WO1988009912A1

WO1988009912A1 - Thickness gauge for plastic film

Info

Publication number: WO1988009912A1
Application number: PCT/GB1988/000454
Authority: WO
Inventors: Tom Bailie
Original assignee: Lambeg Industrial Research Association
Priority date: 1987-06-11
Filing date: 1988-06-10
Publication date: 1988-12-15
Also published as: GB8927819D0; GB8713677D0; GB2231156B; AU1952988A; GB2231156A

Abstract

The technical problem to which the invention relates is the economic measurement of thickness of films. The thickness of a plastic film (18) is measured by passing the film between two heads (10, 12) each comprising a first plate (20) and a plurality of second plates (22) which rest above the surface of the film (18), forming a series of capacitors. The individual capacitances of the series of capacitors are measured. A change in the thickness of the film (18) gives a proportional change in measured capacitance. Each capacitor may be included in a separate oscillator circuit and the oscillators energised successively, so as to obtain a scan of film thickness across the film. The plates (20, 22) may be moved out of the path of the film by rotation about an axis.

Description

Thickness Gauge for Plastic Film This invention relates to a gauge for measuring the thickness of film.

In the production of plastics film by extrusion 5 the thickness of the film is measured and the die adjusted appropriately. Early production methods involved cutting off samples of film and measuring them with a micrometer. This procedure normally had to be repeated a number of times before the die was 10 correctly set and it was therefore time consuming and uneconomic. In order to meet this problem a gauge was developed which comprised a smooth steel reference surface and a measuring head about 50mm square. Film to be measured was passed over the reference surface ^••• and the measuring head rested on the top of the film. Thickness of the film was determined by measuring the capacitance between the reference surface and the measuring head. A change in thickness of the film altered the spacing between the reference surface and 0 e measuring head and consequently changed the capacitance. The capacitor formed by the reference surface and the measuring head was included in an oscillator circuit, the frequency of which was universally proportional to the capacitance. Changes in thickness of the film caused changes in the frequency of the oscillator output. The signal from the oscillator was used indirectly to set the die to produce film of the desired thickness. T.he measuring head was stepped back and forth across the film so that the whole width of the film was monitored. Although the above described gauge was a considerable improvement in the early method involving measurement of film samples with a micrometer it was found to have a number of disadvantages as follows: a) For correct operation the film.had to be contacted on both sides. It was found that the signal representing the thickness was influenced by the pressure applied to the film by the measuring head and reference surface. b) The relationship between the film thickness and the oscillator output was found not to be linear. c) The head had to be moved by hand. d) A profile of the thickness could only be obtained by plotting individual readings of the gauge. e) Small flaws in the film surface would lift the measuring head and cause a false measurement of the film thickness to be obtained.

In other -words the gauge was unreliable except with very smooth film.

Attempts to move the head automatically have been made but drastically increase the cost of the gauge. In addition it was found difficult to ensure precise positioning.

The present invention has been made from a consideration of these problems.

According to the invention there is provided a gauge for measuring the thickness of film moving along a path, comprising a first plate, a plurality of second plates-spaced from the first plate and with the first plate defining a slot through which film whose thickness is to be measured may be passed as said film travels along a path, said first plate and said second plates constituting a plurality of capacitors, means for measuring the capacitance of said capacitors and means for moving the first and second plates out of the path of the film.

In a preferred embodiment of the invention -each capacitor is included in a separate oscillator circuit, the output . of which represents the thickness of the film material in the capacitor- he oscillators are energised successively so that a scan of film thicknesses across the width of the film can be obtained. It is to be noted that no moving parts are necessary to achieve this.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Fig. 1 shows the general arrangement of a gauge in perspective. Fig. 2 shows a gauge head in side elevation; Fig. 2A shows the head of Fig.2 viewed from the left in end elevation; Fig. 2B shows the head of Fig. 2 viewed from the right in end elevation. Fig. 3 is a diagram of the switching circuit. Fig. 4 is a counting circuit; and Fig. 5 is an interface circuit. Referring to the drawings the gauge comprises two heads 10 and 12 mounted on vertical supports 14, 16 arranged on opposite sides of the path along which plastics film material 18 is led from an extruder (NOT SHOWN) in a generally horizontal plane. Each head is mounted on its support for rotation about a vertical axis and comprises a lower datum plate 20 and a plurality of second plates 22 mounted on insulators 24. The plates 22 are equally spaced by a small distance, for example about 1.0mm from plate 20 so as to define a slot 26 and each plate 22 forms a capacitor with plate 20.

As illustrated by head 10 in Fig. 1 the heads can be rotated out of the path of the film. In this condition the assembly can be 'zeroed'. In use the heads are positioned cror-s the film path with the film passing between plate 20 and plates 22. Variations in thickness in the film alters the capacitance of the capacitors. Each capacitor forms part of a separate oscillator circuit. The frequency of the output of the oscillator is proportional to the thickness of the film material in the capacitor formed by plates 20 and 22. The oscillators are energised sequentially so as to_. provide a scan across the film and the signals processed by a computer which actually counts the frequency. (This is to be distinguished from prior art methods of investigating the oscillator output which was to measure the voltage as being proportional to the frequency) . The computer derives the film thickness from the measured frequency and the signal is displayed on a screen together with any deviation from a predetermined thickness. These steps are now described in more detail with reference to Figs. 3 to 5. Switching Units

Signals from an "Apple" 2E computer through an interface unit 28 cause a 5V power supply to be switched to each oscillator in turn by means of electronic switches 30. The output from each oscillator is routed to the counting circuit by a further set of electronic switches 32. Each of the two type 6522 interface adaptors 34 has two eight bit ports - A and B. In one of the interface units port A is arranged to accept data for transmission to the computer while port B is configured to send signals from the computer to various circuits. For the switching unit, bit numbers 0 1 2 3 and 4 from Port B are decoded and used to select which oscillator is energised and to connect its output to the counting unit. Counting unit

This unit counts the pulses from each oscillator in turn for an accurately measured 1/10 second - and m transfers this count to the computer. An accurate timing pulse of 100 milliseconds is derived from the programmable crystal oscillator type PXO-600 36. A signal from the computer via bit 5 of Port B resets two 4040 twelve bit counters 38, 40 and starts the timer - which goes to its low state for 100 ms. When the timer goes to its high state it opens a gate for 100 ms to allow pulses from an oscillator through to the input of the first 4040 counter 38. A machine code routine in the computer monitors the state of the timer through bit 0 of Port A on the second interface circuit. When the timer output goes low after 100 ms the computer reads the number of pulses counted by the two 4040 counters.

The timer continues to give low to high output transitions every 100 ms and the counters continue to count - but this does not matter since the correct values are read into the computer at the first high to low transition. The next reset pulse sent by the computer sets the counter to zero ready to count the pulses from the next oscillator. Two 4040 counters are used in order to be able to count up to a 16 bit number (65536) . When the first counter is full all its outputs are high and this condition is detected by the circuit formed by the two 4068 NAND gates 42 and the 4071 OR or gate 44. The circuit then sends a pulse to the second 4040 counter 40. At the end of the 100 ms timing pulse the count is read by the computer as two 8 bits bytes which are stored in the two 74LS241 tristate latches. the latches use a common data bus to Port A of the interface unit. The high byte is read first by sending an enabling signal from bit 6 of Port B to the appropriate latch. the lower byte is read by an enabling signal from Bit 7. ZERO SETTING AND CALIBRATION

The gauge is set to zero at any time by moving the heads out of the path of the film and pressing the zero push button 46. This causes an interrupt to the program and stores a code number in a specific memory location. Every time the program has finished displaying the current measurements this location is examined. If the code number is present, the computer program branches to a routine which stores a new set of zero values.

In order to calibrate the gauge film of a known thickness is inserted into each capacitor in turn as instructed by the computer. When the calibrate button

48 is pressed it sends a signal via bit 5 of Port A on the second Interface Unit to indicate that film is present in the specified cell, and that calibration for that cell may proceed.

THE COMPUTER PROGRAM

The gauge and the computer communicate by means of the two Interface circuits. When the system is switched on the computer loads the program from the disc and carries out several initialising routines. The user is then asked if zeroing is required. If yes - the computer reads the oscillator frequencies for the empty cells and stores these values on the disc. Otherwise these values are read in from the disc. A calibration option is offered in a similar way and finally the set point is requested.

The main program loop now Ibegrn's.. The oscillator frequencies are measured in sequence and stored in arrays. The actual film thicknesses are now calculated. These values are calculated by subtracting the frequencies of the cells containing film from the stored frequencies of the empty cells and dividing the result by the stored calibration constants. The cell number - the deviation from the set point and the film thickness in microns are then displayed on the screen. Before the next set of measurements the program looks to see if an interrupt had occurred and if so it branches to a rezeroing routine. Otherwise the program pauses for about ten seconds before making a new set of measurements and updating the display.

Claims

1. A gauge for measuring the thickness cf film moving along a path, comprising a first plate, a plurality of second plates spaced from the first plate and with the first plate defining a slot through which film whose thickness is to be measured may be passed as said film travels along a path, said first plate and said second plate constituting a plurality of capacitors, means for measuring the capacitance of said capacitors and means for moving the first and second plates out of the path of the film.

2. A gauge for measuring the thickness of film as claimed in Claim 1, wherein each said capacitor is included in a separate oscillator circuit, the output of which oscillator circuit represents the thickness of the film material in each said capacitor.

3. A gauge as claimed in Claim 2, wherein said oscillators are energised successively so as to obtain a scan of film thickness across the film.

4. A gauge for measuring the thickness of film as claimed in any preceding claim wherein said means for moving said first and second plates out of the path of the film comprises a movable support for said first and second plates.

5. A gauge for measuring the thickness of film as claimed in Claim 4, wherein the means for moving the first and second plates out of the path of the film comprises a plurality of movable supports, each support having mounted thereon a first plate and a plurality of second plates.

6. A gauge for measuring the thickness of film as claimed in any preceding claim, wherein said second plates are mounted on insulators.

7. A gauge for measuring the thickness of film as claimed in any preceding claim, wherein said plurality of second plates are mounted so as to be uniformly spaced from said first plate.

8. A gauge for measuring the thickness of film as claimed in Claim 2, or any of Claims 3 to 7 as appendent to Claim 2, wherein a computer is provided for deriving the film thickness from the measured frequency output of each oscillator.

9. A gauge for measuring the thickness of film substantially as hereinbefore described with reference to the accompanying drawings.