GIThruster wrote:you need to be able to produce large amplitudes by running on resonance regardless of the temperature of the stack, which can change the resonance by a Khz or more.
What other mechanical resonance is there that is changed by the temperature of the stack?
Since when has a piezoelectric resonance not been a mechanical one?
... Always with the clever answers, never with the meaningful ones.
Anyone who studies piezocermaics knows their resonance is a function of temperature, DC offset and several other things. If you doubt my statement, read at least the abstract from the short NASA paper published in 1966 that I linked to just 3 posts back. In that example a stack with a natural resonance of 25 Khz had its resonance shifting up to 820 khz as it heated up, and the PLL was designed to track the natural resonance to +/- 0.5 hz as it went through those significant resonance changes.
Piezo stacks are extremely non-linear as regards extension amplitude (and their consequent accelerations) when on or off resonance. The spike of piezo action available on resonance is very narrow, so tracking it very closely is important as is explained in the NASA paper.
And yes, 92143 is correct. Both the power into the stack needs to be on resonance and the acoustic action of the stack needs to be on resonance. IIRC, Paul March explained this several months ago in this thread.
Last edited by GIThruster on Tue Dec 18, 2012 12:48 am, edited 3 times in total.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis
GIThruster wrote:...the ability to generate large thrusts..you need to be ..running on resonance ...
MSimon wrote:If you put a tuned circuit (the piezo and the ceramics) in the feedback loop of an amplifier you get an oscillation at the resonant frequency.
(GIT = EE fail)
You need electrical and mechanical resonance simultaneously.
They generally go together. If they don't you have a problem. And the problem is your load will be difficult to drive. The load will be reactive.
Engineering is the art of making what you want from what you can get at a profit.
MSimon wrote:Design of such a system is easy. It is the debugging that is difficult.
I believe you. Several EE's have said this over the last few years, but none of them have designed, debugged and built such a system. And this is why I say that despite we want to see new ceramics, new designs, higher frequencies, more thrust, what is needed more than anything else is a salary for an EE in the lab. There are plenty of people able to solve the power system problems we have, but they need to be paid.
This is why I'm so annoyed at NASA that they aren't even interested in a NIAC grant. According to Jay, NIAC is okay with funding TRL 1-3. They're just not available to be the first to fund TRL-1-3. NIAC is a sham.
If you have a good technician and a good scope camera you don't need an engineer on site. At least not full time.
Engineering is the art of making what you want from what you can get at a profit.
MSimon wrote:If you have a good technician and a good scope camera you don't need an engineer on site. At least not full time.
That's true. It depends what you want to do. What we're generally speaking about though is proprietary hardware intended to drive ceramics in unusual ways that don't exist at present. Especially with electrostrictors, no one has ever had a reason to drive a ceramic at 1w+2w before, at least to the best of my knowledge. So when it comes down to it, you're looking at continuing to cobble together stop-gap measures that do indeed call for an EE in the Lab, and designing proprietary power equipment that requires an EE in his own lab working full time. Once the job is done you reappraise and look at the next thing. This has always been evolutionary work that is held back by lack of EE support. Even though there might be weeks or months at a time where an EE wouldn't be needed in the lab, he can always go back to work on whatever is the next thing. So just saying, it's been my opinion based on the weekly updates we've been reading for about 6 years that the highest need is to get an EE in the lab. Sure there are other needs but dishing out big pesos for high quality ceramics doesn't make much sense if you don't have the power system to drive them.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis
Interesting that the crackpot "the aliens are here" folks called out Tau Ceti as one of the systems they claim aliens come from. If we find a planet in the habitable zone around Zeta Reticuli or Rigel I'll start to wonder how they guessed right twice. That would be an astonishing coincidence.
I would note though, we can't visit Tau Ceti ourselves. At 5X our gravity we wouldn't be able to move and it would cause heart failure pretty quickly. Imagine trying to move around with a pair of V8 engines strapped to your back.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis
GIThruster wrote:I would note though, we can't visit Tau Ceti ourselves. At 5X our gravity we wouldn't be able to move and it would cause heart failure pretty quickly. Imagine trying to move around with a pair of V8 engines strapped to your back.
You can't just assume 5 times the mass means 5 times the surface gravity.
The formula is g = (4/3) pi G rho r , where G ~ 6.674e-11 m^3/(kg s^2), rho is mean density (kg/m^3), r is radius (m) and g has units of m/s^2 (1 "gee" ~ 9.8 m/s^2).
Yes well, gas giants are not the thing to compare to. They have a much lower density than solid planets. There are a bunch of different types of solid planets and they vary in density based on things like water content, but if you presume a density similar to Earth with 5 X the mass, and note mass goes with ~4R^3, The surface is not going to be much further from the center than we are here on Earth. I'm sure you can do the calc, but I'd guess it's got to be close to 5 gees on the surface. Suppose it's just 4, or even 3. At 3 Earth Gees you still haven't got a vacation spot. You might manage for a few hours or perhaps even a couple days, but not much beyond that safely.
"Courage is not just a virtue, but the form of every virtue at the testing point." C. S. Lewis
It is believed to have a mass of 3.1 to 4.3 times Earth's and a radius of 1.3 to 2.0 times Earth's (1.3 to 1.5 times larger if predominantly rocky, 1.7 to 2.0 times larger if predominantly water ice). Its mass indicates it is probably a rocky planet with a solid surface. The planet's surface gravity is expected to be in the range of 1.1 to 1.7 times Earth's, enough to hold on to an atmosphere likely to be denser than Earth's.[1]
GIThruster wrote: if you presume a density similar to Earth with 5 X the mass, and note mass goes with ~4R^3, The surface is not going to be much further from the center than we are here on Earth. I'm sure you can do the calc, but I'd guess it's got to be close to 5 gees on the surface. Suppose it's just 4, or even 3. At 3 Earth Gees you still haven't got a vacation spot. You might manage for a few hours or perhaps even a couple days, but not much beyond that safely.
This is not right
With constant density mass increases proportionally to volume. When you buy 2 liters of soda you expect the volume to have mass of 2* 1 liter.That is pretty intuitive.
If density is the same a planet with 5 times mass has 5 times the volume.
Volume of a sphere increases proportionally to 3rd power of radius.
This means that the radius of the body will be roughly 5^(1/3) = 1.7
Put the 1.7 into the g=G*m/r^2 and you get 16.69 m/s^2 which is roughly 1.7 earth gravity.
Not ideal, but definitely livable. I know people who weight 1.7 of my mass and do fine, despite the increased volume
1.7g is a lot of gravity. Take your weight, then stuff 70% of it into a back- pack and try wearing it all day long. Even carrying 15% of your body weight in a backpack gets tiring after a while. I would say that 1.2g is about the maximum that people could live with. A planet with 1.7g gravity is not going to be a comfortable place to be.
