Microprocessor-Controlled Brushless Motor Controller
A microprocessor-controlled brushless motor controller provides improved motion regulation, reduced noise, and optimized durability. The elimination of brushes also reduces eddy currents and wear.
The control circuit uses orientation sensors to determine the rotor’s position and controls power MOSFET drivers that switch the phases and coil currents. It can use different methods of speed control, depending on the application and ambient conditions.
Rotor Position Sensors
The rotor position sensor is an important component of the motor control loop for brushless DC (BLDC) motors. Its signals are critical for accurate torque control to improve motor performance, reduce BOM cost and ensure safety. Errors in the position sensor signal can cause a failure to synchronize the rotor and stator, which can lead to loss of power or possible failure.
There are several different methods to determine rotor position, but the most common is monitoring the back EMF on an undriven phase to detect changes in magnetic field strength and polarity. A more advanced method involves a complex control algorithm to calculate the position of each pole, but this requires a powerful processor and increases the system cost.
KYOCERA AVX offers both inductive and Hall-based position sensors that meet the needs of automotive electric drive systems. Inductive sensors use the physical principles of induction and eddy currents to detect the position of a solid metallic target rotating above a set of coils. These sensors offer a smaller footprint and can be mounted on a through-shaft or end-of-shaft configuration, while providing high accuracy and immunity to stray magnetic fields.
Compared with optical encoders, Hall-based sensors also offer a higher reliability and simpler circuitry. MagAlpha devices, for example, provide outputs directly that emulate UVW hall signals for position control applications and quadrature outputs that can replace the need for an optical encoder in speed/position controlled applications.
Three-Phase Drive Circuits
Three-phase drives require more complicated control circuits than single-phase drives. The voltage of each leg of the three-phase current can vary, which requires a more sophisticated phase-reversal algorithm that ensures one or more phases don’t accidentally reverse their direction. This is a safety issue that can cause damage to equipment or even personal injury. The protection circuit includes two delays (DLY2 and DLY9), a counter CNT0, and a 2-bit LUT0 configured as an XOR gate. The output of the XOR gate turns off the motor when no feedback is received for 100ms or more to avoid accidental starting.
To reduce power loss, the system uses a PWM frequency that is higher than the maximum rotation speed of the motor. The controller can also create sinusoidal PWM signals to improve commutation accuracy and smooth the motor operation.
Most brushless DC motors use sensors to detect the rotor position and send information about the position to the drive circuit. These sensors can be a magnetic element in the rotor or brushless motor controller optical encoders connected to a Hall-effect sensor mounted on the stator. These sensors can complicate the design of a motor, and their arrangement and maintenance can add to costs. In order to simplify the motor design, some brushless DC motor controllers can be programmed to determine rotor position without sensors. They do this by monitoring the back EMF created by the motor.
Sinusoidal PWM Circuits
In brushless motor controller circuits, a sinusoidal drive is achieved through control and driver circuits designed for 3 phases. By utilizing sensors, this drives the current into the motor windings with a sinusoidal waveform shifted by 120° for commutation. This eliminates torque fluctuations caused by non-linear commutation.
In order to create a sinusoidal drive, it is necessary to accurately determine the rotor position six times each revolution and deliver a sinusoidal motor current with varying pulse-width modulation (PWM) duty cycles. The FRDM-KE04Z uses hall-effect sensors to detect the rotor position and then varies Micro brushless motor the PWM duty cycle on a per-rotation basis to create the desired sinusoidal signal.
The two signals used for this operation are a high frequency triangular waveform and a lower sampled sinewave. The opamp then compares the two waves and generates a PWM with a variable width based on the difference between the two waveforms over a given period of time. The oscilloscope figure below shows the coinciding instantaneous voltage levels of both the carrier and the modulating inputs over a single time period.
By using this method, the sinusoidal commutation output voltages are generated with a saddle profile which helps to reduce current surge during commutation and thus eliminates arcing at the brushes and commutator, which in turn significantly decreases switching losses in the power stage. In addition, each phase is energized for only a third of the period, which further reduces current draw.
Speed Control
Almost any model airplane ESC can be used to control your brushless motor, but you should match it to the sort of motor you’re using. For example, a brushed ESC should be used only with a brushed motor and a brushless ESC should be used only with a brushless motor.
A key component to a good brushless motor controller is its processor, which uses PWM to vary the power fed to the motor to control its speed. This requires a precise measurement of rotor position, which Hall effect sensors or a rotary encoder can provide. Some manufacturers also use back-EMF from the undriven coils to infer rotor position.
When this method is used, the rotor current space vector must be measured to determine rotor position, which may require an additional sensor. If the rotor current space vector is measured at higher speeds, the phase shift between it and the quadrature direction can cause the motor to lose efficiency and produce less torque.
Another essential part of a good brushless motor controller is its voltage regulator/BEC (Battery Eliminator Circuit). The BEC separates the electronic speed control from the receiver and transmits a low voltage to the receiver and after that to the servos. It also helps to keep the speed controller from draining too much power from your battery. Finally, your brushless motor controller should include a 250 ohm 10W pre-charge resistor connected across the main contactor. This prevents a large inrush of current across the high current contacts, which could spot weld them in the ‘on’ position and leave them stuck turned on.
