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How to Quickly Start a Brushless DC Motor Control Design Using Highly Integrated ICs

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文章创建人 Bill Schwebe...

Original Title:How to Quickly Start a Brushless DC Motor Control Design Using Highly Integrated ICs

  Due to the accelerating shift toward electronic control of mechanical systems, driven in large part by the Internet of Things (IoT) and the electrification of automobiles, designers are applying low-power motors to basic tasks in applications ranging from home appliances, door locks, and remote-controlled blinds, to automobile pumps, seats, windows, and doors. These DC motors, which range in rating from small, sub-fractional horsepower to multiple horsepower, are ubiquitous yet often unseen.

  While improvements in the motors, as well as better and easier-to-use motor control technology, are aiding this rapid proliferation, designers remain under constant pressure to improve efficiency and lower cost, while also achieving greater accuracy and higher reliability.

  Variations of the brushless DC (BLDC) motor and the stepper motor (another type of brushless DC motor) can help designers meet these increasingly demanding performance and cost objectives, but only with careful consideration of the motor controller and motor-drive circuitry. The controller must deliver suitable drive signals to the motor’s electronic-drive switches (usually MOSFETs), and do so with carefully controlled timing and duration. It must also control motor ramp-up/down trajectory, as well as detect and accommodate the inevitable soft problems and hard failures with the motor or load.

  This article looks at the functions provided by control ICs for BLDC motors. It provides an overall perspective on electrical attributes of BLDC motors and explains how a sophisticated controller enables a BLDC motor to meet the application objectives using the Renesas RAJ306010 series of motor control ICs.

  The motor control path and motor

  The path from the motion-control software to the motor consists of a processor on which the software runs, gate drivers for the motor’s power-switching devices, and the motor (Figure 1). There may also be a path from a sensor at the motor back to the processor via an analog front-end, providing information about the position or speed of the motor rotor to confirm performance and close a feedback loop.


Diagram of today’s motor control begins with software embedded as firmware (click to enlarge)

  Figure 1: Today’s motor control begins with software embedded as firmware in the processor controlling gate drivers which, in turn, switch power to the motor’s windings; there may also be a sensor-driven feedback loop from the motor back to the processor. (Image source: Renesas)

  Designers have two leading choices for their DC-driven brushless motor: the BLDC motor and the stepper motor. Both function due to the magnetic interaction between their internal permanent magnets and the switching of their electromagnetic coils. The choice of which of these two to use is determined by their relative pros and cons with respect to the intended application.

  In general, BLDC motors are highly reliable, efficient, and can deliver large amounts of torque over a range of speeds. The motor stator poles are energized in sequence, which causes the rotor (with its permanent magnets) to turn. BLDC motors typically have three electronically controlled stators around their periphery (Figure 2).


Diagram of BLDC motor’s stators are energized in a sequence

  Figure 2: The BLDC motor’s stators are energized in a sequence such that the permanent magnet rotor turns. (Image source: Renesas)

  Key BLDC motor attributes include responsiveness, quick acceleration, reliability, long life spans, high-speed operation, and a high power density. They are often the choice in applications such as medical equipment, cooling fans, cordless power tools, turntables, and automation equipment.

  The stepper motor works similar to BLDC motors, except that it moves in much smaller rotary motions by dividing a full rotation into a large number of equal-angle steps (typically, 128 or 256). Instead of continuously rotating, the motor rotor is sequentially driven to walk or step it through those small-angle steps (Figure 3). This allows the rotor to be positioned accurately as it is synchronized with the magnetic field produced by the energized stator poles.


Diagram of stepper motion has a large number of stator poles

  Figure 3: The stepper motion has a large number of stator poles that are arranged around its rotor and their permanent magnets; by energizing these poles in a controlled sequence, the rotor turns and is stepped through small angles. (Image source: Renesas)

  Stepper motors are reliable, accurate, and offer quick acceleration and responsiveness. Due to their stepping operation and motor construction, open-loop control and positioning stability are often sufficient even for precision applications like CD drives, flatbed scanners, printers, and plotters. Advanced applications may add a feedback sensor and closed-loop control for additional precision and performance confirmation.

  BLDC motor control options

  Unlike AC induction or brushed DC motors, where the primary means of speed and torque control is by adjustment of the supply voltage, the BLDC motor is controlled by careful timing of the turn-on and turn-off of the power-switching MOSFETs. This allows the motor to efficiently and accurately handle a wide variety of tasks.

  These requirements can range from providing the high revolutions per minute (RPM) needed to move large amounts of air to provide suction in a cordless vacuum, to power tools that must have high start-up torque, especially if the motor is stalled against its load. In many applications, the motor must also be able to handle large load changes which demand fast response times to maintain consistent RPM.

  There are common strategies for controlling the BLDC motor: basic 120⁰ on/off control and vector control. In 120⁰ on/off control, two of the three coils of the BLDC motor are energized, and six energizing patterns are switched in a rotating sequence to support rotation in either direction (Figure 4).


Diagram of stator poles of the BLDC motor (click to enlarge)

  Figure 4: The stator poles of the BLDC motor (left) can be energized in either clockwise or counterclockwise order (right), thus driving the rotor in either direction as required by the application. (Image source: Renesas)

  In this mode, the stator coils are energized with on/off current (a square wave) resulting in a trapezoidal acceleration profile as the motor ramps up to speed, maintains speed, and then ramps down when the coils are de-energized. The benefits of this approach are inherent simplicity and straightforward operation.

  However, it is vulnerable to performance fluctuations with load and other changes, and precision and efficiency are not high enough for some applications. Sophisticated algorithms in the motor controller can overcome these shortcomings to some extent by adjusting the MOSFET turn on/off timing, as well as the use of proportional-integral-derivative (PID) or proportional-integral (PI) control.

  An alternative that has become increasingly attractive is vector control, also called field-oriented control (FOC). In this approach, all three coils are energized via continuous control of the rotating magnetic field, resulting in smoother motion compared to 120-degree control. FOC has advanced to where it is now used in many mass-market products, such as clothes washers.

  In FOC, the current to each stator coil is measured and controlled by advanced algorithms which require complex numerical processing. The algorithm must also continuously transform the three-phase AC values into two-phase DC values (a process called coordinate-phase conversion), simplifying the subsequent equations and computations needed for control (Figure 5). The result of FOC, if done properly, is highly accurate and efficient control.


Diagram of coordinate-phase conversion to simplify the complex numerical-processing computations

  Figure 5: Part of the FOC algorithm requires coordinate-phase conversion to simplify the complex numerical-processing computations. (Image source: Renesas)

  Sensor options for feedback

  BLDC motors can be controlled in an open-loop topology without a feedback signal, or via a closed-loop algorithm with feedback from a sensor at the motor. The decision is a function of the application’s accuracy, reliability, and safety considerations.

  Adding a feedback sensor adds to cost and algorithm complexity but increases confidence in the calculations, making it essential in many applications. Depending on the application, the motion parameter of primary interest is either rotor position or speed. These two factors are closely related: speed is the time-derivative of position, and position is the time-integral of speed.

  Actually, nearly all feedback sensors indicate position and the controller can use their signals directly or develop the derivative to determine speed. In simpler cases, the primary role of the feedback sensor is as a safety-related check on basic motor performance or as a stalling indicator, rather than for closed-loop control.

  Four types of feedback sensors are in common use: Hall-effect devices, optical encoders, resolvers, and inductive sensors (Figure 6). Each offers different performance attributes, resolution, and cost.


Image of wide range of sensor options

  Figure 6: Users have a wide range of sensor options if their system needs a motor-feedback signal, ranging from Hall effect devices to encoders, resolvers, and induction sensors. (Image source: Renesas)

  Hall effect devices are generally considered to be the simplest and easiest to install, and are adequate for many situations. Optical encoders are available with a range of resolutions, from low to moderately high, but have installation challenges and may have some long-term reliability concerns. Resolvers and inductive sensors are larger, heavier, more costly, and come with some interface challenges, but provide very high resolution and long-term performance.

  Delivering the current

  The poles of brushless motors—whether BLDC or stepper—are electromagnetic “coils” and so must be driven by current rather than voltage. To properly energize these poles, the motor control system must deliver this current via on/off switches (MOSFETs in most cases) with accurate timing, pulse width, and controlled slew rates to drive the motor properly and efficiently. The driving arrangement must also protect the MOSFETs against various fault conditions such as motor stalls, excessive current demands, thermal overloads, and short circuits.

  For relatively small motors, typically requiring under 500 milliamperes (mA) to one ampere (A), it is possible to embed the MOSFET gate drivers and even the MOSFETs into the motor control IC package, keeping the footprint as small as possible. While this is convenient and simplifies design-in, it is not a practical choice in many cases for several reasons:

  The semiconductor processes for high-performance MOSFETs are very different than those used for the digital logic of the controller, so the final design of the combination is a compromise (but one which may be acceptable).

  The MOSFET power dissipation and thermal management is dictated largely by the application power needs. As the current and power levels increase, the on-chip MOSFET dissipation and generated heat can soon exceed the package limits. In these cases, a better solution is to separate the digital and power functions, allowing the designer to optimize placement and thermal management of the MOSFETs.

  Finally, as current levels required by the motor increase, the increase in IR-driven voltage drop in the motor supply leads can become an issue. As a consequence, it is advisable to locate the switching devices closer to the load.

  For these reasons, many motor and motion control ICs include all the needed functions, except the power MOSFETs. The topology of the multiple MOSFETs is often called an inverter function. Using discrete MOSFETs gives the designer the flexibility to select devices with the right combination of specifications for factors such as load current, “on” resistance, package type, and switching characteristics.

  Sophisticated ICs meet motor control challenges

  In the past, advanced motor control required an assembly of ICs. Typically, this might involve a low-end processor to issue general commands with a dedicated numeric co-processor to implement the necessary algorithms or a high-end processor to do both, along with the gate-drive circuitry for the power devices. Not only did this require a larger pc board footprint and a longer bill of materials (BOM), but there would often be system integration and associated debugging issues.

  However, today’s motor control ICs can do it all in a single device, as illustrated by the Renesas RAJ306010 (Figure 7). Within the RAJ306010 are the many functional blocks specifically targeting the unique needs of motor control designs.


Diagram of Renesas RAJ306010 IC (click to enlarge)

  Figure 7: The Renesas RAJ306010 IC has the functionality required for highly advanced motor control (except for the power MOSFETs), and so takes less space than a multi-IC solution while simplifying both the BOM and design integration. (Image source: Renesas)

  This general purpose motor control IC is intended for three-phase brushless DC motor applications. It combines and tightly integrates two disparate roles in a tiny 8 × 8 millimeter (mm), 64-lead QFN package: the digital controller function, and the mostly analog pre-driver function. It operates from a 6 to 24-volt supply and targets standalone, largely autonomous applications such as power tools, garden tools, vacuum cleaners, printers, fans, pumps, and robotics. (Note that the otherwise nearly identical RAJ306001 is a 6 to 30-volt version which shares the same datasheet as the RAJ306010.)

  On the digital side, the RAJ306010 incorporates a 16-bit microcontroller (Renesas’ RL78/G1F class) supported by 64 kilobytes (Kbytes) of flash ROM, 4 Kbytes of data flash ROM, and 5.5 Kbytes of RAM. In addition, there is a substantial amount of digital I/O: general purpose I/O (GPIO), SPI, I2C, and a UART. There’s also a nine-channel, 10-bit analog-to-digital converter (ADC) to bring analog signals into the device.

  To use the RAJ306010, the system designer loads the desired operating parameters into the appropriate flash memory control registers to establish the desired operating modes and conditions. The IC is then ready to function on power-up without the need for any additional microcontroller, as seen by the high-level system block diagram of a typical application (Figure 8).


High-level system block diagram of a basic application using the Renesas RAJ306001

  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 source: Renesas)

  The analog side of the RAJ306010 features three half-bridge gate drivers with an adjustable gate-drive peak current up to 500 mA, a self-aligning dead-time generator function to prevent bridge “shoot-through” and damage, a current-sense amplifier, and a back-EMF amplifier. An integral charge pump boosts the delivered gate drive to up to 13 volts from a lower voltage supply.

  There’s 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, there are functions including overtemperature protection, over/under voltage lockout (UVLO), overcurrent detection, and protection against motor-lock conditions.

  The example in Figure 9 shows how the RAJ306010 easily handles a basic standalone application such as a 24-volt cordless blender, although it could be almost any similar small appliance. Note that the bulk of the circuitry is devoted to charging and managing the eight-cell battery pack, while the motor control requires just the control IC, the external three-phase bridge (inverter), a feedback voltage-sense circuit (via a current-sense resistor), and the user’s “start” button.


Diagram of high level of functional integration of the Renesas RAJ306010 (click to enlarge)

  Figure 9: The high level of functional integration of the RAJ306010 clearly shows how little additional circuitry and how few additional components are needed for the core motor control function of a basic appliance, such as this battery-powered blender. (Image source: Renesas)

  Get hands-on with BLDC motor control

  It’s one thing to plan, simulate, evaluate, and adjust a motor control application “on paper” or on a PC using various models of the overall system. However, it’s another thing to run an actual motor and test the performance using real components, real loads, and real dynamics, as well as to learn the impact of setting initial start-up conditions and changes in various performance parameters.

  That’s where the Renesas RTK0EML2C0S01020BJ Motor Control Evaluation System (Figure 10) is a vital asset for the design engineer, along with the Renesas Motor Workbench for ease of debugging. This software tool enables the designer to get familiar with the operation of the RAJ306010, its input and output modes, and the functions of its various control registers.


Image of Renesas RTK0EML2C0S01020BJ Motor Control Evaluation System

  Figure 10: This board at the core of the Renesas RTK0EML2C0S01020BJ Motor Control Evaluation System, when used in conjunction with the Renesas Motor Workbench software, speeds fine tuning of parameters and evaluation of motor performance when using the RAJ306010 motor control IC. (Image source: Renesas)

  To get the product development phase underway even more quickly, the evaluation system includes a 24 volt/420 mA BLDC motor with a no-load speed of 3900 RPM and a rated torque of 19.6 millinewton-meters (mN-m) (equivalent to 200-gram force-centimeters). In addition, Renesas provides sample software control routines for both sensorless and sensor-based control.


  Designers who are incorporating DC motors into their systems have many options beyond the classic brushed DC motor as high-performance, cost-effective BLDC motors are available that offer power and precision in small packages. To fully realize the potential of these BLDC motors, smart controllers incorporate and implement the needed algorithms with the user’s desired parameters. They also provide the necessary drive for the motor’s switching MOSFETs and other analog I/O for a complete motor control solution.

  As shown, ICs like the Renesas RAJ306010, supported by development kits and software, greatly simplify the design challenge of providing high performance, small size, and efficient motor control for applications such as appliances, car seats, and windows, and many other now-common applications.


  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


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