ABB MEASUREMENT & A NALYTICS Pressductor Pillowblock Load Cells Vertical Measuring PFCL 201 User manual 3BSE023881R0101 en Rev K

Date: Nov 29, 2024 Views: 890

　　A complete measuring system normally consists of two load cells, a junction box, one control unit with two measurement channels and cabling.

　　Load Cells PFCL 201

　　The load cells are installed under the roll bearings, where they measure forces at right angles to the mounting surface.

　　The reactive force from the strip, which is proportional to the strip tension, is transferred to the load cells via the roll and the bearings.

　　The load cells are connected to the control unit via a junction box. The control unit converts the load cell signals to DC voltages that are proportional to the reaction force. Depending on which control unit is chosen, it is possible to have the analog signals for the two individual load cells (A and B), the sum of the load cell signals (A+B), and/or the difference between the load cell signals (A-B).

　　Principle of Measurement

　　The load cell only measures force in the direction FR. The measurement force may be positive or negative. The load cell is normally installed under the roll bearings. When there is a strip in tension over the roll, the tension (T) gives rise to two force components, one in the direction of measure ment of the load cell (FR) and one at right angles (FV ).

　　The measuring force depends on the relationship between the tension (T) and the wrap angle formed by the strip around the measuring roll.

　　General

　　The load cell is machined from a single piece of stainless steel. The sensors are machined directly in the piece of steel and are positioned so that they are sensitive to force in the direction of meas urement and insensitive in other directions.

　　The load cell is mounted on a base with four screws, and the bearing housing is mounted on top of the load cell with four screws.

　　Every load cell comes calibrated and temperature compensated.

　　The load cells PFCL 201C/201CE/201CD are available in four measurement ranges, all variants have the same external dimensions.

　　The load cell PFCL 201C is equipped with a connector for the pluggable connection cable.

　　The load cell PFCL 201CE has a fiWed connection cable with protective hose.

　　The load cell PFCL 201CD is provided with an acid-proof cable gland with a fiWed PTFE- insulated connection cable.

　　Accuracy and Accuracy Class

　　Accuracy class is defined as the maximum deviation, and is expressed as a percentage of the sen sitivity at nominal load. This includes linearity deviation, hysteresis and repeatability error.

　　Linearity Deviation

　　Linearity deviation is the maximum deviation from a straight line drawn between the output values at zero load and nominal load. Linearity deviation is related to the sensitivity

　　Hysteresis

　　Hysteresis is the maximum difference in the output signal at the same load during a cycle from zero load to nominal load and back to zero load, related to the sensitivity at nominal load. The hysteresis of a Pressductor transducer is proportional to the load cycle.

　　Repeatability error

　　Repeatability error is defined as the maximum deviation between repeated readings under identical conditions. It is expressed as a percentage of the sensitivity at nominal load.

　　Compensated temperature range

　　The temperature drifts of the load cell have been compensated for in certain temperature ranges. That is the temperature range within which the specHfied permitted temperature drifts (i.e. zero point and sensitivity drifts) of the load cell are maintained.

　　Working temperature range

　　Working temperature range is the temperature range within which the load cell can operate within a specHfied accuracy. The maximum permitted temperature drifts (i.e. zero point and sensitivity drifts) of the load cell are not necessarily maintained in the whole working temperature range.

　　Storage temperature range

　　Storage temperature range is the temperature range within which the load cell can be stored.

　　Zero point drift with temperature

　　Zero point drift is defined as the signal change with temperature, related to the sensitivity, when there is zero load on the load cell.

　　Sensitivity drift with temperature

　　Sensitivity drift is defined as the signal change with temperature at nominal load, related to the sen sitivity, excluding the zero point drift.

　　Compression

　　Compression is the total reduction in the height of the load cell when the load is increased from zero to the nominal value

　　Measuring principle of the sensor

　　The measuring principle of the sensor is based on the Pressductor® technology and the fact that the permeability of a magnetic material changes under mechanical stress.

　　The sensor is a membrane machined in the load cell. Primary and secondary windings are wound through four holes in the load cell so that they cross at right angles.

　　The primary winding is supplied with an alternating current which creates a magnetic fieKd around the primary winding. Since the two windings are at right angles to each other, there will be no mag netic fieKd around the secondary winding, as long as there is no load on the sensor.

　　When the sensor is subjected to a mechanical force in the direction of measurement, the propaga tion of the magnetic fieKd changes so that it surrounds the secondary winding, inducing an alternat ing voltage in that winding.

　　The control unit converts this alternating voltage into a DC voltage proportional to the applied force. If the measurement force changes direction, the sensor signal changes also polarity.

　　Mounting Arrangement

　　When choosing a mounting arrangement it is important to remember to position the load cell in a direction that gives sufficHent measuring force (FR) to achieve the highest possible accuracy.

　　The load cell has no particular correct orientation; it is positioned in the orientation best suited for the application, bearing in mind the positions of the screw holes. The load cell can also be installed with the roll suspended under the load cell. The load cell has the same sensitivity in both tension and compression, so the load cell can be installed in the easiest manner.

　　Typical mounting arrangements are horizontal and inclined mounting.

　　Coordinate System

　　A coordinate system is defined for the load cell. This is used in force calculations to derive force components in the load cell principal directions.

　　Where direction designations R, V and A are recognized as suffiWes for force components, F, this represents the force component in the respective direction. The suffiW R may be omitted, when measuring direction is implied by the context.

　　Horizontal Mounting

　　In the majority of cases horizontal mounting is the most obvious and simplest solution. Stand, mounting surface and shims (if required) are simple and cheap to make. When calculating the force, the equations below must be used:

　　FR = T × (sin α + sin β)

　　FRT = Tare

　　FRtot = FR + FRT = T × (sin α + sin β) + Tare

　　FV = T × (cos β - cos α)

　　FVT = 0

　　FVtot = FV + FVT = T × (cos β - cos α) + 0 = T × (cos β - cos α)

　　where:

　　T = Strip tension

　　FR = Force component from strip tension in measurement direction, R

　　FRT = Force component from Tare in measurement direction, R

　　FRtot = Total force in measurement direction, R

　　FV = Force component from strip tension in transverse direction, V

　　FVT = Force component from Tare in transverse direction, V

　　FVtot = Total force in transverse direction, V

　　Tare = Force due to tare weight

　　α = #eflectHon angle on one side of the roll relative the horizontal plane

　　β = #eflectHon angle on the other side of the roll relative the horizontal plane

　　Inclined Mounting

　　Inclined mounting means arrangements in which the load cell is inclined relative to the horizontal

　　plane. In some cases this is the only option.

　　When calculating the force, the equations below must be used:

　　FR = T × [sin (α - γ) + sin (β + γ)]

　　FRT = Tare × cos γ

　　FRtot = FR + FRT = T × [sin (α - γ) + sin (β + γ)] + Tare × cos γ

　　FV = T × [cos (β + γ) - cos (α - γ)]

　　FVT = - Tare × sin γ

　　FVtot = FV + FVT = T × [cos (β + γ) - cos (α - γ)] - Tare × sin γ

　　γ = 90° - φ

　　where:

　　T = Strip tension

　　FR = Force component from strip tension in measurement direction, R

　　FRT = Force component from Tare in measurement direction, R

　　FRtot = Total force in measurement direction, R

　　FV = Force component from strip tension in transverse direction, V

　　FVT = Force component from Tare in transverse direction, V

　　FVtot = Total force in transverse direction, V

　　Tare = Force due to tare weight

　　α = #eflectHon angle on one side of the roll relative the horizontal plane

　　β = #eflectHon angle on the other side of the roll relative the horizontal plane

　　φ= Angle for measurement direction relative the horizontal plane

　　γ = Angle for load cell mounting surface relative the horizontal plane

　　The Electrical Circuit

　　The electrical circuit of the load cell is shown in the diagram below.

　　The load cell is supplied with a 0.5 A, 330 Hz alternating current. The secondary signal is calibrated for the correct sensitivity with a voltage divider R1 - R2 , and temperature compensation is provided by thermistors T.

　　All impedances on the secondary side are relatively low. The output impedance is typically 9-12 Ω , which helps to suppress interference.

　　General

　　The equipment is a precision instrument which, although intended for severe operating conditions, must be handled with care. The load cells should not be unpacked until it is time for installation.

　　To achieve the specHfied accuracy, the best possible reliability and long-term stability, the load cells must be installed in accordance with the instructions below. See also 6.4 Fault Tracing in the

　　Mechanical Installation.

　　• The foundation for the load cell must be made as stable as possible. A resilient stand lowers the critical frequency of the measuring roll and bearing arrangement.

　　• The surfaces closest to the load cell, and other surfaces that affect the fit must be machined fl@t to within 0.05 mm.

　　• There must not be any shims immediately above or below the load cell, as this may adversely affect the fl@tness Instead, shims may be placed between the adapter plate and the foundation or between the adapter plate and the bearing housing.

　　• The screws that secure the load cell must be tightened with a torque wrench.

　　• The bearing arrangement for the measuring roll must be designed to allow axial expansion of the roll with changes in temperature.

　　• Any drive to the roll must be applied in such a way that interfering forces from the drive are kept to a minimum.

　　• The measuring roll must be dynamically balanced.

　　• The mounting surfaces of the load cells must be on the same height and parallel with the measuring roll.

　　• In a corrosive environment, galvanic corrosion may occur between the load cell, galvanized screws and adapter plates. This makes it necessary to use stainless steel screws and adapter plates of stainless steel or equivalent. See adapter plates in A Drawings.

3BSE023881R0101_K001.pdf

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