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Principle of needle selector for computerized flat knitting machine

Update:28-02-2020
Summary:

Needle selector principle and several factors affecting […]

Needle selector principle and several factors affecting the stability of the needle selector

Discussion: On the problem of needle selector. Study it, and it also explains the problem of six and eight sections:

At present, knitting flat knitting machines have generally adopted computer control technology and the use of electronic needle selection technology to control the flat knitting machine for single-needle selection, thereby forming various designs. In addition, some old machinery also uses electronics when rebuilding equipment. The needle selection replaces the old mechanical needle selection in order to expand the possibility and practicability of its pattern. In our investigation, we found that many manufacturers are in the process of self-development of the needle selection system. Patterns often appear due to improper selection of the needle selector. Distorted deformation may even cause the mechanism to malfunction and damage the mechanical structure.To this end, based on development and experiments, we have carried out some research on several factors affecting the reliability of the needle selection of the computerized flat knitting machine needle selector. Measures to ensure reliable needle selection.

1  The role of needle selector

The structure of most computerized flat knitting machine needle selection control systems is electromagnetic needle selection.Here, the computerized flat knitting machine adopts two levels of upper and lower computer structures. It is responsible for controlling the selection of the needle selector according to the synchronization signal and the control signal of the host computer. It can be seen that the needle selector is a transducer that sequentially drives the needle selector according to the electrical pattern combination of the fabric pattern. The computerized flat knitting machine control system is used to implement the actuators for needle selection control. Therefore, its performance will directly affect the accuracy and reliability of needle selection.

2 Type of needle selector,

At present, commonly used needle selectors are divided into two types: electromagnetic and piezoelectric. The piezoelectric needle selector uses piezoelectric ceramic material as a conversion element, and uses the pulse signal sent by the controller to act on the pressure. Electrical components rely on the inverse piezoelectric effect of piezoelectric materials to make the piezoelectric components bend to the pre-memory shape for needle selection; another type of needle selector uses the electromagnetic coil to pass current in different directions. Make the polarity of the electromagnetic spiral tube tip change correspondingly, or use it in conjunction with a permanent magnet, rely on the attraction and repulsion between the electromagnet and the permanent magnet to achieve the needle selection swing, that is, the needle selection method is used to achieve the needle selection, or directly through electromagnetic The polarity of the iron changes to control the needle selection of the needle selector, so that the knitting needles can be separated into different tracks.

Select the needle.

Compared with the two needle selection methods, the piezoelectric needle selector has the characteristics of fast response speed and low heat generation, but it is slightly inferior in terms of force, shock resistance and vibration of the component, and it is easy for the piezoelectric component to lose memory. Therefore, it is used more on the circular knitting machine to ensure fast response and adapt to multiple channels. At the same time, the power and heat consumption of the needle selector will not be too large. It is more advantageous to use an electromagnetic needle selector on the flat knitting machine. The main reasons are :

① Knitting machine needle

Thicker, the yarn tension is greater when weaving, so the needle selector is subject to greater force.The electromagnetic needle selector can withstand large forces, and has good shock resistance and long service life.

② The operation of flat knitting machine is complicated, and the coordination precision between each part is high. Therefore, the use of electromagnetic needle selector can take advantage of its reliable operation. Therefore, we choose electromagnetic needle selector in the experiment.

3 Needle selector structure

Principle of electromagnetic needle selector:

When selecting a needle, the electromagnetic spiral tube in the needle selector is passed through a certain direction of current to generate a polarity. This polar magnetic field interacts with the magnetic field generated by the permanent magnet, and pushes and pulls to make the needle selection head of the needle feeder around the shaft. The pendulum does not interact with the teeth of the selector, and the selector rises along the triangle of the selector. General needle selector structure:

1-Electromagnetic coil; 2-Permanent magnet; 3-Needle selection; 4-Shaft

Needle selection principle of electromagnetic needle selector:

When the needle is not selected, the electromagnetic spiral tube is passed with the current in the opposite direction to generate the opposite polarity. The needle selection head of the needle selector is swung downward to act on the teeth, and the needle selection piece is pressed into the needle groove. The lower teeth do not expose the surface of the needle bed, and the needle selection piece does not rise along the needle selection triangle. From this, it can be seen that the control of the needle selection is, in short, the control of the needle selection electromagnet so that the needle selection head is in the working and non-working positions. A large number of experiments have proved that the structure of the needle selector must meet the following requirements: when the spiral tube is energized, the needle selection head can quickly swing into place, and it stabilizes within a specified time, but is still controlled by a small current and a permanent magnet, so that it is in a controlled state. ; When the spiral tube is not energized, it should also be held by a permanent magnet, so that the selector of the selector is always in a controlled state, and the selector will not be in an uncertain intermediate position under any state. There will be no knitting errors and needle strikes. We use an electromagnetic needle selector from HARTING in our experiments. The needle selector is a new type of needle selector with a bistable structure.

4 Number of stages of needle selector

Needle selector needle selection, can be divided into single-stage needle selection and multi-stage needle selection. Single-stage needle selection is that each installed selector is only one-level selection position, that is, this selector works on each needle, It has the characteristics of small size, short action time, high needle selection frequency, and low wear of the parts. It is especially suitable for high-speed machines. However, single-stage selection has high requirements for the precision of the parts, and the price is more expensive. Multi-stage needle selectors are more common. Multi-stage needle selectors have multiple needle selection heads, which correspond to multi-stage needle selection positions. Each stage of the needle selection head only corresponds to the selection of needle pieces of equal height. Therefore, for each needle selection For each piece of needle selection on the device, it needs to be spaced multiple times before acting on the needle selection piece once. This increases the needle selection time and makes the needle selection more reliable. However, the multi-stage needle selection mechanism increases the height of the needle bed. , And require different types of needle selection blades, making the mechanism slightly more complicated. At present, most of the needle selectors used in computer flat knitting machines are multi-stage needle selectors. Therefore, how to determine the number of needle selector stages has become an important issue, because each There is an upper limit for the needle selection frequency of this type of needle selector, and the faster the computer flat knitting machine speed, the higher the machine number, the required needle selection frequency The rate is also high, that is, the higher the number of stages of the selector required. Machine number (number of stitches / 25.4 mm) If the number of stages of the selector is selected more than required, it will cause waste; if the number of stages selected is not enough, then The needle cannot be selected correctly, so we calculate as follows:

Setting: G ——

Head speed

T0 — selection cycle of the selected selector

Pitch is T = 25.4 / G

Time required for the machine head to move through each stitch at the highest speed t = T / Vmax

Required needle selection frequency f ′ = 1 000 / t

Consider insurance factor f = 1.5 f ′

And the needle selector frequency f 0 = 1 000 / t0

The number of stages for selecting needles n0 = f / f 0

Six or eight-speed needle selectors are commonly used in flat computer flat knitting machines.

Drive method of needle selector

There are two driving methods of the electromagnetic needle selector: one is the pulse width modulation method to control the switching voltage; the other is the current control method using chopping current limiting. From the perspective of reliable needle selection, the second method should be used In order to stabilize the control method and avoid the influence of temperature. This is because in the electromagnetic induction, in addition to the increase of magnetic energy and mechanical work, a part of the energy is used to generate heat, which will increase the temperature of the electromagnet. The temperature also affects the resistance, thereby affecting the current, and ultimately the electromagnetic force, that is, the reliability of the needle selection. The driving method of the needle selector of Yadi Company is shown in Figure 3; Figure 3 (a) is The waveform of the current flowing into the electromagnet is shown in Figure 3 (b), which shows the movement of the needle selection head of the needle selector. Among them: t1 and t2 are the time of the large and small currents, Ⅰ is the response time of the needle selection, and Ⅱ is the selection time. Needle swing time (including bounce time and jitter time), Ⅲ is the reliable action time of the needle selection, which should be applied to the needle selection time within this time. It can be seen that after the pulse signal is sent, the needle selection needs to lag for some time. Began to swing, and after swinging in place Health short-back jitter, stabilized, after the onset of action and to selector jack. Selectors driving method (current control by limiting the chopping)

6. Concluding remarks

The needle selector is a key component used to implement the needle selection control. Its structure and performance will directly affect the accuracy and reliability of the needle selection control. Therefore, when selecting a needle selector, it should consider its own characteristics, pay attention to the following

A few points:

1 (1) Select the type of needle selector suitable for computer flat knitting machine;

(2) Select the parameters of the needle selector, such as the number of stages;.

(3) Adopt reasonable control methods to reduce unstable factors in the control process;

(4) When selecting the structure, a bistable structure with the needle selection always under control should be adopted to ensure stable performance

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