Analysis of stepper motor open-loop servo system

The stepper motor servo system is a typical open-loop control system, and the command signal flows in one direction. The open-loop system has no position and speed feedback loops, and eliminates the detection device. Its accuracy is mainly determined by the stepper motor, and the speed is also limited by the performance of the stepper motor. The system is simple and reliable, and does not require complex design calculations and test verification like a closed-loop servo system.

The stepper motor open-loop servo system has been widely used in small and medium-sized machine tools and in occasions where speed and precision requirements are not very high due to its simple structure, easy use and maintenance, high reliability, and low manufacturing cost.

1. Types and structures of stepper motors

There are many ways to classify stepper motors. According to different classification methods, stepper motors can be divided into many types, as shown in Table 1.

The stepper motor is structurally divided into two parts: the stator and the rotor. The reactive three-phase stepper motor shown in Figure 2 is used as an example to illustrate. There are six magnetic poles on the stator, each of which is wound with an excitation winding. Each pair of opposite magnetic poles constitutes one phase, which is divided into three phases: A, B, and C. Five small teeth are opened on each magnetic pole of the stator, with equal tooth width and an angle of 9° between teeth. The rotor has no winding and is made of a toothed iron core. There are 40 small teeth evenly distributed, and the angle between teeth is also 9°. In addition, the small teeth on the stator magnetic poles are staggered in space by 1/3 of the tooth pitch.

2. Working principle of stepper motor

The stepper motor works on the principle of electromagnetic attraction. The working principle is explained by taking the reactive stepper motor as an example. The stator of the reactive stepper motor has magnetic poles, each of which has an excitation winding. The rotor has no winding and has teeth evenly distributed in the circumference. It works by the attraction of the magnetic poles to the teeth. As shown in Figure 3, a three-phase stepper motor has three pairs of magnetic poles on the stator, which are divided into three phases: A, B, and C.

Stepper motor open loop servo system

Open loop control CNC machine tool is shown in Figure 1

Features: simple structure, stepper drive, stepper motor, no position and speed feedback.

structure

The stator and rotor cores of the motor are usually made of stacked silicon steel sheets. There are many small teeth evenly distributed on the stator and rotor. There are three pairs of magnetic poles A, B, and C on the stator, and three-way control windings A, B, and C are wound on the corresponding magnetic poles. Their geometric axes are staggered with the rotor tooth axes respectively. See Figure 2-5-6.

How it works

A stepper motor is a motor that converts electrical pulses into angular displacement. A dedicated drive power supply is used to supply a series of regular electrical pulse signals to the stepper motor. Each time an electrical pulse is input, the stepper motor moves forward one step. Its angular displacement is proportional to the number of pulses, the motor speed is proportional to the pulse frequency, and the speed and direction are related to the way each phase winding is energized.

How to power a stepper motor

Its operating modes include: three-phase single three-beat, three-phase double three-beat and three-phase six-beat.

The angle that the stepper motor rotates for each input pulse signal is called the step angle, which is represented by θs.

Step error refers to the difference between the actual angle of the rotor in each step when the stepper motor is running and the theoretical step angle.

Stepper motor power drive

The drive control circuit consists of a ring distributor and a power amplifier.

(1) Ring distributor

The ring distributor is used to control the power-on mode of the stepper motor. Its function is to add a series of command pulses sent by the CNC device to the power amplifier in a certain order and distribution mode to control the power-on and power-off of each phase winding. The ring distributor function can be generated by hardware or software. See Figures 2 and 3.

(2) Driving amplifier circuit

FIG. 4 is a high-low voltage driving circuit.

Features: High voltage charging, low voltage maintenance.

Figure 4 High and low voltage drive principle

Key Features

(1) Maximum static torque Tmax

(2) Startup frequency

(3) Continuous operation frequency

(4) Torque-frequency characteristics

(5) Static step error

Stepper Motor Selection

The stepper motor consists of three major factors: step angle (involving the number of phases), static torque, and current. Once the three major factors are determined, the model of the stepper motor is determined.