Testing the PWM and Braking functions of the DRV8871 driver and 775 DC motor with an AS5600 sensor.

The AS5600 rotary encoder was employed for positional feedback – bit overkill given the 12-bit resolution. There is also another how-to guide here for interfacing the AS5600 here.

A test rig was constructed with PLA, extruded from a 3D printer, and consists of two parts:

• The “nubbin” which connects the motor spindle to the magnet.
• The “cage” that spaces the magnetic end of the nubbin a suitable distance away from the sensor.

The data sheet notes that when both inputs go high, the output goes to ‘brake’, while if both inputs go low then the output goes to ‘coast’. So the first test is to see if there is a difference, and if so, is there much of an affect.

The program spins the motor up to speed* and then at 3000ms the brake is applied. After 4500ms, the test is halted. All data is output to the serial monitor with the time in millis( );

The code is provided for full transparency.

*data suggests that the motor gets to full speed in about 1 to 1.3 seconds.

#include <Wire.h>
#include "AS5600.h"

AS5600 as5600;  

const int IN1 = 5;
const int IN2 = 6;
long totalTimeElapsed = 0;

void setup() {
  Serial.begin(115200);
  Wire.begin();
  as5600.begin(4);  //set direction pin.
  as5600.setDirection(AS5600_CLOCK_WISE);  
  as5600.resetCumulativePosition();
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, LOW);
  Serial.println("time,  rotational speed");
  delay(500);
}

void loop() {
  //break test
   do {
      totalTimeElapsed = millis();
      digitalWrite(IN1, HIGH);
      digitalWrite(IN2, LOW);
      Serial.print(millis());
      Serial.print(",  ");
      Serial.println(as5600.getCumulativePosition());
    } while (totalTimeElapsed < 3000);
    digitalWrite(IN1, HIGH);  //change to LOW for coast
    digitalWrite(IN2, HIGH);  //change to LOW for coast
    do {
      totalTimeElapsed = millis();
      Serial.print(millis());
      Serial.print(",  ");
      Serial.println(as5600.getCumulativePosition());
    } while (totalTimeElapsed < 4500);
  while (1);
}

The datasheet makes note that it’s better to switch between braking and driving; opposed to driving from a ‘coasting’ position. This test is designed to measure that by looping through the PWM range (0-255), pausing at each point for 50ms to take and send a reading.

The setup for the code is identical to above, so this is the pertinent loop.

void loop() {
  for (int i = 0; i < 256; i++) {
    totalTimeElapsed = millis();
    digitalWrite(IN1, HIGH);
    analogWrite(IN2, 255 - i);
    Serial.print(millis());
    Serial.print(",  ");
    Serial.print(i);
    Serial.print(",  ");
    Serial.println(as5600.getCumulativePosition());
    delay(50);
  }
  digitalWrite(IN1, LOW);
  digitalWrite(IN2, LOW);
  while (1);
}