LSM6DSO: Error in Samplingrate and Data lost between the reading

[beeezay]

Hello,
I am a Mechanical Engineering Student writing Thesis on Vibration Analysis of a Robot in a company.
I ordered 3 LSM6DSO to measure the vibration of an Cartesian Robot. The sensor is read by the ESP32 and send it on my computer through Wifi. I first tried with I2C communication with the Code below:

#include <Wire.h>
#include <LSM6.h>
#include <WiFi.h>

const char* ssid = " ";
const char* password = "";

WiFiServer server(8686);

IPAddress local_IP(10, 45, 10, 4);
IPAddress gateway(0, 0, 0, 0);
IPAddress subnet(255, 255, 255, 0);
IPAddress primaryDNS(8, 8, 8, 8);
IPAddress secondaryDNS(8, 8, 4, 4);

unsigned long PreviousTime_acc = 0;

unsigned long AccDelay = 30;

LSM6 imu;

// Calibration values for the accelerometer
const float x_offset = 0.00;
const float y_offset = 0.00;
const float z_offset = 0.00;

// Conversion factor from raw sensor values to g units
const float conversion_factor = 0.061 / 1000.0;  //For +-2G

//const float conversion_factor = 0.122 / 1000.0;                       //For +- 4G : Roboter_acc = 2 m/s/s i.e großer als 2g so 4G ist besser
// const float conversion_factor = 0.488 / 1000.0;                      // For +-6G

void setup() {
  Serial.begin(9600);

  if (!WiFi.config(local_IP, gateway, subnet, primaryDNS, secondaryDNS)) {
    Serial.println("STA Failed to configure");
  }

  Serial.print("Connecting to ");
  Serial.println(ssid);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }

  Serial.println("");
  Serial.println("WiFi connected.");
  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());

  server.begin();

  Wire.begin();

  if (!imu.init()) {
    Serial.println("Failed to detect and initialize IMU!");
    while (1)
      ;
  }
  imu.enableDefault();
}

void loop() {
  WiFiClient client = server.available();
  if (client) {
    Serial.println("New client connected!"); 
    while (client.connected()) {
      imu.read();
      if ((millis() - PreviousTime_acc) >= AccDelay) {


        float x = (imu.a.x * conversion_factor) - x_offset;
        float y = (imu.a.y * conversion_factor) - y_offset;
        float z = (imu.a.z * conversion_factor) - z_offset;

        String report = String(millis()) + "," + String(x, 2) + "," + String(y, 2) + "," + String(z, 2) + ";\n";
        PreviousTime_acc = millis();

        Serial.print(report);
        client.print(report);
      }
    }
    Serial.println("Client disconnected!");
    client.stop();
  }
}

The Problem with I2C communication i am facing is that the data i m getting is not in every 30ms rather in 31ms sometimes. The data as Example looks like this: (millis(Timestamp), x,z, y)
8439 -0.00,-2.00,-0.02;
8469 -0.00,-1.98,-0.00; dt: 30 ms
8500 -0.00,-1.99,-0.01; dt: 31 ms
Similarly using SPI, it has better results but when i try to use in 100 hz or to say trigger read data in less than 30 ms, the data is lost in between. (Reading in 10ms)
232851 0.01,0.02,-0.98;
232861 0.01,0.02,-0.98; dt= 10ms
232985 0.03,0.02,-0.98; dt= 125ms
232995 0.03,0.02,-0.98; st = 10 ms
I dont know if the problem is with the type of communication i am using or i should consider other thing. The Datasheet says that it can read upto 6666hz. Thank you.

[kevin]

Hello,

What is your goal in improving the regularity of your readings? Do you need a certain amount of precision for your application or are you mainly just wondering why it isn’t better?

In general, some amount of jitter in the timing should be expected, but there are several things you could do to improve your code. Probably the biggest issue is that you are calling imu.read() in the top level of your while loop; at the default I2C frequency, it takes nearly 2 ms to get a set of readings from the LSM6, so reading it every time through that loop means that you are checking the elapsed time too infrequently. Since you’re not doing anything with most of those readings anyway, a better solution would be to only read the IMU if the time check indicates it’s time to send another report (inside the if block).

Another minor optimization you could make is to call millis() just once every loop, which could help minimize timing drift if your loop takes a significant amount of time to run. So with both of those changes, your while loop could look like this:

    while (client.connected()) {
      unsigned long time = millis();
      if ((time - PreviousTime_acc) >= AccDelay) {
        imu.read();

        float x = (imu.a.x * conversion_factor) - x_offset;
        float y = (imu.a.y * conversion_factor) - y_offset;
        float z = (imu.a.z * conversion_factor) - z_offset;

        String report = String(time) + "," + String(x, 2) + "," + String(y, 2) + "," + String(z, 2) + ";\n";
        PreviousTime_acc = time;

        Serial.print(report);
        client.print(report);
      }
    }

I am not so sure why your SPI code is sometimes giving you a dt of much greater than 10 ms, but part of the reason could be that the ESP32 is handling other tasks in the background that it periodically needs to service (like wireless communication). Your Serial baud rate could also be a bottleneck if you’re trying to output data at 100 Hz; does it get better if you change 9600 to 115200?

Kevin

[beeezay]

Hello,
Thank you so much for reaching out my post and helping me. The propose of getting precision in Sampling rate is to do the Vibration Analysis of the cartesian Robot. I am doing my Bachelor Thesis in this. I am from mechanical Background and trying to develop a System to determine the Resonance and Maybe try to develop Feedback system. I am planning to do Fast Fourier Analysis. The Problem i am getting is also the noise. But i am trying to solve it.
The solution you provided helped me like a magic. It worked well now for both I2C and SPI communication. However for SPI communication, i used Adafruit 3rd Party library but it worked well. If you have any post, Blogs, or any things,that would help my Research, i would be really grateful.
Thank you and have a great time.
Best Regards,
Bijay Basnet