Encoder configuration giving erroneous speed

Posted on August 16, 2014 at 13:49

Hi, I am trying to measure the rpm of a geared DC motor. The manufacturer claims a speed of 500rpm at 12V but my calculations give a speed of around 1600rpm! The gear ratio is 19:1.

I have written the following code to measure the speed:

//Systick setup for 1ms

void Delay(__IO uint32_t nTime)

{

  TimingDelay = nTime;

  while(TimingDelay != 0);

}

void TimingDelay_Decrement(void)

{

  if (TimingDelay != 0x00)

  {

    TimingDelay--;

  }

}

void SysTick_Init(void){

    while (SysTick_Config(SystemCoreClock / 1000) != 0) {

    }

}

void SysTick_Handler(void){

    TimingDelay_Decrement();

}

//Initializing Timers for encoder interface

void InitENTim(void){

    

    RCC_APB1PeriphClockCmd (RCC_APB1Periph_TIM4, ENABLE);

    RCC_APB1PeriphClockCmd (RCC_APB1Periph_TIM2, ENABLE);

    

    TIM_TimeBaseStructure.TIM_Prescaler = 0;

    TIM_TimeBaseStructure.TIM_Period = 0xFFFF;

    TIM_TimeBaseStructure.TIM_ClockDivision=0;

    TIM_TimeBaseStructure.TIM_CounterMode=TIM_CounterMode_Up;

    TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);

    

    TIM_EncoderInterfaceConfig(TIM4, TIM_EncoderMode_TI12,TIM_ICPolarity_Rising, TIM_ICPolarity_Rising);

   

    TIM_EncoderInterfaceConfig(TIM2, TIM_EncoderMode_TI12,TIM_ICPolarity_Rising, TIM_ICPolarity_Rising);

   

    

    TIM_Cmd(TIM4,ENABLE);

    TIM_Cmd(TIM2,ENABLE);

    encodersReset(); //function to reset counter values

    

}

//The function to read the encoder and get Count

void encoderRead(void){

    

    oldrightEncoder = TIM_GetCounter(TIM4);

    Delay(10); //This should be a 10ms delay (according to my Systick config as written above)

    rightEncoder = TIM_GetCounter(TIM4);

    rightCount = rightEncoder - oldrightEncoder; //I am getting the difference in counts after 10ms

    totalrightCount += rightCount;

    

    

    oldleftEncoder = TIM_GetCounter(TIM4); //Encoder;

    Delay(10);

    leftEncoder = TIM_GetCounter(TIM4);

    leftCount = leftEncoder - oldleftEncoder;

    totalleftCount += leftCount;

    

}

//function to calculate speeds

void encoderCalculate(void){

    volatile float a,b,x,y,z; //just some variables for ensuring I didnt make a mistake in simple maths

    

    x = rightCount/64; //my encoder gives 64 counts per revolution when detecting both edges of both channels.

    y = 1/x;

    z = y*0.01; //This 0.01 is for the 10ms delay that I had for getting the difference in Count

    a = 60/z;

    b = a/19; //accounting for the gear ratio (19:1)

    rightspeed = b;

    

    x = leftCount/64;

    y = 1/x;

    z = y*0.01;

    a = 60/z;

    b = a/19;

    leftspeed = b;

  

}

Is there any chance there is some sort of conflict in timing of the encoder timer and the systick timer? Obviously, there is some sort of a timing problem. Kindly help as I have been stuck on this problem for a quiet a long while.

#stm32 #tim #encoder