DevHeads IoT Integration Server•6mo ago

working on a project that involves monitoring temperature using an LM35 sensor with an STM32F4

Hello everyone, I'm currently working on a project that involves monitoring temperature using an LM35 sensor with an STM32F4 microcontroller. I'm utilizing FreeRTOS for task scheduling and a CAN bus library for communication. However, I'm encountering issues with inaccurate temperature readings. The readings appear to be significantly different from expected values. Could you please assist me in identifying potential issues in my code and suggest corrections to ensure accurate temperature measurements? Here's the relevant snippet from my sensor task implementation:

void sensor_task(void *pvParameters) {
  while (1) {
    // Read temperature sensor data using ADC library function
    uint16_t raw_adc_data = read_temperature_sensor();

    // Convert raw ADC data to temperature value (assuming linear conversion for now)
    float temperature = raw_adc_data * 0.1;

    // Send the temperature data for processing and transmission
    send_temperature_data(temperature);

    // Delay task for a specific period
    vTaskDelay(pdMS_TO_TICKS(1000));
  }
}

void sensor_task(void *pvParameters) {
  while (1) {
    // Read temperature sensor data using ADC library function
    uint16_t raw_adc_data = read_temperature_sensor();

    // Convert raw ADC data to temperature value (assuming linear conversion for now)
    float temperature = raw_adc_data * 0.1;

    // Send the temperature data for processing and transmission
    send_temperature_data(temperature);

    // Delay task for a specific period
    vTaskDelay(pdMS_TO_TICKS(1000));
  }
}

@Middleware & OS

Solution:

That's right , @Dtynin You need to take into account the reference voltage of your ADC and its resolution. For example, if you are using a 12-bit ADC with a reference voltage of 3.3V, the conversion should look something like this:

float temperature = (raw_adc_data * 3.3 / 4096) * 100;

float temperature = (raw_adc_data * 3.3 / 4096) * 100;

This ensures that the raw ADC value is properly converted to the voltage output by the LM35 and then to the corresponding temperature....

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10 Replies

ZacckOsiemo•6mo ago

Your conversion seems to be almost too simplistic are you doing some preprocessing when reading?

Solution

Enthernet Code•6mo ago

float temperature = (raw_adc_data * 3.3 / 4096) * 100;

float temperature = (raw_adc_data * 3.3 / 4096) * 100;

This ensures that the raw ADC value is properly converted to the voltage output by the LM35 and then to the corresponding temperature.

Dtynin•6mo ago

Oh thanks, @Enthernet Code your detailed explanation on the conversion factor is very helpful. I will adjust the conversion to account for the reference voltage and ADC resolution.

UC GEE•6mo ago

In addition to what @Enthernet Code said, @Dtynin make sure your ADC is properly calibrated and configured. Secondly,curtail the noise in the power supply or grounding issues because it can equally affect the moneyreadings.Try as well to add a small capacitor (e.g., 0.1µF) close to the sensor can help stabilize the readings. I believe these points will be helpful to your project.

Dtynin•6mo ago

@UC GEE I'll check the calibration and configuration of my ADC and ensure proper grounding. Adding a capacitor for stabilization is a great idea. thanks

Edison_ngunjiri•6mo ago

Where is the function to read temp sensor?

Great~~~•6mo ago

and also I think that it can be helpful by considering relationship between real temperature and capture adc value. currently it was assumed to linear relationship but I think that via several live experiments, we need to interpolate formula between adc value and real temperature.

Great~~~•6mo ago

There are several alternatives for temperature sensor and I think that it is not problem due to sensor. when selecting any sensor, we have to consider its specification from datasheet and we need to preprocess and interpolate for sensor data. it is same case for LM35 too, I think.

Dtynin•6mo ago

// Function to read temperature sensor using ADC library function
uint16_t read_temperature_sensor() {
    ADC_ChannelConfTypeDef sConfig = {0};
    sConfig.Channel = ADC_CHANNEL_0;  // Assuming the sensor is connected to ADC channel 0
    sConfig.Rank = 1;
    sConfig.SamplingTime = ADC_SAMPLETIME_3CYCLES;
    
    if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK) {
        // Channel configuration error
        printf("ADC Channel Configuration Error\n");
        return 0;
    }

    if (HAL_ADC_Start(&hadc1) != HAL_OK) {
        // ADC start error
        printf("ADC Start Error\n");
        return 0;
    }

    if (HAL_ADC_PollForConversion(&hadc1, HAL_MAX_DELAY) != HAL_OK) {
        // ADC conversion error
        printf("ADC PollForConversion Error\n");
        return 0;
    }

    uint16_t raw_adc_data = HAL_ADC_GetValue(&hadc1);

    if (HAL_ADC_Stop(&hadc1) != HAL_OK) {
        // ADC stop error
        printf("ADC Stop Error\n");
    }

    return raw_adc_data;
}

// Function to read temperature sensor using ADC library function
uint16_t read_temperature_sensor() {
    ADC_ChannelConfTypeDef sConfig = {0};
    sConfig.Channel = ADC_CHANNEL_0;  // Assuming the sensor is connected to ADC channel 0
    sConfig.Rank = 1;
    sConfig.SamplingTime = ADC_SAMPLETIME_3CYCLES;
    
    if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK) {
        // Channel configuration error
        printf("ADC Channel Configuration Error\n");
        return 0;
    }

    if (HAL_ADC_Start(&hadc1) != HAL_OK) {
        // ADC start error
        printf("ADC Start Error\n");
        return 0;
    }

    if (HAL_ADC_PollForConversion(&hadc1, HAL_MAX_DELAY) != HAL_OK) {
        // ADC conversion error
        printf("ADC PollForConversion Error\n");
        return 0;
    }

    uint16_t raw_adc_data = HAL_ADC_GetValue(&hadc1);

    if (HAL_ADC_Stop(&hadc1) != HAL_OK) {
        // ADC stop error
        printf("ADC Stop Error\n");
    }

    return raw_adc_data;
}

Edison_ngunjiri•6mo ago

Try initialize the ADC peripheral once ,and then put all the code to start the adc until get value in read_temperature_sensor().

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working on a project that involves monitoring temperature using an LM35 sensor with an STM32F4