First, the working principle of stepping motor
Stepper motors are open-loop control elements that convert a given electrical pulse signal to angular or linear displacement.
Given an electrical pulse signal, the stepper motor rotor rotates through a corresponding angle, which is called the step angle of the stepper motor. At present, the step angle of a commonly used stepping motor is mostly 1.8 degrees (commonly referred to as one step) or 0.9 degrees (commonly referred to as half steps). For a progressive motor with a step angle of 0.9 degrees, when we give an electric pulse signal to the stepping motor, the stepping motor turns 0.9 degrees; for two pulse signals, the stepping motor turns 1.8 degrees. By analogy, continuously giving a pulse signal, the stepper motor can be continuously operated. Because of the linear relationship between the electric pulse signal and the rotation angle of the stepper motor, the stepper motor has been widely used in speed control, position control and the like.
The use of a stepper motor requires at least three aspects. The first is an electrical pulse signal generator, which repeats the electrical pulse signal for the stepper motor according to a given setting. Most of the current signals are from a programmable controller or a microcontroller. To complete; second is the driver (signal amplifier), which in addition to the electric pulse signal amplification, stepping motor rotation, but also through it to improve the performance of stepper motor, in fact it plays a role in the stepper motor system The important role of a general stepper motor can have a variety of drivers according to different working conditions; three is a stepper motor, it has a variety of control principles and models, now commonly used in the reaction, induction sub-type, hybrid, etc. .
Stepper motor speed control is achieved by the input pulse frequency speed. When the frequency of the pulse is reduced, the speed of the stepping motor decreases; when 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.
Stepper motor position control is controlled by a given number of pulses. Given a pulse, a step angle is turned and when the position of the stop 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 the packaging machinery, in many cases it is necessary to control the operating speed and the operating position of the actuator. It was previously completed by mechanical or other means, and the mechanism is complex and adjustment is not easy. The use of a stepper motor will not only make the packaging machinery simpler, it will be easier to adjust and increase its reliability, and its thickness will be greatly improved.
Second, the application of stepper motor in the measurement of materials
1. Measurement of powdered materials
Screw metering is a commonly used volumetric metering method. It uses the number of revolutions of the screw to achieve the measurement. To achieve the purpose of adjusting the metering size and improving the metering accuracy, the screw speed needs to be adjusted and the positioning is accurate. The motor can meet these two requirements at the same time.
For example, the powder packaging machine uses a stepper motor to control the rotation speed and the 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 rotation speed and stop position of the stepping motor only depend on the frequency and pulse number of the pulse signal, and is not affected by the load change, which has obvious accuracy compared with the screw measurement controlled by the electromagnetic clutch. The advantages are more suitable for the measurement of materials with larger changes in specific gravity.
The stepping motor and the screw adopt a direct connection method, the structure is simple, and the maintenance is convenient. It is worth pointing out that if the overload capability of a stepper motor is greater, then when there is a slight overload, there will be considerable noise. Therefore, after the determination of the metering conditions, it is necessary to use a larger overload factor to ensure the balance of the stepper motor.
2. Viscous material measurement
Stepper motor control gear pumps can also achieve accurate metering. Gear pumps are widely used in the delivery of viscous products 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 difficult adjustment, complicated structure, inconvenient maintenance, large power consumption, and inaccurate metering.
The gear pump metering is based on a pair of gears meshing and rotating, and the space for the material to pass through the teeth and teeth is forced from the feed port to the discharge port. The power comes from a stepper motor. The position and speed of the stepper motor are controlled by a programmable controller. The measurement accuracy is higher than that of the piston pump.
Stepper motors are suitable for operation at low speeds. When the speed is increased, the noise of stepper motors will be significantly increased, and other economic indicators will be significantly reduced. For gear pumps with higher rotational speeds, it is better to use a higher speed structure. We started with a stepper motor direct gear pump on a viscous packaging machine. The resulting sound is difficult to avoid due to noise and the reliability is reduced. Later, a spur gear speed-up method was adopted to reduce the speed of the stepping motor, noise was controlled, reliability was improved, and measurement accuracy was guaranteed.
Third, stepper motor in the supply of packaging film application
In the packaging machines that use bag making, filling, and sealing as a whole, the plastic film for packaging is required to be positioned and supplied in a fixed length, and it can be reliably completed with a stepper motor regardless of intermittent supply or continuous supply.
1. For intermittent packaging machines
The batch type packaging machine uses a stepper motor for the film and the reliability can be improved. In the past, the packaging film was often fed intermittently by a crank connecting rod mechanism. The structure was complicated and adjustment was difficult. Especially when it was necessary to replace the product, not only the adjustment was difficult, but also the packaging film was wasted. The use of a stepper motor and a pull belt roller to directly connect the draw tape not only simplifies the structure but also facilitates adjustment. This can be achieved simply by pressing a button on the control panel, which saves time and saves packaging material.
In the batch type packaging machine, the supply control of the packaging material can adopt two modes: the bag length control mode and the color standard control mode. The bag length control mode is applicable to the packaging film without color standard, and can be achieved by presetting the speed of the stepping motor. The setting of the air-to-air ratio can be achieved through the dial switch. The color mark mode is equipped with a photoelectric switch. The photoelectric switch detects the position of the color mark. When a color mark is detected, a control switch signal is issued. After the stepping motor presses the signal, the rotation stops. After a certain time delay, the film is rotated again and again. , to ensure that the length of the color standard for the film.
2. For continuous packaging machines
In the continuous packaging machine, the stepping motor is continuously rotated, and the packaging film is uniformly conveyed continuously. When the bag length is changed, it can be realized only by dialing the switch.
Fourth, the stepper motor in the horizontal seal application
In the continuous packaging machine, the horizontal seal is a very important actuator and it is also one of the more complicated mechanisms in the packaging machine. Especially for colored packaging film, its sealing and cutting position requirements are extremely stringent. In order to improve the cutting accuracy, people have developed eccentric sprocket mechanism, crank guide rod mechanism, etc., but these institutions have the adjustment is very troublesome and reliable. The disadvantage of low sex. The main reason for these disadvantages is that the process requires a horizontal seal and a horizontal seal at a fixed speed and positioning and cutting.
Stepping motors directly drive the transverse seals to achieve speed synchronization. The film feeding wheel of the continuous packaging machine is continuously supplied with the film, and the lateral speed of the transverse sealing is required to be synchronized with the feeding speed of the film during transverse sealing, so as to avoid the situation of tearing film and film accumulation due to the fact that the diameter of the transverse sealing wheel is constant. When changing the bag length, it needs to be changed by changing the speed of the transverse seal wheel, but the transverse seal requires a certain time, that is to say, the transverse seal wheel and the film need a constant time from contact to departure, otherwise the seal is noisy. The total time for each revolution of the horizontal seal wheel and the time required for the horizontal seal is constant. To meet the speed synchronization requirement, the speed of the stepping motor can be divided into two parts within one revolution, and one part first meets the speed synchronization requirement. In addition, the empty part meets the requirements for the total time of the week.
In order to achieve a good sealing quality, it is also possible to implement a non-velocity control mode for the horizontal seal wheel by means of a stepping motor, that is, speed synchronization is achieved at each point of the horizontal seal. Due to space limitations, it is not described here.
The characteristics of the stepper motor mainly attributed to three aspects, one is the overload. Its rotation speed is not affected by the size of the load. Unlike an ordinary motor, when the load increases, the speed will drop. Therefore, the stepper motor is used in places where speed and position are strictly required. Second, the control is convenient. The stepping motor rotates in units of “stepsâ€. The digital features are obvious. This brings great convenience to computer control. In turn, the emergence of computers has opened up a wider market for stepper motors. Third, the overall structure is simple. The traditional mechanical speed and position control structure are more complex and difficult to adjust. After using a stepper motor, the structure of the complete machine becomes simple and compact.
The use of stepper motors in packaging machinery has just begun. Many conventional mechanical control methods can be replaced by stepper motors. In addition to the above-mentioned applications, they are also used in printing, filling, and packaging. With the application, 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 on the platform of stepper motor technology.