The wiring diagram of a Mitsubishi PLC and a stepper motor driver illustrates how the control system interacts with the motor to achieve precise motion control. 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 by a fixed angle, known as the "step angle." This allows for accurate positioning through the number of pulses, while speed and acceleration can be controlled by adjusting the pulse frequency.
Stepper motors are widely used in open-loop control systems due to their high accuracy and lack of accumulated error. In industrial automation, they play a crucial role in applications like positioning and drilling. To ensure efficient and accurate movement, the motor must go through acceleration, constant speed, and deceleration phases when moving between positions.
If the motor's operating frequency exceeds its starting frequency, direct start-up may cause stepping loss or stalling. Similarly, sudden stops at high speeds can lead to overshoot. Therefore, proper acceleration and deceleration strategies are essential to maintain both accuracy and efficiency.
Two common frequency control methods are linear and exponential ramping. Linear ramping offers smooth transitions and is suitable for fast positioning. This method was chosen in this project for its simplicity and effectiveness in software implementation.
To balance speed and precision, the positioning process is divided into two stages: coarse and fine. During the coarse stage, a larger pulse equivalent (e.g., 0.1 mm/step) is used for faster movement, while the fine stage employs a smaller pulse equivalent (e.g., 0.01 mm/step) for accurate positioning. This approach ensures that the overall positioning time remains reasonable while maintaining high accuracy.
In an example, a 200 mm movement is split into a 196 mm coarse segment and a 4 mm fine segment. The coarse movement uses a linear ramping method with a higher pulse frequency, while the fine movement is done at a lower frequency for precision. The PLC automatically switches the shifting mechanism at the end of each phase.
Modern PLCs, such as the Siemens S7-200 series, offer advanced functions like PTO (Pulse Train Output) and PWM (Pulse Width Modulation). These commands allow for high-speed pulse generation, which is essential for controlling the motor’s movement. In this application, the multi-segment PTO mode is used for coarse positioning, and the single-segment mode is used for fine positioning.
The pulse increment during acceleration and deceleration is calculated using a formula based on the initial and final cycle times. For example, if the motor starts at 2 kHz and accelerates to 10 kHz, the envelope table defines the timing for each segment. The PLC then generates the appropriate pulses to control the motor’s motion.
The source code includes initialization routines, interrupt handling, and control logic for both coarse and fine positioning. Subroutines set up the pulse parameters, define the envelope table, and manage the motor’s operation. Interrupts are used to trigger actions once the positioning is complete, ensuring seamless transitions between stages.
Overall, this system provides a reliable and efficient way to control the motion of a stepper motor in industrial applications, combining speed, accuracy, and flexibility.
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