hydra presses
TRANSCRIPT
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An innovative approach to solve current
problems in advanced forming using
Proportional Hydraulic Technology with PLC(advanced hydraulic presses)
Sanjeev.R1, Prafull V.Joshi
2, Dr. S.V Prakash
3, Prof. K.R Prakash
4
1, 24
thSem., M.Tech-CIM,Dept. of Mechanical Engg., MSRIT, Bangalore.
3Asst.Prof. Dept. of Mechanical Engg., MSRIT, Bangalore
4Special Officer, VTU-BR CoC, Mysore.
Abstract: - Hydraulic presses always offered tremendous advantages to metal formers in terms of flexibility and control of ramspeed and stroke. Fast open-loop controls monitor valve activity every few milliseconds, so that state-of-the-art proportional valves
precisely control and direct hydraulic oil to the press. Proportional hydraulic technology is rapidly convincing metal formers that now
are the time to retire aging presses and bring in new models.
In this paper an attempt is made to improve the performance of the metal forming presses incorporating proportional hydraulic valves
over conventional hydraulic systems. This paper concentrates on position control and velocity control of ram. A circuit has been builtusing 4/3 proportional directional control valve and a load pressure valve to simulate ram movements
Here velocity control of the ram i.e. approaching a part quickly, and then varying speed during the forming process and again
retracting quickly, is achieved by assigning various command values (analog inputs) and ramps through on board electronics to the
valve. Positional control of the ram is through adjusting the braking distance by decelerating the ram movement. This braking
distance can be varied by assigning different ramp values (decelerating time) to the valve.The programmable logic controllers (PLCs) and interfaces available along with sophisticated proportional valves, can deliver more
capabilities. This results in greater precision during forming and greater control of the forming process. In above experimentation
velocity and position control of the ram is achieved using PLC program and sensors.
Index Terms: Hydraulic presses, Proportional Directional control valve, PLC programming, Braking distance, position control,
velocity control
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I. Introduction:
Increasing pressure on metal formers to improve
productivity while decreasing costs has placed stringent
demand on the machines they employ. Hydraulic remains
the power of choice for heavy duty presses, but olderhydraulic control systems often lack the precision to
produce highly accurate and repeatable motion. Also
more and more, hydraulic presses are incorporated into
assembly cells or automation production lines, which put
constant demand on the presses to increase productivity,
improve quality, faster cycle times and decrease
maintenance cost. [1]
Choosing the proper hydraulic elements plays a vital role
to meet the above put requirements. Heavy duty (high
tonnage) presses involve large masses to be moved such
as movable platen, dies etc. Press motions involve
movement of these large masses at various speeds
changing continuously. Conventional hydraulic systemswith ON-OFF valves are inefficient for such applications
since they create large acceleration and deceleration
forces during changeover of directions, which may cause
major system damages. Complex motion requires the
conventional system to incorporate many components(FCVs, check valves etc.) increasing system complexity,
which directly affects product quality and productivity.
Open loop control of hydraulic presses has provided a
new edge in terms of performance and more accurate
control with improved reliability. These open loopcontrols incorporate Proportional Hydraulic valves where
in they monitor valve activity every few milliseconds, so
that state of art proportional valves precisely control and
direct hydraulic oil to press. Proportional hydraulic
technology is rapidly convincing metal formers that now
are the time to retire aging presses and bring in new
models.
Proportional hydraulic ales offer various advantages over
conventional ON -OFF valves that includes thefollowing: - [2]
The technical benefits of the proportional devicescan be primarily found in the controlled transfer
during valve changeover, i.e. infinitely variable
control of command signals (i.e. speed) and the
reduction of hydraulic equipments requirements for
certain control applications. This therefore also
represents an effective contribution to reducing
material requirements in the hydraulic circuits.
Proportional valves and pumps with theirproportional solenoids provide perfect interface for
electronic control, thereby facilitating increased
flexibility in the operating cycles of production
machines as well as freely programmable controlsystems and drives.
Proportional valves permit faster, simpler, andprecise movement cycles while at the same time
improving the reversal process. As a result of
controlled spool cross-over, pressure peaks areavoided resulting in a longer service life of the
mechanical and hydraulic components.
The fact that the signals for direction and flow orhydraulic pressure are provided by electrical means
has made it possible to arrange the proportionaldevices directly on the loads, thereby greatly
improving the dynamic characteristics of the
hydraulic system.
The addition of a Programmable Logic Controller (PLC)with proportional valve technology can be used to
precisely control position, velocity and force which bring
several advantages: [7]
More flexible production results from the ability toeasily change a recipe that specifies different
positions during the press cycle, which also
significantly reduces machine downtime.
Multiple sections or axes of a large press can becoordinated or synchronized.
The press can adapt automatically to handle varyingmaterial consistencies and the effects of differing
environmental conditions such as temperature and
humidity.
Pressure spikes that damage sensors and cause leakscan be reduced or eliminated, decreasing
maintenance costs and extending press life.
In addition, more consistent, smooth motion results in
less wasted or rejected parts, and ensure consistent
quality and production from different machine operators
with varying skill levels. This results in greater precision
during forming and greater control of the forming
process, with a smoother speed/position profile that
minimizes press shock and vibration. We can ramp the
control signal being sent to the hydraulic valves and
create gradual changes in hydraulic-oil flow rate which
More reliable control makes process changes morequickly, so what might have been, using previous control
technology, a step response between speed changes
becomes a smooth response.
II. Proportional Valve Technology
The emerging open and closed-loop control technology
with its high flexibility necessitated electronically
controllable hydraulic valves. Servo-valves meet thisrequirement, but were much to complex and expensive
for most of the applications. Hydraulic valves had
therefore to be developed, which can be steplessly
adjusted by means of an electronic system, while being
simple, rugged and cost-efficient. This was the "birth" ofproportional valve technology. [4] Decisive features of
proportional valves are spools with control lands, specific
solenoids and special electronics for optimum operation
of the valves. With the help of the electronics it was now
possible to steplessly adjust the valve aperture (command
value) and the spool velocity (ramp), which allowed the
machine speed and in particular acceleration to be exactly
controlled. Now, it was possible to adjust the machine
movement continuously and proportionally to electrical
signals. This resulted in the designation "proportional
valve. This technology provided higher speeds and
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hence higher productivity. Another aspect was that the
motion sequences could be programmed with an
electronic control. With today's proportional valves, the
valve electronics can be mounted directly onto the valve.
Fig 1 Signal flow in proportional hydraulics
Fig. 2 Modern proportional directional valve with integrated
On-Board Electronics (OBE)
III. Design and mode of operation of a proportional
solenoid
Depending on the design of the valve, either one or two
proportional solenoids are used for the actuation of an
electrically variable proportional valve.
A. Solenoid design
The proportional solenoid (fig. ) is derived from the
switching solenoid, as used in electro-hydraulics for the
actuation of directional control valves. The electrical
current passes through the coil of the electro-solenoid and
creates a magnetic field. The magnetic field develops a
force directed towards the right on to the rotatable
armature. This force can be used to actuate a valve.
Similar to the switching solenoid, the armature, barrel
magnet and housing of the proportional solenoid are
made of easily magnetisable, soft magnetic material.
Compared with the switching solenoid, the proportional
solenoid has a differently formed control cone, which
consists of non-magnetisable material and influences the
pattern of the magnetic field lines.
B. .Mode of operation of a proportional solenoid
With the correct design of soft magnetic parts and control
cone, the following approximate characteristics are
obtained. The force increases in proportion to the current,
i.e. a doubling of the current results in twice the force on
the armature. The force does not depend on the positionof the armature within the operational zone of the
proportional solenoid.
.
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IV. Characterization of Proportional directional valve 4WREE6E08/2X
Fig 3. Proportional directional valve 4WREE6E08/2X
Fig. 4 Symbol of a proportional valve
A. Position control and velocity control withproportional hydraulic valves
Velocity control [2]
Velocity of the cylinder (ram) is controlled from the
proportional directional valve is controlled by analog
command value module. On the command value
module, the velocities are adjusted by means of
command value potentiometers. Acceleration anddeceleration rates are adjusted by means of ramp
potentiometers.
Potentiometers "w1-4are used for adjusting the
command values and hence also the cylinder velocities.
The set values are activated by means of command
value call-ups. Command values "w1and "w2are
positive, whereas "w3and "w4are negative. In this
way, a cylinder can, for example, extend with "w1and
"w2and retract with "w3and "w4. A ramp is
assigned to each command value "w1 "t1, "w2
"t2, etc. This means that when command value
"w1is active, only ramp "t1" is also activated. Ramp"t5is active, when no command value is active.
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force via the blank holder onto the blank & the draw
die. The die & the ejector are located in the lower die
on the press bed. During forming, the blank holder
brings the sheet metal into contact against the die; the
punch descends from above into the die & shapes the
part while the sheet metal can flow without any
wrinkling out of the blank holding area. In this Case,
the drawing process is carried out with a fixed blank
holder & moving punch. In double action drawing
operations, the drawing slide can only apply a pressing
force. [6]
Fig. 6 Deep Drawing Process
A. Conventional Hydraulic circuit for deep drawing
process control
A hydraulic circuit is built with ON-OFF valves which
are as shown below. This circuit incorporates two
cylinders for typical deep drawing operation. One
cylinder acts as a blank holder and other acts as a draw
punch. The circuit incorporates solenoid operated 4/2
spring return ON-OFF valves.
The conventional hydraulic circuit is as shown below.
Fig. 7 & 8
Electrical circuit for above hydraulic circuit is shown in
Fig. 9
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Fig. 7 Conventional hydraulic circuit for rapid speed- creep speed control
Fig. 8 Hydraulic circuit for draw punch
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Fig. 9 Electrical Circuit for conventional press operation
Operation is as follows-
To reduce the idle time the blank holder cylinderis moved with rapid speed and then with creep
speed to hold the blank. The sensing action at
changeover during rapid speed to creep speed is
achieved with the help of Inductive proximity
sensors.
Once the blank is hold firmly the draw punchcylinder starts operating.
Once the drawing action is complete the drawpunch is retracted and then the blank holder
cylinder so that part can be ejected.
The speed control of cylinders is achieved with the help
of flow control valves. One flow control valve each is
required for both the cylinders.
The circuit involves two major operations-
Rapid speed- creep speed of blank holdercylinder
Advancing of draw punchHowever the deep drawing press with conventional
valves has many limitations. It incorporates three 4/2
solenoid operated spring return DCV and two flow
control valves to perform the operation. The speed
control is very cumbersome as it is with manual flow
control valves which are not accurate. Position control is
also difficult and is inflexible.
B. Hydraulic circuit with proportional valveHence to overcome all these limitations proportional
valve is incorporated through which both velocity control
and position control is achieved with a single valve,
eliminating number of valves and reducing system
complexity.
Hydraulic circuit dig with proportional valve is shown in
Fig.10 & 11
Electrical circuit dig with proportional valve is shown in
Fig.12 & 13
The operation sequence with proportional valve is as
below
The blank holder cylinder is extended rapidlywith command value w1=5V
With activation of command value w2=2V thespeed is reduced to creep speed
Then the draw punch cylinder is extended It is then retracted with sensor actuation Finally blank holder cylinder is retracted rapidly
with command value w3
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Fig.10 Hydraulic circuit with proportional valve for rapid speed creep speed control
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Fig 11 Hydraulic circuit for draw punch
Fig. 12 Electrical Circuit 1 for proportional valve
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Fig. 13 Electrical Circuit 2 for proportional valve (command value module)
VI. Proportional hydrualic system with PLC
PLC programming techniques is used to automate the
above proportional hydraulic system for deep drawing
press applications.IndraWorks/ IndraLogic programming
software with Rexroth Bosch L20 controller is used.
Ladder logic programming method is used to write the
logic.Complexity of the electrical circuitry is reduced by
PLC and allows flexibility in changing both analog and
digital control of the valves.
Fig: 14 Equivalent PLC logic diagram (Program) for above electrical circuit for proportional valve
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Fig: 15 Equivalent PLC logic diagram (Program) for above electrical circuit for proportional valve
Fig: 16 Equivalent PLC logic diagram (Program) for above electrical circuit for proportional valve
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VII. Conclusion:
Forming presses for advanced applications will have
complex hydraulic circuits to achieve determined
characteristics, such as ram movements with creep
speeds. Use of proportional valve avoids multiple valve
incorporation. With open loop systems, velocity controlwill be accurate, yielding greater part accuracy. Position
control with proportional valves reduces large
acceleration and deceleration forces generated during
faster ram movements reducing the risk of system
damages. This enhances product quality and smoother
action of the press increasing reliability.
In recent days proportional technology along with PLC is
being used for applications which involve heavy masses
and require better control, such as deep drawing of fire
extinguishers that are 1 kg and larger, sheet steel with
thicknesses between 1.4 and 2.5 mm.
In this paper an attempt is made to use proportional valve
technology in hydraulic presses for advanced forming
process applications and automated with PLCprogramming which achieves flexible and advanced
forming options.
VIII. References
[1] Bill Savela, Pe, Smarter Motion controlspromise better Hydraulic Press Performance,
Metal forming / May 2008.
[2] R.Ewald, J. Hutter, The Hydraulic TrainerVolume 2 Proportional and Servo Valve
Technology, Rexroth Bosch Pg. 11-50, 77-86.[3] Jansuz Pluta, Hydraulic Press with LS System
for Modeling of Plastic Working Operations
[4] Learning System for Automation andCommunications Proportional hydraulics
Textbook, Festo Diadctic
[5] D. Kretz, M.Reik The Hydraulic TrainerVolume 1 Basic Hydraulis
[6] By F San Martin, F Negroni, TheHydromechanical Deep Drawing Process.
[7] Brad F. Kuvin, Hydraulic Presses -SmoothOperators Metal forming / March 2002