PIC based Pt100 RTD temp sensor circuit

M

MarkMc

Guest
Can anybody give me any hints ideas or circuits to allow me to read the
temperature from a Pt100 RTD probe in to a PIC microcontroller?

Regards,
Mark
 
MarkMc wrote:
Can anybody give me any hints ideas or circuits to allow me to read the
temperature from a Pt100 RTD probe in to a PIC microcontroller?

Regards,
Mark

The signal from an RTD is very small, so you need a low offset, low
Click to expand...
drift amplifier to bring the signal up to a range that is appropriate
for the PIC A/D converter.

If you are determined to roll your own amplifier, I suggest you visit
the various op amp manufacturers' sites and look for application noted
for platinum RTDs.

For example:
http://pdfserv.maxim-ic.com/en/an/AN3450.pdf
http://www.national.com/nationaledge/dec04/article.html
 
I had a read of the MAXIM application notes, which use the MAX197 IC
and a couple of op amps for amplification. This seems to be perfect,
but the only problem is that it costs Ł20 for a single 28-pin DIN
MAX197, which makes it a bit unviable for me. It is a one-off personal
project, but others on my brewing forum may well want to build units
themselves.

I think I understand what's required at a high level, but my
electronics isn't up to it at the low-level. I need a range of 0C to
100C and 0.5C accuracy/resolution for my application.

High level things I think I need (high-level block diagram)
4-wire PT100 RTD
"Something" to generate the excitation current - I think this needs to
be extremely accurate and invariable.
Something to amplify the small voltage across the RTD in to the range
of an ADC. Looks like NS ML4140A-2.500 is used for this in the links
above?

An ADC to convert the analogue signal in to a format for my PIC to use.
Are there not PIC's which have on-board ADC's that I can use for this
purpose or is this not a good idea?

I think I know how to convert from the digital voltage in to a
temperature - divide the known voltage range up in to chunks based on
the bit resolution of the ADC. Say - 0-5v ADC and 10-bit, I would then
5/((2^10)-1) to get the voltage increment per bit. Then map the
'voltage' reading back to resistance (known excitation current), and
use a look-up table to 'linearise' and then interpolate to give a
reasonable approximation of temperature.

One thing I'm not sure about is how to calibrate this process.

Another is that IIRC 0C is 100R and 100C is 138.5R (for Platinum PT100)
which @ 1mA gives a voltage reading range of 100mV-138.5mV. I'm sure I
can scale this up so that the 100C resistance maps somewhere near 5
volts, but the 0C will not be near 0v. so some of the 10-bit signal is
wasted. Could I not get better accuracy (perhaps it's not needed with
10-bits) by translating the 0C voltage/resistance somewhere near 0v and
then scale/amplify so that 100C is near 5v so I'm using a larger
portion of the bit range?

Regards,
Mark
 
I'll have to digest some of this to understand it fully, but one thing
which I'm not too sure how to do is the two point calibration.

IIRC the MAX197 app notes suggest calibrating with a highly accurate
100R and 300R resistors. Say I do this and note the voltage or binary
value from the ADC, what do I then do with these two values? I see how
I can apply a correction factor, but this doesn't take both calibration
points in to consideration.

Cheers,
Mark
 
MarkMc wrote:
(snip)
I think I understand what's required at a high level, but my
electronics isn't up to it at the low-level. I need a range of 0C to
100C and 0.5C accuracy/resolution for my application.

High level things I think I need (high-level block diagram)
4-wire PT100 RTD
"Something" to generate the excitation current - I think this needs to
be extremely accurate and invariable.
Something to amplify the small voltage across the RTD in to the range
of an ADC. Looks like NS ML4140A-2.500 is used for this in the links
above?
Click to expand...

Except for the transposition of LM, yes. It is a precision 2.5 volt
reference regulator. In combination with the opamp, it forces the
voltage drop across the grounded 2.5K resistor to be a constant 2.5
volts, so the RTD current is forced to be a constant 1 mA.

An ADC to convert the analogue signal in to a format for my PIC to use.
Are there not PIC's which have on-board ADC's that I can use for this
purpose or is this not a good idea?
Click to expand...
Most PICs have an internal 10 bit (1024 level) ADC. You just need to
amplify and shift the signal into the ADC measurement range. You may
also want to reference the ADC full scale voltage to a precision
reference, instead of using the 5 volt supply as the full scale value,
or else the accuracy of the PIC supply gets involved in the measurement.

I think I know how to convert from the digital voltage in to a
temperature - divide the known voltage range up in to chunks based on
the bit resolution of the ADC. Say - 0-5v ADC and 10-bit, I would then
5/((2^10)-1) to get the voltage increment per bit. Then map the
'voltage' reading back to resistance (known excitation current), and
use a look-up table to 'linearise' and then interpolate to give a
reasonable approximation of temperature.

One thing I'm not sure about is how to calibrate this process.
Click to expand...
You might use a distilled water ice bath and boiling point as a two
point calibration. Otherwise, you buy a high accuracy liquid
thermometer and use it in a stirred water or oil bath as a temperature
reference.

Another is that IIRC 0C is 100R and 100C is 138.5R (for Platinum PT100)
which @ 1mA gives a voltage reading range of 100mV-138.5mV. I'm sure I
can scale this up so that the 100C resistance maps somewhere near 5
volts, but the 0C will not be near 0v. so some of the 10-bit signal is
wasted. Could I not get better accuracy (perhaps it's not needed with
10-bits) by translating the 0C voltage/resistance somewhere near 0v and
then scale/amplify so that 100C is near 5v so I'm using a larger
portion of the bit range?
Click to expand...
You can add an arbitrary offset to the resistance measurement by
connecting R1 in figure 1 to an adjustable voltage, instead of to
ground. This voltage should be a low impedance source (not just a pot
hooked up between two voltages) like an opamp follower, in order to
not upset the subtracter's ability to measure the voltage across the
RTD. The subtracter circuit is A4 and the resistors around it.

You can use a pot between the 2.5 volt reference and ground to feed
the follower.

When you have zero degrees just above ground and 100 degrees just
below the ADC positive reference voltage (full scale voltage) you are
ready to calibrate and linearize the circuit with stored constants in
the program.

Regards,
Mark
Click to expand...
 
A design to do what you want is a big challange to one of your experience.
You will probably spend considerable money and a great deal of time and
still not achieve your goal

There are a multitude of RTD digital panel meters that do what you want
available in the UK

Just buy one - it will work and probably be cheaper than you can do it
yourself

Search internet for RTD temperature panel meter UK

Dan

--

Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
QuickScore@USSailing.net
www.QuickScoreRace.com


"MarkMc" <mmcnospam@yahoo.co.uk> wrote in message
news:1120650307.753396.12880@g43g2000cwa.googlegroups.com...
I had a read of the MAXIM application notes, which use the MAX197 IC
and a couple of op amps for amplification. This seems to be perfect,
but the only problem is that it costs Ł20 for a single 28-pin DIN
MAX197, which makes it a bit unviable for me. It is a one-off personal
project, but others on my brewing forum may well want to build units
themselves.

I think I understand what's required at a high level, but my
electronics isn't up to it at the low-level. I need a range of 0C to
100C and 0.5C accuracy/resolution for my application.

High level things I think I need (high-level block diagram)
4-wire PT100 RTD
"Something" to generate the excitation current - I think this needs to
be extremely accurate and invariable.
Something to amplify the small voltage across the RTD in to the range
of an ADC. Looks like NS ML4140A-2.500 is used for this in the links
above?

An ADC to convert the analogue signal in to a format for my PIC to use.
Are there not PIC's which have on-board ADC's that I can use for this
purpose or is this not a good idea?

I think I know how to convert from the digital voltage in to a
temperature - divide the known voltage range up in to chunks based on
the bit resolution of the ADC. Say - 0-5v ADC and 10-bit, I would then
5/((2^10)-1) to get the voltage increment per bit. Then map the
'voltage' reading back to resistance (known excitation current), and
use a look-up table to 'linearise' and then interpolate to give a
reasonable approximation of temperature.

One thing I'm not sure about is how to calibrate this process.

Another is that IIRC 0C is 100R and 100C is 138.5R (for Platinum PT100)
which @ 1mA gives a voltage reading range of 100mV-138.5mV. I'm sure I
can scale this up so that the 100C resistance maps somewhere near 5
volts, but the 0C will not be near 0v. so some of the 10-bit signal is
wasted. Could I not get better accuracy (perhaps it's not needed with
10-bits) by translating the 0C voltage/resistance somewhere near 0v and
then scale/amplify so that 100C is near 5v so I'm using a larger
portion of the bit range?

Regards,
Mark
 
MarkMc wrote:
I'll have to digest some of this to understand it fully, but one thing
which I'm not too sure how to do is the two point calibration.

IIRC the MAX197 app notes suggest calibrating with a highly accurate
100R and 300R resistors. Say I do this and note the voltage or binary
value from the ADC, what do I then do with these two values? I see how
I can apply a correction factor, but this doesn't take both calibration
points in to consideration.
Click to expand...
The two point calibration I was referring to is just interpolating 0
to 100 degrees, linearly between the two A/D values you get with an
ice bath and a boiling bath, using your RTD and amplifier. The
linearity errors over that small temperature range are not very big.
If you want to correct them, you need a table or polynomial fit to the
RTD curve, and fit that to your zero and 100 degree points.
 
Had a look, and they seem to be about Ł200. Not what I'd call cheap
for hobby use.

What would be so expensive to make?

I have the PIC microcontrollers and programming hardware + software
etc. The way I see it I just need to find a good OpAmp and ADC?

Half the fun is the journey with this hobby I think. Yep, right now, I
don't know too much, but I think I'm getting somewhere - doesn't seem
too scarey.

Regards
Mark
 
Ah, I see. The calibration I had in mind was to pull things in to line
with the look-up tables.

Cheers,
Mark
 
MarkMc wrote:
Ah, I see. The calibration I had in mind was to pull things in to line
with the look-up tables.
Click to expand...
A couple suggestions. Excite the RTD with a current proportional to
the PIC supply, scaled down to about 1 mA. That would involve
replacing the precision 2.5 volt reference with a 2 to 1 voltage
divider from the 5 volt supply. Now, since the excitation will be
proportional to the supply, if you use the supply as the positive
reference voltage for the A/D, the signal will also be proportional to
the A/D reference. This cancels any scaling errors for changes in the
supply and eliminates the need for a precision component.

Use the two point calibration method (which eliminates the need for .1%
or better standard resistors and compensates for tolerances in your
RTD) and apply this formula to the result.
temperature in degrees C = (counts-Czero)*100/C100
where Czero is the A/D result for ice bath, and C100 is A/D result with
boiling bath. If you do this at sea level, this linear approximation
for the RTD averages about .235 degree error over the range of 0 to 100
degrees C. If done at higher elevations, you can change the constant
100 factor to something lower that represents the local boiling
temperature. If you have precision resistors available, they can be
substituted for the RTD during calibration. Store the calibration
constants in EEPROM.
 
jpopelish@rica.net wrote:
(snip)
If you do this at sea level, this linear approximation
for the RTD averages about .235 degree error over the range of 0 to 100
degrees C.
Click to expand...
I found an error in my math. The average error in the linear
approximation over this range with a best fit is .097 degrees. If you
force the errors to zero at the end points (a sub optimal fit, overall)
the worst error occurs at about the mid point temperature and is about
-.4 degrees. If you fudge the endpoints to read .25 degree high, the
midpoint error is reduced to about -.15 degree.
 
Look at these.

http://ww1.microchip.com/downloads/en/AppNotes/00687b.pdf
http://ww1.microchip.com/downloads/en/AppNotes/00895a.pdf


"MarkMc" <mmcnospam@yahoo.co.uk> wrote in message
news:1120581097.596235.247300@f14g2000cwb.googlegroups.com...
Can anybody give me any hints ideas or circuits to allow me to read the
temperature from a Pt100 RTD probe in to a PIC microcontroller?

Regards,
Mark
Click to expand...
 
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