BLDC Motors and Integrated Motor Drivers Are Key to Efficiency in Robotics and Drones

Since their development in the 1960s, brushless direct current (BLDC) motors have proven to be more efficient and longer lasting than the brushed direct current (DC) motors that preceded them. While high-powered industrial applications turned to synchronous alternating current (AC) motors, many other applications now use BLDC motors.

Today, BLDC motors are part of consumers’ everyday lives. They can be found in battery-operated tools like drills and leaf blowers, in household appliances like washing machines and printers, and in e-bikes and electric cars. In industrial settings, BLDC motors are used in motion control and material handling applications. BLDC motors also power unmanned ground vehicles (UGVs), drones and similar unmanned aerial vehicles (UAVs), and surgical robots and assistive exoskeletons.

While brushed DC motors relied on metal or carbon commutator brushes to conduct power to motor windings, BLDC motors are contactless. The lack of friction and wear makes them more efficient, reduces maintenance requirements, and extends motor life. They also perform better, boasting higher speeds, higher torques, and a higher power-to-weight ratio. Sophisticated control systems allow BLDC motors to almost instantaneously change speed or torque as well as providing precision positioning and ensuring safety.

The performance available from sophisticated BLDC motor drivers makes these motors and their control systems attractive to engineers designing modern robotic and drone applications that require miniaturization, speed, precision, safety, and minimal maintenance.

BLDC motor basics

BLDC motors’ three-component makeup is deceptively simple. A stationary stator hosts two to eight sets of copper windings distributed around a circumference that is surrounding, surrounded by, or parallel to a rotor that contains permanent magnets (Figure 1). A motor controller connects to the stator, accessing position data and powering the windings.

Figure 1: The controller of a three-phase BLDC motor switches which stator windings (U, V, and W) are energized and their polarity, changing the magnetic field orientation. The permanent-magnet-containing rotor (in blue) rotates to align with the stator’s magnetic field. (Image source: Qorvo)

Energizing a set of windings in the stator creates a magnetic field to which the rotor’s permanent magnets react. The attraction between opposite magnetic poles causes the rotor to spin. Before the rotor can align itself with the stator’s magnetic field, the controller switches which windings are energized, shifting the orientation of the magnetic field to keep the rotor in motion.

In practice, the current pulses that the controller sends to the stator change from on to off and switch polarity with such frequency that the current can be represented by a waveform. The switching scheme shown in Figure 1 is represented by a trapezoidal wave. Other motors—including permanent magnet synchronous motors (PMSMs), which are similar to BLDC motors in construction but use a varying current to rotate a magnetic field to which the rotor becomes locked—have sinusoidal waveforms. Adjustments to these waves’ amplitude and phase alter the speed of the motor and the available torque.

The controller also receives constant feedback from position sensors such as Hall effect sensors or optical encoders. In sensor-less BLDC motors, measurement of back electromotive force (BEMF), the current generated in unenergized windings by the magnetic field produced by energized windings, is used to determine the rotor’s position.

Motor driver development

Given the complex architecture needed to monitor, power, and control BLDC motors, it’s no surprise that older BLDC motor controllers using solid-state electronics required their own cabinet space and thick power and data cables running to the motors in industrial settings. Increasingly sophisticated integrated circuits (ICs) have slimmed down motor controllers until they can fit onto a printed circuit board (PCB). Despite the miniaturization, the capabilities of today’s motor controllers continue to expand.

One example is Qorvo’s ACT72350 three-phase BLDC motor driver (Figure 2), which combines a configurable analog front-end (AFE), a power management module that adapts to a variety of supply configurations, and an application-specific motor driver (ASPD) in a single 9 mm by 9 mm quad flat no-lead (QFN) surface-mount device.

Figure 2: The ACT72350 integrated three-phase BLDC motor driver combines AFE circuitry with configurable power management in a compact surface-mount package. (Image source: Qorvo)

The ACT72350’s configurable AFE has three differential programmable-gain amplifiers, four single-ended programmable-gain amplifiers, two 10-bit digital-to-analog converters, and ten comparators that allow it to act as a bridge between sensors and control circuitry. It also receives pulse-width modulation (PWM) control signals from an external microcontroller unit (MCU) via a serial peripheral interface (SPI).

The configurable power management module lets the ACT72350 accept DC input voltages between 25 V and 160 V, including battery power up to a 20S standard (nominally 72 V or 84 V when fully charged). Its high-voltage switching supply provides a stable output voltage of 12 V or 15 V. It also supplies stable 5 V, 200 mA power to the ACT72350’s modules and the MCU.

The ACT72350’s ASPDs can drive the motor with a half-bridge, H-bridge, or three-phase architecture (Figure 3). Three high-side gate drivers at 160 V and three low-side gate drivers at 20 V each have 2 A (source)/2 A (sink) gate driving capabilities to enable fast switching for greater motor speed.

Figure 3: The block diagram of the ACT72350’s ASPD module shows the high-side and low-side gate drivers. The nBRAKE pin is activated by an external controller to stop the motor’s rotation for safety reasons. (Image source: Qorvo)

The ACT72350 reduces the number of electronic components needed to control a BLDC motor. The unit combines modules that manage analog signal inputs, accept and standardize power inputs, and drive the motor in a single, compact surface-mount package. At the same time, the ACT72350 preserves design flexibility by allowing any chosen MCU to provide control signals via SPI.

Drone deployment

Simplifying the control electronics for BLDC motors into one integrated package plus an MCU is key for applications where space and weight are at a premium, like drone aircraft and other UAVs. Designers of these systems choose BLDC motors to make the best use of every square millimeter of space and every gram of weight, and the motor drivers need to contribute to that. BLDC motors’ high torque-to-weight ratio means they are relatively light for the power they provide to drone rotors or propellers. Their energy efficiency of over 85% means they can carry greater payloads or fly longer on a single battery charge.

A space efficient motor driver like the ACT72350 combines multiple functions into a small package while providing high-quality motor performance. Instead of requiring a control cabinet and thick, heavy cables, designers of drones and UAVs can employ several ACT72350s, a battery pack, and the MCU of their choice, all deployed on the vehicle. High-voltage gate drivers on the ACT72350 support high-speed switching for smooth operation, freeing the MCU on the flight control board for higher-level flight instructions.

Space and weight efficiency may not be as important for UGVs but their designers still choose BLDC motors for their high torque capabilities in propulsion applications and their ability to deliver precise motion in steering applications. BLDC motors are also prized in these applications because of their low-maintenance requirements, an especially important consideration in outdoor environments.

Reimagining robotics

A low-maintenance BLDC motor is also advantageous in robotics where it ensures long-term reliability in high cycle applications. BLDC motors move joints in industrial robotic arms, exoskeletons, material grippers and manipulators, prosthetics, and humanoid companion robots.

In all these applications, the lightweight, compact design of BLDC motors contributes to their efficient operation, high precision, and the range of motion they permit. The high torque-to-weight ratio that benefits drones also lets BLDC motors power robotics without adding weight or bulk. With its integrated AFE providing up to 2 A each of source and sink drive capability, the ACT72350 is set up to accept signals from multiple rotor position sensors or to measure BEMF, ensuring precise speed control in a robotics application.

Safety is also paramount in these applications, where equipment is often operating near humans or near high-value goods or equipment. The AFE allows the system to react instantaneously to overtemperature, overvoltage, and overcurrent conditions that could pose a threat to electronics or nearby humans. The ACT72350 can also supply emergency breaking via its ASPD’s nBRAKE pin. A 50 µsec signal from the MCU or a redundant safety MCU to the nBRAKE pin disables all the high-side gate drivers, while low-side gate drivers carry out braking and PWM inputs are ignored.

Conclusion

Designers choose BLDC motors for many applications in arenas such as medical, consumer products, automotive, recreational, industrial, and more. To take advantage of the efficiency, torque capability, high speeds, precision, and low maintenance requirements of BLDC motors, designers must also choose motor drivers that can handle the complex combination of analog sensor inputs, digital commands from the MCU, power sources with varied voltages and currents, and the fast-switching current pulses needed to power the motor windings. Motor controllers like Qorvo’s ACT72350 that combine these capabilities in a compact package contribute to the success of BLDC motors in advanced applications.