Motion control is one of the cores in the field of industrial control, which plays a huge role in printing, packaging, assembly and other industrial scenarios. Motion control originated from motor control, the task of motor control is to control a single motor torque, speed, position and other parameters, so that the motor to complete the specified action. The motion control is based on the motor control to realize the control of multiple motors, the control system automatically coordinates multiple motors to complete the specified movement. The application of complex and precise motion control system greatly reduces the production cost and also reduces the occurrence of misoperation in processing, and improves the quality of products. Nowadays, with the rapid development of industrial production automation technology, a variety of motion control systems are widely used in the logistics industry and large assembly lines.
The mechanical arm that frequently appears in our vision is the most critical link of motion control system to help industrial production. At present, the most advanced mechanical arm in the world has 7 gearless joints, and each motor drives a joint. When the manipulator is in normal operation, the motion control system coordinates seven motors at the same time, so that the manipulator can easily grasp an object at any position in space. Not only that, it can perform other complex functions, it can even help people clean up or play an instrument.
A few years ago, sweeping robots exploded on the Internet as a microcosm of motion control. When the sweeping robot has formulated the motion route, the motion control system will drive the motor to execute different actions, so that the sweeping robot can complete the task efficiently. In the factory, the robot arm is widely used in the assembly line, in the automobile manufacturing assembly line, the robot arm can easily lift tens of kilograms or even hundreds of kilograms of parts to complete the welding and assembly. As we can see, motion control systems are not only used in industry, but also found in the most close to our lives.
In order to understand the motion control system, it is important to understand the executors of the motion command - the motors. Most of the motors used in the motion control system are stepper motors and servo motors. The following Xiaobian will briefly introduce two kinds of motors.
1 step motor
The stepper motor can transform the input pulse signal into angular displacement. In the normal operation of the stepper motor, the speed, position, speed and deceleration of the motor only depend on the frequency and number of pulse signals, and are not affected by the load change. When the stepper motor driver receives a pulse signal, it drives the stepper motor to rotate a fixed Angle in the set direction. It's called "step Angle," and it rotates step by step, one step at a time, and that's where the steeping motor gets its name.
2 Servo motor
The servo motor converts the received electrical signal into the angular displacement output on the motor shaft, and the servo motor driver controls the three-phase electricity to form an electromagnetic field, and the rotor rotates under the action of the magnetic field. The encoder of the servo motor feeds back the signal to the driver, and the driver adjusts the Angle of the rotor rotation according to the comparison between the feedback value and the target value.
Comparison of two motors
1. Different control methods
The stepper motor adopts open-loop control, and the servo motor adopts closed-loop control. The difference between the two control methods is that the closed-loop control will compare the target value with the actual value and adjust the motor position. In contrast, the control accuracy of the servo motor is better than that of the stepper motor.
2 Different control precision
The more phases the stepper motor has, the higher its accuracy will be. The 2-phase motor cost is low, but the vibration is large at low speed, and the torque drops fast at high speed. The 5-phase motor has small vibration and good high-speed performance, which is 30~50% higher than the speed of the 2-phase motor, and can even replace the servo motor in some occasions. The servo motor comes with an encoder, and the more scales the encoder has, the higher the accuracy. In general, the accuracy of the servo motor is equivalent to the stepper motor with a step Angle of 0.036 degrees, of course, there is no such a small step Angle of the stepper motor, the step Angle of the general stepper motor is 1.8, the above is just an analogy, it can be seen that in the implementation of high-precision motion control, the performance of the servo motor is far beyond the stepper motor.
3 Different low-frequency characteristics
Unlike servo motors, stepper motors use damping technology or subdivision technology to overcome the low-speed vibration phenomenon at low speeds. Stepper motor at low speed is still prone to vibration, and servo motor no matter at high speed or low speed will not appear vibration phenomenon.
4. Different motion performance
Stepper motor is open loop control, too high start frequency or load is easy to cause the phenomenon of lost step, too high speed when stopping is easy to cause overshoot phenomenon, servo motor is closed loop control, servo driver can directly sample the feedback signal of motor encoder, internal speed loop and position loop, generally will not appear lost step or overshoot phenomenon.
5 Speed varies accordingly
It takes hundreds of milliseconds for stepper motors to accelerate from static to working speed, while servo motors generally only need a few milliseconds, which can be used in control situations requiring fast start and stop.
From the above comparison, servo motors are better than stepper motors in many aspects of performance. Is it ok to choose servo motors when we choose motor models? No, the price of servo motor will be much higher than that of stepper motor, stepper motor will be in terms of cost performance than servo motor, after mastering the characteristics of the two motors, according to different needs, it is particularly important to choose the right type of motor.
Motion control systems are not only made up of motors and actuators, but more important than these are control schemes or algorithms that coordinate the movement of multiple motors. For example, there is a motion system in which the film is wrapped around a turntable driven by two motors, so that the film can be unrolled from one turntable to the other without breaking, at a set speed of film winding. In the process of film winding, the winding diameter of the two turntable will change constantly. In order to ensure that the film does not break and meet the specified film winding speed, the speed of the two motors needs to be adjusted constantly, which requires the use of PID algorithm to do closed-loop control, so that the controlled object: tension feedback value affects the speed of the motor. In this way, relying on the fast response performance of the servo motor, the speed is reduced when the tension is too large, and the speed is accelerated when the tension is too small. Under constant adjustment, the tension and winding speed of the film reach the requirements.
In addition to the PID algorithm, the motion difference algorithm is also used in the 6 degrees of freedom or even 7 degrees of freedom manipulator control system to ensure that the manipulator runs to the specified position. The quality of the motion control system scheme determines whether the system is safe and reliable, and whether the efficiency is high. Having excellent program design ability will make us more competitive.