Bad Perfectflite altimeter?

[Handeman]

Here's the screen shots of the altimeter data from my Perfectflite CF. It was the first flight. The two test flight that came already in the data looked perfect.

This was set up as the backup altimeter in a 4" rocket. The apogee delay was 1 second and the main was 450 ft. (50 ft. below the primary altimeter main deploy setting).

The flight per the primary was 4064 ft. The back up showed 5061 and the drogue fired at about 1500 ft. the main at 450 ft.

The power trace looks like the only good data recorded.

Anybody have any idea if this is just a bad altimeter, or could there be something in the setup?

 

[SparkyVT]

Did you happen to have a strong radio transmitter onboard? Photo of your setup?

 

[cerving]

Looks like baro noise to me... if your altitude readings are a bit noisy, the velocity readings will be REALLY noisy.

 

[Handeman]

Nothing in the av-by but the two Perfectflite altimeters. There is a T3 in the nose cone. The other Perfectflite CF didn't look at all like this.

Can't figure out why the drogue fired so far after apogee on the back up. There was also a 1000 foot difference between them.

I fly two of the same model Perfectflites in a 3" rocket with the same T3 and they don't have any problems like this. In fact, they actual both read the exact same altitude a couple flight ago.

 

[Zeus-cat]

I would be tempted to fly it again, but not hooked up to charges. If you get the same results contact Perfectflite about having it checked.

 

[mbeels]

That does look suspicious. Some of the drops in temperature line up with the spikes in altitude. I'd try a test flight, or some vacuum tests. Something isn't right.

 

[Ronz Rocketz]

All I know is the Flightsketch Mini does that when exposed to sunlight or gets jarred from swing around. The pouches fix most of that.

 

[BEC]

That looks like it was swinging or spinning pretty fast under 'chute. At least that's how I've gotten barometric data that look like that. Although now that I look more closely, I've never seen periodic pressure change spikes on boost like that.

Does the rocket roll on the way up, and what does your static port arrangement look like? And where was this device relative to the ports? Those nearly two degree swings in the temperature data sure are curious.....

 

[rharshberger]

Static ports large enough/too large? Were the static ports in clean air flow, I have had similar data when the Altimeter was either in the nose or the static ports near a "feature" like a forward fin.

 

[GlueckAuf]

I too recommend a vacuum test. Those are some strange results. I’d suspect a battery issue, but the voltage readings are steady. The problem seems nearly identical on BOTH 6632 and 6636, so maybe it’s a bad production run or calibration issue. In any case, PerfectFlite will, I’m certain, resolve it one way or another. Great small business, despite recent challenges.

Good skies,
GlueckAuf

 

[Handeman]

Thanks for all the suggestions. I was given another by a club member that said the barometric sensor is pizo and sensitive to shock. To make sure the altimeters are not loose and rattling or vibrating against the board. I'm thinking it might be the board vibrating.

The two flight sims that come loaded in the altimeters are vacuum chamber tests and they look perfect.

The vent ports are about 20" below the nose cone and centered on a 1" switch band. I've never had issues like this because of a switch band before and have kind of ruled out the joint between the switch band and the payload tube.

The vent ports were a little smaller than 1/8" and the tube that guides and hold the pull pins were very close to the inside edge of two of the four holes.

I've drilled the vent ports out to 5/32" and am doing another test flight today.

I'll let you know how it goes.

 

[BEC]

Thanks for all the suggestions. I was given another by a club member that said the barometric sensor is pizo and sensitive to shock. To make sure the altimeters are not loose and rattling or vibrating against the board. I'm thinking it might be the board vibrating.

Rattling around does cause spikes in the data - this is for any bard-based altimeter, not just these. I still think, from the plots, that something periodic was going on: rolling (boost) or swinging (descent). But if the unit was free to smack against something, that would certainly give spikes in the data. I have actually had a flight or two with similarly wild spikes in the data from swinging under the parachute on descent...where the altimeter was not properly restrained in the compartment. For example: https://flightsketch.com/flights/1045/

 

[mbeels]

Rattling around does seem very possible, and it could also explain the spikes in the temperature data. Many MCUs have an internal temperature sensor which is also (not by design) sensitive to strain. If it was loose, it wasn't just rattling, it must have been banging around really hard.

 

[Ronz Rocketz]

Rattling around does cause spikes in the data - this is for any bard-based altimeter, not just these. I still think, from the plots, that something periodic was going on: rolling (boost) or swinging (descent). But if the unit was free to smack against something, that would certainly give spikes in the data. I have actually had a flight or two with similarly wild spikes in the data from swinging under the parachute on descent...where the altimeter was not properly restrained in the compartment. For example: https://flightsketch.com/flights/1045/

90F!!! We’re you in Tricities?

 

[BEC]

90F!!! We’re you in Tricities?

No, that was further east - flying at BMR's site above Dayton, WA, at their June launch last year.

 

[Buckeye]

Nothing in the av-by but the two Perfectflite altimeters. There is a T3 in the nose cone. The other Perfectflite CF didn't look at all like this.

Can't figure out why the drogue fired so far after apogee on the back up. There was also a 1000 foot difference between them.

I fly two of the same model Perfectflites in a 3" rocket with the same T3 and they don't have any problems like this. In fact, they actual both read the exact same altitude a couple flight ago.

View attachment 458105

This other altimeter is equally bad as the first one you posted.

A vaccuum test should show fairly nice smooth altitude curves. Ignore the drogue an main events, though. As I learned in another post, the deployment logic cannot keep up with a quick, high-speed suction test.

If the vacuum test looks fine, then something weird is happening in your flight or mounting of the altimeter.

 

[rharshberger]

No, that was further east - flying at BMR's site above Dayton, WA, at their June launch last year.

Sadly we haven't seen much of BEC in the last two years, however the 2021 season has just begun!

 

[BEC]

...and I plan to be there for the June launch. It'll be a rocket trip: NSL on the way out to visit family in NM, June Swoosh on the way back.

 

[rharshberger]

...and I plan to be there for the June launch. It'll be a rocket trip: NSL on the way out to visit family in NM, June Swoosh on the way back.

See you then!

Handeman, looking foward to the new data and maybe a solution to this problem.

 

[Handeman]

I don't have the new data yet. but the altimeters are not "rattling" around. Each battery and altimeter is mounted to their own side of the board. They are mounted with 4 stand offs and all screws are tight. The board is held between two threaded rod with nuts tightened to hold the board in place.

I enlarged the vent holes and I think that may have improved things some. This last flight also had a late secondary drogue charge, about 10 seconds after apogee instead of 30 like the first one. Data coming.... We'll see

 

[Handeman]

This other altimeter is equally bad as the first one you posted.

A vaccuum test should show fairly nice smooth altitude curves. Ignore the drogue an main events, though. As I learned in another post, the deployment logic cannot keep up with a quick, high-speed suction test.

If the vacuum test looks fine, then something weird is happening in your flight or mounting of the altimeter.

The other altimeter isn't as bad, although it does show the same type issues, it is firing the drogue charge at apogee and the main at the desired setting. That is why I decided to make a second flight with the secondary altimeter. I am going to do vacuum tests one each one over the next couple weeks and may swap secondary altimeter between this rocket and another that is also flying a pair of PF CFs and see if the problem follows the altimeter or stays with the rocket.

More to come.

 

[BEC]

I didn't really think you had them loose and "rattling around" but that's the only thing I could think of that would have a healthy device give you plots of pressure (in the form of altitude) and temperature that looked like that. Will be curious to see what you see after swapping devices around.

 

[Handeman]

So after opening the vent holes, things got better. The noise was still there, but less. The back up altimeter fired the drogue about 15 seconds after apogee, right after a fairly smooth section of data. Here's the plots.





An interesting glitch. As I was walking back with the rocket, I plugged in the pull pins to shut off the altimeters and then pulled them out for each altimeter to catch the altitude beeps. The first time I did that, the secondary had a false flight and started warbling an error code of lost power during flight when I powered it up after that. It has two new flights in the data that the primary altimeter doesn't have.

Here's the plot of the first fake flight.

 

[mbeels]

To me it looks like something is wrong with the altimeter. The data is just about as bad with spikes in the altitude reading of about 1,000 feet, and spikes in the temperature reading of about 1 to 1.5 degrees.

 

[Ronz Rocketz]

The temperature and velocity look messed up.

 

[GlueckAuf]

Have you performed the self-test routine on these altimeters?

 

[Reinhard]

The barometer on the Perfectflite (looks like an MS5607 or MS5611) contains a temperature sensor, because it is required to convert the raw sensor reading into a temperature compensated pressure value. So I assume the temperature reading is from this sensor. The correlation between negative temperature spikes and positive altitude points in that direction.
The temperature sensor should not be affected by vibrations and shocks, neither should the value be able to change that fast.
Curiously, the temperature reading on ascent seems to oscillate with a frequency roughly proportional to the rockets speed. I suspect that this shows the increasing and decreasing roll-rate of the rocket.
As @rklapp has pointed out, the exposed silicon on barometric sensors is susceptible to sunlight. If I had to guess, every dip in temperature is caused by sunlight reaching the silicon. This happens once per revolution on ascent and more or less randomly on descent under drogue.
The positive spikes in altitude can be either a direct consequence (sunlight messes with the pressure reading), an indirect consequence (changed temperature reading messes with the result of the pressure calculation), or a combination of both.

If you perform a vacuum chamber test, I'd try to play with exposing the sensor to sunlight. Maybe your problem can be fixed with a small "sunshade"

Reinhard

PS: The Raspberry Pi 2 was a somewhat famous example of that effect
https://www.raspberrypi.org/blog/xenon-death-flash-a-free-physics-lesson/

 

[mbeels]

The barometer on the Perfectflite (looks like an MS5607 or MS5611) contains a temperature sensor, because it is required to convert the raw sensor reading into a temperature compensated pressure value. So I assume the temperature reading is from this sensor.

As @rklapp has pointed out, the exposed silicon on barometric sensors is susceptible to sunlight. If I had to guess, every dip in temperature is caused by sunlight reaching the silicon.

Interesting, this sounds like the most well reasoned and plausible explanation so far.

 

[GlueckAuf]

Not sure if this is all that illuminating, but here's one flight I've done that exceeded Mach and rolled pretty hard on the way up, and rocked on the way down. Here's the on-board video...



...and here's the resulting PerfectFlite graph. (Note: I used a JLCR set for about 700", the graph's Main event is inconsequential.)



The only hiccups I saw were 1) at ignition, probably the usual Loki Red chuff, and 2) a little blip at 1,750 feet. The rest is pretty smooth even with Smoothed Alt turned off. Hope you can solve the mystery, bud. Best of luck.

Good skies,
GlueckAuf

 

[Ronz Rocketz]

The only hiccups I saw were 1) at ignition, probably the usual Loki Red chuff, and 2) a little blip at 1,750 feet. The rest is pretty smooth even with Smoothed Alt turned off. Hope you can solve the mystery, bud. Best of luck.

Good skies,
GlueckAuf

My FS sometimes (lees than half) gets the hiccup at cut off and the acceleration drops. Obviously the rocket doesn’t drop in elevation or freeze in time, so what’s going on here?

https://flightsketch.com/flights/2126/
The altimeter gets the weather data from some nefarious source. I’m not sure if at the time of flight, when it’s downloaded to the phone, or when uploaded to the website.