A stepper motor is an actuator that converts electrical pulses into angular displacement. When the driver receives a pulse signal, it causes the motor to rotate in a specific direction by a fixed angle, known as the "step angle." This rotation occurs in discrete steps, and the total angular movement can be precisely controlled by the number of pulses. Additionally, the speed and acceleration of the motor are regulated by adjusting the frequency of these pulses, making it ideal for applications requiring accurate speed control.
As a specialized motor used in control systems, stepper motors are widely employed in open-loop applications due to their high precision and lack of accumulated error, ensuring 100% accuracy in positioning.
**Positioning Principle and Scheme**
When a stepper motor moves an actuator from one position to another, it typically goes through three stages: acceleration, constant speed, and deceleration. If the motor's operating frequency is below its starting frequency, it can start directly at that frequency and continue running. Similarly, when stopping, it can reduce the frequency to zero without issues. However, if the frequency exceeds the starting frequency, direct start-up may cause the motor to lose steps or become blocked. Sudden stops at high frequencies can also lead to overshooting due to inertia, which affects positioning accuracy.
To avoid this, the motor must be accelerated and decelerated carefully to ensure it reaches the target position quickly without losing steps or overshooting.
There are two common methods for controlling the frequency: linear and exponential. The exponential method offers better tracking but can be unstable during large speed changes. The linear method provides smoother transitions and is more suitable for fast positioning with significant speed variations. It is simple to implement in software and was chosen for this application.
**Positioning Plan**
To maintain high positioning accuracy, the pulse equivalent—the distance moved per step—should be small. However, this can significantly increase the time required for positioning, reducing system efficiency. To address this, the process is divided into two stages: coarse and fine positioning.
In the **coarse positioning** stage, a larger pulse equivalent (e.g., 0.1 mm/step) is used to move quickly. In the **fine positioning** stage, a smaller pulse equivalent (e.g., 0.01 mm/step) ensures accuracy. Since the fine positioning stroke is short, the overall speed remains unaffected. This can be achieved using different mechanical shifting mechanisms.
For example, moving from point A to C (200 mm), the AB segment (196 mm) is for coarse positioning using 0.1 mm/step, while BC (4 mm) is for fine positioning with 0.01 mm/step. At the end of coarse positioning, the PLC automatically switches the shifting mechanism to enable precise movement.
**Overview of Positioning Program Design**
Modern PLCs, such as the Siemens S7-200 series, offer advanced features beyond basic logic instructions. They support high-speed pulse output commands like PTO (Pulse Train Output) and PWM (Pulse Width Modulation). The PTO command allows users to control the period and number of pulses, making it ideal for multi-stage positioning.
In this design, the PTO is used in multi-segment mode for coarse positioning and single-segment mode for fine positioning. For instance, the motor starts at 2 kHz, accelerates to 10 kHz, then decelerates back to 2 kHz. The pulse increment value during acceleration is calculated using the formula:
**Cycle increment = (ECT - ICT) / Q**
Where ECT is the end cycle time, ICT is the initial cycle time, and Q is the number of segments. This ensures smooth acceleration and deceleration.
**Source Program Example**
The main program initializes the coarse positioning parameters, sets the envelope table, and triggers the PTO operation. Subroutines handle the actual pulse generation for both coarse and fine positioning. Interrupts are used to detect when the pulse sequence is complete, allowing the system to transition between stages smoothly.
This detailed approach ensures accurate and efficient positioning, making the system suitable for industrial automation and precision machining applications.
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