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 positioning of the transmission system. Currently, they represent a high-end product in transmission technology.
Basic Introduction
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.
Working 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.
Basic requirements
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.
Common Servo Driver Faults and Troubleshooting
1. The LED light is green, but the motor is not moving.
(1) Cause of failure: Motors in one or more directions are prohibited from operating.
Solution: Check the +INHIBIT and –INHIBIT ports.
(2) Cause of the fault: The command signal is not grounded to the driver signal.
Solution: Connect the command signal ground and the driver signal ground.
2. After powering on, the driver's LED light does not illuminate.
Cause of the fault: The power supply voltage is too low, below the minimum voltage requirement.
Solution: Check and increase the power supply voltage.
3. The LED light flashes when the motor rotates.
(1) Cause of failure: HALL phase error.
Solution: Check if the motor phase setting switch is correct.
(2) Cause of failure: HALL sensor failure.
Solution: Detect the voltage at Hall A, Hall B, and Hall C while the motor is running. The voltage values should be between 5VDC and 0V.
4. The LED light remains red at all times.
Cause of the fault: A fault exists.
Troubleshooting: Causes: Overvoltage, undervoltage, short circuit, overheating, driver disabled, HALL ineffective. 5. Motor stall.
(1) Cause of failure: The polarity of the speed feedback is incorrect.
Solution:
a. If possible, switch the position feedback polarity switch to another position. (This is possible on some drivers.)
b. If using a speed measuring machine, swap the TACH+ and TACH- connections on the driver.
c. If using an encoder, swap the connections of ENC A and ENC B on the driver.
d. In HALL speed mode, swap HALL-1 and HALL-3 on the driver, and then swap Motor-A and Motor-B.
(2) Cause of failure: The encoder power supply is lost during encoder speed feedback.
Troubleshooting: Check the 5V encoder power supply connection. Ensure this power supply can provide sufficient current. If using an external power supply, ensure this voltage is connected to the driver signal ground.
6. A motor runs faster in one direction than in the other.
(1) Cause of failure: The phase of the brushless motor is wrong.
Solution: Detect or identify the correct phase.
(2) Cause of failure: The test/deviation switch is in the test position when not in use for testing.
Solution: Set the test/deviation switch to the deviation position.
(3) Cause of failure: The position of the deviation potentiometer is incorrect.
Solution: Reset.
7. When checking the current monitoring output of the driver with an oscilloscope, it was found to be entirely noise and could not be read.
Cause of the fault: The current monitoring output is not isolated from the AC power supply (transformer).
Solution: Use a DC voltmeter to check and observe.
8. What should I do if a motor deviation counter overflow error occurs when the servo motor is rotating at high speed?
(1) Cause of the fault: The motor deviation counter overflowed during high-speed rotation;
Troubleshooting: Check if the wiring of the motor power cable and encoder cable is correct and if the cables are damaged.
(2) Cause of the fault: The motor deviation counter overflowed when a long command pulse was input.
Solution:
a) The gain setting is too high. Manually adjust the gain or use the automatic gain adjustment function.
b. Extend the acceleration and deceleration time;
c. If the load is too heavy, it is necessary to select a larger capacity motor or reduce the load, and install a speed reducer or other transmission mechanism to improve the load capacity.
(3) Cause of failure: The motor deviation counter overflowed during operation.
Solution:
a. Increase the overflow level setting of the deviation counter;
b. Slow down the rotation speed;
c. Extend the acceleration and deceleration time;
d. If the load is too heavy, it is necessary to select a larger capacity motor or reduce the load, and add a speed reducer or other transmission mechanism to improve the load capacity.
9. How to handle a servo motor that does not run when there is a pulse output?
① Monitor the current value of the pulse output of the controller and whether the pulse output light is flashing to confirm that the command pulse has been executed and the pulse has been output normally;
② Check the control cables, power cables, and encoder cables from the controller to the drive for incorrect wiring, damage, or poor contact;
③ Check if the brake of the servo motor with the brake has been disengaged;
④ Monitor the servo drive panel to confirm whether the pulse command has been input;
⑤ The Run command was executed normally;
⑥ The position control mode must be selected;
⑦ Are the input pulse type and command pulse settings of the servo driver consistent?
⑧ Ensure that the forward drive prohibition signal, reverse drive prohibition signal, and deviation counter reset signal are not input, disconnect the load and run normally under no-load, and check the mechanical system.
