Ways to Reduce Audible Noise in Motion Control Applications

With the advent of open floor plans in homes and offices and the growing shift toward hybrid electric and electric vehicles, there is an increasing need for quieter, more efficient motor control . Even very small acoustic differences can have a significant impact on audible noise.

Leveraging advanced real-time control technologies such as motor control circuits with higher power density, higher integration, and more efficient systems can help you achieve better system acoustic performance. Some other strategies include vector field-oriented control (FOC) algorithms using continuous pulse width modulation (PWM) , specific control algorithms to reduce vibration, and integrated control that applies dead time compensation and PWM generation to reduce audible noise.

Because these different products and strategies can all reduce audible noise in motion control applications, it can be difficult to determine which strategy is better for your application. In this article, I will list three excellent ways to reduce audible noise in motion control applications, using the BLDC integrated control gate driver as an example.

PWM

The first strategy used to reduce audible noise in motor control applications is continuous PWM. PWM
is a technique that turns a transistor on and off to produce an output waveform so that the motor voltage is either high or low at a given time. The inductor in the motor then filters these waveforms so that they essentially average the output waveform. Adjusting the duty cycle (the ratio of the waveform's on time to its off time) will change the average voltage. Figure 2 shows an example of using PWM to generate a sine wave.

Figure 2: Example of using PWM to generate sine waves

For example, the Texas Instruments MCF8315A BLDC Integrated Control Gate Driver is a sensorless FOC motor driver that enables continuous and discontinuous space vector PWM schemes. Continuous modulation helps reduce current ripple in low-inductance motors , but results in higher switching losses because all three phases are interleaved. Discontinuous modulation has lower switching losses (because only two phases are interleaved with each other at a time), but the current ripple is higher. In Figures 3 and 4 you can see the difference between continuous and discontinuous PWM.

Figure 3: Relationship between Phase Current Waveforms and Fast Fourier Transform (FFT) Discontinuous PWM

Figure 4: Relationship between phase current waveform and FFT continuous PWM

Dead time compensation

A second strategy for reducing audible noise in motor control applications is dead time compensation. In motor control applications, breakdown can be avoided by inserting dead time between the switches of the high-side and low-side MOSFETs in the half-bridge. When dead time is inserted, the expected voltage at the phase node will differ from the applied voltage, and the phase node voltage will introduce unwanted distortion in the phase current, resulting in audible noise.

To manage this additional noise, engineers can use resonant controllers to integrate dead-time compensation to control the harmonic components in the phase currents, thereby mitigating the current distortion caused by dead time, as shown in Figure 5.

Figure 5: Sensorless FOC dead time compensation analysis

For example, TI's MCF8316A BLDC integrated control gate driver, a sensorless FOC motor driver, uses this built-in capability to optimize acoustic performance across multiple motor frequencies, as shown in Figure 6.

Figure 6: Implementing PWM modulation and dead time compensation to optimize MCF8316A acoustic performance

Convertible direction mode

A final strategy for reducing audible noise in motor control applications is variable direction mode. In trapezoidal commutation, there are two main configurations: 120 degrees and 150 degrees. The 120-degree trapezoidal commutation may result in more acoustic noise because the longer high-impedance period results in larger torque ripples, as shown in Figures 7 and 8. 150-degree trapezoidal commutation can only operate at low speeds because the window period for detecting zero crossings is very short.

To address these challenges and improve acoustic performance, engineers can build motor drive systems that can dynamically switch between 120-degree trapezoidal commutation and 150-degree trapezoidal commutation. This dynamic modulation can improve
the overall acoustic performance during BLDC motor control.

Figure 7: Phase currents and FFT - 120 degree commutation

Figure 8: Phase currents and FFT - 150 degree commutation

For example, TI's sensorless BLDC integrated ladder control gate drivers such as the MCT8329 and MCT8316 use this built-in capability to optimize acoustic performance across multiple motor frequencies, as
shown in Figure 9.

Figure 9: Implementing a switchable steering mode with dynamic modulation to optimize MCT8316A acoustic performance

Conclusion

TI is increasing its investment in motion control technology to help build more efficient acoustically sensitive systems with building blocks designed to meet acoustic requirements. As you design your system, remember to employ these three excellent ways to reduce audible noise in motor control applications.