Auto leveler is
an additional device which is meant for correcting the linear density
variations in the delivered sliver by changing either the main draft or break
draft of the drafting system, according to the feed variation.
There are two
types of Auto leveling systems. They are
- Open loop system
- closed loop system
Most of the draw
frame auto levelers are open loop auto levelers.
In open loop
auto levelers, sensing is done at the feeding end and the correction is done by
changing either a break draft or main draft of the drafting system.
In closed loop
system, sensing is at the delivery side and correction is done by changing
either a break draft or main draft of the drafting system.
Most of the
earlier card auto levelers are closed loop auto levelers. But the latest cards
have sensing at the feed rollers and as well as at the delivery calendar
rollers. We can say, both closed loop and open loop systems are being used in
such auto levelers.
Open loop system
is very effective, because the correction length in open loop system is
many folds lower than closed loop system. But in case of closed loop
system, it is confirmed that the delivered sliver is of required linear
density. In case of open loop system, since the delivered material is not
checked to know whether the correction has been done or not, Sliver monitor is
fixed to confirm that the delivered sliver has the required linear density
Let us discuss
about an auto leveler system which is being used in most successful draw frames
like RSB-951, RSB-D-30 etc.
This system is
an electronic leveling system. The major components in the system are
·
Scanning roller
·
signal converter
·
leveling CPU
·
servo
drive(servo motor and servo leveler)
·
Differential
gear box(Planetary gear box)
Function of the
scanning roller is to measure the variation in the feed material. All
slivers fed to the machine pass thro a pair of scanning rollers. One of the
scanning rollers is moveable. These scanning rollers are loaded either by a
spring loading system or a pneumatic loading system. Pneumatic loading is always
better, because the pressure in kgs will be always same (consistent),
irrespective of the sliver feed variation. But in the case of spring loaded,
the pressure on scanning rollers may vary depending upon the feed variation.
The variations
in sliver mass of the incoming slivers displace the scanning roller. The
distance moved by the scanning is proportional to the sliver mass fed. This
displacement of scanning rollers are transformed into voltage by a signal
converter and is fed to an Electronic Leveling processor. With analogue system,
electronic leveling processor is a servo amplifier, but in the case of digital
system, it a CPU.
It is the
Electronic Leveling processor which furnishes the correct target value to the
servo drive. (Servo motor and servo leveler).
Delivery speed
of the machine and electric signal values arrived at by the slivers fed are the
two important signals for the correction.
Servo drive
takes the information and is converted in such a way that servomotor RPM and
direction is decided for appropriate correction.
Planetary
gearing (Differential gearing) with its controlled output speed drives the
middle and back roller. I.e. Sliver entry of the drafting system
Because the
servo motor RPM and direction varies according to the feed variation and
the servo motor and servo leveler generates a control speed of the planetary
gearing, the required change in main draft is accomplished, compensating for
the weight variation of the sliver fed.
·
If the slivers
fed are too heavy, the entry speed is reduced i.e. draft increased
·
If the slivers
fed are too light, the entry speed is increased i.e. draft reduced
Delivery speed (the
front roller speed) remains constant and hence the production remains constant.
