Q: There are many types of servo motors that we commonly use. The speed of servo motors varies from 1000, 1500, to 3000. Taking the most commonly used 3000rpm AC servo as an example: If we need a speed of 0~3000 , what methods can we use to change the current servo speed?
A: The adjustment of servo speed depends on the control method used. Whether it's pulse control, analog control, or direct speed adjustment via the driver's internal settings, the corresponding methods differ. Below is a summary of speed changes for these three different control methods.
01. Torque control , speed is free ( varies with load )
Torque control is a commonly used control method. We set the output torque through external analog signals or direct address assignment. However, the corresponding speed is not fixed because changes in the friction coefficient due to equipment aging and changes in load will affect the output speed. In this case, we generally do not need to adjust the speed because it is automatically adjusted. What we need is the stability of the system and the continuous torque stability over a long period of time.
The torque setting can be changed in real time by altering the analog quantity settings, or it can be achieved by changing the corresponding address value via communication. Its main applications are in winding and unwinding devices where there are strict requirements on the stress on the material, such as winding machines or fiber optic drawing equipment. The purpose of using a servo system is to prevent changes in the winding material from altering the stress.
02. Position control ensures precise positioning ; both speed and torque can be strictly controlled .
Position control mode typically determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servos can also directly assign speed and displacement values via communication.
Because position mode allows for precise control over both speed and position, it is generally used in positioning devices. Applications include CNC machine tools, printing machinery, and more. In practice, we need to understand the rated frequency of the PLC or other pulse transmitter, the actual distance to be moved, and the corresponding pulse equivalent of the servo. This allows us to calculate the upper limit of the servo's movement speed and time to reach the specified position.
Servo online speed is something we must calculate; only by selecting the appropriate servo model can we meet the requirements of the field application. Servo online speed = Rated command pulse frequency × Servo upper limit speed
Servo controllers typically include an encoder and can receive feedback pulses from the encoder. The encoder feedback pulse frequency is set on the speed loop: Encoder feedback pulse frequency = Encoder cycle feedback pulse count × Servo motor set speed (r/s)
Furthermore, since the command pulse frequency = encoder feedback pulse frequency / electronic gear ratio, the " command pulse frequency " can also be set to set the servo motor speed.
03. Speed mode : Torque is flexible ( varies with load ) .
Rotation speed can be controlled by analog input or pulse frequency. In the outer loop PID control with a host control device, speed mode can also be used for positioning, but the position signal of the motor or the position signal of the direct load must be fed back to the host for calculation.
The speed mode corresponds to the position mode. The position signal has an error. The position mode signal is provided by the terminal load detection device, which reduces intermediate transmission errors and relatively increases the positioning accuracy of the entire system.
Speed control has a wide range of applications: continuous speed control systems requiring rapid response; positioning systems with a closed-loop control system; and systems requiring rapid switching between multiple speeds. Speed control primarily uses a 0-10V voltage signal to control motor speed. The magnitude of the analog signal determines the given speed, and its sign determines the motor's direction of rotation. The relationship between the analog signal and speed depends on the speed command gain. When using speed control in applications with high load inertia, we need to set the speed loop gain to make the system respond more quickly. Adjustments must take into account equipment vibration; system vibration should not be caused by the increased response speed.
When using speed control, we also need to pay attention to the acceleration and deceleration settings. Without closed-loop control, we need to use zero clamping or proportional control to bring the motor to a complete stop. When using a host computer for position closed-loop control, the analog signal cannot be automatically zeroed.
The speed is controlled by sending ±10V analog voltage commands to the servo driver through the control system . Its advantage is fast servo response, but its disadvantage is that it is more sensitive to field interference and the debugging is slightly complicated.
Q: During the use and debugging of servo systems, various unexpected interferences may occur from time to time, especially in applications involving servo motors that send pulses. What are some effective anti-interference measures?
A: The field application environment of servo systems is usually quite complex. Below, we will analyze the types and generation methods of interference from three aspects: interference from the power supply, interference from a chaotic grounding system, and interference from within the system, in order to achieve targeted anti-interference purposes.
01. Interference from the power supply
Field operating conditions present various limitations, and we often encounter many complex situations. We need to habitually avoid these issues and mitigate the causes of problems as much as possible. In many cases, we reduce power supply interference and prevent servo control system malfunctions by adding voltage regulators, isolation transformers, filters to the power supply module of the rotary encoder and motion controller , reconnecting the driver to a DC reactor , and modifying the low-pass filter time and carrier rate parameters of the driver.
Servo system power lines should be routed in separate cable trays, and measures such as shortening the distance between the driver and motor power lines should be taken to avoid interference with control lines and cause driver failure.
02. Interference from a disordered grounding system
Grounding is an effective way to improve the anti-interference ability of electronic equipment. It can suppress the interference emitted by the equipment and prevent it from being affected by external interference. However, incorrect grounding can introduce serious interference signals, making the system unable to work properly. The ground wire of the control system generally includes system ground, shield ground, AC ground, and protective ground.
If the grounding system is chaotic, the main interference to the servo system is the uneven potential distribution at various grounding points. Potential differences exist between different grounding points, such as the two ends of the cable shield, the grounding wire, the earth, and the grounding points of other equipment, causing ground loop currents and affecting the normal operation of the system.
The key to solving this type of interference lies in distinguishing the grounding method and providing good grounding performance for the system.
When grounding servo motors, ensure environmental electromagnetic compatibility and shield against high-frequency electromagnetic waves and radio frequency devices. Suppress and eliminate sources of power supply noise interference; for example, avoid having high-frequency, medium-frequency, or high-power rectifiers and inverters on the same power transformer or distribution bus .
Here's an unconventional grounding approach: because power distribution lines inevitably contain large interference sources, the driver is installed separately in a cabinet with a non-metallic mounting plate. The ground wires related to the servo driver are all floating, while other measurement systems are reliably grounded. This approach might be better.
03. Interference from within the system
Interference is mainly caused by electromagnetic radiation between internal components and circuits, such as mutual radiation between logic circuits, mutual influence between analog and logic grounds, and mismatched use of components. • Shielded cables should be used for signal and control lines to help prevent interference.
• When the line is long, for example, the distance exceeds 100m , the conductor cross-section should be increased.
• Signal and control lines are best run in conduits to avoid interference with power lines.
• Current signals are primarily used for signal transmission due to their relatively good attenuation and interference immunity. In practical applications, sensor outputs are mostly voltage signals, which can be converted using a converter. Filtering the DC power supply of analog weak circuits can be achieved by adding two 0.01uF (630V) capacitors, one end connected to the positive and negative terminals of the power supply, and the other end connected to the chassis and then to ground. This is very effective.
• When the servo emits a squeaking sound, it is due to high-frequency harmonic interference. You can try connecting a 0.1u/630v CBB capacitor to the P and N terminals of the servo drive bus power supply to the chassis.
• Connect the shielding layer of the control line on the board to the 0V of the board , and do not connect it to the driver end. Simply pull out a section of the shielding layer, twist it into a strand, and expose it to the outside.
• Use an electromagnetic EMI filter, solder an anti-interference resistor onto the control line, or connect a magnetic ring to the motor power line.
