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Figure 1: To keep the rotor in motion, the coils are energized in a particular sequence, 120° out-of-phase from one another.
The process of energizing and de-energizing the coils to keep the rotor spinning is called commutating. What makes a BLDC such an attractive motor is that commutation occurs electrically, without any mechanical interaction between the rotor and coils. This makes the BLDC motor very reliable and at the same time relatively inexpensive to build. The challenge in controlling a BLDC motor is knowing when to commutate, as commutation must occur at precise points during rotation in order for the motor to have maximum torque and smooth operation. Most BLDC motor systems use Hall Effect sensors to detect where the shaft is in its rotation and thus when to commutate. This introduces more cost, in cabling and sensors, and another point of failure. A more advanced system is to eliminate the sensors entirely by sensing the back Electromotive Force (back EMF) generated by the motor’s rotation and calculate when to commutate based on the back EMF of the motor. A digital signal controller, such as a dsPIC DSC, greatly simplifies implementing this kind of “sensorless” motor control system.
Stop right there! Don’t think another thought! “Advanced” doesn’t have to mean complicated or… expensive! Digital signal controllers provide all the functionality of an MCU with these convenient “tools” for digitally controlling virtually any motor type. One example is described in Microchip’s app note AN1160 on sensorless BLDC motor control which includes easy-to-follow example software. In addition, there are many motor control tools on the market that make learning simple but are often cost prohibitive. Breaking this cost barrier is Microchip’s $90 Motor Control Starter Kit (Figure 2). This kit features Microchip’s newest low cost, high performance dsPIC DSC. With advanced motor control systems within reach, let’s take a look at how a sensorless BLDC system works and at what a low cost sensorless BLDC development system looks like.
Take another look at Figure 1. Notice how there is always one coil (or phase) that isn’t driven? The motor’s back EMF will be visible on this phase because it isn’t being driven. By sensing the voltage on this phase, it is possible to reconstruct the back EMF waveform for this phase. With careful timing, this can be done for each phase of the motor, enabling all three back EMF signals to be reconstructed. Since the commutation times are based on the back EMF of the motor, the commutations times can be found from the reconstructed back EMF waveform, eliminating the need for sensors. Details on sampling the back EMF, reconstructing the back EMF waveform, and detecting when commutation should occur can be found in numerous app notes from many suppliers. Microchip’s app note AN1160 uses a zero crossing method to detect when to commutate and also uses a digital filter on the sampled back EMF waveform to reduce noise introduced by switching of the motor driver. Other suppliers use similar methods in their app notes to achieve sensorless BLDC control.
While app notes and example software are very useful in explaining how a sensorless BLDC system works, the real world is a much better teacher. Spinning a real motor and executing software on a real digital signal controller can quickly illustrate many concepts that might easily be misunderstood by just reading an app note. To help with this, there are numerous sensorless BLDC development boards available. The Motor Control Starter Kit (MCSK DM330015-ND) from Microchip is one of the least expensive and most full featured boards available. The MCSK includes a high performance dsPIC digital signal controller, an on-board USB programmer and debugger, a BLDC motor driver and a BLDC motor right on the board. See Figure 2.