Since I had disassembled the Wine-O-Meter I’d made for a friend’s bachelor party I needed to come up with something else for my own, I wanted to do an updated strongman competition. I decided to put together a wireless accelerometer to hopefully measure the speed and impact of various activities such as swinging a baseball bat, a sledgehammer, a hatchet, a tennis racket — get the idea? Sort of like the measurement tools used on shows like MythBusters or Deadliest Warrior. Along the lines of the Wine-O-Meter I dubbed the project the Stab-O-Meter as measuring arm movements reminded me of one of my favourite Futurama characters, Roberto.

My plan was to use an Arduino to read an accelerometer and use a pair of XBees to wireless relay the information to a laptop. The laptop would be running a Processing sketch to handle the high score display, reset and current readings. It took a little bit to find the right Arudino code to read the LIS331 Triple Axis Accelerometer I’d selected but it worked well once I found it. I don’t know a whole heck of a lot about accelerometers, but this one measures g-forces on three axis, x, y and z. After some trial and error I decided to add all positive g-force readings together and then add all negative g-force readings together. If the positive total was higher I used that as the current amalgamated reading otherwise I used the absolute sum of the negative values. Comment if you’re aware of a better way to translate x, y, z g-forces into a single number representing the speed of the motion (see Hank’s comment below).

Hank Cowdog

A neg X Acc means acc along the negative X axis. The magnitude of the acc is the important measurement, so a better approach would be to sum the squares of each X,Y,Z component and then take the square root (as per the Pythagorean Theorem). This computes the magnitude of the Acc regardless of the direction (or orientation of the accelerometer chips).

result = sqrt(xAcc*xAcc + yAcc*yAcc + zAcc*zAcc);

The Arduino sent the single number amalgamated reading in realtime (or as close as possible) via it’s serial connection to a XBee which in turn wirelessly relayed the serial data to a laptop running a processing sketch to read and deal with the data. The Processing sketch displayed a realtime reading bar on the right, the highest reading yet recorded in large numbers in the center and a RESET button to clear the current highest reading. With this system each contestant could reset the high score using the RESET button or the spacebar and the proceed to swing a bat or stab a tree or whatnot to find they’re personal best, which was then ranked against other’s scores on a white board.

This part worked great, however in impact scenarios (actually hitting something) it was too easy to max out the sensor, which has a max of 24g, so we restricted our games to non-impact swings. I had added hand wrap to the sensor case in order to secure it to the implement of choice, however I quickly realized that it also needed a non-slip surface for grip, I epoxied some rubber salvaged from a guitar effect pedal. Even with the hand wrap and the rubber footing the first full-force swing with a baseball bat sent the sensor soaring into a neighbouring house — duct tape provided the necessary upgrade in grip, but downgrade in polish.

The video below is, aside from my Roberto impression, an early test using a preliminary Processing sketch and no cases for the components. When I get a chance I’ll record a video of the finished setup, perhaps as I demolish my garage this weekend. Yes, it’s an odd video, but that’s what YouTube is for, right?

Parts

• Arduino Uno, ~$25
• Triple Axis Accelerometer ±6 / 12 / 24g – LIS331, $28
• 2x XBee Transceiver Module, $25
• XBee Explorer USB, $25
• XBee Breakout Board, $8.75
• 2x Small project box
• Power switch for transmitting XBee
• 9v Battery and Connector
• Rubber/Grip Tape
• Various foam bits
• Everlast Hand Wraps

Arduino Sketch

// 3-axis Accelerometer
// Sparkfun Electronics Triple Axis Accelerometer Breakout - LIS331
// Arduino UNO

/* Wiring:
    UNO LIS331

    3.3V VCC
    GND GND
    10 CS
    11 SDA/SDI
    12 SA0/SDO
    13 SCL/SPC
    */

#include <SPI.h>
#include <stdlib.h>
#include <stdio.h>

#define SS 10 // Serial Select -> CS on LIS331
#define MOSI 11 // MasterOutSlaveIn -> SDI
#define MISO 12 // MasterInSlaveOut -> SDO
#define SCK 13 // Serial Clock -> SPC on LIS331

#define SCALE 0.0007324; // approximate scale factor for full range (+/-24g)
// scale factor: +/-24g = 48G range. 2^16 bits. 48/65536 = 0.0007324

// global acceleration values
double xAcc, yAcc, zAcc;

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

  // Configure SPI
  SPI_SETUP();

  // Configure accelerometer
  Accelerometer_Setup();
}

void loop()
{
  readVal(); // get acc values and put into global variables

  int pos = 0;
  int neg = 0;

  if(xAcc > 0)
  {
    pos = pos + xAcc;
  }
  else
  {
    neg = neg + abs(xAcc);
  }

  if(yAcc > 0)
  {
    pos = pos + yAcc;
  }
  else
  {
    neg = neg + abs(yAcc);
  }

  if(zAcc > 0)
  {
    pos = pos + zAcc;
  }
  else
  {
    neg = neg + abs(zAcc);
  }

  int result = neg;

  if(pos > neg)
    result = pos;

  Serial.println(result,1);

   /*
    Serial.print(xAcc, 1);
    Serial.print(",");
    Serial.print(yAcc, 1);
    Serial.print(",");
    Serial.println(zAcc, 1);
  */

  delay(10);
}

// Read the accelerometer data and put values into global variables
void readVal()
{
  byte xAddressByteL = 0x28; // Low Byte of X value (the first data register)
  byte readBit = B10000000; // bit 0 (MSB) HIGH means read register
  byte incrementBit = B01000000; // bit 1 HIGH means keep incrementing registers
  // this allows us to keep reading the data registers by pushing an empty byte
  byte dataByte = xAddressByteL | readBit | incrementBit;
  byte b0 = 0x0; // an empty byte, to increment to subsequent registers

  digitalWrite(SS, LOW); // SS must be LOW to communicate
  delay(1);
  SPI.transfer(dataByte); // request a read, starting at X low byte
  byte xL = SPI.transfer(b0); // get the low byte of X data
  byte xH = SPI.transfer(b0); // get the high byte of X data
  byte yL = SPI.transfer(b0); // get the low byte of Y data
  byte yH = SPI.transfer(b0); // get the high byte of Y data
  byte zL = SPI.transfer(b0); // get the low byte of Z data
  byte zH = SPI.transfer(b0); // get the high byte of Z data
  delay(1);
  digitalWrite(SS, HIGH);

  // shift the high byte left 8 bits and merge the high and low
  int xVal = (xL | (xH <<8));
  int yVal = (yL | (yH <<8));
  int zVal = (zL | (zH <<8));

  // scale the values into G's
  xAcc = xVal * SCALE;
  yAcc = yVal * SCALE;
  zAcc = zVal * SCALE;
}

void SPI_SETUP()
{
  pinMode(SS, OUTPUT);

  // wake up the SPI bus
  SPI.begin();

  // This device reads MSB first:
  SPI.setBitOrder(MSBFIRST);

  /*
  SPI.setDataMode()
  Mode    Clock Polarity (CPOL) Clock Phase (CPHA)
  SPI_MODE0    0    0
  SPI_MODE1    0    1
  SPI_MODE2    1    0
  SPI_MODE3    1    1
  */
  SPI.setDataMode(SPI_MODE0);

  /*
  SPI.setClockDivider()
  sets SPI clock to a fraction of the system clock
  Arduino UNO system clock = 16 MHz
  Mode SPI Clock
  SPI_CLOCK_DIV2 8 MHz
  SPI_CLOCK_DIV4 4 MHz
  SPI_CLOCK_DIV8 2 MHz
  SPI_CLOCK_DIV16 1 MHz
  SPI_CLOCK_DIV32 500 Hz
  SPI_CLOCK_DIV64 250 Hz
  SPI_CLOCK_DIV128 125 Hz
  */

  SPI.setClockDivider(SPI_CLOCK_DIV16); // SPI clock 1000Hz
}

void Accelerometer_Setup()
{
  // Set up the accelerometer
  // write to Control register 1: address 20h
  byte addressByte = 0x20;
  /* Bits:
  PM2 PM1 PM0 DR1 DR0 Zen Yen Xen
  PM2PM1PM0: Power mode (001 = Normal Mode)
  DR1DR0: Data rate (00=50Hz, 01=100Hz, 10=400Hz, 11=1000Hz)
  Zen, Yen, Xen: Z enable, Y enable, X enable
  */
  byte ctrlRegByte = 0x37; // 00111111 : normal mode, 1000Hz, xyz enabled

  // Send the data for Control Register 1
  digitalWrite(SS, LOW);
  delay(1);
  SPI.transfer(addressByte);
  SPI.transfer(ctrlRegByte);
  delay(1);
  digitalWrite(SS, HIGH);

  delay(100);

  // write to Control Register 2: address 21h
  addressByte = 0x21;
  // This register configures high pass filter
  ctrlRegByte = 0x00; // High pass filter off

  // Send the data for Control Register 2
  digitalWrite(SS, LOW);
  delay(1);
  SPI.transfer(addressByte);
  SPI.transfer(ctrlRegByte);
  delay(1);
  digitalWrite(SS, HIGH);

  delay(100);

  // Control Register 3 configures Interrupts
  // Since I'm not using Interrupts, I'll leave it alone

  // write to Control Register 4: address 23h
  addressByte = 0x23;
  /* Bits:
  BDU BLE FS1 FS0 STsign 0 ST SIM
  BDU: Block data update (0=continuous update)
  BLE: Big/little endian data (0=accel data LSB at LOW address)
  FS1FS0: Full-scale selection (00 = +/-6G, 01 = +/-12G, 11 = +/-24G)
  STsign: selft-test sign (default 0=plus)
  ST: self-test enable (default 0=disabled)
  SIM: SPI mode selection(default 0=4 wire interface, 1=3 wire interface)
  */
  ctrlRegByte = 0x30; // 00110000 : 24G (full scale)

  // Send the data for Control Register 4
  digitalWrite(SS, LOW);
  delay(1);
  SPI.transfer(addressByte);
  SPI.transfer(ctrlRegByte);
  delay(1);
  digitalWrite(SS, HIGH);
}

Processing Sketch

 import pitaru.sonia_v2_9.*;
 import processing.serial.*;

 Sample beep;

 float high;
 int count;

 int inside = -1;
 int bx=850; // position in X of the up corner of the botton
 int by=460; // position in Y of the up corner of the botton
 int h=40;
 int w=100;

 float inByte=0;
 float drawByte=0;

 PFont f;

 Serial myPort;         // The serial port
 int xPos = 10;         // horizontal position of the graph

public void stop()
{
  Sonia.stop();
  super.stop();
}

 void setup () {
   // set the window size:
   size(1024, 550);

   high = 0;
   count = 0;

   f = createFont("Verdana",6,true);

   // List all the available serial ports
   println(Serial.list());
   // I know that the first port in the serial list on my mac
   // is always my  Arduino, so I open Serial.list()[0].
   // Open whatever port is the one you're using.
   myPort = new Serial(this, Serial.list()[0], 9600);
   // don't generate a serialEvent() unless you get a newline character:
   myPort.bufferUntil('\n');
   // set inital background:
   background(0);

  Sonia.start(this);
  beep = new Sample( "beep-02.wav" );
 }

void draw()
{
  if(keyPressed)
  {
    if(key == ' ')
    {
      high = inByte;
    }
  }

  background(0);

  //stroke(255,0,0);
  rect(xPos, 500 - inByte, xPos+20, inByte);

  textFont(f,25);
  fill(255);
  text(inByte, xPos - 10, 500 - inByte - 25);

  count = count + 1;

  if(count > 2)
  {
    count = 0;

    if(drawByte < high - 200)
    {
     beep.play();
     drawByte = drawByte + 100;
    }
    else if(drawByte < high - 10)
    {
       beep.play();
       drawByte = drawByte + 10;
    }
    else if(drawByte < high - 1)
    {
      beep.play();
      drawByte = drawByte + 1;
    }
    else if(drawByte < high - .1)
    {
      beep.play();
      drawByte = drawByte + .1;
    }
    else if(drawByte < high - .01)
    {
      beep.play();
      drawByte = drawByte + .01;
    }
    else if(drawByte < high - .001)
    {
      beep.play();
      drawByte = drawByte + .001;
    }
    else if(drawByte < high)
    {
      drawByte = high;
    }
  }

  if(drawByte > high)
  {
    drawByte = high;
  }

  textFont(f,140);
  fill(255);
  text(drawByte, 200, 325);

  rect(bx,by,w,h); // Button 

  textFont(f,25);
  fill(0);
  text("RESET", bx+10, by+30);
  fill(255);
}

void mousePressed(){
  if(!(((mouseX > (bx+w))
  ||(mouseY > (by+h)))
  ||((mouseX < bx)
  ||(mouseY < by))))
  {
      high = inByte;
  }
}

void serialEvent (Serial myPort) {
   String inString = myPort.readStringUntil('\n');

   if (inString != null)
   {
     // trim off any whitespace:
     inString = trim(inString);
     // convert to an int and map to the screen height:
     inByte = float(inString);
     inByte = map(inByte, 0, 1023, 0, 500);

     if(inByte > high)
     {
       high = inByte;
     }
   }
 }