Edge BafflesAutomation

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Galvanizing line is a continuous line, which means that particular metal strip plates are weld together at an initial section of line. Welds are marked by cut-out and their movement is precisely monitoring during whole progress in a production line. The plate area just before and after the weld has not a sufficient parameters, therefore it is not galvanized. During such section the edge baffles move away from the plate edge and the air knives are stopped. After weld passed edge baffles moves back towards the plate edges. The detection of strip edge is provided by the contact roll. Complex kinematical layout of currently used contact system for turbulences prevention during removing excess liquid coating metal as a continuous steel strip exits a coating bath is in Figure 2 ^[1].

Description of all parts with their parameters required for next calculations ^[1]:

1. compressor- compressed air of 0,6MPa pressure, possible to use nitrogen instead

2. pneumatic piston

Ÿ two way, controlled by regulator

Ÿ length: 1210mm

Figure 2. Kinematic layout [1]

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Ÿ cylinder diameter: 25mm

Ÿ in the end position effect of constant downforce 49N invoked by constant pressure 0,1MPa on piston surface in cylinder

Ÿ movement: 300mm.s^-1

Ÿ signal for moving aside: 2s before detected weld

Ÿ signal for moving forward: 2s after weld passed

3. linear guidance for bearing carriage

Ÿ guidance length, respectively the length from the first end position (aside baffles) to another one (closed baffles): 1000 mm

4. carriage

Ÿ bearing movement with guidance

Ÿ magnitude of force needed to overcome rolling friction and actuate carriage carrying the baffle is F=4,3N

5. gauge rod

Ÿ serve for determining the baffle end positions

6. baffle with holder

Ÿ replaceable set in case of running coating process

Ÿ total weight in range 35 to 40kg

Ÿ distance between baffle and plate in closed position: 3-5mm

7. contact roll

Ÿ contact with the strip edge, not supplied with end switch; constant downforce in closed position which causes undesired strip edge deformation

8. coating sheet metal

9. control unit

Ÿ PLC SIEMENS 416-2XK04-0AB0

Ÿ CPU 416-2, 2,8MB

Ÿ control of piston movement automatically, based on information about weld position acquired from main PLC, operator inputs available

Ÿ unit is placed in box well isolated from negative outside conditions, near air knives system

Ÿ process only digital signals

The contact system is quick and reliable. The problem is the pressure of contact roll on the strip edges. This downforce damages the edges which need to be cutoff as a scrap at the end of line. Contactless strip edge guiding system for edge baffles is a solution for this problem. It covers replacement of the contact roll by a contactless sensor, replacement of used pneumatic actuator by an electromechanic one and new control algorithm.

2. Contactless Sensor

Necessary parameters and factors for choosing the right contactless sensor are:

Ÿ temperature nearby the sensor is 72°C, in case of emergency up to 100°C,

Ÿ sheet metal thickness: 0,3-2,0mm,

Ÿ possibility of pollution,

Ÿ continuous output signal required for sheet metal monitoring,

Ÿ high reliability, accuracy and fast response,

Ÿ low costs.

Due to the system parameters it is necessary to put the sensors 876 mm above the edge baffles, therefore they will detect the changes of a sheet edge profile with small time deviation. This had to be also considered. From the line datasheets we know that the deviation due to wrong centering of the strip is max. 50mm from both sides of strip sheet. This deviation occurs gradually, not by steps ^[2].

Edge profile discontinuities in the range of ±0,5mm ^[2] can also occur.

Fork shaped inductive sensor BMI 4 produced by EMG ^[3], based on Faraday law was chosen as a most suitable option for the required conditions. Figure 4 shows the model of sensor with its placement on existing roll holder, with the contact rolls left in case of sensor failure.

Figure 3. Continuously changing edge profile: a- increasing deviation, b- decreasing deviation [1]

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Figure 4. Sensor assembly [1]

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The chosen sensor is powered by AC voltage, two shielded cables with the cross-section 2x0,5mm² and two for end switching (4x0, 75mm²) are supplied by the manufacturer and are easily connected by the plugs. Sensor holder for preventing the sensor damage by the moving strip sheet was designed.

3. Positioning

Contactless strip edge guiding system for Galvanizing line requires replacement of using pneumatic piston by an electromechanic actuator. This will enable precise positioning according the continuous output signal from contactless sensor. Input requirements for positioning according the measurements on used system are:

Ÿ downforce: greater than 20N,

Ÿ linear speed: greater than 300mm.s^-1,

Ÿ length of guidance: more than 700mm,

Ÿ precision, reliability, low cost.

Linear stage represents the actuator in which the rotary movement is transformed into a linear one using screw and moving nut. This reduces general actuator dimensions and makes it more compact. Sufficient product is motorized linear stage T-LST1000D from Zaber Company.Main parameters are ^[4]:

Ÿ length of guidance: 1000mm,

Ÿ accuracy: ±125μm,

Ÿ maximum speed: 420mm.s^-1,

Ÿ minimal speed: 0,0186 mm.s^-1,

Ÿ maximum force: 80N,

Ÿ maximum load capacity: 1000N.

Linear stage is powered by stepping motor with 200 steps per revolution. Control unit and the sensor of actual position are integrated into device. High speed movement and very short response enables to follow small strip edge profile deviations detected by the sensor. Figure 5 shows the subassembly consisting of linear stage with the carriage for edge baffles.

Figure 5. Linear stage assembly [1]

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4. Controlling

The most important part of whole contactless strip edge guiding system is a control algorithm. The selection of appropriate control unit has to consider following conditions:

Ÿ capability to receive and process the analog signal from sensor,

Ÿ analog data output for controlling the actuator,

Ÿ active communication with parent PLC in production line.

In the selection it is important to count with the unit protection against outside noise, pollution, temperature and vibrations. Product SPC 16 (strip position controller) from EMG Company is a specially designed control unit for using with sensor BMI4. Main parameters are ^[5]:

Ÿ power supply: 220V, 50Hz,

Ÿ energy consumption: 70VA,

Ÿ range of outside temperature: 0-50°C, unit will be placed in special box situated in a sufficient position ,

Ÿ 8 digital inputs, 6 digital outputs, 5 analog inputs,

Ÿ 16 bit microprocessor.

Replacement of currently used contact system requires completely new control algorithm what will not be possible without new calibrations and testing, therefore it is necessary to use new control unit instead of the old one. Figure 6 shows the description of a logical structure for new system using UML language.

Figure 6. Logical structure in UML language [1]

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5. Final Assembly

The entire conception of contactless strip edge guiding system on Galvanizing line number 3 is based on used contact system from FOEN Company. The contactless solution is designed with an effort for minimum construction changes in existing system. Current pneumatic cylinder and gauge rod need to be replaced in case of using contactless sensors. For placing the sensor cables, the cable carrier was designed according the instructions from Kabelschlepp Company ^[6]. The cable carrier is placed in a tray and connected with existing carriage which moves the edge baffles and will also move contactless sensors Figure 7.

For new system positioning was chosen linear stage T-LST1000D with regards to the performance parameters and also the construction ones. The dimensions of actuator require reduction of one of the carrier guidance. The width of specified profile will decrease by 5mm, which will have no effect on the function and construction of whole system at all.

Placing of proposed cable carrier requires changes of system cover. New conception in detail with appropriate construction drawings was created. By using all selected parts of new contactless system a 3D model of strip edge guiding system was created in Solidworks 2011 Figure 8.

The cross-section view of a system cover with the description is shown in Figure 9.

Figure 7. Cable carrier assembly [1]

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Figure 8. 3D design of contactless strip edge guiding system [1]

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Figure 9. Cross-section of the system cover [1]

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