METHOD AND APPARATUS FOR MONITORING AND CONTROLLING A

HEATER ZONE

Cross-Reference to Related Applications

This application claims the benefit of U.S. Provisional Patent Application No. 60/634,554 filed on December 10, 2005 entitled "Method and Apparatus for Monitoring and Controlling a Moulding Machine Heater Zone"

Field of the Invention

The present invention relates generally to temperature control and more specifically, to method and apparatus for monitoring and controlling a heater zone.

Background of the Invention

Equipment and machinery having heating zones that require accurate temperature control are well known in the art. For example, moulding machines which use heat and pressure to melt, reshape and mould granular material into moulded articles, must be accurately controlled to ensure the integrity of the produced articles. Moulding machines of this nature include for example, injection moulding machines, blow moulding machines, extrusion moulding machines, die-cast moulding machines and thixo-moulding machines. Such moulding machines make use of heater elements positioned in heater zones to generate the heat that is necessary to melt and mix the granular material. The heater elements may be of the resistive, inductive or infrared type. For example, blow moulding machines use infrared heater lamps arranged within an oven at various profiling positions to profile a preform to be stretched. Thixo-moulding machines operate injection barrels at temperatures of up to 1000 ⁰ F using calrod heater elements to melt and mix the granular material.

Regardless of type, heater elements used in moulding machines unfortunately degrade over time due to environmental factors, such as for example, shock, vibration, high temperature, hydro-scopics, and misuse during replacement. As the heater elements degrade, their current carrying characteristics change. In less severe cases, degradation of heater elements can result in temperature fluctuations in the heater zones. In more severe cases, heater element degradation can result in short

and open circuits rendering the heater zones inoperative. Whether less or more severe, temperature fluctuations can adversely affect moulded article quality. For example, temperature fluctuations can affect crystallinity in the moulded article, cause a chemical byproduct referred to as AA to be created, which may transfer to the moulded articles or cause carbonated elements in products held by the moulded articles to be released.

To maintain heater zones at desired temperature levels and avoid such temperature fluctuations, controllers are typically provided to control the supply of power to the heater elements in the heater zones to maintain the temperature in the heater zones. Commonly, heater zone controllers employ solid state relays (SSRs) that are activated in a cyclical manner as temperature demands, set by moulding machine operating parameters, are met. The SSRs are responsive to digital signals output by a processor that uses logarithmic formulae to generate the digital signals. As a result, the SSRs are cycled until stable temperatures in the heater zones are obtained. Monitoring of the heater zones to detect when the heater zones drop below desired levels can be carried out either actively or passively. In some cases, thermocouples are employed in the heater zones, to provide active feedback to the processor concerning the temperature in the heater zones. In other cases, passive feedback methods are employed to determine if the heater zones are maintaining their desired temperature levels. Such passive feedback methods include for example visual and/or chemical tests of moulded articles and predictive heater element analysis.

Controlling heater zones in moulding machines presents challenges. Typical moulding machines have many heater zones, each requiring different heat control. For example, the unit of the moulding machine that homogenizes the granular material has different heat control requirements than the moulding unit of the moulding machine even though these units work in conjunction. Also, some areas of moulding machines are restricted in terms of space due to mechanical stress issues, machining cost restraints, and input costs. As a result, heater zones having restricted space may be unable to accommodate thermocouples. In such cases, passive temperature feedback methodologies are the only options.

In some moulding machine environments, plural heater zones are grouped together with the temperature of the heater zone group being monitored using one of the above-described passive temperature feedback methods. In this case, immediate temperature feedback over the entire heater zone group and at each individual heater zone within the group is simply unavailable.

As will be appreciated, the above-described temperature feedback methodologies lack accuracy and responsiveness. These temperature feedback methodologies also fail to detect heater element degradations that affect moulding machine performance and provide advance warning of potential heater zone failures. Monitoring of heater zones to ensure desired temperatures are maintained is important. It is however, also desired to monitor heater zones so that undesired operating conditions can be detected before moulding machine failures occur.

Under certain conditions, elements such as steel braid overlaid conductors, or semiconductor devices such as solid state relays (SSRs) that are used in moulding machines may cause fires or explode. For example, if the heater elements in a heater zone are wired to a power circuit using steel braid overlaid conductors, under harsh environments the metal sheath or braid surrounding the conductors can break down to the extent that an arc may appear between the steel braid and the surface of the moulding machine being heated. This may result in a fire or explosion leading to failure of the moulding machine. Ensuring that a fire or explosion does not occur is particularly important in thixo-moulding machines, which use magnetism, a highly flammable material that requires constant monitoring.

Failure of a moulding machine can prove to be costly. PET-type moulding machines, which create preforms for such uses as bottles, often include a significant number of heater zones, in some cases as many as one-hundred and twenty- four (124) heater zones. Should a failure happen, the scrap rate of the moulding machine is 124 parts, typically per every nine seconds resulting in significant losses. Blow moulding machines often operate at rates of up to 60,000 parts/hr. Again should a failure happen, significant losses can result. Moulding machines used to produce articles for use in the automotive field typically operate on

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a twenty-four (24) hour basis. Down time due to moulding machine failure often results in penalties to the article producer.

As will be appreciated, being able to detect conditions that signify potential heater zone and/or moulding machine failures is desired. It is therefore not surprising that attempts have been made to predict heater failure in a moulding machine.

For example, U.S. Patent Nos. 6,718,283 and 6,336,083 to Lanham, et al. disclose a method and apparatus for predicting heater failure by using a laboratory model heater element against collected field data. The apparatus compares a laboratory model heater element bench marked against existing heater elements, to estimate the life of existence of the heater elements in order to predict their failure. Although the apparatus predicts heater element life expectancy, the predictions are based on a laboratory model and thus, they can be inaccurate. Also, since the predictions are based on collected field data, the apparatus is unable to provide real- time feedback concerning the operating conditions of heater elements.

Improvements to enable heater zones and/or heater elements to be monitored substantially continuously so that heater element degradations can be detected prior to failure are desired.

It is therefore an object of the present invention to provide a novel method and apparatus for monitoring and controlling a heater zone.

Summary of the Invention

Accordingly, in one aspect there is provided an apparatus for monitoring and controlling a heater zone comprising: a sensing device monitoring at least one heater element within said heater zone and outputting a signal representing the operation of said heater element; and a controller communicating with said sensing device and signaling an alarm condition when operation of said heater element deviates from normal operation.

In one embodiment, the sensing device is a Hall-effect sensor that senses current drawn by the heater element. The controller in this case signals the

alarm condition when the current carrying characteristics of the heater element change signifying a potential failure condition. The controller also adjusts current flow to the heater element to compensate for the change in heater element current carrying characteristics. According to another aspect there is provided an apparatus for monitoring and controlling a heater zone having at least one heater element, said apparatus comprising: a switching device coupling said heater element to a power source and being actuable to allow power to be supplied to said heater element; a current sensing device sensing current supplied to said heater element when said switching device is actuated; and a controller coupled to said switching device and said current sensing device, said controller signaling an alarm condition when current drawn by said heater element deviates from normal operation and adjusting actuation of said switching device to compensate for the deviant heater element operation.

According to yet another aspect there is provided a method of monitoring and controlling a heater zone comprising: generally continuously monitoring the current drawn by at least one heater element in said heater zone; determining when the drawn current signifies heater element degradation and a potential failure condition; and signaling an alarm condition.

According to still yet another aspect there is provided a circuit comprising: a housing having terminals for coupling between a power supply and load and having an output terminal; and a series circuit within said housing comprising a switching device and a current sensor, said current sensor monitoring current drawn by said load when said circuit is electrically coupled between said power supply and said load and said switching device is actuated, said current sensor providing an output signal on said output terminal representative of said sensed current.

The apparatus and method provide advantages in that the operating condition of the heater element is monitored on a generally continuous basis allowing heater element degradation that may lead to a failure to be detected prior to occurrence of such a failure. In this manner, the heater zone can be serviced prior to the occurrence of a moulding machine failure. This of course avoids the significant expense associated with moulding machine failures.

Brief Description of the Drawings

Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which:

Figure l is a schematic circuit diagram of a heater zone monitoring and controlling circuit;

Figure 2 is a block diagram of the heater zone monitoring and controlling circuit of Figure 1; Figure 3 are graphs showing adaptive heater zone control using the heater zone monitoring and controlling circuit of Figure 1;

Figure 4a is a plan view of another embodiment of a heater zone monitoring and controlling circuit;

Figures 4b and 4c are plan views of yet another embodiment of a heater zone monitoring and controlling circuit;

Figure 5 is a side elevation view of yet another embodiment of a heater zone monitoring and controlling circuit;

Figure 6 is a side elevation view of still yet another embodiment of a heater zone monitoring and controlling circuit; Figure 7 is a perspective view of still yet another embodiment of a heater zone monitoring and controlling circuit; and

Figure 8 is a schematic circuit diagram of still yet another embodiment of a heater zone monitoring and controlling circuit.

Detailed Description of the Embodiments

An apparatus and method for monitoring and controlling a heater zone are provided. The apparatus comprises a sensing device which monitors at least one heater element within the heater zone and outputs a signal representing the operation of the heater element. A controller communicates with the sensing device and signals an alarm condition when operation of the heater element deviates from normal operation. Specific embodiments of the apparatus incorporated into a moulding machine will now be described with particular reference to Figures 1 to 8.

Turning now to Figures 1 and 2, a heater zone monitoring and controlling circuit is shown and is generally identified by reference numeral 10. As can be seen, heater zone monitoring and controlling circuit 10 is coupled to a heater zone 12 within a moulding machine to monitor and control the heater zone. In this particular example, heater zone 12 is shown as including a single heater element 14. Those of skill in the art will appreciate that this is for ease of illustration only. Heater zone 12 may include a plurality of heater elements 14. The heater element 14 is coupled to lines Li and L ₂ of an ac or dc power supply 16 via supply conductors 18 and 20 and protection circuitry 21. A switching device 22 in the form of a silicon controlled rectifier (SCR) is provided along conductor 18. SCR 22 is coupled to a controller 24 in the form of a personal computer (PC) or programmable logic controller (PLC). PC/PLC 24 provides switching signals to the SCR 22 to control current flow to the heater element 14.

Heater zone monitoring and controlling circuit 10 includes a Hall- effect sensor 30 positioned along supply conductor 18 between the heater element 14 and the SCR 22. The Hall-effect sensor 30 senses current flow to the heater element 14 and generates signal output I _HES that is proportional to the level of the sensed current. The signal output I _HES generated by the Hall-effect sensor 30 is conveyed to the controller 24 via a Field-bus 34 such as for example, Profibus or CANbus. Alternatively, the signal output IH _ES generated by the Hall-effect sensor 30 may be provided to the controller 24 via a multiplexer 36. SCR 22, Hall-effect sensor 30 and multiplexer 36 are mounted on an open face printed circuit board 38. Although not shown, a suitable power supply provides the necessary power for the components on the circuit board 88.

The controller 24 executes a heater zone monitoring and controlling program. During program execution, the controller 24 uses the analog output signal I _HES received either from the Field-bus 34 or multiplexer 36 to determine the current supplied to the heater zone 12 and the operating condition of the heater element 14. When the temperature of the heater zone 12 drops below a desired level, the controller 24 adjusts the switching signal output provided to the SCR 22 thereby to change the duty cycle of the SCR 22 and alter the current supplied to the heater element 14 until the temperature within the heater zone 12 returns to the desired level. The controller 24 also monitors the level of current drawn by the heater element 14 to detect situations where the current carrying characteristics of the heater element 14 have changed signifying a potential heater element failure condition. In the event of such a condition, an alarm signal S _A is generated by the controller 24 to provide advance warning to the moulding machine operator allowing preventive maintenance to be performed on the moulding machine before a heater element failure occurs. If the current carrying characteristics of the heater element 14 have changed dramatically signifying an unsafe operating condition, the controller 24 in addition to generating the alarm signal S _A , conditions the SCR 22 to electrically isolate the heater element 14 from the power supply 16.

The operation of the heater zone monitoring and controlling circuit 10 will now be further described. During operation, the controller 24 provides switching signals to the SCR 22 causing the SCR to switch on and off according to a programmed duty cycle so that the heater zone 12 is heated in a manner to meet the moulding machine operating demands. When the SCR 22 is actuated and turns on, current is supplied to the heater element 14 causing the heater element to be energized and thus, to generate heat. The duty cycle is selected so that the heater element 14 heats the heater zone 12 to a desired temperature. When the SCR 22 is on and current flows to the heater element 14, the Hall-effect sensor 30 generates the signal output I _HES , which as mentioned previously, is proportional to the current drawn by the heater element 14. The controller 24, which receives the signal output I _HES generated by the Hall-effect sensor 30, compares the sensed current level to a set-point current level, representing the current level that should be drawn by the heater element 14

when it is operating properly. When the sensed current level is at or near to the set- point current level, the controller 24 continues to operate the SCR 22 at the programmed duty cycle.

When the sensed current level deviates from the set-point current level by a threshold lower margin, the controller 24 changes the switching signal output to alter the duty cycle of the SCR 22 so that the heater element 14 is energized more or less depending on whether the sensed current level signifies an increase or decrease of the heater zone temperature from the desired heater zone temperature. If the sensed current level deviates from the set-point current level by more than a threshold upper margin, the controller 24 also generates an alarm signal S _A signifying a potential heater element failure condition. If the sensed current level signifies an unsafe operating condition, the controller 24 conditions the SCR 22 to electrically isolate the heater element 14 from the power supply 16 in addition to generating the alarm signal

SA Figure 3 presents a number of graphs showing heater zone temperature

HZ _T , SCR switching and Hall-effect sensor signal output I _HES during operation of the heater zone monitoring and controlling circuit 10. In the heater zone temperature graph 100, line 102 represents the desired heater zone temperature. During normal operation, the SCR 22 is switched according to the programmed duty cycle 104 as shown in graph 106 to maintain the temperature HZ _T in the heater zone at or near to the desired temperature 102. When the SCR 22 is switched in this manner, the Hall- effect sensor 30 generates signal output I _HES that in effect mirrors the SCR duty cycle as identified by reference number 108 in graph 110.

Point 112 represents a condition where the current I _HE drawn by the heater element 14 drops below the threshold upper margin resulting in a temperature drop in the heater zone 12 below the desired temperature 102. In this case, the signal output I _HES of the Hall-effect sensor 30 reflects the drop in drawn current as identified by reference number 114 in graph 110 allowing this operating condition to be detected by the controller 24. The controller 24 in turn adjusts the duty cycle of the SCR 22 as identified by reference number 116 in graph 106 to increase the current I _HE drawn by the heater element 14 until the temperature of the heater zone 12 returns to the desired temperature level 102 as shown at point 118. Once the temperature of the

heater zone 12 returns to the desired temperature level 102 as signified by the signal output I _HES of the Hall-effect sensor 30 and shown in graph 110 by reference number 120, the controller 24 alters the switching signal output to the SCR 22 so that it again is switched according to the programmed duty cycle 104. Graph 130 shows an alternative method of altering the duty cycle of the SCR 22 during the interval between points 112 and 118 to return the heater zone temperature HZ _T to the desired temperature level 102.

Graph 140 in Figure 3 shows the signal output I _H E _S of the Hall-effect sensor 30 when a partial short circuit condition occurs at point 112 that is not compensated for by the protection circuitry 21. In this case, the controller 24 generates the alarm signal S _A and conditions the SCR 22 to electrically isolate the heater element 14 from the power supply 16 as identified by reference number 152 in graph 150 allowing the heater zone to be repaired. After repair at point 118, the operation of the heater zone monitoring and controlling circuit returns to normal operation.

In the above embodiment, the SCR 22, Hall-effect sensor 30 and multiplexer 36 are described as being mounted on an open face printed circuit board 38. Those of skill in the art will however appreciate that the physical layout of the heater zone monitoring and controlling circuit components may vary depending on the environment in which the circuit 10 is to be employed.

For example, Figure 4a shows an alternative layout of the heater zone monitoring and controlling circuit 10. In this case, the components of the heater zone monitoring and controlling circuit 10 are mounted within a housing 200 rather than on an open face printed circuit board. Terminal 202 on the housing 200 receive conductor 18 coupled to the power supply. Terminal 204 on the housing 200 is connected to the conductor leading to the heating element 14. SCR 22 and Hall-effect sensor 30 are in series and connected between terminals 202 and 204. In this manner, the Hall-effect sensor 30, which is in series with the SCR 22 and heater element 14, generates the signal output I _HES that is supplied to the controller 24 via output terminal 205. Terminals 206 and 208 are coupled to the controller allowing the switching signal input to be provided to SCR 22.

Figures 4b and 4c show yet another layout of the heater zone monitoring and controlling circuit 10. hi this embodiment, the components of the heater zone monitoring and controlling circuit are mounted in two separate housings 200a and 200b. Housing 200a includes terminals 202a to 208a. SCR 22 is accommodated by the housing 200a and is connected across terminals 202a and 204a. Housing 200b includes terminals 250b to 256b. Hall-effect sensor 30 is accommodated by the housing 200b and is connected across terminals 252b and 254b. As shown in Figure 4c, housing 200b is mechanically and electrically coupled to housing 200a via terminals 204 and 252b. Terminals 254b and 256b are for a connection to a power source from the Hall-effect sensor 30. Terminals 206a and 208a receive the switching signal output of the controller to drive the SCR 22. Terminal 202 receives the conductor 18 coupled to the power supply. Terminal 250b is connected to the conductor leading to the heating element 14. Similar to the previous layout, the Hall-effect sensor 30 is connected in series with the SCR 22 and heater element 14 allowing the signal output I _HES generated by the Hall-effect sensor to be supplied to the controller 24 via output terminal 205b. Terminals 206a and 208a are coupled to the controller allowing the switching signal input to be provided to SCR 22.

Figure 5 shows another embodiment of the heater zone monitoring and controlling circuit 10. In this embodiment, the heater zone 12 includes a plurality of SCRs 22 and heater elements 14. The heater zone monitoring and controlling circuit 10 includes a plurality of Hall-effect sensors 30. Each Hall-effect sensor 30 is connected in series with an SCR 22 and heater element 14 allowing the current drawn by each heater element 14 in the heater zone 12 to be monitored. Similar to the embodiment of Figure 1, the SCRs 22 and Hall-effect sensors 30 are mounted on an open face printed circuit board 38.

Figure 6 shows yet another heater zone monitoring and controlling circuit 10 similar to that of Figure 5. In this embodiment however, the components of the heater zone monitoring and controlling circuit are accommodated within a housing 300. Multi-terminal connectors 302 and 304 are provided to enable the Hall-effect sensors 30 to be connected in series with the SCRs 22 and heater elements 14 and to

allow the signal output I _HES of the Hall-effect sensors 30 to be supplied to the controller 24.

Figure 7 shows still yet another embodiment of the heater zone monitoring and controlling circuit 10. In this embodiment, the Hall-effect sensor 30 is accommodated by a terminal housing 400 mounted on a DIN rail 402. The housing 400 includes terminals 404 to 414. Terminals 404 and 408 receive power supply conductors that provide power for the Hall-effect sensor. Terminal 410 leads to the heater zone 14. Terminal 412 is coupled to the SCR 22 and terminal 414 provides the signal output I _HES to the controller 24. Terminal 406 is a spare. In this manner, the Hall-effect sensor 30 is connected in series with the SCR 22 and heater element 14 thereby to allow the signal output I _HES to be supplied to the controller 24.

Figure 8 shows still yet another embodiment of a heater zone monitoring and controlling circuit 10. hi this embodiment, the SCR and Hall-effect sensor are integrated and packaged in a single common housing 500. Housing 500 has an input terminal 502 receiving conductor 18 coupled to the power supply. A series circuit within the housing including SCR 22 and Hall-effect sensor 30 extends across input terminal 502 and terminal 504. Terminal 504 is connected to the conductor leading to heating element, hi this manner, SCR 22 and the Hall-effect sensor 30 are in series with the heating element, similar to the previous embodiments. SCR 22 is driven by a control circuit 550 including an optoisolator

552, a light emitting diode (LED) 554 and a filtered trigger switch 556. Optoisolator 552 and LED 554 are in parallel and extend between terminals 506 and 508 on the housing 500. Terminals 506 and 508 are coupled to the controller 24 and receive the switching signal output thereof. Switching signals received on the terminals 506 and 508 drive the optoisolator 552 and switch 556 resulting in switching pulses being supplied to the SCR 22. A voltage regulator 560 and power source 562 are also provided.

Hall-effect sensor 30 is shown comprising a magnetic concentrator 530 surrounding the conductor on which the load current drawn by the heating element appears. The output of the Hall-effect sensor 30 is fed to amplifying and filtering circuitry 570. Amplifying and filtering circuitry 570 performs dynamic offset cancellation, amplifies and filters the Hall-effect sensor output. The output of the

amplifying and filtering circuitry 570 is then applied to signal conditioning circuitry 572, which conditions the input signal prior to providing the signal output I _HES on output terminal 505.

Although the Hall-effect sensor is described as being connected in series between the SCR and heater element, those of skill in the art will appreciate that the Hall-effect sensor may be positioned at any suitable location that permits the current drawn by the heater element to be sensed. Also, other sensing devices that provide real-time output concerning the operating state of the heater element can be used. Further, those of skill in the art will appreciate that other suitable switching devices, such as far example, triacs may be used instead of SCRs.

In the above examples, the heater zone monitoring and controlling circuit 10 is described with reference to moulding machines. It will however be appreciated by those skilled in the art that the heater zone monitoring and controlling circuit 10 can be used in other machines and equipment having heater zones, such as for example, printing and thermography devices, extruders etc.

Although embodiments have been described, those of skill in the art will appreciate that the variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.