The accelerometer(LSM6DSL) on the BerryIMUv3 has built in double tap detection, which makes it very easy to detect double taps without the need for any fancy code.

When the LSM6DSL detects a double tap, it can fire an interrupt pin on the BerryIMUv3.  We will use a Raspberry Pi to monitor the interrupt pin and turn a LED off and on when a double-tap is detected.

Double-Tap event recognition has special registers which control tap recognition functionality, these are the tap threshold and the Shock, Quiet and Duration time windows

The Raspberry Pi will configure the BerryIMUv3 for double tap recognition. It will also monitor for double taps, which will be used to turn a LED on and off.

INT1 On the BerryIMUv3 will go high when a double tap is detected.

GPIO18 (physical pin 12) on the Raspberry Pi will be used to monitor INT1 , using an interrupt.

GPIO20 (physical pin 28) will be used to drive the LED.

The resister below is 330 Ohms

Here is the hock up diagrams

import signal
from LSM6DSL import *
import sys
import RPi.GPIO as GPIO 
import smbus
bus = smbus.SMBus(1)
LED_ON = 0                                                           #Used to track of the current state of the LED
INTERRUPT_PIN = 12                                                   #The interrupt pin which will be connected to the IMU
LED_PIN = 38                                                         #The pin which will be driving the LED
#Used to clean up when Ctrl-c is pressed
def signal_handler(sig, frame):
    GPIO.cleanup()
    sys.exit(0)
    
#Used to write to the IMU
def writeByte(device_address,register,value):
    bus.write_byte_data(device_address, register, value)


def LEDnotification(channel):
	global LED_ON
	if LED_ON:
		GPIO.output(LED_PIN,0)
		LED_ON = 0
	else:
		GPIO.output(LED_PIN,1)
		LED_ON = 1

writeByte(LSM6DSL_ADDRESS,LSM6DSL_CTRL1_XL,0b01100000)                 #ODR_XL = 416 Hz, FS_XL = +/- 2 g
writeByte(LSM6DSL_ADDRESS,LSM6DSL_TAP_CFG,0b10001110)                  #Enable interrupts and tap detection on X, Y, Z-axis
writeByte(LSM6DSL_ADDRESS,LSM6DSL_TAP_THS_6D,0b10001100)               #Set tap threshold
writeByte(LSM6DSL_ADDRESS,LSM6DSL_INT_DUR2,0b01111111)                 #Set Duration, Quiet and Shock time windows
writeByte(LSM6DSL_ADDRESS,LSM6DSL_WAKE_UP_THS,0b10000000)              #Double-tap enabled 
writeByte(LSM6DSL_ADDRESS,LSM6DSL_MD1_CFG,0b00001000)                  #Double-tap interrupt driven to INT1 pin
        
GPIO.setmode(GPIO.BOARD) # Use physical pin numbering
GPIO.setup(INTERRUPT_PIN, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
GPIO.setup(LED_PIN, GPIO.OUT)
GPIO.output(LED_PIN, 0)
GPIO.add_event_detect(INTERRUPT_PIN, GPIO.RISING, callback=LEDnotification, bouncetime=300)

while True:
    signal.signal(signal.SIGINT, signal_handler)
    signal.pause()

We will cover specific code which relates to double-tap recognition.

Line 37 , LSM6DSL_TAP_CFG is used to enable tap recognition on the X, Y, Z directions.  It is also used to enable the interrupt function for double-tap recognition.

Line 38 , LSM6DSL_TAP_THS_6D is used to set the tap thresholds. a lower value will result in softer taps being detected.

Line 39 , LSM6DS_INT_DUR2 is used to set the duration, quiet and shock time window.  A larger duration will result in a longer time between 1st and 2nd tap.

Line 40, LSM6DSL_WAKE_UP_THS. Set the left most bit to enable double tap recognition.

Line 41,  LSM6DSL_MD1_CFG is used to set which interrupt pin ont he BerryIMUv3 is used. In this instance, it is set to INT1.

BerryIMU also works great with Windows IoT Core on the Raspberry Pi.

Our Git repository contains the source files needed to get the BerryIMU up and running on Windows IoT.

The code will print out the following values to the screen;

• Raw values from the gyroscope, accelerometer and magnetometer.
• Accelerometer calculated angles.
• Gyro tracked angles.
• Fused X and Y angles.
• Heading.
• Tilt compensated heading.

Connecting BerryIMU to a Raspberry Pi

BrryIMU can connect via the jumper cables to the Raspberry Pi as shown below;

Or BerryIMU can sit right on top of the GPIO pins on a Raspberry Pi. The first 6 GPIOs are used as shown below.

Get the Code

Download the BerryIMU code for Windows IoT from our GIT repository. The files you need are under the WindowsIoT-BerryIMU folder.

You will need to download the entire git repository as GIT doesn’t allow downloading individual folders.

Once downloaded, double-click  the file WindowsIoT-BerryIMU.sln to open up the project in Visual Studio.

About the code

The project code outputs all of the needed values to the screen and a complementary filter is used to fuse the accelerometer and gyroscope angles.

We have a number of guides already documented on how to get the BerryIMU working with the Raspberry Pi.
https://ozzmaker.com/berryimu/
These are based on Raspbian, however the principals and math are the same for Windows Iot.

The final values which should be used are the fused X &Y angles and the tilt compensated heading.

The sensor on the BerryIMU is the LSM9DS0 and all the I2C registers for this sensor can be found in LSM9DS0.cs

The main code can be found in MainPage.xaml.cs

Complementary Filter

A complementary filter is used to fuse the angles. Is summary, the complementary filter trusts the gyroscope for short periods and trusts the accelerometer for longer periods;

CFangleX = AA * (CFangleX + (rate_gyr_x * DT / 1000)) + (1.0f - AA) * AccXangle;
CFangleY = AA * (CFangleY + (rate_gyr_y * DT / 1000)) + (1.0f - AA) * AccYangle;

Changing how much trust is given for each of the sensors can be changed by modify the complementary filter constant at the start of the code.

const float AA = 0.03f;     // Complementary filter constant

Loop Speed

The loop speed is important as we need to know how much time has past to calculate the rotational degrees per second on the gyroscope.
A time delta is set at the start of the code.

const int DT = 100;         //DT is the loop delta in milliseconds.

This is then used to specify a new timer method.

periodicTimer = new Timer(this.TimerCallback, null, 0,DT);

Here you can see where DT is used to keep track of the gyroscope angle. You can also see it in the above calculation for the complementary filter.

//Calculate the angles from the gyro
gyroXangle += rate_gyr_x * DT / 1000;
gyroYangle += rate_gyr_y * DT / 1000;
gyroZangle += rate_gyr_z * DT / 1000;

BerryIMU orientation

The calculations in the code are based on how the BerryIMU is orientated. If BerryIMU is upside down, then some of the angles need to be reversed. It is upside down when the skull logo is facing up(or to the sky).
If it is upside down, set the below value to true. Otherwise, set it to false.

bool IMU_upside_down = true;

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