In addition, with the advancement of integrated circuit manufacturing technology and the development of power electronics technology, AC servo has also been greatly developed. The combination of three-phase inverter and protection circuit, isolation circuit, energy-consuming braking circuit and other intelligent power modules, advanced power electronic devices, making AC servo control more convenient, lower power consumption, switching time Short, wide frequency range and superior performance. These all make the AC servo have obvious advantages over the DC servo.
1 System Overview
The hardware of the AC servo digital system is used as the signal processor by the DSP. The feedback signal is provided by the rotary encoder and the current sensor. The intelligent power module IPM is used as the inverter. The signal from the sensor is processed by filtering and shaping, and then fed back to the DSP for calculation. The DSP processes the reference signal and the feedback signal to adjust the current loop, speed loop, and position loop of the servo system. Finally, the output PWM signal is controlled by the IPM module to realize the servo closed loop control of the motor. The hardware structure of the system is shown in Figure 1.
Figure 1 hardware structure diagram
The control of the system is the three-loop control mode. The position control is the outer ring and the final target. The speed control is the middle ring and the current control is the inner ring. In order to ensure dynamic response speed and no oscillation during positioning, the current loop and speed loop are adjusted by PID, and the position regulator is adjusted by PI. The control block diagram of the system is shown in Figure 2:
Figure 2 control system block diagram
The actual position of the rotor position detected by the encoder is compared with the given position signal of the system, and the compared difference is outputted by the position adjuster PI to output a rotor speed reference signal, and the given speed signal is then compared with the actual speed signal detected by the encoder. After comparison, the compared difference is adjusted by the speed regulator, and the given current command value is output, and the PWM control is performed after the actual value of the current feedback is compared.
2 vector control
In a synchronous machine, the spatial angle between the excitation magnetic field and the armature magnetomotive force is not fixed, so adjusting the armature current cannot directly control the electromagnetic torque. Through the external control system of the motor, the armature magnetomotive force is spatially oriented with respect to the excitation magnetic field, and the angle between the two is controlled to maintain a fixed value, and the amplitude of the armature current is also controlled. This control method is called For vector control.
Vector control is equivalent to controlling the three-phase current of the armature by controlling the current of the two-phase rotor reference coordinate dq axis. This equivalent can be clearly understood through the previous system control block diagram, which can be expressed by the following formula:
The encoder mounted on the non-loaded shaft end of the motor detects the position of the rotor magnetic pole at any time, continuously obtains the position angle information, and knows the Î¸ by detecting the real-time, that is, the real-time coordinate change can be performed, and the converted current is performed on the inverter. Control, generate PWM waveform to control the motor.
3 Position and speed detection
The AC servo motor is equipped with an encoder for position and speed measurement. In most cases, the signal waveform directly from the encoder is irregular and cannot be directly used for control, signal processing and long-distance transmission. Therefore, the signal should be shaped. And the filter becomes a rectangular wave and then feeds back to the DSP. The processed two orthogonal encoder signals A and B are directly sent to the QEP pin of the DSP through voltage conversion, and the turn signal is generated by the decoding logic unit and 4 times. Frequency pulse signal. The turn signal is determined based on the phase lead lag of the two signals. Since there is a problem of forward and reverse, the counter is required to be reversible, so the general-purpose timer 2 is set to the directional up-down counting mode, and the multi-coded orthogonal coded pulse is counted as the input clock of the timer 2, and the direction of counting is performed. Determined by the turn signal, if the input phase of QEP1 is ahead, the count is incremented, otherwise it is counted down. The position and speed can be determined by the number of pulses and the pulse frequency. The total number of pulses per revolution is represented by M. The number of pulses at time T1 is m1, and the angle at which the motor rotates can be calculated according to the following equation.
If it is multi-turn, and then match the count value of the encoder's Z-phase zero pulse and the corresponding timer 2 clear, you can know how many turns the motor shaft turns. The calculation of the rotor speed of the motor can be based on the MT speed measurement method to determine the speed formula of the encoder as follows:
M1â€”the number of encoder pulses recorded by the counter within a fixed time;
M2â€”the number of clock pulses of the DSP recorded in a fixed time;
Nâ€”the number of encoder lines, that is, the number of pulses of the encoder before multiplying;
Fclkâ€”The clock frequency of the DSP.
In summary, the digital AC servo drive studied in this paper implements modular design, simple hardware structure and easy software programming. It can easily realize the communication between PC or PLC and controller, which realizes that the host computer can accept the real-time parameters of the control system and transfer parameters to the servo control system, and directly control the servo system.
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