I like the idea of connecting them all together to get a single reading --
much simpler wiring. However, it seems to me that it would be better to
wire them in parallel, so one or a couple of dead diodes don't kill it
dead. That also reduces the peak voltage from ~n*Vf to ~Vf.
-p
On Mon, Sep 14, 2015 at 5:08 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
On Monday, September 14, 2015, Pierce Nichols <piercenichols@xxxxxxxxx>
wrote:
Nifty idea! My understanding is that all plain LEDs shift their
wavelength with temperature because the size of the band-gap is temperature
dependent. It seems to me like the same physics would affect the receive
side of the optocoupler as well... so it's not clear to me that you'd
actually get the response to expect.
That would certainly kill it if true. I'll have to get some ln2 and try
it.
However, the current through a diode is also temperature dependent, which
points the way to a more elegant implementation. Perhaps you could just
have a line of plain diodes and sense the change in current through each
one as the liquid level passes it. Thoughts?
If it's a large enough change then you could have a bunch of them in
serial and have basically the same analog sensor as centaur but with a
current change rather than capacitance. Easier to turn into something a uc
reads.
-p
On Mon, Sep 14, 2015 at 2:14 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
One of the few disadvantages liquids have over solids is propellant
depletion. A solid burns nearly all its propellant, though some of it tends
to be at lower than nominal pressures during the tail off. For a liquid
stage to get maximum impulse it is imperative that it deplete both
propellants simultaneously. The last kilogram of propellant is the most
important it. A real world example of this is that Stiga flew to ~90km but
had 30lb of fuel remaining after it burned out the LOX; with equal
depletion it would have reached roughly 120km.
Centaur accomplishes this with capacitive sensors in the propellant
tanks. A rod is inside and electrically insulated from a tube that is open
at bottom and top. This forms a capacitor. As the level of the propellant
goes down, more of the capacitor contains gas than liquid and its
capacitance changes. This is used as an input to the mixture ratio
controller, which drives a harmonic drive on a throttle valve to slightly
tweak the mixture. With this control a stage carrying thousands of
kilograms of propellant is able to deplete down to tens of kg, helping give
the stage it's incredible (and consistent) performance.
One downside of this system is that it is analog and it requires some
extract structures and mass to hold up the tube and wire, and keep them
electrically isolated.
Cryo and electronics geeks know that some types of light emitting diodes
(LEDs) change their emission wavelength when they are cooled to cryogenic
temperatures.
An optocoupler is a solid state semiconductor device essentially
composed of an IR LED mechanically coupled to an IR transistor. The two
components aren't electrically coupled, so it's a way to send information
between two isolated DC systems without them needing to share a ground. I
used optocouplers to run some of the solenoids on Xombie and Xoie; it was a
convenient way to amplify a TTL signal to coil current while isolating the
noise associated with quickly turning on and off inductive loads.
Background complete. You could make a chain of optocouplers inside a
cryogenic tank, with them turned on all the time. It would consume just a
few mA per sensor. If the wavelength alteration trick works on IR LEDs, the
optocouplers would only pass a signal when they're above the level of the
cryogen.
By watching the timing as the sensors toggle, you can then measure the
depletion rate of the propellant in the tank and adjust mixture ratio
accordingly. With SMD optocouplers, the whole thing could be quite light.
There is a similar concept with small heaters coupled to temperature
sensors, where sensors that get warmer are above the liquid level. But I
think my concept would put less heat into the tank and has digital outputs,
rather than analog.
So that's my idea of the afternoon.
Ben
