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Design and analysis of a method for monitoring felled seat seam characteristics utilizing TRIZ Methods

| On 10, Dec 1999

Timothy G. Clapp, PhD, PE Professor NC State University Raleigh, NC 27695 tclapp@tx.ncsu.edu

Brad A. Dickinson Design Engineer 3-TEX Incorporated Cary, NC 27511 Dickinsonb@stex.com

Abstract

Felled seams are used extensively in the construction of apparel and industrial textile products. Defects can be formed in the seaming operation that cannot be detected visually. These “internal” defects cause the product to fail when used. This paper describes the design of an on-line monitoring system to detect internal defects created during the felled seaming operation.

Theory of Inventive Problem Solving (TRIZ) methods were used to efficiently design a commercially acceptable monitoring system for the seat seaming operation in jeans production.

Introduction

Modern companies compete in a global marketplace. Advances in information technology have made marketing products and communicating with customers a simple task. In fact, the customer now has a great impact on decisions regarding improvements to existing products and the design of new products. Often, engineering design goals focus on meeting the needs and requirements of intended customers. Companies that wish to excel in the global market must be prepared to generate quickly products that meet the customer’s current needs and desires. They must also produce highly innovative products to discourage competition. Thus, engineers are now faced with the need to solve engineering design problems rapidly, cost effectively, and with a high level of innovation. The goal of this paper is to demonstrate the utility of TRIZ methods for rapidly creating innovative solutions at the concept generation and prototyping phases of the engineering design process. These methods are applied to a critical problem in the apparel manufacturing industry. Specifically, a sensor system will be designed to monitor the quality of a joining operation in the assembly of denim jeans.

FELLED SEAM CONSTRUCTION


Figure 1: Cross-sectional View of Correct
and Incorrect Felled Seatseam Constructions

In denim jean construction, the felled seam comprises more seam length than any other seam type. The U.S. government developed Federal Standard 751a (FS751a), which classifies seams and stitches used in apparel garments. Lapped seam type C-2 (denoted LSc-2) is generally the seam type used in jeans construction. This is the strongest of the lapped seams since two rows of stitching are used to secure it. In this seam “lapped bottom and top plies fold toward each other and interlock, displaying no raw edges on either the outside or inside of the garment”[1]. Two rows of type 401 chain stitch bind the seam. Figure 1a is a cross-sectional end view schematic of an LSc seam.

Seam quality problems occur when one or both of the two lapped fabric ends that are used to form the seam fail to be formed or “stuffed” completely, such as in Figure 1b. Although a raw fabric edge may not be visible, an “understuffed” seam exhibits less strength than a properly formed seam. When the completed garments undergo the post-finishing (washing) process, improperly stuffed seams tend to “blowout” resulting in a total rejection of the garment. During the post-finishing process it is likely to “blowout” as illustrated in Figure 1c.

In jeans, the “seatseam” has a high instance of failure. This seam runs from the crotch to the top of the back of the jeans. This seam was selected as the first application for the new monitoring system.

Clapp [2] developed a method for identifying defective seams by measuring the compressed thickness in the center of the seam. Figure 1a shows a correct seam with four layers of fabric in the center of the seam. Figure 1b shows a defective seam that has only three layers of fabric in the center of the seam.

Table 1: Engineering Design Specifications
for the Seatseam Monitoring System
with Completed Design Benchmark
Rating (5-highest, 1-lowest)

Engineering Design Factor

Rating

PHYSICAL CONSTRAINTS

1.-Sensing Point/Location

5

2.-Control Unit Location

5

3.-User interface

3

OPERATION

4.-Environmental Conditions

4

5.-Durability

4

6.-Fabric Style

4

7.-Seam Length

5

8.-Sewing Machine Speed

5

9.-Sewing Machine Maintenance

3

10.-Electrical Power

4

QUALITY

11.-Resolution

4

12.-Edge Detection

5

13.-Fault Location

5

14.-Operator Feedback

5

FINANCIAL

15.-Product/Development Costs

4

ENGINEERING DESIGN

The process of engineering design begins with understanding the problem and converting customer requirements into design specifications. Table 1 summarizes the engineering specifications that govern the design of the felled seatseam monitoring system.

Theory of Inventive Problem Solving (TRIZ) methods were used to accelerate and optimize the design of the monitoring system.

INITIAL DESIGN


Figure 2: Sewing Machine to be modeled

Three-dimensional solid modeling is used to minimize development time and cost [3]. The first step in the modeling process is to model the existing components. These components establish the geometrical constraints of the sewing machine. Several components of the Pfaff model 5489-H sewing machine, shown in Figure 2, were modeled in the SolidWorks®. As new components are designed and added to the existing model, the solid modeling software allows the designer to check for mating part alignment, dynamic operation, and interference problems. The designer reduces time and cost by not having to physically make the parts and test them.


Figure 3: Three-dimensional Solid Model
of the Initial Mechanical Prototype.

The initial mechanical design is shown in Figure 3. This design used a “ski(1)” to contact the center of the seam. Structural components, bearings, and screws were designed to transform rotational displacement of the ski caused by the seam thickness into an amplified displacement. A linear variable displacement transducer (LVDT) converted the displacement into an electrical voltage for analysis. The LVDT is housed in bracket(11) shown in Figure 2.

Figure 3 shows the initial prototype that was constructed and proven to accurately measure compressed seam thickness. Unfortunately the initial design did not meet the design goals for cost, reliability, and jeans size variation.

TRIZ METHODS

The Theory of Inventive Problem Solving (TRIZ) is a collection of concepts and methods formulated by Genrich Altshuller [4,5,6] to systematically improve the engineering designer’s ability to solve difficult problems. The theory may be best described as “a unique way of thinking that enhances creativity by getting individuals to think far beyond their own experience, to reach across disciplines, to resolve problems using extracted knowledge from other areas of business, science, or technology.”[5]


Figure 4: Modified Mechanical Design
after Ideality Concept was applied to the Design

TRIZ concepts, “Ideality” and the “utilization of resources” were used to analyze and improve the design of the initial monitoring system. “Ideality” is defined as the sum of the useful functions divided by the sum of the harmful functions plus the sum of the costs. “Ideality” approaches infinity as the design approaches the Ideal Final Result (IFR). The ideal system meets all of the desired useful functions with no harmful effects and at no cost. The primary function of the monitoring system is to measure compressed seam thickness. The “Ideal” system measures seam thickness and identifies all defects in the seam using no parts and at no cost.

The initial prototype was comprised of 14 parts. The function of each part was documented. The “Ideality” method was used to systemically eliminate each part from the system by eliminating or combining functions. This design exercise resulted in the elimination of 6 parts from the original system. The “Ideality” of the system was increased. The modified design reduced cost and improved reliability of the system. Figure 4 shows the modified design.

The design of the sensor system is improved by identifying resources that can be used to increase “Ideality.” Resources are categorized as substance, space, field, time, functional, and informational resources[6]. Table 2 shows a partial list of resources available to the designer.

The sensor system was optimized using the informational resource produced by the LVDT displacement signal shown in Figure 4. The voltage trace of the LVDT contained much more information than just the presence of a defect in the seam. Careful analysis of the trace clearly showed when the leading edge of the fabric was approaching the sensor. This triggered the encoder to start counting and record the distance traveled. A second, large dip in the signal identified the presence of a riser seam and prevented the system from classifying this as a defect. At the end of the fabric, a sudden rise indicated the presence of the end of the fabric. The information contained in the voltage trace was used to eliminate external sensors and make the sensing system independent of seam length and speed.

Table 2: Resources Available to Increase Ideality

Substance Resources

-Sewing machine components

-Lower lever arm

-Sensing ski

-Lower mounting plate

-LVDT mount

-LVDT sensor

-The fabric seam

-Fabric

-Lint/Dust

-Thread

-Thread Waste

Space Resources

-Sewing Machine Mounting Locations

-Empty Space Adjacent and Above Puller Wheel

Field Resources

-220 VAC Electrical Supply

-Compressed Air

-Gravity

Time Resources

-Repetitive Sewing Machine Motion

-Time to Produce One Stitch

Informational Resources

-Fabric Sensor Signal

-Encoder Signal

-Linear Potentiometer Signal

-User Interface

-Red Indicator Lamp

Functional Resources

-Rotational Motion of the Lever Arm

-Vertical Motion of the Linear Potentiometer Shaft

-Rotational Motion of the Puller Wheel

-Rotational Motion of the Sewing Machine Motor

-Cyclic Motion of the Sewing Machine Feed Dogs

-Vertical Motion of the Sewing Machine Presser Foot

-Vertical Motion of the Sewing Machine Needles


Figure 5: Information Contained Voltage Trace

CONCLUSIONS

The use of TRIZ concepts, “Ideality” and “utilization of resources,” resulted in an improved sensor system. The concepts directed the designer’s thinking to systematically eliminate components of the design through the use of resources available to the designer. Design objectives were achieved using 40% fewer components.

A production prototype was constructed and tested. A rating system of 1-5 (5 – meets goal) was used to evaluate the system against the original design goals. Table 1 shows the very favorable ratings of the performance of the production prototype. A commercial version of this system is now being used in manufacturing to eliminate the internal defects that cannot be seen visually.

This research proves that TRIZ methods are valuable tools for rapidly designing robust systems.

References

Hudson, Peyton B., Guide to Apparel Manufacturing, Spark Press, Raleigh North Carolina.

Clapp, Timothy G. et al., United States Patent Number 5,671,689, issued September 30,1997.

LaCourse, Donald E., Handbook of Solid Modeling, McGraw-Hill 1995.

Altshuller, G. (H. Altov), And Suddenly the Inventor Appeared TRIZ, the Theory of Inventive Problem Solving, Technical Innovation Center, Inc., Worcester, Massachusetts 1996.

http://www.jps.net/triz/triz0000.htm

Terninko, John, Alla, Zusman, Zlotin, Boris, Step-by-Step TRIZ: Creating Innovative Solution Concepts, Responsible Management Inc., Nottingham, New Hampshire 1996.