项目设计

本次参加了硬禾与digikey推出的fastbond活动，选择了可穿戴测量题目，做一个心率测量的原型机，要求使用到ADI与MAXIM的芯片。

那我选择的传感器是MAX30102是一款集成脉搏血氧饱和度和心率监测传感器，在网络上很容易就能找到它的硬件和软件开发资料，测量的原理是用传感器内部的两个不同波段led向人体组织上发射灯光，并测量反射的信号强度，测算出心跳速率和血氧浓度。

那么我又搭配采购了一个ADI的LDO稳压芯片ADP7158，用于整个传感器部分的供电，这样可以减小因为供电不稳定造成的测量误差。

MAX30102相关介绍：用于可穿戴健康设备的高灵敏度脉搏血氧仪和心率传感器

官网：https://www.maximintegrated.com/cn/products/interface/sensor-interface/MAX30102.html

ADP7158相关介绍：2 A、超低噪声、高PSRR、固定输出、RF线性稳压器

官网:https://www.analog.com/cn/products/adp7158.html

开发过程

确定好传感器和电源部分，接下来就是选择MCU将各部分信号收集起来，并作出相应的决策。我找出了MAX32660评估板，（顺便一提，这也是一款来自美信的产品哦）

查阅有关数据手册得知，芯片接口为I2C协议，从机地址为0b1010111（0x57），当然手册中使用的8位地址模式，即为0xAE和0xAF

下一步，写相关传感器驱动程序

美信提供了全套的算法代码包括噪声滤除，求峰峰之间的距离，血样饱和度映射等等

void maxim_heart_rate_and_oxygen_saturation(uint32_t *pun_ir_buffer,  int32_t n_ir_buffer_length, uint32_t *pun_red_buffer, int32_t *pn_spo2, int8_t *pch_spo2_valid, 
                              int32_t *pn_heart_rate, int8_t  *pch_hr_valid)
/**
* \brief        Calculate the heart rate and SpO2 level
* \par          Details
*               By detecting  peaks of PPG cycle and corresponding AC/DC of red/infra-red signal, the ratio for the SPO2 is computed.
*               Since this algorithm is aiming for Arm M0/M3. formaula for SPO2 did not achieve the accuracy due to register overflow.
*               Thus, accurate SPO2 is precalculated and save longo uch_spo2_table[] per each ratio.
*
* \param[in]    *pun_ir_buffer           - IR sensor data buffer
* \param[in]    n_ir_buffer_length      - IR sensor data buffer length
* \param[in]    *pun_red_buffer          - Red sensor data buffer
* \param[out]    *pn_spo2                - Calculated SpO2 value
* \param[out]    *pch_spo2_valid         - 1 if the calculated SpO2 value is valid
* \param[out]    *pn_heart_rate          - Calculated heart rate value
* \param[out]    *pch_hr_valid           - 1 if the calculated heart rate value is valid
*
* \retval       None
*/
{
    uint32_t un_ir_mean ,un_only_once ;
    int32_t k ,n_i_ratio_count;
    int32_t i, s, m, n_exact_ir_valley_locs_count ,n_middle_idx;
    int32_t n_th1, n_npks,n_c_min;      
    int32_t an_ir_valley_locs[15] ;
    int32_t an_exact_ir_valley_locs[15] ;
    int32_t an_dx_peak_locs[15] ;
    int32_t n_peak_interval_sum;
    
    int32_t n_y_ac, n_x_ac;
    int32_t n_spo2_calc; 
    int32_t n_y_dc_max, n_x_dc_max; 
    int32_t n_y_dc_max_idx, n_x_dc_max_idx; 
    int32_t an_ratio[5],n_ratio_average; 
    int32_t n_nume,  n_denom ;
    // remove DC of ir signal    
    un_ir_mean =0; 
    for (k=0 ; k<n_ir_buffer_length ; k++ ) un_ir_mean += pun_ir_buffer[k] ;
    un_ir_mean =un_ir_mean/n_ir_buffer_length ;
    for (k=0 ; k<n_ir_buffer_length ; k++ )  an_x[k] =  pun_ir_buffer[k] - un_ir_mean ; 
    
    // 4 pt Moving Average
    for(k=0; k< BUFFER_SIZE-MA4_SIZE; k++){
        n_denom= ( an_x[k]+an_x[k+1]+ an_x[k+2]+ an_x[k+3]);
        an_x[k]=  n_denom/(int32_t)4; 
    }

    // get difference of smoothed IR signal
    
    for( k=0; k<BUFFER_SIZE-MA4_SIZE-1;  k++)
        an_dx[k]= (an_x[k+1]- an_x[k]);

    // 2-pt Moving Average to an_dx
    for(k=0; k< BUFFER_SIZE-MA4_SIZE-2; k++){
        an_dx[k] =  ( an_dx[k]+an_dx[k+1])/2 ;
    }
    
    // hamming window
    // flip wave form so that we can detect valley with peak detector
    for ( i=0 ; i<BUFFER_SIZE-HAMMING_SIZE-MA4_SIZE-2 ;i++){
        s= 0;
        for( k=i; k<i+ HAMMING_SIZE ;k++){
            s -= an_dx[k] *auw_hamm[k-i] ; 
                     }
        an_dx[i]= s/ (int32_t)1146; // divide by sum of auw_hamm 
    }

 
    n_th1=0; // threshold calculation
    for ( k=0 ; k<BUFFER_SIZE-HAMMING_SIZE ;k++){
        n_th1 += ((an_dx[k]>0)? an_dx[k] : ((int32_t)0-an_dx[k])) ;
    }
    n_th1= n_th1/ ( BUFFER_SIZE-HAMMING_SIZE);
    // peak location is acutally index for sharpest location of raw signal since we flipped the signal         
    maxim_find_peaks( an_dx_peak_locs, &n_npks, an_dx, BUFFER_SIZE-HAMMING_SIZE, n_th1, 8, 5 );//peak_height, peak_distance, max_num_peaks 

    n_peak_interval_sum =0;
    if (n_npks>=2){
        for (k=1; k<n_npks; k++)
            n_peak_interval_sum += (an_dx_peak_locs[k]-an_dx_peak_locs[k -1]);
        n_peak_interval_sum=n_peak_interval_sum/(n_npks-1);
        *pn_heart_rate=(int32_t)(6000/n_peak_interval_sum);// beats per minutes
        *pch_hr_valid  = 1;
    }
    else  {
        *pn_heart_rate = -999;
        *pch_hr_valid  = 0;
    }
            
    for ( k=0 ; k<n_npks ;k++)
        an_ir_valley_locs[k]=an_dx_peak_locs[k]+HAMMING_SIZE/2; 


    // raw value : RED(=y) and IR(=X)
    // we need to assess DC and AC value of ir and red PPG. 
    for (k=0 ; k<n_ir_buffer_length ; k++ )  {
        an_x[k] =  pun_ir_buffer[k] ; 
        an_y[k] =  pun_red_buffer[k] ; 
    }

    // find precise min near an_ir_valley_locs
    n_exact_ir_valley_locs_count =0; 
    for(k=0 ; k<n_npks ;k++){
        un_only_once =1;
        m=an_ir_valley_locs[k];
        n_c_min= 16777216;//2^24;
        if (m+5 <  BUFFER_SIZE-HAMMING_SIZE  && m-5 >0){
            for(i= m-5;i<m+5; i++)
                if (an_x[i]<n_c_min){
                    if (un_only_once >0){
                       un_only_once =0;
                   } 
                   n_c_min= an_x[i] ;
                   an_exact_ir_valley_locs[k]=i;
                }
            if (un_only_once ==0)
                n_exact_ir_valley_locs_count ++ ;
        }
    }
    if (n_exact_ir_valley_locs_count <2 ){
       *pn_spo2 =  -999 ; // do not use SPO2 since signal ratio is out of range
       *pch_spo2_valid  = 0; 
       return;
    }
    // 4 pt MA
    for(k=0; k< BUFFER_SIZE-MA4_SIZE; k++){
        an_x[k]=( an_x[k]+an_x[k+1]+ an_x[k+2]+ an_x[k+3])/(int32_t)4;
        an_y[k]=( an_y[k]+an_y[k+1]+ an_y[k+2]+ an_y[k+3])/(int32_t)4;
    }

    //using an_exact_ir_valley_locs , find ir-red DC andir-red AC for SPO2 calibration ratio
    //finding AC/DC maximum of raw ir * red between two valley locations
    n_ratio_average =0; 
    n_i_ratio_count =0; 
    
    for(k=0; k< 5; k++) an_ratio[k]=0;
    for (k=0; k< n_exact_ir_valley_locs_count; k++){
        if (an_exact_ir_valley_locs[k] > BUFFER_SIZE ){             
            *pn_spo2 =  -999 ; // do not use SPO2 since valley loc is out of range
            *pch_spo2_valid  = 0; 
            return;
        }
    }
    // find max between two valley locations 
    // and use ratio betwen AC compoent of Ir & Red and DC compoent of Ir & Red for SPO2 

    for (k=0; k< n_exact_ir_valley_locs_count-1; k++){
        n_y_dc_max= -16777216 ; 
        n_x_dc_max= - 16777216; 
        if (an_exact_ir_valley_locs[k+1]-an_exact_ir_valley_locs[k] >10){
            for (i=an_exact_ir_valley_locs[k]; i< an_exact_ir_valley_locs[k+1]; i++){
                if (an_x[i]> n_x_dc_max) {n_x_dc_max =an_x[i];n_x_dc_max_idx =i; }
                if (an_y[i]> n_y_dc_max) {n_y_dc_max =an_y[i];n_y_dc_max_idx=i;}
            }
            n_y_ac= (an_y[an_exact_ir_valley_locs[k+1]] - an_y[an_exact_ir_valley_locs[k] ] )*(n_y_dc_max_idx -an_exact_ir_valley_locs[k]); //red
            n_y_ac=  an_y[an_exact_ir_valley_locs[k]] + n_y_ac/ (an_exact_ir_valley_locs[k+1] - an_exact_ir_valley_locs[k])  ; 
        
        
            n_y_ac=  an_y[n_y_dc_max_idx] - n_y_ac;    // subracting linear DC compoenents from raw 
            n_x_ac= (an_x[an_exact_ir_valley_locs[k+1]] - an_x[an_exact_ir_valley_locs[k] ] )*(n_x_dc_max_idx -an_exact_ir_valley_locs[k]); // ir
            n_x_ac=  an_x[an_exact_ir_valley_locs[k]] + n_x_ac/ (an_exact_ir_valley_locs[k+1] - an_exact_ir_valley_locs[k]); 
            n_x_ac=  an_x[n_y_dc_max_idx] - n_x_ac;      // subracting linear DC compoenents from raw 
            n_nume=( n_y_ac *n_x_dc_max)>>7 ; //prepare X100 to preserve floating value
            n_denom= ( n_x_ac *n_y_dc_max)>>7;
            if (n_denom>0  && n_i_ratio_count <5 &&  n_nume != 0)
            {   
                an_ratio[n_i_ratio_count]= (n_nume*100)/n_denom ; //formular is ( n_y_ac *n_x_dc_max) / ( n_x_ac *n_y_dc_max) ;
                n_i_ratio_count++;
            }
        }
    }

    maxim_sort_ascend(an_ratio, n_i_ratio_count);
    n_middle_idx= n_i_ratio_count/2;

    if (n_middle_idx >1)
        n_ratio_average =( an_ratio[n_middle_idx-1] +an_ratio[n_middle_idx])/2; // use median
    else
        n_ratio_average = an_ratio[n_middle_idx ];

    if( n_ratio_average>2 && n_ratio_average <184){
        n_spo2_calc= uch_spo2_table[n_ratio_average] ;
        *pn_spo2 = n_spo2_calc ;
        *pch_spo2_valid  = 1;//  float_SPO2 =  -45.060*n_ratio_average* n_ratio_average/10000 + 30.354 *n_ratio_average/100 + 94.845 ;  // for comparison with table
    }
    else{
        *pn_spo2 =  -999 ; // do not use SPO2 since signal ratio is out of range
        *pch_spo2_valid  = 0; 
    }
}

实验现象

如截图所示：

可以看见心跳的原始数据和一阶差分数据，算法就是基于此计算出实际心率信息，右侧数据也输出了心跳BPM

总结

Fastbond是一个综合性的项目，既有软件编程，也有硬件设计，对于平时专注于布线的硬件工程师来说，这个机会非常棒，走出舒适圈，接触到一些程序设计及算法知识。