Difference And Selection Of Stepper Drive And Servo Drive

- Mar 05, 2019-

Difference and selection of stepper drive and servo drive


1. The main difference between stepper drive and servo drive.

2. Several practical problems in the specific selection process.

 

Overview:

Stepper motors are mainly classified according to the number of phases, and two-phase and five-phase stepping motors are widely used in the market. The two-phase stepping motor can be divided into 400 equal parts per revolution, and the five-phase can be divided into 1000 equal parts. Therefore, the characteristics of the five-phase stepping motor are better, the acceleration/deceleration time is shorter, and the dynamic inertia is lower. .

With the advent of all-digital AC servo systems, AC servo motors are increasingly being used in digital control systems. In order to adapt to the development trend of digital control, stepping motor or all-digital AC servo motor is mostly used as the execution motor in the motion control system. Although the two are similar in control (burst and direction signals), there are large differences in performance and application.


Now compare the performance of the two.


First, the control accuracy is different

The two-phase hybrid stepping motor step angle is generally 3.6 degrees and 1.8 degrees, and the five-phase hybrid stepping motor step angle is generally 0.72 degrees and 0.36 degrees. There are also some high performance stepper motors with smaller step angles. For example, a stepping motor for a slow wire-cutting machine produced by some domestic companies has a step angle of 0.09 degrees; the step angle of a three-phase hybrid stepping motor produced by some international companies can be set by a dial switch. It is 1.8, 0.9, 0.72, 0.36, 0.18, 0.09, 0.072, and 0.036, and is compatible with the step angle of two-phase and five-phase hybrid stepping motors.

The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear of the motor shaft. Take our all-digital AC servo motor as an example. For a motor with a standard 2500-line encoder, the pulse equivalent is 360 degrees/10000=0.036 degrees due to the quadruple frequency technology inside the driver. For a motor with a 17-bit encoder, the drive receives one revolution per 217 = 131072 pulse motors, ie its pulse equivalent is 360 degrees / 131072 = 9.89 seconds. It is 1/655 of the pulse equivalent of a stepping motor with a step angle of 1.8 degrees.


Second, the low frequency characteristics are different

Stepper motors are prone to low frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally considered that the vibration frequency is half of the take-off frequency of the motor no-load. This low frequency vibration phenomenon, which is determined by the working principle of the stepper motor, is very detrimental to the normal operation of the machine. When the stepper motor works at low speed, damping technology should generally be used to overcome low-frequency vibration phenomena, such as adding a damper to the motor or subdividing the drive.

The AC servo motor runs very smoothly, and vibration does not occur even at low speeds. The AC servo system has a resonance suppression function, which can cover the rigidity of the machine, and has a frequency analysis function (fft) inside the system to detect the resonance point of the machine and facilitate system adjustment.


Third, the difference in frequency characteristics

The output torque of the stepping motor decreases as the speed increases, and it drops sharply at higher speeds, so the maximum operating speed is generally 300 to 600 rpm.

The AC servo motor is a constant torque output, that is, it can output the rated torque within its rated speed (generally 2000 rpm or 3000 rpm), and it is a constant power output above the rated speed.


Fourth, different load capacity

Stepper motors generally do not have overload capability. The AC servo motor has a strong overload capability. Take our AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is three times the rated torque and can be used to overcome the moment of inertia of the inertia load at the moment of starting. Because there is no such overload capability in stepping motors, in order to overcome this moment of inertia during the selection, it is often necessary to select a motor with a large torque, and the machine does not need such a large torque during normal operation, and a torque appears. The phenomenon of waste.


Five, different operating performance

The control of the stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it may be lost or blocked. If the speed is too high during the stop, the overshoot may occur. Therefore, in order to ensure the control accuracy, it should be handled well. The problem of raising and lowering speed. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder. The internal position loop and speed loop are formed. Generally, the stepless motor is lost or overshooted, and the control performance is more reliable.


Sixth, speed impact performance is different

It takes 200 to 400 milliseconds for the stepper motor to accelerate from standstill to the operating speed (typically several hundred revolutions per minute). The AC servo system has better acceleration performance. Taking the Panasonic msma400w AC servo motor as an example, it takes only a few milliseconds to accelerate from standstill to its rated speed of 3000 rpm. It can be used in control applications requiring fast start and stop.


How to choose?

1. How to choose the servo motor and stepper motor correctly

Mainly depending on the specific application, it is simple to determine: 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 In terms of requirements, the main control method is position, torque or speed. 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.


2. 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.


3.What is the difference between  2 phase and 5 phase stepper motors? How to choose?

The 2-phase motor has a low cost, but the vibration at a low speed is large, and the torque at a high speed is rapidly lowered. The 5-phase motor has less vibration and high-speed performance, which is 30~50% higher than that of the 2-phase motor. It can replace the servo motor in some occasions.


4. 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 convenient maintenance (replacement of carbon brushes), electromagnetic interference, and 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. Currently, synchronous motors are generally used in motion control. It has a large power range and can achieve a large power. 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.


5. Problems to be aware of when using the motor

Check the following before powering up:

1) Is the power supply voltage suitable (overvoltage is likely to cause damage to the drive module); the +/- polarity of the DC input must not be connected incorrectly, and the motor model or current setting value on the drive controller is appropriate (do not start at the beginning) Too big);

2) The control signal line is firmly connected, and the industrial site should preferably consider the shielding problem (such as using twisted pair);

3) Do not connect the wires that need to be connected when starting. Only connect to the most basic system. After running well, connect them step by step.

4) Be sure to find out the grounding method, or use the floating.

5) Closely observe the state of the motor within half an hour of starting operation, such as whether the motion is normal, sound and temperature rise, and immediately stop the adjustment if the problem is found.


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