GB2130761A

GB2130761A - Temperature control

Info

Publication number: GB2130761A
Application number: GB08233505A
Authority: GB
Inventors: Robert Yardley; Ewen Rothnie Cameron
Original assignee: British United Shoe Machinery Ltd
Current assignee: Noxet UK Ltd
Priority date: 1982-11-24
Filing date: 1982-11-24
Publication date: 1984-06-06
Also published as: IT8323860A1; IT1167019B; BR8306550A; GB2130761B; IT8323860A0

Abstract

For enabling a body, e.g. a melt chamber and/or nozzle for a hot-melt composition, to be heated to and maintained precisely at a desired temperature, a predetermined temperature value is stored as a numerical value in a computer, and the output of a sensor, e.g. each of the thermistors (TS 1-3), mounted on the body, whose output is in the form of an analogue signal, is supplied to an A-D convertor which converts the signal to a numerical value, this output value and the stored value being then compared, at predetermined intervals, and a command being supplied to the respective heater circuit (H1-3) appropriate to the difference between the two values. When the temperature enters a predetermined band about the desired temperature, the heater circuit is energised by regular pulses of variable duration. <IMAGE>

Description

SPECIFICATION Temperature control This invention concerns a method of controlling the temperature of a body which is being heated.

One known method of this type comprises determining a target temperature using a thermostatic device, and sensing the temperature of the body by means of a sensor which supplies a signal for comparison with the setting of the thermostatic device, a command being supplied to a heater circuit when the compared values differ.

In using such a method, however, the temperature of the body cannot be accurately controlled, since usually a thermostatic device operates within a band, causing power to be supplied to the heater circuit when the lower limit of the band is reached and thereafter switching off the power only when the upper limit of the band is attained. Consequently, the temperature of the body is maintained not at a target temperature, but rather within a band which extends at either side of the target temperature.

It is the object of the present invention to provide an improved method of controlling the temperature of a body which is being heated, which method maintains the temperature of the body at or near the target temperature, within significantly closer tolerances than are available using a conventional thermostatic device.

The invention thus provides a method of controlling the temperature of a body which is being heated comprising determining a target temperature and storing in a computer a numerical value corresponding to the target temperature, sensing the temperature of the body by means of a sensor which supplies an analogue signal to an analogue-to-digital convertor, which, in turn converts the analogue signal into a numerical value, comparing the numerical value of said signal with that corresponding to the target temperature, and supplying a command to a heater circuit appropriate to the difference between the compared numerical values.

It will thus be appreciated that, by using numerical values and comparing the numerical value as aforementioned, it becomes possible to direct the controls specifically to the target temperature, rather, than to a band of temperatures centred on the target temperature.

Furthermore, the method in accordance with the invention lends itself more readily to computer control, which enables the comparison of numerical values to take place at very frequent intervals, thereby reducing still further any deviation from the target temperature and increasing the frequency with which any such deviation can be corrected.

Furthermore, in carrying out the method in accordance with the invention, at regular intervals the analogue-to-digital converter may be interrogated as to the state of the sensor and the numerical values may be compared as aforesaid.

In addition, conveniently the command to the heater circuit for each time interval may be divided into "on" and "off" periods, when respectively the power supply to the heater circuit is switched on and off, such division being proportional to the difference between the numerical values being compared. Preferably, furthermore, in accordance with the invention, when the target temperature has been determined and the corresponding numerical value stored, a band of such values is established at either side of the "target temperature" value, the command to the heater for each time interval being divided as aforesaid only when the numerical value being compared with the "target temperature" value lies within said band.It will, of course, be appreciated that, when the numerical value corresponding to the sensed temperature lies below the band, the heater circuit will be constantly supplied with power, while should the numerical value corresponding to the sensed temperature be above the band, the power supply to the heater circuit will be shut off.

The method in accordance with the invention for controlling the temperature of a body also lends itself to the diagnosis of faults, whether in the heater circuit or the control circuit. Thus, if, after a predetermined time following the switching on of the heater circuit, the numerical value being compared with the "target temperature" value is spaced from the latter value by a predetermined amount (indicating too high or too low a temperature) a warning is indicated and, further, in the event that in a subsequent comparison, the difference between the two numerical values increases to a further predetermined amount, the heater circuit is switched off. Such a warning may be provided by illuminating a light-emitting diode (LED), e.g. with a constant illumination.

Similarly, in accordance with the invention, if, after the target temperature has been reached, the numerical value corresponding to the analogue signal from the sensor increases to a maximum value, or substantially so, (indicating that the sensor has entered an "open circuit" condition), a warning is indicated. In this case, the warning may be provided on the same LED as in the previous case, but using a signal which is different, e.g. a flashing signal. Alternatively, the two different warnings may be provided using different colours in the same LED.

Furthermore, where a warning is indicated, in the event of the sensor entering an "open circuit" condition, conveniently the command to the heater circuit for each subsequent time interval is divided into equal "on" and "off" periods for a predetermined time, whereafter the heater circuit is switched off. In this way, the heater is maintained operational for a limited period, on the basis that no detriment to the various integers will be occasioned thereby.

It may be desirable for some indication to be given, e.g. when the heater circuit is first switched on, that the switching on has in fact taken place, and further that the target temperature has been reached. To this end, therefore, preferably the heating up of the body by the heater is indicated by a first signal supplied by an indicator device, and the attaining of the target temperature is indicated by a second signal supplied by said device. If desired, the first signal may be e.g. a flashing signal supplied by an LED, and the second signal e.g. a constant illumination of such LED.

In carrying out the method in accordance with the invention, the control circuit may, if desired, be provided with a variable potentiometer by which the target temperature can be selected.

The method in accordance with the invention has been found to be useful in controlling the temperature of various integers of an apparatus for supplying a hot melt composition, e.g. a socalled thermo-cementing and folding machine.

The invention thus provides, in another of its several aspects, apparatus for supplying a hot melt composition, comprising a nozzle connected by a delivery tube to a melt chamber in which the composition is rendered molten, three heater circuits, one associated with each of said integers (viz. the nozzle, delivery tube and melt chamber), and three computer-controlled temperature sensing circuits, one associated with each of the heater circuits, wherein each temperature sensing circuit comprises a sensor by which the temperature of the integer being heated by its associate heater can be sensed, and by which an analogue signal can be supplied to an analogueto-digital converter, which convert such signal into a numerical value, and a computer which, at regular intervals, interrogates the analogue-todigital converter, compares each numerical value supplied as aforesaid with a corresponding further numerical value representing a pre-set "target" temperature for each of the integers, and issues a command to each heater circuit appropriate to the difference between the compared numerical values for that circuit.

Furthermore, in using an apparatus as set out above, conveniently, in an initial heating up period, the heater circuit for the melt chamber is first switched on, thereafter, when the signal from its associated sensing circuit has reached a predetermined numerical value, the heater circuit for the delivery tube is switched on, and subsequently, when the signal from the iatter's sensing circuit has reached a pre-determined numerical value, the heater circuit for the nozzle is switched, the arrangement being such that all three integers reach their target temperatures at approximately the same time.

There now follows a detailed description, to be read with reference to the accompanying drawings, of one machine in accordance with the invention.

This machine, it will be appreciated, has been selected for description merely by way of exemplification of the invention and not by way of limitation thereof.

In the accompanying drawings:~ Figure 1 is a front view of the machine to be described; and Figure 2 is a block diagram of an electronic control circuit of said machine.

The machine now to be described is a so-called thermo-cementing and folding machine, which finds use in the shoe industry and allied trades, where it is desired to fold the edge of the workpiece over on itself and secure it in a folded condition. To this end, the machine comprises a work table 10 on which is supported a block 12 having a work-guiding surface 14 which curves upwardly, out of the plane of the work table 10, so as to provide a smooth fold-initiating surface for a workpiece the edge of which is to be folded. For limiting the movement of the workpiece edge of the surface 14, a gauge finger 16 is provided which is adjustable heightwise by means of an adjustment knob 18. For raising the gauge finger 16 out of its operative position, furthermore, a manually operable lever 20 is provided.

For assisting in the formation of a fold, furthermore, a creaser foot 22 is mounted with its end adjacent the block 1 2. The creaser foot has a central passage through which hot-melt adhesive can be fed, the foot having an outlet through which adhesive can be fed onto the central region of the part of the workpiece to be folded. The passage in the creaser foot is supplied through a delivery tube 24 which is connected "upstream" to a gear pump 26 which in turn is fed from a melt chamber 28. Because the adhesive is a hot-melt, the melt chamber 28, delivery tube 24 and creaser foot 22 are each provided with a separate heater, respectively Hi, H2 and H3, of the electric cartridge type. The delivery tube, furthermore, is clad with appropriate thermal insulation.

For moving the creaser foot 22 out of its operative position a manually operable lever 38 is provided, which together with the lever 20, thus facilitates the introduction of a workpiece to the operating locality of the machine. The heightwise position of the creaser foot 22 is adjustable by an adjustment knob 40.

The machine, as so far described above, is conventional. Furthermore, also as is conventional, the machine comprises a snipping knife arrangement generally designated 30 and comprising a fixed blade 32 and a movable blade 34 mounted on the fixed blade, the blades being so arranged, "downstream" of the block 1 2, that they can cut the upstanding edge portion of the workpiece which is supported by the block 12. In general, the snipping knife arrangement 30 is used where the edge of the workpiece defines an "outside" curve.

For feeding a workpiece past the block 12 and the creaser foot 22, and also for completing and consolidating the fold, a work feed arrangement is provided comprising a hammer and anvil (not shown) which are moved orbitally, the arrangement being such that over a given part of the orbit, the hammer and anvil trap the workpiece therebetween as they move rearwardly over a given distance (feed length) and at a given speed (feed speed). The hammer and anvil are driven through a main drive shaft (not shown) of the machine, by means of an electric motor (not shown) through a clutch. The motor speed, and thus the feed speed, is controlled by a first treadle (not shown); a second treadle (also not shown) also is provided for operating two switches S6, S7, the arrangement being such that only one of said switches can be operated at any one time.

Switch S6 is effective to reduce the feed length, which thereby causes pleating of the folded over margin of the workpiece (and is thus especially use ul on sharp "inside" curves). For cor ,ling the feed length, "maximum" and "minimum" stops 46, 48 are provided, said stops being arranged to project through an appropriate slot 50 in the control panel to facilitate operator setting thereof. Switch S7 is effective to cause the snipping knife arrangement 30 to operate.

For switching the supply of adhesive on and off, a main switch S4 is provided on a control panel 36 of the machine, and, for controlling the supply of adhesive during the operation of the machine, a knee-operated switch S5 is provided.

The control panel 36 of the machine has, in addition to the main "adhesive supply" switch S4, a mains on-off switch S1 and a motor on-off switch S2. Mains power is thus supplied to two solenoids SOL1, SOL2 and to heaters H 1, H2, to be referred to hereinafter, and also to a transformer (not shown) which steps down the voltage to 12 volts. A 12 V a.c. supply is thus supplied to a work lamp (not shown) which can be switched on by switch S3, also on the control panel 36. In addition, this circuit supplies power to a further heater H3. From this 12 V a.c. circuit, furthermore, is derived an unsmoothed 12 volt d.c.

circuit which supplies power to a mains-controlled control box Ml supplying a "mains interrupt" signal to be referred to hereinafter. In addition, there is derived from the 12 V a.c. circuit a smoothed 12 V d.c. circuit which supplies power to an n.c. motor M (in casu a stepping motor), which will be referred to hereinafter. From the smooth 12 V d.c. circuit, furthermore, is derived a 5 V circuit, which drives a central processor unit (CPU) and circuits, and supplies power to switches S4, S5, S6 and S7 thermisters'TS 1, TS2, TS3 and potentiometers VR4, VR5 and VR6, each of which will be referred to hereinafter.

The control panel 36 also is provided with various indicator devices, including light-emitting diodes LED 1, LED 2, and LED 3, associated respectively with heaters H 1, H2 and H3, and LED 4 and LED 5, associated respectively with an "adhesive supply" circuit and with the kneeoperated switch S5, also as to be described in detail later.

As already mentioned, switches S6 and S7, which are operated under the control of the second treadle of the machine, cannot be operated simultaneously, the one switch being operated by depression of the toe of the operator on the treadle, and the other by depression of his or her heel. In some cases, however, it is desirable that snipping should take place while the feed length is reduced, and to this end a further control switch S8 is provided on the control panel 36 which is effective, in combination with switch S6, to cause snipping to take place simultaneously with the reduced feed length.

The machine in accordance with the invention is computer-controlled and comprises a central processor unit (CPU) in the form of a single chip 8-bit micro-computer (in casu, a Zilog Z8681 which in addition to a micro-processor, also incorporates a random access memory (RAM) (shown separately in Fig. 2) and scratch pad; th s microcomputer is obtainable from Zilog Inc.) For the internal timing of the CPU a system clock C, comprising a free-running 8 MHZ crystal, is provided.

The CPU is connected via I/O bus l/OB with input and output ports IP, OP and via a memory address and data bus DB with a non-volatile memory in the form of an EPROM (erasable programmable read-only memory), which is accessed by the CPU via the data bus DB for instructions to execute. A conventional decoder D is also provided for controlling the functioning of the input and output ports IP, OP.

The control circuit also includes an analogueto-digital convertor (ADC) to which signals are supplied by the potentiometers VR4, VR5, VR6, thermisters TS1 , TS2, TS3, and switches S4, S8.

The ADC is interrogated by the CPU, by the I/O bus, each time a mains interrupt signal is supplied to the CPU by the control box MI. More particularly, the various channels of the ADC are interrogated in turn, one in response to each mains interrupt in a so-called "wrap around" sequence. The ADC, in response to a signal from the decoder D, supplies information as to the state of the interrogated channel via the input port IP.

Also supplying information via the input port in response to an enabling signal from the decoder D, are switches S6, S7, while switch S5 provides a direct "interrupt" signal to the CPU.

The electronic control circuit also comprises a re-set sub-circuit R by which, upon starting up of the machine, the CPU is enabled to set the controls to their correct state in a rapid manner.

This sub-circuit R is directly connected into the CPU for this purpose.

A further, direct, "interrupt" input is provided to the CPU from a shaft encoder E which is driven by the main drive shaft of the machine. The shaft encoder E is conveniently a disc having a plurality of (in casu sixteen) equally spaced notches, with which are aligned two opto-switches operating through a flip-flop (set re-set) to supply pulses to the CPU. The switches are spaced apart from one another by a distance more than the width of a notch, so that if, for example, the main drive shaft is arrested in a position in which the edge of a notch is aligned with one of the switches, any vibration of the disc, e.g. caused by vibrations of the machine, will not result in the generation and supply to the central processing unit of a series of pulses, but rather that switch, having once emitted a pulse, will be disabled until the flip-flop is re-set by the other switch having been actuated.

In response to the various signals thus supplied to the CPU, the CPU supplies outputs, via output port OP, to sub-circuits controlling the heaters Hi, H2, H3, to sub-circuits controlling the solenoids SOL1, SOL2, to motor drive SMD and to the various LEDs referred to above.

Dealing now more specifically with particular features of the invention, the n.c. motor M is operatively connected to the gear pump 26 and serves to control the rate of feed of adhesive through the adhesive-supply system.

To this end, switch 54, which is a threeposition switch, is provided for manually switching the adhesive supply system on and off; the third position will be referred to later. Switch S4 is an overall control for switching at the start and finish of a working shift. In addition, knee-operated switch S5 is provided for switching the system on and off in each working cycle. Both switches S4, S5 serve, through the CPU, to switch motor M on and off.

The operating speed of the motor M is controlled by the shaft encoder E, as will now be described. Thus, in response to each pulse generated by the shaft encoder E, a digital "increment" value is added to an accumulator stored in the RAM of the CPU. This increment value is determined by an operator setting of the potentiometer VR4, which is provided with an adjustment knob 42 on the control panel 36 for this purpose. The potentiometer VR4 forms part of a metering circuit which supplies a signal through the ADC to the CPU. The range of adjustment of the potentiometer VR4 corresponds to a range of ratios of rotation of the main drive shaft to rotation of the motor M. In the machine described, the range of ratios is approximately 40:1 to 400:1 and this range of ratios corresponds to an output from the ADC of 255 to O (FF to 0 hex).The value of the signal from the ADC constitutes the increment value. The accumulator adds the increment value to the accumulated total in response to each pulse from the shaft encoder, and each time the accumulator "overflows", the motor M receives a drive pulse; in the case of a stepping motor, it is stepped through one step.

It will thus be appreciated that, by altering the position of adjustment knob 42, the rate of feed of adhesive in relation to the rotational speed of the shaft can be adjusted by the operator.

In a thermo-cementing and folding machine, as mentioned above, it is sometimes desirable to reduce the feed length, irrespective of the feed speed, in order to steer round "inside" curves. To this end, as is conventional, solenoid SOL1 is provided which switches the feed length between maximum and minimum as determined by the stops 46, 48, referred to above. More particularly, as is conventional, the solenoid SOL1 serves to cause the geometry of a linkage system to be so varied as to consequently vary also the distance through which the hammer and anvil move in feeding the workpiece. In the machine in accordance with the invention, solenoid SOL1 is operated by actuation of treadle-operated switch S6.

Reducing the feed length is of course effective to reduce the rate at which the workpiece is fed through the machine, but without reducing the feed speed, as measured at the main drive shaft, so that the amount of adhesive fed is not generally affected by a reduction of feed length. This can lead to excessive adhesive being supplied, which adhesive may of course be squeezed from beneath the fold, leaving an unsightly amount of adhesive visible in the finished workpiece. To overcome this problem therefore, a "metering modify" circuit is provided which supplies an appropriate signal through the input port.This circuit operates in combination with the "metering" circuit and with the circuit incorporating the switch S6 so that, upon operation of the switch S6, the increment value referred to above is reduced, thereby reducing the frequency of "overflow" of the accumulator, and thereby increasing the ratio between the main drive shaft and the output shaft of the motor M. It has been found that a reduction of 50% of the increment value is appropriate when operating with reduced feed length.

For enabling the "metering modify" circuit, switch S4 is provided with contacts which are closed when in its third position. Of course, in the third position, the adhesive supply is still switched on At the end of an operating cycle of the machine, it is regarded as desirable to avoid drooling of the adhesive from the passage in the creaser foot 22.

This is achieved conventionally by a "suck back" arrangement. In the machine in accordance with the invention, "suck back" is achieved by reversing the direction of rotation of the motor M through a pre-determined distance. This takes place independently of the rotation of the main drive shaft. Conveniently, this reversing of the motor drive takes place upon operation of the knee-operated switch S5, the main function of which is to cause the supply of adhesive to be terminated. Furthermore, in the machine in accordance with the invention, the amount of "suck back" can be adjusted by the operator and to this end the potentiometer VR5 is provided in a "suck back setting" circuit, the potentiometer having an adjustment knob 44 on the control panel 36.As already mentioned, the "suck back setting" circuit supplies a signal through the ADC so that the pre-determined distance can be varied according to operator preference. In the particular case, where a stepping motor is used to drive the gear pump 26, a range of O to 128 steps in the reverse direction has been found to be suitable, and the ADC serves to provide a "suck back" value in the range 0 to 255 (0 to FF hex) in response to the setting of the potentiometer VR5.

In order, furthermore, to avoid a deficiency of adhesive at the start of the next following machine cycle, the motor M driving the gear pump 26 is actuated, upon actuation of the knee-operated switch S5, and operates through a pre-determined distance at a fast speed. The pre-determined distance may be the same as the "suck back" distance, or, if desired, may be a proportion (whether greater or smailer) of that distance. The adhesive sucked back in the preceding machine cycle is thus restored at the start of each machine cycle. It is to be noted that, in order to prevent accidental switching on of the adhesive when the machine is not operating, the operation oF the gear pump 26 at a fast speed is enabled as aforesaid only if the main drive shaft is rotating.More especially, the "fast speed" operation of the pump is initiated only after two pulses having been generated by the shaft encoder E.

As is also conventional in thermo-cementing and folding machines, the operation of the snipping knife arrangement 30 is controlled by solenoid SOL2, which is operated upon actuation of the treadle-operated switch 57.

As already mentioned above, switches S6 and S7 cannot be operated simultaneously under the control of the treadle. In some cases, however, it is desirable that snipping should take place while the feed length is reduced. To this end, therefore, control switch 58 is provided, actuation of which is effective, when switch S6 is also actuated to cause snipping to take place simultaneously with the reduced feed length; that is to say, actuation of switch S8 causes solenoid SOL2 to be energised when switch 56 is actuated.

When the machine is switched on at the start of a working shift, the CPU is first enabled and ensures that any incorrect settings of the various operating elements are corrected. Thereafter, signals are supplied via the output port OP to heaters Hi, H2 and H3, which respectively supply heat to the melt chamber 28, delivery tube 24 and creaser foot 22. Because of the construction of the various elements, it is likely that the creaser foot 22 will heat up considerably more rapidly than the melt chamber 28, while the delivery tube 24 will heat more rapidly than the melt chamber but less rapidly than the creaser foot. Consequently, initially only the heater H1 for the melt chamber 28 is switched on.The heater H2 for the delivery tube 24 is then switched on at a pre-determined stage in the heating up of the melt chamber, and finally the heater H3 for the creaser foot 22 is switched on at a pre-determined stage in the heating up of the delivery tube.

For sensing the temperature of.the melt chamber thermistor TS 1 is provided, incorporated in a sub-circuit by which a signal is supplied to the ADC, which converts the signal to a numerical value between 255 and 0 (FF hex and 0). The switching on of the heater H2 for the delivery tube 26 takes place when the value of the ADC output reaches a predetermined number. Similarly, the temperature of the delivery tube 24 is also sensed by thermistor TS2, incorporated in a sub-circuit identical with that for the melt chamber, and at a given numerical value, the heater H3 for the creaser foot is switched on. In the case of both heaters H1, H2, a "target" temperature is pre-set and cannot be varied by the operator.The temperature control sub-circuits operate, once the target temperature has been achieved, to maintain the temperature at the target, in a manner described below.

The temperature of the creaser foot is also sensed by thermistor TS3, incorporated in a subcircuit which is generally similar to the aforementioned sub-circuits, but which also includes potentiometer VR6, having an adjustment knob 52 on the control panel. The maximum resistance of the potentiometer VR6 is relatively small in relation to that of the thermistor TS3, but is sufficient to enable the temperature of the creaser foot to be varied over a range of some 200 C, at the level of temperatures at which it is expected the machine will normally operate; the normal temperature range would be expected to be within approximately 130 to 1500 C.

For maintaining the temperatures at the "target", a programme stored in the EPROM establishes a band of numerical values extending at either side of the target value, this band representing a band of temperatures at either side of the target temperature. When the numerical value as sampled lies within the band, the appropriate duty cycle for the heaters over the next time interval (determined by the mains interrupt) is calculated; more specifically, the difference between the actual and target temperatures is calculated and, depending upon this difference, a proportion of the time interval is determined during which the heater is to be switched on, and appropriate instructions are issued, which are then executed during the time interval.Thus, for example, if the target value is almost achieved, the programme could calculate that the heater need be switched on for only 55% of the time interval, in which case after 55 main interrupts, the heater will be switched off for the remainder of that time interval.

It will thus be appreciated that, especially once the target value has been achieved, maintenance of the target temperature is much more accurately achieved than would be the case with a conventional thermostatic device.

During the heating up period, LED4 on the control panel 36 flashes to indicate that heating up is taking place. When the target temperatures in all three areas have been achieved, LED4 is constantly illuminated.

If, after the heating up period, the numerical value corresponding to the temperature of any heater is observed to have moved outside the band, and if it remains so for a pre-determined number of (in casu ten) consecutive interrogations, a warning signal is supplied to an appropriate one of the three light-emitting diodes LED1, LED2, LED3, which, as mentioned above, are associated respectively with the heaters H1, H2, H3. In the event that the fault has arisen as a consequence of the corresponding thermistor entering an "open circuit" condition, in which condition it will of course supply a permanent maximum signal (FF hex) to the ADC, the appropriate LED will flash. In such a case, furthermore, in order that the operator can continue to use the machine for a limited period, e.g. in order to finish a batch of work being operated upon, even though the monitoring of the performance of the heaters is no longer being correctly carried out, while at the same time ensuring that the machine will not be damaged by continued use, once a malfunction of the thermistor is detected, the machine will continue to operate for a further pre-determined period (in casu ten minutes), during which period a 50% duty cycle is implemented for the heater associated with the malfunctioning thermistor.

That is to say, during each time interval the heater will be switched on and off for equal proportions.

At the end of the pre-determined period, the CPU instructs a relay RL1 to drop out, whereby the mains power supply is cut off and thus the machine operation is terminated and all the heaters are de-energised.

The diodes LED1, LED2, LED3 are also used to diagnose any "heater channel" failures, in which case the appropriate LED is constantly illuminated.

Such failures include failure of the heating elements and of the triacs controlling the heater elements, and also if one of the thermistors falls out of or is removed from the pocket in which it is to be located. In such circumstances, the warning is indicated when the numerical value moves outside the band (and in this case the observed change in signal is likely to take place more slowly than in the case of a thermistor going into "open circuit" condition - which feature of course is utilised to distinguish between the failure of the sensing circuit and that of the heater circuit or heater control circuit). If thereafter the change in numerical value continues to take place away from the target value, an "interlock" signal is supplied by the thermistor, causing the power supply to the machine to be switched off, again by relay RL1 dropping out.

The relay RL1 also serves as a general "watch dog" over the whole of the control circuit. To this end, it is maintained in a "made" condition during normal operation of the machine by a control subcircuit which is "refreshed" at regular intervals, failure to refresh the sub-circuit causing the relay RL1 to drop out. More particularly, the sub-circuit receives a signal at each mains interrupt, the signal serving to change the state of the circuit between "1" and "0", the arrangement being such that switching to the "1" state constituting the "refresh" signal. The sub-circuit is arranged to become de-energised, in the absence of a refresh signal, after a time interval which is greater than the interval between two "1" signals. Deenergisation of the sub-circuit of course switches off the relay, thereby terminating the power supply to the machine.

Claims

1. A method of controlling the temperature of a body which is being heated comprising~ determining a target temperature and storing in a computer a numerical value corresponding to the target temperature, sensing the temperature of the body by means of a sensor which supplies an analogue signal to an analogue-to-digital convertor, which in turn converts the analogue signal into a numerical value, comparing the numerical value of said signal with that corresponding to the target temperature, and supplying a command to a heater circuit appropriate to the difference between the compared numerical values.

2. A method according to Claim 1 wherein at regular intervals the analogue-to-digital convertor is interrogated as to the state of the sensor and the numerical values are compared as aforesaid, and further wherein the command to the heater circuit for each time interval is divided into "on" and "off" periods, when respectively the power supply to the heater circuit is switched on and off, such division being proportional to the difference between the numerical values being compared.

3. A method according to Claim 2 wherein, when the target temperature has been determined and the corresponding numerical values stored, a band of such values is established at either side of the "target temperature value, the command to the heater for each time interval being divided as aforesaid only when the numerical value being compared with the "target temperature" value lies within said band.

4. A method according to any one of the preceding Claims wherein if, after a predetermined time following the switching on of the heater circuit, the numerical value being compared with the "target temperature" value is spaced from the latter value by a pre-determined amount (indicating too high or too low a temperature) a warning is indicated and further, in the event that, in a subsequent comparison, the difference between the two numerical values increases to a further pre-determined amount, the heater circuit is switched off.

5. A method according to any one of the preceding Claims wherein if, after the target temperature has been reached, the numerical value corresponding to the analogue signal from the sensor increases to a maximum value, or substantially so (indicating that the sensor has entered an "open circuit" condition), a warning is indicated.

6. A method according to Claim 5 when tied directly or indirectly to Claim 2 wherein, upon a warning being indicated as aforesaid, the command to the heater circuit for each subsequent time interval is divided into equal "on" and "off" periods for a pre-determined time, whereafter the heater circuit is switched off.

7. A method according to any one of the preceding Claims wherein the heating up of the body by the heater is indicated by a first signal supplied by an indicator device, and the attaining of the target temperature is indicated by a second signal supplied by said device.

8. A method according to any one of the preceding Claims wherein the target temperature is selected by setting a potentiometer arranged in the sensor circuit.

9. Apparatus for supplying a hot melt composition, comprising:~ a nozzle connected by a delivery tube to a melt chamber in which the composition is rendered molten, three heater circuits, one associated with each of said integers (viz. the nozzle, delivery tube and melt chamber), and three computer-controlled temperature sensing circuits, one associated with each of the heater circuits, wherein each temperature sensing circuit comprises::~ a sensor by which the temperature of the integer being heated by its associated heater can be sensed, and by which an analogue signal can be supplied to an analogue-to-digital convertor, which converts such signal into a numerical value, and a computer which, at regular intervals, interrogates the analogue-to-digital convertor, compares each numerical value supplied as aforesaid with a corresponding further numerical value representing a pre-set "target" temperature for each of the integers, and issues a command to each heater circuit appropriate to the difference between the compared numerical values for that circuit.

10. Apparatus according to Claim 9 wherein, in an initial heating up period, the heater circuit for the melt chamber is first switched on, thereafter, when the signal from its associated sensing circuit has reached a pre-determined numerical value, the heater circuit for the delivery tube is switched on, and subsequently, when the signal from the latter's sensing circuit has reached a predetermined numerical value, the heater circuit for the nozzle is switched, the arrangement being such that all three integers, reach their target temperatures at approximately the same time.