Discussion and research on multi-stage subdivision driving method of stepping motor - News - Global IC Trade Starts Here Free Join

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Stepper motors are used as electromagnetic mechanical devices, and the resolution of their feed depends on the subdivision drive technology. Adopting software subdivision driving method, due to the flexibility and versatility of programming, the cost of stepping subdivision driving is low and the efficiency is high. It is also easy to modify the scheme. At the same time, it can also solve the low frequency vibration and the running noise that the stepping motor is prone to at low speed. However, a single software subdivision driver will have contradiction between accuracy and speed. The more steps the subdivision has, the higher the accuracy, but the rotation speed of the stepper motor is reduced. To increase the rotation speed, the number of steps is subdivided. Have to reduce. To this end, a multi-stage subdivision drive system has been designed to achieve subdivision of unsynchronized numbers through different subdivision gear settings, while ensuring different rotational speeds.

1 subdivision drive principle

The mechanism of subdivision has been implied in stepper motor control. For example, the three-phase stepping motor rotates in the order of A→B→C......, and the stepping motor works in the whole step. When the power is turned on in the order of A→AC→C→CB→B→BA→A..., the stepping motor works in half step. Taking A→B as an example, if the phase current is regarded as a vector, the transformation from the whole step to the half step is to insert the transition vector IAB between IA and IB, because the direction of the current vector is determined by the stepping motor. The direction of the resultant magnetic potential, and the angle of rotation of the resultant magnetic potential itself is the stepping angle of the stepper motor. Obviously, the insertion of the IAB changes the rotation of the resultant magnetic potential, so that the stepping angle of the stepping motor is changed from θb to 0.5θb, thereby achieving a 2-step subdivision. It can be seen that the subdivision principle of the stepping motor is to insert the current synthesis vector regularly through equal angles, thereby reducing the rotation angle of the synthetic magnetic potential and achieving the purpose of stepping motor subdivision control.

By inserting the composite vector AB between the A phase and the B phase of the three-phase stepping motor, a 2-step subdivision is achieved. To realize the four-step subdivision, only three vectors I1, I2, and I3 are inserted between A and AB, so that the rotational angle of the resultant magnetic potential θ1 = θ2 = θ3 = θ4, and the four-step subdivision is realized. However, the 4-step subdivision is different from the 2-step subdivision. Since the insertion of I1, I2, and I33 vectors is a decomposition of the current vector IB, the control pulse has become a staircase wave. The higher the degree of subdivision, the more complex the staircase wave.

When the three-phase stepping motor works in the whole step, the 2-step subdivision synthetic magnetic potential rotation process is IA→IAB→IB; the 4-step subdivision rotation process is IA→I2→IAB......; and the 8-step subdivision is realized. Then the rotation process is IA→I1→I2→I3→IAB.... It can be seen that when different subdivision steps are selected, different current synthesis vectors are inserted.

2 Multi-level subdivision drive system implementation

2.1 system composition

The system consists of a host computer, a keyboard input system, a step display system, and a step control system. The host adopts AT89C51 single-chip microcomputer, which is a low-power 8-bit single-chip microcomputer. It has a 4K byte Flash programmable, erasable and read-only memory on the chip, which simplifies the system configuration and can satisfy the data storage space of this system. Claim. The host receives the step control data from the serial port and processes it to implement step control. The keyboard input system is used to enter the subdivision gears required for control. When designing the system, considering the precision of the subdivision, such as the 128-step subdivision, the step angle is small enough to meet various stepping requirements, so the integer power of 2 is used as the subdivision reference. The step display system displays parameters such as the current subdivision gear position and the step angle after subdivision by the liquid crystal display. In order to reduce the complexity of the circuit, the minimum unit displayed by the display is specified to be 0.01°. The step control system consists of a D/A conversion section and a drive system. The D/A conversion section includes three DAC0830 integrated chips and a data latch system. The conversion resolution of the DAC0830 is 8 bits. The chip has the advantages of being compatible with the microprocessor, low price, simple interface, and easy conversion control. The function of the D/A conversion part is to convert the step wave value represented by the binary code into a corresponding current value output, which is amplified by the driving system to control the rotation of the stepping motor.

The drive system uses a three-stage tube for current amplification.
2.2 Subdivided generation of step wave

The implementation of the subdivision is the process of inserting the current synthesis vector and converting the current synthesis vector. The premise of the current synthesis vector transformation is the insertion of the composite vector. In the system, the host calculates the relevant parameters according to the set subdivision gear position, and generates a corresponding step wave through the look-up table, that is, the current synthesis vector is inserted. Under the forward or reverse control signal, the staircase wave pulse is sent from the output port to the D/A converter DAC0830 through the latch system to convert the current synthesis vector, output the corresponding current value, and drive the amplification stepper motor. Thereby implementing the subdivision drive.

The insertion of the current synthesis vector is the key to realizing the subdivision, and to obtain the current synthesis vector, the staircase wave must first be generated. It is known from Fig. 1 that in the case of a three-phase motor half-step operation, in order to realize four-step subdivision, it is necessary to divide the B-phase current into four parts, but not equally, and it is necessary to ensure that θ1 = θ2 = θ3 = θ4. If the current vectors corresponding to θ1, θ2, θ3, and θ4 are IB1, IB2, IB3, and IB4, then in the triangle corresponding to θ1, the step angle is θb, then α=180°-θb, β=θb- Θ1, from the sine theorem to take into account the general situation, because the stepping motor control pulse waveform is stepped when subdivided, such as the 4-phase subdivision of the B phase, the current input is IB1, IB1 + IB2, IB1 + IB2 +IB3, IB1+IB2+IB3+IB4, the angle at which the corresponding synthetic magnetic potential turns is θ1, θ1+θ2, θ1+θ2+θ3, θ1+θ2+θ3+θ4, and IBk is set in the current synthesis vector. The current value of the kth step in the B-phase staircase wave, and θk is the angle at which the resultant magnetic potential is rotated accordingly. Therefore, for the B phase, when the stepping angle of the stepping motor is θb, considering the IA=IB, the current value of any step of the stepped wave type is the same, and the phase A and the C can be obtained. Corresponding step wave current value when phased. For the solution of (1), consider that the conversion accuracy of D/A device DAC0830 is 8 bits, and the conversion stabilization time is 1μs, so the maximum operation of 128 steps is performed, and the corresponding subdivided current value is obtained accordingly. The corresponding conversion is obtained, and a corresponding list of binary values ​​is obtained. At this time, all the values ​​in the list are the current values ​​corresponding to the steps of the step wave when the 128-step subdivision is implemented.

2.3 Multi-level subdivision driver implementation

To achieve multi-level segmentation based on subdivision, different stepped waves must be generated for different subdivisions. To this end, the system uses a cyclic incremental look-up table method. First, the ladder wave value storage table is established. There are two methods. One is to create a corresponding table for each subdivision method, which is characterized by various subdivisions, but the table takes up a large space; the other is that The system uses only one table in the number of steps corresponding to the largest subdivision gear, which greatly reduces the required storage space and reduces the unstable factors in the running of the program. In the specific control, the system realizes multi-level subdivision driving by setting the cyclic increment base so that different subdivision gears correspond to different subdivision steps.

The cyclic increment base is the number of steps that the pointer jumps to when it is equally spaced. The cyclic increment base is obtained by the corresponding calculation formula after the subdivision gear is set. Since the maximum number of subdivision steps of the system is 128 steps, that is, the maximum length of the table is 128 bytes, if the number of subdivision steps is m steps, the cyclic increment base is LB=(128/m)-1. Different gear positions correspond to different cyclic increment bases, and the same table produces the step wave required for multi-level subdivision.