brushless DC motor
A brushless DC motor (or BLDC motor for short) is a type of motor that uses a DC power supply and achieves electronic commutation through an external motor controller. Unlike brushed motors, BLDC motors rely on an external controller for commutation. Simply put, commutation is the process of switching the current in each phase of the motor to generate motion. Brushed motors have physical brushes, allowing for two commutation processes per revolution, while BLDC motors are brushless, hence their name. Due to their design characteristics, brushless motors can achieve any number of commutated pole pairs.
Compared to traditional brushed motors, BLDC motors offer significant advantages. These motors typically offer 15-20% higher efficiency; they eliminate the physical wear of brushes, reducing maintenance; and they provide a flat torque curve at all rated speeds. While BLDC motors are not a new invention, their widespread adoption has been slow due to the need for complex control and feedback circuitry. However, recent advancements in semiconductor technology, improved permanent magnet quality, and the growing demand for higher efficiency have led to BLDC motors replacing brushed motors in numerous applications. BLDC motors have found a market niche in many industries, including white goods, automotive, aerospace, consumer electronics, medical, industrial automation, and instrumentation.
As the industry moves towards requiring BLDC motors in more applications, many engineers are forced to turn their attention to this technology. While the fundamental elements of motor design remain applicable, the addition of external control circuitry introduces another set of design considerations. Among these numerous design challenges, the most critical is obtaining feedback on motor commutation.
Motor commutation
Before delving into the feedback options for BLDC motors, it's crucial to understand why they are necessary. BLDC motors can be configured as single-phase, two-phase, and three-phase; three-phase is the most common configuration. The number of phases matches the number of stator windings, while the number of rotor poles can be arbitrary depending on the application requirements. Because the rotor of a BLDC motor is affected by the rotating stator poles, the stator pole positions must be tracked to effectively drive the three motor phases. This requires a motor controller to generate a six-step commutation pattern across the three motor phases. These six steps (or commutation phases) move the electromagnetic field, which in turn causes the rotor permanent magnets to move the motor shaft.
Figure 1: Six-step commutation mode of BLDC motor
By employing this standard motor commutation sequence, the motor controller can effectively reduce the average voltage across the motor using a high-frequency pulse-width modulation (PWM) signal, thereby altering the motor speed. Furthermore, this setup significantly enhances design flexibility by allowing a single voltage source to be used for a wide variety of motors, even when the DC voltage source is significantly higher than the motor's rated voltage.
To maintain its efficiency advantage over brushed technology, a very tight control loop is required between the motor and the controller. This is where feedback technology becomes crucial; for the controller to maintain precise control of the motor, it must always know the exact position of the stator relative to the rotor. Any misalignment or phase shift between the expected and actual positions can lead to unexpected situations and performance degradation. Many methods can be used to achieve this feedback for BLDC motor commutation, but the most common are using Hall effect sensors , encoders, or resolvers. Additionally, some applications rely on sensorless commutation technology for feedback.
Location feedback
Since the advent of brushless motors, Hall effect sensors have been the primary means of achieving commutation feedback. Because three-phase control requires only three sensors and has a low unit cost, they are often the most economical choice for commutation from a purely BOM cost perspective. Hall effect sensors that detect rotor position are embedded in the motor stator, allowing the transistors in the three-phase bridge to be switched to drive the motor. The outputs of the three Hall effect sensors are typically labeled as U, V, and W channels. While Hall effect sensors effectively solve the commutation problem of BLDC motors, they only meet half of the requirements of a BLDC system.
Figure 2: Three-phase bridge driver circuit
While Hall effect sensors enable controllers to drive BLDC motors, unfortunately, control is limited to speed and direction. In three-phase motors, Hall effect sensors can only provide angular position within each electrical cycle. As the number of pole pairs increases, the number of electrical cycles per mechanical rotation also increases, and as BLDC becomes more widespread, the demand for precise position sensing increases accordingly. To ensure a robust and complete solution, BLDC systems should provide real-time position information, allowing the controller to track not only speed and direction but also travel distance and angular position.
To meet the demand for more stringent position information, a common solution is to add an incremental rotary encoder to the BLDC motor. Typically, an incremental encoder is added to the same control feedback loop system, in addition to a Hall effect sensor. The Hall effect sensor is used for motor commutation, while the encoder is used to track position, rotation, speed, and direction more precisely. Because the Hall effect sensor only provides new position information with each Hall state change, its accuracy is limited to six states per electrical cycle; for a bipolar motor, it's only six states per mechanical cycle. The necessity of both is obvious compared to the incremental encoder, which provides resolution in the thousands of PPRs (pulses per revolution) and can decode four times the number of state changes.
Figure 3: Six-step Hall effect output and trapezoidal motor phase
However, since motor manufacturers currently have to assemble both Hall effect sensors and incremental encoders onto their motors, many encoder manufacturers have begun offering incremental encoders with commutation outputs, often simply referred to as commutator encoders. These encoders are specifically designed to provide not only the traditional quadrature A and B channels (and in some cases, the index pulse channel Z "once per revolution"), but also the standard U, V, and W commutation signals required by most BLDC motor drives. This allows motor designers to avoid the unnecessary step of installing both Hall effect sensors and incremental encoders simultaneously.
While the advantages of this method are undeniable, it also involves significant trade-offs. As mentioned above, accurate commutation of a BLDC motor requires precise knowledge of the rotor and stator positions. This necessitates careful alignment of the commutation encoder's U/V/W channels with the BLDC motor's phase.
For optical encoders with fixed patterns on optical discs and Hall effect sensors that must be manually positioned, achieving proper alignment of the BLDC motor is both iterative and time-consuming. The alignment method also requires additional equipment, including a second motor and an oscilloscope. To align an optical encoder or a group of Hall effect sensors, a second motor must be used to drive the BLDC motor in reverse; then, as the motor rotates at a constant speed under the action of the second motor, the oscilloscope is used to monitor the back electromotive force (also known as reverse electromotive force or back EMF) of the three motor phases.
The U/V/W signals subsequently emitted by the encoder or Hall effect sensor must be compared with the back EMF waveform on an oscilloscope. If there are any discrepancies between the U/V/W channels and the back EMF waveform, phase adjustment is necessary. This process takes more than 20 minutes per motor and requires a significant amount of laboratory equipment, making it a major source of frustration when using BLDC motors. While optical commutator encoders solve the installation burden by using only one technology, their implementation also suffers from a lack of versatility. Because optical encoders use a fixed pattern on their optical discs, the number of motor poles, quadrature resolution, and motor shaft dimensions must be clearly understood before purchase.
Figure 4: Ideal alignment of commutation channel and motor phase
Capacitive commutator encoder
CUI Inc.'s enhanced commutation encoder solves both problems simultaneously. This encoder is based on the patented capacitor technology used in its AMT series. Optical encoders use very small LEDs that emit light through a disc (with regularly spaced slots) to generate an output pattern. The AMT encoder works similarly, but instead of transmitting light through LEDs, it transmits an electric field. A PCB rotor replaces the disc, containing sinusoidal patterned metal traces that modulate the electric field. The modulated signal is then sent back to the transmitter, where it is compared to the original signal via a proprietary ASIC. This technology, similar to that of digital vernier calipers, offers excellent reliability and accuracy.
Figure 5: Working principle of a capacitive encoder
The AMT31 series commutator encoders offer incremental outputs A/B/Z and commutator outputs U/V/W. With a design incorporating a capacitive ASIC and an onboard MCU, the encoder can generate digital outputs. This is crucial because it allows users to digitally set the encoder's zero position with the touch of a button. Simply lock the BLDC motor to the desired phase state and zero the AMT31 encoder using the AMTOneTouchZero™ module or the AMTViewpoint™ programming GUI. This eliminates the need to reverse the motor or use an oscilloscope to view the output signal, significantly reducing assembly time by 20 minutes. Thanks to capacitive technology, quadrature resolution and commutator output can be dynamically adjusted.
Users simply connect the AMT31 encoder to the AMTViewpoint GUI, select from a list of 20 orthogonal resolutions (maximum 4096 PPR) and 7 standard pole pair options (maximum 20 poles), and then click "Program." This offers advantages in the development process, allowing engineers to quickly and easily modify prototypes and use a single stock unit (SKU) for various motor controls with different resolutions and BLDC pole numbers, improving production supply chain management efficiency. In addition to supporting multiple resolutions and pole pair numbers per unit, the encoder housing is easy to assemble and offers various mounting options and sleeve sizes to accommodate common motor shaft diameters. Furthermore, the AMTViewpoint GUI provides unprecedented design support for the AMT31 series encoders. When connected to AMTViewpoint, diagnostic data can be downloaded from the AMT31 encoder to avoid potential field failures and reduce downtime.
Summarize
The high-precision, rigorous control loops of BLDC motors enable them to excel in numerous fields. Increased precision means less power loss, higher accuracy, and better control over BLDC operation by end users. Currently, BLDC motors are widely used in a variety of fields, including surgical robotic arms, autonomous vehicles, and assembly line automation, and will soon gain a place in many other unimaginable areas.
The BLDC motor market continues to grow, but the demands on BLDC motors remain unchanged: the market needs high-efficiency, durable motors with low cost and high-precision position sensing feedback. When used with BLDC motors, the AMT31 series encoders save valuable time during installation while simplifying development and manufacturing processes. With its versatility, ability to program and zero-set in seconds, and compatibility with the AMTViewpoint GUI, the AMT31 encoder perfectly meets the needs of the rapidly growing BLDC market.
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