Technical articles
Pressductor Pillowblock Load Cells Vertical Measuring PFCL 201
Date: May 14, 2024 Views: 96
Function and Design
General
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 measurement 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.
Description
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 measurement 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.
Definitions
Nominal load
Nominal load, Fnom, is the maximum load in the measurement direction for which the load cell is dimensioned to measure within the specHfied accuracy class. The load cell is calibrated up to Fnom.
Sensitivity
Sensitivity is defined as the difference in output values between nominal load and zero load.
Accuracy and Accuracy Class
Accuracy class is defined as the maximum deviation, and is expressed as a percentage of the sensitivity 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 sensitivity, 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 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.
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