21 Key Issues With Servo Motors

The general structure of an industrial robotic electric servo system is three closed-loop control, namely current loop, speed loop and position loop.

21 Key Issues With Servo Motors

In general, for the AC servo drive, various functions such as position control, speed control, and torque control can be realized by manually setting the internal function parameters.

How to choose the servo motor and stepper motor correctly?

It depends mainly on the specific application.

Simply say, you need to determine several things:

  • the nature of the load (such as horizontal or vertical load),
  • torque, inertia, speed, accuracy, acceleration and deceleration requirements,
  • upper control requirements (such as port interface and communication requirements), the main control method Is the position, torque or speed mode.
  • Whether the power supply is DC or AC, or battery-powered, voltage range.

This is used to determine the model of the motor and the associated drive or controller.

Choose the stepper motor or servo motor system?

In fact, what kind of motor should be selected depends on the specific application, each has its own characteristics.

How to use stepper motor driver?

Depending on the current of the motor, a driver greater than or equal to this current is used.

A subdivided drive can be used if low vibration or high precision is required.

For high torque motors, use high voltage drives as much as possible to achieve good high-speed performance.

What is the difference between 2-phase and 5-phase stepper motors? How do I choose?

The 2-phase motor has a low cost, but the vibration is high at low speeds and the torque at high speed is fast.

The 5-phase motor has less vibration and high-speed performance, which is 30~50% higher than that of the 2-phase motor and can replace the servo motor in some occasions.

When is the DC servo system selected, what is the difference between it and AC servo?

DC servo motors are divided into brushed and brushless motors.

The brush motor has low cost, simple structure, large starting torque, wide speed regulation range, easy control and maintenance, but with convenient maintenance (replacement of carbon brushes), electromagnetic interference, and higher environmental requirements.

It can, therefore, be used in cost-sensitive general industrial and residential applications.

The brushless motor is small in size, light in weight, large in output, fast in response, high in speed, small in inertia, smooth in rotation and stable in torque.

The control is complex, and it is easy to realize intelligence.

The electronic commutation method is flexible and can be square wave commutation or sine wave commutation.

The motor is maintenance-free, has high efficiency, low operating temperature, low electromagnetic radiation and long life, and can be used in various environments.

AC servo motors are also brushless motors, which are divided into synchronous and asynchronous motors.

At present, synchronous motors are generally used in motion control, and its power range is large, and a large power can be achieved.

High inertia, the highest rotational speed is low, and it decreases rapidly as power increases.

Therefore, it is suitable for applications with low speed and smooth operation.

What should I pay attention to when using the motor?

Check the following before powering using it:

  • Whether the power supply voltage is suitable (over-voltage is likely to cause damage to the drive module);

For the +/- polarity of the DC input, it must not be connected incorrectly.

Is the motor model or current setting value on the drive controller suitable (not too big at the beginning);

  • The control signal line is firmly connected, and the industrial site should preferably consider the shielding problem (such as using twisted pair);
  • Do not connect the wires that need to be connected at the beginning, only connect to the most basic system. After running well, connect them step by step.
  • Be sure to find out the grounding method, or use floating without connection.
  • Closely observe the state of the motor within half an hour of starting operation, such as whether the motion is normal, the sound and whether the temperature rise, and immediately stop the adjustment after finding the problem.

When the stepping motor starts running, sometimes it will not move or move back and forth in the same place? Sometimes it will lose the step when running. What is the problem?

Generally, consider the following aspects for inspection:

  • Whether the motor torque is large enough to drive the load.

Therefore, we generally recommend that the user select a motor with a torque greater than 50%~100% than the actual one, because the stepping motor can’t run under overload.

Even if it is instantaneous, it will cause step losing, and if it is serious, it will stop or repeatedly moving irregularly.

  • Whether the current of the input walking pulse from the upper controller is large enough (generally >10mA) to make the optical coupler stably turn on.

The input frequency is too high, resulting in no reception.

If the output circuit of the host controller is a CMOS circuit, the CMOS input type driver should also be used.

Micro-signal training is worth your attention.

  • Is the start-up frequency too high? Whether the acceleration process is set on the launcher.

It is best to start the acceleration from the starting frequency specified by the motor to the set frequency.

Even if the acceleration time is short, it may be unstable or even in a passive state.

  • When the motor is not fixed, this condition sometimes occurs, which is normal.

Because, in fact, this time caused a strong resonance of the motor and caused to enter the out-of-step state.

The motor must be fixed.

  • For 5-phase motors, the phase is incorrectly connected and the motor does not work.

I want to directly control the servo motor by means of communication. Is it OK?

Yes, it is more convenient, just a speed issue, for applications that require less response speed.

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

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

Such as S acceleration, multi-axis interpolation and so on.

Is it good to supply the stepping and DC motor system with a switching power supply?

Generally, it is best not to use a large torque motor, unless a switching power supply that is more than twice as large as the required power is used.

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

The overload performance of the switching power supply is not good, it will protect the shutdown, and its precision voltage regulation performance is not needed, and sometimes it may cause damage to the switching power supply and the driver.

A DC power supply that can be transformed with a conventional annular or R-type transformer.

I want to use a DC voltage of ±10V or 4~20mA to control the stepper motor. Is it OK?

Yes, but additional conversion modules are required.

There is a servo motor with encoder feedback, can it be controlled by a servo drive with only the speedometer port?

Yes, you need to configure an encoder to tachometer signal module.

Can the servo motor’s code disc part be disassembled?

It is forbidden to disassemble because the quartz piece inside the code wheel is easily broken.

After entering the dust, life and accuracy will not be guaranteed.

Professional maintenance is required.

Can the stepper and servo motor be disassembled for repair or modification?

No, it is best to let the manufacturer do it.

After disassembling, it is difficult to install back to the original if there’s no professional equipment, and the gap between the rotating stators of the motor cannot be guaranteed.

The performance of the magnetic steel material is destroyed, and even the loss of magnetism, the motor torque is greatly reduced.

Can the servo controller sense changes in external load?

If encountered, stop, return or maintain a certain thrust follow-up when setting resistance.

Can I use a domestic drive or motor with a foreign high-quality motor or driver?

In principle, it is fine, but it must be used to understand the technical parameters of the motor.

Otherwise, it will greatly reduce the effect it should have, and even affect long-term operation and longevity.

It is best to consult with the supplier before deciding.

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 is running within the set speed and current limits.

Since the motor speed is proportional to the motor line voltage, selecting a power supply voltage does not cause over speed, but a driver failure may occur.

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

In fact, it is good if you can make the motor run slower in the device you are designing (below the rated voltage).

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

On the other hand, if the motor size limit and performance requirements require additional torque and speed, over-driving the motor is also possible, but it will reduce the life of the product.

How do I choose the right power supply for my application?

It is recommended to select a supply voltage value that is 10%-50% higher than the maximum required voltage.

This percentage varies depending on Kt, Ke, and the voltage drop across the system.

The current value of the driver should be sufficient to deliver the energy required by the application.

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

To determine the proper supply current, all power requirements for this application need to be calculated, plus an additional 5%.

Calculate according to the formula I = P/V to get the required current value.

What kind of work mode can I choose for a servo drive?

Different modes do not all exist in all models of drives.

How is the driver and system grounded?

If there is no isolation between the AC power source and the drive DC bus (such as a transformer), do not connect the non-isolated port of the DC bus or the ground of the non-isolated signal to the ground, which may result in equipment damage and personal injury.

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

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

A variety of ground loops generated by grounding are susceptible to noise and generate flow at different reference points.

  • In order to keep the command reference voltage constant, connect the signal ground of the driver 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 the drive (eg: 5V power supply for the encoder).

  • Shield grounding is more difficult, there are several ways.

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

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

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

Why can’t the gearbox match the motor exactly at the standard torque point?

If the maximum continuous torque through the reducer generated by the motor is taken into consideration, many reduction ratios will far exceed the torque level of the reducer.

If we are designing each reducer to match the full torque, there will be too many combinations of internal gears (larger, more material).

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

How to choose an electric cylinder, sliding table and precision platform products? How is the cost calculated?

The key to selecting an actuator-type product depends on your requirements for the motion parameters.

You can determine the technical conditions such as specific motion parameters based on the application you need.

These parameters should meet your actual needs, both to meet the application requirements and leave room for it, and not to set too high, otherwise, the cost may be several times the standard product.

For example, if the 0.1mm accuracy is sufficient, do not choose a parameter of 0.01mm. Others such as load capacity, speed, etc. are also true.

Another suggestion for the user is that if not necessary, the three main parameters of push-pull or load-bearing, speed, and positioning accuracy are not required at the same time.

Because the actuator is a high-precision and high-tech mechatronics product, we need to consider the mechanical structure, electrical properties, material properties, materials and processing methods in design and manufacture.

And select the appropriate components of the motor, drive controller and feedback device, as well as guide rails, lead rods, support bases and other mechanical systems of different accuracy levels.

To achieve the required overall exercise parameters, it can be said that the whole body of the product is launched.


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2 thoughts on “21 Key Issues With Servo Motors”

  1. ACS355 Series VFDs

    Great information! Servo motor issues might arise at any time. Many applications are there for the servo motors and that is the reason why people usually faces troubleshooting and repair issues. Thanks for sharing these tips and information here.

  2. When replacing dc servos with ac servos
    Is ok if we keep the same rated torque, rated rpm and inercia?

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