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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 magnetic 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 propagation of the magnetic fieKd changes so that it surrounds the secondary winding, inducing an alternating 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 GeneralA

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 lowersthe 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.

Unpacking

When the equipment arrives, check against the delivery document. Inform ABB of any complaint, so that errors can be corrected immediately and delays avoided.

Preparations

Prepare the installation in good time by checking that the necessary documents and material are available, as follows:

• Installation drawings and this manual.

• Standard tools, torque wrench and instruments.

• Rust protection, if additional protection is to be given to machined surfaces. Choose TECTYL 511 (Valvoline) or FERRYL (104), for example.

• Load cells, adapter plates, bearing housings, etc.

Cabling for Load Cell PFCL 201CE

Cabling with protective hose shall be mounted so that the forces related to the weight of the cable/ hose do not act in the measuring direction of the load cell. A cable clamp is therefore necessary. If the load cell is prevented from movement in the measuring direction- it will shunt force, and the measured force will differ from the actual.

The favourable direction of the cable/hose is the horizontal direction to the left or right as indicated in Figure 16. Position of cable from factory page 20. This as possible forces in the longitudinal direction of the cable/hose due to temperature, will act perpendicular to the measuring direction of the load cell (the direction in which the load cell is insensitive to loads).

For achievable cable directions, see Figure 17. Possible directions of cable for PFCL 201CE page.

The direction of the cable and protective hose can be changed by unscrewing the two screws in the connection box and turning the cable to a suitable direction. Make sure to re-install the screws in the connection box.

Fault Tracing in the Mechanical Installation

There are a number of parts in the mechanical arrangement that can cause faults. The extent to

which these faults are repeatable differs. Possible causes fall into the following groups.

• Defective mounting surface, stand or adapter plates.

• Force shunting.

• (nsufficHent mounting of load cell and adapter plates.

• Rolls and bearings.

• Driven roll.

Defective Mounting Surface, Support or Adapter Plates

An unmachined or poorly machined mounting surface, which is uneven, may cause bending or twisting of the load cell. This may result in instability of the zero point.

Force Shunting

Force shunting means that some of the force is diverted past the load cell. This may be caused by some kind of obstruction to the force through the load cell. The connecting cables, for example, have been incorrectly installed and are preventing movement. Another possible cause is that the roll is not free to move in the direction of measurement, possibly because something is mounted too close to a bearing housing, or because an object has worked loose and become trapped between the bearing housing and adjacent parts.

Force shunting causes the strip tension indication to be lower than the actual strip tension.

Fastening of Load cell and Adapter Plates

Screw joints that have not been properly tightened or have lost their pre-tightening force, cause sliding at the mating surfaces. Fastening of the load cell is especially critical. If a load cell is not properly secured, the zero point will be unstable. Sliding between other surfaces may cause the same symptoms.

Rolls and Bearings

An incorrectly designed bearing arrangement may give rise to high axial forces. The roll should be

fiWed at one end and free at the other.

If both ends are fiWed there will be a high axial (thrust) force due to expansion of the shaft with

rising temperature.

Even a correctly designed bearing arrangement may deteriorate with time; bearings become worn, and so on. This may give similar symptoms, such as slow zero point drift between cold and hot machine, or sudden jumps in the signal.

Driven Roll

A source of error that is seldom suspected is the roll itself. The effect is especially critical when measuring forces on the load cell are relatively low. Long drive shafts with their associated universal joints may cause unstable signals if they are not properly maintained. It is important to lubricate universal joints. Longitudinal expansion of the drive shaft should also be taken into account. Since such expansion is often taken up by splines, these must also be lubricated. The symptoms are instability of the signal, for instance jumps in the signal during slow running.
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