Why do robots use DC Servo Stepper Motors instead of AC Step

time:2019-06-11 Browse:

  Why do robots use DC servo stepper motors instead of AC stepper motors?


  

STEPPER MOTOR

 

  At present, the four major robot manufacturers and other robot manufacturers have introduced man-machine industrial robots, KUKA exits LBR II wa robots, and UR robots collaborate with UR3, 5, 10. ABB and others also have human-machine collaboration. However, all man-machine industry robots use DC stepper stepper motors, and none of them use AC stepper motors. What is the reason why the AC stepper motor is not used?

  The continuous rotation must have a rotating magnetic field, and the rotating magnetic field necessarily requires an alternating power source. In the early days, modern power electronics technology did not directly control the waveform (amplitude, frequency, phase, sine/square wave) of the alternating power supply according to the needs of the stepper motor speed and torque. Therefore, to get an alternating power supply to rotate the rotor is limited to two ways:

  1)Directly connected to the AC power grid and driven by industrial frequency AC. The structure is simple and reliable, but the speed of the stepper motor depends on the frequency of the AC grid and cannot be controlled by electrical methods.

  2)Connected to the DC power supply, using mechanical brush to commutate, the structure is complex, the brush will inevitably wear out, and the life is limited, but the advantage is that the stepper motor speed can be controlled by the power supply voltage.
 

  Stepper Servo Motor

  These two methods are widely used in various fields. According to the characteristics of the power supply, the first one is called AC stepper motor, and the second is called DC stepper motor.

  However, modern stepper motor drive technology has long since moved away from that era. The development of embedded systems and power electronics has enabled people to arbitrarily generate the required electromagnetic fields through controlled high-power high-frequency switching circuits. With such technology, what form of power supply is no longer important, no matter what power supply can be converted into the required alternating power supply through the drive to power the stepper motor coil, so the classification of DC stepper motor AC stepper motor has been no longer strict. However, due to the historical inertia, it is still used to refer to the power frequency AC stepper stepper motor that is still widely used without power electronic driver control, and the brush commutating DC stepper stepper motor. Two stepper motors are used for high-power industrial applications that do not require speed regulation, and one for low-power, the other for low-cost applications.

  In the context of modern drive control technology, new vocabularies including permanent magnet brushless stepper motors and stepper motors have emerged (and many more, but these two are common in industrial robot applications). Among them, the permanent magnet brushless stepper motor basically adopts the structure of the DC stepper motor, but replaces the worn brush by the power electronic circuit to generate a controlled alternating current. If an advanced space vector algorithm algorithm is used to accurately calculate the current waveform, then such a stepper motor drive system can accurately control the speed and torque, improve efficiency, and achieve a wider working range, which is Servo stepper motor. Strictly speaking, servos can only be used to refer to systems including stepper motors and controllers, but most people still call stepper servo motors.

  From the characteristics of the servo drive system, the control is precise, the torque is controllable, the efficiency is high, the working range is wide, the reliability is high, and the efficiency is high. It is obviously the first choice for industrial robots, especially cooperative robots that are highly dependent on torque control. In fact, it is also a solution for commercial products, including the four major manufacturers mentioned by the title. Stepper motors or permanent magnet brushless stepper motors that do not use space vector algorithms, combined with appropriate sensors and improved algorithms, can meet such application needs to a certain extent, so as a low-cost solution, they can also be used in the market, but not the mainstream direction of development.

  Stepper motors and their drive systems are confusing because of their long history, revolutionary technological changes, and excessively conservative inertia in the industrial sector. In addition, the quality of translation of Chinese technical documents is generally not high, and this confusion is more serious. Electrical engineers must see the essence of technology through chaotic terms, and they can't be seen.

  The key to the collaborative robot is to identify the external force exerted by the operator on the robot, so that when the collision occurs, the robot will adjust the motion state to ensure the safety of the user; or judge the intention according to the operator's power, and the robot will follow this intention. Exercise (drag teaching).

  The boundary of the AC/DC stepper motor is increasingly blurred in principle, and it has less influence on identifying external forces and achieving human-machine cooperation. ABB's several collaborative robots use AC stepper motors. As for the current wide range of DC stepper motors, there are two possible reasons:

  1.Kollmorgen's hollow stepper motor is easy to route and easy to make collaborative robots. It is difficult for small robots to use wiring without a stepper motor. ABB's Yumi does not seem to use a hollow stepper motor, and the wiring is very delicate.

  2.Possible certification of DC will be easier. This kind of need to contact people often, the certification should be stricter than the ordinary, the system leakage of communication is more dangerous.

  At this stage, the difference between AC and DC stepper motors has been blurred. For example, brushless DC stepper motor, switched reluctance stepper motor, you say it is DC is right, said the exchange seems to be right. Because the drives are all DC-AC implemented.

  When applied, this difference is secondary. The preferred points are power density, structural complexity, cost, accuracy, etc. The advantages of ordinary DC stepper motors are simple control and accuracy, but the brush structure is not reliable. Brushless can be seen as an AC stepper motor.

  The stepper motor power density required by the robot is not high, so the DC stepper motor with simple control and precision is a suitable choice.

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