NAU7802

Contributed by Daniel Byshkin.

The Adafruit NAU7802 is a high-resolution 24-bit ADC with an integrated load cell amplifier. It is designed for use with load cells and other sensors that require high precision and low noise measurements. The NAU7802 features a built-in programmable gain amplifier (PGA) that allows for easy calibration and adjustment of the sensor’s output.

The driver uses the uorb interface.

Application Programming Interface

#include <nuttx/sensors/nau7802.h>

The NAU7802 registration function allows the driver to be registered as a UORB driver. Registering this driver will cause the /dev/uorb/sensor_force<n> topic to appear, where n is the value of devno.

Registering the device in polling mode will create a kernel thread to poll the sensor

int err = nau7802_register(i2c_master, 0, 0x2a);
if(err < 0){
  printf("Failed to register NAU7802: %d\n", err);
}

The following are available commands for the NAU7802 driver:

SNIOC_RESET

Performs a reset of all registers on the NAU7802.

orb_ioctl(sensor, SNIOC_RESET);

SNIOC_SET_GAIN

This command sets the gain of the NAU7802. The possible values are dictated by the nau7802_gain_e enum. The default value is 128x.

orb_ioctl(sensor, SNIOC_SET_GAIN, NAU7802_GAIN_128);

SNIOC_SET_INTERVAL

This commands sets the polling interval of the NAU7802. The possible values are dictated by the nau7802_odr_e enum. The default value is 10HZ.

orb_ioctl(sensor, SNIOC_SET_INTERVAL, NAU7802_ODR_10HZ);

SNIOC_SET_LDO

This command sets the LDO voltage of the NAU7802. The possible values are dictated by the nau7802_ldo_e enum. The default value is 3.0V.

orb_ioctl(sensor, SNIOC_SET_LDO, NAU7802_LDO_3V0);

SNIOC_CALIBRATE

This commands performs one of the calibration procedures of the NAU7802. The possible calibration modes are:

• NAU7802_CALMOD_INTERNAL: Removes internal PGA gain and offset errors.

• NAU7802_CALMOD_OFFSET: Calibrates the zero point of the sensor.

• NAU7802_CALMOD_GAIN: Calibrates the max value of the sensor.

orb_ioctl(sensor, SNIOC_CALIBRATE, NAU7802_CALMOD_INTERNAL);

For the gain calibration mode the user must place a known weight on the sensor. Unfortunately the NAU7802 records it as the maximum value, thus if your loadcell supports up to 100kg you shall put a 100kg weight on.

A workaround would be to do a manual calibration by placing a smaller known weight and polling the sensor to get an average point, then using such point to offset the recorded values. An example is provided below.

#include "stdio.h"
#include <errno.h>
#include <fcntl.h>
#include <nuttx/sensors/nau7802.h>
#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <uORB/uORB.h>
#include <unistd.h>

int get_data(const struct orb_metadata *imu_meta, int imu, struct sensor_force *data) {
  int err = 0;
  bool update = false;
  err = orb_check(imu, &update);
  if (err < 0) {
      return err;
  }

  err = orb_copy(imu_meta, imu, data);
  if (err < 0) {
      return err;
  }
  return err;
}

int main(int argc, char **argv) {
  int err;
  int imu;
  char *name = "sensor_force0";

  const struct orb_metadata *imu_meta = orb_get_meta(name);
  if (imu_meta == NULL) {
      fprintf(stderr, "Failed to get metadata for %s\n", name);
      return EXIT_FAILURE;
  }

  imu = orb_subscribe(imu_meta);
  if (imu < 0) {
      fprintf(stderr, "Could not subsribe to %s: %d\n", name, errno);
      return EXIT_FAILURE;
  }

  struct sensor_force data;

  // flush 10 readings
  for (int i = 0; i < 10; i++) {
      err = get_data(imu_meta, imu, &data);
      if (err < 0) {
          printf("Error reading data\n");
      }
      usleep(100000);
  }

  long zero_point = 0;
  for (int i = 0; i < 10; i++) {
      err = get_data(imu_meta, imu, &data);
      if (err < 0) {
          printf("Error reading data\n");
      } else {
          zero_point += data.force / 10;
      }
      usleep(100000);
  }
  printf("Zero point: %ld\n", zero_point);

  printf("Place weigth on the sensor... you have 5 seconds from when you see this message\n");
  usleep(5000000);
  printf("Starting gain calibration\n");

  long weight_point = 0;
  for (int i = 0; i < 10; i++) {
      err = get_data(imu_meta, imu, &data);
      if (err < 0) {
          printf("Error reading data\n");
      } else {
          weight_point += data.force / 10;
      }
      usleep(100000);
  }
  printf("Weight value: %ld\n", weight_point);
  float known_weight_val = 15000; // 1.5kg

  while (true) {
      err = get_data(imu_meta, imu, &data);
      if (err < 0) {
          printf("Error reading data\n");
      } else {
          printf("Force: %.3f\n", known_weight_val * (data.force - zero_point) / (weight_point - zero_point));
      }
      usleep(50000);
  }

  orb_unsubscribe(imu);
  return EXIT_SUCCESS;
}

SNIOC_GET_CALIBVALUE:

This commands gets the gain calibration value when set by the SNIOC_CALIBRATE command.

uint32_t cal_value;
orb_ioctl(sensor, SNIOC_GET_CALIBVALUE, (unsigned long)&cal_value);

SNIOC_SET_CALIBVALUE:

This commands sets the gain calibration value, useful when you calibrated the sensor with the gain calibration mode once and want to reuse it later on.

uint32_t cal_value;
orb_ioctl(sensor, SNIOC_SET_CALIBVALUE, cal_value);