Application of stepper motor in packaging machinery

Application of stepper motor in packaging machinery

Author: Wulong Qi

After nearly 20 years of development, China's packaging machinery has made great progress, but compared with foreign countries, there is still a relatively large gap. Where is the gap? We believe that it is mainly in terms of control. In view of this, two experts from Yang Fu, Wenzhou University and Wu Longqi from Henan University of Science and Technology, wrote a series of articles on the research content they are engaged in, hoping to inspire the relevant industry insiders.

First, the working principle of stepper motor

A stepper motor is an open-loop control element that converts a given electrical pulse signal into an angular displacement or line displacement.

Given an electrical pulse signal, the stepper motor rotor is rotated through the corresponding angle, which is called the step angle of the stepper motor. At present, the step angle of commonly used stepping motors is mostly 1.8 degrees (commonly known as one step) or 0.9 degrees (commonly known as half step). For a progressive motor with a step angle of 0.9 degrees, when we give the stepper motor an electrical pulse signal, the stepper motor turns over 0.9 degrees; for two pulse signals, the stepper motor turns over 1.8 degrees. By analogy, the stepping motor can be continuously operated given the pulse signal continuously. Due to the linear relationship between the electric pulse signal and the stepping motor's rotation angle, the stepping motor has been widely used in speed control and position control.

The use of stepper motors requires at least three aspects. (1) An electrical pulse signal generator that repeatedly transmits electrical pulse signals to a stepper motor according to a given setting. Currently, most of this signal is controlled by a programmable controller or The MCU is used to complete; (2) the driver (signal amplifier), which not only amplifies the electric pulse signal, but also drives the stepping motor to rotate, and can also improve the performance of the stepping motor through it, in fact, it is in the stepping motor system. It plays an important role. Generally, a stepping motor can have multiple drivers according to different working conditions. (3) Stepper motor, which has various control principles and models, and now commonly used reactive, inductive, Hybrid and so on.

The speed control of the incoming motor is achieved by the input pulse frequency. When the frequency of the pulse decreases, the speed of the stepper motor decreases; as the frequency increases, the speed increases. It is also possible to increase the speed or positional accuracy of the stepper motor by changing the frequency.

The position control of the stepper motor is controlled by the given number of pulses. Given a pulse, a step angle is turned. When the stop position is determined, the number of pulses required for the stepper motor is determined. In the packaging machine control, the number of given pulses is also related to the parameters of the mechanism, such as the diameter of the screw.

In packaging machinery, in many cases it is necessary to control the operating speed and operating position of the actuator, which was previously done by mechanical or other means, and the mechanism is complicated and the adjustment is not easy. When the stepping motor is used, not only can the packaging mechanical structure be simple, the adjustment is convenient, the reliability is increased, and the thickness is greatly improved.

Second, the application of stepper motor in material measurement

(1) Measurement of powdered materials

Screw metering is a commonly used volumetric metering method. It is how much the metering is achieved by the number of turns of the screw. In order to achieve the purpose of adjustable metering size and improved metering accuracy, the screw speed is adjustable and the position is accurately positioned. The motor can meet both of these requirements.

For example, the metering machine uses a stepping motor to control the speed and number of revolutions of the screw, which not only simplifies the mechanical structure, but also makes the control very convenient. In the case of no overload, the speed and stop position of the stepping motor depend only on the frequency and pulse number of the pulse signal, and are not affected by the load change. This has obvious accuracy compared with the screw meter controlled by the electromagnetic clutch. Advantages, more suitable for material measurement with large changes in specific gravity.

The stepping motor and the screw adopt a direct connection method, and the structure is simple and the maintenance is convenient. It is worth pointing out that, as the stepping motor has a higher overload capacity, when there is a slight overload, considerable noise will appear. Therefore, after the measurement condition is determined, a larger overload factor is selected to ensure the stepping motor balance work.

(2) Measurement of viscous materials

Stepper motor control gear pumps can also achieve accurate metering. Gear pumps are widely used in the delivery of viscous bodies, such as syrup, bean paste, white wine, oil, ketchup, etc. At present, piston pumps are mostly used for the measurement of these materials, which have the disadvantages of difficulty in adjustment, complicated structure, inconvenient maintenance, large power consumption, and inaccurate measurement.

The gear pump metering is measured by a pair of gear meshing rotations, and the space of the material passing through the teeth and teeth is forcibly sent from the feed port to the discharge port. The power comes from the stepping motor. The position and speed of the stepping motor are controlled by the programmable controller, and the measurement accuracy is higher than the measurement accuracy of the piston pump.

The stepper motor is suitable for running at low speed. When the speed is increased, the noise of the stepping motor will increase significantly, and other economic indicators will drop significantly. For a gear pump with a relatively high speed, it is better to use a speed-up structure. We started to use the structure of the stepping motor direct-coupled gear pump on the viscous packaging machine. As a result, it is difficult to avoid the noise and the reliability is degraded. Later, the spur gear speed-up method was adopted to reduce the speed of the stepping motor, the noise was controlled, the reliability was also improved, and the measurement accuracy was guaranteed.

Third, the application of stepper motor in the supply of packaging film

In the packaging machine which integrates bag making, filling and sealing, it is required to position the fixed length supply of the plastic film for packaging, and whether it is intermittent supply or continuous supply, it can be reliably completed by a stepping motor.

(1) for intermittent packaging machine

The intermittent packaging machine uses a stepping motor to supply the film, and the reliability can be improved. In the past, the packaging film was fed by the intermittent pulling mechanism of the crank linkage mechanism, and the structure was complicated and the adjustment was difficult. Especially when the product needs to be replaced, not only the adjustment was difficult, but also the packaging film was wasted a lot. The stepping motor and the draw belt roller are directly connected to the drawstring, which not only simplifies the structure, but also is very convenient to adjust, as long as the button on the control panel can be realized, which saves the adjustment time and saves the packaging material.

In the intermittent packaging machine, the feeding control of the packaging material can adopt two modes: a bag length control mode and a color mark control mode. The bag length control mode is applicable to the packaging film without color standard, which is realized by setting the speed of the stepping motor in advance, and the setting of the turning ratio can be realized by the dial switch. The color mark mode is equipped with a photoelectric switch, and the photoelectric switch detects the position of the color mark. When the color mark is detected, the control switch signal is issued, and the stepping motor stops rotating after the signal is pressed, and after a certain time delay, the film is rotated again and again. , to ensure that the film is fixed according to the position of the color mark.

(two) for continuous packaging machine

In the continuous packaging machine, the stepping motor is continuously rotated, and the packaging film is uniformly and continuously conveyed. When the length of the bag is changed, it can be realized only by the dial switch.

Fourth, the application of stepper motor in horizontal seal

In continuous packaging machines, cross-sealing is an important actuator and one of the more complex mechanisms in packaging machines. Especially for the packaging film of colored files, the sealing and cutting position requirements are extremely strict. In order to improve the cutting accuracy, people have developed eccentric sprocket mechanism, crank guide mechanism, etc., but these mechanisms are very troublesome and reliable. The disadvantage of low sexuality. The main reason for these shortcomings is that the process requires a fixed-width transverse seal and a positioning cut.

The stepper motor directly drives the transverse seal wheel to achieve speed synchronization. The film supply wheel of the continuous packaging machine is continuously supplied with film. The horizontal speed required for the horizontal sealing is synchronized with the speed of film feeding to avoid the occurrence of tear film and film accumulation, because the diameter of the transverse sealing wheel is constant. When changing the length of the bag, it needs to be changed by changing the rotation speed of the horizontal sealing wheel, but the horizontal sealing requires a certain time, that is, the transverse sealing wheel and the film need a constant time from contact to leaving, otherwise the sealing is unduly. The total time per revolution of the transverse sealing wheel and the time required for the horizontal sealing are constant. To meet the requirements of speed synchronization, the speed of the stepping motor can be divided into two parts in one week, and some of them first meet the requirements of speed synchronization. In addition, the unloaded part meets the requirements of the total time of the week.

In order to achieve good sealing quality, the non-equal speed control mode can be realized by the stepping motor to the transverse sealing wheel, that is, the speed synchronization is realized at every point of the horizontal sealing, which is limited to the length, and will not be stated here.

The characteristics of the stepper motor are mainly attributed to three aspects, (1) the overload is good. The speed is not affected by the load size. Unlike ordinary motors, when the load is increased, the speed will drop. Therefore, the stepping motor is used in places where the speed and position are strictly required. (2) The control is convenient. The stepping motor rotates in units of “steps”, and the digital features are obvious. This brings great convenience to computer control. Conversely, the emergence of computers has opened up a broader market for stepper motors. (3) The structure of the whole machine is simple. The traditional mechanical speed and position control structure is complicated and difficult to adjust. After using the stepping motor, the structure of the whole machine is simple and compact.

The use of stepper motors in packaging machinery has just begun. Many traditional mechanical control methods can be replaced by stepper motors. In addition to some of the applications described above, they are still in printing, filling, and packaging. It has been applied, and it is believed that in a relatively short period of time, China's packaging machinery industry will be able to leap to a new level in the platform of stepper motor technology.