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Rob Tillaart 394b509528 0.4.4 MAX31855_RT

2023-11-12 10:16:51 +01:00

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MAX31855_RT

Arduino library for MAX31855 chip for K type thermocouple.

The library has experimental support for other types of thermocouples E, J, N, R, S, T.

Description

The MAX38155 is a chip to convert the reading of a K-type thermocouple to a temperature. The working of thermocouples (TC) is based upon Seebeck effect. Different TC's have a different Seebeck Coefficient (SC) expressed in µV/°C. See http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.html

For every type of TC there exist an MAX31855 variant, this library is primary developed for the K-type sensor. However it has experimental support for all other types of TC's. See details below.

Library tested with breakout board.

         +---------+
     Vin | o       |
     3V3 | o       |
     GND | o     O | Thermocouple
      D0 | o     O | Thermocouple
      CS | o       |
     CLK | o       |
         +---------+

Hardware SPI vs software SPI

Default pin connections. ESP32 can overrule with setGPIOpins().

HW SPI	UNO	ESP32 VSPI	ESP32 HSPI	Notes
CLOCKPIN	13	18	14
MISO	12	19	12
MOSI	11	23	13	not used...
SELECT	4	5	15	can be others too.

Performance read() function, timing in us. (ESP32 @240MHz)

mode	clock	timing UNO	timing ESP32	Notes
HW SPI	32000000	ni	~15	less reliable
HW SPI	16000000	~68	~16
HW SPI	4000000	~72	~23
HW SPI	1000000	~100	~51
HW SPI	500000	~128	~89
SW SPI	bit bang	~500	~17 (!)

Interface

#include "MAX31855.h"

Constructor

MAX31855() create object.
void begin(const uint8_t select) set select pin => hardware SPI
void begin(const uint8_t sclk, const uint8_t select, const uint8_t miso) set clock, select and miso pin => software SPI

Hardware SPI

To be used only if one needs a specific speed.

void setSPIspeed(uint32_t speed) set SPI transfer rate.
uint32_t getSPIspeed() returns SPI transfer rate.
void setSWSPIdelay(uint16_t del = 0) for tuning SW SPI signal quality. Del is the time in micros added per bit. Even numbers keep the duty cycle of the clock around 50%.
uint16_t getSWSPIdelay() get set value in micros.

ESP32 specific

void selectHSPI() must be called before begin()
void selectVSPI() must be called before begin()
bool usesHSPI()
bool usesVSPI()
void setGPIOpins(uint8_t clk, uint8_t miso, uint8_t mosi, uint8_t select) to overrule ESP32 default hardware pins

Reading

To make a temperature reading call read(). It returns the status of the read which is a value between 0..7 The function getStatus() returns the same status value.

Table: values returned from uint8_t read() and uint8_t getStatus()

value	Description	Action
0	OK
1	Thermocouple open circuit	check wiring
2	Thermocouple short to GND	check wiring
4	Thermocouple short to VCC	check wiring
7	Generic error
128	No read done yet	check wiring
129	No communication	check wiring

There are six functions to check the individual error conditions mentioned above. These make it easier to check them.

bool openCircuit()
bool shortToGND()
bool shortToVCC()
bool genericError()
bool noRead()
bool noCommunication()

After a uint8_t read() you can get the temperature with float getTemperature() and float getInternal() for the internal temperature of the chip / board itself. Normally these are (almost) equal.

Repeated calls to getTemperature() will give the same value until a new read(). The latter fetches a new value from the sensor. Note that if the read() fails the value of getTemperature() can become incorrect. So it is important to check the return value of read().

Offset

The library supports a fixed offset to calibrate the thermocouple. For this the functions float getOffset() and void setOffset(float offset) are available. This offset is "added" in the getTemperature() function.

Note the offset used is a float, so decimals can be used.

Delta analysis

As the tc object holds its last known temperature it is easy to determine the delta with the last known temperature, e.g. for trend analysis.

  float last = tc.getTemperature();
  int state  = tc.read();
  if (state == STATUS_OK)
  {
    float new  = tc.getTemperature();
    float delta = new - last;
    // process data
  }

Last time read

The tc object keeps track of the last time read() is called in the function uint32_t lastRead(). The time is tracked in millis(). This makes it easy to read the sensor at certain intervals.

if (millis() - tc.lastRead() >= interval)
{
  int state = tc.read();
  if (state == STATUS_OK)
  {
    float new = tc.getTemperature();
    // process read value.
  }
  else
  {
    // handle error
  }
}

GetRawData

The function uint32_t getRawData() allows you to get all the 32 bits raw data from the board, after the standard uint8_t tc.read() call.

Example code can be found in the examples folder.

  int state = thermocouple.read();              
  uint32_t value = thermocouple.getRawData();  // Read the raw Data value from the module

This allows one to compact the measurement e.g. for storage or sending over a network.

Pull Up Resistor

To have proper working of the MAX31855 board, you need to add a pull-up resistor (e.g. 4K7 - 1K depending on length of the wires) between the MISO pin (from constructor call) and the VCC (5Volt). This improves the signal quality and will allow you to detect if there is proper communication with the board. Without pull-up one might get random noise that could look like real data.

Note: the MISO pin can be different from each board, please refer to your board datasheet.

If the MAX31855 board is not connected tc.read() will return STATUS_NO_COMMUNICATION.

You can verify this by tc.getRawData() which will give 32 HIGH bits or 0xFFFFFFFF).

You can use a simple code to detect connection error board:

  uint8_t status = thermocouple.read();
  if (status == STATUS_NO_COMMUNICATION)
  {
    Serial.println("NO COMMUNICATION");
  }

  uint8_t status = thermocouple.read();
  if (thermocouple.getRawData() == 0xFFFFFFFF)
  {
    Serial.println("NO COMMUNICATION");
  }

Operation

See examples

breaking change 0.4.0

In issue #21 it became clear that the code in the constructor is not always executed correctly. Therefore this code + parameters is moved to the begin() function.

Experimental part (to be tested)

NOTE: The support for other thermocouples is experimental use at your own risk.

The MAX31855 is designed for K type sensors. It essentially measures a voltage difference and converts this voltage using the Seebeck Coefficient (SC) to the temperature. As the SC is linear in its nature it is possible to replace the K-type TC with one of the other types of TC.

Datasheet Table 1, page 8 SC = Seebeck Coefficient

Sensor type	SC in µV/°C	Temp Range in °C	Material
E_TC	76.373	-270 to +1000	Constantan Chromel
J_TC	57.953	-210 to +1200	Constantan Iron
K_TC	41.276	-270 to +1372	Alumel Chromel
N_TC	36.256	-270 to +1300	Nisil Nicrosil
R_TC	10.506	-50 to +1768	Platinum Platinum/Rhodium
S_TC	9.587	+50 to +1768	Platinum Platinum/Rhodium
T_TC	52.18	-270 to +400	Constantan Copper

The core formula to calculate the temperature is (Datasheet page 8)

Vout = (41.276µV/°C) x (Temp_R - Temp_internal)

As we know the internal temperature and the returned temperature from the sensor the library can calculate the Vout measured (as the chip assumes that a K-type thermocouple is connected. Having that Vout we can redo the math for the actual thermocouple type and calculate the real temperature.

The library has two functions setSeebeckCoefficient(float factor) and float getSeebeckCoefficient() to get/set the Seebeck Coefficient (== thermocouple) to be used. One can adjust the values to improve the accuracy of the temperature read.

The float getTemperature() has implemented this algorithm, however as long as one does not set the Seebeck Coefficient it will use the K_TC as default.

Future

Must

Should

investigate other TC's

Could

move code to .cpp

Wont

Support

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Thank you,

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