AUTO LEVELLING SYSTEM

AUTO LEVELLING SYSTEMPRIMARY TASKS:The main task of auto leveling is to eliminate deviations in mass.To control the consistency of output from a process by deliberately altering the input. Aimed at achieving the minimum possible variations in the linear density of output sliverTo eliminate the Short, Medium and Long term variations in the sliver. CLASSIFICATION:The autolevellers are classified on the basis ofSpectrum of length variation it controls Short term : 0.25 to 2.5 mMedium term : 2.5 25 mLong term : 25 250 mVery long term : >250 m Principle of operationOpen loopClosed loopMixed loopOPEN-LOOP AUTOLEVELLINGA measuring sensor is provided in the region of the in feed for continuous detection of the actual value (volume) mechanically, optically, pneumatically, or otherwise.

A regulator compares the result with the set reference value, amplifies the difference signal, and feeds it to an adjusting device (actuator), which then finally converts the impulse into a mechanical adjustment.

The principle of open-loop control; A, measuring sensor; B, store; C, amplifier; D, adjusting device; E, adjustment point; F, set-value inputIts used for correction of short-term variationsInput material variation is measuredSignal is compared to a reference signal control unit measures the difference control unit sends a signal to a draft control unit indicating the necessary action to be taken.

Control by this chain of steps requires an additional element, namely a storage device. This additional requirement represents a second disadvantage of open-loop control in addition to the lack of self-monitoring. There is a third disadvantage, since very exact values of the adjustment are required at all times.

The open loop system results in a quicker response time to the deliberate changes, since the lag time of the process is avoided.There is no feedback from the output to ensure that corrections made achieve minimum variation of the output characteristics.

CLOSED-LOOP AUTOLEVELLINGThe measuring sensor is usually arranged in the delivery region, i.e. downstream from the adjusting device. In contrast to open loop control, the sensing point is located in front of the controlling point. Its used for correcting long-term variation

The principle of closed-loop control; A, measuring sensor; C, amplifier; D, adjusting device; F, set-value input; G, dead-time distance

Autoleveling in cardingIn modern carding machines combination of Open-loop & Closed loop autoleveling is used for controlling short & long term variations.

1 Input signal obtained by measuring the thickness of the matt being fed to the card. 2 Input signal from the sliver delivery rollers. 3 Input signal from the light sensor in the A 70 chute. SCU control unit. A) Control for the drive of the feed rollers in the A 70 chute. B) Inverter controlled drive to vary the feed roller speed according to the measured matt thickness and the sliver output signal.

what it contains ?sensor - pneumatic tongue and groove roller data convertorcontrol element

Carding levelling

AUTOLEVELLING IN DRAW FRAMEOPEN-LOOP SYSTEM:The total volume of all slivers is measured at the in feed and adjustment is effected with the appropriate time delay in the main drafting zone, i.e. the extent of the change is retained in a storage device until the measured deviation arrives at the drafting point.Detection is usually carried out mechanically (rollers with grooves, bores or steps) or by capacitive sensors.

This system permits very precise leveling of very short lengths. A second advantage is the ability to measure far greater sliver masses due to the lower in-feed speed (corresponding to the amount of draft). Recording becomes more precise. In practice, draw frame leveling using open-loop control is now predominant

Heavy place in slivers from creel.Scanning rolls sense the heavy place before the draft zone.Autoleveler adjusts the main draft to equalize the heavy place.Autoleveler Operation

CLOSE-LOOP SYSTEM:In this system, the evenness of the sliver delivered is measured rather than the in feed sliver, as is the case with open loop control. In contrast to the open-loop control system, where the adjusting point is located after the measuring point, the adjusting point in the closed-loop control system is located behind the measuring point

Control point:The control can be exercised either in the front or back zone, through adjustment of draft.Adjusting the draft in front zone by regulating the speed of middle roller is preferred.While manipulating the draft in back zone, the danger of draft falling in stick-slip zone of drafting exist which may cause additional irregularity.In a particular zone, the draft change can be brought about by either changing feed or delivery speed.A change in delivery speed would result a change in production, hence feed roller speed is always changed.

Correction Length:If there is a sudden deviation from the set volume as the material passes through, a corresponding signal is sent to a regulating device to correct the fault. Owing to the mass inertia of the system, compensation cannot be effected suddenly, but must be carried out by gradual adjustment. A certain time (the correction time: I) elapses before the sliver delivered has returned to the set volume.

During this time, faulty sliver is still being produced, although the deviation is being steadily reduced. The total length that departs from the set value is referred to as the correction length (I).The term correction length is used to describe the efficiency of a leveling device.The current interpretation is: The correction length is the length of the product which would be produced when leveling a rectangular deviation of the product.The length therefore refers to an amplitude of the fault of 1%. As they cannot be checked in the spinning mill, the quality of the delivered sliver is usually taken as the standard of comparison, and sliver evenness can be determined by any evenness tester.

The correction length depends uponInertia of the regulating system and hence on its design.Delivery speedDraftExtent of mass variation of sliver from the set value.Sense of change of mass i.e. whether it is fromNormal level to lighter sideLighter level to Normal sideNormal level to heavier sideHeavier level to normal sideIf a system takes t sec to level a certain percent increase in mass variation of a sliver that is being delivered at V m/min, the correction length (l) would be

Correction length (l) = 100 V t mm 60TESTING OF AUTOLEVELLER:Two important parameters for quality leveling areLeveling action point (LAP time of correction)Leveling intensity (LI)Leveling Intensity (LI):Levelling Intensity is to decide the amount of draft change required to correct feed variation. The correlation between mass and volume for different fibres is not same. Therefore the levelling intensity may be different for different fibres. Levelling intensity is selected based on the following trial. Wrapping of the delivered sliver should be checked with "n", "n+1", "n-1" sliver at the feeding side.

Produce 100 m of sliver with normal doubling (say 6)Produce 100 m of sliver keeping one sliver off (i.e. 5) and then another 100 m with one extra sliver ( i.e. 7). This will simulate a situation of light and heavy feed.Each of sliver produced, should be checked for count determination based on 5 - 10 samples.A% = ((gms/mt(N-1) - gms/mt(N))/ gms/mt(N) ) x 100 A% = ((gms/mt(N+1) - gms/mt(N))/ gms/mt(N)) x 100 A % should be below 0.5%.Most of the auto levelers can correct 25% of feed variation.

Levelling Action Point (LAP Correction Time):Both feed variation sensing and correction are being done when the machine is running (continuous process) at two different places(i.e. sensing is at one place and correction is at an other place). Hence the calculated correction should be done on the corresponding defective material. This is decided by Levelling action Point.

Leveling Action too soonenter higher number.LAP timingSliver deflection influences LAP

The time required for the defective material to reach the correction point should be known and correction should be done at the right time. Levelling action point depends upon break draft main draft roller setting &delivery speed

Advantages of high performance levelling:IN THE SPINNING MILL:reducing count variations;fewer short-term mass variations in the yarn (CV %);improving the coefficient of variation of yarn strength (CV % cN/tex);fewer yarn imperfections (IPI and Classimat);improving the efficiency of roving frame and spinning machine by reducing the ends down rates;fewer cuts on the winding machine.IN THE SUBSEQUENT PROCESS STAGES:reduction of ends down rates in weaving preparation and weaving;even appearance of the finished cloth;reducing the cost for claims by eliminating a remarkable number of faults.

Norms for U% & CV%RatingCV% (1 m)U %GoodO.5 Coarse & medium Fine countsCarded 2.25 2.5Combed 1.75 2.0

Average0.75Poor1.0Influence of machine & process parameters on evennessThe short term irregularity in the processed material and yarn is generally by assessing the Uster U%.The U% is a measure of the variation in the weight of pieces of 20 mm in the case of sliver and 12 mm in the case of roving and 8 mm in the case of yarnVARIATION IN BLOW ROOM LAP WEIGHT:

Insufficient opening of cotton and wide variation in tuft size.Use of excessive soft waste in the mixingMalfunctioning of the length measuring motionCage choked with dust and dirt on their surfaceIneffective working of feed regulating motion.Card sliver U%:The contribution of card sliver evenness to the count variation is about 10%.The norms for card sliver U% areSHP 4.0%HP 3.5%VHP 3%As a routine control, the cards should be checked once in a month for sliver irregularity.

U% influencing factors in Carding machine :1. Doffer wire condition Doffer grinding can be done2. Tongue setting Transfer to be set right3. Web tension draft optimum web draft to be selected for that particular speed in the transfer zone4. Drives Drive transmission to be perfected ( no loose belts, no slippage, minimum play between gears)5. Feed Draft optimum feed draft between lap-to-feed roller or chute-to-feed roller6. Trumpet/ condenser selection as per hank of sliverVariation in flat speed between cards processing the same mixingObstruction in the movement of aprons during doffing in modern cards.Bent/damaged back and front plates.Difference in drafts between cardsComber sliver U%:As in the case of card sliver, a high irregularity in comber sliver could have a detrimental effect on the yarn count variation.Norms for comber sliver U% are

RatingSliver Hank0.12 to 0.16> 0.16Good3.03.5Average3.54.0Poor4.04.5HIGH COMBER SLIVER VARIATION:Difference in waste extraction between headsVariation in settings between back detaching roller and nipper.Unicomb choked with seed coats or immature cotton.Wider setting between unicomb and brushImproper needle spacing, broken or bent needlesVariation in detaching roller diameter and improper timing of top combsDraw frame sliver U%:Control of draw frame sliver irregularity is one of the important points in reducing yarn count variation.The short term variation in draw frame sliver contributes to 50% of the lea count variation.An uneven draw frame sliver would increase both the lea strength variation and end breaks in spinning.RatingSliver Hank0.12 to 0.16> 0.16Good2.02.5Average2.53.0Poor3.03.5Norms for combed draw frame sliver U%:Influence of draw frame sliver U%:An increase in draw frame sliver U% from 3% to 5% will increase the roving U% from 5 to 6.4%.An increase in draw frame sliver U% from 3% to 5% will increase the yarn U% by 0.5%.The count cv% shows an increase with increasing draw frame sliver irregularity.The lea strength variation is significantly greater at very high levels of draw frame irregularity

Factors affecting the draw frame sliver U%Setting between the rollers:Roller settings based on span length (now-a-days based on AFIS 5% length) is useful.Such settings are known to confer improvements in the performance of preparatory and spinning machines as well as in sliver & yarn quality.

Draw frame passageRoller settings (nip to nip in mm)FrontBackBreaker5% AFIS Length5% AFIS Length + 4Finisher5% AFIS Length + 25% AFIS Length + 6Break, Web & Creel Drafts:The break draft is determined by a number of factors such as the fibre properties of raw material, type of draw frame, first or second passage etc.

Draw frame passageCarded countCombed countMan-made fibresBreaker1.71.31.7Finisher1.31.31.3The web tension draft , which is governed by the type of material used must be slightly lower at the breaker drawing than at the finisher.Excessive web tension draft would lead to an increase in the sliver irregularity as well as lea count variation.

CountWeb draftCotton:Upto 24s24s 36sAbove 36sMan-made fibres0.96 0.970.98 1.001.00 1.021.00 1.02Trumpet size:Use of a proper trumpet size helps to obtain a sliver of sufficient compactness necessary for subsequent processing.

For combed counts, the diameter will be less by 0.5 mm

Sliver hankCarded countsBreakerFinisher0.25 & above3.02.50.18 0.243.02.50.15 0.173.53.00.12 0.143.53.5Machinery condition:The mechanical condition of the draw frame is also an important factor determining sliver irregularity.Improper pressure on top rollersImproperly meshed or worn gear wheelsVariation in top roller diameter.

ROVING U%:Under normal working conditions, roving process contributes for about 15% of the yarn irregularity.

RatingRoving hank1.2 to 1.6> 1.6Good3.53.8Average4.04.3Poor4.54.8Periodic irregularity in roving affects lea strength variation adversely. The effect is more pronounced in fine counts.Short term irregularity in roving (U%) influences medium term variation in yarn which is mainly responsible for end breaks in ring spinning.Simplex machine should be checked once in 15 days for roving irregularity (U%).Factors influencing Roving U%:1. Setting between the rollersFront & middle zone settings are fixed.Back zone settings should lie within 2.5% span length + 12 to 15 mm.For two zone drafting Effective length + 8 to 10 mm2. Total draft & Break draftDecided based on factors like type of drafting system, quality of back material & condition of machine.

Break draft has to maintained in the range of 1.08 to 1.3 for satisfactory performanceCount (Ne)DraftCount (Ne)Draft20s9.070s13.530s, 40s & 50s10.080s14.569s12.090s & 100s15.03. Wrong size of sliver guidesSelection of sliver guides of proper size helps to condense the sliver effectively and reduce uneven rate of feedingParticularsSliver hank0.09 to 0.120.121 to 0.140.141 to 0.170.171 to 0.2Inlet condenser (mm)18 x 6 or 16 x 415 x 312 x 2.58 x 2Middle condenser (mm)14 or 1111 0r 99 or 69 or 6Hank of sliver (Ne)Hank of roving (Ne)Size of floating condenser (mm)Size of spacer (mm)0.09 0.120.5 1.01.1 1.61.7 2.511 189 167.5 146 95.5 85 70.121 0.140.6 1.01.1 1.61.7 2.511 169 147.5 116 95.5 85 70.141 0.170.7 1.21.3 1.61.7 3.09 -147.5 116 95.5 85.5 85 70.171 0.21.0 1.61.7 3.06 96 7.55 75 - 74. Slipped or Missing apronsSpindles running without bottom aprons create uneven yarn because the materials are being stretched in loose state and without any guidance.Irregularity of roving will increase by 2 to 2.5 U%.

5. Top roller loading, Shore hardness, and improper spacers

A shore hardness of 80o proves to be ideal both for cotton as well as for man-made fibres.6. Mechanical condition of machine

PositionMaterialCottonMan-madeFront line2225.0Middle line12 1316.5Back line12 - 1316.5Yarn U%:1. Roller settingsIn order to avoid the creation of drafting waves and to reduce U% of yarn, proper roller settings must be adopted.

Count groupRoller settings (mm)Up to 20s5521s to 60s6060s & above652. Top roller pressure & Shore hardnessInsufficient loading of top rollers leads to erratic movement of the fibres due to fibre slip between the drafting rollers.This, in turn, will lead to high level of short term unevenness of yarn.Top roller pressure of 18 kgs improves the U% and reduces the thick & thin places.

Use of softer cots (shore hardness of 60o to 75o) generally improves yarn quality by reducing slip between the cot and the bottom fluted roller.Soft cots with a top roller pressure of 18 kg in counts below 50s and 15 kg in counts finer than 50s will result in improved yarn quality.PositionPressure (kg)Front line16 - 18Middle line10 12Back line12 143. Draft distributionThe total draft and break draft employed in spinning influence the amount of irregularity added in spinning and they depend on the quality of roving and condition of the ring frame.Break draft in ring frame is mainly to break the mild twist in the roving.While using higher break draft, the back zone settings should be wider to obtain optimum performance.

4. Apron spacingCradle opening contributes to the tune of 60 to 80% on the incidence of thick and thin places in the yarn.Wider cradle opening, lesser will be the control of fibres between aprons leading to thin places in the yarn,Narrower the cradle opening, greater will be the strain to the fibres between the aprons, leading to undrafted ends in the yarn.5. Roving twistThere is a high degree of interaction between apron spacing, break draft and roving twist.Closer spacing between the aprons will improve the yarn irregularity.Too close setting will leads to un drafting. So we have to increase the break draft / reducing the twist level in the input roving,INDEX OF IRREGULARITYThe most uniform strand of material which our present machinery can produce is one which the fibre ends are laid in a random order in the sliver, roving and yarn this is an ideal yarnEven this ideal strand would have some irregularity in the structure.Thus, for a particular type of fibre and count of yarn, there is an irregularity limit which cannot be improved upon by the present machinery limiting irregularity.By calculating this limit irregularity and then measuring actual irregularity, we can judge the spinning performance.

Index of irregularity I = CV CV lim

CV = actual measured irregularityCV lim = calculated limit irregularity

I = 1 for best possible yarnI > 1 more irregular yarn.For cotton fibres, the limit irregularity is given by,CV lim = 100 = 100 (Tf / T) (n)

The higher the number of fibres, the lower the irregularity.Fine fibres produce a more regular yarn for a given count than coarser fibres.80Fiber Micronaire Vs Yarn EvennessThe evenness can be lowered by using fine fibers. Thetheoretical background is Martindales formula:

CV(Lim) = 100/ n

CVlim = Limit irregularity n = Number of fibers in the cross-section C

Yarn EvennessFibre FinenessCoarseFineLower the micronaire, lower U%81Fiber Micronaire Vs Yarn Evenness Irregularity Index = CVmass/CV(lim) = b/a

Irregularity Index is the ratio between the evenness values obtained under practical condition and limit irregularity.

Finer the fibre, more number of fibres in the cross section and lesser limit irregularity

Yarn EvennessFibres in cross section1003000Measured evennessLimit irregularityabMachine induced irregularity.Natural irregularity due to Nature of cotton.This is the minimum Cv m we can get.