Brushless DC motors (BLDC motors) are widely utilized in household appliances and factory automation (FA) machinery, etc. Development of the RL78/G1F microcontroller (MCU) now broadens the sphere of potential usage of these motors.
This low-end MCU is fitted with a high-speed programmable gain amplifier (PGA) and comparator, enabling initial rotor position detection and vector control.
Product designers have been voicing the desire for an affordable MCU that delivers more precision with less effort for brushless DC motor designs.
The new RL78/G1F was developed to answer this market need as the latest addition to the RL78 family of low power consumption MCUs.
Application of the brushless DC motor (BLDC motor) continues to expand, servicing everything from major household appliances such as refrigerators, air conditioners and washing machines, to industrial equipment such as electrical tools, robots, conveyance equipment, fans, and pumps.
The MCU expanding BLDC motor potential
The expanded use of brushless DC motors is not only attributed to the enhanced efficiency and durability it provides in comparison to brush-type DC motors, but also to the advent of MCUs that enable the brushless motor to offer increasingly intricate control more easily and with less cost. One such MCU broadening BLDC potential is the RL78/G14.
RL78/G14 products comprise a group within the RL78 family, Renesas’ top entry-level MCU family. In addition to the low power consumption and diverse functions characterizing the RL78 family, these compact MCUs enable low power BLDC motor control and have been chosen by many users for various applications.
However, we now see an increasing demand for further cost reductions coupled with performance gains, and specific needs for more precision control and ease of circuit design.
Easily realize higher performance required of even entry-level MCUs
The RL78/G1F was developed based on the concept of realizing n MCU that would further simplify BLDC motor control while broadening the range of possible applications. The result is a group of MCU products that provide effective functions to enhance BLDC motor control.
Three BLDC motor control functions have been enhanced by the RL78/G1F. The first is the strengthened overcurrent detection/output forced cutoff, boosting system-level safety. The second is support for sensorless vector control through single shunt current detection, allowing for low-cost and efficient motor driving. The third is expansion of the sensorless rotor position detection function, enabling the RL78/G1F to deliver sensorless, low-cost, and high-performance control. RL78/G1F can be expected to expand the usage of BLDC motors due to these enhancements. The following is a more detailed look at the three BLDC motor control features enhanced by the RL78/G1F.
Built-in high-speed overcurrent detection function
Overcurrent, which leads to motor burning and breakdown, sometimes happens with overload in motor operations, and can be spotted with the overcurrent detection function. The RL78/G1F not only provides highspeed overcurrent detection but is equipped with a function that forcibly shuts down the motor’s control output signal (PWM output) without CPU processing.
The conventional RL78/G14 was not equipped with overcurrent detection capability, making it necessary to introduce an external overcurrent detection circuit which would input the corresponding signal. The RL78/G1F, on the other hand, has built-in hardware enabling high-speed overcurrent detection. Specifically, it uses a high-slew rate programmable gain amplifier (henceforth, PGA) and quick-responding comparator. The RL78/G1F is also equipped with a PWM option unit, called the “PWMOPA,” which forcibly cuts off the PWM signal output from the I/O port that is supplied as a control signal to the inverter. The PWMOPA supports swift forced cutoff in response to the overcurrent detection output signal from the built-in comparator, and various release methods.
The PGA can amplify the minute voltage change in the current detection shunt resistor by amplification factors of 4, 8, 16, and 32. The fast slew rate of 3.5 V/μs or more (when VDD≧4V, amplification factor of 32x is 3.0V/μs or greater), and dedicated ground input (PGAGND port) enables amplification without noise influence from internal MCU operations.
The comparator that compares the amplified voltage signal to the reference voltage and determines the overcurrent state offers a response time of 0.07μs, 8 times faster than the RL78/G14 built-in comparator. In addition to the external input and internal reference voltage (1.45 V), output of the dedicated D/A converter for generating comparison reference voltage can be used for the reference input of the comparator.
Achieving single-shunt vector control
Vector control, also called field-oriented control (FOC), is a motor control method achieving optimal control by ongoing awareness of rotor position and enables greater BLDC motor efficiency and lower noise level. However, this method, especially sensorless vector control, is difficult for use in entry-level MCUs due to restrictions accompanying the need for multiple calculations and timing of current detection.
On the other hand, since the RL78/G1F has high throughput as an entry-level device and also features a high-speed PGA, the MCU easily enables “sensorless vector control with single-shunt current detection” with a one shunt resistor. Achieving vector control with few external parts and lower costs expands the range of potential applications for the BLDC motor.
Expanded functionality of sensorless rotor detection
The relatively simple 120-degree conducting control (rectangular wave control) has been widely used as a BLDC motor control method. Sensorless 120-degree conducting control reduces component costs by avoiding use of sensors such as rotor position-detecting Hall elements or encoders, and instead estimating rotor position through the motor’s back electromotive force.
In conventional low-end motor control MCUs such as the RL78/G14, the voltage of the three phases of the BLDC motor are measured by the internal A/D converter. The calculated average value (midpoint value) of the 3 voltages and the value of the back electromotive forced voltage of a phase with no applied voltage are compared by software processing, enabling sensorless detection of the rotor position (zero-cross point).
The RL78/G1F, on the other hand, has a built-in high-speed comparator for rotor position detection featuring the same performance as that of high-speed overcurrent detection. This comparator performs high-speed comparisons of the comparison reference three-phase midpoint voltage and the voltage of each phase, and enables zero-cross point detection quickly by hardware, without three exterior comparators. Like the G14, the RL78/G1F has a built-in AD converter which can be used for rotor position detection, but since hardware-based zero-cross detection with the comparator minimizes CPU processing, the software load is reduced. Moreover, with no need to take A/D conversion time into account, the RL78/G1F offers excellent support for high-speed motors.
Also supports initial rotor position detection function
The built-in high-speed comparator also enables the RL78/G1F to detect the initial rotor position in a short time span with low power consumption.
The BLDC motor’s sensorless control, which determines rotor position using back electromotive force, cannot detect the initial rotor position when the motor is stopped and back electromotive force is not generated. When the motor is forcefully started without confirmation of rotor position, neither smooth start-up nor start-up with strong torque are possible, resulting in problems such as flow of a wasted electric current. If initial position can be determined and that information reflected by start-up controls, these issues can be resolved.
RL78/G1F initial rotor position detection incorporates two types of processing for optimum efficiency.
For further information on initial rotor position detection, please refer to the following material.
[White paper] Is smooth, high-speed start-up possible even with sensorless control?
Application note featuring new functions
This application note describes the initial rotor position detection feature of the MCU, a key feature for sensorless brushless DC motor control. This method resolves issues unique to motor start-up using the sensorless control method.
This application presents a smooth and speedy high-torque start-up, realizing an efficient system.