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This paper mainly solves two problems:
1. The main difference between stepping driver and servo driver. 2. Several practical problems in the process of specific type selection. summary
Stepper motors are mainly classified according to the number of phases, and two-phase and five phase stepper 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 stepping motor can be divided into 1000 equal parts. Therefore, the five phase stepping motor has better characteristics, shorter acceleration and deceleration time and lower dynamic inertia.
With the emergence of all digital AC servo system, AC servo motor is more and more used in digital control system. In order to adapt to the development trend of digital control, stepping motor or all digital AC servo motor are mostly used as executive motor in motion control system. Although they are similar in control mode (pulse train and direction signal), there are great differences in service performance and application occasions. The performance of the two is compared.
1、 Different control accuracy
The step angle of two-phase hybrid stepping motor is generally 3.6 degrees and 1.8 degrees, and the step angle of five phase hybrid stepping motor 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 slow wire walking machine tool produced by stone company has a step angle of 0.09 degrees; The step angle of the three-phase hybrid stepping motor produced by bergerlahr in Germany can be set to 1.8, 0.9, 0.72, 0.36, 0.18, 0.09, 0.072 and 0.036 through the dial switch, which is compatible with the step angle of two-phase and five phase hybrid stepping motors.
2: Different low frequency characteristics
Stepper motor is prone to low-frequency vibration at low speed. The vibration frequency is related to the load condition and driver performance. It is generally considered that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon determined by the working principle of stepping motor is very unfavorable to the normal operation of the machine. When the stepping motor works at low speed, damping technology should generally be used to overcome the phenomenon of low-frequency vibration, such as adding damper on the motor or subdivision technology on the driver.
The AC servo motor runs very smoothly and will not vibrate even at low speed. The AC servo system has resonance suppression function, which can cover the lack of mechanical rigidity, and the system has frequency analysis function (FFT), which can detect the mechanical resonance point and facilitate the system adjustment.
3: Different torque frequency characteristics
The output torque of stepping motor decreases with the increase of speed, and will decrease sharply at higher speed, so its maximum working speed is generally 300 ~ 600 rpm.
AC servo motor is constant torque output, that is, it can output rated torque within its rated speed (generally 2000rpm or 3000rpm), and constant power output above the rated speed.
4: Different overload capacity
Stepper motors generally do not have overload capacity. AC servo motor has strong overload capacity. Taking Panasonic AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is three times of the rated torque, which can be used to overcome the inertia torque of inertia load at the moment of starting. Because the stepping motor does not have this overload capacity, in order to overcome this inertia torque, it is often necessary to select the motor with large torque, and the machine does not need so large torque during normal operation, so the phenomenon of torque waste appears.
5: Different operation performance
The control of stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose step or stall. If the speed is too high when stopping, it is easy to overshoot. Therefore, in order to ensure its control accuracy, the problems of speed increase and speed decrease should be handled well. The AC servo drive system is a closed-loop control. The driver can directly sample the feedback signal of the motor encoder. It forms a position loop and a speed loop. Generally, there will be no step loss or overshoot of the stepping motor, and the control performance is more reliable.
6: Speed affects performance differently
It takes 200 ~ 400 milliseconds for the stepping motor to accelerate from static to working speed (generally hundreds of revolutions per minute). The acceleration performance of AC servo system is good. Taking Panasonic msma400w AC servo motor as an example, it takes only a few milliseconds to accelerate from static to its rated speed of 3000rpm, which can be used in control occasions requiring rapid start and stop.
How to select?
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), the requirements of torque, inertia, speed, accuracy, acceleration and deceleration, the upper control requirements (such as the requirements for port interface and communication), and whether the main control method 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 motor and matched driver or controller can be determined.
2. How to use 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.
3. What is the difference between 2-phase and 5-phase stepping motors and how to choose them?
The cost of 2-phase motor is low, but the vibration is large at low speed and the torque decreases quickly at high speed. 5-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. 4. When to choose DC servo system and what is the difference between it and AC servo?
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, 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 synchronous motor and asynchronous motor. At present, 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.
5. Problems needing attention when using 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 into 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 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.