3 ms·

> Have you ever wished for a 500 Hz, millimeter-precise linear position sensing system Kind of, but I'd like an 0.01mm precision please. It can be just a few H

by deskr 2y ago

> Have you ever wished for a 500 Hz, millimeter-precise linear position sensing system Kind of, but I'd like an 0.01mm precision please. It can be just a few Hz, I don't need 500 Hz. Great project though!

tonyarkles 2y ago
I'd be super curious to see how the accuracy changes with averaging out/low-pass filtering the measurements. Accuracy usually improves proportional to sqrt(N) when you take N samples so your higher precision desire might just be a bit of code to write. The other side of it though is that you're starting to get down into the "everything needs to be temperature controlled" region as you squeeze that precision number. FR-4 and copper have thermal expansion coefficients around 15-20ppm/C. If I'm doing this mental math correctly, a 5 deg temperature rise would make a 1m long piece of FR4 expand by 0.1mm, or a 10cm piece of FR4 expand by 0.01mm.
- foobarian 2y ago
  One time I wanted to demonstrate thermal expansion to my kid (1st grade or so) so I made some marks with a steel ruler and put it in the freezer. Imagine my surprise when we took it out and there was no perceptible difference :-D
  - dekhn 2y ago
    did you measure the freezer after? It expanded :)
- satiric 2y ago
  With some microcontrollers you can do this "averaging out" just by changing ADC parameters, you don't even have to write the low-pass-filter code.
  - auxym 2y ago
    That is specifically sigma-delta type ADCs: https://www.analog.com/en/resources/technical-articles/sigmadelta-adcs-tutorial.html https://www.analog.com/en/resources/technical-articles/sigma...
    - magicalhippo 2y ago
      No, several MCUs, like a lot of STM32s, support hardware oversampling with optional division. Here's[1] a document describing it for the L0 and L4 series, which like most of the STM32 series have SAR[2] ADCs. [1]: https://www.st.com/resource/en/application_note/an4629-adc-hardware-oversampling-for-microcontrollers-of-the-stm32-l0-and-l4-series-stmicroelectronics.pdf https://www.st.com/resource/en/application_note/an4629-adc-h... [2]: https://en.wikipedia.org/wiki/Successive-approximation_ADC https://en.wikipedia.org/wiki/Successive-approximation_ADC
namibj 2y ago
That at 500Hz is called "optical mouse on absolute positioning patterned surface". On that note: I'm looking for a mouse style camera sensor unit that can export full frame rate raw to a system where I can actually decode such an absolute positioning code. Anyone got something in the sub-100$ range?
- bigiain 2y ago
  The DYI Drones and Ardupilot comminities were using mouse sensors to do "optical flow sensing" for accurate low altitude position hold around 10 or so years back. https://ardupilot.org/copter/docs/common-mouse-based-optical-flow-sensor-adns3080.html https://ardupilot.org/copter/docs/common-mouse-based-optical... https://github.com/RCmags/ADNS3080_frame_capture https://github.com/RCmags/ADNS3080_frame_capture https://www.pixelelectric.com/sensors/distance-vision/adns-3080-optical-flow-sensor-module/?setCurrencyId=2 https://www.pixelelectric.com/sensors/distance-vision/adns-3...
- Karliss 2y ago
  I once looked into mouse sensors for similar purpose, but the raw frame export capabilities were disappointing. Looking at the datasheet of PMW3360 which claims to have up to 12000 FPS, but the timing sequence for raw frame dump suggests ~20ms if not 40ms to transfer the whole frame which would give only 50-25FPS. That doesn't necessarily mean the main spec is lying, it just isn't capable of dumping the frames over SPI as fast as it is producing and processing them internally. Frame dumping seems to be more of debug/test feature, so no big surprise it's not capable of fully utilizing the sensor and there is little motivation for sensor manufacturer to improve it. But there is also a chance that the manufacturer has put very conservative timing for it in the datasheet, since it's not the main functionality of sensor. If anyone has explored further how much the timing can be optimized, or maybe there are other mice sensors for with better frame dumping capabilities, I am curious to hear about it as well. With that said even if you remove all the unnecessary delays data transfer over SPI will likely be a limiting factor for very high framerates. Assuming 4MHz SPI and 32x32 8bpp image that would be at most 500FPS. Few hundred FPS would still be nice, and you might push it a bit higher with increasing frequency but it's probably not possible to push it much further than 1000FPS without MIPI or some kind of other parallel data transfer protocol. The ardupilot project in sibling comment seems to be using the mouse movement output directly instead of capturing the frame and then doing optical flow analysis on main CPU. The image dump is only being used to verify camera focus.
krisoft 2y ago
> It can be just a few Hz, I don't need 500 Hz. Tldr: understood that you think you don’t need 500 Hz, but there is a technological limitation why you need that frequency for these systems to work. The reason these measurements need to be so fast is because the measurement is not absolute but periodic. Meaning that when you measure something it can’t tell you how large it is in absolute terms just how much larger it is than the closest integer period. In mathematical notation you can measure x where the whole distance is k*period+x, where k is an unknown integer, and period is a design parameter. It can’t natively tell you what is the value of k. So to figure out the whole measurement you need to calculate k. And you can do that by keeping track of it. You know it is zero when they zero out the caliper, and you know that it just increased by one every time there was a falling discontinuity of x. (Meaning that every time x approaches the period and then suddenly drops to a low number it just crossed over one of these period boundaries.) Similarly you decrease k every time x had a positive discontinuity. And you can only do that trick if the sampling rate is much higher than the speed the caliper is moved. If you sample too slow suddenly the caliper might move multiple whole periods between two samples and the code would loose track of the value of k.
- deskr 2y ago
  Thanks, appreciated! Yes, I took the Hz as meaning the rate at which I see an accurate measurement on my screen.

by deskr 2y ago

tonyarkles 2y ago

I'd be super curious to see how the accuracy changes with averaging out/low-pass filtering the measurements. Accuracy usually improves proportional to sqrt(N) when you take N samples so your higher precision desire might just be a bit of code to write. The other side of it though is that you're starting to get down into the "everything needs to be temperature controlled" region as you squeeze that precision number. FR-4 and copper have thermal expansion coefficients around 15-20ppm/C. If I'm doing this mental math correctly, a 5 deg temperature rise would make a 1m long piece of FR4 expand by 0.1mm, or a 10cm piece of FR4 expand by 0.01mm.

foobarian 2y ago
One time I wanted to demonstrate thermal expansion to my kid (1st grade or so) so I made some marks with a steel ruler and put it in the freezer. Imagine my surprise when we took it out and there was no perceptible difference :-D
- dekhn 2y ago
  did you measure the freezer after? It expanded :)
satiric 2y ago
With some microcontrollers you can do this "averaging out" just by changing ADC parameters, you don't even have to write the low-pass-filter code.
- auxym 2y ago
  That is specifically sigma-delta type ADCs: https://www.analog.com/en/resources/technical-articles/sigmadelta-adcs-tutorial.html https://www.analog.com/en/resources/technical-articles/sigma...
  - magicalhippo 2y ago
    No, several MCUs, like a lot of STM32s, support hardware oversampling with optional division. Here's[1] a document describing it for the L0 and L4 series, which like most of the STM32 series have SAR[2] ADCs. [1]: https://www.st.com/resource/en/application_note/an4629-adc-hardware-oversampling-for-microcontrollers-of-the-stm32-l0-and-l4-series-stmicroelectronics.pdf https://www.st.com/resource/en/application_note/an4629-adc-h... [2]: https://en.wikipedia.org/wiki/Successive-approximation_ADC https://en.wikipedia.org/wiki/Successive-approximation_ADC

namibj 2y ago

That at 500Hz is called "optical mouse on absolute positioning patterned surface". On that note: I'm looking for a mouse style camera sensor unit that can export full frame rate raw to a system where I can actually decode such an absolute positioning code. Anyone got something in the sub-100$ range?

bigiain 2y ago
The DYI Drones and Ardupilot comminities were using mouse sensors to do "optical flow sensing" for accurate low altitude position hold around 10 or so years back. https://ardupilot.org/copter/docs/common-mouse-based-optical-flow-sensor-adns3080.html https://ardupilot.org/copter/docs/common-mouse-based-optical... https://github.com/RCmags/ADNS3080_frame_capture https://github.com/RCmags/ADNS3080_frame_capture https://www.pixelelectric.com/sensors/distance-vision/adns-3080-optical-flow-sensor-module/?setCurrencyId=2 https://www.pixelelectric.com/sensors/distance-vision/adns-3...
Karliss 2y ago
I once looked into mouse sensors for similar purpose, but the raw frame export capabilities were disappointing. Looking at the datasheet of PMW3360 which claims to have up to 12000 FPS, but the timing sequence for raw frame dump suggests ~20ms if not 40ms to transfer the whole frame which would give only 50-25FPS. That doesn't necessarily mean the main spec is lying, it just isn't capable of dumping the frames over SPI as fast as it is producing and processing them internally. Frame dumping seems to be more of debug/test feature, so no big surprise it's not capable of fully utilizing the sensor and there is little motivation for sensor manufacturer to improve it. But there is also a chance that the manufacturer has put very conservative timing for it in the datasheet, since it's not the main functionality of sensor. If anyone has explored further how much the timing can be optimized, or maybe there are other mice sensors for with better frame dumping capabilities, I am curious to hear about it as well. With that said even if you remove all the unnecessary delays data transfer over SPI will likely be a limiting factor for very high framerates. Assuming 4MHz SPI and 32x32 8bpp image that would be at most 500FPS. Few hundred FPS would still be nice, and you might push it a bit higher with increasing frequency but it's probably not possible to push it much further than 1000FPS without MIPI or some kind of other parallel data transfer protocol. The ardupilot project in sibling comment seems to be using the mouse movement output directly instead of capturing the frame and then doing optical flow analysis on main CPU. The image dump is only being used to verify camera focus.

krisoft 2y ago

> It can be just a few Hz, I don't need 500 Hz. Tldr: understood that you think you don’t need 500 Hz, but there is a technological limitation why you need that frequency for these systems to work. The reason these measurements need to be so fast is because the measurement is not absolute but periodic. Meaning that when you measure something it can’t tell you how large it is in absolute terms just how much larger it is than the closest integer period. In mathematical notation you can measure x where the whole distance is k*period+x, where k is an unknown integer, and period is a design parameter. It can’t natively tell you what is the value of k. So to figure out the whole measurement you need to calculate k. And you can do that by keeping track of it. You know it is zero when they zero out the caliper, and you know that it just increased by one every time there was a falling discontinuity of x. (Meaning that every time x approaches the period and then suddenly drops to a low number it just crossed over one of these period boundaries.) Similarly you decrease k every time x had a positive discontinuity. And you can only do that trick if the sampling rate is much higher than the speed the caliper is moved. If you sample too slow suddenly the caliper might move multiple whole periods between two samples and the code would loose track of the value of k.

deskr 2y ago
Thanks, appreciated! Yes, I took the Hz as meaning the rate at which I see an accurate measurement on my screen.