Foot / Palm pressure sensing and 3D plot in MATLAB

 The 3d plot used to show the pressure distribution is a shown using the surf()  funtion of MATLAB. The plot will display pressure variations from low pressure ares with a lighter color/higher peaks and higher pressure areas with darker colors/ lower peaks. The design would involve many flat pressure sensors(FSR) distributed to form a sensor bed.

The sensors used can be a FSR (Force sensitive resistor shown above) or other sensors which you find suitable. The sensors used must be selected small enough to provide better sensing resolution in a given area. Smaller the size of pressure sensor, more will be the number of sensors in a given area, which will give a more detailed plot of pressure areas. If you are using an FSR then you can interface them to the microcontroller via an ADC and convert the pressure variations to digital values and transmit them to a PC running MATLAB via UART or other interface.

The microcontroller PIC16F877A reads the 8-pressure sensor values using an 8 bit ADC and sends them to the PC COM port. A piece of the microcontroller code is shown below,

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  #include  #include "UART.h"   unsigned char channel;  unsigned char ADCReadComplete = 0; unsigned char Temp; unsigned short Data[8]; unsigned char channel = 3; void ReadADCCH(unsigned char ch) {           Temp = ADRESL; 	Temp |= (ADRESH<<8);  	ADRESL = 0; 	ADRESH = 0; 	channel = ch;  	switch(ch) 	{ 		case 0: 		case 1: 		case 2: 		case 3: 			TRISA = 1<<ch; 		break; 		case 4: 			TRISA = 1<<5; 		break; 		case 5: 			TRISE = 1; 		break; 		case 6: 			TRISE = 2; 		break; 		case 7: 			TRISE = 4; 		break;			 	}	 		 	ADCON1= 0xC0; 	ADCON0 = 0xC0 | ch<<3;	 	ADCON0 |= 0x01;			 	 	ADCON0 = 0xC5 | ch<<3;     while((ADCON0 & 0x04) == 0x04);     while(ADIF == 0);	     Data[channel]  = ADRESL; 	Data[channel] |= (ADRESH<<8);  }    extern unsigned short Counter; unsigned char rxdata; void main() { 	unsigned char i;      	InitUART();     	RB4 = 1;       while(1) 	{ 		ReadADCCH(0); 		ReadADCCH(1); 		ReadADCCH(2); 		ReadADCCH(3); 		ReadADCCH(4); 		ReadADCCH(5); 		ReadADCCH(6); 		ReadADCCH(7); 	  		if(PIR1bits.RCIF != 0)         {             // Clear the interrupt flag             PIR1bits.RCIF = 0;             if(RCSTAbits.OERR == 1)             {                                  //                 // Clear Over run flag by clearing CREN                 // and enable again for reception                 //                 RCSTAbits.CREN = 0;                 RCSTAbits.CREN = 1;             }             rxdata = ReadUART();             if(rxdata == '#')             {  		    	for(i = 0; i < 8; i++) 		    	{ 		    		PrintNumber3(Data[i]); 		    		Print((unsigned char*)" "); 		    	}	 		    	Print((unsigned char*)"\r\n"); 		    }         }   	    		 	} 	 }   

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The MATLAB program running on the PC will read the sensed values and plot the pressure distribution map. MATLAB has inbuilt serial port interface library function’s, which can be used to open,close, set baud rate for serial port. The surf() 3d plot function in MATLAB is used to display the individual peaks. Each peak represents a pressure point/pressure sensor value. The MATLAB 3d plot,serial port code used is shown below,

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  s = serial('COM1');  set(s,'BaudRate',9600); set(s,'DataBits',8); set(s,'Parity','none');  fopen(s);  while 1      try         fprintf(s,'#');     catch         fclose(s);     end      try     line = fgets(s)     catch         fclose(s);     end         try     out = strread(line)     catch         fclose(s);     end           zsign(1) = 1-out(1)/1024;     zsign(2) = 1-out(2)/1024;     zsign(3) = 1-out(3)/1024;     zsign(4) = 1-out(4)/1024;     zsign(5) = 1-out(5)/1024;     zsign(6) = 1-out(6)/1024;     zsign(7) = 1-out(7)/1024;     zsign(8) = 1-out(8)/1024;       zsign            hold on              [x,y] = meshgrid([-3:0.5:3]);     z = exp(-x.^2-y.^2);      surf(x+5,y+5,(z * 0));         surf(x+5,y+15,(z * 0));         surf(x+5,y+20,(z * 0));         surf(x+15,y+20,(z * 0));                            surf(x+10,y+20,(z * zsign(1)));          surf(x+10,y+15,(z * zsign(2)));          surf(x+15,y+15,(z * zsign(3)));      surf(x+5,y+10,(z * zsign(4)));      surf(x+10,y+10,(z * zsign(5)));        surf(x+15,y+10,(z * zsign(6)));      surf(x+10,y+5,(z * zsign(7)));      surf(x+15,y+5,(z* zsign(8)));                    hold off      shading interp     drawnow           box off     grid off     axis off     caxis([0 1])     colorbar;     axis([0 20 0 25 0 2]);            display('f');     cla end     

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