Servo systems are a crucial component in electromechanical products, offering the highest levels of dynamic response and torque density. Therefore, the development trend of drive systems is to replace traditional hydraulic, DC, stepper, and AC variable frequency drives with AC servo drives, achieving a new level of system performance, including shorter cycle times, higher productivity, better reliability, and longer lifespan. To realize the superior performance of servo motors , it is essential to understand their specific characteristics.
This article will briefly analyze common problems encountered in the use of servo motors:
Problem 1: Noise, instability
When customers use servo motors on certain machines, they often encounter problems such as excessive noise and unstable operation of the motor driving the load. When this happens, many users' first reaction is that the servo motor is of poor quality, because sometimes when stepper motors or frequency converter motors are used to drive the load, the noise and instability are much less. On the surface, it does seem to be the servo motor's fault, but after carefully analyzing the working principle of servo motors, we find that this conclusion is completely wrong.
An AC servo system includes a servo drive, a servo motor, and a feedback sensor (typically, the servo motor has a built-in optical encoder). All these components operate within a closed-loop control system: the drive receives parameter information from the outside, then supplies a certain current to the motor, which converts this current into torque to drive the load. The load moves or accelerates/decelerates according to its own characteristics. The sensor measures the load's position, allowing the drive to compare the setpoint information value with the actual position value. It then adjusts the motor current to maintain consistency between the actual position value and the setpoint information value. When a sudden change in load causes a speed change, the encoder detects this speed change and immediately sends a response to the servo drive. The drive then adjusts the current supplied to the servo motor to accommodate the load change and returns to the set speed. An AC servo system is a highly responsive, fully closed-loop system. The time lag between load fluctuations and speed correction is very fast. Therefore, the real limiting factor for the system's responsiveness is the transmission time of the mechanical connection devices.
To give a simple example: Imagine a machine that uses a servo motor to drive a load with constant speed and high inertia via a V-belt. The entire system needs to achieve constant speed and fast response characteristics; analyze its operation process.
When the driver sends current to the motor, the motor immediately generates torque. Initially, due to the elasticity of the V-belt, the load will not accelerate as fast as the motor. The servo motor will reach the set speed ahead of the load. At this time, the offset encoder mounted on the motor will reduce the current, and thus reduce the torque. As the tension of the V-belt increases, the motor speed will slow down. At this time, the driver will increase the current again, and so on.
In this example, the system oscillates, the motor torque fluctuates, and the load speed fluctuates accordingly. The result, of course, is noise, wear, and instability. However, these are not caused by the servo motor itself. This noise and instability originate from the mechanical transmission mechanism, caused by a mismatch between the servo system's high response speed and the mechanical transmission or response time, i.e., the servo motor responds faster than the time required for the system to adjust to new torque.
Once the root cause of the problem is found, solving it becomes much easier. For the example above, you can:
(1) Increase mechanical rigidity and reduce system inertia, and reduce the response time of mechanical transmission parts, such as replacing V-belt with direct screw drive or replacing V-belt with gearbox;
(2) Reduce the response speed of the servo system and reduce the control bandwidth of the servo system, such as reducing the gain parameter value of the servo system.
Of course, the above is only one of the reasons for noise and instability. Different solutions will be available for different reasons. For example, noise caused by mechanical resonance can be addressed by resonance suppression and low-pass filtering in the servo motor. In short, the causes of noise and instability are generally not due to the servo motor itself.
Question 2: Inertial Matching
Inertia issues are frequently encountered during the selection and debugging of servo systems!
Specifically, this manifests as follows:
1. When selecting a servo system, in addition to considering factors such as the motor's torque and rated speed, we also need to calculate the moment of inertia of the mechanical system converted to the motor shaft, and then select a motor with a suitable moment of inertia based on the actual motion requirements of the machine and the quality requirements of the processed parts.
2. During debugging (manual mode), correctly setting the inertia ratio parameter is a prerequisite for fully utilizing the optimal performance of the mechanical and servo systems. This is particularly important for systems requiring high speed and high precision (Delta servo inertia ratio parameters are 1-37, JL/JM). This leads to the issue of inertia matching!
So what exactly is "inertia matching"?
1. According to Newton's second law: "The torque required for the feed system T = system inertia J × angular acceleration θ"
Angular acceleration θ affects the dynamic characteristics of the system. The smaller θ is, the longer the time from when the controller issues a command to when the system completes its execution, and the slower the system response. If θ changes, the system response will fluctuate, affecting machining accuracy. Since the maximum output T value remains constant after the motor is selected, J should be as small as possible if a small change in θ is desired.
2. The total moment of inertia of the feed axis "J" = rotational inertial momentum of the servo motor JM + load inertial momentum converted from the motor shaft JL
The load inertia JL consists of the inertia of the worktable and the fixtures, workpieces, screws, couplings, and other linear and rotary moving parts mounted on it (taking a machine tool as an example), which is then referred to the inertia on the motor shaft. JM is the rotor inertia of the servo motor, which is a fixed value once the servo motor is selected, while JL changes with the load, such as the workpiece. If a smaller rate of change of J is desired, it is best to make the proportion of JL smaller. This is what is commonly referred to as "inertia matching".
Now that we know what inertia matching is, what specific impact does it have, and how is it determined?
Influence:
Transmission inertia affects the accuracy, stability, and dynamic response of a servo system. A large inertia results in a large mechanical constant, slow response, and a decrease in the system's natural frequency, making it prone to resonance. This limits the servo bandwidth and affects servo accuracy and response speed. Appropriately increasing the inertia is only beneficial when improving low-speed crawling. Therefore, in mechanical design, the inertia should be minimized as much as possible without affecting the system's stiffness.
Sure:
When evaluating the dynamic characteristics of a mechanical system, the smaller the inertia, the better the system's dynamic response; the larger the inertia, the greater the load on the motor, and the more difficult it is to control. However, the inertia of the mechanical system must be matched with the inertia of the motor. Different mechanisms have different choices of inertia matching principles, and these choices have different effects. For example, when a CNC machining center performs high-speed cutting using a servo motor, an increase in load inertia will result in the following:
(1) When the control command changes, the motor needs to take a long time to reach the speed requirement of the new command;
(2) When the machine tool performs rapid arc-shaped cutting along two axes, a large error will occur:
① Under normal circumstances, the above problem will not occur with servo motors when JL≦JM.
② When JL = 3 × JM, the controllability of the motor will decrease slightly, but it will not affect normal metal cutting. (For high-speed curve cutting, it is generally recommended that JL ≤ JM.)
③ When JL ≥ 3 × JM, the controllability of the motor will decrease significantly, which is particularly noticeable during high-speed curve cutting.
Different mechanical actions and processing quality requirements have different requirements for the relationship between JL and JM. The determination of inertial matching needs to be based on the mechanical process characteristics and processing quality requirements.
Question 3: Servo Motor Selection
After selecting the mechanical transmission solution, it is necessary to select and confirm the model and size of the servo motor.
(1) Selection criteria—Generally, the following conditions must be met when selecting a servo motor:
● Maximum motor speed > Maximum required operating speed of the system;
● The rotor inertia of the motor is matched with the load inertia;
●Continuous load operating torque ≤ motor rated torque;
● Maximum output torque of the motor > maximum torque required by the system (torque during acceleration).
(2) Selection calculation:
● Inertia matching calculation (JL/JM)
●Slewing speed calculation (load end speed, motor end speed)
● Load torque calculation (continuous load working torque, acceleration torque)
