The Motor Controller in a Brushless DC Motor
The motor controller in a brushless DC motor controls the current to the rotor windings. It also detects the rotor position and provides other functions.
Brushed motors use hard switching to move current from one winding to another. This can create motor vibration and mechanical noise.
A BLDC motor controller uses sinusoidal commutation to reduce these effects. This control method also eliminates the need for a separate back EMF sensor.
Sinusoidal Commutation
The commutation methods used with BLDC motors significantly impact their performance and efficiency. By selecting an optimal commutation method, engineers can improve speed control, position accuracy, acoustic noise, and other important motor characteristics. However, determining the best method is not always obvious as different manufacturers offer various commutation technologies for their drives and some applications have specific commutation requirements.
One option is to use sinusoidal commutation, which uses Hall Effect sensors to detect the position of the rotor and energizes groups of windings in a sequence that corresponds with the rotor’s rotational position. This allows for accurate current delivery and reduces torque ripple. However, sinusoidal commutation can cause a lag between the desired rotor current and the actual winding current, which results in wasted energy and reduced torque output.
Another option is sensorless field-oriented control (FOC), which utilizes back EMF signals to determine the rotor position. FOC is more complex than sinusoidal commutation, but it offers superior performance for many applications, including increased torque, wide-range speed regulation, and efficient operation.
The HPSC module 24/5 (High Performance Sensorless Control) from maxon is designed to provide both sinusoidal and FOC commutation depending on the motor/encoder combination*. It starts immediately with block commutation and switches to sinusoidal commutation within the first electrical turn of the motor shaft when an absolute encoder has been mounted and aligned in a defined way concerning the rotor position.
Hall Effect Sensors
Hall effect sensors are solid state devices used for detecting position, velocity or directional movement. They operate on the principle that a magnetic field will induce an electric current in a semiconductor when the magnetic field passes across it, with the oppositely charged sides of the semiconductor producing oppositely charged voltages (Hall voltage). This current can then be detected by measuring the difference in the Hall voltage on both sides of the chip.
Hall sensors are popular with motor control applications as they can be installed without contact and are robust and immune brushless motor controller to vibration and dust. They are also a cost-effective alternative to mechanical breaker points.
BLDC motors use hall position sensors as a replacement for mechanical encoders for speed measurement and control. They generate digital pulses when a magnetic field passes across them, with each Hall sensor output generating an independent binary number. When combined, the outputs of all three Hall sensors (CHA, CHI and CHC) can give an accurate reading of the rotor angle.
To calibrate the position of your BLDC Motor you will need to connect the CHA, CHI and CHC outputs of your HALL sensors to the corresponding drive inputs. You will need to short these leads and then revolve the armature one turn while monitoring the corresponding outputs from the drives. The resulting signal should match the picture shown below.
Angle Feedback
The rotor of a sensorless BLDC motor is static, and therefore doesn’t generate any back EMF to communicate with the driver. This leaves the drive circuitry with only a one-in-six chance of guessing the correct rotor position. To overcome this limitation, some manufacturers use an alignment method to force the rotor into a known alignment. This adds weight, complexity and cost to the system.
Another option is to use angle feedback, which uses an internal oblique angle sensor to provide a current command value based on the actual rotor position. This enables the control system to adjust the motor input based on the rotor position, optimizing energy consumption and improving performance.
In this approach, a single phase of the motor is powered to power the rotor, and the other two phases are used to sense back EMF and detect zero-crossings. The resulting voltage signals are then passed through a loop filter, which can be a complex piece of hardware that requires significant analog and digital signal processing.
The most efficient way to do this is with a microcontroller that has fast ADCs and comparators built brushless motor controller manufacturer in. You could also use a dual-core MPU to independently handle time-sensitive communications and motor driving functions, which will save on computing resources.
PWM
Pulse width modulation is the simplest way to control a motor and works by sending a signal that has varying durations of time between ‘on’ and ‘off’ states. The more the duration of the ‘on’ state, the faster the motor will run.
The pulses of the PWM signal are generated by changing the frequency. If the frequency is increased, the pulses will appear shorter and repeat faster, while if the frequency is decreased the pulses will appear longer and repeat slower.
These voltage drops are detected by the ESC’s microcontroller, which then compares them to a table to generate commutation signals that are delivered to the gate driver. The gate drivers take the pulses and deliver them to each of the three phases of the motor. The more voltage they receive during the ‘on’ pulses, the faster the rotor spins.
Within the rotor and stator there are copper coils that develop a pair of North and South poles. When current is delivered to a coil it turns the electromagnet into that polarity, which then attracts or repels the permanent magnets in the rotor and moves them around. ESCs use a programmable PWM waveform to control the timing of these signals, which are sent at very high frequencies (up to 500,000 Hz) to ensure they can operate as fast as possible.