PIC32MX: Interfacing with Force Sensors from a Scale

Original Assignment

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Your assignment is to use four analog inputs to continuously read four force sensors taken from a bathroom scale, and to display them on a PC screen as bars whose length corresponds to the force at the sensors. You will use Processing for the PC data acquisition and graphics.

Overview

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Circuit

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Code

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The Processing portion of our lab involves 4 sections:

1. colorbars:

continuously displaying the force applied onto each force sensor by using 4 bars that will grow or shrink depending on the amplitude of mass (force). The bars will also show the amplitude of the mass by changing colors. When 0 or little force is applied, then corresponding bar is black colored, at the max force, the bar is orchid (pink) colored.

2. xy-plot:

continuously displays the x- and y- position of the center of balance calculated between all 4 sensors with a dot on a xy-coordinate plane. So, if the person shifts his weight to the right side of scale, then the dot will move to the right in the x direction. If the person shifts his weight to the front of the sensor, then the dot will move up in the y direction.

3. x-position vs. time:

continuously plots a dot for the x-position along a time axis. This plot will be useful to see how much the person shifts his/her weight to the left or right (sagittal plane) over time. Due to the window size and the speed of processing, the points will be plotted over a time period of about 20 seconds.

4. y-position vs. time:

continuously plots a dot for the y-position along a time axis. This plot will be useful to see how much the person shifts his/her weight forward or backwards (coronal or frontal plane) over time. The code for this section is essentially the same as the x-position but edited to show y-position.

General Notes:

Our code was written with the assumption that the inputs from the PIC (that initially comes from the 4 force sensors) will come in the form of a 4x1 array. It does not actually arrive in that form from the PIC, it actually arrives as a string of floats separated by spaces ending with a new line. The new line is deleted and the floats are extracted out, normalized between 0-255, and put into the "mass" array. All of the normalization, window dimension parameters, rectangle dimensions, etc. were set up with the 0-255 range and 1000x700 window size in mind. So if the values are not between 0-255 or the window size is altered, almost all of following code will be messed up. The x-position vs. time and y-position vs. time plots are not implemented as independent functions. The code for these plots are found at the very end of colorbars();.

Global Variables

 //initialization of the serial variable that will hold the values sent from the PIC via RS232
 import processing.serial.*; 
 Serial[] myPorts = new Serial[1];

 //the PIC will output 4 floats and we will first store the values into these 4 variables
 float input0;
 float input1;
 float input2;
 float input3;

 //dimensions of our window will be 1000x700
 float window_height = 700;
 float window_width = 1000;
 PFont fontA;  //initializing our text font

 //the main "mass" 4x1 array that holds the values continuously sent from the 4 force sensors
 //we will refer to this array as the "mass array" in our comments
 float[] mass = new float[4]; 

 //window dimension variable that is used to make the bargraph.  The bx and by variables are
 //used so that the bargraph section could be more easily formatted in case we change the
 //window size.
 float bx = window_width/2/8;
 float by = window_height/2/8;
 int index;  

 //color variables used to indicate the amplitude of the force applied to each sensor
 color c0;
 color c1;
 color c2;
 color c3;

Main Setup Function

void setup() {
  
  index = 0;
  InitSerial();
  smooth();

  //splitting the window into 5 separate areas and filling them with a 102 gray color
  size(int(window_width),int(window_height));
  fill(102);
  rect(0,0,window_width/2,window_height/3);
  fill(102);
  rect(0,window_height/3,window_width/2,window_height/3);
  fill(102);
  rect(0,window_height*2/3,window_width/2,window_height/3);
  fill(102);
  rect(window_width/2,0,window_width/2,window_height/2);
  fill(102);
  rect(window_width/2,window_height/2,window_width/2,window_height/2);

  //filling inside of the x-position vs. time and y-position vs. time plots with white color
  fill(255);
  rect(25, int(window_height/3)+29, int(window_width/2)-49, int(window_height/3)-59);
  fill(255);
  rect(25, int(window_height*2/3)+29, int(window_width/2)-49, int(window_height/3)-59);
}