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 electromechanical devices that operate based on electromagnetic principles. Their movement resolution is determined by the subdivision drive technology, which allows for more precise control over the motor’s position. Using a software-based subdivision driving method offers flexibility and versatility in programming, reducing costs and improving efficiency. It also makes it easier to modify the system configuration. Additionally, this approach helps minimize low-frequency vibrations and running noise that are common in stepping motors at lower speeds.

However, a single software subdivision driver faces a trade-off between accuracy and speed. The more subdivisions you have, the higher the precision, but the slower the motor's rotation. To balance this, a multi-stage subdivision drive system was developed. This system enables different levels of subdivision through various gear settings, ensuring both high precision and efficient speed control.

1. Subdivision Drive Principle

The concept of subdivision is embedded in the control mechanism of stepper motors. For example, a three-phase motor typically rotates in steps like A→B→C... when operating in full-step mode. But when the current is applied in a sequence such as A→AC→C→CB→B→BA→A..., it operates in half-step mode. By inserting intermediate current vectors between the main phases, the motor can achieve finer control. This process effectively divides the step angle into smaller increments, allowing for smoother and more accurate motion.

For instance, inserting a vector IAB between phase A and B changes the step angle from θb to 0.5θb, resulting in a two-step subdivision. Similarly, adding more vectors between phases leads to higher subdivision levels, such as four or eight steps. Each additional vector reduces the step angle further, but increases the complexity of the current waveform.

2. Implementation of Multi-Level Subdivision Drive System

2.1 System Composition

The system includes a host computer, keyboard input, step display, and control modules. The host uses an AT89C51 microcontroller, known for its low power consumption and 8-bit architecture. It features 4KB of Flash memory, making it ideal for storing data and instructions. The system receives control signals via serial communication and processes them to manage the motor's operation.

The keyboard input system allows users to set the desired subdivision level. For high-precision applications, such as 128-step subdivision, the system uses powers of two as reference points. The step display uses an LCD to show parameters like the current subdivision setting and step angle. To simplify the design, the smallest unit displayed is 0.01°.

The control system consists of a D/A converter and a driver circuit. Three DAC0830 chips are used for digital-to-analog conversion, with an 8-bit resolution. These chips are compatible with microprocessors and offer cost-effective performance. The converted signal is amplified and used to drive the motor, enabling precise control.

2.2 Generation of Step Waves

Subdivision involves generating step waves that represent the current vectors inserted during the motor's operation. The host calculates these values based on the selected subdivision level and stores them in a lookup table. When a forward or reverse command is issued, the system sends the corresponding staircase wave to the D/A converter, which converts it into an analog current signal. This current drives the motor, achieving the desired subdivision effect.

To generate these step waves, the system must first determine the appropriate current values for each step. For example, in a four-step subdivision, the B-phase current is divided into four equal parts, ensuring that each step contributes equally to the motor's rotation. The same principle applies to higher subdivisions, where more vectors are added to reduce the step size.

2.3 Multi-Level Subdivision Driver Implementation

To support multiple subdivision levels, the system uses a cyclic incremental lookup table method. Instead of creating separate tables for each subdivision level, a single table is used for the highest possible subdivision (e.g., 128 steps). This approach saves memory and improves system stability. The system adjusts the step size dynamically by changing the cyclic increment base, allowing different subdivision gears to be implemented using the same table.

The cyclic increment base determines how many steps the pointer skips during each cycle. It is calculated based on the selected subdivision level. For example, if the maximum subdivision is 128 steps, and the current setting is 16 steps, the increment base would be (128/16)-1 = 7. This ensures that the correct current values are retrieved for each step, producing the required waveform for the chosen subdivision level.