Direct control of stepper motor PLC

A stepper motor is a digital electromagnetic actuator that converts a pulse signal into an angular displacement. The angular displacement of the stepping motor is proportional to the number of input pulses, and its rotational speed is proportional to the pulse frequency. The steering is related to the phase sequence of the pulse distribution to the phase windings of the stepping motor. Since the stepping motor's rotation angle, rotation speed and steering can be controlled by digital (pulse), stepping motors are widely used in the field of digital servo. Figure 1 shows a typical application of a stepper motor.

Figure 1 Typical application of stepper motor

In the figure, the input signal is generated by a sensor in the servo system. The command pulse controller is determined by the specific servo control process. A dedicated logic circuit can be used, and currently a multi-chip microcomputer and an interface circuit are used. The ring distributor distributes pulses of the input single pulse train in turn to the power amplifiers connected to the phase windings of the stepper motor in order to form a rotating magnetic field. The ring pulse distributor is mostly composed of an application specific integrated circuit such as CH250. The weak signals of the phases thus formed are amplified by the power amplifiers of the respective phases, generating sufficient electromagnetic torque to rotate the motor. The design, selection, and connection of the various parts in the figure often require the designer of the control system to spend a lot of energy and labor. The matching of the interface signals and the quality of the components have a great influence on the reliability of the entire system.

Method for directly controlling stepping motor by PLC

This paper presents a method for directly controlling a stepper motor with a programmable controller (PLC), as shown in Figure 2. The advantage of this technical route is that the workload of the system design is greatly reduced, there is no matching problem of the interface signals of various parts, and the reliability of the system is improved. The entire control system consists of a PLC and a stepper motor. As an industrial control computer PLC, the function is getting stronger and stronger. Not only for switching logic control, but also for closed-loop process control, and can be combined with other computers to form a multi-level control system. With the support of the powerful functions of PLC, the tasks of different command pulse controllers of various control systems can be completed by different control programs of PLC. For the function of the ring pulse distributor and power amplifier, two characteristic requirements are imposed on the PLC. First, the PLC used in this application is preferably a PLC with real-time refresh technology, so that the frequency of the output signal can reach several thousand Hz or higher. The purpose is to enable the ring pulse distribution to have a higher distribution speed, fully utilize the speed response capability of the stepping motor, and improve the rapidity of the entire system. Second, the output port of the PLC itself should use high-power transistors to meet the driving requirements of tens of volts pulse voltage and several ampere pulse current of each phase winding of the stepping motor. It should be pointed out that PLC with relay or thyristor as output port, even if the software ring pulse distribution can achieve high speed requirements, but because the output port device is difficult to turn on and off the DC power at high speed, it can not provide the phase winding of the stepper motor. The pulse current is driven, so it cannot be used for PLC direct control of stepper motors. For PLCs that meet the requirements of one or two, such as the IP1612DC-220 programmable controller of US IPM, direct control of the stepper motor can be performed.

Figure 2 PLC direct control of stepper motor

The system sends the sensor's measured bag length error signal from the PLC's input port, as shown in Figure 3. According to the error and compensation algorithm, the PLC program automatically calculates the number of steering and corner steps that the stepper motor should compensate, and the ring distribution is performed by the ring distribution program through the output ports Y9, Y10, and Y11, so that the control is connected to the stepping motor. The 48V DC power supply of the three-phase winding is turned on and off in sequence to form a rotating magnetic field, so that the stepping motor rotates. The rotation of the stepper motor is superimposed on the main drive train via a mechanical differential to compensate for the bag length error. Since the stepping motor is an inductive load and the DC resistance is small, the current limiting resistor is connected to prevent the pulse current from being too large to damage the PLC port, that is, the high power transistor corresponding to Y9, Y10, and Y11. When the high-power transistors corresponding to Y9, Y10, and Y11 are turned on and off in the following order: Y9-Y9Y10-Y10-Y10Y11-Y11-Y11Y9-Y9..., the stepping motor rotates forward. Press: Y9-Y9Y11-Y11-Y11Y10-Y10-Y10Y9-Y9... When the battery is turned on and off in turn, the stepping motor is reversed. That is, the stepper motor works according to three-phase six-shot. Whenever the stepping motor takes one step, the number of steps of the ring pulse distribution program is decremented by one. When the number of steps is reduced to zero, the ring pulse distribution is stopped, waiting for the input of the next measurement error.

Figure 3 specific application diagram

The error compensation system that directly controls the stepping motor by PLC has perfect functions, high flexibility and high reliability. The production line has won the National Science and Technology Progress Award. The special emphasis is on the method of directly controlling the stepping motor by PLC, which reduces the workload of the system design, can greatly shorten the development cycle, and has a high promotion and practical value within a certain range.