Machines have freed people from labor, and many factories nowadays are automated and no longer require human labor. Automation cannot be achieved without electric motors, which are the power source of machines.
From the discovery of the magnetic effect of electric current in 1820 to 200 years of innovative development now, scientists have manufactured a variety of motors.
Today we will analyze the difference between servo motors and stepper motors.This post is intended to provide clarity on the differences between stepper and servo motors.
What is the difference between a stepper motor and a servo motors?
Stepper motors and servo motors are both control motors, both can accurately control the speed.
But the two control speed principle is different: servo motor is closed-loop control (through the encoder feedback, etc. to complete), that is: the speed of the motor will be measured in real time and then automatically adjusted; stepper motors is open-loop control, input a pulse stepper motors will turn a fixed angle, but not to determine the speed.
I. What is stepper motor?
stepper motors operate in the open-loop, constant current mode.That converts an electrical pulse signal into an angular or linear displacement.
In the case of non-overload, the motor speed, stop position depends only on the frequency of the pulse signal and the number of pulses, and is not affected by changes in load, when the stepper driver receives a pulse signal, it drives the step motor to set the direction of rotation of a fixed angle, called the “step angle”, its rotation is to a fixed angle Step by step operation.
The number of pulses can be controlled to control the amount of angular displacement, so as to achieve the purpose of accurate positioning; at the same time, the pulse frequency can be controlled to control the speed and acceleration of motor rotation, so as to achieve the purpose of speed regulation.
Stepper motor systems operating in a constant current mode (open loop) creates a significant amount of heat in both the motor and drive, which is a concern for some.
At high speeds, stepper motors typically have poor torque characteristics.
What are the characteristics of servo motors?
1.The servo motor is fast to start. The rated speed can be reached in a very short time. Suitable for frequent starting and stopping and have a start torque requirements, while the power of the servo motor can do a lot. Stepper motor start, it is slower, to go through the process of frequency from low to high.
Servo accuracy is slightly higher than the torque stepper motors, but most occasions, the resolution of the stepper has been much higher than the accuracy of mechanical requirements (such as Narushi high-precision hybrid stepper motor can be accurate to 0.9 ° small incremental step angle movement, with the use of differential stepper driver can be further improved).
In contrast, servo motors in high resolution low speed will repeatedly to the position, so that micro-jitter, in some very high-precision equipment, such as precision jade carving machine, high-precision low-speed robot, such jitter will cause jagged on the arc, or blurred; and the special treatment of stepper motor can indeed solve this problem.
2, In some indexing control applications, stepper motors turn to the angle stop, locking torque is large, the servo is lost in the position of external forces and then find the position, this process may cause processing errors.
3, Debugging complex: servo drive involves many parameters, long manuals, and different brands of drivers may be very different.
4, The most critical, servo cost is higher.
Servo system responsiveness, high-speed performance is outstanding, through the analysis of its control method to know that it is mainly used vector control technology and closed-loop control to obtain high performance, but it is limited in some cost-sensitive applications.
And through the subdivision drive, add closed-loop control, stepper can also achieve high control accuracy, the development of the two is not opposing, the continuous development of information technology for stepper motors and servo motors have won a wide range of market space, in some occasions, the two interactive development.
Servo motor, also called actuator motor, is used as an actuating element in automatic control system to convert the received electrical signal into angular displacement or angular velocity output on the motor shaft.
They are divided into two categories: DC and AC servo motors. Its main features: no self rotation when the signal voltage is zero, the speed decreases uniformly with the increase of torque.
The characteristics of the servo point here and the stepper motor compared to say more clearly
1.The control accuracy is different
Two-phase hybrid stepper motor step angle is generally 3.6 °, 1.8 °, five-phase hybrid stepper motor step angle is generally 0.72 °, 0.36 °. There are also some high-performance stepper motor step angle is even smaller.
For example, a stepper motor for slow-walking machine tools produced by SCT, its step angle is 0.09 °; Germany BERGERLAHR company (BERGERLAHR) produced a three-phase hybrid stepper motor whose step angle can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 ° through a dip switch, compatible with step angle of two-phase and five-phase hybrid stepper motors.
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear of the motor shaft. For a Panasonic fully digital AC servo motor, for example, the pulse equivalent is 360°/10000=0.036° for a motor with a standard 2500-line encoder due to the quadruple frequency technology used inside the driver.
For a motor with a 17-bit encoder, the drive receives 217=131072 pulses per motor revolution, i.e., its pulse equivalent is 360°/131072=9.89 seconds. Is the step angle of 1.8 ° of the stepper motor pulse equivalent 1/655.
2.Low frequency characteristics are different
Stepper motor at low speeds are prone to low-frequency vibration phenomenon. Vibration frequency and load conditions and drive performance, it is generally believed that the vibration frequency is half of the motor no-load starting frequency.
This low-frequency vibration phenomenon determined by the working principle of the stepper motor is very unfavorable to the normal operation of the machine. When stepper motors work at low speeds, damping techniques should generally be used to overcome the low-frequency vibration phenomenon, such as adding dampers to the motor, or using subdivision technology on the drive.
AC servo motors run very smoothly and do not show vibration phenomena even at low speeds. AC servo system has resonance suppression function, which can cover the lack of rigidity of machinery, and the system has internal frequency resolution function (FFT), which can detect the resonance point of machinery, and facilitate system adjustment.
3.Different torque and frequency characteristics
The output torque of the stepper motor decreases with the increase of speed, and will drop sharply at higher speeds, so its maximum working speed is generally 300-600RPM. AC servo motor for constant torque output, that is, in its rated speed (generally 2000RPM or 3000RPM), can output the rated torque, above the rated speed for constant power output.
4.Overload capacity is different
Stepper motors generally do not have overload capability. AC servo motor has a strong overload capacity. Take Panasonic AC servo system as an example, it has speed overload and torque overload capability.
Its maximum torque is three times the rated torque, which can be used to overcome the moment of inertia of inertial loads at the moment of start-up. Stepper motor because there is no such overload capacity, in order to overcome this moment of inertia in the selection, often need to choose a larger torque motor, and the machine does not need so much torque during normal operation, there will be a waste of torque phenomenon.
5.Operating performance is different
Stepper motor control for open-loop control, starting frequency is too high or too large a load prone to step loss or blocking the phenomenon, stopping the speed is too high prone to overshoot the phenomenon, so in order to ensure its control accuracy, should deal with the problem of speed up and down.
AC servo drive system for closed-loop control, the driver can directly sample the motor encoder feedback signal, the internal position loop and speed loop, generally will not appear in the stepper motor step loss or overshoot phenomenon, the control performance is more reliable.
6.Speed response performance is different
Stepper motor from standstill acceleration to working speed (generally a few hundred revolutions per minute) takes 200 to 400 milliseconds. AC servo system acceleration performance is better, to Panasonic MSMA400W AC servo motor, for example, from standstill acceleration to its rated speed of 3000RPM only a few milliseconds, can be used to require fast start-stop control occasions
In summary
The biggest difference between the two, I think the servo motor is a closed system, easy to automate control, stepper motor is an open-loop system, the action is not controllable. At the same time, servo motors are superior to stepper motors in many performance aspects.
But the same servo motor price is also much more expensive than the stepper motor. Therefore, in some less demanding occasions also often use stepper motors to do the execution motor. Therefore, in the design process of the control system to consider the control requirements, cost and other factors, the choice of appropriate control motor.
How to choose the motor ?
1.Look at the required working speed, if the speed is lower than 300 RPM, you can consider stepper motor, if higher than this speed, the basic servo motor is the main.
2.See what the requirements for the smoothness of operation, stepper motors are non-continuous work, vibration and noise problems, servo no.
3.See what the requirements for control accuracy, stepper motors are generally open-loop system, all the accuracy is calculated, not necessarily true, stepper motors have lost step problem, subdivision has theoretical and practical separation problems. Servo can be made into a fully closed-loop, reflecting the position is true.
4 See if there is no instantaneous overload work, stepper motors do not have the ability to overload. Servo has the ability to overload. After the above are met and then look at the required torque and installation space, to confirm the motor volume.
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