When you’re striving to drive high efficiency from your motor control systems, you need very accurate and quick winding current measurement results from the current-sense amplifier. But not all of the measurement methods available are tightly in sync with this objective.
Many of the traditional measurement tactics have limitations on accuracy. They tend to involve complex calculations that require a lot of signal processing and, as a result, significant computing power. For example, you could use three current sensors, applying one on each leg of the driver circuit. The signals, which include flyback currents and return currents from the other phases, are fast, bidirectional, and complex. The waveforms have to be sampled, disassembled, and combined to calculate the winding currents. There’s a low-side current-sensing approach, but this comes with ground variations due to additional resistance between load and ground. It measures flyback and return current, but the winding current has to be calculated in the microcontroller. High-side current sensing is another method, where the current-sense amplifier inputs must sense at the supply voltage. But this approach can’t measure flyback current nor can it measure the winding current.
Now, there’s a way to measure current in the windings directly. This in-line current-sensing approach uses Maxim’s new MAX40056 bidirectional current-sense amplifier with pulse-width modulation (PWM) rejection. The MAX40056 connects directly to motor windings to provide 0.3% accurate, full-scale direct winding current measurements 4x faster than competitive offerings. No special calculations are needed, and the PWM rejection feature yields faster measurement results. Since the IC has bidirectional sensing, it can work with any motor application, including servo motors, autonomous and high-precision cars, battery stack monitors, and robotics.
To learn more about DC motor current sensing in a PWM environment, watch the video featured below. Ashwin Badri Narayanan, a senior member of the technical staff for signal chain products at Maxim, demonstrates how the MAX40056 performs rejection of high common-mode PWM signals to accurately measure torque and speed in a three-phrase brushless DC motor system.