GB2084735A

GB2084735A - Device for measuring layer thickness

Info

Publication number: GB2084735A
Application number: GB8127684A
Authority: GB
Original assignee: Buehler AG
Current assignee: Buehler AG
Priority date: 1980-09-15
Filing date: 1981-09-14
Publication date: 1982-04-15
Also published as: JPS5779405A; DE3039532C2; FR2490334B1; DE3039532A1; GB2084735B; CH650077A5; FR2490334A1

Abstract

A device for measuring the thickness of a layer (13) on a moving support surface, e.g. a roller (11) comprises first and second gauges (24,23) which may be electrically and air operated respectively, mounted together on a movable jointed frame or mount (19) (20) for adjustable positioning above the support surface (11), the gauges (24) (23) being adapted to produce by noncontact methods first and second signals proportional respectively to the distance above the moving support surface (11) and to the distance from the top surface of the layer (13). The second signal is fed to an air amplifier (44) and thereafter to a transducer or air cylinder (49) to produce a force acting on a membrane 50 and the lever (19) to change the pivotal position and the height of the mount and the two gauges (23,24) above the support surface, so that the position of the mount (20) is kept at a constant height above the top surface of layer (13), the first signal from 24 after balancing being arranged to vary in dependence on the layer thickness. The device is useful for controlling a roller mill for grinding chocolate or the like. <IMAGE>

Description

SPECIFICATION Device for measuring layer thickness The invention relates to a device for measuring layer thickness, in particular of the product layer on a roller of a roller mill for fine grinding cocoa, chocolate, pigments, soap and other masses of similar consistency.

Efforts are long known to have been made to allow for the product layer when adjusting, regulating or feedback controlling roller mills. An optical device disclosed in DE-PS 1 204052 has proved particularly successful and has become widely known. However, this device is concerned only with detecting whether or not the product layer is present or absent, the purpose being to quickly stop the roller mill in the absence of any product layer. The device thus represents a safety measure against dry running.

Later solutions to the same problem possess various disadvantages. The device according to DD-PS 53 531 provides for wipers, which in normal operation bear against the product layer, in the absence of such layer, on the other hand, against the outer surface of the roller, so as to provide a stop signal for the roller mill. As time goes on an unwanted product layer deposits on the wiper itself and impairs its proper functioning.

The device according to DE-OS 1 607618 utilizes a load change of the drive motor as a stop signal for the roller mill. This signal, however, is not very accurate. It fails to measure the layer thickness itself and is subject to frictional disturbances in the bearing and the drive and to varying values of mass viscosity and toughness.

The device according to DD-PS 81 305 comprises a flap provided on its feed chute, which responds to the presence or absence of product in the chute. The roller mill is stopped if no product is present. With this device, too, the measurement is indirect and fails to determine the thickness of the product layer on the roller itself.

A common feature of the solutions discussed so far is that they only control the stopping operation of the roller mill. An attempt has been made in DE-PS 1 218854 to measure the thickness of the product layer, the purpose being to control the gap between the rollers. A feeler roller is pressed against the product layer and its movements are amplified by means of a linkage mechanism with the purpose of feeding the control device actual values. Since a mechanism of this kind works inaccurately, a proposal was made in the supplementary patent application DE-OS 1 507 583 to transmit the feeler roller movements by electrical means. We do not know, whether or not this design has in fact been used in practice.In spite of the great progress involved, from a theoretical point of view, in this proposal (determining the thickness of the product layer allows a direct inference regarding the fineness of the product), this solution has not been accepted by the technical world.

Another device for the non-contact measurement of the thickness of a web travelling on a metallic roller is disclosed in GB-PS 1 188 083. A sensing head movably suspended on levers and balanced out by balancing weights is supplied with air at constant pressure. The air escapes through the space between the periphery of the sensing head and the web, thus holding the sensing head at constant distance from the web. The sensing head fixedly supports an electrical distance measuring probe, the distance of which from the roller being a measure of the thickness of the web. The air acts directly upon the sensing head, and any deviations from the desired distance between the sensing head and the web result in that the small pressure changes caused thereby directly effect the entrainment of relatively large masses. Accuracy and sensitivity are not very high.They do not suffice for a five-roller mill for chocolate. In a five-roller mill of this kind the product layer thickness may be as high as 45Fm at the second roller and may progressively decrease to 13m atthefifth roller.

The device according to DE-OS 26 09 387 comprises a throttle feeding a blast nozzle mounted together with an electrical distance measuring probe on a common movable support. A pneumatic piston pushes against the force of a spring in the direction of an extruded plastic tube. This tube is arranged to slide on a calibration plug fixedly mounted on the extruder. The exit pressure of the blast nozzle is transmitted by way of an amplifier to the pneumatic piston, so that the distance of the blast nozzle and of the electrical distance measuring probe to the outer surface of the tube remains constant. Thus the distance measured by the electrical probe to the outer surface of the calibration plug represents a measure of the thickness of the tube.The blast nozzle is in itself a very accurate distance measuring probe, however, the accuracy of the entire device is adversely affected by the hysteresis resulting from piston friction. The application of the device disclosed in DE OS 2609387 is thus limited, and it is not suitable for the type of roller mills referred to in the introduction.

It is an object of the present invention to provide an improved device, which can be made sufficiently reliable and accurate for the needs arising in practice with roller mills for fine grinding cocoa, chocolate, pigments, soap and other masses of similar consistency.

According to the invention there is provided a device for measuring the thickness of a layer on a moving support surface comprising first and second gauges mounted together on a movable mount for positioning above the support surface, the mount being linked to a lever so that the position of the mount above the support surface depends on the pivotal position of the lever, the first gauge being adapted to produce by a non-contact method a first signal proportional to its distance above the moving support surface and the second gauge being adapted to produce by a non-contact method a second signal proportional to its distance from the top surface of the layer which second signal is fed to an amplifier and thereafter to a transducer to produce a force acting on the lever to change its pivotal position and thereby the height of the mount and the two gauges above the support surface. The effect of friction acting upon a lever of this kind is relatively small, particularly because the friction forces exert their action at the relatively small radius of its pivot pin. As a result, the frictional turning moments of perturbing nature which arise are very small compared to the turning moments resulting from the adjusting forces to be balanced out.Moreover, the effect of this frictional turning moment can be further reduced by employing low-friction orfrictionless articulations as used for balances.

The lever is also conveniently acted on by a spring bias restoring force which together with the force applied to the lever by the transducer serves to position the lever in a position dependant on the second signal. Apart from allowing perturbing frictional forces to be reduced to a minimum, the provision of a lever on which forces due the transducer, the spring bias and the link to the mounting can act at selected distances from the pivot, provide for a device whose response to the second signal produced by the second gauge can be precisely determined. In particular the sytem can be designed to keep the distance of the mount and the two gauges substantially constant above the top surface of the layer without frictional forces undesirably delaying response to changes in layer thickness or causing hysteresis effects.This means that the first signal produced by the first gauge will give a measure of changes in the layer thickness precise eriough to be of use in controlling a roller mill to produce the desired layer thickness.

According to a preferred embodiment of the invention there is provided a device for measuring the thickness of the product layer on a roller of a roller mill for fine grinding cocoa, chocolate, pigments and other masses of similar consistency, comprising two non-contact distance measuring devices fixedly mounted on a common movable support namely: (a) a pneumatic distance measuring device for measuring the distance to the surface of the product layer, said device being constructed in the form of a pneumatic blast nozzle having its exiting jet directed against the outer surface of the product layer and provided with a signal output for the pressure prevailing therein and dependent upon the distance of the nozzle from the outer surface of the product layer, said signal output being arranged to act by way of an amplifier upon a device for positioning said movable support; and (b) an electrical distance measuring device for measuring the distance to the metallic surface of the roller, wherein the movable support is carried by a lever, said lever being acted upon by the positioning device constructed in the form of a pneumatic lifter.

By arranging the lever to run essentially horizontally and, in particular, parallel to the axis of the roller of a roller mill, it becomes possible to choose the direction of movement of the distance measuring devices to run essentially perpendicular to the roller axis.

Each distance measuring probe remains parallel to itself if the movable support is a member of a parallelogram linkage connected by way of the aforedescribed and an additional lever to the fixed member mounted on the frame of the roller mill, preferably by way of a base plate.

The design of the device becomes particularly simple if the movable support is pulled toward the roller by reason of its own weight. Such is the case, too, if a spring is arranged to act upon the movable support to pull the same toward the roller. A combination of the two forces is frequently made use of for the purpose in actual practice. The spring makes it possible to mount the device instead of above the roller, on the side, to make the lever move in a horizontal plane.

Particularly good accuracy and very short re sponsetimes may be obtained by having the amplifier function in an essentially digital or stepwise manner with the blast nozzle.

The operation of the roller mill becomes particularly uniform if the electrical distance measuring probe is operationally connected with the control device of the roller mill. Specifically, the grinding gap located before the measuring point is controlled between the two rollers preceding the measuring point. Interpreting the measurement may be simplified by means of a zero-point shift circuit.

The drawing shows an exemplary embodiment of the invention in schematic form.

Figure 1 shows a diagram of the device together with information relating to the spatial arrangement of the mechanical and the electrical parts thereof; Figure 2 shows a sectional view of the blast nozzle shown in Figure 1 as pneumatic measuring element, art a largerscale; and Figure 3 shows a sectional view of the amplifier shown in Figure 1 at a larger scale.

The roller 11 of a roller mill appears in Figure 1. It consists of a metallic material, in particular of a cast iron known as chilled cast iron. It comprises an axial bore 12 used for cooling, most frequently by circulat- ing a cooling liquid through it. A product layer 13 is shown on the outer surface of the roller 11; this layer originated as the product was pulled-in through the gap provided between the two rollers, while at the same time intensive grinding took place therein. As is known, the individual rollers of a roller mill always rotate at a larger peripheral speed than the rollers immediately preceding them.

Afive-roller roller mill for chocolate, for example, may have the thickness of the product layer at the second roller amount to approximately 45 cm. This thickness progressively decreases to approximately 13 ism atthefifth roller.

A base plate 15 for the measuring device is fixedly mounted on the roller mill frame not shown in the drawing. The base plate 15 carries the fixed member 16 of a parallelogram linkage 17, which has a movable support 20 connected to its horizontal levers 18, 19 running essentially parallel to the axis of the roller 11. The movable support 20 carries a stop 21. Atension spring 22 is provided between the lever 19 and the base plate 15.

A blast nozzle 23 and an inductive distance measuring probe 24 are fixedly mounted on the support 20 in the vicinity of the roller 11. The blast nozzle 23 is connected to a blower 30 by way of the conduits 27, 28 and a pressure regulating valve 29.

The details of the blast nozzle 23 are shown in Figure 2. The bore 33 serves as connector to the conduit 28. This bore 33 is connected by way of a throttling bore 34 with the bore 35 which leads to the blast orifice 36. The cross-sectional area of the bore 35 widens suddenly as compared to that of the throttling bore 34. Another bore 37 communicates with the bore 35 and leads to a lateral signal output 38. A conduit 41 leads from the signal output 38 to a pressure limiting valve 42, the outlet of which is connected by way of a conduit 43 to an amplifier 44.

The amplifier 44 receives pressure from the pressure regulating valve 29 by way of the conduit 45. A conduit 46 extends from the amplifier 44 to an adjustable throttling valve 47 connected at its output end by way of the conduit 48 to a pneumatic lifter 49.

The lifter 49 consists of a container 51 closed off by a membrane 50. The membrane 50 transmits the pneumatic force to the lever 19 of the parallelogram linkage 17.

The details of the amplifier 44 are shown in Figure 3. The housing 52 comprises a bore 53 to be connected to the conduit 45 for receiving the system pressure. The bore 53 opens into a bore 54 connected at its one end with the conduit 46 leading to the throttle valve 47 and comprising at its other end a valve seat 55, which accommodates a control membrane 56 fixedly mounted within the housing 52. The control membrane 56 defines, on one hand, an exhaust air chamber 57 provided with an exhaust bore 58 and, on the other hand, a control chamber 59 provided with a control bore 60 intended to be connected with the conduit 41 for receiving the measuring pressure.

The inductive distance measuring probe 24 is connected to the measurement amplifier 62 by way of the conductor 61. The measurement amplifier 62 is connected at its output end to a zero-point shift circuit 64 by way of the conductor 63. A conductor 65 leads from the zero-point shift circuit 64 to a visual display 66 and the conductor 67 to the control device 71 of the roller mill.

During operation the pressurized air supplied by the blower 30 is regulated at the pressure regulating valve 29 to a valve of 0.2 bar gage pressure. The pressurized air passes through the conduit 28, the bore 33, the throttling bore 34 and the bore 35, to reach the blast orifice 36 in a way to direct the exiting jet againt the outer surface of the product layer 13.

The location of the bore 37 in the very proximity of the transition from the throttling bore 34 to the bore 35 causes a measuring pressure at the signal output 38, which may be positive or negative, depending upon the distance between the blast orifice 36 and the product layer 13.

When the blast nozzle 23 approaches the product layer 13 the measuring pressure which passes to the amplifier 44 through the conduit 41 and the pressure limiting valve 42, increases. This measuring pressure keeps the membrane 56 closed, so that the amplifier 44 transmits the full system pressure it receives from the pressure regulating valve 29 through the conduit 45 to the container 51 of the lifter 49, by way of the conduit 46, the throttling valve 47 and the conduit 48.

The force generated by the pressure acting on the membrane 50 overcomes the force resulting, on one side, from the combined weight of the movable support 20 and the parallelogram linkage 17 and, on the other side, from the spring force of the tension spring 22. The movable support 20 moves the blast nozzle 23 away from the product layer 13 and causes the membrane 56 to move away from the valve seat 55 of the amplifier, provided that the measuring pressure at the signal output is negative. The amplifier 44 discharges both the pressurized air from the system and the pressurized air from the container 51 by way of the exhaust chamber 57 and the exhaust bore 58. The pneumatic force of the lifting device 40 is overcome by the weight and the spring force, and the movable support 20 moves the blast nozzle nearer the product layer 13.The pressure at the signal output thus keeps mounting, with the result that the blast nozzle is again moved away from the product layer 13. This game is repeated at high frequency and small amplitude, so that the distance between the blast nozzle 23 and the product layer 13 oscillates with very small amplitude around a constant value.

The following forces act upon the membrane 56 of the amplifier 44: first, the pressure from the signal output 38 integrated over the area of the control membrane 56 and second, the system pressure in the bore 54 integrated over the area abutting against the control membrane 56.

Atmospheric pressure prevails in chamber 57. The system pressure could be in the order of magnitude of 0.2 bar gage. Since the area of the valve seat 55 is much smaller than the area of the membrane 56 it follows that the total pressure acting on the membrane 56 and resulting from the pressure on the side of the valve seat 55 and the pressure in the chamber 57 lies but very little above atmospheric.

The membrane 56 closes (or opens, as the case may be) the bore 54 at a pressure prevailing at the signal output 38 of only slightly above (or below) the atmospheric pressure, and thus the amplifier 44 functions in digital manner.

The measuring pressure prevailing at the signal output 38 is very accurately dependent upon the distance of the blast orifice 36 from the outer surface of the product layer 13, so that the blast orifice 23 functions as a non-contact distance measuring device. In no case will the pressure of the exiting jet disturb the product layer 13 on the rotating roller 11.

The lifter 49 functions as a positioning device for the movable support 20. The throttling valve 47 attenuates the control movements, with the result, that no appreciable fluctuations will arise, even if the product layer 13 suddenly disappears. If the blast nozzle 23 comes too close to the product layer 13, the pressure limiting valve 42 will blow out the air coming from the signal output 38, thus protecting the movable parts of the amplifier 44 from damage.

The stop 21 prevents the blast nozzle 23 from making contact with the product layer 13.

The inductive distance measuring probe 24 measures its own distance from the metallic surface of the roller 11. The corresponding measuring signal is fed by way of the conductor 61 to the measurement amplifier 62, which transfers the distance value to the zero-point shift circuit 64 by way of the conductor 63. This circuit 64 is set before the roller mill receives any product. The jet exiting from the blast nozzle 23 impinges upon the bare outer surface of the roller 11, whereas the previously described pneumatic control holds the blast nozzle 23 at a practically constant desired distance value from the outer surface of the roller 11.The distance value measured by the distance measuring probe 24 to the bare outer surface of the roller and fed by way of the conductor 63 to the measurement amplifier 62 is balanced out at the zero-point shift circuit 64, so that the visual display 66 will show zero. When balancing the circuit, the stop 21 must be removed. For this reason it is of advantage to provide it adjustable.

Applying a product layer 13 to the roller 11 causes the blast nozzle 23 and together with it the distance measuring probe 24to move away a distance equal to the thickness of the layer. In this way, the visual display 66 will show the actual thickness value of the layer. This same value is fed by way of the conductor 67 to the control device 71 of the roller mill.

The control device 71 of the roller mill may assume one of many various forms. It may meet an increase in the thickness of the product layer beyond its desired value by decreasing the width of the gap between rollers. Or, it may increase the forces pressing the rollers against one another. If the thickness value of the product layer is too small, and particularly if it is zero, the control device may, in analogy to a device used heretofore for protection against dry operation, stop the drive of the rollers and cause a movement of the rollers away from each other.

Such use of the product layer thickness measurement is particularly suited for controlling the roller gap width by changing the roller temperature, as described in the German patent application P30 16 786.3. A change in temperature while maintaining constant the force pressing the rollers toward each other causes a change in the viscosity of the product with a resulting change in the roller gap width.

Various othertypes of non-contact measuring devices may be provided. Thus for example, the blast nozzle 23 may be replaced by an optical distance measuring device. Instead of using the inductive measuring probe 24 one could use a distance measuring probe based upon a magnetic principle.

Claims

1. A device for measuring the thickness of a layer on a moving support surface comprising first and second gauges mounted together on a movable mount for positioning above the support surface, the mount being linked to a lever so that the position of the mount above the support surface depends on the pivotal position of the lever, the first gauge being adapted to produce by a non-contact method a first signal proportional to its distance above the moving support surface and the second gauge being adapted to produce by a non-contact method a second signal proportional to its distance from the top surface of the layer which second signal is fed to an amplifier and thereafter to a transducer to produce a force acting on the lever to change its pivotal position and thereby the height of the mount and the two gauges above the support surface.

2. A device for measuring the thickness of the product layer on a roller of a roller mill for fine grinding cocoa, chocolate, pigments and other masses of similar consistency, comprising two noncontact distance measuring devices fixedly mounted on a common movable support namely: (a) a pneumatic distance measuring device for measuring the distance to the surface of the product layer, said device being constructed in the form of a pneumatic blast nozzle having its exiting jet directed against the outer surface of the product layer and provided with a signal output for the pressure prevailing therein and dependent upon the distance of the nozzle from the outer surface of the product layer, said signal output being arranged to act by way of an amplifier upon a device for positioning said movable support; and (b) an electrical distance measuring device for measuring the distance to the metallic surface of the roller, wherein the movable support is carried by a lever, said lever being acted upon by the positioning device constructed in the form of a pneumatic lifter.

3. A device as claimed in Claim 2, wherein said lever is arranged to run essentially horizontally.

4. A device as claimed in Claim 3, wherein said lever is arranged to run essentially parallel to the axis of the roller.

5. A device as claimed in Claim 2, wherein said movable support is a member of a parallelogram linkage and is connected by way of said lever and an additional lever to the fixed member fixedly mounted to the frame of the roller mill, preferably by way of a base plate.

6. A device as claimed in Claim 2, wherein the movable support is pulled by forces acting on it, e.g.

its own weight, towards the roller.

7. A device as claimed in Claim 5, wherein a spring is arranged to act upon the movable support and pull the same towards the roller.

8. A device as claimed in Claim 2, wherein the amplifier functions with the blast nozzle in an essentially digital manner.

9. A device as claimed in Claim 2, wherein an electrical distance measuring probe is connected with the control device of the roller mill, preferably by way of a zero-point shift circuit.

10. A device according to Claim 1,whereinthe lever is also acted on by a spring bias force.

11. A device according to claim 10, wherein the leverformsone arm of a parallelogram linkage connected to the mount.

12. A device according to any one of Claims 1, 10, or 11 wherein the first gauge is an inductive measuring probe producing an electrical signal.

13. A device according to any one of Claims 1, 10 or 11 wherein the first gauge is a magnetic comparator gauge producing an electrical signal.

14. A device according to any one of claims 1, or 10 to 13, wherein the second gauge is a pneumatic distance measuring device which produces a pneumatic signal, and the amplifier is a pneumatic amplifier.

15. A device according to any one of Claims 1 or 10 to 13, wherein the second gauge is an optical measuring device producing an electrical signal.

16. A device according to any one of Claims 1 or 10 to 16 wherein the forces acting on the lever are controlled by the second signal so as to keep the distance of the mount and the first and second gauges from the top surface of the layer substantially constant.

17. A device for measuring layerthicknesssubstantially as hereinbefore described with reference to the accompanying drawing.

18. A roller mill comprising a device as claimed in any preceding claim wherein the first signal is used to control the roller mill to produce a desired thickness of the layer.