Analysis of key points of non-inductive scheme control of brushless motor
Brushless and non-inductive control is extremely extensive in practical applications, and people have studied it for a long time. Its control difficulties mainly include two points: first, the start of the motor; second, the detection of the rotor position.
For the high-voltage non-inductive solution, in addition to the difficulties in software, the hardware design can not be ignored. If the hardware design is slightly inappropriate, the interference of the entire control board will be great, which will increase the difficulty of the entire solution.
Below we mainly discuss the low-pressure non-inductive scheme. For the low-voltage non-inductive scheme, the hardware design on the market is similar, and the method of detecting the position of the rotor is almost always using the back electromotive force detection method.
1. Why is it so difficult to start the motor without feeling?
For the brushless motor, the operation of the motor is controlled by the electronic switch. Therefore, if the motor is to operate normally and efficiently, it is necessary to know the position of the rotor before the normal commutation. The problem is that the motor has no sensor. Did not turn, so the position of the rotor is not known, so the start of the senseless rotation will start, let the motor rotate at a certain rate, in the process of the motor automatically, we know the position of the rotor by detecting the back electromotive force To get the correct phase of the commutation.
The self-starting of the motor is simple and difficult to do. I have summarized the following experiences for reference during the process of debugging many non-inductive solutions:
(1) First, it is self-rotating. The rotation must make the motor run smoothly, can not shake, and can not cause large current. This is a very critical step in the success of the startup. How to achieve this effect, we must adjust the PWM duty cycle and the length of the commutation time during the debugging process.
(2), the number of starting steps can not be too small, not too much, generally ten steps is enough, after the motor runs ten steps, it starts to detect the back electromotive force. When the correct back electromotive force is detected, the motor will run normally. Woke up.
2. How to detect back electromotive force
There are two ways to detect the back electromotive force. The first is to use the internal AD sampling back EMF signal for comparison, and the second is to compare directly with the comparator. The idea of the two methods is the same, but according to personal experience, the comparator scheme is more reliable and the performance is better. Especially when the motor speed requirement is very high, it is almost impossible to use the AD sampling method.
Although the comparator solution is more advantageous, why is it common to use AD sampling on the market? This is mainly due to the problem of product cost. Do not use a comparator IC to increase the cost and increase the layout space of the PCB. The second is to find a microcontroller with an internal AD. Higher.
Taking the comparator scheme as an example, when the motor is rotated, the comparator interrupt is turned on (the comparison object is: the midpoint voltage and the voltage value of the suspended phase). When the comparison interrupt arrives, the phase is immediately commutated, and the comparator is set after the phase change. The comparison object, that is, the midpoint voltage and the current dangling phase voltage value, waits for the next comparison interrupt to arrive.
The above control method is a control method with no delay of 30 degrees. In the general control system, this control method is feasible, especially for the scheme of no large current and large torque, and the control method without delay will be more stable, and the load will be more stable. The ability will be stronger. Of course, there are drawbacks to this, that is, the efficiency is not as high as the 30-degree delay. In what way to do it, you have to look at the actual product.
