Hi all,

I am designing a small 6-DOF IMU with digital outputs that uses the newest MEMs devices on the market (some of which haven't been released yet) to keep the footprint small and the cost low. I'm hoping to make a bunch of them and sell them at low margins so that DIY'ers like us can afford to tinker without breaking the bank. If they sell well enough, I'll be able to keep the cost on the order of $100 for the full 6-DOF with digital outputs. :-)

If you had the choice, what do you all think the "best" pinout would be?

My current design has two .1" headers mounted on opposite ends of the board, so that prototyping and mounting is easy.

Alternatively, I could add mounting holes, and then put little shrouded headers that you'd plug into; then, you wouldn't have to mount the IMU on a PCB - you could put it anywhere on the airframe.

Another option would be to use low-profile board-to-board connectors, but they wouldn't be easy to use (have really fine pitch).

What do you think?

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Hi Anthrax,

By oversampling and decimating I reduce quantization noise from the ADC itself. It is true that the noise floor of the sensors is high, but it is better, I think, to remove quantization noise from the equation. :-)
ST hasn't posted specs on the resonant frequency of the sensing elements. I would guess that they are similar to the LISY, but I don't know for sure.
Hi Roy,

I went with an FIR filter because it is easier to implement than an IIR filter, and it has guaranteed stability properties (no feedback). It is also super-easy to come up with an optimal FIR filter using Parks-McClellan. I don't think an IIR filter takes less processing, but I am not a signals guy, so I'm not sure.

I'm actually thinking of using a minimum phase FIR filter instead of a Parks-McClellan filter... less phase delay = higher allowable feedback gains = better performance.

The IMU communicates over a TTL UART. i2c pins are also routed out, but I haven't added code to use them.

I won't be releasing code that turns this board into an all-in-one control solution, but I routed four extra GPIO pins out in case anyone wanted to do that. The timers on the STM32 are very, very flexible, and it would be easy to implement a hobby PWM signal on one or all of the four extra GPIO pins (you could do it using one timer and no bit-banging). I am toying with the idea of making a miniature quadrotor with this as the control board.
Why don't you use IDG gyros, they use high resonant frequency - 27 KHz and higher?
And I think price for them is comparable to ST
They are roughly the same price now, but they weren't when I started. They were $40 each at first, then they dropped to $20, now they are $9 each.

Yesterday the stencils for my design came in the mail, and last night I made a batch of 16 of them... so I feel a little less inclined to switch parts at this point. :-)
Anyway most efforts goes into software and filters tuning
I'm a software guy so I thought that replacing an IC is similar to putting a new dll with similar interface :)

How much more time will it take you to use IDG gyros instead of ST?
The extra time comes from ordering new PCBs, which may or may not work the first time through, ordering new stencils, working out unforseen HW bugs, etc.

Given manufacturer lead times, it would probably take a little over a month to switch the design now.
1 - 1.5 month still not bad. I'm not in a hurry I can wait if you say you are going to make it :)
(not to mention > $200 for new stencils, ~ $400 in unused ST parts, plus maybe $100 for boards from my PCB manufacturer. If I were a big company and not a hobbyist...)
Ok, finish the first batch and sell it. I will buy 1 for testing.
I hope this is not the last batch you are planning to manufacture. In the next one you may use suggestions from guys from this forum.
Sergey,

Yes, I imagine I will switch to the IDG rate gyros once my bank account recovers slightly...

Thanks for the suggestions - I wasn't aware that the IDG prices dropped again.
that makes sense to me now, tks for your explanation

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