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Position feedback for brushless DC motor
2021/01/11

Position feedback for brushless DC motor

Since the birth of brushless DC motor, Hall effect sensor has been the main force of realizing commutation feedback.Since three-phase control requires only three sensors and has a low unit cost, they are often the most economical choice for reversing from a purely BOM cost perspective. Hall effect sensors embedded in the stator detect the position of the rotor so that transistors in the three-phase bridge can be switched to drive the motor.The three Hall effect sensor outputs are generally labeled as U, V, and W channels.Although Hall effect sensors can effectively solve the problem of BLDC motor commutation, they only meet half of the requirements of BLDC system.


Although the Hall effect sensor enables the controller to drive the BLDC motor, its control is unfortunately limited to speed and direction. In a three-phase motor, the Hall effect sensor can only provide an angular position within each electrical cycle.As the number of pole pairs increases, so does the number of electrical cycles per mechanical rotation, and as the use of BLDCs becomes more widespread, so does the need for precise position sensing.To ensure that the solution is robust and complete, the BLDC system should provide real-time position information so that the controller can track not only speed and direction, but also travel distance and angular position.


To meet the need for more stringent position information, a common solution is to add an incremental rotary encoder to the BLDC motor.Typically, incremental encoders are added to the same control feedback loop system in addition to the Hall effect sensor.Hall effect sensors are used for motor reversing, while encoders are used for more precise tracking of position, rotation, speed and direction.Since the Hall effect sensor only provides new position information at each Hall state change, its accuracy only reaches six states for each power cycle.For bipolar motors, there are only six states per mechanical cycle.The need for both is obvious when compared to an incremental encoder that offers resolution in thousands of PPR (pulses per revolution), which can be decoded into four times the number of state changes.


However, since motor manufacturers currently have to assemble both Hall effect sensors and incremental encoders into their motors, many encoder manufacturers are starting to offer incremental encoders with commutating outputs, which we commonly refer to simply as commutating encoders.These encoders have been specially designed to provide not only the traditional orthogonal A and B channels (and in some cases the "once per turn" index pulse channel Z), but also the standard U, V, and W commutation signals required by most BLDC motor drivers.This saves the motor designer the unnecessary step of installing both the Hall effect sensor and the incremental encoder at the same time.


Although the advantages of this approach are obvious, there are significant trade-offs.As mentioned above, the position of rotor and stator must be mastered for the BLDC brushless motor to be effectively commutated.This means that care must be taken to ensure that the U/V/W channels of the commutator encoder are correctly aligned with the phase of the BLDC motor.

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