SparkFun Forums

[Woehrle]

Hello everyone,
I am trying to create a system capable of taking a spectrum at a specific point. I believe this can be accomplished with a spectrometer, IMU, and Microcontroller to determine where the spectrometer is pointing. I might add, this is for a sub-orbital rocket, spinning at 5Hz. So my question to you guys and girls is, Is this feasible? and Could I possibly obtain some ideas on how I would code the microcontroller?
Thanks,
Woehrle

[cpolley]

I assume that your spectrometer is fast enough that the 5Hz motion won't blur it all together, and that you know how to trigger it to read & save the save the data. If not you need to give more information.

Maybe somebody better qualified than me can chime in, but my understanding is that an accelerometer does not do well at interpreting position by integrating acceleration.

You might be better off with one of the tilt compensated compass modules (?)

[Woehrle]

I believe it will work at the 5Hz, but over the next couple of weeks it will be put through tests to confirm that. The reason for using the IMU is that we would also like to characterize the rocket's flight too. but if the IMU doesn't get the job done, we will look at other methods of triggering the spectrometer.
Thanks,
Woehrle

[UofMEShop]

A linear array detector could be used to grab an "image" of all wavelengths at the same time. i.e. use many detectors, each acquiring a sub-wavelength region of the entire spectrum. There are array detectors that are simply multiple photodiodes close to each other. Digikey carries some. Also, a 2D array detector can be used (CCD).

[Shifted]

A 3 axis gyro would do it, but you would probably need FOG's.

What kind of budget are you working with?

[Woehrle]

Money is not a problem for this project. I would love to hear more from you shifted. the IMU's gyros won't do the same thing?

[Shifted]

No, unfortunately, the bias and drift over time will accumulate faster than you can deal with. You may be able to accomplish it with some extremely expensive MEM's gyro's, but you are still looking at a bias and drift.

What you need are much higher quality FOG's, start looking at suppliers like Lockheed Martin. They have some strap-down gyro's that are made specifically for what you are looking at. I know you said money wasn't an issue, but be prepared for the cost, you are looking somewhere between $60-$70k. You'll also have to go through an approval process because FOG's and high quality accelerometers from Lockheed are on the ITAR restrictions list, basically you have to guarantee you won't export or use it illegally (you can read about ITAR in the CFAR's, contractor federal aquisition rules).

We deal a lot with these kinds of things where I work. I've used strap-down MRU's (an IMU on steroids) from other suppliers like TSS Solardyne and Kongsberg. Those probably wouldn't work since they are specialized for the marine industry, but those sensors start at $25k.

I've been playing with some very expensive IMU's (take a look at MemSense uIMU), and they don't perform accurately enough for a long enough period of time, even despite a Kalman filter.

[Woehrle]

I only need it to be accurate for 20-25min max. I was aware of the drift, but I'm trying to figure out when that actually happens.

[Shifted]

My experiments showed they were accurate for a much shorter period of time. Your biggest problem is the rate at which you need to calculate the position, 5hz = 1800deg/sec. You just aren't going to get that kind of bandwidth and speed on MEMS devices.