Sensorless BLDC control relies on the characteristics of the BLDC motor to calculate the rotor position and in this position the motor is commutated at the appropriate time. To explain how it works, let's look back at the BLDC motor itself and the basic sensor control. Fundamentally, a BLDC motor uses an excitation coil (called a stator) to create a magnetic field parallel to the coil axis on the rotor (or shaft) that causes the rotor to rotate and generate torque. In a three-phase BLDC motor, three coils (or phases) in the stator are continuously turned on and off to rotate the rotor and generate torque. In order to keep the rotor rotating, the relevant phase must be turned on and off before the rotor is rotated to the appropriate position. In order to smoothly rotate the rotor, each winding or phase constituting the motor may be composed of a plurality of sets of coils. Each phase must be turned on and off in a specific sequence to rotate the rotor. The position of the rotor determines which phase needs to be turned on or off. Therefore, understanding the rotor position is critical to the operation of the motor. In order for the BLDC motor to operate, the controller must actively turn these phases on or off. The controller must maintain the magnetic field inside the stator in front of the rotor to keep the rotor rotating. The easiest way to get the rotor position is to use Hall effect sensors that generate pulses to inform the controller of the rotor position. Once the rotor position is known, the basic BLDC controller simply looks for which of the three phases corresponds to the rotor position and switches these phases to the appropriate mode.
Relying on the operation of the sensor is very easy to implement, but removing the sensor can reduce system cost and increase reliability. To understand how the sensorless algorithm calculates the rotor position, let's take a closer look at the three phases of the BLDC motor.
In the "ladder" control, one phase is pulled high (+VBUS) at any time, one phase is pulled low (-VBUS), and the third phase is inactive. Since the waveform of each phase is like a trapezoid (see Figure 1), the "trapezoid" control is named after it. When the rotor passes a phase, the permanent magnets on the rotor induce a current in the phase, which in turn produces a voltage called back electromotive force (EMF). The back electromotive force depends on the number of turns of each phase winding, the angular velocity of the rotor, and the strength of the rotor's permanent magnetic field. The back EMF waveform of each phase is related to the rotor position, so the back EMF can be used to determine the rotor position.
Figure 1 BLCD motor windings and trapezoidal waveforms
There are many different ways to determine rotor position using back EMF, the most common and reliable of which is zero crossing detection. When one of the back EMF signals is converted and crosses zero, the controller needs to switch the phase mode. This process is called commutation (see Figure 2). In order to keep the rotor moving forward, a phase shift must be made during the time between zero crossing and commutation, and the motor controller must calculate and compensate for this phase shift. An easy way to achieve zero crossing is to assume that a zero crossing occurs whenever the back EMF of either phase reaches VBUS/2.
Figure 2 Back EMF zero crossing
This method is easily implemented with several op amps configured as comparators. However, there are several problems in this method. First, the back EMF is usually less than VBUS, so the zero crossing event does not necessarily occur at VBUS/2. In addition, the characteristics of each phase may be different, so the zero-crossing back EMF voltage of one phase may be different from the zero-crossing EMF voltage of other phases. Finally, this overly simple detection method causes a positive and negative phase shift in the detected back EMF signal.
In actual motors, the zero-crossing threshold voltage varies greatly. Fortunately, the threshold voltage for this change is equal to the motor neutral point voltage because the motor neutral point is the average of all three opposite electromotive forces. Therefore, as long as the back electromotive force of either phase is equal to the neutral point of the motor, a zero crossing event occurs and the controller needs to commutate. This can be done with resistors and op amps, or with the controller's own ADC module and software. With programmable controllers such as the dsPIC family of DSCs, the back EMF of each phase can be sampled using the ADC module, and the average of the three back EMF signals makes it easy to reconstruct the neutral point using software. The software can then compare this value to the detected three-phase back EMF and detect when a zero crossing event occurs. After a zero-crossing event occurs, the controller reverses the motor and the entire process repeats. Thus, by using the back EMF of the motor and detecting zero crossings, the sensor can be removed from the system while maintaining the same level of performance.
In the actual system, sensorless operation will encounter other difficulties. First, at low speeds, the back EMF is very small and difficult to detect. Therefore, the controller must guess the rotor position before the motor begins to spin quickly enough to produce a sufficiently large back EMF to operate in sensorless mode. A software-programmable controller reduces the impact of this problem by allowing the system startup mode to be tailored to the specific application. Another issue is the switching noise of the MOSFET. Since the MOSFET changes the voltage of each phase by switching operations, this introduces noise into the back EMF detected by the controller ADC module. This noise needs to be filtered out to accurately reconstruct the back EMF of each phase. The DSC's built-in DSP engine makes it easy to handle the calculations needed to implement digital filtering and eliminate switching noise. Other challenges come from the characteristics of a particular design. However, using software-programmable controllers often makes these challenges easier to solve, just like the solutions to the two problems mentioned in this article.
Research and experimentation through examples make learning new technologies easier. Customized development tools for sensorless BLDC control greatly simplify the learning process and speed product development. In the past, learning with development tools required expensive money and time costs. New tools on the market are changing this situation. For example, Microchip's Motor Control Starter Kit costs less than $100 and includes detailed application notes, sample software, and hardware schematics (see Figure 3). Motor controller suppliers (including Microchip) typically provide free software and hardware files on their website to make the learning process easier.
Figure 3 Low-cost sensorless BLCD development kit
In short, as the electronic motor market continues to grow, the demand for BLDC motor systems will also increase, and cost pressures will rise. DSC-based sensorless technologies are taking the lead in meeting these new demands and addressing cost challenges.
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