Design of Electronic Weft-Insertion System for Three-Dimensional Loom

QIAN Yong-ming (钱永明)，YAN Hong-xia (闫红霞)，WANG Yong-xing (王永兴)，CHEN Ge (陈革)*

1 Research Center of Advanced Textile Machinery，Ministry of Education，Donghua University，Shanghai 201620，China

2 Mechanical Engineering College，Nantong University，Nantong 226019，China

Introduction

As the prefabrication for textile structure composite material，three-dimensional (3-D)fabric has good integrity and profile modeling.The mechanical properties of 3-D fabric composite material are superior to the traditional laminated composite material including interlaminar shear strength，impact resistance， damage tolerance， fracture toughness， and reliability.The 3-D fabric composite material can be widely applied in aerospace，military project，transportation，universal machine，building energy fields，and so on，so it is the research hot spot how to weave 3-D fabric and develop the efficient，automated 3-D loom［1-2］.

To produce the 3-D fabric of high thickness，there are several sheds in the vertical direction on the 3-D loom，so the process of weft insertion normally need several pairs of rapier work simultaneously，or a pair of rapier insert weft through each shed one by one.As the 3-D fabric with special-shaped profiled cross-section normally requires the changeable travel of weft insertion，and the range of weft insertion motion is different for each pair of rapier，the flexibility and reliability of weft insertion system become the important factor for weaving special 3-D fabric.Cui［3］proposed a multi-rapier weft insertion system with pneumatic control， which simplifies the weft insertion mechanism，but could not satisfy weft-insertion of variable travel.Li et al.［4］designed a multilayer weft insertion system incidental finding device which could be satisfied with multi-layer weft insertion according to the change of shedding position and different layers，but its control system is not based on the servocylinder technology.Though Ref.［5］proposed a novel weft insertion mechanism named eccentric conjugate non-circular gear crank-rocker gears train weft insertion mechanism to meet the requirements of rapier loom's weft insertion mechanism as well as reduce the manufacturing difficulty，it cannot satisfy weftinsertion of variable travel.Zhao et al.［6］put forward a kind of computer-controlled electronic weft insertion system which can realize weft-insertion with variable travel online，but it is only for weaving flat fabric without multiple-shedding weft insertion system for weaving 3-D fabric.Reference ［7］introduced the feasibility for application of the driving system in 3D textile machinery with servo-cylinder，however，the specific electronic weft-insertion system based on servo-cylinder was not present.

In this paper，the law of rapier's weft-insertion motion and the characteristic of rapier loom's weft-insertion system are analyzed.According to the special requirements for rapier loom's weft-insertion system to weave 3-D fabric，we have designed a new rapier weft-insertion system controlled by computer and driven by servo-cylinder.

1 The Motion Law for Rapier Weft Insertion Mechanism

Rapier's motion law must meet three requirements.

(1)The displacement law

When the rapier moves forward，the rapier head is required reaching a specified maximum travel from the shed sides to the shed central;when the rapier moves backward，the rapier head should reach a prescribed position in the shed side.

The maximum travel of entry rapier Ssmaxis:

and the maximum travel of exit rapier Sjmaxis:

where，a is the distance from entry rapier to shed;b is the distance from exit rapier to shed;w is looming reed space;d is the transfer stroke which presents the overlap distance of the clamp yarn point displacement of the two rapier heads;s0is the relay distance that the entry rapier follows the exit rapier while it moves back (in this way，some rapier ensure weft transfer firm).

(2)The velocity law

When the rapier moves forward，it starts from static state and increases gradually to the maximum，then decreases to speed of zero when the rapier head reaches the maximum travel;when the rapier moves backward，it starts from static state and increases gradually to the maximum，then decreases to speed of zero when the rapier head reaches the specified position in the shed sides.For the speed of the rapier is influenced by the performance of weft picking and transfer，the speed at the ends of the gripper head should be small and smooth.

(3)The acceleration law

From the velocity law，it's shown that the first half part is a acceleration phrase while the rapier moves forward or backward，and the second half of the process is actually a deceleration phrase.The starting acceleration cannot be too large because the weft would be broken with an excessive acceleration.

According to the requirements of rapier motion law，the existing rapier motion laws generally are the celebration motions of the sine acceleration，the quintic polynomial or the modified trapezoid［8］.

1.1 The sine acceleration motion law

Sine acceleration motion law can basically meet the performance requirements of rapier's movement.The movement of each section is continuous，smooth transition without rigid impact and flexible impact.

The equations for the rapier moves forward are:

The equations for the rapier moves backward are:

where，l is the rapier stroke，φ is the shaft angle，φ0is the shaft angle when the rapier moves forward，and φ'0is the shaft angle when the rapier moves backward.

1.2 The quintic polynomial motion law

The equations of quintic polynomial motion are:

where pi(i = 0，1，2，3，4，5)is undetermined coefficient，which is decided by boundary conditions.At the start point，φ = 0，s = 0，v = 0，a = 0.

At the end point，φ = φ0，s = l，v = 0，a = 0.

Take these values into the above three equations，the following can be calculated:

The velocity and acceleration in this motion type can be solved.

1.3 The modified trapezoid acceleration motion law

For the modified trapezoid acceleration motion law，the maximum value of acceleration can be determined in advance，thus the inertial impact could be eliminated to fit the high speed rapier loom.We should pay special attention to designing the transition curve which should be smooth and continuous without rigid impact and flexible impact.

As shown in Table 1，the comparisons for extreme values of velocity and acceleration of three kinds of motion laws are presented.The largest acceleration value of sine acceleration motion limits its application in the high speed rapier loom，and it is mainly used in low-speed rapier loom (speed less than 300 r/min).The quintic polynomial motion law is similar to the sine acceleration law，but it makes the extreme acceleration reduced，rapier moving smoothly，vibration being lower，and the mechanical property optimized，so it is generally applied in mediate high speed movement.The extreme acceleration value of the modified trapezoid acceleration motion law is least which is usually applied in modern high-speed rapier loom.

Table 1 The extreme values of velocity and acceleration of three kinds of motion laws

For the 3-D loom works with low velocity (speed less than 300 r/min)，all of the three motion laws are adaptive for electronic weft-insertion system of 3-D loom.However，the quintic polynomial motion law is the most ideal option for 3-D loom's electronic weft-insertion system，because it satisfies the application of medium high speed and medium load，and is easy to program in software.

2 Characteristics of Typical Weft Insertion Mechanism

There are various types of weft insertion mechanism such as conjugate cam weft-insertion mechanism，variable-pitch screw weft-insertion mechanism，and space four-links weftinsertion mechanism［9］.

In theory，the motion of the rapier-head in a cam rapier transition mechanism can be designed by requirement，which always has complex structure and is required high accuracy of cam profile.Once the cam profile is determined，the rapier's motion is defined，which is unchangeable according to the fabric woven.

The rapier transition mechanism of screw type has the advantages of short transmission chain，compact structure，and less occupied area，but it is hard to manufacture the screw especially the variable pitch screw.Moreover，variable pitch screw cannot be commonly used in other systems with different reed width.

The rapier transition mechanism of connecting rod type is of simple assembles and convenient to maintain，but its motion law can't be designed according to the perfect demand.This kind of mechanism usually has many kinematic pairs，large motion errors，and noises.

Among the mechanisms above，the motion of the rapierhead mainly depends on the mechanism itself，which is hard to adjust to meet the different structures of 3-D fabric.Therefore，the limits of traditional weft insertion mechanism are obvious in weaving various 3-D fabrics.

3 Design of a 3-D loom's Electronic Weft Insertion System

In weaving 3-D fabric，there are multi-layers warp with several sheds，so the process of weft insertion normally needs several pairs of rapier work simultaneously，or a pair of rapier insert weft through each shed in turn.The 3-D fabric of profiled cross-section normally requires the weft insertion process change online，and range of weft insertion motion is different for each pair of rapier.The flexibility and reliability of weft insertion system are important for weaving special 3-D fabric.Fortunately，servo-cylinder provides a new way to design the weft insertion system for 3-D weaving.For the 3-D loom works with low velocity，the mechanical response caused by electronic weftinsertion system could fit the 3-D weaving stably.

3.1 The theory of servo-cylinder

Servo-cylinder consists of servo-motor，reducer，cylinder block，lead screw，push rod，etc.(shown in Fig.1).The rotation of the servo-cylinder is transformed to linear motion of push rod via lead screw (shown in Fig.2).

Fig.1 The external structure of servo-cylinder

Fig.2 The inside structure of cylinder

By combining the servo-cylinder and computer controlling technology，we can control the acceleration， velocity， and displacement of push rod with high precision.This method provides a good support for weft insertion system with multi-rapier.

3.2 General design of the 3-D loom's weft insertion system

In this paper，a new 3-D loom's weft insertion system is designed.As shown in Fig.3，the servo-cylinder［7，10］is applied to controlling the 3-D loom's weft insertion.According to the structure of 3-D fabric，with help of computing program，the computer controls the servo-cylinder's motion;the rapier linked with the servocylinder's pushes rod，and reciprocating motion servo-cylinder's push rod makes the rapier to accomplish weft insertion.

As shown in Fig.3，the rapier-head is controlled by servocylinder to accomplish the weft insertion.The gear rack mechanism controls the motion of the servo-cylinder in vertical direction，and the gears are driven by a servo-motor.The rotation amount of servo-motor makes the push rod aim at the sheds one by one to accomplish the weft insertion separately.To weave the 3-D orthogonal fabric as shown in Fig.4，there are 4 sheds in vertical direction［11］.The rapier moves vertically，and inserts weft one by one.

Compared with the traditional rapier weft insertion mechanism，the weft insertion mechanism with servo-cylinder simplifies the mechanism，whose moving range of insertion can vary according to different 3-D fabrics.Obviously，it has many advantages in weaving variable 3-D fabrics.

Fig.3 Weft-insertion system of 3-D loom based on the servo-cylinder

Fig.4 The structure of a 3-D fabric

In addition，by comparing with the constant insertion range of traditional mechanism，servo-cylinder-controlled mechanism can complete the weft insertion with variable travel，providing a new method to weave the abnormal cross section fabric.All in all，the servo-cylinder has much advantage such as simple compositions，high reliability，and easy to maintain，which make it possible to achieve higher speed，automation，and digitization in the weft insertion system of 3-D loom.

Fig.5 Weft-insertion system based on the servo-cylinder

In servo-cylinder mechanism，the rotation angle of the servo-motor has linear relation with the displacement of push rod.As rapier is linked with the push rod，its displacement goes linearly with the motor's rotation angle as well.For 3-D looms generally operate at low speed，the sine acceleration motion law is suitable to the rapier weft insertion.Therefore，the servo-motor could operate in accord with the sine acceleration motion law.As shown in Fig.5，the servo-motor is controlled by a special controller to drive screw rotate;the nut push slipway moves back and forth along the straight linear guideway.The entry rapier fixed to the slipway moves in the horizontal direction to insert weft.According to operating principle，the variation of servo-motor's rotation has linear proportional relationship with rapier's displacement，so the motion law of the servo-motor could be determined based on the rapier weft's motion law.With help of setting software，the motor program and the parameters of location，velocity，and acceleration can be entered the controller，thus the motion of the rapier head is controlled accurately，to realize the weft insertion.

With the advantage of exact velocity，position and push force，servo-cylinder has easily accessed to controlling system such as programmable logic controller，by which the motion of weft insertion can be controlled with high precision.

In the 3-D loom designed，the maximum scale of the reed width is 500 mm，so the maximum stroke of each rapier is recommended as 300 mm.Relevant parameters of servocylinder are shown in Table 2.In the control system，the superior control units are computers with peripheral component interconnect control card.

Table 2 The concrete parameters of servo-cylinder

3.3 Design of control software for the new weft insertion system

In order to meet the motion law of the rapier，3-parameters-mode is used to control the movement of rapier in the weft insertion mechanism.The working rule of weft insertion mechanism is shown in Fig.6.When computer receives the relevant weft insertion signals along with the displacement，velocity，and acceleration feedback from rapier weft，it produces the control signal and motivate the driver to control the push rod to insert weft.

Fig.6 Diagram of control principle of weft-insertion system

According to different structure of the 3-D woven fabric structure，there are three control parameters of PID to adjust to realize the rapier motion law required.

Figure 7 is the main flow of the control program for the new weft insertion system.The important parameters need reset at first.The status of rapier which moves forward or backward is determined by the rotation angle of the principle axis.PC sends the relevant pulse to the servo-amplifier to drive the rapier to accomplish the operation of weft insertion.

Fig.7 Diagram of control program for weft-insertion

4 Conclusions

Based on the analysis of the movement of rapier and the characteristics of the rapier's weft insertion mechanism，according to the special requirements to rapier loom's weftinsertion system for weaving three-dimensional fabric，a new electronic weft insertion system is presented.For the new weft insertion system is controlled by computer with CPI and driven by servo-cylinder，it not only greatly simplifies the weftinsertion system，but also achieves weft-insertion with variable travel，and improves the ability of weaving different kinds of 3-D fabric.For the 3-D loom works with low velocity，the mechanical response of electronic weft-insertion system could fit the 3-D weaving reliably.The weft insertion system designed has been applied in a 3-D loom successfully.

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