概述

一般儀器儀表的信號電流都為4-20mA，指最小電流為4mA,最大電流為20mA。傳輸信號時候，要考慮到導線上也有電阻，如果用電壓傳輸則會在導線的產生一定的壓降，那接收端的信號就會產生一定的誤差，所以使用電流信號作為變送器的標準傳輸。為什么選擇4-20mA而不是0-20mA呢？4ma而不是0ma是用來檢測線路開路的，如果0是最小，那么開路故障就檢測不到了。為了解決上述問題和避開相關噪聲的影響，我們用電流來傳輸信號，因為電流對噪聲并不敏感。4～20mA的電流環(huán)便是用4mA表示零信號，用20mA表示信號的滿刻度，而低于4mA高于20mA的信號用于各種故障的報警。

很多控制器接受來自各種檢測儀表的0~20mA或4~20mA電流，制作一個0-20mA的的信號發(fā)生器，可以給很多儀器儀表做校準或測試。

2. 總體方案設計

2.1. 設計目標

單片機控制4路ＰＷＭ，輸出的PWM信號控制輸出電壓在0-3.0V之間，經過電流電壓轉電流電路變成電流信號，輸出電流在0-20mA之間。按鍵調節(jié)輸出電流，oled實時顯示４路電流值。

2.2. 設計思路

芯片選擇STM32F030C8T6，帶4路PWM輸出，性價比高，同時支持rt-thread操作系統(tǒng)，這里我們選用2.1的版本。OLED選用中景園電子0-96寸OLED。

2.2.1.硬件電路原理

圖1是一個電壓轉電流的典型電路。單片機輸出PWM，控制Vi電壓電平在0-3V之間，RL流過的電流為0-20ma。

圖1 電壓轉電流電路

圖2 OLED顯示電路

由于MCU內部可配置上拉電阻，所以可以直接將按鍵接到MCU上。

圖3 按鍵電路

2.2.2.軟件設計流圖

2.2.3.關鍵代碼

按鍵部分代碼。

/* key thread entry */

staticvoid key_thread_entry(void*parameter)

{

KEY_e i;

uint8_t key_state1[KEY_NUM];

uint8_t key_state2[KEY_NUM];

uint8_t key_counter[KEY_NUM];

rt_base_t level;

memset(key_counter, 0, sizeof(key_counter));

while(1)

{

for (i=KEY1; i

{

key_state1[i] = rt_hw_key(i);

}

rt_thread_delay(RT_TICK_PER_SECOND / 20);

for (i=KEY1; i

{

key_state2[i] = rt_hw_key(i);

}

for (i=KEY1; i

{

if (key_state1[i] == key_state2[i] &&

key_state1[i] == 0)

{

level =rt_hw_interrupt_disable();

if (key_counter[i] == 0)

{

switch(i)

{

case KEY2:

if (pwm_channel< 3)

{

pwm_channel++;

}

break;

case KEY1:

if (pwm_channel> 0)

pwm_channel--;

break;

case KEY3:

if(pwm_value[pwm_channel] < 20000)

pwm_value[pwm_channel]++;

break;

case KEY4:

if(pwm_value[pwm_channel] > 0)

pwm_value[pwm_channel]--;

break;

case KEY5:

if(pwm_value[pwm_channel] < 16000)

pwm_value[pwm_channel] += 4000;

else

pwm_value[pwm_channel] = 20000;

break;

case KEY6:

if(pwm_value[pwm_channel] >= 4000)

pwm_value[pwm_channel] -= 4000;

else

pwm_value[pwm_channel] = 0;

break;

}

rt_kprintf("key %dclicked ", i);

}

if (key_counter[i] >= 5)

{

switch(i)

{

case KEY2:

if (pwm_channel< 3)

{

pwm_channel++;

}

break;

case KEY1:

if (pwm_channel> 0)

pwm_channel--;

break;

case KEY3:

if(pwm_value[pwm_channel] < 20000)

pwm_value[pwm_channel]++;

break;

case KEY4:

if(pwm_value[pwm_channel] > 0)

pwm_value[pwm_channel]--;

break;

case KEY5:

if(pwm_value[pwm_channel] < 16000)

pwm_value[pwm_channel]+= 4000;

else

pwm_value[pwm_channel] = 20000;

break;

case KEY6:

if(pwm_value[pwm_channel] >= 4000)

pwm_value[pwm_channel] -= 4000;

else

pwm_value[pwm_channel] = 0;

break;

}

rt_kprintf("key%d pressed ", i);

}

if (key_counter[i] < 5)

{

key_counter[i]++;

}

rt_hw_interrupt_enable(level);

}

else

{

key_counter[i] = 0;

}

}

rt_thread_delay(RT_TICK_PER_SECOND / 100);

}

}

oled顯示部分代碼。

/* oled thread entry */

staticvoid oled_thread_entry(void*parameter)

{

uint8_t i;

rt_base_tlevel;

char str_pwm[64];

OLED_Init();

OLED_Clear();

PWM_TIM1(999, 1); //48MHZ/(999+1)/(1+1) = 24KHZ

while(1)

{

//OLED_ShowString(0, 3,"1.3' OLED TEST");

if ((memcmp(pwm_value_temp, pwm_value, sizeof(pwm_value)) != 0) ||

(pwm_channel_temp != pwm_channel))

{

level = rt_hw_interrupt_disable();

memcpy((char *)pwm_value_temp, (char *)pwm_value, sizeof(pwm_value));

pwm_channel_temp = pwm_channel;

rt_hw_interrupt_enable(level);

for (i=0; i<4; i++)

{

if (pwm_channel == i)

{

snprintf(str_pwm, 64,"* %2d.%03d ma", pwm_value[i]/1000, pwm_value[i]%1000);

}

else

{

snprintf(str_pwm, 64," %2d.%03d ma",pwm_value[i]/1000, pwm_value[i]%1000);

}

OLED_ShowString(0, i*2,(uint8_t *)str_pwm);

}

TIM_SetCompare1(TIM1, 0.915 *(pwm_value_temp[3] * 999) / 20000);

TIM_SetCompare2(TIM1, 0.915 *(pwm_value_temp[2] * 999) / 20000);

TIM_SetCompare3(TIM1, 0.915 *(pwm_value_temp[1] * 999) / 20000);

TIM_SetCompare4(TIM1, 0.915 *(pwm_value_temp[0] * 999) / 20000);

rt_thread_delay(RT_TICK_PER_SECOND / 10);

}

else

{

//OLED_ShowString(63,6,"CODE:");

rt_thread_delay(RT_TICK_PER_SECOND / 10);

}

}

}

2.2.4.RTT使用情況

按鍵的初始化函數(shù)可以不用再main函數(shù)中添加，而是在key.c中調用下面的函數(shù)就可以了INITDEVICEEXPORT(rthwkey_init)，代碼看起來干凈整潔。通過scons來裁剪配置系統(tǒng)，不需要人為刪減代碼，非常好用。Finsh組件，方便調試，方便自定義添加串口命令。代碼類linux編程風格。提供POSIX 標準接口，上層代碼移植方便。

3. 實物展示