A brief analysis of common problems in the use of servo motors

The servo system is an important part of electromechanical products. It can provide the highest level of dynamic response and torque density. Therefore, the development trend of the drive system is to replace the traditional hydraulic, DC, stepping and AC variable frequency speed drive with AC servo drive, so as to make the system performance reach a new level, including shorter cycle, higher productivity, better reliability and longer life. In order to achieve better performance of servo motor, it is necessary to understand some of the usage characteristics of servo motor.

Problem 1: Noise, instability

When customers use servo motors on some machines, they often experience excessive noise and unstable operation of the load driven by the motor. When this problem occurs, many users' first reaction is that the servo motor is of poor quality, because sometimes when a stepper motor or variable frequency motor is used to drive the load, the noise and instability are much smaller. On the surface, it seems that the servo motor is indeed the cause, but after a careful analysis of the working principle of the servo motor, we will find that this conclusion is completely wrong.

The AC servo system includes: servo drive, servo motor and a feedback sensor (generally the servo motor has its own optical encoder). All these components operate in a control closed-loop system: the driver receives parameter information from the outside, and then transmits a certain current to the motor, which is converted into torque to drive the load. The load moves or accelerates and decelerates according to its own characteristics. The sensor measures the position of the load, so that the drive device compares the set information value and the actual position value, and then changes the motor current to keep the actual position value consistent with the set information value. When the load suddenly changes and causes the speed to change, the encoder will immediately respond to the servo driver after learning of this speed change. The driver changes the current value provided to the servo motor to meet the load change and return to the set speed. The AC servo system is a fully closed-loop system with a very high response. The time lag response between load fluctuation and speed correction is very fast. At this time, what really limits the system response effect is the transmission time of the mechanical connection device.

Let's take a simple example: there is a machine that uses a servo motor to drive a constant speed and large inertia load through a V-belt. The entire system needs to obtain a constant speed and a faster response characteristic, and analyze its action process.

When the driver sends current to the motor, the motor immediately generates torque. At the beginning, 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 earlier than the load. At this time, the encoder installed on the motor will weaken the current and then weaken the torque. As the tension of the V-belt continues to increase, the motor speed will slow down. At this time, the driver will increase the current again, and the cycle will repeat.

In this case, the system is oscillating, the motor torque is fluctuating, and the load speed is also fluctuating. The result is of course noise, wear, and instability. However, this is not caused by the servo motor. This noise and instability come from the mechanical transmission device, which is caused by the mismatch between the servo system reaction speed (high) and the mechanical transmission or reaction time (long), that is, the servo motor responds faster than the time required for the system to adjust to the new torque.

Once you find the root cause of the problem, it will be much easier to solve it. For the above example, you can:

(1) Increase mechanical rigidity and reduce system inertia to reduce the response time of mechanical transmission parts, such as replacing the V-belt with a direct screw drive or using a gear box instead of a V-belt;

(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. There will be different solutions for different reasons. For example, for noise caused by mechanical resonance, resonance suppression and low-pass filtering can be adopted in the servo. In short, the causes of noise and instability are basically not caused by the servo motor itself.

Problem 2: Inertia Matching

In the selection and debugging of servo systems, inertia problems are often encountered! The specific manifestations are:

1. When selecting a servo system, in addition to considering factors such as the motor's torque and rated speed, we also need to first calculate the inertia of the mechanical system converted to the motor shaft, and then select a motor with a suitable inertia size based on the actual mechanical action requirements and workpiece quality requirements;

2. During debugging (in manual mode), correctly setting the inertia ratio parameters is the prerequisite for giving full play to the best performance of the machinery and servo system. This is particularly prominent in systems that require high speed and high precision (Delta servo inertia ratio parameters are 1-37, JL/JM). In this way, there is the problem of inertia matching! So what exactly is "inertia matching"?

1. According to Newton's second law: "The torque T required by the feed system = system transmission inertia J × angular acceleration θ

Angular acceleration θ affects the dynamic characteristics of the system. The smaller θ is, the longer it takes for the controller to issue a command and the system to complete the execution, and the slower the system response is. If θ changes, the system response will be fast or slow, affecting the processing accuracy. Since the maximum output T value does not change after the motor is selected, if you want to keep θ small, J should be as small as possible.

2. The total inertia of the feed axis "J = the rotational inertia momentum of the servo motor JM + the load inertia momentum converted by the motor shaft JL

The load inertia JL is composed of the inertia of the workbench and the fixtures and workpieces, screws, couplings and other linear and rotary moving parts (taking the machine tool as an example) converted to the inertia of the motor shaft. JM is the servo motor rotor inertia. After the servo motor is selected, this value is a fixed value, while JL changes with the load such as the workpiece. If you want a smaller change rate of J, it is best to make the proportion of JL smaller. This is the popular sense of "inertia matching".

Now that we know what inertia matching is, what are the specific effects of inertia matching and how to determine it?

Influence:

Transmission inertia affects the accuracy, stability, and dynamic response of the servo system. Large inertia means large mechanical constants of the system, slow response, and lower natural frequency of the system, which is prone to resonance. This limits the servo bandwidth and affects the servo accuracy and response speed. An appropriate increase in inertia is only beneficial when improving low-speed creep. Therefore, when designing the machine, the inertia should be minimized without affecting the stiffness of the system.

Sure:

When measuring the dynamic characteristics of a mechanical system, the smaller the inertia, the better the dynamic characteristics of the system; the larger the inertia, the greater the load on the motor, and the more difficult it is to control, but the inertia of the mechanical system must match the inertia of the motor. Different mechanisms have different choices for the inertia matching principle and have different performances. For example, when a CNC center machine uses a servo motor for high-speed cutting, when the load inertia increases, the following will occur:

(1) When the control command changes, the motor takes a long time to reach the speed requirement of the new command;

(2) When the machine performs arc-shaped rapid cutting along two axes, large errors will occur:

① In general, under normal conditions of servo motors, when JL≦JM, the above problem will not occur.

②When JL=3×JM, the controllability of the motor will be slightly reduced, but it will not affect normal metal cutting. (For high-speed curve cutting, it is generally recommended that JL≦JM)

③When JL≧3×JM, the controllability of the motor will decrease significantly, and it will perform well in high-speed curve cutting.

Different mechanism actions and processing quality requirements have different requirements on the size relationship between JL and JM. The determination of inertia matching needs to be determined based on the process characteristics of the machine and the processing quality requirements.

Question 3: Servo motor selection

After choosing the mechanical transmission solution, the model and size of the servo motor must be selected and confirmed.

(1) Selection conditions - Generally speaking, the following conditions must be met when selecting a servo motor:

The maximum motor speed > the maximum moving speed required by the system;

The motor's rotor inertia matches the load inertia;

Continuous load working torque ≦ motor rated torque;

The maximum output torque of the motor is greater than the maximum torque required by the system (torque during acceleration).

(2) Model selection calculation

Inertia matching calculation (JL/JM)

Rotation speed calculation (load end speed, motor end speed)

Load torque calculation (continuous load working torque, acceleration torque)