Current Limiting Stepper Driver with DRV8871

Got big motors to drive? Call in the choppers!

by Bill Earl

published August 22, 2016, last edited August 22, 2016

posted in Adafruit Products Robotics & CNC

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Beginner

Project guide

Overview

A Stepper Smörgåsbord

The Maker movement has created a growing demand for stepper motors to build into projects like 3D printers and CNC machines. The market has responded and now there is a huge variety of reasonably priced stepper motors to choose from.

But not all motors are created equal. Two identical looking motors may have the same NEMA size designation, yet have completely different electrical specifications. So it is important to choose an appropriate driver for your stepper. At best, a poor match will result in poor performance. At worst, a motor or driver may go up in smoke.

Basic Motor Drivers

The first generation of hobby-level motor controllers like the V1 Motor Shield were almost all based on the ubiquitous L293D chip - a very basic dual H-bridge chip. These bridge chips have no current limiting, so you need to apply Ohm's Law to determine a supply voltage that would not exceed the chip's maximum current rating.

Many high-performance steppers have a very low phase resistance so the maximum 'safe' voltage is impossibly low. These motors are simply not compatible with those first-generation hobbyist controllers.

The Adafruit V2 motor shield uses the more modern TB6612B dual bridge chips. These are more efficient and have a higher current rating. But the TB6612 has no current limiting either, so many high-performance motors are still off-limits for even the V2 shield.

Choppers to the rescue!

Current limiting drivers - also known as "Chopper" drives - are capable of handling a wider range of stepper motor and power supply combinations. Current limiting was previously only available in expensive industrial controllers. But thanks largely to the increased demand from Makers, current limiting drivers are now becoming available at reasonable prices

Motor drivers like the DRV8871 allow you to use much higher drive voltages to get best results from these high performance motors. The higher drive voltage helps to overcome the inductance of the motor windings to produce more torque at the start of a step. But to keep current to a safe level, it 'chops' the current output when it exceeds the preset limiting level.

The DRV8871 bridge chip is capable of much higher supply voltages and currents than an L293D. However, it is only a single bridge chip. We will need to combine two of them to drive the two phases of a typical stepper motor. In this tutorial, we'll wire up 2 DRV8871 breakout boards to drive a high-torque bipolar stepper motor.

For more information on how a current-limiting "chopper" driver interacts with the motor, please read this guide: Matching the driver to the stepper

Matching the Driver to the Stepper

Page last edited August 21, 2016

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Materials

For this tutorial you will need the following:

2x DRV8871 Breakout Boards

Each DRV8871 breakout board contains a single H-bridge. Since the stepper motor has 2 sets of windings, we will need two of them: One DRV8871 for each winding.

A Microcontroller

We are using an Arduino UNO R3. But most Arduino compatibles will work as well.

Breadboard

We are using a full-size board here. But the circuit would fit on a half-size breadboard just as well.

Hookup Wire or Jumpers

A Stepper Motor

In the example circuit, we are using a bipolar stepper with the following specs:

NEMA 17
Torque: 76 oz-in - 5.47 kg-cm
Step Angle: 1.8
Winding Resistance: 1.8 Ohms/Phase
DC 2 A/Phase
12-24VDC (Recommended)

But this tutorial will work with many NEMA-17 and NEMA-23 bipolar stepper motors -including the NEMA-17 motor from our store.

Just having current limiting is not enough to protect your motor. You need to make sure that the current limit is set to an appropriate level for your motor. The DRV8871 comes configured to limit the current at 2A. Check your motor specifications. If it is rated for less than 2A current, you will need to change the 'RLIM' resistor to reduce the current limit for the DRV8871.

A Power Supply

We are using a 12v/5A supply in our example circuit.

Using our example motor with a 1.8 ohm phase resistance, this would draw 6.7A per phase and would quickly burn out a V1 or V2 motor shield.

But the DRV8871 breakout is configured to automatically limit the current to a maximum of 2A. This is a safe level for both the motor and the driver.

With some motors, the DRV8871 chips will get quite hot. Sometimes hot enough to trip the thermal shutdown circuit. Adding heat sinks and/or a fan will help dissipate the extra heat.

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Wiring

Wiring up the 2 DRV8871s is farly straightforward:

First assemble the DRV8871 breakouts as instructed in this tutorial:

From the Arduino to the DRV8871 Breakouts (we'll call them "A" and "B"), make the following connections:

GND -> (both breakouts)
Pin 2 -> IN1 (breakout A)
Pin 3 -> IN2 (breakout A)
Pin 4 -> IN1 (breakout B)
Pin 5 -> IN2 (breakout B)

On the motor side:

Connect one pair of motor leads to the MOTOR terminal block on breakout A.
Connect the other pair of motor leads to the MOTOR terminal block on breakout B.
Connect the power supply ground to the "-" terminal of the POWER terminal block on both breakouts.
Connect the power supply positive wire to the "+" terminal of the POWER terminal block on both breakouts.

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Testing

To test out our current limiting stepper driver, we'll use the standard Arduino Stepper library. This is included with the Arduino IDE, so no additional libraries need to be installed.

Copy the code below and upload it to your Arduino.

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#include <Stepper.h>

// A 200-step motor connected on pins 2-5
Stepper stepper(200, 2,3,4,5);

void setup() 
{
  // Rotate at 20 rpm:
  stepper.setSpeed(20);

  Serial.begin(9600);
  Serial.println("Stepper Test!");
}

void loop() 
{
  // Step forward 1000 steps
  Serial.println("forward");
  stepper.step(1000);

  // Step in reverse 1000 steps
  Serial.println("backward");
  stepper.step(-1000);
}

#include <Stepper.h>

// A 200-step motor connected on pins 2-5
Stepper stepper(200, 2,3,4,5);

void setup() 
{
  // Rotate at 20 rpm:
  stepper.setSpeed(20);

  Serial.begin(9600);
  Serial.println("Stepper Test!");
}

void loop() 
{
  // Step forward 1000 steps
  Serial.println("forward");
  stepper.step(1000);

  // Step in reverse 1000 steps
  Serial.println("backward");
  stepper.step(-1000);
}

The motor should start turning back and forth - 1000 steps at a time.