, critical speed and shaft deflection spring to mind, as do mass,
velocity, and acceleration, but my education is almost 35 years old
when I learned about this. Professionally I deal with statics and
make sure things don't fall on people or break unexpectedly:-))

Since the magnetic pick-up is really a velocity transducer, and the
velocity of the vibrating mass lags the rotor position with the amount
of lag probably a function of rotational velocity, I thought that a
position transducer would make the problem more tractable by removing
some variables from the system.

Hence my interest and queries on capacitive transducers plus
circuitry.

And thanks for your efforts on this.

Wolfgang

In article
,
wrote:

On Apr 10, 5:52*pm, Joseph Gwinn wrote:
In article ,
*Mark Rand wrote:



On Fri, 10 Apr 2009 13:55:39 -0700 (PDT), wrote:

One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. *Buying commercial sensors is
not warranted because of the cost of the associated electronics.

The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

Wolfgang

Solid state accelerometers are smaller. lighter, cheaper, simpler for that
sort of application.

For example:-

Analog Devices *ADXL322JCP

ADI makes very nice MEMS accelerometers, but for turbine rotor
balancing, I would ensure that the bandwidth is sufficient, as turbines
spin pretty fast, especially small turbines.

I have to mention that MEMS accelerometers use differential capacitor
bridges to sense the motion of the silicon proof mass. *Silicon is a
very good mechanical material, almost as good as quartz. *The
capacitances are in the femtofarads, which makes for a noisy output
signal if wide bandwidth is needed.

So, there is a tradeoff study to be made.

The usual bolt-on accelerometer used on big machines is a proof mass
glued to a piezoelectric crystal, often quartz, with bandwidth in the
tens of kilohertz. *Sensitivity is often low. *The commercial sensors
are expensive, and home manufacture is difficult. *However, I bet they
turn up on evilbay.

I've got some similar ones waiting for a two plane balancer project.

What are you balancing?

Joe Gwinn


Joe et all,

There was a discussion on this topic a while ago here where I promised
to post some pics of the wee beastie....

Basically it is a functioning 1/16" scale turbo generator for live
steam locomotives, with the prototype for my own. The alternator
rotor is 3/8" dia. x 3/8" long, with permanent magnet. The turbine
rotor is 5/8" diameter x 3/16" wide with buckets around the perimeter
as per Stumpf design. Turbine is overhung design with the ball
bearings on each side of the generator rotor. Rotational speed is
about 56,000 rpm loaded. We've run it on steam and it easily lights
up 4 flashlight bulbs at 80 psig steam pressure. It'll also burn them
all out if the pressure is increased due to malfunctioning steam
pressure regulator but this problem has been resolved:-))

I was thinking only 40,000 RPM, like the rotors in a jet engine. But
56,000 RPM is 933 RPS, call it 1,000 RPS. To see the details within a
revolution, you need minimum five and better ten harmonics, which means
that one must have bandwidth to 9,333 Hz, call it 10 KHz. And more is
better.


I've searched the 'net and there is much information on dynamic
balancing; but because of the small size most of the methodology is
difficult to implement. I did jury-rig a crude machine to measure
frequency, amplitude, and rpm. I used an inductive pick-up (old
magnetic headphone) with the steel diaphragm glued to the vibrating
part. This produced a very good sine wave output signal, displayed
on an oscilloscope, but...

My son rigged up a LED timing light that would illuminate an ink mark
on the rotor, but...

What we observed was that the timing mark would rotate from its
original position. In fact I was able to move this timing mark over
360 degrees by increasing the turbine speed from about 5,000 rpm (as
low as it would run) to over 60,000 rpm where the old bearings would
howl (I have new ones with high temp light grease, not yet installed).

This moving of the timing mark threw me; I suspect I know why it
moves, critical speed and shaft deflection spring to mind, as do mass,
velocity, and acceleration, but my education is almost 35 years old
when I learned about this. Professionally I deal with statics and
make sure things don't fall on people or break unexpectedly:-))

Visible LEDs are quite slow, and the delay to max light could cause the
rotation. As one spins faster, a constant delay in time becomes an
increasing delay in angle terms. LEDs are also not that bright, forcing
use of long pulses to get enough light to be visible, which causes
blurring. At high speeds, this blurring will become severe.

The standard way to address this is to use a very short pulse xenon
flash. This is very easy if powered from the AC line. The key trick is
to use an optocoupler to deliver the Fire! signal to the flash. Pulse
transformers won't work, for an odd reason. They fire the flash well
enough, but too much flash energy comes backwards through the pulse
transformer and discombobulates the trigger logic. Been there done
that. Ended up ripping the pulse transformer circuit out and replacing
it with an optocoupler circuit.

The pulse must be very short, with very low jitter. At 1,000 RPS, make
the rotor appear to stand still, with resolution of 1/1000 of a
rotation, requires light pulses about 1 microsecond wide, with jitter no
larger.

There are flashtubes intended for this, but one can short-pulse almost
any tube, probably well enough.


Since the magnetic pick-up is really a velocity transducer, and the
velocity of the vibrating mass lags the rotor position with the amount
of lag probably a function of rotational velocity, I thought that a
position transducer would make the problem more tractable by removing
some variables from the system.

The only way this can work is a one-tooth gear and a magnetic pickup.
Drill a hole under the one tooth to maintain dynamic balance.

What I use is an HP/Agilent (now Avago) optical encoder. Look for
motion control encoders at http://www.avagotech.com/pages/home/. But
these may be too big. For such a small rotor, some other approach may
be needed. The signal from the generator might be used.

Well, the alternative is to integrate the velocity signal to get a
position signal. This can be accomplished with an opamp with a
capacitor (and parallel resistor) in the feedback path.


Hence my interest and queries on capacitive transducers plus
circuitry.

And thanks for your efforts on this.

It's an interesting albeit tiny project, but with high power-to-weight
ratio.

Joe Gwinn

In article
,
Jim Wilkins wrote:

On Apr 10, 6:33*pm, Joseph Gwinn wrote:

(No left-right commentary today:-))

There must be a way ... which way did you say those rotors turn?

Joe Gwinn-

Widdershins

Damn leftists! Hmm. Damn rightists! Depends on which end you view.

Joe Gwinn

"Joseph Gwinn" wrote in message
...
In article
,
wrote:

On Apr 10, 12:53 pm, Joseph Gwinn wrote:
In article ,



Wes wrote:
Joseph Gwinn wrote:

In article
,
wrote:

[snip]


Interesting concept, and buildable by any competent toolmaker.

Yes. I think that Kahlmann and colleagues were worrying about the
same
things as I had been. But they started in 2000, and it was more than
a
home project for them.

If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very
worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. It will be
a
traditional transformer capacitance bridge with synchronous
detector,
which is what Kahlmann probably uses as well.

See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors"
by
Larry K. Baxter for the general theory. The usual implementation of
the
two voltage sources is a centertapped transformer. I built one of
these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. One winding became the primary, the
other two were connected together to make a center-tapped secondary.
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eSd zGJJ2EyATi4_3ODg

If I'm understanding this correctly, the distance between the plates
has
a linear effect
on capacitance so if the pendulum leans to one side, using what is
essentially two
capacitors in series will cancel since one side will decrease, the
other
side
increase.

It's arranged so that for small displacements, the change in
capacitance
is the same, except that one increases and the other decreases. With a
suitable circuit, one does get linear response.

When I was in Sweden, one thing I did was to design a 3-plate
capacitance sensor for tracking of the movements of small animals in
pharmacological research. I found some of my notes from then, dated
1975, and later notes from 1977 (when I built a sample of such a
sensor).

Thanks for pointing out this article, and the previous
discussions.

Welcome. I don't know why this so drew my attention.

You are doing an excellent job of researching this. Your threads on
this
has
been the high point of R.C.M. for the last week or so.

Thanks. There has to be a way to find some left-right politics in this.
But I guess that the idea of building such a level has tickled more
than
just my fancy.

Joe Gwinn

Joe,

One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. Buying commercial sensors is
not warranted because of the cost of the associated electronics.

There is no reason that this cannot be done, and there are variants of
the 3-plate differential capacitor bridge that allow the moving plate to
be grounded, so the moving plate could be the rotating shaft (if round
enough) or the housing.

One also needs a way to sense rotor angular position, so one can tell
where the heavy spots are. A simple display would use the rotor angle
signal to trigger an oscilloscope, which scope would display the housing
motion signal. This yields a stable display of displacement versus
angle, regardless of rotation rate (although the horizontal scale will
change with speed).

How big are these rotors, what are they made of, and how fast do they
spin?

If there are patent numbers on the commercial sensor units, it would be
worthwhile to read those patents, if only for the litany of practical
effects to be considered. And to educate me.


The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

There must be a way ... which way did you say those rotors turn?


Joe Gwinn

What about using a record player( remember them) cartridge as a
displacement sensor? A stereo one could supply a signal from two
dimmensions.

On Fri, 10 Apr 2009 08:40:10 -0400, Wes wrote:

Joseph Gwinn wrote:

In article
,
wrote:

On Apr 9, 9:08*am, Joseph Gwinn wrote:
The relevant article is "Hochprazise Neigungsmessung mit dem
elektronischen Einachspendelsystem HRTM" by Timo Kahlmann (ETH Zurich),
Christian Hirt (Universitat Hannover), and Hilmar Ingensand (ETH
Zurich), Ingenieurvermessung 2004, 14th International Conference on
Engineering Surveying Zurich, 15.-19. Marz 2004.

http://www.geometh-data.ethz.ch/downloads/Kahlmann_HRTM_IV2004.pdf

Even if you don't read any German, the figures and photos tell the story.

That was a very interesting try to read.


Some details: *


Joe:

Damping of the pendulum is accomplished by an eddy current brake. A
powerful permanent magnet is placed below the pendulum in the base.

According to the article this reduces the damping time from about 20
seconds to 2 seconds.

Yes. I didn't have the energy to translate the entire article, given my
schoolboy German.

I was looking at the drawing and noticed that magnet. A few winters ago one of the
regulars mentioned how neat it is to drop a neodymium magnet in a length of copper pipe
and watch it slither rather than drop through.

My school boy German was one year when I wasn't paying attention during the first grade.
Dad was stationed in Germany at the time. I should have paid attention.

If you hit a section that you really want to understand (need translation), I suspect an
email to Nick Mueller who posts on u.r.m.e. would get a reply.


Interesting concept, and buildable by any competent toolmaker.

Yes. I think that Kahlmann and colleagues were worrying about the same
things as I had been. But they started in 2000, and it was more than a
home project for them.


If one of the electron-savvy members here is willing to prepare a
circuit diagram for the electronics, this could be a very worthwhile
project for some of us. The read-out could be arranged such that a
commercial digital voltmeter could be utilized.

I will publish the circuit when I learn it or develop it. It will be a
traditional transformer capacitance bridge with synchronous detector,
which is what Kahlmann probably uses as well.



See "Three-Plate Micrometer" (section 5.1.2) in "Capacitive Sensors" by
Larry K. Baxter for the general theory. The usual implementation of the
two voltage sources is a centertapped transformer. I built one of these
in 1975 by winding three strands of #30 wire to fill the bobbin of a
10mm by 18mm ferrite cup core. One winding became the primary, the
other two were connected together to make a center-tapped secondary.
The transformer primary was driven with 20 volts at 500 KHz.

http://books.google.com/books?id=Tjd...22Three-Plate+
Micrometer%22++%22Capacitive+Sensors%22&ei=E8_eS dzGJJ2EyATi4_3ODg

If I'm understanding this correctly, the distance between the plates has a linear effect
on capacitance so if the pendulum leans to one side, using what is essentially two
capacitors in series will cancel since one side will decrease, the other side increase.

An explanation of how these things work can be seen at
http://tinyurl.com/c53teq

This cite says there is a linear region, which I think means linear
enough for many purposes. The transfer function is of the form
f(x) = k*x/(a^2 -x^2) which obviously is not linear anywhere but may
be close enough to linear in regions where a x.

You are doing an excellent job of researching this. Your threads on this has been the
high point of R.C.M. for the last week or so.

Indeed! I would not have thought it possible to sense microinch
displacements this way but Jones (1973) demonstrated detectable
displacement of 10^-10 mm.

On Fri, 10 Apr 2009 13:55:39 -0700 (PDT),
wrote:



Joe,

One of my other interests is a small vibration balancing machine for
tiny steam turbogenerator rotors.

Do you think that this circuitry you are contemplating would serve as
a capacitive position sensor?

I'm thinking completely home-built here. Buying commercial sensors is
not warranted because of the cost of the associated electronics.

The level is certainly in the cards for a home shop project.

Thanks for all your work in this regard.

(No left-right commentary today:-))

Wolfgang

Not Joe, but not bashful either...

Something to think about is that sensing of imbalance may be easier to
do by strongly constraining motion and sensing vibratory force rather
than velocity or accel. This greatly reduces phase shift between
imbalance vector and observed response, which can be very significant
and somewhat chaotic at speeds near a mechanical resonance.

Strain gages are quite inexpensive. Another possibility might be to
make piezo sensors using the piezo elements in cigarette lighters or
those long-neck grill and fireplace lighters. I would think that a
couple of kilohertz bandwidth would be easy enough to get from those
using a transimpedance amplifier made with a jellybean opamp. That's
120K RPM.

Joseph Gwinn wrote:

That's a thought. What is u.r.m.e. ?

uk.rec.models.engineering

Wes

On Apr 10, 10:04*pm, wrote:

My son rigged up a LED timing light that would illuminate an ink mark
on the rotor, but...

What we observed was that the timing mark would rotate from its
original position. *In fact I was able to move this timing mark over
360 degrees by increasing the turbine speed from about 5,000 rpm (as
low as it would run) to over 60,000 rpm where the old bearings would
howl (I have new ones with high temp light grease, not yet installed).

Wolfgang-

If you can characterise the pickup, circuit and LED delay, perhaps
with a temporary contact sensor on a second scope channel, you can
make a calibration chart to compensate for it.

In article ,
Wes wrote:

Joseph Gwinn wrote:

That's a thought. What is u.r.m.e. ?

uk.rec.models.engineering

Of course. RCM with an accent. Thanks,

Joe Gwinn

On Fri, 10 Apr 2009 18:52:58 -0400, Joseph Gwinn wrote:

In article ,
Mark Rand wrote:




I've got some similar ones waiting for a two plane balancer project.

What are you balancing?


Toolpost grinder...


Mark Rand
RTFM

On Apr 11, 12:53*pm, Mark Rand wrote:
On Fri, 10 Apr 2009 18:52:58 -0400, Joseph Gwinn wrote:
In article ,
Mark Rand wrote:

I've got some similar ones waiting for a two plane balancer project.

What are you balancing?

Toolpost grinder...

Mark Rand
RTFM

I have a Quorn T&C grinder for which I balance the cup wheel
experimentally. Not perfect but perfectly adequate. Usually 3 or 4
tries suffice.

I tried to implement one of those automatic balancing features which
has some steel balls in a circular track (a variant of this is sold
for tire balancing by blowing it in through the valve). Sometimes it
appeared to improve the balance, at times it appeared worse. Tried it
on the Quorn before doing it to the turbogenerator, but the results
were too iffy to mangle the turbine.

Wolfgang

wrote in message
...
On Apr 11, 12:53 pm, Mark Rand wrote:
On Fri, 10 Apr 2009 18:52:58 -0400, Joseph Gwinn
wrote:
In article ,
Mark Rand wrote:

I've got some similar ones waiting for a two plane balancer project.

What are you balancing?

Toolpost grinder...

Mark Rand
RTFM

I have a Quorn T&C grinder for which I balance the cup wheel
experimentally. Not perfect but perfectly adequate. Usually 3 or 4
tries suffice.

I tried to implement one of those automatic balancing features which
has some steel balls in a circular track (a variant of this is sold
for tire balancing by blowing it in through the valve). Sometimes it
appeared to improve the balance, at times it appeared worse. Tried it
on the Quorn before doing it to the turbogenerator, but the results
were too iffy to mangle the turbine.

Wolfgang

sitting next to me I have a thing called a Davey Vibrometer - a pretty old
piece of machinery that may have had something to do with balancing - I
found the patent but it didn't really give a lot of insight into how it
would be useful. I could upload a photo to the drop box if there is
interest (or of course I could sell it to someone who had an interest) - it
looks to me like it is the same kind of mechanism found in a mechanical
comparator - it has a high gain mechanical path and an optical beam.