search

iliasam / Laser_tape_reverse_engineering

Alternative firmware for a cheap X-40 laser tape measure
https://habrahabr.ru/post/327642
MIT License
121 stars 46 forks source link

Question on source code #15

Closed epsi1on closed 2 months ago

epsi1on commented 2 months ago

I was looking through source code, in line 10 of file distance_calc.c it seems that there are three frequencies used:

#define WAVE_L1         59036 //wavelength for 162.5Mhz in mm * 32
#define WAVE_L2         50095 //wavelength for 191.5Mhz in mm * 32
#define WAVE_L3         49578 //wavelength for 193.5Mhz in mm * 32

which is 162.5, 191.5 and 193.5. Could you please give information about whether these are modulated frequencies? if yes, do you advice for me to edit the code to use multiplies of 2?

For example 193.5 and 193.5/2=96MHz and so on until it gets to say 755 KHZ (193.5/256) . then one would be able to tell the distance more precisely and possibly without brute force algorithm. Thanks for sharing great experience...

epsi1on commented 2 months ago

Assuming we know that the distance is less than 150m far, one would first try with 1Mhz modulation, with wavelength of 300m. then from phase diff, one will understand distance is about 70 meter. retry with wavelength of 50 and will turn out phase diff is about to 20.5m. and so on until it gets to the maximum frequency which result best resolution. this way the maximum range could extend to more that 37.5m (currently) and integer ambiguity could be resolved simpler i think (at least if i'm not missing anything) Thanks

iliasam commented 2 months ago

Sorry, I don't understand your questions well. Have you read this article: https://habr.com/ru/articles/327642/ ?

162.5 Mhz, 191.5 Mhz, 193.5 Mhz are modulation frequencies. They are set here: https://github.com/iliasam/Laser_tape_reverse_engineering/blob/master/Code/Firmware_dist_calculation_fast/src/pll_functions.c It is not possible to set frequency less that a certain limit - it is limited by hardware. I did some experiments earlier, 25Mhz gave too small signal.

Also, 40m limit is based on hardware sensitivity - original laser tape measure was measuring such distance slowly, because signal is very weak. I don't think that it is possible to make measurements better.

epsi1on commented 2 months ago

Thanks for information.

It is not possible to set frequency less that a certain limit - it is limited by hardware. I did some experiments earlier, 25Mhz gave too small signal.

Could you please give more information about what are the minimum and maximum possible modulate frequencies which hardware allow me to set? what you mean by 25MHz gave small signal?

Also, 40m limit is based on hardware sensitivity - original laser tape measure was measuring such distance slowly, because signal is very weak. I don't think that it is possible to make measurements better.

I was thinking to use a corner prism to reflect back the laser beam, this way the returned laser will not get weak so hopefully i could be able to measure larger distances. say 200m with a quality hand made (!) corner reflector.

Thanks again

iliasam commented 2 months ago

"what are the minimum and maximum possible modulate frequencies " I don't remember them well, but I know that they are dependent on hardware type: https://github.com/iliasam/Laser_tape_reverse_engineering/wiki/Known-modules Maximum limit is Si5351 limit - near 250MHz, if I not mistaken.

"what you mean by 25MHz gave small signal?" Signal amplitude is 2-5 times smaller when 25MHz modulation is used, comparing to "traditional" 150-200MHz modulation frequency.

If you are planning to use corner prism, you can try to make L3 frequency 193.5MHz more closer to L2, 192.0Mhz for example. Yo will need to change "WAVE_L3" value and "pll_set_frequency_3()" implementation.

Notice that it can be problematic to find laser tape measure hardware supported by this project now.

epsi1on commented 2 months ago

Notice that it can be problematic to find laser tape measure hardware supported by this project now.

Actually i'm trying to find one.
I was thinking to use frequencies like 1MHZ :)

epsi1on commented 2 months ago

The modulation frequency is ~193MHZ, but how stm32's ADC with relatively low sample rate (like few Msps, if at all) is used to find phase diff such relative high frequency signal? the schematics it shows that a 5khz signal is passed to micro controller.

Could you please briefly describe it for a newby electronic guy like me? any links would be highly appreciated... Thanks

epsi1on commented 2 months ago

let me read the article carefully, then will ask question if any. Thanks

iliasam commented 2 months ago

It is all described here - https://habr.com/ru/articles/327642/ Use goggle translate if you need it. See this image: https://habrastorage.org/r/w1560/getpro/habr/post_images/b38/81b/cc5/b3881bcc5a8babc7e9d23821da51061e.png And read this: https://en.wikipedia.org/wiki/Heterodyne

epsi1on commented 2 months ago

It is all described here - https://habr.com/ru/articles/327642/ Use goggle translate if you need it. See this image: https://habrastorage.org/r/w1560/getpro/habr/post_images/b38/81b/cc5/b3881bcc5a8babc7e9d23821da51061e.png And read this: https://en.wikipedia.org/wiki/Heterodyne

I got all answers by reading the article and your post. the key for conversion from high frequency to low frequency was Heterodyne. Thanks you for sharing knowledge...