Tension Control in the Fabric Dyeing and Finishing Process

The dyeing and finishing multi-unit combined machine is equipped with an “elastic frame” between the units, which is used to coordinate the speed difference between the units; buffer the dynamic process to prevent the accumulation or breakage of the cloth; set the initial tension of the unit feeding cloth; and “offside” of the fault to safely stop the machine to prevent the accident from expanding. The elastic frame line speed difference adjustment device is actually a simple tension control device. When the line speed difference between adjacent units occurs, tension is generated. When it is greater than the initial tension (set by the elastic frame), the swing arm roller of the elastic frame swings down (tension roller action); when it is less than the initial tension, the tension roller swings up to adjust the speed difference between the units and maintain the fabric feeding tension around the initial tension.

When the tension frame is set between the two units and the fabric exits the rolling car and enters the flat washing tank (steam washing box, multi-cloth guide roller device), the tension setting of the tension frame only needs to set the initial tension to ensure that the fabric does not wrinkle during the flat feeding process; when the tension frame is set at the fabric exit of the flat washing tank (steam washing box, multi-cloth guide roller device), the tension setting of the tension frame for the fabric should be based on the initial tension, and the additional tension increased by the friction resistance torque during the fabric feeding process should be added. The initial tension of the fabric feeding in the multi-unit tight working condition cannot be reduced to prevent the generation of cloth guide wrinkles, and the additional tension should be controlled as small as possible to reduce the burden of the post-finishing pre-shrinkage process.

The roll dyeing and roll-up and stacking of the fabric in the dyeing and finishing process all need to be coordinated with the change of the roll diameter to implement constant linear speed operation. From the perspective of the automatic adjustment principle, no matter how large the speed difference between the two units is, or how large the roll diameter changes, it will cause fluctuations in fabric tension, which are all disturbances to the tension. Therefore, the production process needs to monitor and control the tension variable difference of the fabric on the machine.

Equipment for full tension control during jig dyeing

The tension of fabric in the jig dyeing process is caused by the difference in linear speed between winding and unwinding, the friction resistance torque in the fabric guiding process and the additional tension of the tension frame, which causes the fabric to extend, that is, elastic deformation. The elastic modulus of the fabric determines the tension generated by the elastic deformation. The fluctuation of fabric tension will determine the amount of dye liquid carried in the jig dyeing process, resulting in uneven liquid carrying, resulting in defective products with head and tail and left, middle and right color differences. Therefore, the jig dyeing process must implement full online tension control.

Current Status of Tension Control of Giant Dye Jiggers on the Market

1.   Speed ​​control does not meet the requirements of winding mechanical characteristics

Figure 1 is a schematic diagram of the characteristic curve of the winding machine and the characteristic curve of the transmission machine.

In the figure, curve 1 is the winding characteristic curve; curve 2 is the speed curve of the winding shaft according to the change of the cloth roll diameter. The two curves intersect at points A and B. When the cloth wheel rolls from D0 (winding roller diameter) to D1, the speed required by the cloth roll should be n1. At this time, the speed of the cloth roll is controlled to be n1′. The speed of the winding cloth is high, and the speed difference between the unwinding and the unwinding will cause the fabric to stretch and increase the tension. In Figure 1, between points A and B, the closer curve 2 is to curve 1, the smaller the process tension difference is. If the two curves overlap, the process tension is the same. In this way, during dyeing, the speed difference between unwinding and winding is controlled, which can effectively prevent color difference defects caused by tension changes due to speed difference fluctuations.

The giant dyeing machines currently on the market are not sensitive enough in adjusting the dynamic speed deviation, which causes curve 2 to deviate far from curve 1. This is reflected in the problem of uneven color at the head and tail of the cloth roll and in the middle of the production.

2.   Mid-process control of tension in the process

1. Winding roller 2. Cloth roll 3. Cloth pressing roller 4. Expanding roller 5. Arm 6. Tension spring 7. Rotating shaft 8. Submerged cloth guide roller

Figure 2 is a schematic diagram of the working principle of the tension spring type tension frame. The cloth roll pushes the cloth pressing rod 3 to make the frame arm 5 move to ∠BOB’, and the tension spring 6 moves to B’A (point A is the fixed point of the tension spring on the machine body, and point B is the fixed point of the tension spring on the frame arm). It is assumed that the tension of the tension spring is linearly constant from BA to B’A.

OB=AB·∠COSα , OB’=AB’· ∠COSβ, ∵∠α>∠β,∴OB’>OB

This shows that as the roll diameter increases, the vector force of the tension spring on the tension frame increases. The expansion roller 4 on the tension frame is a fixed, non-rotating bend. After being combined with the cloth pressing roller 3, it produces an extrusion force on the upper roll fabric. In this way, as the diameter of the cloth roll increases, the dye liquid on the cloth surface tends to spread to the edge area due to the increase in extrusion force, which seriously affects the color difference between the left, middle and right. At present, there are two tension spring tension frames on the giant dyeing machine on the market. According to the above analysis, both will affect the quality of dyeing. Some manufacturers set speed measuring cloth guide rollers in the middle of unwinding and winding, and float pressure sensors under the bearing seats at both ends of the cloth guide rollers. The purpose is to implement online tension monitoring. In fact, from unwinding cloth roll → tension spring tension frame → cloth guide roller (under liquid) → speed measuring tension roller, half of the feeding route of the dyeing fabric, under the feedback control of the pressure sensor sampling signal, the control of the half-way tension is solved. The tension change in the half-way from speed measuring tension roller → cloth guide roller (under liquid) → tension spring tension frame → winding cloth roll cannot be controlled.

Equipment for full tension control during jigger dyeing

1. Cloth roller 2. Swing arm device 3. Submerged cloth guide roller 4. Speed ​​tension cloth guide roller 5. Speed ​​encoder 6. Pressure sensor

Figure 3 is a schematic diagram of tension control during the whole process of jigger dyeing.

1. Fuzzy control AC servo drive system

Based on the requirements of speed control accuracy (line speed is less than or equal to 0.5%), a digital control system is selected and fuzzy control technology is applied to conduct fast deviation feedback between the measured line speed and the process setting value and the rate of change of the deviation is fed back to the servo driver to control the servo motor. The motor has a speed closed-loop control speed encoder that is resistant to torque disturbances to ensure the stability of speed control. The system has a fast dynamic follow-up response and does not produce oscillation, which is in line with the mechanical characteristics of the winding machine and the winding line speed is constant. The fuzzy control winding line speed is constant, and there is no need to establish a digital model on the computer to indirectly measure the winding diameter, making the control simple, fast and accurate, which meets the requirements of the winding nonlinear time-varying system.

2. Full control of process tension

One of the major technical breakthroughs of the new giant dye jigger is to delete the two tension springs of the traditional giant dye jigger and use a constant liquid swing arm device (see Figure 4). Since the cloth guide roller 1 on the constant liquid swing arm always maintains a certain distance from the cloth roll during the process, the friction resistance torque of the two expansion rollers 2, 3 and the cloth guide roller only produces a constant tension on the fabric. The pressure sensor only needs to detect the tension (resistance torque) of the unwinding roller and control it to be constant, so that the total tension of the process can be controlled throughout the process.

1. Fabric guide roller 2, 3. Expanding tube 4. Left and right sleeves 5. Frame 􀀂7, 17. Gear

In the process, the servo motor of the constant liquid swing arm device receives the cloth roll increase signal, and through the meshing of gears 7 and 17 (servo transmission output shaft gear), the transmission swing arm angular displacement is driven to keep the cloth guide roller 1 always at a distance from the cloth roll (without pressing the cloth rewinding shaft), thus completing the cloth guide without additional tension. Since the constant liquid swing arm device solves the problem of color difference increase corresponding to the increase of cloth roll, the roll diameter of the new giant dyeing machine can be increased to 1500mm.

Composition of winding control system with constant stress (tension) distribution

Indirect constant stress distribution winding control system

The composition of the indirect constant stress distribution winding control system is shown in Figure 5. In the figure, TC is the winding tension controller, PC is the photoelectric encoder, NS is the photoelectric switch, LH is the magnetic powder clutch, and M is the winding motor. Its working principle is: the linear speed signal measured by PC and the winding speed signal measured by NS􀀂 are sent to the winding tension controller, and the current winding radius r is calculated in real time. This non-contact radius detection method is much superior to direct detection.

The advantages of the indirect constant stress distribution winding control system are simple structure, low cost, easy operation and maintenance. The disadvantage is that since the actual tension value is controlled indirectly through the output torque of the magnetic powder clutch, the nonlinear characteristics of the magnetic powder clutch and the wear of the mechanical part will affect the actual winding tension value, thereby affecting the control accuracy. However, in occasions where the requirements are not particularly high, this method can basically meet the control requirements. Figure 6 is a block diagram of the indirect constant stress distribution winding control system.

The advantages of the indirect constant stress distribution winding control system are simple structure, low cost, easy operation and maintenance. The disadvantage is that since the actual tension value is controlled indirectly through the output torque of the magnetic powder clutch, the nonlinear characteristics of the magnetic powder clutch and the wear of the mechanical part will affect the actual winding tension value, thereby affecting the control accuracy. However, in occasions where the requirements are not particularly high, this method can basically meet the control requirements. Figure 6 is a block diagram of the indirect constant stress distribution winding control system.

Closed-loop constant stress distribution winding control system

The composition of the closed-loop constant stress distribution winding control system is shown in Figure 7 .

In Figure 7, TE is a tension sensor that directly measures the online tension of the fabric. The TC controller feeds in the speed signal NS and the line speed signal PG to calculate the winding radius, and obtains the control speed of the constant line speed of the fabric roll; the TE feedback signal controls the constant tension, and the two are calculated to give the inverter VVVF main command signal, so that the motor running speed meets the preset process line speed and constant tension. The control system shown in Figure 7 has high control accuracy.

Tension control system

The system is divided into unwinding group, feeding group and receiving group.

(1) Unwinding group: The tension of unwinding is determined by the braking torque of the magnetic powder brake installed in the unwinding group. In order to maintain a constant tension, the braking torque must be reduced as the coil diameter decreases. In this case, a tension detector is used to detect the tension of the material and control the torque of the magnetic powder brake.

(2) Feeding group: The feeding motor is used to feed long-sized materials from the left to the right. The conveying line speed is determined by the motor speed, but it has nothing to do with the tension. However, if the tension is large, the output horsepower of the motor will increase.

(3) Receiving group: The receiving speed of the receiving group needs to be faster than the line speed to determine the input speed of the magnetic powder clutch. Since the line speed is determined by the transfer reel, the clutch will slip. Therefore, the tension of the receiving group is determined by the transmission torque of the magnetic powder clutch.

In order to maintain a constant tension, the transmission torque must be increased or decreased according to the size of the coil diameter. In this case, the winding diameter is detected by the rotation amount of the reel, and the magnetic powder clutch is controlled accordingly. Moreover, the manual speed of the magnetic powder clutch is greater than the maximum output speed (at the minimum winding diameter). Therefore, the larger the winding diameter, the lower the output speed, and the clutch slip differential increases.

Tension controller selection process

Figure 9 is a flow chart of the selection of the tension controller.

Working principle of the tension detector

The tension detector applies load through a roller, causing the leaf spring to move as shown in the figure below, and then detects the tension through a differential transformer. Since the displacement of the leaf spring is extremely small, about ±200μm, it is also called a micro-displacement tension detector. See Figure 10.

ince many detection rollers are used, the tension detector is actually loaded even if the tension is zero. This load is not constant, but changes with the changes of mechanical factors, such as the load calculation of the detector of the detection roller with different installation types.

The mass, the installation angle of the detector, etc., and due to the different types and materials of various machines and the different angles of the detection rollers, it is necessary to adjust the settings of the tension meter or tension controller side, which is called zero adjustment or span adjustment.

Tension detector installation method

The tension detector can be installed flat or on the wall. It can be divided into compression type and lifting type according to the direction of force applied to the product. Figure 11 shows the load calculation of different installation types of tension detectors. The magnetic powder clutch on the unwinding side of the cloth inspection machine must be fixed on the side of the motor, so a motor with brake should be used. The magnetic powder clutch installed between the motor and the reel can control the torque of the reel. The cloth tension is fed back through the tension detector, and the magnetic powder clutch is controlled by the tension controller.

Note: Load G must be less than 1.6 times the rated weight.

F = tension of the coil G = load of the tension detector K = constant W = weight of the roller + = compression direction – lifting direction, θ1, θ2 = angle between the detector and the coil

The original author of this article is Chen Liqiu. If there is any infringement, please contact [email protected]