20948 9DoF using SPI - need help getting acc y etc values

Sensors IMU & Motion
1

I have this Qwiic breakout board and would like to get scaled accelerometer x, y, z values to perform some basic math on. The board is connected to a UNO by SPI. I am new to arduino so am struggling to figure this out from the examples. How do I get scaled acc y? Is there a variable list? Not looking for pages of code, just the basics. Thanks in advance for any help.

2

If you have the raw sensor values, then while held still, a strictly vertical accelerometer axis reports +/- 1 g, regardless of the range selected. Use the reported value as the scale factor to scale to g units.

An additional multiplicative factor of 9.8 scales the data to m/s^2.

3

Thank you.

I was able to muddle through studying the KEYWORDS.TXT and the below object etc:

myICM.getAGMT();

myICM.accX();

myICM.gyrX();

etc.

My next question regards calibration. I am getting accX values of around 50milli G’s sitting still. One of the examples used a SCALED variable but I could not figure out how that worked. How do you calibrate this sensor?

Thanks in advance

John

4

An excellent general tutorial and overview of accelerometer and magnetometer calibration can be found at https://thecavepearlproject.org/2015/05 … r-arduino/

For the accelerometer, the simple axial min/max approach works well:

https://learn.adafruit.com/adxl345-digi … rogramming

https://chionophilous.wordpress.com/201 … e-methods/

5

Thank you. That second article was an excellent read.

I am getting accZ values between 995 and 1035. Should I be calibrating this to exactly 1000?

John

6

It is reasonable to use a scale factor that leads to a value of 1000 milli-g, when averaging many individual measurements.

7

Ok, new question. I want to use the RTC to timestamp the file I am writing to the SD card.

I cannot get any of the Example RTC sketches to work. They all report “Couldn’t find RTC”. As I understand it the RTC time is set once the GPS gets its first fix [and the battery is present]. That happened several times already. I don’t want the “live” GPS time [in case I don’t have a fix] but rather the RTC time.

Another problem is I cannot find any documentation one what clock chip is on this “Ultimate GPS Data Logger” shield. Therefore I have to blindly try every example.

Thanks

John

8

I’m not seeing any RTC chip on adafruits shield, you’re going to have to either add your own external RTC or just pull time from the GPS. (Assuming the GPS has a clear view of the sky or enough reception to return current time. )