Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly.
In automatic control systems, servo motors are used as actuators and possess characteristics such as small electromechanical time constant, high linearity, and low starting voltage. They convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors. Their main characteristic is that they exhibit no self-rotation when the signal voltage is zero, and their speed decreases uniformly as torque increases. As the power source of automated factories, servo motors are unavoidable in industrial control design and maintenance. Therefore, today we will summarize and learn about servo motor speed control and anti-interference measures.
There are many types of servo motors commonly used, and selecting the right one is not a simple task. Being proficient in every type would be very challenging for learning. Our best approach is to focus on the models we encounter most frequently in our work, while also learning about the most commonly used models and brands on the market. Servo motor speeds vary from 1000 RPM to 1500 RPM and 3000 RPM; we'll use the most common 3000 RPM AC servo as an example. In practical use, if we've selected or are currently using a 3000 RPM servo and need a speed range of 0-3000 RPM, how can we change the current servo speed? Adjusting the servo speed depends on the control method used. Whether we're using pulse control, analog control, or direct internal driver settings, the corresponding methods will differ. We'll summarize speed changes for three different control methods.
1. Torque control, where the 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. The corresponding speed is not fixed because changes in the friction coefficient due to equipment aging and load variations will affect the output speed. In this case, we generally don't need to adjust the speed because it's automatic; what we need is system stability and sustained torque stability over a long period. The set torque can be changed in real-time by altering the analog signal setting, or it can be achieved by changing the corresponding address value via communication. Applications are mainly in winding and unwinding devices where there are strict requirements on the force applied to the material, such as winding machines or fiber optic drawing equipment. The purpose of using a servo motor is to prevent changes in the winding material from altering the force applied.
2. Position control, precise positioning, and strict control over both speed and torque: Position control mode generally 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 offers very strict control over both speed and position, it is generally used in positioning devices. Application areas include CNC machine tools, printing machinery, etc. When using servos, we need to know the rated frequency of the pulses sent by the PLC or other system (20kHz, 100kHz, 200kHz), 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 operating speed and time to move to the specified position. The servo's on-line speed is something we must calculate; only by selecting the appropriate servo model can we meet the requirements of the field. Servo online operating speed = command pulse rated frequency × servo upper limit speed. Servo controllers generally have an encoder and can receive encoder feedback pulses. The encoder feedback pulse frequency is set on the speed loop. The encoder feedback pulse frequency setting = encoder weekly feedback pulse count × servo motor set speed (r/s). 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.
3. Speed Mode: Torque is free (varies with load). Rotation speed can be controlled via analog input or pulse frequency. With external PID control from a host control device, speed mode can also be used for positioning, but the motor position signal or the position signal of the direct load must be fed back to the host for calculation. Speed mode corresponds to position mode. Position signals have errors; the position mode signal is provided by the terminal load detection device, reducing intermediate transmission errors and relatively increasing the positioning accuracy of the entire system. Our speed control mode mainly uses a 0-10V voltage signal to control the motor speed. The magnitude of the analog signal determines the given speed, and the sign determines the motor response. The relationship depends on the speed command gain. When using speed mode with high load inertia, we need to set the speed loop gain for a faster system response. Adjustments must consider equipment vibration; system vibration should not be caused by the response speed. When using speed control, attention must also be paid to 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 automatically zero. Speed control via a servo drive using +/-10V analog voltage commands from the control system offers advantages such as fast servo response, but it is also sensitive to field interference and requires slightly more complex debugging. 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. During the use and debugging of servo systems, various unexpected interferences can occur from time to time, especially in applications involving pulse-transmitting servo motors.
The following analysis will examine the types and generation methods of interference from several aspects in order to achieve targeted anti-interference measures. We hope everyone will study and research this together.
1. Interference from power supply: On-site operating conditions are subject to various limitations, and we often encounter many complex situations. We need to habitually avoid these issues and minimize the potential causes of problems. In many cases, we reduce interference caused by power supply inputs 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 the distance between the driver and motor power lines should be shortened, among other measures, to avoid interference with control lines and prevent driver failure.
2. Interference from a Disorganized Grounding System: Grounding is an effective way to improve the anti-interference capabilities of electronic equipment, suppressing external interference and preventing it from being affected by external interference. However, incorrect grounding can introduce serious interference signals, making the system unable to function properly. Control system grounding typically includes system ground, shield ground, AC ground, and protective ground. If the grounding system is disorganized, the interference to the servo system is mainly due to 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 clearly identifying the grounding method and providing good grounding performance for the system. When grounding the servo drive, pay attention to environmental electromagnetic compatibility and shield against high-frequency electromagnetic waves and radio frequency devices. Power supply noise interference sources should be suppressed and eliminated. For example, avoid having high-frequency, medium-frequency, or high-power rectifiers and inverters on the same power transformer or distribution bus. An unconventional grounding method is introduced because power distribution lines inevitably contain large interference sources. The driver is installed separately in a cabinet with a non-metallic mounting plate. All grounding wires related to the servo driver are floated, while other measurement systems are reliably grounded. This approach may be better.
3. Interference from within the system mainly arises from 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 prevent interference. When the line is long, for example, more than 100m, the conductor cross-section should be increased. Signal and control lines are best placed in conduits to avoid interference with power lines. Current signals are preferred for transmission, as they have relatively good attenuation and anti-interference capabilities. 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 indicates high-frequency harmonic interference. Try connecting a 0.1uF/630V CBB capacitor to the P and N terminals of the servo drive bus power supply to the chassis. The shielding layer of the control line on the board should be connected to 0V on the board, but not on the driver side. Simply pull out a section of the shielding layer, twist it into a strand, and expose it. Use an electromagnetic EMI filter, solder an anti-interference resistor to the control line, or connect a ferrite core to the motor power line. Actual field conditions are much more complex, requiring specific analysis of each problem. However, there will always be a satisfactory solution; only the process may differ!
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