autocal.py

##
## This CircuitPython code auto-calibrates the reference resistors and
## RTD Harness for the attached SenseTemp or SenseTemp TEC PCB.
##
## Before running auto_calibrate() or auto_calibrate_loop(),
## YOU MUST have all four RTD elements at the same temperature.
## The recommended way of doing this is to press all four onto
## a large body of metal with compliant material. 
##
## If pressing them with your fingers, that complaint material 
## must have enough thermal resistance to insulate the sensor
## elemets from the heat generated by your fingers.
##
## SenseTemp : https://www.crowdsupply.com/capable-robot-components/sensetemp
##

## CircuitPython Imports
import digitalio
import analogio
import pulseio
import busio

import board

## Local Imports
import adafruit_max31865 as max31865

def mean(data):
    return sum(data)/float(len(data))

def _ss(data):
    c = mean(data)
    ss = sum((x-c)**2 for x in data)
    return ss

def stddev(data, ddof=0):
    ss = _ss(data)
    pvar = ss/(len(data)-ddof)
    return pvar**0.5

RTD_NOMINAL   = 1000.0  ## Resistance of RTD at 0C
RTD_REFERENCE = 4300.0  ## Value of reference resistor on PCB

import adafruit_onewire.bus as onewire_bus

bus = onewire_bus.OneWireBus(board.D12)
spi = busio.SPI(board.SCK, MOSI=board.MOSI, MISO=board.MISO)

## Create SPI Chip Select pins
cs1  = digitalio.DigitalInOut(board.D10)
cs2  = digitalio.DigitalInOut(board.D9)
cs3  = digitalio.DigitalInOut(board.D6)
cs4  = digitalio.DigitalInOut(board.D5)
css  = [cs1, cs2, cs3, cs4]

sensors = []

## Create array for the RTD sensors on the SenseTemp Wing
for idx, cs in enumerate(css):
    cs.switch_to_output(digitalio.DriveMode.PUSH_PULL)

    sensors.append(
        max31865.MAX31865(
            spi, cs,
            wires        = 4,
            rtd_nominal  = RTD_NOMINAL,
            ref_resistor = RTD_REFERENCE
        )
    )

def collect_samples(count):
    idx = 0
    readings = [[], [], [], []]

    while idx < count:
        idx += 1

        for ch, sensor in enumerate(sensors):
            readings[ch].append(sensor.temperature)

    return readings

def auto_calibrate(count=10, reset=True):
    if reset:
        for sensor in sensors:
            sensor.ref_resistor = RTD_REFERENCE

    presample = collect_samples(count)
    readings = [mean(channel) for channel in presample]

    print("Means         : %s" % " ".join(["%.3f" % f for f in readings]))

    overall_mean  = mean(readings)
    overall_limit = stddev(readings)

    print("Overall Range : %.3f" % (max(readings)-min(readings)))
    print("Overall Mean  : %.3f" % overall_mean)
    print("Overall STD   : %.3f" % overall_limit)

    for ch, reading in enumerate(readings):
        if reading > overall_mean + overall_limit or reading < overall_mean - overall_limit:
            
            error = reading - overall_mean
            idx   = 0

            print("CH%d is outlier at %.3f" % (ch+1, reading))
            print("  %d %.3f %.3f %.3f" % (idx, sensors[ch].ref_resistor, reading, error))

            while abs(error) > overall_limit*0.25:
                idx += 1

                if idx > 15:
                    break

                step = error * -2.0
                sensors[ch].ref_resistor += step

                reading = sensors[ch].temperature
                error = reading - overall_mean

                print("  %d %.3f %.3f %.3f" % (idx, sensors[ch].ref_resistor, reading, error))

    postsample = collect_samples(count)
    readings = [mean(channel) for channel in postsample]

    print("Means         : %s" % " ".join(["%.3f" % f for f in readings]))
    print("Resistors     : %s" % " ".join(["%.1f" % s.ref_resistor for s in sensors]))

    overall_mean  = mean(readings)
    overall_limit = stddev(readings)

    print("Overall Range : %.3f" % (max(readings)-min(readings)))
    print("Overall Mean  : %.3f" % overall_mean)
    print("Overall STD   : %.3f" % overall_limit)

    return presample, postsample

def auto_calibrate_loop(times=4, sample_count=20):
    idx = 0
    reset = True

    while idx < times:
        if idx > 0:
            reset = False

        print()
        print("Calibration Cycle %d" % idx)
        auto_calibrate(count=sample_count, reset=reset)
        idx += 1

    return [s.ref_resistor for s in sensors]