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FOC/DTC of permanent magnet synchronous motor, can you figure it out?
1. What is torque control
Permanent magnet synchronous motors are more and more widely used in automobiles, and their traces can be seen in actuators from power drives to steering brakes. Today I want to talk about the control of permanent magnet synchronous motor.
Anyone who does control knows that the control of any motor is nothing more than three different control goals:
Position control: it turns as many degrees as you want the motor to turn
Speed control: let the motor spin as fast as you want it to
Torque control: as much force as you want the motor to exert
But no matter what kind of control target, it is nothing more than the difference between one closed loop, two closed loops or three closed loops. As the innermost loop, torque control is essential. Today we will talk about what is torque control?
To control a motor, the first understanding of the controlled object is necessary. Let's use the following animation to help understand how a permanent magnet synchronous motor moves. After passing alternating voltages with a difference of 120 degrees on the three phases of the stator, a rotating magnetic field can be seen on the stator iron core (the red and green colors representing the direction of the magnetic field in the animation rotate counterclockwise). Under the action of the force generated by the magnetic field of the rotor, the rotor is driven to rotate.
How is the motor torque generated? In the previous article "Motor torque, speed and power of the motor", we analyzed that the over torque is proportional to the armature (stator) current;
So how is the current generated? We can think of each winding of the motor as a resistor + inductor rotating in a magnetic field
Assuming open-loop operation of the motor, when a rotating magnetic field is established for a given three-phase voltage of the motor stator with a difference of 120 degrees, if there is no load at this time, the motor will rotate rapidly (no load) until the back EMF and the given voltage Completely equal; at this time, the current in the stator winding is still 0, and the rotating magnetic field of the stator can be imagined (virtual/equivalent) into a magnet that rotates around the axis of the motor, and the imaginary south pole of the magnet is the same as the rotor magnet the North Pole axes coincide;
When there is a load on the rotor, according to Newton's theorem of motion, the speed of the motor must have a deceleration process, which means that the back EMF in the above equivalent circuit is reduced, and when the given voltage remains unchanged, the remaining Those voltages below will generate current in the resistor. What else happened during this period of deceleration? Because it was dragged by the load, the axis of the rotor magnet is at a later angle than the virtual axis of the stator magnet. This angle is what we call "power". Horn".
Regarding the vector model of the motor, various diagrams can be found on the Internet, but these diagrams are either too abstract and cannot be compared with the real objects after reading for a long time; It is of little significance for practical work guidance.
Therefore, in the actual work process, the author likes to rub together a large number of related vectors.
The small circle in the middle is the rotor (N pole and S pole), and there are AX, BY, CZ three-phase stators arranged 120 degrees away from each other on the periphery of the rotor.
Static two-phase coordinate axis: α coincides with stator A, β is 90 degrees ahead of α (green coordinate axis in the figure)
Rotate the two-phase coordinates to find: the d axis coincides with the N pole of the rotor, and the q axis is 90 degrees ahead of the d axis (the purple coordinate axis in the figure)
X-axis: stator rotating magnetomotive force ψs, which can be decomposed into rotor magnetomotive force ψf, id*Lq and Iq*Ld (red vector in the figure)
Voltage vector: the representation of the voltage in space that can be represented by the switch combination of the three-phase full bridge (yellow arrow)
Say one thousand, say ten thousand, the so-called torque control of the motor is to find a combination of some switch tubes (the yellow part in the figure) through a certain control algorithm to synthesize a given voltage for the motor stator (the large one in the figure). Red arrow), the torque corresponding to the current generated after the voltage cancels the back EMF is just balanced with the external load.
2. FOC and DTC
The current two main schools of motor torque control are field oriented control FOC and direct torque control DTC. Of course, these two control algorithms are applicable to all AC motors in principle. This article only talks about their use in permanent The similarities and differences of magnetic synchronous motor control.
FOC
The FOC control theory was first proposed by Siemens engineers in the 1970s. We mentioned above that the magnetic field generated by the stator can be virtualized as a high-speed rotating magnet around the rotor.
The stator magnetic potential can be decomposed into the d-axis magnetic potential and the q-axis magnetic potential. The d-axis magnetic potential is coaxial with the rotor magnetic potential, which cannot generate tangential torque, but will affect the magnetic field generated by the permanent magnet of the permanent magnet synchronous motor rotor; q The shaft is 90 degrees out of the rotor's magnetic potential, thus producing a tangential moment (similar to the interaction force produced by two perpendicular bar magnets).
The basic idea of FOC control is to convert the relevant variables in the three-phase static ABC coordinate system to the rotating coordinate system (d, q) for mathematical operations. the goal of. However, in the end, the three-phase voltage of the motor can only be the voltage in the static coordinate system, so in the control algorithm, the voltage of the dq axis needs to be converted into the ABC three-phase voltage again to the drive bridge. That is, there is a process from physical model→mathematical model→control algorithm→physical model.
To achieve FOC, the following inputs are required:
1. Three-phase current of the motor (two current sensors as shown in the figure above can be used, or a low-side or high-side bus current sensor can be used, and the three-phase current can be restored by the method of time-sharing sampling current reconstruction)
2. The position signal of the motor is indispensable
The following control modules are required:
1. Clark-Park transformation
2. PI adjustment of d-axis and q-axis
3. Inverse Clark-Park Transformation
4. SPWM/SVPWM (of course using SVPWM)
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