PT100 with amp board  

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guy.k2
(@guy-k2)
Noble Member

Yes, the additional resolution would be nice.

Posted : 28/08/2019 3:59 pm
chocki
(@chocki)
Honorable Member

Hopefully the parts should arrive soon, then I can play with it, I'll hook it up to my Arduino MEGA first to see what ADC values I get, then mod the amp and see what I get again, then test with a K type thermocouple hooked up with the PT100 sensor to compare values and adjust the thermistor (Thermocouple) table for the PRUSA.

The rest of my re-build is going bit by bit, new thicker cables from new PSU to reverse mounted Einsy, also adjusted the new PSU to 26 volts output, I'm going with the PIN10 P3 connector analog in for the PT100 amp.

Designing new cover plate with FAN casing for the LRS-350 PSU with a customisable finger guard to suit fitting an Arctic F9 Silent 92mm fan to the PSU.

The idea is so that the fan housing can be printed separately to the rest of the cover, this way anyone can make their own design to suit their taste, and maybe a skin for the rest of the PSU so that the colour matches with whatever else you want, again customisable designs.

And I'm still learning about the firmware, I may have to break it down into chunks to load onto the MEGA to fully learn what different parts do, but this is a long term project, even crossed my mind that this may be a good way to tune the trinamics, just buy a trinamics module, run barebones software on the MEGA, plug it into one of the steppers on the printer and drive it, should be easier to connect a scope and no danger of frying your EINSY.

 

Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet....
Posted : 29/08/2019 12:20 pm
Tim
(@tim-m30)
Illustrious Member

should be easier to connect a scope and no danger of frying your EINSY.

?? You should be able to ground at the PS common and not do any damage. At least my first pass at looking at the EINSY schematics show it should be safe - all of the GND points float, so should just drop to earth when attaching the scope. Or are you wanting to look at the stepper drive differentially?

 

This post was modified 5 months ago by Tim
It is always wise to get more than one opinion......
Posted : 29/08/2019 4:23 pm
chocki
(@chocki)
Honorable Member

Thanks Tim.

The hot end parts have not arrived yet, so nothing happening yet ☹️ 

 

Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet....
Posted : 01/09/2019 12:05 am
chocki
(@chocki)
Honorable Member

My parts arrived, and I have glued a 10k multiturn potentiometer to the board ready to solder, so that I can adjust the gain from 10 to 16. Potentiometer pins will be soldered directly to the ends of R5.

Currently there is a glass block on it pressing down until the clear gorilla glue dries, and tomorrow I will solder the ends.

Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet....
Posted : 10/09/2019 12:11 pm
chocki
(@chocki)
Honorable Member

All soldered and set to 3.300K Ohms. I had to buy a new multimeter when I realised my existing 30+ years old Fluke one was no longer accurate, tried new battery, but no good, guess I can't complain really.

Got a Beha Amprobe37XR-A so should last a good number of years, pointless getting a cheap Chinese meter, it would have been worse than my faulty 30+ years old meter.  🙂

Just creating a little snap shut case for the board and testing on an Arduino MEGA can start.

Normal people believe that if it ain’t broke, don’t fix it. Engineers believe that if it ain’t broke, it doesn’t have enough features yet....
Posted : 12/09/2019 2:28 pm
metacollin
(@metacollin)
Eminent Member
Posted by: @chocki

True, but the resolution is poor. The ATMEGA 2560 only has a 10 bit ADC input so that's 1024 values between 0v and 5v.

This isn't strictly true.  The ATMEGA 2560 has a SAR (successive approximation) ADC, which thanks to the built in 0.5LSB of quantization noise simply by bring a SAR ADC, plus thermal noise gives it enough noise for oversampling to be very effective.  The Prusa firmware uses 16x oversampling, which means it takes 16 samples each time the temperature is measured.  This increases the dynamic range by 16 fold, or 4 bits.  Unfortunately, this also increases the noise amplitude by the square root of the samples, or 4 fold.  All in all, you need 4^n samples, where n is the number of additional bits of resolution you want.  

This isn't free of course, it causes measurement time to increase exponentially.  These 2 extra bits require 16 samples per measurement, and if we wanted just one more bit, it would take 64 samples.  

 

As long as the signal being measured changes slowly enough relative to the total oversampling time, it works very well and does actually get you extra effective bits of resolution that are actually accurate and not just more bits of noise.  

 

So for all intents and purposes, the Prusa firmware is squeezing 12 bits out of that 10 bit ADC.  The real number of values is roughly 4096, and remember, that is AFTER factoring in the increased noise from oversampling as well.  It really is 4096, not kinda sorta 4096.  

Source: http://ww1.microchip.com/downloads/en/Appnotes/doc8003.pdf

 

So in reality, even with a voltage swing of only 1.58V, you still have 1295 possible values.  This translates into a resolution slightly better than 1/3rd of a degree. 

This is more than enough.   In fact, whether you use the PT100 + amplifier board and the narrower voltage swing or the wider swing of the thermistor, in both cases the variation on VCC (which is used as the reference) will completely dominate any temperature measurement errors, NOT the measurement resolution.  

 

In other words, there is no point to increasing the voltage swing or removing the 4K7 resistor.  You won't get better measurement accuracy because the voltage swing is not the dominant source of error and is not the limiting factor.  

Posted : 07/12/2019 2:02 pm
metacollin
(@metacollin)
Eminent Member

Of course, if you were to use a dedicated external reference instead of VCC... that's a different story 🙂

Posted : 07/12/2019 2:14 pm
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