The control principle of stepping motor and its single-chip control realization

The stepping motor can perform high-precision control of the rotation angle and rotation speed. As a control actuator, stepping motor is one of the key products of mechatronics, and it is widely used in various automation control systems and precision machinery and other fields. For example, in instrumentation, machine tool equipment, and computer peripherals (such as printers and plotters, etc.), where precise control of the angle of rotation is required, stepping motors are the most ideal. With the development of microelectronics and computer technology, the demand for stepper motors is increasing day by day, and they are applied in various fields of national economy.

The stepping motor can perform high-precision control of the rotation angle and rotation speed. As a control actuator, stepping motor is one of the key products of mechatronics, and it is widely used in various automation control systems and precision machinery and other fields. For example, in instrumentation, machine tool equipment, and computer peripherals (such as printers and plotters, etc.), where precise control of the angle of rotation is required, stepping motors are the most ideal. With the development of microelectronics and computer technology, the demand for stepper motors is increasing day by day, and they have applications in various national economic fields.

The control principle of stepping motor and its single-chip control realization

The stepping motor appeared in the last century. It is an electromagnet that can rotate freely. The principle of action is no different from that of today’s reactive stepping motor. It also relies on the change of the air gap permeance to generate electromagnetic torque. At the beginning of this century, due to capitalist powers competing for colonies, the shipbuilding industry developed rapidly, and at the same time, the technology of stepping motors has been greatly improved. After the 1980s, the control methods of stepping motors became more flexible and diverse due to the appearance of cheap microcomputers in a multi-functional posture. The original stepper motor control system used discrete components or integrated circuit control loops, not only complicated debugging and installation, but also consumed a lot of components, and once the model was finalized, the circuit must be redesigned to change the control scheme. The computer controls the stepper motor through software to better tap the potential of the motor. Therefore, it has become an inevitable trend to use a computer to control a stepper motor, and it is also in line with the trend of the digital age.

The difference between a stepper motor and an ordinary motor is that the stepper motor is controlled by a pulse signal. The stepping motor relies on a kind of Electronic switching device called a ring distributor, through the power amplifier, the excitation winding is connected to the DC power supply in turn in order. Since the field windings are arranged in a certain law in space, after turning on the DC power supply in turn, a step-change rotating magnetic field will be formed in the space, causing the rotor to rotate step by step. As the pulse frequency increases, the speed Will increase. The rotation of the stepper motor is also related to the number of phases, the number of distributions, and the number of rotor gears.

The more commonly used stepper motors include reactive stepper motors, permanent magnet stepper motors, hybrid stepper motors and single-phase stepper motors. Among them, the rotor magnetic circuit of the reactive stepping motor is made of soft magnetic material, and the stator has a multi-phase excitation winding, which uses the change of magnetic permeability to generate torque. At this stage, reactive stepping motors have the most applications.

The control principle of stepping motor and its single-chip control realization

The main difference between stepper motor and ordinary motor lies in its pulse drive form. It is precisely this feature that stepper motor can be combined with modern digital control technology. However, the stepping motor is not as good as the traditional closed-loop control DC servo motor in terms of control accuracy, speed variation range, and low-speed performance. The stepper motor can be used in occasions where the accuracy is not particularly high. The stepper motor can give play to its simple structure, high reliability and low cost. When used properly, it can even rival the performance of a DC servo motor.

Stepper motors are widely used in various fields of production practice. Its biggest application is in the manufacture of CNC machine tools. Because stepper motors do not require A/D conversion and can directly convert digital pulse signals into angular displacements, they are considered to be the ideal actuators of CNC machine tools. The output torque of early stepper motors was relatively small and could not meet the needs. In use, it forms a hydraulic pulse motor together with a hydraulic torque amplifier. With the development of stepper motor technology, stepper motors have been able to be used alone in the system and have become irreplaceable actuators. For example, a stepping motor is used as the driving motor of the feed servo mechanism of a CNC milling machine. In this application, the stepping motor can complete two tasks at the same time, one is to transmit torque, and the other is to transmit information. The stepping motor can also be used as the driving motor of the synchronous system of the CNC worm grinding wheel edge grinding machine. In addition to the application on CNC machine tools, the stepper motor can also be used in other machinery, such as as the motor in the automatic feeder, as the motor of the general floppy disk drive, and can also be used in printers and plotters.

With its remarkable characteristics, stepping motors are playing an important role in the era of digital manufacturing. With the development of different digital technologies and the improvement of stepper motor technology, stepper motors will be used in more fields.

1. The control principle of stepper motor

The angle between two adjacent magnetic poles of the stepper motor is 60. . The coil goes around two opposite magnetic poles to form a phase (A-A’, B-B’, C-C’). There are 5 uniformly distributed rectangular small teeth on the magnetic pole, no windings on the rotor, and 40 small teeth evenly distributed on its circumference, and the angle between two adjacent teeth is 9. .

When a certain group of windings is energized, the corresponding two magnetic poles respectively form NS poles, generate a magnetic field, and form a magnetic circuit with the rotor. If the small teeth of the stator are not aligned with the rotor at this time, the rotor will rotate a certain angle under the action of the magnetic field to align the rotor teeth with the stator teeth, so that the stepping motor “goes” one step forward.

1. The control method of stepping motor

If an orderly pulse current is applied to the windings in sequence through the single-chip microcomputer, the rotation of the motor can be controlled to realize the conversion of digital angles. The angle of rotation is proportional to the number of pulses applied, the speed of rotation is proportional to the pulse frequency, and the direction of rotation is related to the sequence of pulses. Taking a three-phase stepping motor as an example, there are three ways to apply current pulses.

(1) Single-phase three-beat mode-applying current pulse according to single-phase winding

The control principle of stepping motor and its single-chip control realization

(2) Two-phase three-beat mode—current pulse is applied according to the two-phase winding

The control principle of stepping motor and its single-chip control realization

(3) Three-phase six-beat mode-single-phase winding and dual-phase winding alternately apply current pulses

The control principle of stepping motor and its single-chip control realization

The single-phase three-beat mode has a step angle of 3. , The step angle of three-phase six-beat is 1.5. , Therefore, under the three-phase six-beat, the stepping motor runs smoothly and softly, but at the same operating angle and speed, the frequency of the three-phase six-beat drive pulse needs to be doubled, which has an impact on the switching characteristics of the drive switch tube. The requirements are higher.

2. The driving method of stepping motor

The common driving method of stepper motor is full voltage drive, that is, the rated voltage is applied when the motor moves and locks. In order to prevent the motor from over-current and improve the drive characteristics, a current-limiting resistor is required. Because the current-limiting resistor consumes a lot of power when the stepper motor locks in step, the current-limiting resistor must have a larger power capacity, and the switch tube must also have a higher load capacity.

Another driving method of stepping motor is high and low voltage drive, that is, when the motor is moving, a voltage of rated value or exceeding the rated value is applied, so that the motor can move quickly under a larger current drive; while in lock step , Then apply a voltage lower than the rated value, and only allow the motor winding to flow through the current value required for lock step. In this way, the power consumption of the current limiting resistor can be reduced, and the running speed of the motor can be increased, but the circuit of this driving mode is more complicated.

The distribution of driving pulses can be realized by a hardware method, that is, a pulse distributor. Now, the pulse distributor has been standardized, chip-based, and can be bought on the market. However, the hardware method has a complicated structure and a high cost.

Stepper motor control (including the generation and distribution of control pulses) can also use software methods, that is, realized by a single-chip microcomputer, which simplifies the circuit and reduces the cost. Using a single-chip microcomputer to drive a stepper motor in software, not only can the speed of the stepper motor, the angle of reciprocating rotation and the number of rotations be set freely within a certain range through programming methods, but also the operation of the stepper motor can be conveniently and flexibly controlled Status to meet the requirements of different users. Therefore, the single-chip stepper motor control circuit is often referred to as a programmable stepper motor control driver.

Second, the single-chip microcomputer control of the stepping motor

The biggest feature of stepper motor control is open-loop control, which does not require feedback signals. Because the movement of the stepper motor does not produce the accumulation of error in the amount of rotation.

The stepping motor control system realized by the single-chip microcomputer is shown in the figure.

The control principle of stepping motor and its single-chip control realization

Assume that the P1 port of 8051 is connected to the windings of the stepping motor, and the control current pulse is output. Among them, P1.0 is connected to A, P1.1 is connected to B, and P1.2 is connected to C.

1. Two-phase three-beat control

The two-phase three-beat control model is shown in the table below.

The control principle of stepping motor and its single-chip control realization

Assume the following work unit and work position definitions:
R0 is the step number register;
In PSW, F0 is the direction flag bit, F0=0 forwards, F00 reverses.
The reference procedure is as follows:

The control principle of stepping motor and its single-chip control realization

2. Three-phase six-beat control program

In the two-phase three-shot program, the control word output by the P1 port is given in the program. In the three-phase six-beat control, because there are many control words, these control words can be stored in the internal RAM unit in the form of a table in advance, and they are taken out and outputted one by one by looking up the table when the program is running.

Assuming that the forward and reverse control words are sequentially stored in the internal RAM with POINT as the first address, the content of the table is as follows:
POINT: DB 01H; forward A
DB 03H; AB
DB 02H; B
DB 06H; BC
DB 04H; C
DB 05H; CA
DB 00H; cycle mark
DB 01H; Reverse A
DB 05H; AC
DB 04H; C
DB 06H; CB
DB 02H; B
DB 03H; BA
DB 00H; cycle mark

The reference procedure is as follows:

The control principle of stepping motor and its single-chip control realization

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