Stepper motor control methods are generally divided into two types: open-loop control and closed-loop control. The simplest control method for open-loop stepper motors is the closed-loop control system. In this control method, the input of the stepper motor control pulse does not depend on the position of the rotor. Instead, the control pulse is issued according to a fixed pattern. The stepper motor works by relying solely on this series of predetermined pulses. This control method is more suitable for controlling stepper motors due to their unique characteristics.
What is a closed-loop stepper motor?
Closed-loop control is a fundamental concept in cybernetics. It refers to a control relationship in which the controlled output returns to the control input in a certain way, exerting a control effect on the input. Closed-loop control of a stepper motor uses position feedback and/or speed feedback to determine the phase transitions adapted to the rotor position, which can greatly improve the performance of the stepper motor.
In a closed-loop controlled stepper motor system, the operating speed range can be expanded while tracking and feedback are performed with a given accuracy, or the tracking and positioning accuracy can be improved at a given speed, or the limit speed and limit accuracy indicators can be obtained.
Closed-loop stepper motor control method
There are two main types of closed-loop control methods for stepper motors :
1. To keep the phase relationship between the excitation flux and the current consistent, so that it generates electromagnetic torque that can drive the load torque, this method of controlling the motor current is the same as the brushless DC motor control method, and is called brushless drive method or current closed-loop control method.
2. Maintaining a constant motor current and controlling the phase angle between the excitation flux and the current is called the power angle closed-loop control method. The power angle is the phase angle formed by the attraction between the rotor poles and the stator excitation phase (or, alternatively, the axis of the rotating magnetic field of the stator in a synchronous motor). This power angle is smaller at low speeds or light loads, and larger at high speeds or heavy loads. Referring to the diagram below from the open-loop control principle section above, phase "A" attracts the rotor poles. Then, phase "B" is energized at an angle of π/2. When the rotor poles are at the leading edge of phase "A" (the rotor's S pole is to the left of phase A in the diagram), phase "B" begins to be energized.
Why? Because at high speeds, the inductance of the coil extends the turn-off time of the A-phase current and the rise time of the B-phase current. As a result, the angle that generates the maximum torque acceleration increases with speed.
Working principle diagram of closed-loop stepper motor
This control method directly or indirectly detects the position or speed of the rotor (or load), and then automatically provides a drive pulse sequence for the stepper motor through feedback and appropriate processing. This drive pulse sequence changes constantly according to the position of the load or rotor. There are many ways to implement this control method. In applications requiring high precision, combining microstepping drive technology and microcomputer control technology can achieve very high position accuracy requirements.
Advantages of closed-loop stepper motors
a. As the output torque increases, the speed of both decreases in a non-linear manner; however, closed-loop control improves the torque-frequency characteristics.
b. Under closed-loop control, the output power/torque curve is improved because, under closed-loop control, the motor excitation conversion is based on the rotor position information, and the current value is determined by the motor load. Therefore, even in the low speed range, the current can be fully converted into torque.
C. Under closed-loop control, the efficiency-torque curve is improved.
d. By adopting closed-loop control, a higher operating speed and a more stable and smoother rotation speed can be obtained than that of open-loop control.
e. Using closed-loop control, stepper motors can be automatically and effectively accelerated and decelerated.
f. A quantitative evaluation of the speed improvement of closed-loop control compared to open-loop control can be obtained by comparing the time taken to traverse a certain path interval within IV steps:
g. By applying closed-loop drive, efficiency can be increased by up to 7.8 times, output power by up to 3.3 times, and speed by up to 3.6 times. Closed-loop driven stepper motors outperform open-loop driven stepper motors in all aspects. Closed-loop drive of stepper motors combines the advantages of open-loop drive and brushless DC servo motors. Therefore, closed-loop controlled stepper motors will be widely used in position control systems with high reliability requirements.
What is the difference between open-loop and closed-loop stepper motors?
1. Open-loop control simply requires the other party to execute the commands. There is no feedback. Closed-loop control requires the other party to execute the commands and report back to you. There must be feedback.
2. Does it affect the current control? Open-loop control is generally a control activity completed instantaneously, while closed-loop control will definitely last for a certain period of time, which can be used to judge.
