Servo systems are an integral part of electromechanical products, providing the highest level of dynamic response and torque density.
Consequently, the trend in the development of drive systems is to replace traditional hydraulic, DC, stepper and AC variable speed drives with AC servo drives.
This transition aims to take system performance to a new level, including shorter cycles, greater productivity, greater reliability and longer service life.
To maximize the performance of servo motors, it is essential to understand some of their unique usage characteristics.
Issue 1: Noise, Instability
Customers often encounter excessive noise and unstable operation when using servo motors in certain machines. When these problems arise, the first reaction of many users is to question the quality of the servo motor.
This is because when they switch to stepper motors or variable frequency motors to drive the load, noise and instability usually decrease significantly.
At first glance, it actually appears to be a problem with the servo motor.
However, a careful analysis of the working principle of the servo motor reveals that this conclusion is completely wrong.
The AC servo system consists of a servo drive, a servo motor and a feedback sensor (generally, the servo motor comes with an optical encoder).
All of these components operate within a closed-loop control system: the inverter receives external parameter information and then supplies a specific current to the motor, which converts it into torque to drive the load.
The load performs actions or accelerates/decelerates based on its characteristics.
The sensor measures the position of the load, allowing the drive device to compare the set information value with the actual position value. It then adjusts the motor current to keep the actual position value consistent with the set information value.
When a sudden load change causes a speed variation, the encoder will immediately relay this speed change to the servo drive.
The inverter then changes the current supplied to the servo motor to accommodate the load change and return to the preset speed.
The AC servo system is a highly responsive closed-loop system, where the response time interval between load fluctuation and speed correction is very fast. At this point, the main limitation on system response is the transmission time of the mechanical connecting device.
To provide a simple example: consider a machine that uses a servo motor to drive a high-inertia, constant-speed load via a V-belt. The entire system needs to maintain constant speed and fast response characteristics. Let's analyze your operation process.
When the inverter supplies current to the motor, the motor immediately generates torque. Initially, due to the elasticity of the V-belt, the load does not accelerate as quickly as the engine.
The servo motor reaches the set speed before loading, at which point the encoder mounted on the motor weakens the current, subsequently reducing torque. As the tension on the V-belt continually increases, the engine speed decreases.
Then, the driver increases the current again and this cycle repeats. Official account: SolidWorks non-standard mechanical design.
In this case, the system oscillates, the motor torque fluctuates, and the load speed fluctuates accordingly.
The resulting noise, wear and instability are inevitable. However, these are not caused by the servo motor. Such noise and instability originate from the mechanical transmission device and are due to a mismatch between the high speed response of the servo system and the mechanical transmission or longer response time.
In other words, the servo motor's response is faster than the time needed for the system to adjust to the new torque.
Once you identify the root of the problem, solving it becomes much easier. Referring to the previous example, you could:
(1) Increase the mechanical rigidity and reduce the inertia of the system, thereby decreasing the response time of mechanical transmission parts. For example, replace V-belts with direct screwdrivers or use gearboxes instead of V-belts.
(2) Decrease the response speed of the servo system and reduce the control bandwidth of the servo system, such as decreasing the gain value of the servo system.
Of course, these are just a few reasons for noise and instability. There are different solutions for different causes. For example, noise caused by mechanical resonance can be resolved by implementing resonance suppression or low-pass filtering in the servo system. In conclusion, the causes of noise and instability are generally not due to the servo motor itself.
Issue 2: Inertia Matching
During the selection and adjustment of servo systems, the problem of inertia often arises. Specifically, it manifests itself as follows:
1. When choosing a servo system, in addition to considering factors such as motor torque and rated speed, we first need to calculate the inertia of the mechanical system converted into the motor shaft.
We then choose a motor with suitable inertia size based on the actual machinery action requirements and part quality requirements.
2. During tuning (in manual mode), correctly setting the inertia ratio parameter is a prerequisite for maximizing the performance of the machinery and servo system.
This point is particularly prominent in systems that require high speed and high precision (servo inertia ratio parameter Delta is 1-37, JL/JM). Thus, the problem of inertia matching arises! So what exactly is “inertia matching”?
1. According to Newton's second law, the torque required for the power system, T, is equal to the system's moment of inertia, J, multiplied by the angular acceleration, θ. The angular acceleration θ impacts the dynamic characteristics of the system. The smaller θ is, the longer the time from controller command to system execution, resulting in slower system response. If θ fluctuates, the system response will vary in speed, affecting machining accuracy. Given that the maximum output T remains constant once the motor is selected, if we want minimal changes in θ, J must be as small as possible.
2. The total inertia of the power shaft, J, is equal to the rotational inertia of the servo motor, JM, plus the converted load inertia of the motor shaft, JL. JL load inertia consists of the inertia of linear and rotating components such as the work table (in case of machine tools), the accessories and parts on it, the screw, the coupling, etc., all converted to the inertia of the motor stem. JM represents the rotor inertia of the servo motor, which is a constant when the servo motor is selected, while JL fluctuates with changes in load, such as on the workpiece. If you want the rate of change in J to be smaller, it is better to minimize the proportion that JL occupies. This is commonly called “inertia matching”.
Now that we understand what inertia matching is, what specific impacts does it have, and how is it determined?
Impact:
Inverter inertia affects the precision, stability and dynamic response of the servo system. Greater inertia results in a greater system mechanical constant, slower response, and a reduced system natural frequency, potentially leading to resonance.
This limits servo bandwidth and affects servo accuracy and response speed.
An appropriate increase in inertia is only advantageous when improving low-speed tracking. Therefore, in mechanical design, efforts must be made to minimize inertia without compromising the rigidity of the system.
Determination:
When evaluating the dynamic characteristics of a mechanical system, the lower the inertia, the better the dynamic response of the system. On the other hand, greater inertia leads to greater motor load, making control more challenging.
However, the inertia of the mechanical system must correspond to the inertia of the engine. Different engines have varying selections for inertia matching principles, each with unique functional displays.
For example, during high-speed cutting with a CNC machining center using a servo motor, when the load inertia increases, the following occurs:
(1) When control commands change, the motor takes a considerable amount of time to reach the speed requirements of the new instruction;
(2) Significant errors may occur when the machine operates along two axes to perform rapid arcuate cuts:
I. Under normal circumstances with typical servo motors, if JL is less than or equal to JM, the above problems will not occur.
ii. If JL is equal to 3 times JM, the controllability of the engine will decrease slightly, but this will not affect routine metal cutting. (For high-speed curve cutting, it is generally recommended that JL be less than or equal to JM).
iii. When JL is 3 times JM or more, engine controllability will decrease significantly, which is particularly noticeable during high-speed curve cutting.
Different mechanical actions and machining quality requirements require different relationships between JL and JM.
The determination of inertia matching needs to be based on the technological characteristics of the machine and the quality requirements of the machining process.
Issue 3: Servo Motor Selection
After finalizing the mechanical transmission diagram, it is necessary to select and confirm the model and size of the servo motor.
(1) Selection criteria
In general, the selection of a servo motor must satisfy the following situations:
- The maximum rotational speed of the motor > the highest required movement speed of the system;
- The inertia of the motor rotor corresponds to the inertia of the load;
- The continuous load working torque ≤ the rated torque of the motor;
- The maximum motor output torque > the maximum required system torque (acceleration torque).
(2) Selection calculations
- Inertia correspondence calculation (JL/JM);
- Calculation of rotational speed (load final rotation speed, engine final rotation speed);
- Calculation of load torque (continuous load working torque, acceleration torque).
