With the development of microelectronics and computer technology, the demand for stepper motors is increasing day by day, and they will be widely used in the control field of various industries.
As an industrial control computer, the PLC has a modular structure, flexible configuration, high-speed processing speed, and precise data processing capabilities. The PLC also has good control capabilities for stepper motors. By utilizing its high-speed pulse output function or motion control function, the stepper motor can be controlled.
For specific equipment whose movement distance and speed are fixed during operation, engineers at Shanshe Electric believe that using a PLC to control the operation of a stepper motor through a stepper motor driver is an ideal technical solution.
The characteristics of stepper motors are: (1) The angular displacement of a stepper motor is strictly proportional to the number of input pulses. There is no cumulative error after one revolution of the motor, and it has good following performance. (2) The open-loop digital control system composed of a stepper motor and a driver circuit is very simple, inexpensive, and very reliable. At the same time, it can also be combined with an angle feedback loop to form a high-performance closed-loop digital control system. (3) Stepper motors have fast dynamic response and are easy to start, stop, reverse, and speed change. (4) The speed can be smoothly adjusted within a fairly wide range, and large torque can still be obtained at low speeds. (5) Stepper motors can only operate when powered by a pulse power supply. They cannot use AC or DC power directly.
The highest stepping frequency at which a stepper motor can respond without losing steps is called the "starting frequency." Similarly, the "stopping frequency" refers to the highest stepping frequency at which the stepper motor will not overshoot the target position if the system control signal is suddenly turned off. The motor's starting frequency, stopping frequency, and output torque must all be adapted to the load's rotational inertia. With this data, the stepper motor can be effectively controlled for speed variations.
When using a PLC to control a stepper motor, the system's pulse equivalent, upper limit of pulse frequency, and maximum number of pulses should be calculated using the following formulas to select the appropriate PLC and its corresponding functional modules. The pulse frequency determines the frequency required for high-speed pulse output by the PLC, and the number of pulses determines the PLC's bit width. Pulse equivalent = (stepper motor step angle × pitch) / (360 × transmission ratio); Upper limit of pulse frequency = (moving speed × stepper motor microstepping) / pulse equivalent; Maximum number of pulses = (moving distance × stepper motor microstepping) / pulse equivalent.
The first step in controlling a stepper motor using a PLC is to establish a coordinate system, which can be either a relative or absolute coordinate system. The coordinate system is set in the DM6629 word; bits 00-03 correspond to pulse output 0, and bits 04-07 correspond to pulse output 1. Setting it to 0 indicates a relative coordinate system; setting it to 1 indicates an absolute coordinate system.
By employing a PLC to control the operation of a stepper motor via a stepper driver, the application of PLCs in stepper motor control has become more widespread. For example, in controlling single-axis and dual-axis motion, parameters such as movement distance, speed, and direction are set on the control panel. After reading these settings, the PLC generates pulse and direction signals through calculations to control the stepper motor drive, achieving the purpose of controlling distance, speed, and direction. Practical testing has proven the system's reliability, feasibility, and effectiveness.
