Dangerous error produced in speed calculation for EBiCS display method #28
Description
In case of 2.2 m of wheel circumference, single magnet in the spokes:
#define WHEEL_CIRCUMFERENCE 2200
#define PULSES_PER_REVOLUTION 1
When the speed reaches 4r/s (4 * 2.2 * 3.6 = 31.68 km/h), the interval measured by TIM3 is (1/f)/[1/(64M/7814)] = 8190/f = 8190/4 = 2048; the value in EBiCS is [(int)2200 / ((int)2048 >> 3)] * 36 / 10 = 28.8, nearly 2 km/h error produced.
you can draw two functions in the calculator
y = 2.2*3.6*x
y = 3.6*floor(2200/floor(8190/x/8))
to see the maximum error 4.3 km/h. 29 km/h will be shown as 25 km/h!
I have extracted related code from your project (with many unindented code):
int main(void)
{
...
while (1)
{
...
// SPEED signal processing
if (ui8_SPEED_flag) {
if(uint32_SPEED_counter > 200) { //debounce
MS.Speed = uint32_SPEED_counter;
uint32_SPEED_counter = 0;
ui8_SPEED_flag = 0;
}
}
...
if (ui32_tim3_counter > 500) {
if(uint32_SPEED_counter > 127999)
MS.Speed = 128000;
...
ui8_slowloop_counter++;
if (ui8_slowloop_counter > 3) {
ui8_slowloop_counter = 0;
send_ant_page(0, &MS, &MP);
}
ui32_tim3_counter = 0;
}
...
}
}
// elapse frequency of TIM3 is 64M/7814 (Hz), interval is about 122.1 us
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
if (htim == &htim3) {
if(ui32_tim3_counter < 32000)
ui32_tim3_counter++;
...
if (uint32_SPEED_counter < 128000)
uint32_SPEED_counter++; //counter for external Speed sensor
}
}
void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin)
{
...
//Speed processing
if(GPIO_Pin == Speed_EXTI5_Pin)
ui8_SPEED_flag = 1; //with debounce
}
void send_ant_page(uint8_t page, MotorState_t* MS, MotorParams_t* MP)
{
...
// *3,6 for km/h then *10 for LEV standard definition.
uint16_t speedx10 = MP->wheel_cirumference / ((MS->Speed*MP->pulses_per_revolution) >> 3) * 36;
ui8_tx_buffer[9] = speedx10 & 0xFF; //low byte of speed
ui8_tx_buffer[10] = speedx10>>8 & 0x07; //lower 3 Bytes(needs correction) of high byte
...
HAL_UART_Transmit_DMA(&huart1, (uint8_t *)&ui8_tx_buffer, 12);
}
Thanks.