Process and device for measuring slice gap spacing at a paper machine headbox

The invention refers to a process for measuring spacing at a paper machine headbox. It is primarily characterized by constant magnetic fields being generated at the headbox slice gap area, the intensity of these fields being measured and these measurements being used to determine the gap between the individual lips. The invention also refers to a measuring device to determine spacing in the headbox 1 area of a paper machine, where a magnetic field generator 10 for a constant magnetic field is provided in one of the two elements 11 that determine the gap and a measuring device for magnetic fields 9 is contained in the second of the two elements 7, 8 determining the gap.

BACKGROUND OF THE INVENTION
 The present invention relates to a process and a device for measuring
 spacing at a paper machine headbox.
 The water content and thus, the consistency of the pulp mixture in the
 headbox of a paper machine are normally set at the headbox feed by
 altering the slice gap. In a multi-layer headbox, measuring the slice gap
 is even more important because it means that dewatering behavior in the
 wire section can be controlled even more efficiently for multi-layer
 operations. The following methods of measuring, all of which take
 measurements indirectly because of the prevailing difficulties, are
 already known for the tissue machine: (a) measuring at the lip adjustment
 drive, without taking account of any deformation of the thin lips, which
 are subject to enormous loads because of the high pressures in the
 headbox, and (b) measuring using thin, mechanical levers which either
 relay the position of the lip back out of the slice gap area or through
 the side wall to the outside. These levers are complicated to install and
 the results of measurements taken correspondingly unreliable.
 SUMMARY OF THE INVENTION
 The object of the invention is to provide a measuring method which measures
 the slice gap with the greatest possible accuracy.
 The invention is thus characterized by constant magnetic fields being
 generated at the headbox slice gap area, the intensity of these fields
 being measured and these measurements being used to determine the gap
 between the individual lips. In this way, the consistencies in the
 constant part to the paper machine can be determined exactly, thus
 fulfilling the requirements for exact control of a multi-layer headbox
 (top layer, intermediate layer, back layer). The pulp quantities in the
 individual layers can be set precisely at the headbox area.
 An advantageous further development of the invention is characterized by
 the magnetic field being generated by a permanent magnet. With this
 arrangement, stainless steel walls can also be penetrated without the
 magnetic field being affected. This penetration is particularly important
 because the lips must be smooth, i.e., not have any irregularities, on the
 side facing the pulp. Thus the measurement can be taken through this
 uniform layer. Conventional inductive gap sensors or even eddy current
 sensors cannot penetrate through stainless steel walls.
 A favorable configuration of the invention is characterized by a Hall
 generator being used to measure the magnetic field. These are particularly
 suitable for installation in the thin lips because of their small physical
 dimensions.
 A favorable further development of the invention is characterized by
 connecting non-linear electrical elements to the signal output of the
 magnetic field sensor in order to linearize the measuring result. These
 non-linear elements have a non-linear characteristic curve which is
 inverse to the characteristic curve of the magnetic field. Thus, where is
 no need to linearize the magnetic field.
 An advantageous configuration of the invention is characterized by
 non-linear signals from a transducer being converted into a linear signal
 using a data table. This data table can be adapted very easily to the
 appropriate characteristic curve of the magnetic field. Any non-linearity
 from the Hall generator or the signal processing can also be taken into
 account in this way.
 An alternative configuration of the invention is characterized by
 non-linear signals from a transducer being converted using a data table
 into a signal which is an intended non-linear signal. One of the
 advantages of this is that small gaps can be measured with greater
 accuracy and larger gaps with less accuracy. The relative accuracy
 (resolution/current signal level) could be kept at a constant level, for
 example, over the entire measuring range.
 The invention also refers to a measuring device to determine spacing in the
 headbox area of a paper machine. It is characterized by a magnetic field
 generator for a constant magnetic field being provided in one of the two
 elements that determine the gap and a measuring device for magnetic fields
 being contained in the second of the two elements determining the gap.
 This type of measuring equipment is particularly easy to install in the
 thin lips of a headbox.
 A favorable further development of the invention is characterized by a
 permanent magnet being provided as the source of the magnetic field; as an
 alternative, a current coil can also be provided. The permanent magnet can
 operate without an electrical supply voltage and, therefore, does not
 require any type of cable connection to the remaining parts of the
 measuring set-up. A current coil can generate a particularly strong
 magnetic field which is constant, even at extreme temperature
 fluctuations.
 An advantageous configuration of the invention is characterized by a Hall
 generator being provided as a device to measure magnetic fields. This is
 particularly easy to accommodate in the headbox lips thanks to its small
 physical dimensions.
 An advantageous further development of the invention is characterized by
 the source of the magnetic field, particularly the permanent magnet, being
 located in the center lip of a two-layer headbox with center lip, and a
 measuring device for magnetic fields being placed in each outer lip (top
 and bottom). Thus, the gaps between the top and the center lip and between
 the center and the bottom lip can be measured particularly well.
 A particularly favorable further development of the invention for a
 multi-layer headbox with several intermediate lips is characterized by the
 sources of the magnetic fields, particularly the permanent magnets, being
 installed in an offset arrangement in the intermediate lips of a
 multi-layer headbox and one pair of magnetic field measuring devices being
 provided for each magnetic field and placed in each outer lip (top and
 bottom). With this offset arrangement, the position of the individual
 intermediate lips between the top and bottom lips can be determined
 exactly, and thus, the gap between the intermediate lips can be calculated
 from the differences in the spacing. In this way, all of the gaps are
 registered in order to determine the individual material flows.
 An advantageous configuration of the invention is characterized by the
 magnets being arranged such that they generate a linear magnetic field.
 The signal from the magnetic field detectors can then be used as a
 measuring signal without any further processing.
 A favorable further development is characterized by the magnetic field
 measuring devices being connected to a microprocessor. In this way, the
 signals from the measuring transducer can be converted in a particularly
 favorable manner into the linear or non-linear signals required for
 further processing.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 In FIG. 1, a two-layer headbox 1 for a tissue machine is shown. The paper
 web is formed here between two rolls 2, 3, which have wire cloths 4,4'.
 There is a gap 5 between these wires 4,4' running over the rolls 2,3 and
 the suspension supplied to the headbox 1 through feed pipes 6,6' is
 injected through nozzles into this gap 5. The speed of the wires 4,4' and
 thus, the required speed of the suspension injected into the gap 5 can be
 up to approximately 2000 m/min.
 The area over which the suspension enters into the gap 5 between the wires
 4,4' on the rolls 2,3 is shown in the detailed drawing in FIG. 2. A
 magnetic field receiver 9 is installed in the moveable top lip 7 and
 moveable bottom lip 8, respectively, and a magnetic field transmitter 10
 is installed in the intermediate lip 11. The transmitter 10 generates a
 reference magnetic field of constant intensity, which depending on the lip
 material, can be influenced by the movement of lip 7 or 8. The intensity
 of the transmitter's 10 magnetic field depends on the measurement position
 within the field. The signal from the magnetic field receiver 9
 corresponds to the magnetic field received and thus, also depends on the
 position, particularly the distance separating it and the magnetic field
 transmitter 10. As a result, the signal from the magnetic field receiver 9
 is commensurate with, and can be used as a measure of, the distance
 separating the intermediate lip 11 and the top lip 7 or bottom lip 8. As a
 general rule, there is normally no linear connection between the distance
 separating the transmitter 10 and the receiver 9, however, there is a
 clear relationship in that a certain signal corresponds to a certain
 intensity of magnetic field and thus, to a certain distance.
 Taking account of this special characteristic, where the intensity of the
 magnetic field depends on its location, a subsequent microprocessor (not
 shown) can be used to linearize the characteristic curves. This
 linearization generates, for example, a value for the actual spacing by
 means of a data table, using the magnetic field receiver signal in each
 case. The output can take the form of a display or also a normalized
 current signal, e.g., 4 . . . 20 mA. The current signal is particularly
 suitable for further processing in a process control system.
 FIG. 3 shows a view from the wire section looking towards the lips in a
 three-layer headbox. In order to measure the position of two intermediate
 lips 12, 13, two measuring arrangement set-ups are installed adjacent to
 one another, but with sufficient space in between to prevent the signals
 from interfering with one another. The measuring arrangement 14 on the
 left measures the position of the lower intermediate lip 13 and the
 measuring arrangement 14' on the right measures the position of the upper
 intermediate lip 12. These signals can also be used to determine the space
 between the two intermediate lips 12, 13 from the difference calculated.
 The measuring arrangements 14, 14' are structured in the same way as the
 measuring arrangements for a two-layer headbox and each comprises two
 magnetic field receivers 9 and one transmitter 10.
 FIG. 4 shows a perspective of lips 7, 8, 12, 13 and of the built-in in
 measuring arrangements 14, 14' for a three-layer headbox. If the
 transmitters 10 are permanent magnets, no electrical connection to a power
 source is required. The receivers 9 are electrically connected via
 conductors 15 to non-linear electrical elements 16, for linearizing the
 receiver output signals. A microprocessor based display unit 17 generates
 a visual indication of the gaps, and can deliver gap width data to a
 controller (not shown). In an alternative embodiment, linearization can be
 achieved by a "look up" data table or other digital techniques, in unit
 17.
 If the gap between the individual headbox lips is to be determined across
 the running direction of the web in order to take account of lip
 deflection, several measuring arrangements can also be installed across
 the web width. This is possible both in a two-layer and in a three-layer
 headbox.