This article mainly introduces servo drives, and focuses on a detailed explanation of the pulses of servo drives.
Table of contents
What is a servo driver?
How a servo driver works
Basic requirements of servo feed system
What kind of pulses does a servo driver require?
Other troubleshooting techniques for servo motors
What is a servo driver?
Servo drives , also known as servo controllers or servo amplifiers, are controllers used to control servo motors. Their function is similar to that of a frequency converter for a regular AC motor. They are part of a servo system and are primarily used in high-precision positioning systems. Generally, they control the servo motor through position, speed, and torque to achieve high-precision transmission system positioning. Currently, they represent a high-end product in transmission technology.
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.
In the speed closed loop of a servo drive, the real-time speed measurement accuracy of the motor rotor is crucial for improving the dynamic and static characteristics of the speed loop's speed control. To achieve a balance between measurement accuracy and system cost, incremental photoelectric encoders are generally used as speed sensors, and the commonly used speed measurement method is the M/T (Mean Transmission/Turning) method. While the M/T method offers a certain level of measurement accuracy and a relatively wide measurement range, it has inherent drawbacks, primarily including:
1) At least one complete encoder pulse must be detected within the speed measurement cycle, which limits the minimum measurable speed;
2) The timer switches of the two control systems used for speed measurement are difficult to keep strictly synchronized, and speed measurement accuracy cannot be guaranteed in measurement situations with large speed variations. Therefore, the traditional speed loop design scheme using this speed measurement method is difficult to improve the speed tracking and control performance of the servo driver.
How a servo driver works
Currently, most mainstream servo drives use digital signal processors (DSPs) as their control core, enabling the implementation of 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 also features 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-source 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.
Basic requirements of servo feed system
Requirements for servo feed systems:
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.
What kind of pulses does a servo driver require?
Positive and negative pulse control (CW+CCW); pulse plus direction control (pulse+direction); AB phase input (phase difference control, commonly used in handwheel control).
The servo driver main program is mainly used to complete system initialization, LO interface control signals, and settings of registers of various control modules in the DSP.
After all the initialization work of the servo driver is completed, the main program enters a waiting state and waits for an interrupt to occur so that the current loop and speed loop can be adjusted.
The interrupt service routines mainly include the four-timer interrupt routine, the photoelectric encoder zero pulse capture interrupt routine, the power drive protection interrupt routine, and the communication interrupt routine.
Other troubleshooting techniques for servo motors
(1) Motor running: When running occurs during feeding, the speed measurement signal is unstable, such as cracks in the encoder; poor contact of the wiring terminals, such as loose screws; when running occurs at the moment of reversal between forward and reverse movement, it is generally due to the reverse backlash of the feed transmission chain or excessive servo drive gain.
(2) Motor creep: This mostly occurs during the starting acceleration phase or low-speed feed. It is generally caused by poor lubrication of the feed transmission chain, low gain of the servo system, and excessive external load. In particular, it is important to note that the coupling used to connect the servo motor and the ball screw may be loose or have defects such as cracks, causing the ball screw and the servo motor to rotate out of sync, resulting in inconsistent feed speed.
(3) Motor vibration: When the machine tool is running at high speed, vibration may occur, which will trigger an overcurrent alarm. Machine tool vibration problems are generally speed-related, so the speed loop problem should be investigated.
(4) Reduced motor torque: When the servo motor goes from its rated stall torque to high speed, the torque may suddenly decrease. This is caused by damage to the heat dissipation of the motor windings and overheating of the mechanical parts. At high speeds, the temperature rise of the motor increases. Therefore, the load of the motor must be verified before using the servo motor correctly.
(5) Motor position error: When the servo axis movement exceeds the position tolerance range (KNDSD100 factory standard setting PA17:400, position over-tolerance detection range), the servo drive will generate a position over-tolerance alarm "4". The main reasons are: the system's set tolerance range is too small; the servo system gain setting is improper; the position detection device is contaminated; the cumulative error of the feed transmission chain is too large, etc.
(6) The motor does not turn: In addition to the pulse + direction signal, the CNC system also has an enable control signal to the servo drive, which is generally the DC + 24V relay coil voltage. Common diagnostic methods for the servo motor not turning include: checking whether the CNC system has a pulse signal output; checking whether the enable signal is connected; observing whether the system input/output status meets the starting conditions of the feed axis through the LCD screen; confirming that the brake is open for servo motors with electromagnetic brakes; driver malfunction; servo motor malfunction; failure of the coupling between the servo motor and the ball screw or key disengagement, etc.
summary
In summary, the correct use of servo drives for CNC machine tools, in addition to setting parameters correctly according to the user manual, also requires flexible operation based on the usage environment and load conditions. In practice, only users with strong parameter understanding and practical skills can explore the techniques for debugging drives and motors, and effectively utilize servo drives and servo motors.
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