PRIMARY 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 at
linear density of output sliver To eliminate the Short, Medium and Long term variations in
the sliver.
CLASSIFICATION:
The autolevellers autolevellers are classified on the basis of Spectrum
of length variation it controls
Short term
: 0.25 to 2.5 m
Medium term
: 2.5 – 25 m
Long term
: 25 – 250 m
Very long term
: >250 m
Principle of
Open loop
Closed loop
Mixed loop
operation opera tion
OPEN-LOOP AUTOLEVELLING A
measuring sensor is prov provided ided in the region of the in
feed for continuous detection of the actual value (volume)
– mechanical mechanically ly,, optically opt ically,, pneumatica pn eumatically lly,, or otherwise. otherwis e. A
regulator compar compares es the result with the set refer reference ence
value, amplifies the differ difference ence 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 input
It’s used
for correction of short-term variations
Input material variation is measured Signal 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 AUTOLEVELLING The
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. It’s 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 carding In
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 convertor control element
Carding levelling
AUTOLEVELLING IN DRAW FRAME OPEN-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
Autoleveler Operation
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.
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 As
length therefore refers to an amplitude of the fault of 1%.
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 upon Inertia of the
regulating system and hence on its design.
Delivery speed Draft Extent of mass variation of sliver from the set value. Sense of change of mass i.e. whether it is from
Normal level to lighter side
Lighter level to Normal side
Normal level to heavier side
Heavier level to normal side
If 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 60
mm
TESTING OF AUTOLEVELLER: Two important parameters for quality leveling are Leveling
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.
LAP timing
Leveling Action too soon enter higher number.
n P o A i t L c e s l e f c e n d e r u e l v f i n l i S
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 improving
yarn (CV %);
the coefficient of variation of yarn strength (CV %
cN/tex); fewer yarn improving
imperfections (IPI and Classimat); 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
number of faults.
remarkable
Norms for U% & CV% Rating
CV% (1 m)
Good
O.5
Average
0.75
Poor
1.0
U%
Coarse & medium Carded 2.25 Combed 1.75
Fine counts 2.5 2.0
Influence of machine & process parameters on evenness The
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 8
roving and
mm in the case of yarn
VARIATION IN BLOW ROOM LAP WEIGHT: Insufficient
opening of cotton and wide variation in tuft
size. Use
of excessive soft waste in the mixing
Malfunctioning of the Cage
length measuring motion
choked with dust and dirt on their surface
Ineffective
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% are
SHP – 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 done 2. Tongue setting – Transfer to be set right 3. Web tension draft – optimum web draft to be selected for that particular speed in the transfer zone 4. 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 roller 6. Trumpet/ condenser selection as per hank of sliver
7. Variation in flat speed between cards processing the
same mixing 8. Obstruction in the movement of aprons during doffing
in modern cards. 9. Bent/damaged back and front plates. 10. Difference in drafts between cards
Comber 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 Rating
Sliver Hank 0.12 to 0.16
> 0.16
Good
3.0
3.5
Average
3.5
4.0
Poor
4.0
4.5
HIGH COMBER SLIVER VARIATION: 1. Difference in waste extraction between heads 2. Variation in settings between back detaching roller and
nipper. 3. Unicomb choked with seed coats or immature cotton. 4. Wider setting between unicomb and brush 5. Improper needle spacing, broken or bent needles 6. Variation in detaching roller diameter and improper
timing of top combs
Draw 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.
Norms for combed draw frame sliver U%:
Rating
Sliver Hank 0.12 to 0.16
> 0.16
Good
2.0
2.5
Average
2.5
3.0
Poor
3.0
3.5
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 passage
Roller settings (nip to nip in mm)
Front
Back
Breaker
5% AFIS Length
5% AFIS Length + 4
Finisher
5% AFIS Length + 2
5% AFIS Length + 6
Break, 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 passage
Carded count
Combed count
Man-made fibres
Breaker
1.7
1.3
1.7
Finisher
1.3
1.3
1.3
The
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. Count
Web draft
Cotton:
Upto 24s
0.96 – 0.97
24s – 36s
0.98 – 1.00
Above 36s
1.00 – 1.02
Man-made fibres
1.00 – 1.02
Trumpet size: Use of a proper
trumpet size helps to obtain a sliver of
sufficient compactness necessary for subsequent processing. Sliver hank
Carded counts Breaker
Finisher
0.25 & above
3.0
2.5
0.18 – 0.24
3.0
2.5
0.15 – 0.17
3.5
3.0
0.12 – 0.14
3.5
3.5
For combed counts, the diameter will be less by 0.5 mm
Machinery condition: The
mechanical condition of the draw frame is also an
important factor determining sliver irregularity. Improper
pressure on top rollers
Improperly Variation
meshed or worn gear wheels
in top roller diameter.
ROVING U%:
Under
normal
working
conditions,
roving
contributes for about 15% of the yarn irregularity.
Rating
Roving hank 1.2 to 1.6
> 1.6
Good
3.5
3.8
Average
4.0
4.3
Poor
4.5
4.8
process
Periodic
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 rollers
Front & 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 mm
2. Total draft & Break draft
Decided based on factors like type of drafting system, quality of back material & condition of machine.
Count (Ne)
Draft
Count (Ne)
Draft
20s
9.0
70s
13.5
30s, 40s & 50s
10.0
80s
14.5
69s
12.0
90s & 100s
15.0
Break draft has to maintained in the range of 1.08 to
1.3 for satisfactory performance
3. Wrong size of sliver guides Selection
of sliver guides of proper size helps to condense
the sliver effectively and reduce uneven rate of feeding Particulars
Inlet condenser
Sliver hank 0.09 to 0.12
0.121 to 0.14
0.141 to 0.17
0.171 to 0.2
18 x 6 or 16 x 4
15 x 3
12 x 2.5
8x2
14 or 11
11 0r 9
9 or 6
9 or 6
(mm) Middle condenser (mm)
Hank of sliver
Hank of roving
Size of f loating
Size of spacer
(Ne)
(Ne)
condenser (mm)
(mm)
0.09 – 0.12
0.5 – 1.0
11 – 18
6 – 9
1.1 – 1.6
9 – 16
5.5 – 8
1.7 – 2.5
7.5 – 14
5 – 7
0.6 – 1.0
11 – 16
6 – 9
1.1 – 1.6
9 – 14
5.5 – 8
1.7 – 2.5
7.5 – 11
5 – 7
0.7 – 1.2
9 -14
5.5 – 8
1.3 – 1.6
7.5 – 11
5.5 – 8
1.7 – 3.0
6 – 9
5 – 7
1.0 – 1.6
6 – 9
5 – 7
1.7 – 3.0
6 – 7.5
5-7
0.121 – 0.14
0.141 – 0.17
0.171 – 0.2
4. Slipped or Missing aprons Spindles
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 Position
A
Material
Cotton
Man-made
Front line
22
25.0
Middle line
12 – 13
16.5
Back line
12 - 13
16.5
shore hardness of 80o proves to be ideal both for cotton
as well as for man-made fibres. 6. Mechanical condition of machine
Yarn U%: 1. Roller settings
In order to avoid the creation of drafting waves and to reduce U% of yarn, proper roller settings must be adopted.
Count group
Roller settings (mm)
Up to 20s
55
21s to 60s
60
60s & above
65
2. Top roller pressure & Shore hardness Insufficient
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
Position
Pressure (kg)
Front line
16 - 18
Middle line
10 – 12
Back line
12 – 14
of softer cots (shore (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 50s will result in improved yarn quality.
3. Draft distribution The
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 spacing Cradle
opening contributes to the tune of 60 to 80%
on the incidence inciden ce of thick and thin places in the yarn. Wider
cradlee opening cradl opening,, lesser will be the con control trol 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 twist There
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 IRREGULARITY The
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 yarn Even
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 irregularity
CV lim = calculated limit irregularity
I
= 1 for best possible yarn
I
> 1 more irregular yarn.
For
cotton fibres, the limit irregularity is given by, = 100 (Tf / T)½ CV lim = 100 (n)½ The
higher the number of fibres, the lower the
irregularity. Fine
fibres produce a more regular yarn for a given
count than coarser fibres.
Fiber Micronaire Vs Yarn Evenness
Lower the micronaire, lower U% Yarn Evenness
Coarse
Fine
Fibre Fineness
The evenness can be lowered by using fine fibers. The theoretical background is Martindale’s formula:
CV(Lim) = 100/√ n CVlim = Limit irregularity n = Number of fibers in the cross-section C
