CA2136093A1

CA2136093A1 - Heating control means

Info

Publication number: CA2136093A1
Application number: CA 2136093
Authority: CA
Inventors: Gerald Buttery
Original assignee: Individual
Current assignee: CEETAK DEVELOPMENTS Ltd
Priority date: 1992-05-27
Filing date: 1993-05-26
Publication date: 1993-12-09
Also published as: GB9211249D0; WO1993024302A1; AU4085293A; EP0642409A1

Abstract

Apparatus for controlling heating of heat sealing dies (12) relative to a target temperature has one or more electrical resistance heating elements (10) switched between "on" and "off" states for getting electrical power, according to frequency and durations of control signals. A first signal (from 42) proportional to said target temperature and a second signal (from 18) proportional to actual temperature to be controlled are compared (36, 38) to gauge prevailing temperature difference. Means (28, 30; 24, 26) responsive to comparison results is operative for varying said durations and said frequency of the control signals, both according to said temperature difference and to reduce same.

Description

~:; W093/~302 213 6 Q 9 3 PCT/GB93/01086 TITLE:HEATING CONTROL MEANS
DESCRIPTION
~ he invention relates to heating means and more particularly, but not exclusively, to means for ~ontrolli.ng electric heaters for use in apparatus and heat sealing plastics packaging film.
It is known to seal or join flexible plastics packaging material by welding using heat supplied for example by hot air, or by heated metal dies often nowadays faced with other materials. It is usually impo~tant that the temperature of the dies is closely controlled since too low a temperature will result in poor or no sealing of the plastics film, whereas too high a temperature can damage even destroy, the plastic film again resulting in pvor or no sealing. Historically, heated metal dies have comprised quite large metal blocks having a high thermal inertia in an attempt to provide temperature stability. This however means that it is impossible quickly to change or even correct the die temperature relative to a required target t~mperature.
At least for heat sealing plastics film it is known to use heated dies faced with thermally conductive silicone rubber able to conform more closely to shape for what is to be heat-sealed than is the case for bare rigid metal dies.
Although rubber-faced dies, or other faced dies, can be desirable, they are often adversely susceptible to temper~tures higher than target, particularly to protraGted such over-heating, so their heating should be closely controlled. It is advantageous for such dies, particularly rubber-faced dies, to Aave low thermal inertia, otherwise there can be ea~ly and unpredictæble failure of the ~ond betwee~ the facing, such as rubber, and the ele~trical heating element or other carrier, which can create undesira~le temperature gradients along the die with consequent inefficiency of resulting heat seals.
It is an object of the inven~ion to provide means of controlling the temperature of a low thermal inertia heat ?

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~ ~ W093/24302 2~36~93 PCT/GB93/01086 temperature. At and above the target temperature, variable control can be by way of further reductions of both of said frequency and said durations, say with minimum (short of complete switch off as could be further provided) for said durations at a higher further prescribed temperature difference above the target temperature than applies to minimum for said frequency.
Relative to te~perature increasing through the target temperature, preferred reductions of ~oth of said frequency and said duration for control purposes are progressive, i.e. ~o that less elQctrical power is applled at and above the target temperature than below it; and duration reductions can start before and continue after frequency reductions. The reductions in either or both of said frequ~ncy and said duration could be continuous and follow any desired characteris~ic, but are particularly readily implemented in a step-wise manner relative to the above and other suitable or desired prescribed temperature differences, so~e of which can coincide for both of ," 1 .- frequency and duration reductions tor increases considered against falling temperature of the dies).
~ Thus, both of frequency and dur~$ion reductions may -- take place at or close to achie~ing the target temperature ~, on heating UPJ and~or at the prescribed temperature . difference for frequency reductions ~o start on heating up, - and/or at a prescribed temperature difference representing ~, minimum frequency; and there can be intermediate further !.'.~,'~. prescribed temperature differences at which at least . frequency reduction occurs.
~ Other preferred embodiments of this invention can ; readily and advantageously operate with fewer reductions of said durations, even only one such reduction, usually at or ~- preferably before actual temperature reaches said target temperat1-re; depending, of course, on whether or not . reductions of said frequency beg n before or at the one duration reduction.
' I~ is practical and highiy advantayeous for .h~ same ~ .
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W093/24302 : PCT/GB93/01086 c ~ ~ ~provision to be operati~e in respect of more than one heater element. Examples include situations where ~ut or melted ~everance or tear-off weakenin~ is required between two good ~eals, or beyond one good seal, normally requiring two or three heater elements; or long heating dies where end and intermediate dies or die parts may have ~ifferent he~t loss or heat take-off characteristics, particularly if used for difference widths of articles to be sealed; or an~led heating dies, typically in an L- or V- or U-configuration, but not limit~d thereto.
Embodiments of the invention thus allow such operation to control plural heat sealing dies or die parts, particularly where control involves use of electrical pulses.
~ Whilst it is feasible to assign consecutive periods of said frequency indiYidually to different ones of plural die or die part heaters to be controlled in whatever sequence may be desired, it is preferred to deal with powering each of the die heaters concerned in desired sequencP within each period of said frequency. It is convenient for each of a stream of pulses at said frequency to trigger pulse length determining means effectively setting said durations, and further to have ~uch dur~ion setting pulses each trigger~d within the sequence from the trailing edge of the immediately preceding duration setting pulse in the sequence.
Control of plural heat-sealing dies or die parts can be relative to individual control temperature sensing means associated therewith. This approach is perhaps preferable : where characteristics of different dies or die part~ and their heating elements are not sufficiently similar for a single temperature sensor to suffice, or their temperature requirements differ and they are individually available for association with temperature sensors. Alternatively, pulse lengths corresponding to application of electrical power, f or eYen a proportioning of applied or available power, could be set for different dies or die parts, whether because their ~aid characteristic~ are different in known ,~
.

!`-". wo 9~/24302 2 1~ ~ O 9 3 Pcr/GB93/0l086 relative ways or because one or more of the dies or die ~arts are to be heated differentially. This approach is perhaps preferable whe~e it is impractical to have individu21 temperature sensors, say where a desiredly hotter die part is in a sandwich structure between desiredly less hot die parts.
Specific implementation for this invention is diagrammatically illustrated by way of example in the accompanying drawings, in ~hich:
Figure 1 is an overall block diagram for heat sealing die provision including three side-by-side heaters;
Figure 2 is a diagram for plural heat sealing dies or die parts each with a heater;
Figure 3 is a circuit diagram for one stage of means for setting voltage by reference to temperature difference; and ~igure 4 .is another circuit diagram for changing control pulse duration directly according to temperature difference.
Generally, it is desired accurately to control the conversion of electrical energy into heat energy in the heating elements of a heat-sealing ~die, particularly a heat-se~ling die having low thermal inertia, usually where the mass of material of the or each heating die itself is low. A conventional heating system operating simply to switch-on and switch-off electrical power under the control Or a thermostat would result in unacceptably inaccurate heating of the die, partlcularly for its surface, relative to a target temperature. I~ is proposed herein to modulate the electrical current ~low1ng through the heating element to enable more accurate temperature control and stability to be maintained Xor the h~ating die concerned. The ci-cuitry which will now be described meets these requirements and can be considered as a power-modulating circuit.
Operation is based on measuring the difference between 1 voltage signals arranged to represent a ~elected o~ target ,~, ~3 6 PCT/GB53/01086 temperature and actual sensed temperature of a heat sealing die, and controllin~ -switching signals for solid-state switching circuitry for supplyin~ either AC or DC current to at least one heating element.
The block diagram of Figure 1 ls first described for main components involved in controlling one heating element lOB of a heat sealing die 12 that actually has three heating elements as will be further described later. An electrical power source 14 is shown supplying the heater element lOB through a solid-state electronic switch 16B.
The heat-sealing die is shown with an actual temperat~lre sensor 18.
Solid state electronic power sw}tches are well-known operative for determining 'on' states by durations of control p~lse signals, see line 20B for switch 16B. l~hese control pulse signals are controlled as to both frequency and duration, see idealised pulses 22B that can vary in a ~ontrclled way as to length or duration of each and as to intervals between them, i.e repetition rate or frequency~
Duratisns of the control pulse signals 22B are shown determined by voltage controlled pulse le~ngth/width setting circuit 24B, and initiations and repetition rate by voltage controlled oscillator 26. The circuit 24B and the voltage controlled oscillator 26 can and preferably do ha~e base states, i.e. with no voltage signals on their control lines 28 and 30, for pre-set maximum control pulse duration and preset maximum frequency or repetition rate.
Voltage control signals for the pulse duration setting circuit 24B and the voltage controlled oscillator 26 are shown coming from circuits 28 and 30, respectively. The circuits 28 and 30 can be of si~ilar types, convenie~tly resistance ladder network~ that give voltage output changes in steps and according to sequential energisation of plural inputs. For each of the circuits 28 and 30, five inputs are shown, see 32A-E and 34A-E, respectively. For pulse-length/width and frequency control (at circuit 24B and ozcillatcr 25) ~ropo-t~ c3~1 tc cor.trol v^~tag~s f~llo~.7ing W 0 93/243U2 213B~9~ P(-r/GB93/01086 decreasing temperature difference to the target temperature then increasing beyond, resistance ladder networks (as circuits 28 and 30) can be so arranged that more of their inputs (32A-E, 34A-E) are energised, see furthe~ below regarding Figure 3.
Energisation of inputs ~2A-E and 34A-E of ~he vo~tage setting circuits 28 and 30 for the circuit 24B and oscillator 26, respectively, is shown in accordance with outputs of comparator circuity 36 and 38, respectively.
These comparator circuits 36 and 38 receive both of signals representing a selected or target temperature at branches from line 40 from temperature selector 42, and at branches ~rom line 44 for actual heat-sealing die temperature from -the temperature sensor 18. For energisation of resistive ladder networks as voltage setting circuits 28 an 30, the comparator circuits 36 and 38 operate on, as mentioned above, a ~asis of energising lines 32A-E and lines 34A-E
sequentially according to prescribed temperature differences ~etween its inputs, which prescribed voltages extend from the actual temperature being below through to being above the target temperature. For one suitable ~scheme for particularly close die temperature control -jprescribed temperature differences and responses are as follows:-1. First prescribed temperature difference, say 10C below target temperature, for energisation of output 32A, thus first reduction of the control pulse duration by circuit 24B during heat-up.
2. Second prescribed temperature difference, say 2C ~elow ~the target temperature! ~or energisation of outputs -~32B and 34A, thus second reduction of the control ipulse duration by circuit 24B, and first red~ction of the frequency or repetition rate of oscillator 26.

3. Third prescribed temperature difference, say 1C below ~the target temperature, for energisation o o~tput 34B
s~thus second reduction of the fre~uency or repetition r.-te at osc-llator 2~.

,, wo 93~24302 3~93 8 PCT/GB93/01086 4. Fourth prescribed temperature difference, say zero, i.e. when the actual temperature first matches the target temp~rature during heating up, for energisation of outputs 32C and 34C, thus third reduction of the control pulse duration by circui~ 24~, and third reduction o~ the frequency or repetition rate at oscillator 26~

5. Fifth prescribed temperature difference, say 1C above the target temperature, for energisation of output 34D
thus fourth reduction of the frequency or repetition rate at oscillator 26.

6. Sixth prescribed temperature difference, say 2C a~ove the target temperature, for energisation of outputs 32D and 34E, thus fourth reduction of the control puls~ duration by circiut 24B and fifth reduction of the frequency or repetition rate at oscillator 26.

7. Seventh prescri~ed temperature difference, say 5C or 10C above the target tempera~ure~ for energisation of output 32E, thus fifth reduction of the control pulse duration by circuit 24B.
Such a control regime gives good accuracy of actual temperature relative to target tempe~ture, even for dies with very low thermal inertias, but should not be taken as ruling out alternatives, whether fo~ more duration and/or frequency reductions, or for less as will be described or Figure 4, and including as to temperature differences, if any, at which both of the comparators 36 and 38 change their output states.
For a heating element made from an alloy of 80% Nickel and 20% Chrome, with an effec~ti-~e electrical resistance of 0.5 ohms, and a DC supply of 30 Volts normal, effective ~vailable control pulse durations as above can be 30, 20, 10, 8 and 5 milliseconds and available frequencies can have periods as above corresponding to repetition rates of 30, 17, 15, 10 and 2 per minute.
The comparators 36 and 38 can, as indicated, usefully be ba~ed on ~perational amplifiers, one for each cu~rut, ~ W093/24302 ~ 3~9 3 PCT/GB93/01086 say each arranged to change state for detect~d e~ualit.y of input sign~ls, then with the target temperature signal off line 40 subject to successive- offsets. Suitable such offsets can be minus lO0, minu~ 20, zero, plus 20 and plus 50 or 100 millivolts for the comparator 36; and minus 20, minus 10, zero, plus 10 and plus 20 millivolts, where the temperature sensor 18 and the temperature setting means 42 operate or have their outputs adjusted on the basi.s of 10 millivolts per degree Centigrade tCelsius).
It will be appreciated that illustrated components as so far described can service a heat sealing die having a single heating element, or a heat sealing die or dies having plural heating elements all to be energised together. However, particular pr~ctical advantage is seen ~9 arising from separate individual driving of plural heating eiements sequentially within each of single periods of the Yoltage controlled oscillator 26, see dashed pulses 22C, 22A following first solid pulse 22B in Figure 1.
That is readily achieved where all heating elements are to ~e ~im1larly dri~en as indicated by solid lines 20A,B,C to solid-state switches 16A,B,C for heating ~lements lO~,B,C in Figure 1, and with control according to a single die surface temperature sensor (18), but with control signals for the lines 20A,B,C coming from successive stages (T8, TC, TA) of the pulse generator 24B, each prefer~bly and conveniently simply trigge~ed from trailing edge of the control pulse from the preceding active stage, as is readily achieved, say using Schmitt Trigger circuit~y 24S between those stages. Alternatively, the ~oltage controll`ed os'cillator 26 could have a successive phasing or stage operation or structure, see dashed extensions therefrom in Figure 1 with dashed connection to pulse generating stages that could then be independent of each other. Repeated use of a single pulse generator 24 is feasi~le with switching of its output connection to the lines 20A,B,C and/or its input connection to outputs of the voltage controlled oscillator 26. More W093~24302~ ~ b~ PCT/GB93/DI086 individual control is possi~le using more temperature sen~ors, up to one per heating elernent concerned, then with switching of actual temperature signal lines at input of the duration setting circuit 36, but one chosen to set the frequency or repetition rate ~ia the circuit 38, and clearly permitting maintaining any desired or required ~, temperature differences for the die or dies concerned.
Another way to achieve differential die/die part temperatures is and to adjust the lengths of control signal pulses on the lines 20A,B,C after they have left the circuitry 24, whether in accordance with different actual temperature sensors, or by pulse shortening, even pulse stretching, circuitry to utilise a single temperature sensor 18 and operate according to offsets representing ~nown relative parameters or simply ad~usted to be effective. Figure 1 shows such offsetting by way of pulse shortening circuitry, see dashed at 46A,C in lines 20A,C as is appropriate for a three heater elements and die part system (lOA,B,C;12) as used for heat sealing and severing~
weakening within overall sealing or between two distinct seals, where the central die part (lOB) will ordinarily require a higher temperature and it i~ convenient to set - the flanking die part (lOA,C) relative thereto. Ramp-action blocking amplifiers are suitable and may well be best provided as indicated adjustable units 46A,C.
Alternatively, adjus~ment could be within the extended stages of the circuitry 24.
At least provisions such as control pulse length adjustors can also be particularly useful in coping with a dies or die-parts system where there is differential heat losstta~e-off, and ln order to seek to maintain a steady desired temperature, as -~ensed at 18 (or some other characteristic if more appropriate).
Figure 2 shows a die system 52 with four heating elements 50A,B,C,D as may be applicable simply to a long heat sealing die, or to a die structure with more than one direction of reyuired se-lin~, say an ~symmetric L- or a U-,'-k~, WO 93/24302 3 - P~T/GB93/0l086 or C- or V- configuration, and each with its own soli.d-state pulse Length controlled switch 56~,~,C,D from common power supply 54. This arrangement is readily controlled by circuitry as described for Figure l, indeed that Figure shows dashed extension of its control pulse generator circuitry 24 ~nd/or voltage con~rolled oscillator 26 for four-way use.
Figure 3 shows part of one s~Ltable resistive ladder type control for the voltage controlled oscillator 26, for which its control voltage needs to decrease for lower frequencies. Specifically, as shown ~or the output 34A, the related operational amplifier 60A has lnputs 61A,62A f~r voltage signals repxesenting actual and appropriately offset target temperatures, respectively, and output 34 going high when the actual temperature (6lA) exceeds the target temperature (62A) as appropriately offset. Then, transistor 65A will be switched into conduction and adjustable resistance of potentiometer 66A will be shorted out and replaced by lower resistor 67A in the emitter/collector circuit of the transistor 65A going to earth rail 68. It will be appreciated that there will be other similar circuit provisions at th~ indicated break 69, on~ for each of the operational amplifiers and outputs 34B-E, and that the voltage seen at 64 ~or controlling the frequency Qf the voltage controlled oscillator 26 will depend on how many of the transistors are sequentially switched on and their variable potentiometers shorted out.
Adjustability of potentiometers 66A-E allows individual set-up of changes of frequency for particular controlled systems.
It will`be appreciated that a multi-stage cascaded ladder-type circuit ll~e Figure 3 can be used for control pulse length/duration setting as at 28, which might then operate on an up or down ramping basis to convert the voltage concerned to time, specifically duration of control pulse.
However, there are many instances where up to five or W 0 93/24302 ~ ~ ~ 12 PCT/GB93/01086 more frequency changes is adequate relative to controlling ~ealing dies with low thermal inertia even where deployed with a lesser numb~r of changes of control pulse duration, even as low as two. Figure 4 shows a particularly simple way of handling just two available control pulse lengths or durations. A single operational amplifier 70 has two inputs 7l and 72 for signals with voltage levels representing actual and target temperatures, actually conveniently some suitable offset from the target temperature, which will usually be below. The operational amplifier 70 has its output 74 taken to bases of transistors 75 and 76, the latter ~ia in~erter 77. The transistors 75 and 76 are connected with their emitter/collector circuits going in common to ground at 78 from different capacitors 79 and 80 alternatively cooperating with resistor 8l to set t.ime constant for a suitable pulse duration setting circuit 24.
It will be appreciated that the operational amplifier 10 will operate at some prescribed ~emper~ture difference to switch from capacitor 8~ to capacitor 79 for a single reduction in control pulse length~duration. A change from 12 to lO millisecond duration is found to be enough, with the switching at as low as 5C below ~arget, or at about 2C, or as otherwise desired relative to any useful overlap with fre~uency changing, say as a~ove descri~ed.
It would, of course, ~e feasible to control fre~uency and duration according to sin~le sequential output signals for each change required, and for temperature comparator circuitry to operate accordingly, further feasibly for a single temperature comparator provision to provide outputs selecti~ely gated for frequ,ency and/or duration control purposes.

Claims

13

1. Apparatus for controlling heating of heat sealing dies (12) relative to a target temperature, comprising at least one electrical resistance heating element (108) means (42) for generating a first signal proportional to said target temperature, means (18) for generating a second signal proportional to actual temperature to be controlled by said at least one heating element (10B), and means (24-38) for varying electrical power applied to said at least one heating element (10B) according to comparison between the first and second signals, characterized in that variation of electrical power applied to said at least one heating element (10B) is by way of means (16B) for switching between "on" and "off" states for supply of said electrical power according to frequency and durations of control signals (22B) representing successive energisations of said at least one heating element (1?B), which control signals (22B) are produced by means (36,38) for making comparisons of the first and second signals to gauge prevailing temperature difference between said actual and target temperatures and means (28,24B;30,26) responsive to results of said comparisons for varying said frequency (via 30,26) and said durations (28,24B) of the control signals (22B) both according to said temperature difference and to reduce same.

2. Apparatus according to claim 1, wherein changes of said frequency (via 30,26) and of said durations (via 28,24B) serve for relatively fine and relatively coarse control, respectively.

3. Apparatus according to claim 1 or claim 2, wherein changes of said frequency and said durations overlap.

4. Apparatus according to any preceding claim, wherein said means (36,38) for making comparisons and said means (28,30) responsive are operative relative to prescribed temperature differences for step-wise changes in said frequency and durations of the control signal.

5. Apparatus according to claim 4, wherein largest prescribed temperature difference below the target temperature corresponds to maxima for said frequency and said durations, and largest prescribed temperature difference above the target temperature corresponds to minima for said frequency and said durations.

6. Apparatus according to claim 4 or claim 5, wherein reductions of said frequency and of said durations, considered for increasing actual temperature, are progressive for prescribed temperature differences from below through and above the target temperature.

7. Apparatus according to claim 6, wherein at least one change of said frequency coincides with a change of said duration.

8. Apparatus according to any one of claims 4 to 7, wherein temperature comparator circuitry (36,38) is operative to provide an input for each of prescribed temperature differences corresponding to required changes of said frequency and/or said durations.

9. Apparatus according to claim 8, wherein distinct temperature comparator circuits (36,38) of said circuitry are operative to control setting of said frequency (38) and of said durations (38), respectively.

10. Apparatus according to claim 8 or claim 9, wherein said comparator circuitry (36,38) includes operational amplifiers, one for each prescribed temperature difference required.

11. Apparatus according to claim 10, wherein each operational amplifier is operative for equality of input signals representing the actual and target temperatures, and inputs thereto concerning the target temperature are after being subjected to offsets as required.

12. Apparatus according to claim 11, wherein representation of the actual and target temperatures and offsetting the latter are at 10 millivolts per degree centigrade.

13. Apparatus according to any preceding claim, wherein control of said frequency is by way of a voltage controlled oscillator (26).

14. Apparatus according to any preceding claim, wherein control of said durations is by way of a voltage controlled pulse duration setting circuit (24B).

15. Apparatus according to claim 13 or claim 14, wherein said voltage controlled oscillator (25) and/or pulse duration setting circuit (24B) are controlled by outputs from resistive ladder networks (in 36,38).

16. Apparatus according to claim 14, wherein said voltage controlled pulse duration setting circuit (Fig 4) is controlled by a circuit alternatively operative to set one of two different time constants.

17. Apparatus according to any preceding claim, wherein plural heating elements (10A-C:50A-D) of plural heat sealing dies or die parts are controlled sequentially within each period of said frequency.

18. Apparatus according to claim 17, wherein a set sequence of energising the plural heating elements (10A-C:50A-D) is followed in each said period.

19. Apparatus according to claim 18, wherein trailing edges of the control signals (22A-C) for preceding heating elements in said sequence serve to trigger the control signals (22A-C) for the next of the heating elements in said sequence.

20. Apparatus according to claim 17, 18 or 19, wherein control signal duration adjustment means (46A,C) is operative relative to different ones of the heater elements (10A-C) and relative to single actual temperature sensing.