Motor Speed Fluctuations with DRV8870DDAR : Fixing the Issue

Motor Speed Fluctuations with DRV8870DDAR: Fixing the Issue

Motor speed fluctuations in a system using the DRV8870DDAR motor driver can be frustrating, especially when you require consistent performance. These fluctuations can stem from several possible sources. Let’s break down the potential causes and provide a step-by-step guide to diagnosing and fixing this issue.

1. Understanding the DRV8870DDAR Motor Driver

The DRV8870DDAR is a popular motor driver IC for controlling brushed DC motors. It offers features such as adjustable speed and direction control. However, issues like speed fluctuations can occur if the driver or its surrounding components are not correctly set up or maintained.

2. Possible Causes of Motor Speed Fluctuations

Here are the main causes of motor speed fluctuations in systems using the DRV8870DDAR:

a. Power Supply Issues

One of the most common causes of motor speed instability is an insufficient or noisy power supply. The DRV8870 needs a stable voltage and current supply to maintain consistent motor operation.

Cause: A fluctuating or noisy power source can lead to inconsistent motor speed. Solution: Ensure that the power supply provides a stable voltage within the required range. Use Capacitors close to the motor driver to smooth out any voltage spikes or dips. A high-quality, regulated power supply is key. b. Inadequate Decoupling capacitor s

Capacitors are essential in filtering noise and stabilizing voltage fluctuations. If the decoupling capacitors near the DRV8870 are missing or of insufficient value, the motor may experience speed fluctuations.

Cause: Lack of proper decoupling can cause electrical noise or voltage dips that interfere with motor control. Solution: Add adequate decoupling capacitors (e.g., 0.1 µF ceramic and 10 µF electrolytic) close to the DRV8870 power pins. These capacitors help filter out noise and stabilize the power supply to the motor driver. c. Incorrect PWM Frequency

The DRV8870 uses Pulse Width Modulation (PWM) to control the motor speed. If the PWM frequency is set too low or too high, it can lead to motor speed instability.

Cause: A low or unstable PWM frequency might result in uneven motor performance or jittering. Solution: Adjust the PWM frequency to a range that suits your motor's specifications (typically around 20 kHz to 100 kHz for most motors). Check the datasheet of the DRV8870 for recommendations on optimal PWM settings. d. Motor Back EMF (Electromotive Force) Interference

When the motor is running, it generates a back EMF, which can cause speed fluctuations if not properly managed.

Cause: The back EMF from the motor can interfere with the motor driver’s ability to maintain a constant speed. Solution: Ensure that proper flyback diodes are installed to protect the DRV8870 from voltage spikes caused by the back EMF. Also, check that the motor wiring is securely connected and that no shorts or bad connections are present. e. Faulty or Loose Wiring

Loose connections or faulty wiring can cause intermittent behavior in the motor control system, leading to inconsistent motor speed.

Cause: Poor connections can cause intermittent power loss or signal instability, leading to motor fluctuations. Solution: Inspect all wiring and connections for tightness and proper soldering. Ensure that the connections are secure and there is no visible damage or wear. f. Overheating of the Motor Driver

Excessive heat can cause the DRV8870 to throttle back its output power, leading to fluctuations in motor speed.

Cause: Overheating can cause thermal shutdown or a reduction in output current, impacting motor speed. Solution: Ensure that the DRV8870 is properly heat-sinked, especially if you're using a high-current motor. If necessary, use a fan or improve airflow around the motor driver.

3. Step-by-Step Solution for Fixing Motor Speed Fluctuations

If you're experiencing motor speed fluctuations, follow these steps to diagnose and fix the issue:

Step 1: Check the Power Supply Verify that the power supply is providing a stable voltage within the range specified in the DRV8870 datasheet. Use a multimeter to measure the voltage at the power input pins of the DRV8870 and ensure it stays constant during motor operation. If you notice any instability, replace or upgrade the power supply. Step 2: Inspect and Add Decoupling Capacitors Check if there are any capacitors (e.g., 0.1 µF ceramic, 10 µF electrolytic) near the DRV8870’s power input. If capacitors are missing or improperly rated, add the correct values as specified in the datasheet. Step 3: Adjust the PWM Frequency Using a microcontroller or PWM signal generator, ensure that the PWM frequency is set to an appropriate range (typically 20 kHz to 100 kHz). If the PWM frequency is too low, increase it to avoid noticeable speed fluctuations caused by low-frequency noise. Step 4: Check the Motor’s Back EMF Protection Ensure that proper flyback diodes are in place to protect the motor driver from back EMF. If necessary, install additional diodes to reduce interference from the motor’s back EMF. Step 5: Verify Wiring and Connections Inspect all wiring, ensuring that no loose or damaged connections exist between the motor, driver, and power supply. Re-solder any suspect connections and tighten any loose wires. Step 6: Ensure Proper Cooling If the motor driver is overheating, consider adding a heatsink or improving ventilation around the DRV8870. Check the current being drawn by the motor and ensure it does not exceed the DRV8870's maximum rated output.

4. Conclusion

Motor speed fluctuations with the DRV8870DDAR can usually be traced back to power supply issues, inadequate capacitors, incorrect PWM settings, or motor driver overheating. By following these diagnostic steps and making the necessary adjustments, you can eliminate these fluctuations and achieve smooth, consistent motor control.

If the problem persists after addressing these factors, consider testing with a different motor or motor driver to rule out any component-specific issues.