“As mechanical systems accelerate toward Electronic control, driven in large part by the Internet of Things (IoT) and vehicle electrification, designers are implementing low-power motors for everything from home appliances, door locks, and remote blinds to automotive oil pumps, seat Basic tasks in applications such as chairs, windows and doors. Rated from as little as a fraction of a horsepower to as much as multiple horsepower, these DC motors are ubiquitous and often unknown.
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As mechanical systems accelerate toward electronic control, driven in large part by the Internet of Things (IoT) and vehicle electrification, designers are implementing low-power motors for everything from home appliances, door locks, and remote blinds to automotive oil pumps, seat Basic tasks in applications such as chairs, windows and doors. Rated from as little as a fraction of a horsepower to as much as multiple horsepower, these DC motors are ubiquitous and often unknown.
While motors are rapidly expanding as they continue to improve and motor control technology becomes better and easier to use, designers are under constant pressure to increase efficiency and reduce costs, while also achieving greater accuracy and improved reliability. High reliability.
Variations of brushless DC (BLDC) motors and stepper motors (another type of brushless DC motor) can help designers meet these increasingly demanding performance and cost goals, but motor controllers and motor drive circuits must be carefully considered. The controller must provide the appropriate drive signals to the motor’s electronic drive switches (usually MOSFETs), and do so with carefully controlled timing and duration. It must also control the motor’s ascent/descent trajectory and be able to detect and adapt to the inevitable soft and hard faults of the motor or load.
This article explores the features offered by control ICs for brushless DC motors, provides the reader with a holistic view of the electrical properties of brushless DC motors, and explains how a complex controller can use the Renesas RAJ306010 series of motor control ICs to enable brushless DC motors meet application goals.
Motor Control Path and Motor
The path from the motion control software to the motor includes a processor that runs the software, gate drivers for the motor power switching device, and the motor (Figure 1). There may also be a path from the motor’s sensors back to the processor through the analog front end, facilitating information on the position or speed of the motor’s rotor to confirm performance and close the feedback loop.
Figure 1: Today’s motor control starts with software embedded as firmware in the processor to control the gate drive, which in turn switches power to the motor’s windings; there may also be a Sensor-driven feedback loop. (Image credit: Renesas)
Designers have two leading DC drive brushless motor choices: BLDC motors and stepper motors. The functions of both are achieved by the magnetic interaction between the internal permanent magnets and the switching of their electromagnetic coils. The choice of which of these two motors to use is determined by their relative advantages and disadvantages in the intended application area.
Generally speaking, BLDC motors are highly reliable, efficient, and can provide high torque within a certain speed range. The motor stator poles are energized in sequence, causing the rotor (and its permanent magnets) to turn. A BLDC motor typically has three electronically controlled stators around its periphery (Figure 2).
Figure 2: The stators of a BLDC motor are sequentially energized, causing the permanent magnet rotor to turn. (Image credit: Renesas)
Key attributes of BLDC motors include responsiveness, fast acceleration, reliability, long life, high speed operation and high power density. They are often the choice for applications such as medical equipment, cooling fans, cordless power tools, turntables and automation equipment.
A stepper motor works similarly to a brushless DC motor, except that its rotational motion is much smaller, and it divides a full rotation into a large number of equiangular steps (usually 128 or 256 steps). Instead of rotating continuously, the motor rotor drives it sequentially over or over those small angled steps (Figure 3). This enables accurate positioning of the rotor as it synchronizes with the magnetic field produced by the energized stator poles.
Figure 3: A stepping motion has a large number of stator poles arranged around its rotor and its permanent magnets; by energizing these poles in a controlled sequence, the rotor is turned and small angular steps are performed. (Image credit: Renesas)
Stepper motors are reliable, precise, and provide fast acceleration and responsiveness. Due to its stepper operation and motor structure, open-loop control, and positioning stability, it is often sufficient for precision applications such as CD drives, flatbed scanners, printers, and plotters. Advanced applications can add a feedback sensor and closed-loop control for greater accuracy and performance confirmation.
BLDC motor control options
Unlike AC induction motors or brushed DC motors, where the primary means of speed and torque control is by adjusting the supply voltage, brushless DC motors are differentiated by careful switching of the power switching MOSFETs on and off. timed to control. This allows the motor to handle a variety of tasks efficiently and accurately.
These task requirements range from delivering the high number of revolutions (RPM) needed to move large volumes of air to provide the suction power of a cordless vacuum to power tools that must have high starting torque, especially when the motor is stalled relative to its load. In many applications, the motor must also be able to handle large load changes, which requires a fast response to maintain a stable rotational speed.
Common strategies for controlling BLDC motors are: basic 120⁰ on/off control and vector control. In 120⁰ on/off control, two of the three coils of the brushless DC motor are energized, and the six energization modes are switched in rotational order to support rotation in any direction (Figure 4).
Figure 4: The stator poles (left) of a brushless DC motor can be energized in a clockwise or counterclockwise sequence (right), driving the rotor to rotate in either direction, depending on the application. (Image credit: Renesas)
In this mode, the stator coils are energized with an on/off current (square wave), creating a trapezoidal acceleration profile as the motor ramps up, maintains speed, and slows down after the coil is de-energized. The benefits of this approach are inherent simplicity and direct manipulation.
However, it is prone to performance fluctuations with load and other changes, and is neither accurate nor efficient enough for some applications. Sophisticated algorithms in the motor controller can overcome these drawbacks to some extent by adjusting the on/off times of the MOSFETs and using proportional-integral-derivative (PID) or proportional-integral (PI) control.
An alternative that has become increasingly attractive is vector control, also known as field-oriented control (FOC). In this method, all three coils are energized by continuously controlling the rotating magnetic field, resulting in smoother motion compared to 120-degree control. FOC has now grown to be used in many mass market products such as washing machines.
In FOC, the current of each stator coil is measured and controlled by advanced algorithms, which require complex digital processing. The algorithm must also constantly convert three-phase AC values to two-phase DC values (a process called coordinate phase transformation), simplifying the equations and calculations required for subsequent control (Figure 5). When done correctly, FOC generation is a highly precise and efficient control.
Figure 5: Part of the FOC algorithm requires a coordinate phase transformation to simplify complex digital processing calculations. (Image credit: Renesas)
Sensor options for feedback
BLDC motors can be controlled in an open-loop topology with no feedback signal, or through a closed-loop algorithm with feedback from sensors on the motor. This decision depends on the accuracy, reliability, and safety considerations of the application.
Adding a feedback sensor increases cost and algorithm complexity, but increases confidence in the computation and is therefore essential in many applications. Depending on the application, the motion parameter of primary interest is the position or speed of the rotor. These two factors are closely related: velocity is the time derivative of position, and position is the time integral of velocity.
In fact, almost all feedback sensors indicate position, and the controller can use their signals directly or develop derived information to determine velocity. In simpler cases, the primary role of the feedback sensor is as a safety-related check of basic motor performance or as a stall indicator, rather than for closed-loop control.
Four types of feedback sensors are commonly used: Hall-effect devices, optical encoders, resolvers, and inductive sensors (Figure 6). Each offers different performance attributes, resolutions and costs.
Figure 6: If a user’s system requires a motor feedback signal, they have a wide selection of sensors, from Hall-effect devices to encoders, resolvers, and inductive sensors. (Image credit: Renesas)
Hall effect devices are generally considered to be the simplest and easiest to install, and are adequate in many cases. Optical encoders come in a range of resolutions, from low to medium, but have installation challenges and may have some long-term reliability issues. Resolvers and inductive sensors are larger, heavier, more expensive, and have some interface challenges, but offer very high resolution and long-term performance.
supply of current
Whether it’s a BLDC or a stepper motor, the poles of a brushless motor are electromagnetic “coils” and therefore must be driven by current rather than voltage. In order to properly energize these poles, the motor control system must provide this current through on/off switching (MOSFETs in most cases) with accurate timing, pulse width and controlled slew rate for correct and Drive the motor efficiently. The drive arrangement must also protect the MOSFETs from various fault conditions, such as motor stall, excessive current demand, thermal overload, and short circuits.
For relatively small motors, typically requiring less than 500 milliamps (mA) to 1 amp (A), MOSFET gate drivers or even MOSFETs can be embedded into the motor control IC package to keep the package size as small as possible . While this is convenient and simplifies design import, it is not a practical option in many cases for several reasons:
The semiconductor process for high-performance MOSFETs is very different from that used for controller digital logic, so the combined final design is a compromise (but probably acceptable).
The power dissipation and thermal management of a MOSFET is largely determined by the application power requirements. As current and power levels increase, the dissipated and generated heat of the on-chip MOSFET can quickly exceed the package’s limits. In these cases, separating the digital and power functions is a better solution, allowing designers to optimize MOSFET placement and thermal management.
Finally, as the current level required by the motor increases, the increased IR drive voltage drop in the motor power leads can become a problem. Therefore, it is recommended to place the switchgear closer to the load.
For these reasons, many motor and motion control ICs include all the required functions except for the power MOSFETs. The topology of multiple MOSFETs is often referred to as the inverter function. Using discrete MOSFETs gives designers the flexibility to choose a device with the right combination of specifications, such as load current, “on” resistance, package type, and switching characteristics.
Complex ICs Address Motor Control Challenges
In the past, advanced motor control required an IC assembly. Typically, this might involve a low-end processor to issue general-purpose instructions and a dedicated digital coprocessor to implement the necessary algorithms, or a high-end processor to do both, plus There are gate drive circuits for power devices. Not only does this require a larger printed circuit board area and a longer bill of materials (BOM), but there are often system integration and associated debugging issues.
However, today’s motor control ICs can do all of this in one device, as shown in the Renesas RAJ306010 (Figure 7). There are many function blocks within the RAJ306010 that are specific to the unique needs of motor control designs.
Figure 7: The Renesas RAJ306010 IC has the functions (except power MOSFETs) required for highly advanced motor control, thus taking up less space than multi-IC solutions while simplifying BOM and design integration. (Image credit: Renesas)
This general purpose motor control IC is designed for three-phase brushless DC motor applications. It combines and tightly integrates two distinct roles in a tiny 8 x 8 millimeter (mm), 64-pin QFN package: digital controller functions and most analog pre-driver functions. It operates from 6 to 24 volts and is aimed at self-contained, largely autonomous applications such as power tools, garden tools, vacuum cleaners, printers, fans, pumps and robots. (Note that another nearly identical device, the RAJ306001, is a 6 to 30 volt version that shares the same datasheet as the RAJ306010.)
On the digital side, the RAJ306010 integrates a 16-bit microcontroller (Renesas’ RL78/G1F class) backed by 64KB of flash, 4KB of data flash, and 5.5KB of RAM. In addition, there are plenty of digital I/Os: general purpose I/O (GPIO), SPI, I2C, and a UART. There is also a nine-channel, 10-bit analog-to-digital converter (ADC) to introduce analog signals into the device.
To use the RAJ306010, the system designer loads the desired operating parameters into the corresponding flash control registers to establish the desired operating mode and conditions. As can be seen from a high-level system block diagram of a typical application (Figure 8), the integrated circuit can function at power-up without any additional microcontroller.
Figure 8: This high-level system block diagram of a basic application using the RAJ306001 shows how the high level of integration minimizes the need for additional discrete components. (Image credit: Renesas)
The analog side of the RAJ306010 features three half-bridge gate drivers with adjustable gate drive peak current up to 500 mA, a self-adjusting dead time generator function to prevent bridge “shoot-through” and damage, and There is also a current sense amplifier and a back EMF amplifier. An integrated charge pump boosts the supplied gate drive voltage from a lower voltage source to 13 volts.
There is direct support for Hall effect sensors, and the analog front end (AFE) can also be used to support other types of feedback sensors. As with any properly designed motor control, features include thermal protection, over/under voltage lockout (UVLO), overcurrent detection, and protection against motor lockout conditions.
The example in Figure 9 shows how the RAJ306010 can easily handle a basic stand-alone application such as a 24-volt cordless blender, although it could be just about any similar small appliance. Note that most of the circuitry is used to charge and manage the eight-cell battery pack, while motor control only requires the control IC, external three-phase bridge (inverter), feedback voltage sensing circuit (via current sense resistors), and User’s “Start” button.
Figure 9: The advanced functional integration of the RAJ306010 clearly shows how little extra circuitry and extra components are required for the core motor control functions of a basic appliance such as this battery-operated blender. (Image credit: Renesas)
Hands-on experience with BLDC motor control
It’s one thing to plan, simulate, evaluate, and tune a motor control application using various models of the entire system on paper or on a PC. Running an actual motor and testing performance with real components, real loads, and real power, as well as understanding the effects of setting initial start-up conditions and changes in various performance parameters, is another story.
That’s why the Renesas RTK0EML2C0S01020BJ motor control evaluation system (Figure 10) is an important asset for the design engineer, and it also comes with the Renesas Motor Workbench for easy debugging. This software tool enables designers to become familiar with the operation of the RAJ306010, its input and output modes, and the functions of its various control registers.
Figure 10: This board is the heart of the Renesas RTK0EML2C0S01020BJ motor control evaluation system, and when used with the Renesas Motor Workbench software, speeds up the fine-tuning of parameters and the evaluation of motor performance when using the RAJ306010 motor control IC. (Image credit: Renesas)
To get to the product development phase faster, the evaluation system includes a 24V/420mA BLDC motor with a no-load speed of 3900 RPM and a torque rating of 19.6 millinewton-meters (mN-m) (equivalent to 200 gram force •centimeter). In addition, Renesas provides sample software control programs for sensorless and sensor-based control.
Epilogue
When designers consider using DC motors in their systems, there are many options in addition to traditional brushed DC motors. High-performance, cost-effective BLDC motors are available for high power and precision requirements in small packages. To realize the full potential of these BLDC motors, intelligent controllers have been developed that implement the required algorithms using the parameters required by the user. These devices also provide the necessary drive for the motor’s switching MOSFETs and other analog I/Os, enabling a complete motor control solution.
As mentioned above, ICs like the Renesas RAJ306010, supported by development kits and software, greatly simplify the need to provide high performance, small size, and efficient motor control for appliances, car seats and windows, and many other applications that are now commonplace. Design challenges.
references
BLDC Motor Control Algorithms
RTK0EML2C0S01020BJ BLDC Motor Control EvaluaTIon System for RAJ3060xx Motor Control ICs
ApplicaTIon Note R01AN3786EJ0102, “Sensorless Vector Control for Permanent Magnet Synchronous Motor (Algorithm)”
Portable Power Tools SoluTIon
24V Cordless Blender
Motor SoluTIons: User-Friendly Motor Control Development Environment to Shorten Time to Market
The Links: MSG100U43 G150XTN066