BLDC motors drive efficiency in robotics

Brushless DC (BLDC) motors offer some significant advantages over other categories of electric motors, such as brushed motors. Compared with the latter, which are cheaper, BLDC motors require a hardware/software control system for correct speed and torque regulation. However, their main advantage is an accurate regulation of the engine performance under any load condition along with higher efficiency and lower response times.

Unlike brushed motors, where the interaction between brushes and commutator creates friction and the formation of electric arcs, BLDC motors use a rotating magnetic field generated by a suitable electronic circuit for commutation. This achieves several significant advantages, including:

• Improved energy efficiency: This aspect has today become paramount, considering that a recent report produced by the International Energy Agency (IEA) found that 40% of all global electricity is used to power electric motors.
• Accurate speed and torque control
• High durability: Unlike brushed motors, in which performance suffers a degradation due to brush wear and the formation of arcs, the BLDC motor maintains its performance over time.
• Low noise
• Reduced electromagnetic interference (EMI)
• Reduced weight and size

Thanks to these properties, BLDC motors are widely used in applications ranging from the smallest electric drives, such as computer hard drives, to motors used in electric vehicles (EVs) and up to the largest robotic arms used in robotics and industrial automation (Figure 1).

The main disadvantage of BLDC motors is the complexity of the control circuit and the resulting increase in costs. In fact, the electronic switching technique requires driver circuits capable of generating electrical signals with precise timing.

Today, however, designers can count on wide commercial availability of power devices for BLDC motor drivers, both as standalone and integrated with a microcontroller (the controller), which simplify the design and reduce the overall cost of the solution.

The controller

In addition to regulating speed and torque, the controller can start, stop and reverse the motor’s rotation. It can also directly receive information related to the rotor position (sensor-type control) or deduce it by applying suitable algorithms, implemented at a firmware or hardware level (sensorless control). The electrical impulses (amplitude and duration) applied to the power MOSFETs are determined according to the position of the rotor.

Sensored BLDC motors are typically used in applications that require starting the motor under load, as the exact position of the motor is immediately available. The position can be determined using:

• Hall-effect magnetic sensors: These sensors are very robust and capable of operating even in the harshest operating conditions, such as automotive applications.
• Rotary encoders: By translating mechanical motion into electrical impulses, encoders provide essential data like position, speed and direction. Depending on the technology used, the encoders can be optical (Figure 2), capacitive or magnetic.
• Variable reluctance sensors: These sensors produce an electrical signal proportional to the displacement of a magnetically conductive or permeable object relative to a coil.

The sensorless controller instead detects the current position of the rotor by estimating the back electromotive force (EMF), which is the voltage created in the stator windings by the rotating armature. By measuring the back EMF, the position of the rotor can be determined. The closer the magnet is to the rotor, the higher the back EMF.

The controller is a component whose design or selection must be conducted with high accuracy. The main functions performed by a motor controller are as follows:

• Regulation of the speed and direction of the motor
• Torque regulation: By monitoring the load applied to the motor, the controller can determine the corresponding torque value to be delivered for the most efficient use of the motor.
• Management of the engine start and stop phases, which normally takes place gradually, following precise acceleration/deceleration profiles
• Protection against overvoltage or overcurrent
• Management of engine parameters to maximize the efficiency

In the past, controllers were circuits based on the use of many discrete components, as well as a microcontroller capable of executing the firmware with the control algorithms. The current trend is to use highly integrated solutions that incorporate both power and control functions in a single chip.

BLDC applications

The main advantages of a BLDC motor are derived from its construction characteristics. For example, the electronic commutation provides better management of the current, with consequent increase in torque, better speed control and therefore better overall motor performance.

Because there are no mechanical parts subject to wear, BLDC motors are virtually maintenance-free and guarantee high durability and efficiency. They also offer extremely low levels of EMI and noise. For all these reasons, BLDC motors are widely used in devices and systems like:

• Industrial applications
• EVs and e-mobility
• Unmanned aerial vehicles and drones
• Hard drives
• Consumer electronics
• Robotics and industrial automation

In particular, by enabling the automation of critical processes that improve worker safety, expedite production and increase productivity, industrial robotic arms are assisting businesses in strengthening their competitive advantage and maintaining low prices.

Robotic arms are quick, accurate and programmable to carry out a limitless number of jobs in a range of applications, including factories, warehouses and even farms and agriculture. In addition to providing improved safety, robotic arms achieve great efficiency and productivity thanks to their flexibility and accuracy in movement and positioning.

Thanks to the ability to accurately regulate rotational speed and torque, BLDC motors are widely used in this type of application, in which other essential requirements are reliability, durability and good immunity to electromagnetic radiation.

Design considerations

The design of a BLDC controller requires significant technical skills, both in terms of hardware and software. Although commercially available integrated solutions can meet the requirements of most applications, there may be specific cases in which it is necessary to adopt a custom solution.

Driver

To implement electronic commutation, power transistors capable of withstanding high voltages and currents are normally used, such as silicon (Si) MOSFETs and IGBTs, silicon carbide (SiC) MOSFETs and gallium nitride (GaN) HEMTs.

The most classic BLDC controller uses a half-bridge topology, which, compared with a full H-bridge solution, uses only two switches: one transistor on the high side and one on the low side. Because three-phase BLDC motors are used in most robotics and industrial automation applications (better-performing and more efficient than single- or dual-phase versions), the typical controller scheme will include three half-bridges. Figure 3 shows the diagram of a three-phase driver that uses a highly integrated gate driver.

Control of a three-phase BLDC motor is divided into six steps, required by a complete commutation cycle, to energize all three stator windings. Each pair of transistors shown in Figure 3 manages the commutation of a motor phase.

By sequentially turning the high-side and low-side transistors on and off, current can flow through the stator windings, causing them to rotate. High-side switches are typically controlled using the pulse-width modulation (PWM) technique, which converts the DC input voltage into a modulated drive voltage. The use of PWM allows for more efficient management (especially during startup) and ensures more precise control of speed and torque.

The choice of the PWM frequency is very important. Most designers generally opt for a compromise between switching losses (more sensitive at high frequencies) and ripple currents (which are mainly generated at low frequencies). The use of GaN devices can drastically reduce switching losses even at high frequencies.

Control scheme

Controlling a BLDC motor can be accomplished by using different current-switching techniques. The three main techniques adopted by designers are:

• Trapezoidal: It is the simplest technique, in which two windings (one “high” and one “low”) are energized at each step, while the other winding remains floating. The disadvantage of the trapezoidal method is that the stepped commutation causes the torque to oscillate, especially at low speeds, causing annoying motor vibrations.
• Sinusoidal: This technique provides smoother current switching between phases and reduces torque ripple. Unlike the trapezoidal technique, all three coils remain energized with a drive current 120˚ out of phase from each other. The result is a much smoother power delivery, at the expense of greater implementation complexity, especially at high speeds.
• Field-oriented control (FOC): This technique, based on the measurement and regulation of the stator currents, ensures that the angle between the rotor and the stator flux is always 90˚. Compared with the sinusoidal method, this technique is more efficient at high speeds and offers the capability to adapt to dynamic variations of the dynamic load. The result is virtually zero torque ripple at both low and high engine speeds.

Closed and open loop

To determine the current position of the rotor, designers have two available techniques: closed-loop control and open-loop control.

• Closed-loop control: As previously mentioned, this technique uses positional sensors, such as Hall sensors integrated into the stator and arranged at regular intervals, typically 60˚ or 120˚. In this case, the switching sequence is determined based on the state of the Hall sensors. At any instant, at least one of the sensors is activated by one of the magnetic poles of the rotor, generating an associated voltage pulse.
• Open-loop control: This sensorless technique uses the back EMF. As a result of the motor’s coils rotating in relation to a magnetic field, back EMF is a voltage that operates in the opposite direction to current flow. As the EMF is proportional to the angular speed of the motor, its monitoring performed by a properly programmed microcontroller can determine the relative positions of the stator and rotor without the need for Hall-effect sensors. This simplifies motor construction, reducing costs and eliminating additional wiring and connections to the motor, thus improving reliability. The limitation of this technique is that it is not possible to detect the position of the motor at startup. A solution to this problem is to run the motor with an open-loop control until sufficient EMF is generated to determine the position of the rotor and stator.

Discrete vs. integrated controller

Choosing between a discrete or integrated motor control solution presents a major dilemma for the designer. Generally, commercially available driver circuits can cover the needs of most applications, with the advantage of simplifying the design, shrinking time to market and reducing application development and testing costs. Potential disadvantages to consider are the maximum usable power level (commercial components have a power limit) and the need to replace the entire controller in case of failure.

As a rule, if the powers involved are high, a design based on discrete components makes more sense, where Si-, SiC- or GaN-based switches, more suitable for the specific application, can be used. For low-power applications, commercially available integrated solutions are often preferable to a custom design.

It should also be emphasized that the non-negligible complexity of the control algorithms (trapezoidal, sinusoidal or FOC) can be overcome by the designer in two ways: by using software libraries provided by many chip manufacturers and using ICs with an integrated driver.

Together with evaluation kits, these software libraries speed time to market by solving potential motor driver integration issues at the source.

One example of a device with an integrated driver is Texas Instruments’ MCT8316Z, a 40-V max, 8-A peak, sensored trapezoidal-control, three-phase BLDC motor driver. For users running 12- to 24-V BLDC motors, the MCT8316Z offers a sensored trapezoidal solution on a single chip without any code. To allow high-power–drive capability, the MCT8316Z includes three half-H bridges with a 40-V absolute maximum capacity and an extremely low R_DS(on) of 95 mΩ (high side and low side combined). External sense resistors are not required because current is measured internally utilizing a current-detecting function.

A fixed-function state machine in the MCT8316Z implements sensored trapezoidal control (Figure 4), eliminating the need for an additional microcontroller to turn the BLDC motor. To enable sensored trapezoidal BLDC motor control, the device combines three analog Hall comparators for position detection.