Author Topic: EM Drive Developments - related to space flight applications - Thread 10  (Read 618148 times)

Offline oyzw

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.... test whether vibrating devices can produce false-positive thrust results on a torsional pendulum. ...
 
Hello, your experiment is very rigorous, but I suggest you use the air suspension platform as an experimental vehicle as soon as possible.

O, no, he shouldn't.  :o The air suspension platforms are very unreliable instruments to work with. For critiques, see, e.g., Marc Millis, Nonviable mechanical “antigravity devices, in: M.G. Millis and E.W. Davis (eds.), Frontiers of propulsion science, AIAA, 2009, pp. 249–261.

Monomorphic started his experiment with an air suspension rail. There were reasons why it was not good.
I think it would be more appropriate to use a boat as a test vehicle in a static pool.

Offline otlski

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.... test whether vibrating devices can produce false-positive thrust results on a torsional pendulum. ...
 
Hello, your experiment is very rigorous, but I suggest you use the air suspension platform as an experimental vehicle as soon as possible.

O, no, he shouldn't.  :o The air suspension platforms are very unreliable instruments to work with. For critiques, see, e.g., Marc Millis, Nonviable mechanical “antigravity devices, in: M.G. Millis and E.W. Davis (eds.), Frontiers of propulsion science, AIAA, 2009, pp. 249–261.

Be careful not to generalize too much on this subject.  I will say that without a doubt a professionally manufactured, well characterized air bearing will out perform all but perhaps a perfectly executed torsion balance.  And at that, I would have to think about it some more.  The air bearing is much more expensive yes, but inferior, no.  I say this having built hundreds of air bearings as part of our mass properties instruments, and having built hundreds of torsion instruments for measuring MOI.  The torsion instruments measure to within 0.1% for parts as small as a few grams and as large as 24,000 lbs.  The air bearings support similarly sized payloads.  I have purchased perhaps fifty air bearings from three different competitors that perform similarly to ours.

I have reliably measured torques down to tens of millions of a lb-inch. Extracting said signal out of motoring torque, viscous drag, turbulent drag, etc.  If my company were tasked with construction of an instrument to test EM Drives, our design kick-off meeting would settle on an air bearing as the heart of the instrument in two minutes.  The only complicating factor would be the application in a vacuum chamber where vacuum level required and pump capacity, I might go to one of our competitors for solutions.

The one Mono made was a valiant attempt but a linear bearing would be my last choice and the execution was the best he was willing to do at the time.  There is (both literally and performance-wise) a sizable gap between that attempt and one professionally made.

All that said, I would love to read the paper you cited if you had a link.  Always looking to learn more...

Offline flux_capacitor

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About problems with air bearings: do you remember Paul March's remarks back to thread 8, after Eagleworks has tested their cavity on a rotary low-friction Cavendish balance supported on a spherical air-bearing?

Emphasis mine:
Quote from: Star-Drive
the EW team ran the same Integrated Copper Frustum Test Article (ICFTA) on a battery powered, spherical air-bearing supported, Cavendish-Balance (C-B) last summer, and it self-accelerated in both directions when the ICFTA was reversed on its mount.  Past that I can't reveal anymore on the C-B test campaign until Dr. White gets around to publishing those test results after some improvements are made to the spherical air bearing, which had some annoying swirl torques that disturbed the data runs, but did not hide the already noted results.



TheTraveller calculated the required force to accelerate the platform to the observed speed is only 17 µN, so he admitted the results of these tests are very uncertain.
« Last Edit: 12/10/2017 11:39 AM by flux_capacitor »

Offline otlski

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What Paul March is calling swirl torque we call motoring a.k.a turbine effect. Motoring can be virtually nonexistent or problematic of course depending on magnitude.  Well made air bearings can motor but the motoring will be repeatable and easily characterized (using two different methods). 

in order for TT to calculate the torque required to achieve a particular angular acceleration rate, he would need to know the MOI of the test rig.  I did not see that posted anywhere back then.  Furthermore he would have to separate the retarding torque caused by viscous air drag and perhaps added mass.  The bare minimum required would be one full revolution of the test rig.

Offline xyzzy

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Monomorphic,

Looking at your vibrational thrust test graphs, I think there is a significant experimental error: You seem to have neither eliminated nor accounted for the changing offset / DC component in the movement of the plunger in your shaking mechanism. The reason is that you are using a unipolar signal source for the excitation and that the source turns off (rather than maintaining the average bias current through the solenoid) between the test pulses. This change in biasing conditions and the corresponding mass shift in the shaker mechanism results in a mass shift of the entire torsional balance, which, together with the long time constant of the balance, makes it look like a thrust signal.

To perform such an experiment "cleanly" it would need 2 signal sources for the shaker:
1. A fixed, DC, current (not voltage) regulated, constant current bias supply that is always on for the duration of the experiment.
2. A AC-only signal source with no DC component in the output (preferably also a current source, but voltage would still be OK in a pinch) for the shaking signal.

Both sources need to drive the shaker in parallel but to prevent signal flows in undesired directions the bias DC source should be coupled through an inductor while the signal AC source is coupled through a capacitor. This makes sure that each signal component (DC and AC) can only take its appropriate path.

It's important that the DC source is current regulated (not voltage) because the solenoid coil will change resistance as it warms up and from a source that stays on a long time it will be warming up. The AC signal can be voltage regulated if need be, but beware that its absolute current magnitude must never exceed the DC bias, otherwise the polarity of the coil will start flipping at the peaks and this will make the average B-field and the average plunger position no longer fixed and well-defined.

Also beware that the entire signal must never drive the mechanical spring-mass-system out of linear operation (like by compressing a spring too much) and this also includes effects of resonance (the control of which is not always trivial).

P.S. It's also important for the bias current source to consistently maintain correct bias even when resonance effects drive the voltage across the solenoid coils to high peak levels (possibly negative and/or even higher than the voltage of the power supply that supplies the bias current source in the first place) - the bias source needs to maintain linear operation all the time and this also may not always be trivial.
« Last Edit: 12/09/2017 11:33 AM by xyzzy »

Online Monomorphic

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Looking at your vibrational thrust test graphs, I think there is a significant experimental error: You seem to have neither eliminated nor accounted for the changing offset / DC component in the movement of the plunger in your shaking mechanism. The reason is that you are using a unipolar signal source for the excitation and that the source turns off (rather than maintaining the average bias current through the solenoid) between the test pulses. This change in biasing conditions and the corresponding mass shift in the shaker mechanism results in a mass shift of the entire torsional balance, which, together with the long time constant of the balance, makes it look like a thrust signal.

I'm using a Voice Coil Actuator (VCA), not a solenoid. The VCA is driven by an analog audio signal. I use mono WAV files created in Audacity to generate the various signal frequencies and waveform shapes to test. Those are stored on microSD, which is accessed by the arduino, which then converts it to analog, sends it to the 3W amplifier, and finally to the VCA. 

In the image below, the two bottom waveforms are sinusoidal chirps, in case anyone was wondering what those looked like.

the corresponding mass shift in the shaker mechanism results in a mass shift of the entire torsional balance, which, together with the long time constant of the balance, makes it look like a thrust signal.

The shakers were designed to test the following criticism of the mach effect: "Due to the asymmetric motion inside the box, the center of mass of the box and its contents shifts relative to the box. But the center of mass must still remain where it was before (relative to the laboratory). So the box moves aside, while its center of mass stays put. Newton's laws were working properly, as they always do." 

The idea being that it is fairly easy to produce a false-positive thrust trace on a torsional pendulum with a vibrating device. I certainly do not believe I'm producing actual thrust with the shaker.
« Last Edit: 12/09/2017 12:57 PM by Monomorphic »

Offline xyzzy

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OK, a voice coil is quite another story. Sorry for the confusion.

A question remains however: how does the arduino output the analog signal and how does the power amplification work? From the graphs (especially the lower frequencies) it looks like the signal is not entirely DC-free.

A voice coil does not need a bias source, so forget everything I was telling about a bias supply. But a voice coil needs to be free of any DC bias however.

It looks like the digital-to-analog conversion is somewhere introducing an offset, each time that the shaker is operating. Maybe it's not a real DAC but some sort of PWM output mechanism with a filter - and the PWM is being switched off entirely (output going to zero) between the periods of shaker operation, rather than continuing to drive out a "neutral" 50% duty cycle.

Can you check this: what is the average voltage that comes out of the amplifier? Best to check with an analog DC voltmeter. If the signal is fully symmetric and has the correct "neutral" output between the periods of shaker operation (and if each period is itself DC-free) then the meter needle should not visibly move away from zero (it would vibrate in place, but not move on average).

Regards

P.S. Of course it's clear that the shaker does not produce thrust. I did not imply any such thing or belief  ;)

But it's easy to introduce a DC offset in the drive circuitry - either with each pulse or in the time between the pulses. The effect on the measurement of such a mistake would be enormous, orders of magnitude more than the effect you were testing for (mechanical nonlinearities in the materials when subject to vibration), and it would easily dwarf such nonlinearities in comparison if it happened.

P.P.S. If the meter needle makes a (visible) swing in opposite directions at the start and end of each pulse (of each period of operation) then something is clearly wrong with the DC offset difference between operation and non-operation.
« Last Edit: 12/09/2017 01:17 PM by xyzzy »

Online Monomorphic

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Can you check this: what is the average voltage that comes out of the amplifier? Best to check with an analog DC voltmeter. If the signal is fully symmetric and has the correct "neutral" output between the periods of shaker operation (and if each period is itself DC-free) then the meter needle should not visibly move away from zero (it would vibrate in place, but not move on average).

This is something I can easily check. And if present, I can compensate for it by adjusting the bias of the waveform in software rather than requiring physical biasing. That is much exaggerated in the image below to illustrate.
« Last Edit: 12/09/2017 01:48 PM by Monomorphic »

Offline xyzzy

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This is something I can easily check. And if present, I can compensate for it by adjusting the bias of the waveform in software rather than requiring physical biasing. That is much exaggerated in the image below to illustrate.

Please do. And if you happen to find anything strange, please also check the code logic for simple configuration mistakes - like forgetting to output a "neutral" 50% duty PWM (or to set a DAC to an appropriate "neutral" value) between output pulses (or generating the output pulses themselves with a wrong midpoint in amplitude). Note that some "hard" cases of such code mistakes cannot be completely compensated in the waveform offset (e.g. when the compensation would have required the output to go below a power supply rail).

Offline WarpTech

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Can you check this: what is the average voltage that comes out of the amplifier? Best to check with an analog DC voltmeter. If the signal is fully symmetric and has the correct "neutral" output between the periods of shaker operation (and if each period is itself DC-free) then the meter needle should not visibly move away from zero (it would vibrate in place, but not move on average).

This is something I can easily check. And if present, I can compensate for it by adjusting the bias of the waveform in software rather than requiring physical biasing. That is much exaggerated in the image below to illustrate.

If your chirp is shifting the frequency, to avoid a DC offset you must maintain the same Volt-seconds in each half cycle. The period should only change like a staircase, shift some Hz ONLY after 1 complete cycle, not a continuous ramp. Otherwise, the DC will be inherent in the chirp input.

I know this because it's what causes transformer saturation in DC to AC Inverters that use power transformers. DC offset of the AC output when adjusting frequency can be a big problem and has to be accounted for in the design.

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