Microcontroller-Based Fabric Bending-Length Tester for Knitted Fabrics

Microcontroller-Based Fabric Bending-Length Tester is a diploma Textile Technology project that builds a digital version of the classic cantilever stiffness test and then uses it on real knitted fabrics from the Tirupur market. Stiffness — how a fabric drapes and handles — is measured in the standard way: a fabric strip is pushed over a horizontal platform until its own weight bends the leading edge down to the standard's 41.5° inclination line, and the distance travelled is the bending length; flexural rigidity follows directly from it.

Commercial testers with engraved scales strain a polytechnic budget, and reading a drooping tip against an inclined scale varies from reader to reader. Our rig replaces the eye with an IR break-beam sensor pair set exactly on the 41.5° line: a geared motor drives a lead-screw carriage that pushes the strip at constant speed, and when the drooping tip breaks the beam, the AT89S52 stops counting pulses from a slotted-opto wheel on the feed roller and displays the bending length in millimetres on a 16x2 LCD. Resolution is one millimetre, against several millimetres of scale-reading scatter.

The second half is the study that makes it viva-worthy: five knitted fabrics (single jersey, 1×1 rib, interlock, piqué and loopknit) are tested in machine and cross directions, GSM measured with a cutter and balance, and flexural rigidity G = W × c³ correlated against GSM and structure. Every circuit block is simulated in Multisim/TINA before assembly, and the rig is validated by repeat measurements.

Knitted-fabric stiffness affects drape, handle, cutting and sewing behaviour, so buyers and dyers care about it — but the standard cantilever bending-length test is still read by eye in most polytechnic and small mill laboratories. The observer must decide exactly where the drooping strip tip touches an inclined 41.5° scale, and two observers, or the same observer over a long batch, disagree by several millimetres. Commercial digital stiffness testers solve this but cost far more than a department can spend, and imported units cannot be maintained or modified locally.

At the same time, diploma students need a laboratory-scale project that connects their electronics syllabus to their branch: a measurement instrument they build, understand end to end, and then use to produce real textile data. The problem is to design and build a low-cost digital cantilever bending-length tester — motorised feed, infrared tip detection on the standard's 41.5° geometry, and a microcontroller readout with 1 mm resolution — and to demonstrate its value with a comparative stiffness study of five locally sourced knitted fabrics, reporting repeatability honestly.

Abstract

Fabric stiffness controls drape, handle and sewing behaviour, and the standard laboratory method is the cantilever bending-length test of ASTM D1388. This project builds a low-cost digital cantilever tester in which a motor-driven carriage pushes a fabric strip until its drooping tip breaks an infrared beam on the standard's 41.5° line. An 8051 microcontroller counts pulses from a slotted-opto encoder on the feed roller, converts them to millimetres and displays the bending length on an LCD. The rig then runs a comparative study of five knitted fabrics from Tirupur: bending length in both directions, GSM by cutter and balance, flexural rigidity G = W × c³, and stiffness correlation with GSM and knit structure. Repeatability is quantified over five measurements per strip, and digital readings are cross-checked against a manual inclined scale.

Introduction

Tirupur and the Coimbatore–Erode belt knit and export lakhs of tonnes of cotton fabric, and buyer specifications routinely include handle and drape. In college and mill laboratories the scale must be read where the drooping tip touches an inclined line — a judgement that differs between observers. A digital readout removes that subjectivity at component cost and exercises exactly the sensor, motor and microcontroller interfacing our syllabus teaches.

Existing system

• Manual cantilever testers (Shirley-type): the reading depends on the observer's eye and tires over a long batch.
• Heart-loop and drape-meter methods: related but different indices; drape meters are costly and rare in polytechnic labs.
• Imported digital testers: automated but priced far beyond a department budget.
• Limitations: observer variation, no digital record, and no affordable instrument students can maintain.

Proposed system

• Motorised lead-screw carriage pushing the strip at a constant speed, so loading rate no longer varies between tests.
• IR break-beam detection on the standard's 41.5° geometry, removing human judgement entirely.
• Slotted-opto encoder on the feed roller giving 1 mm travel resolution through the AT89S52.
• LCD display of bending length in millimetres with a repeat-test key; readings are noted into the measurement sheet.
• A five-fabric study: bending length both directions, GSM, flexural rigidity and structure-wise comparison with repeatability statistics.

Feasibility

• Technical: IR pairs, slotted wheels, geared motors and 8051 interfacing are standard diploma-level electronics; the platform is aluminium sections with printed or carpentered brackets.
• Economic: indicative cost is about ₹5,500–6,200 including the motor, lead screw and electronics — well below any commercial digital tester.
• Operational: a test takes under a minute — lay the strip, press start, record the reading.
• Schedule: fits the 14-week semester: rig working by week 10, fabric study complete by week 13.

Standard and validity boundary

The rig follows the cantilever principle and geometry of ASTM D1388 but is a student-built instrument; the report compares digital versus manual readings on the same fabrics and quotes repeatability, rather than claiming laboratory-grade certification. Fabric specimens are conditioned in the lab atmosphere before testing, and every value is the mean of five measurements.

In scope

• One cantilever platform with 41.5° inclined reference and motorised carriage; strip width 25 mm, specimen length up to 200 mm.
• Woven-cotton and knitted-cotton fabrics; the headline study covers five knitted structures from the local market.
• Digital readout of bending length in millimetres; manual cross-check readings on the same strips.
• GSM measurement with a circular cutter and precision balance; flexural rigidity computation and correlation tables in Excel.
• Repeatability study: five measurements per strip, per direction.

Out of scope

• Heart-loop procedure of ASTM D1388 and hot-wire/autoclave variants for very soft fabrics.
• Temperature and humidity conditioning chambers — specimens are tested in the lab atmosphere, which the report states.
• Drape-coefficient measurement on a drape meter; the report discusses drape as related but distinct.
• Multi-directional (bias) bending and nonwoven fabrics.

We follow a design, build, validate, study sequence organised as a V-model: each design block has a matching verification planned before assembly, and the instrument itself must pass validation before it is allowed to produce study data.

Measurement plan: each strip is laid flat on the platform with its free end at the start mark; the carriage pushes at constant speed; the beam-break instant latches the encoder count; bending length is displayed and noted. Validation criteria: the encoder calibration agrees with the steel rule within 1 mm over 150 mm, and the pooled standard deviation of five repeats is below the manual method's observed scatter. Analysis: means, ranges and standard deviations per fabric and direction; flexural rigidity computed as G = W × c³ with W in mg/cm² and c in cm; correlation of G against GSM and knit structure discussed against expectation.

The rig has three subsystems: a mechanical platform, a sensing-and-drive electronics layer, and the microcontroller that ties them together.

1. Mechanical platform: a flat aluminium platform ends at the test edge; beneath and beyond the edge sits the 41.5° inclined reference matching the standard's geometry. The IR transmitter–receiver pair is aligned so the drooping tip breaks the beam exactly on that line. A carriage on two guide rails is driven by a lead screw from a geared DC motor, with microswitch limit stops.
2. Drive and counting: the motor driver (transistor H-bridge or relay-reversing arrangement) runs the carriage forward at fixed duty; the slotted-opto wheel on the roller produces one pulse per millimetre of travel into an 8051 counter input; the beam-break signal latches the count.
3. Microcontroller: AT89S52 with 11.0592 MHz crystal runs the test sequence — home, start, count, latch, display — and the 16x2 LCD shows bending length in millimetres and the test number.

flowchart TD
  PS["Power supply (12 V motor, 5 V logic)"] --> MCU["AT89S52 microcontroller"]
  START["Start key"] --> MCU
  MCU -->|"forward run command"| DRV["Motor driver (H-bridge)"]
  DRV --> MOT["Geared DC motor + lead screw"]
  MOT --> CAR["Carriage pushes fabric strip on platform"]
  ENC["Slotted-opto wheel on feed roller"] -->|"1 pulse per mm"| MCU
  IR["IR break-beam pair on the 41.5 degree line"] -->|"beam-break latch"| MCU
  MCU --> LCD["16x2 LCD: bending length in mm"]
  LS["Limit switches (home / end)"] --> MCU

flowchart TD
  A["Press START"] --> B["Carriage returns to home limit"]
  B --> C["Operator lays strip, confirms READY"]
  C --> D["Run motor forward, count encoder pulses"]
  D --> E{"IR beam broken by drooping tip?"}
  E -->|"No"| D
  E -->|"Yes"| F["Latch count, stop motor"]
  F --> G["Show bending length in mm"]
  G --> H{"Repeat key pressed?"}
  H -->|"Yes"| B
  H -->|"No"| I["End of test"]

Firmware notes (Embedded C, Keil C51): encoder pulses are counted on Timer 1 in counter mode; the beam signal is debounced over 10 ms so motor vibration cannot latch a false tip; after latch, the count is converted to millimetres with the calibrated pulse-per-mm constant and shown with one decimal. A timeout aborts a run where the strip never reaches the beam (extremely floppy fabric), and the LCD prompts the operator to record it as beyond range.

• Automatic data logging of every reading to a memory card, with Excel-compatible CSV export, removing manual transcription errors.
• Hot-wire / very-soft-fabric handling per the standard's provisions for fabrics that cannot support their own weight on the cantilever.
• Conditioned-atmosphere testing with a simple humidity-logged enclosure, so results carry their test atmosphere.
• Bending-hysteresis studies by adding a return cycle (push and pull) to observe recovery behaviour.
• Woven-fabric extension and a comparison of starch-finished versus grey cloth to show finish effects on stiffness.
• A second instrument cross-check with a drape-meter coefficient on the same fabrics, building a stiffness–drape mapping for local knits.