Servo drives are an important component of modern motion control and are widely used in automated equipment such as industrial robots and CNC machining centers. Servo drives for controlling AC permanent magnet synchronous motors, in particular, have become a research hotspot both domestically and internationally. Current AC servo drive designs commonly employ a three-loop control algorithm based on vector control, encompassing current, speed, and position. The rationality of the speed closed-loop design within this algorithm plays a crucial role in the overall performance of the servo control system, especially in terms of speed control performance.
Basic requirements for servo drives
Requirements of servo feed system
1. Wide speed range
2. High positioning accuracy
3. It has sufficient transmission rigidity and high speed stability.
4. Fast response, no overshoot
In order to ensure productivity and processing quality, in addition to high positioning accuracy, good fast response characteristics are also required. That is, the response to tracking command signals must be fast, because the CNC system requires sufficient acceleration and deceleration when starting and braking to shorten the transition time of the feed system and reduce contour transition error.
5. High torque at low speeds, strong overload capacity
Generally speaking, servo drives have an overload capacity of more than 1.5 times for several minutes or even half an hour, and can be overloaded by 4 to 6 times in a short period of time without damage.
6. High reliability
The feed drive system of CNC machine tools is required to have high reliability, good working stability, strong adaptability to environmental conditions such as temperature, humidity, and vibration, and strong anti-interference ability.
Requirements for motors
1. The motor can run smoothly from the lowest speed to the highest speed with little torque fluctuation. Especially at low speeds such as 0.1 r/min or lower, it still maintains a stable speed without creeping.
2. The motor should have a large and long-term overload capacity to meet the requirements of low speed and high torque. Generally, DC servo motors are required to withstand overloads of 4 to 6 times the rated load for several minutes without damage.
3. In order to meet the requirements of fast response, the motor should have a small moment of inertia and a large stall torque, and have the smallest possible time constant and starting voltage.
4. The motor should be able to withstand frequent starting, braking and reversing.
Servo driver principle
Currently, most mainstream servo drives use digital signal processors (DSPs) as their control core, enabling complex control algorithms and achieving digitalization, networking, and intelligence. Power devices generally employ drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuitry and includes fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added to the main circuit to reduce the impact on the driver during startup. The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power or mains power is then frequency-converted by a three-phase sinusoidal PWM voltage-type inverter to drive the three-phase permanent magnet synchronous AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.
With the large-scale application of servo systems, the use, debugging, and repair of servo drives are important technical issues in servo drive technology today, and more and more industrial control technology service providers are conducting in-depth research on servo drives.
Servo drives are an important component of modern motion control and are widely used in automated equipment such as industrial robots and CNC machining centers. Servo drives for controlling AC permanent magnet synchronous motors, in particular, have become a research hotspot both domestically and internationally. Current AC servo drive designs commonly employ a three-loop control algorithm based on vector control, encompassing current, speed, and position. The rationality of the speed closed-loop design within this algorithm plays a crucial role in the overall performance of the servo control system, especially in terms of speed control performance.
Servo driver parameter settings
Servo motors are frequently used in automated equipment, especially for position control.
Most brands of servo motors have position control functions, which control the operation of the servo motor by sending pulses through the controller.
The number of pulses corresponds to the rotation angle, and the pulse frequency corresponds to the speed (related to the electronic gear setting).
When a new system fails to function correctly, first set the position gain, ensuring the motor operates without noise, and then set it to the highest possible value.
The rotational inertia ratio is also very important and can be referenced using the value set through self-learning.
Then set the speed gain and speed integration time to ensure continuous operation at low speeds and controllable position accuracy.
1. Position Proportional Gain: Sets the proportional gain of the position loop regulator. A higher value results in higher gain, greater stiffness, and less position hysteresis under the same frequency command pulse conditions. However, excessively high values may cause oscillation or overshoot. The parameter value is determined by the specific servo system model and load conditions.
2. Position Feedforward Gain: Sets the feedforward gain of the position loop. A higher setting indicates a smaller position lag under any command pulse frequency. A larger feedforward gain of the position loop improves the high-speed response characteristics of the control system, but it can also make the system's position unstable and prone to oscillations. When a high response characteristic is not required, this parameter is usually set to 0. Range: 0~100%
3. Speed Proportional Gain: Sets the proportional gain of the speed regulator. A higher value results in higher gain and greater stiffness. The parameter value is determined based on the specific servo drive system model and load conditions. Generally, the larger the load inertia, the higher the set value. Set a larger value if the system does not oscillate.
4. Speed Integral Time Constant: Sets the integral time constant of the speed regulator. The smaller the value, the faster the integral speed. The parameter value is determined based on the specific servo drive system model and load conditions. Generally, the larger the load inertia, the larger the set value. Set the smallest possible value while ensuring the system does not oscillate.
5. Speed Feedback Filter Factor: Sets the characteristics of the speed feedback low-pass filter. A higher value results in a lower cutoff frequency and less noise from the motor. If the load inertia is large, the setting can be reduced appropriately. A value that is too high will slow down the response and may cause oscillation. A lower value results in a higher cutoff frequency and a faster speed feedback response. If a higher speed response is required, the setting can be reduced appropriately.
6. Maximum Output Torque Setting: Sets the internal torque limit value of the servo drive. The setting value is a percentage of the rated torque, and this limit is effective at all times. Positioning Completion Range Setting: Sets the positioning completion pulse range under position control mode. This parameter provides the basis for the drive to determine whether positioning is complete under position control mode. When the remaining number of pulses in the position deviation counter is less than or equal to the setting value of this parameter, the drive considers positioning complete, and the position switch signal is ON; otherwise, it is OFF.
In position control mode, the output position positioning completion signal is displayed. The acceleration/deceleration time constant setting represents the acceleration time of the motor from 0 to 2000 r/min or the deceleration time from 2000 to 0 r/min. The acceleration/deceleration characteristic is linear. The speed range setting determines the speed reach. In non-position control mode, if the servo motor speed exceeds this setting, the speed reach switch signal is ON; otherwise, it is OFF. This parameter is not used in position control mode. It is independent of the rotation direction.
7. Manually adjust the gain parameters
Adjust the speed proportional gain (KVP) value. After the servo system is installed, parameters must be adjusted to ensure stable rotation. First, adjust the speed proportional gain (KVP) value. Before adjustment, the integral gain (KVI) and derivative gain (KVD) must be adjusted to zero. Then, gradually increase the KVP value; simultaneously observe whether oscillation occurs when the servo motor stops, and manually adjust the KVP parameter to observe whether the rotational speed fluctuates significantly. If the KVP value increases to the point where the above phenomenon occurs, the KVP value must be reduced back to eliminate oscillation and stabilize the rotational speed. This KVP value is the initially determined parameter value. If necessary, after adjusting KVI and KVD, repeated corrections can be made to achieve the ideal value.
Adjust the integral gain KVI value. Gradually increase the integral gain KVI value to allow the integral effect to gradually emerge. As can be seen from the previous introduction to integral control, the KVP value, when increased to a critical value in conjunction with the integral effect, will produce oscillations and instability. Similar to the KVP value, reduce the KVI value to eliminate the oscillations and stabilize the rotational speed. The KVI value at this point is the initially determined parameter value.
Adjust the differential gain (KVD) value. The main purpose of the differential gain is to smooth the rotation of the speed and reduce overshoot. Therefore, gradually increasing the KVD value can improve speed stability.
Adjust the position proportional gain (KPP) value. If the KPP value is set too high, the servo motor will experience excessive overshoot during positioning, causing instability. In this case, the KPP value must be reduced to decrease the overshoot and avoid the unstable region; however, it cannot be set too low, as this will reduce positioning efficiency. Therefore, careful adjustment is necessary.
8. Automatically adjust gain parameters
Modern servo drives are all microcomputer-based, and most offer automatic gain adjustment (autotuning) to handle most load conditions. When adjusting parameters, you can first use the automatic parameter adjustment function, and then adjust manually if necessary.
In fact, automatic gain control also has settings options, and the control response is generally divided into several levels, such as high response, medium response, and low response, which users can set according to their actual needs.
Servo driver bandwidth testing
The driver inputs a sinusoidal speed command with an amplitude of 0.01 times the rated speed command value. The frequency gradually increases from 1Hz, and the corresponding motor speed curve is recorded. As the frequency of the command sinusoidal wave increases, the phase lag of the motor speed waveform relative to the command sinusoidal wave gradually increases, while the amplitude gradually decreases. The frequency at which the phase lag increases to 90° is taken as the bandwidth of the 90° phase shift; the frequency at which the amplitude decreases to 1/√2 times is taken as the -3dB bandwidth, whichever condition is reached first.
The following section provides an explanation of the test method. For ease of description, this standard will be referred to as the "General Technical Conditions".
Among the control signal terminals of a servo drive, there are 1 to 2 analog voltage signal command input terminals for external input of speed or torque commands, commonly referred to as the "AD port of the servo drive". Generally, the speed command is a sine wave signal with an amplitude of ±10V, as shown in Figure 1.
If the servo driver does not set a threshold (voltage dead zone) for the input command, ideally +10V corresponds to the rated speed of the motor in forward rotation, and -10V corresponds to the rated speed of the motor in reverse rotation. The motor speed changes linearly with the change in the amplitude of the command voltage, as shown in Figure 2.
The General Technical Conditions state: "The driver inputs a sinusoidal speed command, the amplitude of which is 0.01 times the rated speed command value."
Assuming the speed command amplitude is ±10V and the rated speed of the motor is 6000RPM, that is, when a sine wave voltage signal with an input amplitude of 0.1V is applied, the speed of the servo motor is 60RPM.
The "General Technical Conditions" state: "As the frequency gradually increases from 1 Hz, record the corresponding speed curve of the motor."
The frequency of the sine wave gradually increases from 1 Hz, as shown in Figure 3.
Under the control of this command, the servo motor performs an action with four movements in one cycle: forward acceleration, forward deceleration, reverse acceleration, and reverse deceleration. When the sine wave command signal is at a certain frequency, the maximum speed of the servo motor is 60 RPM. As the frequency of the command sine wave increases, the phase lag of the motor speed waveform curve relative to the command sine wave curve gradually increases, while the amplitude gradually decreases.
The General Technical Conditions state: "The frequency at which the phase lag increases to 90° shall be taken as the bandwidth of the 90° phase shift; the frequency at which the amplitude decreases to 1/√2 times shall be taken as the -3dB bandwidth, whichever condition is met first shall prevail."
Taking the amplitude-frequency curve as an example, as shown in Figure 4. Here, the frequency f is the bandwidth of the servo driver.
