Need: Indicator of small voltages on a rapidly rotating platform

[PhysicsProf]

  I'm thinking of an experiment involving a rapidly rotating platform  (roughly 3" or 8cm in diameter) and I want to determine visually or by sound that a small voltage is being generated (or not).  Say 0.1-0.2V.   I've been thinking of a JT circuit, but that requires somewhat higher voltages IMHO. 

  Due to high rotational speeds, the system needs to be robust, but it can sit near the axis so that may help.  Just a qualitative measure would work, initially.  OK, maybe some idea of the voltage - that would be helpful.  Is it approx 0.1V or twice that? for example.

Oh -- and NO BRUSHES of any sort; that would defeat the purpose of the experiment (which I would rather explain at a future time).

  Any ideas?

   

[EMdevices]

I know exactly what you mean.   I've thought about this a long time ago when I was playing with homopolar motors and other novel devices   Why?  Because it would be nice to know if a voltage is developed in that rotational frame of reference, because if it is measured by stationary objects, e.g. brushes, probes, etc., magnetic induction is at work.

You could create a small circuit with a voltage comparator, and an LED, and when it lights up you know it crossed the voltage level you set.

EM

[BEP]

It can be done using a small PIC microcontroller, op-amp and a ZigBee or other wifi type connection. The size of the circuit can be as small as a US quarter.

There are solutions out now that use sensors of most types connected to a computer or smart phone via BlueTooth or the Apple interface.

The first time I needed to perform a similar test I used a 741 op-amp and a bi-color LED powered by some button cells. As long as the input was zero the LED was yellow. Even the slightest shift positive or negative changed the color to red or green. It was sensitive down to a few millivolts.

I believe the circuit was provided on the 741 application notes.

[Groundloop]

You may try a inductive transfer of energy from the moving platform.

GL.

[PhysicsProf]

  Very helpful comments.  " It was sensitive down to a few millivolts." - Wow!   

We may be thinking along similar lines, EMD.

I mentioned an LED, but a device to hold and store the numerical value of the voltage generated, to be read after the moving platform stops, would be fine.  Radio transmission of these data to the outside lab would be great also; but the scale is small and limited, so this is not necessary...  I would prefer to have no battery or other energy source available, just that which might be generated ON the moving platform...

From a relativity point of view, rotational motion is absolute - e.g., the Foucault pendulum experiment.  It is rectilinear motion that is evidently relative (not absolute), by contrast.  This difference is what I'd like to explore experimentally.

Again, thanks for comments.

[muDped]

The first time I needed to perform a similar test I used a 741 op-amp and a bi-color LED powered by some button cells. As long as the input was zero the LED was yellow. Even the slightest shift positive or negative changed the color to red or green. It was sensitive down to a few millivolts.

I believe the circuit was provided on the 741 application notes.

These kinds of simple but very innovative
circuits can be extremely useful.  With
surface mount devices it could be made
very small indeed.

There are available now low voltage
comparators which could perform better
than the aged 741.  There are also now
available very low voltage boosters which
operate in the 100 millivolt range.

Since the indicator circuit would be rotating
upon the disk what sort of isolation would the
circuit need from whatever forces may be
producing the small voltage?

[PhysicsProf]

These kinds of simple but very innovative
circuits can be extremely useful.  With
surface mount devices it could be made
very small indeed.

There are available now low voltage
comparators which could perform better
than the aged 741.  There are also now
available very low voltage boosters which
operate in the 100 millivolt range.

Thanks for this - but can you be more specific?  about these "low voltage boosters" or other "innovative circuits"?  would appreciate.

Since the indicator circuit would be rotating
upon the disk what sort of isolation would the
circuit need from whatever forces may be
producing the small voltage?

Good question - and would depend I think on the specific circuit.

[Paul-R]

I want to determine visually or by sound that a small voltage is being generated (or not).  Say 0.1-0.2V.

I fear I may have missed the point here but:

Isn't this what the oscilloscope was designed for?

Unless the frequencies are surprisingly high, the free Winscope software, which uses
a sensor which plugs into the sound card should do.

[Peterae]

I would use a pic micro, they can have 12bit A/D's built in and eeprom for storage and can run on a few 100uA, use a cap for supply storage and sample every 100 mili seconds and store the data straight to eeprom, when full get it to shut down, stop the motor and read the eeprom data.

If the generated waveform is ac use a diode and charge a cap then sample that voltage.

1 Chip & 1 cap needed.
12bit A/D virtually rail to rail will give 3V/4096 732uV per bit

Something like the 16F1788 has a built in OP AMP, 4 Comparators,16K of self write flash memory & operates at 1.8V, although it does have 28 pins
It's also a very low power consumption device.

[PhysicsProf]

Paul-R:  pls see the OP -- I'm trying to measure the voltage reached on a RAPIDLY ROTATING PLATFORM, 1000's of RPM to be specific -- and no brushes of any sort.  So DSO's are out.

Peterae:  "If the generated waveform is ac use a diode and charge a cap then sample that voltage."  Works also for DC -- and this is what I've come to also, as the easiest solution.  Just that the voltage has to be high enough for the diode to operate.  Ge diode, I'm thinking (also recommended by Slider)

[muDped]

Here is a resource for devices
which will operate at about 80 mV:

Custom Thermoelectric

Scroll down to about the middle of the page
where you'll see Boost Converter Circuits.

A custom unit has been developed which
operates with as little as 20 milliVolts.  I'd
like to know more about this one.

[Peterae]

Just that the voltage has to be high enough for the diode to operate.  Ge diode, I'm thinking (also recommended by Slider)

Ah yes the voltage drop LOL.
Well that chip has op amps and a DAC voltage reference source, so add an offset DAC voltage to the op amp input, then stick your diode on the output of the op amp, this way you can adjust the offset so the diode just starts conducing, feed your voltage to be measured into the op amp, adjust the gain of the op amp for sensitivity, you now have a fully configurable system, all using the one chip.

Or the other way to do this.

Use a comparator in the pic to monitor the generated voltage, use an internal reference, start the reference at a low volts setting, when your coil voltage goes above this threshold the comparator will fire an interrupt in code, you then write to eeprom noting this voltage has been reached, you then up the threshold and wait for an interrupt again to happen, and back round the loop.
So you will in effect be looking for comparator triggering and the max voltage it triggered at, giving you an indication of the pk-pk voltage of your generated waveform,again if it's a really small voltage then use the built in op amp stage.

[BEP]

In order to measure a very low voltage you need two contact points. In doing this, in any form, you must have a loop.

Won't this loop counter the loop of the measured circuit with an inverse and countering loop, rendering the measured voltage zero?

(assuming the DUT uses induction)

[PhysicsProf]

In order to measure a very low voltage you need two contact points. In doing this, in any form, you must have a loop.

Won't this loop counter the loop of the measured circuit with an inverse and countering loop, rendering the measured voltage zero?

(assuming the DUT uses induction)

Consider the Faraday disk generator (see 1800's drawing below).  The rotating disk is copper, a good conductor,, so one might expect the potential between axis and rim to be zero.  But it is not zero  when the disk rotates -- there is a current which flows between the rim and axle which can be quite large in fact.  Yes, there is a complete loop. 

If you go to a ring magnet instead of a horseshoe magnet depicted, it still works (perhaps better).  If you attach the ring magnet to the disk, so that they spin together, it still works - a current flowing between rim and axle showing that their is a potential difference between rim and axle.

[BEP]

I cannot think of any measuring circuit that will not strongly effect the experimental results.

If you seek to prove a potential is created from disk axis to periphery without the use of brushes, etc, it should be better to have an accurate way to measure disk drag due to Lenz.
Test with no electrical connection from the shaft to edge. Then test with a solidly connected stiff wire between shaft end and disk edge.

A difference in the amount of power needed to maintain a fixed RPM should confirm current is induced from that reference frame.

[BEP]

I'll think you will see that any conductor rotating with the disk will have the same potential induced upon it. This means that the difference between the end of the wire and the edge of the disk will be zero.

[BEP]

There is one rule that should be part of faraday induction:

When a conductor is in motion perpendicular to the lines of magnetic force 'AND' there is a magnetic force density gradient from one end of that conductor to the other, there will be a potential difference created between the two conductor ends. Whether this induced potential is in addition to or subtracts from the potential induced during the same situation with no magnetic field gradient depends upon the orientation of that gradient to the other factors.

In the case of a rotating disk... there is less magnetic force density at the edge of the disk than the center (for the usual results). It must be confirmed but having a ring magnet with an inner diameter slightly less than the disk diameter should produce the highest output voltage.

I was drawn to a more interesting question and never continued the above line of experiments. 

[PhysicsProf]

There is one rule that should be part of faraday induction:

When a conductor is in motion perpendicular to the lines of magnetic force 'AND' there is a magnetic force density gradient from one end of that conductor to the other, there will be a potential difference created between the two conductor ends. Whether this induced potential is in addition to or subtracts from the potential induced during the same situation with no magnetic field gradient depends upon the orientation of that gradient to the other factors.

In the case of a rotating disk... there is less magnetic force density at the edge of the disk than the center (for the usual results). It must be confirmed but having a ring magnet with an inner diameter slightly less than the disk diameter should produce the highest output voltage.

I was drawn to a more interesting question and never continued the above line of experiments. 

Not the same as what I'm pursuing now, but very interesting -- and I'd encourage you to continue your experiments.