[Introduction to accelerometer] Click here for the series list

• Accelerometer introductory series / Arduino source code now available!

What is the self-test function?

MEMS accelerometers with mechanical parts generally have a self-test function as a self-diagnosis function. By using the self-test function, you can check the operation of the sensor element and signal processing circuit inside the IC.

When the ADXL345 self-test function is enabled, an electrostatic force acts on the sensor element. This electrostatic force moves the sensor element in the same way as if acceleration were input, resulting in changes in the x, y, and z axis outputs. By confirming whether this output change is within the "SELF-TEST" value specified in the electrical characteristics of the data sheet, it is possible to utilize it as a self-diagnostic function.

In general, accelerometers have data sheets that specify the upper limit of the allowable acceleration value (absolute rating), and applying an excessive shock may damage the IC. The self-test function does not require any intentional impact or force, and the self-diagnosis function is performed only by controlling electrical signals.

Please refer to the following application note for the sensor configuration and the operating principle of the IC.
・ ANJ-0005: What is an Accelerometer?

・Sonic Nirvana: Using MEMS Accelerometers as Acoustic Pickups in Musical Instruments

For reference, here are the waveforms when the ADXL335 self-test function is enabled. Since it is an analog output type accelerometer, changes in acceleration can be checked with an oscilloscope. When voltage is applied to the ST pin, the self-test function turns ON during that period, and you can see how the output changes.

Try using the self-test function

We will create a sample program for the "self-test function" using the acceleration sensor "ADXL345" and the hardware open platform "Arduino".

Things to prepare

Here is what I prepared to evaluate the accelerometer this time.

 

・PC with Arduino IDE installed (Download Arduino IDE from here)

・ Arduino Nano compatible board 

・ Accelerometer ADXL345 

・Others (USB cable (for Arduino and PC connection), breadboard, wires)

 

Combine the above parts to form a circuit as shown in the figure below. Power supply to Arduino Nano is supplied by USB bus power from PC. The ADXL345 supports SPI and I2C interfaces, but this time we will use the I2C interface. It also uses the INT1 pin of the interrupt pin.

Download the sample code verified this time

We provide the Arduino project file that we implemented this time. Please apply from here and give it a try.

About Accelerometer ADXL345

The ADXL345 used this time is a 3-axis digital output acceleration sensor. The main features are as follows.

・A standard accelerometer that is very easy to use with built-in ADC, operation function block, and FIFO
・Acceleration data adopts general I2C/SPI in digital serial method
・The 3-axis type sensor is a rectangular coordinate (X, Y, Z), and the acceleration acting on each axis can be obtained.
・The maximum detectable acceleration can be set in the range of 2g to 16g, and the sampling range is as wide as ~3.2kHz, so it can be applied to various applications such as impact, tilt, and motion detection.
・Flexible mode to reduce current consumption

For more information on the ADXL345, visit www.adxl345.com. data sheet Please refer to. Also, this accelerometer is very easy to use, so if you want to evaluate an accelerometer from now on, please try it on the evaluation board.

At the end

If you have any questions about the contents of this article, or if you have any problems with selecting or using accelerometers, please contact us from the following.