Common faults of servo drives and their solutions

Servo drives, also known as "servo controllers" or "servo amplifiers", are controllers used to control servo motors. They function similarly to frequency converters acting on ordinary AC motors. They are part of the servo system and are mainly used in high-precision positioning systems. They generally control servo motors in three ways: position, speed, and torque to achieve high-precision transmission system positioning. They are currently high-end products in transmission technology.

Basic Introduction

Servo drives are an important part of modern motion control and are widely used in automation equipment such as industrial robots and CNC machining centers. In particular, servo drives used to control AC permanent magnet synchronous motors have become a research hotspot at home and abroad. The current AC servo drive design generally adopts a three-loop control algorithm based on vector control: current, speed, and position. Whether the speed closed-loop design in this algorithm is reasonable or not plays a key role in the entire servo control system, especially the speed control performance.

In the servo drive speed closed loop, the real-time speed measurement accuracy of the motor rotor is crucial to improving the dynamic and static characteristics of the speed control of the speed loop. In order to find a balance between measurement accuracy and system cost, an incremental photoelectric encoder is generally used as a speed sensor, and the corresponding commonly used speed measurement method is the M/T speed measurement method. Although the M/T speed measurement method has certain measurement accuracy and a wide measurement range, this method has its inherent defects, mainly including:

1) At least one complete encoder pulse must be detected during the speed measurement cycle, which limits the minimum measurable speed;

2) It is difficult to keep the two control system timer switches for speed measurement in strict synchronization, and the speed measurement accuracy cannot be guaranteed in measurement situations with large speed changes. Therefore, the traditional speed loop design scheme using this speed measurement method is difficult to improve the speed following and control performance of the servo drive

How it works

At present, mainstream servo drives all use digital signal processors (DSP) as the control core.

It can realize relatively complex control algorithms, and realize digitalization, networking and intelligence. Power devices generally adopt drive circuits designed with intelligent power modules (IPM) as the core. The IPM integrates the drive circuit internally, and also has fault detection and protection circuits such as overvoltage, overcurrent, overheating, and undervoltage. A soft start circuit is also added to the main circuit to reduce the impact of the start-up process on the driver. 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 direct current. After the rectified three-phase power or mains power, the three-phase permanent magnet synchronous AC servo motor is driven by a three-phase sinusoidal PWM voltage inverter. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The main topology circuit of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.

With the large-scale application of servo systems, servo drive use, servo drive debugging, and servo drive maintenance are all relatively important technical topics for servo drives today. More and more industrial control technology service providers have conducted in-depth technical research on servo drives.

Servo drives are an important part of modern motion control and are widely used in automation equipment such as industrial robots and CNC machining centers. In particular, servo drives used to control AC permanent magnet synchronous motors have become a research hotspot at home and abroad. The current AC servo drive design generally adopts a three-loop control algorithm based on vector control: current, speed, and position. Whether the speed closed-loop design in this algorithm is reasonable or not plays a key role in the entire servo control system, especially the speed control performance.

Basic Requirements

Requirements for servo feed systems

1. Wide speed regulation range

2. High positioning accuracy

3. Sufficient transmission rigidity and high speed stability

4. Fast response, no overshoot

In order to ensure productivity and processing quality, in addition to requiring high positioning accuracy, it is also required to have good rapid response characteristics, that is, the response of the tracking command signal must be fast, because when the CNC system starts and brakes, the acceleration and deceleration must be large enough to shorten the transition time of the feed system and reduce the contour transition error.

5. Low speed, high torque, strong overload capacity

Generally speaking, a servo drive has an overload capacity of more than 1.5 times within a few minutes or even half an hour, and can be overloaded 4 to 6 times in a short period of time without being damaged.

6. High reliability

The feed drive system of the CNC machine tool is required to have high reliability, good working stability, strong adaptability to environments such as temperature, humidity, vibration, and strong anti-interference ability.

Requirements for motors

1. The motor can run smoothly from the lowest speed to the highest speed, and the torque fluctuation is small, especially at low speeds such as 0.1r/min or lower, there is still a stable speed without creeping.

2. The motor should have a large overload capacity for a long time to meet the requirements of low speed and high torque. Generally, DC servo motors are required to be overloaded 4 to 6 times within a few 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 reversal.

Common faults of servo drives and their solutions

1. The LED light is green, but the motor does not move

(1) Cause of the fault: The motor is prohibited from moving in one or more directions.

Solution: Check the +INHIBIT and –INHIBIT ports.

(2) Cause of the fault: The command signal is not to the driver signal ground.

Solution: Connect the command signal ground and the driver signal ground.

2. After power-on, the LED light of the driver is not on

Cause of failure: The power supply voltage is too low, less than the minimum voltage requirement.

Solution: Check and increase the power supply voltage.

3. When the motor rotates, the LED light flashes

(1) Cause of failure: Hall phase error.

Solution: Check whether the motor phase setting switch is correct.

(2) Cause of failure: HALL sensor failure.

Solution: Detect the voltage of Hall A, Hall B, and Hall C when the motor is rotating. The voltage value should be between 5VDC and 0.

4. The LED light always stays red

Cause of failure: There is a failure.

Solution: Cause: overvoltage, undervoltage, short circuit, overheating, driver disabled, HALL invalid.

5. Motor stall

(1) Cause of the fault: The polarity of the speed feedback is wrong.

Treatment method:

a. If possible, turn the position feedback polarity switch to another position. (This is possible on some drives)

b. If using a tachometer, swap TACH+ and TACH- on the driver.

c. If an encoder is used, swap ENC A and ENC B on the driver.

d. In the HALL speed mode, swap HALL-1 and HALL-3 on the driver, and then swap Motor-A and Motor-B.

(2) Cause of the fault: Encoder power supply lost power during encoder speed feedback.

Solution: Check the 5V encoder power supply. Make sure it can provide enough current. If using an external power supply, make sure the voltage is relative to the driver signal ground.

6. The motor runs faster in one direction than in another

(1) Cause of the fault: The phase of the brushless motor is wrong.

Solution: Detect or find out the correct phase.

(2) Cause: The test/deviation switch was in the test position when not in use for testing.

Solution: Turn the test/deviation switch to the deviation position.

(3) Cause of the fault: The deviation potentiometer position is incorrect.

Solution: Reset.

7. When checking the current monitoring output of the driver with an oscilloscope, it is found that it is full of noise and cannot be read.

Cause of the fault: The current monitoring output is not isolated from the AC power supply (transformer).

Treatment method: You can use a DC voltmeter to detect and observe.

8. How to deal with the motor deviation counter overflow error when the servo motor rotates at high speed?

(1) Cause of the fault: Motor deviation counter overflow error occurs during high-speed rotation;

Solution: Check whether the wiring of the motor power cable and encoder cable is correct and whether the cables are damaged.

(2) Cause of the fault: Motor deviation counter overflow error occurs when a longer command pulse is input

Treatment method:

a. The gain setting is too large. Adjust the gain manually or use the automatic gain adjustment function.

b. Extend the acceleration and deceleration time;

c. The load is too heavy, so you need to reselect a motor with a larger capacity or reduce the load, or install a reducer or other transmission mechanism to increase the load capacity.

(3) Cause of the fault: A motor deviation counter overflow error occurred during operation.

Treatment method:

a. Increase the deviation counter overflow level setting value;

b. Slow down the rotation speed;