This article aims to explain the basic principles of PMSM motor control and the method for obtaining motor position offset values, providing some assistance to beginners in PMSM motor control.
PMSM stands for Permanent-magnet Synchronous Motor.
It needs to meet several characteristics:
(1) The alternating voltage with a phase sequence difference of 120 degrees between the three phase stators generates a rotating stator magnetic field.
(2) The rotor is excited by permanent magnets, regardless of whether the excitation material is AlNiCo, ferrite or NdFeB; regardless of whether it is embedded or surface-mounted, through special stator and rotor shape design, a sinusoidal NS magnetic field is finally presented in the air gap space.
(3) Based on the above two points, the back EMF must be a sine wave, which is the biggest difference between PMSM and BLDC (back EMF trapezoidal wave).
Since the rotor of the PMSM is already fixed, the question of how to control the motor can be simplified to: how to make the rotor shaft of the motor generate a certain torque?
(1) If I were from a distant era and unfortunately traveled to the modern era, the knowledge I possessed in my time would be this: magnets have two polarities, N and S, like poles repel each other, and unlike poles attract each other.
I would imagine taking another magnet close to the rotor and then making circles around the stator with the magnet in my hand. Then, a force would be generated between the magnet in my hand and the rotor, causing the rotor to rotate.
(2) If I came from a slightly more recent era, when the great Mr. Oersted and Mr. Ampère had already discovered that electricity can generate magnetism, and that sinusoidal alternating current can generate a circular magnetic field that moves in space (isn't this the same effect I had when I drew circles with a magnet before?), then my work would be simple. I would just need to arrange stator coils around the rotor and pass an alternating current through the stator coils to make the rotor move under the influence of the magnetic field.
(3) If I came from a more recent era, the brilliant Mr. Park and Mr. Clark had already begun scientific research. They discovered a problem: a three-phase alternating current with a constant rotation speed and a phase difference of 120° has only two degrees of freedom. However, if we observe the three-phase voltage from the perspective of the moving magnetic field generated by the three-phase voltage, we will find that its projection in the direction perpendicular to the magnetic field does not change with time. This greatly facilitates the solution of the control problem.
The digression has gone on a bit, but in any case, (2) and (3) both require the use of motor rotation angle to generate alternating stator voltage.
The block diagram of the simplest standard permanent magnet synchronous motor control algorithm is as follows:
This control algorithm block diagram looks very complex, but in essence, its purpose can be described in the following sentences:
(1) Because there is a moving magnetic field in the air gap, the stator coil will generate a back electromotive force when the rotor rotates;
(2) The easiest way to establish a stator magnetic field is to define a voltage with the same phase angle as the back EMF in the inverter. The stator magnetic field generated by the stator current after overcoming the back EMF interacts with the rotor magnetic field and does work on the outside.
In short, the so-called motor control algorithm is nothing more than generating three stator voltages. The algorithm requires that the generated sinusoidal voltage be in phase with the back electromotive force generated by the stator cutting the magnetic lines of force.
(The reason for using the word "easiest" is that the differential equations for motor control actually have infinitely many solutions, and the one we commonly use in engineering is just one particular solution. To put it simply, there are countless combinations of voltage applied to the stator coils that can achieve the same torque output while the rotor is moving at a constant speed. In engineering applications, we simply use the easiest method to obtain.)
Therefore, there are several methods for determining the initial position angle of the motor controller regarding the motor position:
(1) Back potential method
a) The motor phase wires are not connected to the ECU, but the motor position signal is connected to the ECU. The software in the ECU only needs to calculate the angle; no other control software is required.
b) Rotate the motor shaft by hand; the motor angle signal observed in the software should change from 0 to 360 degrees.
c) Use a servo motor or other equipment to drive the motor under test to rotate at a constant speed and measure its back EMF. The back EMF and the motor rotation angle after software processing should have the following correspondence (0 degrees corresponds to A back EMF from positive 0 to negative 0, 120 degrees corresponds to B back EMF from positive 0 to negative 0, 240 degrees corresponds to C back EMF from positive 0 to negative 0).
d) If the relationship between back EMF and motor rotation angle is not satisfied, the following methods should be used to make it conform to this relationship:
i. Adjust the motor position angle Offset in the software.
ii. Change the definition of the motor's A, B, and C phases (reverse the wiring order)
(2) Rotor-driven method
(3) Torque calibration method
