21 Key Points About Servo Motor (Note to Engineers)

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Key Points About Servo Motor

1. How to correctly select servo motor and stepper motor?

It mainly depends on the specific application.

In short, it is necessary to determine the nature of the load (such as horizontal or vertical load), torque, inertia, speed, accuracy, acceleration and deceleration, and upper control requirements (such as requirements for port interface and communication).

And whether the main control mode is position, torque, or speed.

Whether the power supply is DC or AC power supply, or battery power supply, voltage range.

Based on this, the model of the motor and matched driver or controller can be determined.

2. Stepper motor or servo motor system, which one to choose?

In fact, the choice of what kind of motor should be determined according to the specific application, because they have their own characteristics.

3. How to use the stepper motor driver?

According to the current of the motor, it is equipped with a driver greater than or equal to this current.

If low vibration or high precision is required, it can be equipped with subdivision driver.

For high torque motors, high voltage drives should be used as much as possible to obtain good high-speed performance.

4. What is the difference between 2-phase and 5-phase stepping motors and how to choose them?

The cost of a 2-phase motor is low, but the vibration is large at low speed and the torque decreases quickly at high speed.

Phase motor has less vibration and good high-speed performance, which is 30 ~ 50% higher than that of 2-phase motor, and can replace servo motor in some occasions.

5. When to select DC servo system and what is the difference between DC servo system and AC servo system?

DC servo motor is divided into brushless motor and brushless motor.

The brush motor has the advantages of low cost, simple structure, large starting torque, wide speed regulation range, easy control and needs maintenance, but it is convenient to maintain (replace carbon brush), generates electromagnetic interference and has requirements for the environment.

Therefore, it can be used in common industrial and civil occasions sensitive to cost.

Brushless motor has the advantages of small volume, light weight, large output, fast response, high speed, small inertia, smooth rotation and stable torque.

The control is complex, easy to realize intellectualization, and its electronic commutation mode is flexible, which can be square wave commutation or sine wave commutation.

The motor is maintenance-free, high efficiency, has low operating temperature, low electromagnetic radiation and long service life.

It can be used in various environments.

AC servo motor is also a brushless motor, which is divided into the synchronous motor and asynchronous motor.

At present, the synchronous motor is generally used in motion control.

It has a large power range and can achieve great power.

Large inertia, low maximum rotational speed, and rapidly decreases with the increase of power.

Therefore, it is suitable for the application of low-speed and smooth operation.

6. What should we pay attention to when using the motor?

The following checks shall be made before power on operation:

(1). Whether the power supply voltage is appropriate (overvoltage is likely to cause damage to the drive module);

The + / – polarity of DC input must not be connected incorrectly, and whether the motor model or current setting value on the drive controller is appropriate (not too large at the beginning);

(2). The control signal line shall be firmly connected, and the shielding problem (such as twisted pair) shall be considered at the industrial site;

(3). Do not connect all the wires to be connected at the beginning, only connect them to the most basic system, and then connect them step by step after running well.

(4). Be sure to find out the grounding method or floating non-grounding.

(5). Within half an hour after the start of the operation, closely observe the state of the motor, such as whether the movement is normal, sound, and temperature rise.

If any problem is found, stop the machine immediately for adjustment.

7. When the stepping motor starts and runs, sometimes it stops moving or returns in place, and sometimes it will lose step during operation. What’s the problem?

Generally, the following aspects shall be considered for inspection:

(1). Whether the motor torque is large enough to drive the load.

Therefore, we generally recommend that users select motors with a torque 50% ~ 100% greater than the actual demand when selecting models, because stepping motors cannot operate under excessive load, even for an instant, it will cause out-of-step, stop or move irregularly and repeatedly in place in serious cases.

(2). Whether the current of the input walking pulse from the upper controller is large enough (generally > 10mA).

Whether the optocoupler can be turned on stably and whether the input frequency is too high, resulting in failure to receive.

If the output circuit of the upper controller is CMOS circuit, CMOS input driver shall also be selected.

(3). Is the starting frequency too high?

Whether the acceleration process is set in the starting procedure.

it is best to accelerate from the specified starting frequency of the motor to the set frequency, even if the acceleration time is very short, otherwise, it may be unstable or even in the idle state.

(4). When the motor is not fixed properly, this condition sometimes occurs, which is normal.

Because, in fact, the strong resonance of the motor is caused at this time, resulting in the out-of-step state.

The motor must be fixed.

(5). For a 5-phase motor

if the phase is connected incorrectly, the motor cannot work.

8. I want to directly control the servo motor through communication, is that OK?

Yes, it’s also convenient. It’s just a matter of speed. It’s used for applications that don’t require high response speed.

If fast response control parameters are required, it is best to use servo motion control card.

Generally, it has DSP and high-speed logic processing circuit to realize high-speed and high-precision motion control.

Such as s acceleration, multi-axis interpolation, etc.

9. Is it good to use the switching power supply to supply power to the stepping and DC motor system?

Generally, it is best not to, especially high torque motor, unless switching power supply with more than twice the required power is selected.

Because, when the motor works, it is a large inductive load, which will form an instantaneous high voltage on the power end.

The switching power supply has poor overload performance, which will protect the shutdown, and its precise voltage stabilizing performance is not required, which may sometimes cause damage to the switching power supply and driver.

Conventional ring or R-type transformers can be used to transform the DC power supply.

10. Do you want to use ± 10V or 4 ~ 20mA DC voltage to control the stepping motor?

Yes, but another conversion module is required.

11. There is a servo motor with encoder feedback. Can it be controlled by a servo driver with only a tachometer port?

Yes, it needs to be equipped with an encoder tachometer signal module.

12. Can the code disk part of the servo motor be disassembled?

It is forbidden to disassemble because the quartz sheet in the code disc is easy to break.

And after entering dust, the service life and accuracy will not be guaranteed, so it needs professional maintenance.

13. Can the stepping motor and servo motor be disassembled for maintenance or refitted?

No, it’s best to let the manufacturer do it.

Without professional equipment after disassembly, it’s difficult to install it back as it is.

And the gap between the rotating stators of the motor can’t be guaranteed.

The performance of magnetic steel material is damaged, even resulting in loss of excitation, and the motor torque is greatly reduced.

14. Can the servo controller sense the change of external load?

Stop, return or maintain a certain thrust follow-up in case of set resistance.

15. Can domestic drives or motors be used with foreign high-quality motors or drivers?

In principle, it is possible.

However, it can be used only after the technical parameters of the motor are clear, otherwise, it will greatly reduce the due effect and even affect the long-term operation and service life.

It is best to consult the supplier before making a decision.

16. Is it safe to drive the motor with a DC supply voltage greater than the rated voltage?

Normally, this is not a problem as long as the motor operates within the set speed and current limits.

Because the motor speed is directly proportional to the motor line voltage, selecting a certain power supply voltage will not cause overspeed, but faults such as driver may occur.

In addition, it must be ensured that the motor meets the requirements of the minimum inductance coefficient of the driver and that the set current limit is less than or equal to the rated current of the motor.

In fact, if you can make the motor run slowly (below the rated voltage) in the device you design, this is good.

Operating at a lower voltage (and therefore at a lower speed) will result in less brush operation rebound, less brush/commutator wear, lower current consumption and longer motor life.

On the other hand, if the motor size constraints and performance requirements require additional torque and speed, overdriving the motor is also possible, but it will sacrifice the service life of the product.

17. How to select the appropriate power supply for the application?

It is recommended that the power supply voltage value be 10% – 50% higher than the maximum required voltage.

This percentage varies with KT, Ke, and the voltage drop within the system.

The current value of the driver should be sufficient to transmit the energy required for the application.

Remember that the output voltage of the driver is different from the supply voltage, so the output current of the driver is also different from the input current.

In order to determine the appropriate supply current, all power requirements of this application need to be calculated and increased by 5%.

The required current value can be obtained by calculating according to I = P / V formula.

It is recommended that the power supply voltage value be 10% – 50% higher than the maximum required voltage.

This percentage varies with KT, Ke, and the voltage drop within the system.

The current value of the driver should be sufficient to transmit the energy required for the application.

Remember that the output voltage of the driver is different from the supply voltage, so the output current of the driver is also different from the input current.

In order to determine the appropriate supply current, all power requirements of this application need to be calculated and increased by 5%.

The required current value can be obtained by calculating according to I = P / V formula.

18. What kind of working mode can I choose for the servo driver?

Different modes do not exist in all models of drives.

19. How are drives and systems grounded?

If there is no isolation between the AC power supply and the driver DC bus (such as the transformer), do not ground the non-isolated port of the DC bus or the non-isolated signal, which may cause equipment damage and personal injury.

Because the AC common voltage is not to the earth, there may be a high voltage between the DC bus ground and the earth.

In most servo systems, all common ground and ground are connected together at the signal end.

The ground loop generated by various ways of connecting the ground is easily affected by noise and generates current at different reference points.

In order to keep the command reference voltage constant, the signal ground of the driver shall be connected to the signal ground of the controller.

It will also be connected to the ground of the external power supply, which will affect the operation of the controller and driver (such as the 5V power supply of the encoder).

Shielding layer grounding is difficult.

There are several methods.

The correct shield ground is at the reference potential point inside the circuit.

This point depends on whether the noise source and receiver are grounded at the same time or floating.

Ensure that the shield is grounded at the same point so that the ground current does not flow through the shield.

20. Why can’t the reducer match the motor exactly at the standard torque point?

Considering the maximum continuous torque generated by the motor through the reducer, many reduction ratios will far exceed the torque level of the reducer.

If we design each reducer to match the full torque, the internal gears of the reducer will have too many combinations (large volume and many materials).

This will make the product price high and violate the principle of “high performance and small volume”.

How to select electric cylinder, slide table and precision platform products? How is the cost calculated?

The key to selecting actuator products depends on your requirements for motion parameters.

You can determine the specific motion parameters and other technical conditions according to the application you need.

These parameters should meet your actual needs. They should not only meet the application requirements and leave room, but also not be raised too high, otherwise, the cost may be several times higher than that of standard products.

For example, if the accuracy of 0.1mm is enough, do not select the parameter of 0.01mm.

The same is true for others such as load capacity, speed, etc.

Another type selection suggestion for users is that if it is not necessary, the three main parameters of push and pull or load, speed and positioning accuracy should not be very high at the same time, because the actuator is a high-precision and high-tech mechatronics product.

Thus, we need to consider and select the corresponding composition motor, drive controller and feedback device, as well as guide rail, screw rod, support seat and other mechanical systems with different accuracy levels from the aspects of mechanical structure, electrical performance, material characteristics, material and treatment methods.

To achieve the required overall motion parameters, it can be said to be a product to pull and launch the whole body.

Of course, if you have high requirements for products, we can still meet them, but the cost will increase accordingly.

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