制作一个0-20mA的的信号发生器，可以给很多仪器仪表做校准或测试

概述

一般仪器仪表的信号电流都为4-20mA，指最小电流为4mA,最大电流为20mA。传输信号时候，要考虑到导线上也有电阻，如果用电压传输则会在导线的产生一定的压降，那接收端的信号就会产生一定的误差，所以使用电流信号作为变送器的标准传输。为什么选择4-20mA而不是0-20mA呢？4ma而不是0ma是用来检测线路开路的，如果0是最小，那么开路故障就检测不到了。为了解决上述问题和避开相关噪声的影响，我们用电流来传输信号，因为电流对噪声并不敏感。4～20mA的电流环便是用4mA表示零信号，用20mA表示信号的满刻度，而低于4mA高于20mA的信号用于各种故障的报警。

很多控制器接受来自各种检测仪表的0~20mA或4~20mA电流，制作一个0-20mA的的信号发生器，可以给很多仪器仪表做校准或测试。

2. 总体方案设计

2.1. 设计目标

单片机控制4路ＰＷＭ，输出的PWM信号控制输出电压在0-3.0V之间，经过电流电压转电流电路变成电流信号，输出电流在0-20mA之间。按键调节输出电流，oled实时显示４路电流值。

2.2. 设计思路

芯片选择STM32F030C8T6，带4路PWM输出，性价比高，同时支持rt-thread操作系统，这里我们选用2.1的版本。OLED选用中景园电子0-96寸OLED。

2.2.1.硬件电路原理

图1是一个电压转电流的典型电路。单片机输出PWM，控制Vi电压电平在0-3V之间，RL流过的电流为0-20ma。

图1 电压转电流电路

图2 OLED显示电路

由于MCU内部可配置上拉电阻，所以可以直接将按键接到MCU上。

图3 按键电路

2.2.2.软件设计流图

2.2.3.关键代码

按键部分代码。

/* 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显示部分代码。

/* 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使用情况

按键的初始化函数可以不用再main函数中添加，而是在key.c中调用下面的函数就可以了INITDEVICEEXPORT(rthwkey_init)，代码看起来干净整洁。通过scons来裁剪配置系统，不需要人为删减代码，非常好用。Finsh组件，方便调试，方便自定义添加串口命令。代码类linux编程风格。提供POSIX 标准接口，上层代码移植方便。

3. 实物展示