Design and manufacture of simple digital display electronic scale

Course design of sensor and detection technology 【 Case study 1】—— Design and manufacture of simple digital display electronic scale

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Mission objectives

Weighing technology is widely used in industry , Agriculture , traffic , Important technologies in the fields of trade and scientific research , And social development 、 Industrial production is closely related to people's life , Electronic scales belong to electronic scales , The level of technology , It directly affects the quality of production and life and the development of economic benefits .

Technical indicators	requirement
range	0-10kg
The resolution of the	1g
Maximum reference error	<0.1%
Absolute error	<5g

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The following is the main body of this article , Cases can be used for reference

One 、 Working principle of digital display electronic scale

The electronic scale adopts a single-chip microcomputer processing system , The measured signal is converted into an electrical signal by a load cell , After processing and analyzing the sampled signal , Realize weight signal acquisition and conversion , And the single-chip microcomputer system is used for calculation and analysis , Finally, display through the display device , Can achieve fast , Convenient weight information display .

The overall design scheme is shown in the figure below ：

Two 、 Design of each module

1. Strain sensors

Sensor module is selected 10kg Range resistance strain type pressure sensor , Internal integrated Wheatstone bridge . The physical drawing and specific technical parameters are shown in the figure below .


The sensor sensitivity is 1mV/V, If the excitation voltage of the bridge is 10V, Then the voltage difference at full scale is 10mV, It is assumed that the voltage difference is linear with the force on the pressure sensor , Rate of change 1uV/g. The wiring between the sensor module and the signal processing circuit is as follows ：

2. Signal processing circuit

The signal conditioning circuit can be roughly divided into bridge differential amplification 、 Follower 、 Reverse magnification 、 Low pass filtering four parts .OPA2333 Build bridge amplifier ,AAD227 Build a secondary amplifier .

Schematic diagram
	First, through differential amplification , The bridge difference is obtained by the differential connection method and amplified to a suitable value . Set the magnification according to the chip data manual, about 100 times
	Voltage follower can be used as isolator in circuit , It has the characteristics of high input impedance and low output impedance , It can prevent the front and rear circuits from interfering . Due to the characteristics of the chip , The input current shall not be greater than 20mA, Therefore, a in-phase terminal is connected in series 5kΩ resistance . This resistance has no effect on the magnification .
	Because the difference of the front bridge is negative voltage , The inverter needs to be connected to convert the voltage into a positive voltage that can be measured by the single chip microcomputer , At the same time, the differential amplifier circuit restricted by the amplifier performance can be further amplified to make the measurement range large enough . Amplification factor of inverting amplifier Au = - ( Rf / Rin )

Final magnification = (R2/(R1+350))(IN2-IN1)1（-R7/R6）≈300

3. SCM and AD sampling

The single-chip microcomputer is selected STM32F103C8T6 Minimum system board , Commonly used , Easy to write quickly .
Due to the need to collect uv Level signal , The chip comes with 12 position ad Not competent for the task , To write SPI signal communication 24 position AD modular ADS1256.

Driver reference -》ADS1256 drive

The main program code is as follows （ Example ）：

#include "led.h"
#include "delay.h"
#include "sys.h"
#include "key.h"
#include "usart.h"
#include "exti.h" 
#include "ADS1256.h"

#define Au 300.0
int KeyState = 0;
char str[15],str2[15];

void Delay(vu32 nCount)
{
    
  for(; nCount != 0; nCount--);
}
// Timer 3 500ms An interruption 
void TIM3_Int_Init(u16 arr,u16 psc)
{
    
    TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
	NVIC_InitTypeDef NVIC_InitStructure;

	RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE); // Clock enable 

	TIM_TimeBaseStructure.TIM_Period = arr; // Set the value of the auto reload register cycle for the next update event load activity   Count to 5000 by 500ms
	TIM_TimeBaseStructure.TIM_Prescaler =psc; // Set as TIMx Prescaled value of clock frequency divisor  10Khz The counting frequency of  
	TIM_TimeBaseStructure.TIM_ClockDivision = 0; // Set the clock split :TDTS = Tck_tim
	TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //TIM Upcount mode 
	TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure); // according to TIM_TimeBaseInitStruct The parameter specified in TIMx Unit of time base 
 
	TIM_ITConfig(  // To enable or disable specified TIM interrupt 
		TIM3, //TIM2
		TIM_IT_Update ,
		ENABLE  // Can make 
		);
	NVIC_InitStructure.NVIC_IRQChannel = TIM3_IRQn;  //TIM3 interrupt 
	NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;  // Take precedence 0 level 
	NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;  // From the priority 3 level 
	NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE; //IRQ The channel is energized 
	NVIC_Init(&NVIC_InitStructure);  // according to NVIC_InitStruct The parameter specified in NVIC register 

	TIM_Cmd(TIM3, ENABLE);  // Can make TIMx peripherals 
							 
}

void TIM3_IRQHandler(void)   //TIM3 interrupt 
{
    
	if (TIM_GetITStatus(TIM3, TIM_IT_Update) != RESET) // Check the specified TIM Whether the interruption occurs or not :TIM  Interrupt source  
	{
    
		TIM_ClearITPendingBit(TIM3, TIM_IT_Update  );  // eliminate TIMx Interrupt pending bit of :TIM  Interrupt source  
	  printf(str);
	}
}

int main(void)
{
    	
	long double vr,vout,vsum,v[50];
	unsigned char i=0,j=0;
	long ulResult;
	long double ldVolutage;
	
	delay_init();	    	 // Delay function initialization  
	NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);//  Set interrupt priority group 2
	uart_init(9600);	 // The serial port is initialized to 9600
	
	LED_Init();		  	 // Initialization and LED Connected hardware interface  
  LEDx=0;				// Lighten up LED
	
	EXTIX_Init();		// External interrupt initialization 
	
	printf("OK\r\n");	
	
	Init_ADS1256_GPIO(); // initialization ADS1256 GPIO Pin  

	Delay(0x1ffFF);
	GPIO_SetBits(GPIOB, GPIO_Pin_11 );  
	ADS1256_Init();
	
	TIM3_Int_Init(4999,7199);//10Khz The counting frequency of , Count to 5000 by 500ms 
	
	for(i = 0;i < 20;i++) v[i]=0;
	
	while(1)
	{
    	    
		 for(i = 0;i < 1;i++)
		{
    
			 ulResult = ADS_sum( (i << 4) | ADS1256_MUXN_AINCOM);	
			//ulResult = ADS_sum( ADS1256_MUXP_AIN0 | ADS1256_MUXN_AINCOM); 
			if( ulResult & 0x800000 )
			{
    
			 	ulResult = ~(unsigned long)ulResult;
				ulResult &= 0x7fffff;
				ulResult += 1;
				ulResult = -ulResult;
			}
			ldVolutage = (long double)ulResult*0.59604644775390625;
			//printf(" The first %d passageway :",(i & 0x07)?(i & 0x07) - 1:7);
			//sprintf(str,"%lf\r\n",ldVolutage); //double
			//printf("uV\r");
			//printf("%x",(unsigned long)ulResult);//16
			Delay(0x3fFFF);
		}
		vsum = 0;
		for(i = 1;i < 20;i++)
		{
    
			v[i] = v[i-1];
			vsum += v[i];
			vsum /= 2.0;
		}
		v[0] = ldVolutage;
		vsum += v[0];
		vsum /= 2.0;
		if(j<20)
		{
    
			vr = vsum;
			j++;
		}
		vout = (vsum - vr) / Au;///20.0;
		sprintf(str,"%.0lf -> ",vsum);
		sprintf(str2,"%.0lf g\r\n",vout);
	} 
}
//END

The filtering algorithm adopts continuous sampling 20 It's worth （ Small sample ） Averaging , The actual performance is good .

3、 ... and 、 Simulation and testing

1. Hardware simulation

The magnification of the scheme during simulation is 240, After that, the actual scheme is modified to 300

Bridge output voltage （mv）	Corresponding object weight	Analog output voltage
0.001	1g	241.999uV
0.01	10g	2.402mV
0.1	100g	24.001mV
1	1kg	239.997V
10	10kg	2.4V

2. Actual test

3. Improve and optimize

a） It can be considered to modify the scheme of three operational amplifiers to build differential instrument amplification .
b） The test accuracy is improved by fitting the actual test data .
……

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summary

This paper simply introduces the design and manufacture of simple digital display electronic scale , It provides samples and cases for the follow-up of other teaching projects .