3 This invention is concerned particularly with the control of

A apparatus for cutting at regular intervals a web such as, for example,

5 used for joining cigarettes to filters in a filter attachment machine. Web

6 cutting in this context involves cooperation at regular intervals between

7 two members at least one of which preferably rotates. Cutting may, for a example, be achieved by a crushing action between a knife on one 9 member and an anvil on the other member. Alternatively, cutting may be io achieved ultrasonically, for example as described in our PCT patent π application WO 94/14583 (case 3643).

12 In both crush cutting and ultrasonic cutting it is desirable to n control the relative positions of the two members which cooperate to i« perform each cut in order to achieve a sufficient but not excessive is cutting force. The present invention is concerned particularly with an ie automatic control system for use particularly in such web cutting

17 arrangements. iβ Apparatus according to this invention for periodically applying a

19 controllable force to a moving part (for example a web of paper to be

20 cut) comprises a force-applying member, and means for producing a

21 contro l el ectrical s ignal i nd icative of the fo rce app l ied by the

22 force-applying member and for controlling the force applying member so

23 as to control the forces applied by it to the moving part, the control signal 2< being produced with the aid of a lock-in amplifier to which is applied a

25 force-responsive electrical signal together with a reference signal,

26 whereby any noise element in the control signal is eliminated or reduced

27 by the lock-in amplifier.

28 By this means , despite the noise which almost inevitably

29 accompanies the signals from a sensor, for example in this context, the

30 average cutting force or the force generated at each cut (for example) 1 can be obtained to enable the position of one of the cutting members to 2 be adjusted in order to achieve a desired level of cutting force. For 3 example, in apparatus for cutting a filter attachment web (commonly 4 termed "cork paper"), the cutting members can be set at positions such 5 that, in the absence of the web, there would be merely light contact 6 between the cutting members; thus the presence of the web during use 7 results in a significant but not excessive force being applied sufficient to 8 cut the web, whether this is achieved by an ultrasonically driven cutting

i member or by means of a simple crush cutting operation as used

2 conventionally in connection with cork cutting in a filter attachment

3 machine.

4 Lock-in amplifiers are known per se and are commercially

5 available. For example, reference is directed to a book entitled "The Art

6 of Electronics" by Paul Horowitz and Winfield Hill, published by

7 Cambridge University Press. In the second edition section 1 5.15 β contains a general description of "Lock-in detection" and some uses 9 thereof. An example of a commercially available lock-in amplifier which

10 may be used in carrying out the present invention is that made by ii Analog Devices, of 1 Technology Way, P.O. Box 9106, Norwood, MA

12 02062-9106, USA and identified as their AD630 product. n Examples of ultrasonic apparatus for cutting a web of filter u attachment paper are shown in the accompanying drawings. In these

15 drawings: i6 Figure 1 is a diagrammatic perspective view of one web cutting π apparatus; is Figure 2 is a longitudinal sectio n though part of a similar

19 apparatus;

20 Figure 3 is a circuit diagram of a control system for the apparatus;

21 Figure 4A-4D are graphs showing various signals produced

22 during use of the apparatus; and

23 Figure 5 is a longitudinal section through a modified form of the

24 apparatus.

25 Figure 1 shows a drum 10 around which a web of filter attachment

26 paper is to be conveyed and from which leading end portions are

27 severed by means of an ultrasonic cutting arrangement. This cutting

28 arrangement comprises an ultrasonically driven horn 12 which cuts the

29 web in cooperation with slightly raised members 14 mounted at regular

30 intervals around the circumference of the drum 10. The arrangement ₃ i may more specifically be as described with reference to Figures 16 and

32 17 of our above-mentioned PCT patent application, the raised members

33 14 being relatively sharp-edged knives while the cooperating surface of

34 the horn 12 serves as an anvil; alternatively, as illustrated by Figure 1 ,

35 the members 14 may constitute anvils and the cooperating edge of the

36 horn 12 may effectively constitute a "knife".

37 The horn 12 is driven ultrasonically so as to vibrate towards and

38 away from the drum 10 by an ultrasonic piezo actuator 16 via a booster

1 18. In the region 19 there is a node of zero displacement at which the

2 horn may be clamped to support the whole assembly. As a result of the

3 ultrasonic excitation of the piezo actuator, a 60-70 micron displacement

4 at 20 kHz (for example) occurs at the cutting end of the horn 12.

5 Figure 2 is a longitudinal section through a housing 20 of a

6 different arrangement including a hom 21 mounted in an inner housing

7 22 and having a "cutting" edge 21 A. The inner housing 22 is slidably β mounted in the housing 20 by means of linear bearings 24 (e.g. PTFE) 9 so that the inner housing can move vertically with respect to the main io outer housing 20. Such movement is controlled by a device 23, which ii has a main body carried by a beam 26 mounted on the housing 20, and

12 a movable part 28 extending from its lower end and connected to an end π wall of the inner housing 22. By this means, the position of the anvil 10

14 can be finely adjusted. The device 23 may comprise a piezo actuator or

15 translator, for example one of those manufactured by Physik Instrumente ie (PI GmbH, D-76337 Waldbronn, Germany), one suitable example being π that identified as the LVPZ translator model P-841.30. is The force transmitted to the anvil during each cutting operation is

19 detected by an eddy current sensor 30 mounted inside the inner

20 housing 22 and slightly spaced from an opposed surface of the anvil. An

21 electrical signal derived from the sensor 30 is fed to the control circuit

22 shown in Figure 3.

23 In Figure 3 certain of the circuit components are identified as

24 follows:

25 C = Comparator (10% hysteresis) 6 S = Summation amplifier 7 P = Programmable logic device 8 X = Crystal oscillator 9 R = Retriggerale monostable 0 A = Analog switch 1 D = Peak detector. 2 With reference to Figure 3, the analogue voltage output of the 3 eddy current sensor 30 is proportional to the displacement of the anvil 4 from the reference plane of the sensor. The first stage of the control 5 circuit conditions the sensor signal, removing the large amounts of noise 6 associated with the signal. This part of the circuit is based around a 7 lock-in amplifier 32. The circuit demodulates the sensor signal with the 8 use of an ultrasonic reference signal and requires a controlled phase

1 difference between the reference and sensor signals. The reference

2 signal is received from a source 38 and is fed via a phase adjusting

3 circuit component 40; there is also a phase adjusting provision 34 in the

4 path from the sensor 30 to the lock-in amplifier. Any necessary signal

5 gains are provided by circuit components 31 , 36, 39 and 42. The

6 remaining noise is filtered out by mean of a low pass filter 44. Unwanted

7 portions of the signal are removed by a half-wave rectifier 46 and the β gain is adjusted by circuit component 45. A typical waveform of the 9 conditioned sensor signal is shown is Figure 4A, which depicts peaks of io amplitude which are proportional to the displacement of the anvil from ii the sensor, and are thus proportional to the force reacted by the cutting

12 member.

13 Contact between the knife and anvil is detected when the sensor

14 signal exceeds a pre-determined threshold level. A knife hit envelope

15 signal can then be generated for the period during which the knife and i6 anvil are in contact (see Figure 4B).

17 Individual knife/anvil disturbance forces are detected by a peak is detector 47 that captures the maximum level of the conditioned sensor

19 signal occurring during the knife hit envelope period (see Figure 4C).

20 Excessive cutting forces are monitored by circuit component 61 to

21 prevent premature failure of the cutting apparatus through automatic

22 adjustment of the cutting members, or by stopping the machine.

23 A sample and hold circuit 50 stores the peak of the disturbance

24 signal (corresponding to the force on the anvil at each cutting operation)

25 to produce a maximum disturbance signal (see Figure 4D). The circuit

26 component 50 stores the disturbance signal obtained during each cut,

27 but is reset once during every revolution of the drum 1 0 by a circuit 2β component 52; assuming that there are 16 knives on the drum 10, which

29 would be typical in practice, the component 52 would be a /1 6

30 component.

31 A further sample-and-hold circuit 54 stores the maximum signal

32 obtained during each revolution of the drum 10, and a differential

33 amplifier 56 compares each maximum with a reference. The resulting

34 information is used to control a circuit component 58 which drives the

35 piezo actuator 23, the movement produced by the actuator 23 for

36 controlling the position of the anvil being proportional to the voltage

37 applied to the actuator. The amount of movement is controlled to keep 3β the disturbance signal within an acceptable range.

1 Figure 3 actually shows a motor driver 60 for positioning the anvil,

2 which is a possible alternative to the piezo actuator.

3 The circuit shown in Figure 3 monitors both the maximum

4 minimum disturbance signals and can be used to keep both within an

5 acceptable range. When the minimum signal, for example, drops below

6 the acceptable range and cannot be brought into the acceptable range

7 by movement of the anvil 10, without taking the maximum outside the s acceptable range, an alarm signal is produced or alternatively the 9 machine is automatically switched off to allow the knives to be reground io or manually adjusted.

1 1 An average of the disturbances is obtained through smoothing of

12 the maximum disturbance signal with a low pass filter 51.

13 By adjusting the position of the horn assembly, the average

14 cutting force can be controlled by maintaining the average disturbance

15 level at about a fixed value. The circuit component 58 effectively serves i6 as a window comparator, with two threshold values set about a desired 17 fixed value, and this monitors the average disturbance signal. When the is level of the average disturbance analog signal strays outside of the

19 window thresholds, the horn actuator is adjusted to restore the average

20 disturbance to within the window. Thus, a substantially constant cutting

21 force can be achieved through maintaining the average disturbance at

22 about a fixed level. The aperture of the window is set to correspond to

23 the required tolerance of the average cutting force variation of the duty

24 cycle capability of the actuator.

25 Excessive variance of wear between individual knives is monitored 6 to keep both the maximum and minimum disturbances of one cycle 7 within an acceptable range. The maximum disturbance level over one 8 cycle (revolution) of the drum is stored in the sample and hold circuit 54. 9 During the next cycle of the knife drum, the peak disturbance level 0 associated with each cut is compared with the maximum disturbance 1 stored from the previous cycle. The difference between these two 2 signals is then the variation of the cutting disturbances. The circuit 3 provides a means for automatic detection of worn out knives and an 4 indication that the system is outside the limits of automatic adjustment of 5 the cutting apparatus and is therefore no longer able to maintain a 6 consistent quality of cut from all knives. 7 Instead of the horn 12 being maintained in a set position during 8 each revolution of the drum 10, it may be controlled in position by the

1 piezo actuator so as to produce the required disturbance signal during

2 each cutting operation. This modification would require the actuator to

3 receive a disturbance signal for each individual knife, and to control the

4 position of the anvil during, for example, one sixteenth of a revolution of

5 the drum 1 0. More specifically, the circuit would need to store the

6 disturbance signal for each knife during one revolution of the drum, so

7 that the anvil can be positioned appropriately when the knife next β engages the anvil.

9 A generally similar control circuit may be used to control the io position of each knife in a conventional crush-cutting apparatus, for n example of the type generally described in our US patent No. 4372327.

12 For this purpose the rotating knife carrier which cooperates with anvil π surfaces on the drum would preferably have as many knives as there are

14 anvil surfaces on the drum. The interference between each knife and

15 the corresponding anvil surface on the drum is then adjusted by a piezo i6 actuator controlling the position of the knife carrier. This position may be 17 adjusted gradually in response to the average or maximum disturbance ie signal detected in this case by a sensor adjacent to the knife carrier or a

19 part carrying the knife carrier. Alternatively, the piezo actuator may be

20 capable of resetting the position of the knife carrier in order to achieve

21 the required interference between each knife and its corresponding

22 anvil. A further possibility in principle is that the knife carrier may include

23 a separate piezo actuator for each knife.

24 Figure 5 shows a different arrangement which includes a cork

25 cutting drum 62 fitted with 1 6 tungsten carbide knives 63 mounted at

26 regular intervals around the circumference of the drum and presenting

27 knife edges which project from the surface of the drum. An ultrasonic

28 horn 64 serves as an anvil which cooperates with each knife edge to cut

29 the filter attachment "cork" paper (65) as it passes around the drum 60 to

30 produce individual cork patches 65A.

31 The ultrasonic horn assembly, which is generally referenced 64A,

32 is driven sinusoidally at 20 kHz, its oscillating displacement being

33 amplified by a booster as described with reference to the first example.

34 As a result of the ultrasonic excitation, a 60-70 micron displacement at 20

35 kHz occurs at the cutting end of the anvil 64.

36 The ultrasonic horn assembly is mounted on a base 66 by two

37 flexure devices 68 which set the lateral position of the horn assembly

38 while allowing axial displacement. Such axial displacement for

adjustment purposes can be made by a coarse adjustment arrangement

70 and by a fine adjustment 72, as well as by a piezo actuator 74. There is also provision for adjusting the squareness of the anvil with respect to the knife drum by means of an adjustment arrangement 76.

5 The piezo actuator 74 is capable of expanding up to 90 microns e and of producing a force of up to 800N. A control circuit such as that

7 shown in Figure 3 is used to control the actuator 74 in order to achieve β the desired level of cutting force between the anvil 64 and each of the

9 knives 62. Connections to the control circuit are made from the piezo io actuator 74 by leads 78, and a filtered air supply and electric connection

11 are provided to the horn assembly 64A by means 80. π The following modification has proved to be feasible. The sensor n 30 is omitted. Instead, we have found that the force effect during each

14 cutting operation is reflected in the ultrasonic drive signal used to power

15 ultrasonic horn. Accordingly an input signal to the lock-in amplifier can i6 be tapped off the drive signal. A reference signal can also be tapped off 17 the drive signal and (with suitable phase adjustment) can be applied to is the lock-in amplifier.

19 Another possible modification is as follows: instead of the

20 ultrasonically driven member being adjusted in position to control the

21 force, the force can be controlled at least partly by varying the amplitude

22 of the ultrasonic vibrations.

23 24 25 26 27 8 9 0 1 2 3 4 5 6 7 8