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A microprocessor control system for thermal sterilisation operations
Journal of Food Engineering
5 (1986)
31-53
A Microprocessor Control System for Thermal Sterilisation Operations B. P. Lappo and M. J. W. Povey* Procter Department
of Food Science, The University, Leeds LS2 9JT, UK
ABSTRACT
A development facility, comprising a steam sterilising retort, a microprocessor development system and associated instrumentation and control equipment, is described. Features of the system are its modular design and the use of high level language software. The performance of the facility is reported and the influence of instrument accuracy on the control of sterilisation is explored. A sterilisation monitor capable of scanning IO thermocouples and computing individual sterilisation or ‘cook’ values for each channel is described.
NOMENCLATURE Algorithm A set of instructions for the execution of a calculation Analogue signal A continuous signal which may vary in magnitude Analogue to digital (A to D) Conversion of an analogue signal into a digital signal Assembler An aid to programming in machine code Basic A programming language, very widely used on small computers Decimal reduction time (II) The time during which the bacterial count decreases ten-fold Differential A signal which depends on the difference between two signals Digital signal A signal consisting of a stream of pulses representing numbers *To whom correspondence
should be addressed. 31
of Food Engineering 0260-8774/86/$03.50 - 0 Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain.
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B. P. Lappo, M. J. W. Povey
Digital to analogue (D to A) Conversion of a digital signal into an analogue signal F The time, in minutes, of a hypothetical process which is brought instantaneously to a reference temperature after which it is instantaneously cooled and which is equivalent in lethality to the actual process under consideration F0 Fis designatedF,,ifz = 10 and Tref= 121.1”C f’, j In Fig. 1 the h ea t penetration factor fi, is the time for the straight line plot to traverse one log cycle of temperature deficit and j is the ratio of the pseudo-initial deficit (p.i.d.) to the initial temperature deficit (r). .fr, is related to the thermal diffusivity (a) in conduction packs through the expression fi, = 6.63 X 1O-“/[ (l/ Y’ + 1 .708/h2)c\l] where f,, is in minutes, r and h are the radius and height of the can in metres and (x is in m2 s-’ Hardware The physical components which comprise the equipment Interface Connection between the computer and the rest of the equipment Lethal rate (L) If the destruction rate for microorganisms is arbitrarily defined as 1 at cef then, from the definition of z, it follows that the rate at any other temperature is given by L = 10 -(T=f-W Machine code The digital instructions used by the microprocessor itself. All instructions to the computer ultimately take this form Mainframe A computer capable of supporting many users simultaneously Microprocessor A microelectronic circuit capable, in response to externally supplied machine code, of rapidly executing the addition and subtraction of digital signals Multiplexer Apparatus which selects any one of many signals Optical isolation Electrical isolation by means of light signals PID Proportional, integral and derivative - an algorithm frequently used to control process inputs and outputs to equipment via valves RAM Random access memory ~- erasable electronic memory ROM Read only memory - non-erasable electronic memory Serial port A computer connection to other equipment admitting a stream of digital signals, one at a time Set point A selected value of a variable which it is desired to achieve Software That part of the computing process which can be altered without any apparent change in its physical form. Information in
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machine code is software as are the instruction manuals for the equipment Source code The code in which the software was originally written. If the source code is in a high level language such as Basic it will require translation into machine code Tref The temperature at which sterilisation processes are compared. It is 12 1.1 “C in this paper z The temperature rise required to reduce the decimal reduction time ten-fold
fH
__ :
j :
p.i.d. I
I ; 20 Process
40 time,
min
Fig. 1. A typical heat penetration curve plotted as the logarithm of the temperature deficit versus process time. The positions of the initial temperature deficit (I) and the pseudo initial deficit (p.i.d.) are indicated on the graph.
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B. P. Lappo, M. J. W. Povey
INTRODUCTION
A number of authors have drawn attention to the benefits which may accrue through optimisation and automatic control of the thermal sterilisation operation in food preservation. These potential advantages include minimising process deviations (Teixeira and Manson, 1982) on-line computation of the degree of thermal sterilisation (Holdsworth, 1974; Hayakawa, 1977) and maximising nutrient retention (Teixeira et al., 1969; Saguy and Karel, 1979). Other potential benefits include automatic documentation of the process (Holdsworth, 1983) optimisation of textural, flavour and colour characteristics and on-line measurement of heat penetration. The evidence for an optimum process for nutrient retention (Saguy and Karel, 1979) is repeated in perhaps more convincing terms for textural and flavour characteristics in the ‘cook value’ (Ohlsson, 198Oa, b; van Boxtel and de Fielliettaz Goethart, 1982). However, the great majority of these contributions have been theoretical in content. Examples of experimental verification of the theory include work by Teixeira ef al. (1975) and Ohlsson (1980b). However, nutrient or cook value optimisation is only a part of the story so far as automatic control of the sterilisation process is concerned. For instance, considerable reduction in process times might be achieved through improved instrumentation, with consequent increases in product throughput in batch retorting operations. In the present authors’ view it is necessary to approach the problem of improving the thermal sterilisation step differently. All the advantages detailed above imply the use of an automatic control system for a thermal steriliser and if the best control strategy is to be found then a development system is required which allows all aspects of the strategy to be employed. Aspects to be considered include the factory-installed capital cost of the system, interaction between operators and instruments, maintenance of hardware and software, reliability, instrumentation accuracy, the effects of fluctuations in services on operation and the interaction between management and the system. A number of general features, desirable in any control system, should be noted. For ease of maintenance, both the hardware and the software should be modular. Thus, if a fault develops in any module it can be traced easily, the module removed and returned to its manufacturer and replaced with a spare. In the case of software, modularity implies a high degree
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of documentation and, ideally, source code written in a high level language. In this way many people can work on the software and changes can be incorporated in individual modules, leaving other modules unaffected. Consequently, one of the main benefits of a software-based system, ease of modification, can be achieved. In addition, both hardware and software can be tailored to particular requirements, without rewriting all the software or rebuilding the hardware. An earlier approach to the problem of building an automatic retort system is described by Steele (19804b) and a more recently developed system is described by Bown (1982). For a large-scale system, consisting of two or more steam retorts, high level language software becomes essential. In addition, the option of distributed control should be available, as opposed to real-time control of every operation in every element of the system by one computer. A distributed control system may consist of a number of local controllers each sufficient to control automatically an individual retort. Communication with the central controller would be by a pair of wires with consequent dramatic reduction in cabling costs. Communication would consist of initiation by the central computer of the required process, probably following a start command from those responsible for loading the retort. The local controller would then carry out the required process without any further intervention from the central computer, so that if any fault developed centrally the retorting operation would continue to completion. The local controller would report progress back to the central computer, drive a local mimic diagram display and control local operator intervention. Such a system would also allow each retort to be driven in the conventional, semiautomatic manner. A development system should permit exploration of a number of available control strategies and allow the accumulation of considerable experience with the particular hardware/software combination chosen. The system described below is capable of following a prescribed temperature-time profile and of altering its profile according to an Fb value computed from any of 16 within-can temperature measurements or by an FO value derived from retort temperature. Control of pressure is available during the cooling process in order to reduce stresses on the container. The hardware is modular and the software is written almost entirely in an advanced form of Basic, allowing user documentation of the program and a modular construction.
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B. P. Lappo, M. J. W. Povey
Experimental data is presented to characterise the performance of the retort and its instrumentation. The importance of instrumentation accuracy is discussed and a computer calculation of heat penetration into a can is used to investigate the significance of errors in temperature sensing with regard to process times.
EXPERIMENTAL The development system is outlined in the form of a block diagram in Fig. 2. The facility was designed and assembled by the authors. Computer system The computer is the Motorola Exorset 30 microprocessor development system for which a wide range of interfaces and software are available. The hardware consists of a standard typewriter keyboard together with 16 function keys, a visual display unit with graphics capability, a dot matrix printer for printing of screen images, 32 digital outputs, 32 digital inputs, 16 analogue inputs and two analogue outputs. The 16 analogue inputs are multiplexed into a dual input, 12 bit analogue-todigital converter with a conversion time of 25 ps maximum and a sensitivity of 10 mV full scale. A serial and a tape port are also available. Permanent data storage is achieved through two floppy disc units and programs and experimental data are also stored on one of the University mainframe computers through a serial link between the Exorset and the mainframe. Retort and instrumentation The retort is a vertical, steam-heated retort containing two crates. The pipe layout, services and control valves are indicated in Fig. 3, and a visual impression of the entire system appears in Fig. 4. The retort itself is provided with two over-pressure relief valves and a safety device which prevents lid release until the retort is vented. The lid is opened and closed pneumatically. The retort maximum operating pressure is 350 kPa, the steam pressure 275 kPa and the supply line is 4 in diameter. Compressed air, available at 550 kPa, is reduced to 275 kPa, entering the retort via a 4 in line. Water enters at 400 kPa via a 1 in line. Con-
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B. P. Lappo, M. J. W. Povey
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Fig. 4. Arrangement of the complete development facility. Key: 1, high quality compressed air for modulating valves; 2, safety release valve; 3, blow-off valve; 4, main vent; 5, water level detector; 6, emergency vent; 7, water top; 8, blow-off valve; 9, vent; 10, vent; 11, steam; 12, water; 13, compressed air steam top; 14, NPL calibrated mercury-in-glass thermometer, IOO-150°C; 15, main drain; 16, drain; 17, water/bottom; 18, compressed air steam bottom; 19, mains control box; 20, computer interface; 21, computer; 22, thermocouples; 23, vent; 24, printer; 25, platinum resistance thermometer; 26, Honeywell Versapak controllers; 27, Honeywell chart recorder; 28, syphon; 29, top drain; 30, pressure transducer; 3 1, compressed air.
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B. P. Lappo, M. J. W. Povey
densate is removed from the base of the retort by a steam trap and the retort is vented through a 1 in line and a ) in petcock on the instrument pocket. Instrumentation includes a platinum resistance thermometer (PRT), a diffused silicon pressure transducer, 16 thermocouple connectors of the Ecklund type, suitable for attachment to within-can thermocouples, a mercury-in-glass thermometer calibrated as a secondary standard to +O-05°C and a Bourdon-type pressure gauge. The thermocouples are multiplexed into one analogue input, the signal first having been amplified and linearised in hardware. The radio-frequency filtered, linearised and amplified differential output from each thermocouple is presented to the A-to-D converter and read by the computer. For each thermocouple, this is done every O-5 s and 20 readings are taken and averaged before being accepted as an authentic temperature reading. Thus the data from 16 thermocouples are available, every 10 s, based on the average of 20 measurements per thermocouple. The thermocouple wire runs in a grounded sheath and detection is by a differential amplifier. Cold junction compensation is carried out by the platinum resistance thermometer inside the connector block to which the thermocouples are attached. The thermocouple wire runs unbroken from the amplifier connection block to the connector at the can, entering the retort through a compression gland with silicone rope packing. Each wire is insulated with PTFE sheath and the two wires for each thermocouple run within a PTFE sheath also. The sheath is cut just outside the retort to prevent water being forced back to the thermocouple connector block by the pressure within the retort. The PRT communicates with the computer via a 4-20 mA current loop running through a standard 2.50 s2 resistor, the voltage drop across which is fed in differential form to the analogue input to the computer. The sensing element of the pressure tranducer directly controls the current loop running through it, and this loop passes through another 250 a standard resistor for transmission of the signal to the computer in a similar manner to that of the PRT. Both current loops pass through a standard circular chart recorder providing a permanent record of temperature and pressure within the retort. Control equipment Two controllers supply a 4-20 mA current converters on the two pneumatic positioning
to electro-pneumatic valves, which in their
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turn control the steam and air inputs and the main vent (see Figs 2 and 3). These controllers will operate with either an internally generated set point or an external set-point generated by the computer. The controllers include PI control which can be replaced by PID control generated by the computer. The electro-pneumatic converters are of the pneumatic amplifier type and therefore require a clean, dry air supply. Either conventional, semi-automatic control is provided by manual setting of the controllers, or fully automatic control is available from the computer. The controllers can operate independently of the computer if necessary. It should be noted that it is not necessary to control both temperature and pressure since controlling the temperature of saturated steam will automatically control the pressure, and vice versa. However, having separate controllers gives much greater flexibility in controlling the process when the heat transfer medium is not saturated steam. Seven solenoid valves (see Fig. 2) are switched using 240 V AC which is controlled by the computer’s digital outputs through optically isolated solid state relays. The solenoid valves are arranged so that, in the case of a failure in the power supply, the retort can be operated manually. Great care was taken to isolate the mains supply from the analogue and digital circuitry. RF suppression was included in the mains supply to all circuitry. In order to permit the controller to be operated without switching on the computer, the analogue circuitry power supply is separate from that of the computer. This is necessary in order to avoid earth loops. Earthing is further complicated by the need to keep digital and analogue earths separate and by the need to earth the current loops at the analogue input so that the voltage of the loop was within range of the analogue input circuitry. Power-on detection is provided by an optically isolated connection between mains and a digital input to the computer. Detection of low water level and high water level is by ACexcited conductivity sensors, which trigger relays when water is detected. Relay closure is detected by a 5 to 0 V transition at a digital input to the computer. A 24 V AC supply provides lights on two small viewing ports set into the retort. Software
All of the software is written in an advanced form of Basic called BasicM which is compiled into machine code prior to loading into the machine, ready for execution. This approach allowed us to write clear,
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B. P. Lappo, M. J. W. Povey
well-documented code which could easily be understood by other programmers and which employed the resources of the computer very efficiently. Three software packages have been written for the retort development system so far. The first, called CMIMIC and written to instruct students in the operation of the retort, leads them through the initial operations of powering up the retort, connecting thermocouples, turning on the steam supply, etc. Once all the services have been activated, a mimic diagram is displayed showing the location and state of each solenoid valve. Each solenoid valve is also associated with one of 16 function keys on the computer keyboard whereby the valve can be opened or closed. Retort temperature and pressure are displayed at the top of the screen, together with the set points for temperature and pressure. The set points may be altered at any time by pressing the appropriate function key and typing in the new set points. Elapsed time is displayed, starting from the time the steam is let into the retort. Thermocouple temperatures are printed out, for up to 16 thermocouples, every 10 s. Because Simpsons rule is used to integrate the lethal rate, the respective F,, values are printed out every 20 s. Retort temperature and pressure are printed every 10 s. An estimated time to the end of the process is also given based on an extrapolation of the rate of accumulation of F, to the target FO. This enables the process to be controlled to achieve a target F. instead of the more usual predetermined temperature-time profile. The process is brought to a halt by another function key. The second program is called PROFILE and provides totally automatic control for the retort. The operator is asked for information about the product and the thermal process required or alternatively this information can be obtained automatically from disc, together with a predetermined temperature-time profile. At present the information required of the user includes 2 and &, target F,, value, heating time. total cooling and pressure cooling times, thermocouples required and retort temperature. Once this information has been supplied a mimic diagram is displayed, similar to that described for CMIMIC, as is the printout from PROFILE and unless automatic operation is initiated PROFILE is in all respects similar to CMIMIC. However, once automatic operation is initiated by depression of the appropriate function key, operator intervention is unnecessary and all solenoid valve sequencing, etc., is carried out automatically. This includes the pressure
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cooling phase in which the exterior pressure is balanced against the estimated internal pressure of the can. In the event of any difficulties, the operator can assert manual control to greater or lesser degree, depending on the problem, optionally returning to automatic operation once the problem has been solved. With a simple modification, this program can be run for a specified product, the only operator intervention being loading and closing the retort, inserting the disc into the disc drive and typing a single command on the keyboard. The third program, called RTP, extends the capabilities of CMIMIC to function key control of proportional, integral and derivative constants and the integral term itself. An on-line plot of temperature and pressure against time is provided and this plot can be printed if required. RTP is used to determine the proportional, integral and derivative terms required for optimum retort control and for demonstrating control theory to students. The fourth program, BATH, assists in the calibration of the thermocouples, the pressure transducer and the PRT by first logging the readings from these sensors, requesting the operator to input a reading from the secondary standard mercury-in-glass thermometer and, after repeating the process at a series of temperatures, applying a linear regression fit to determine the calibration constants for the sensors. Whilst logging the data it controls the retort in the manner described for CMIMIC. Analysis of data from retort runs is carried out on the retort computer or two similar computers. Alternatively, the data can be transmitted to the University’s central computer. Other software available on the retort development system include a program which aids the computation of fh and j by employing linear regression and computer graphics, a finite difference model of unsteady-state heat transfer to and from finite cylinders and a program which predicts nutrient retention in canned foods subjected to any temperature-time profile. All the software is in the form of modules, mostly written in BasicM with a very few, short, time-critical routines written in ASSEMBLER. All the programs share common elements so that the development of new routines amounts to selection of the appropriate modules, together with new code as necessary. For example, the same linear regression routines are used in two different programs. All the software is stored on the University Amdahl mainframe computer as well as locally, on disc.
B. P. Lappo, M. J. W. Povey
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Experimental procedure
In order to determine the performance of the retort and its instrumentation, the following procedure was followed. Six months before taking the results described in this paper, the thermocouples and the PRT were calibrated against the secondary standard mercury-in-glass thermometer. This was done by immersing the thermocouples and the PRT, together with the thermometer, in an oil bath with a temperature controller capable of holding the temperature steady to within 0.05’C. Using the calibration program BATH, already described, the sensors were calibrated over the range 100-l 50°C. The pressure transducer was calibrated in the retort by measuring its output as a function of temperature as determined by the calibrated PRT, and then using the steam tables to obtain the pressure. Athough the calibration was regularly checked over the following 6 months, the calibration constants in the software were not changed so that a check on long-term drift in calibration could be made. For the retort performance trial, Ecklund needle thermocouples were attached to the thermocouple plugs but were not placed in cans. Instead, the thermocouples were distributed throughout the retort according to the plan shown in Fig. 5. Thus a check could be made on the temperature distribution within the retort. The retort was filled with 50 cans, each containing water so that the temperature distribution observed was typical of a loaded retort.
RESULTS AND DISCUSSION Retort performance
The temperature/time and pressure/time profiles for one retort run are plotted in Fig. 6 and the data presented are for the worst case of 10 runs, each employing a different temperature profile. The retort is controlled so that it follows a predetermined temperature profile over the whole process. The profile is supplied as data off disc and the required profile is plotted in the figure. The mean and standard error of the mean of 10 thermocouples is also plotted. The temperature is determined from the PRT and pressure is determined from the
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T-
7 00 Iim
m-any-in-glass
thermmeter
PRT
txktmlrasket
Instrument psket
f
L
f _I
Fig. 5. Retort dimension and arrangement of thermocouples and $ represent the positions of thermocouples 90” behind, principal plane of the retort, respectively.
in the retort: 0, X in front and in the
transducer. The retort was controlled by the PROFILE program employing PID control for which the control constants had been determined for a step input from room temperature to 121’C. For a constant set point, the temperature was maintained to within O.l’C of the set point. With a variable set point, the maximum deviation was 3°C. This deviation occurred during the venting phase when sequencing of the venting process overrode the temperature required by the profile pressure
B. P. Lappo, M. J. W. Povey
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which had been predetermined. Apart from this, the worst deviation occurred at the sudden gradient change at 2 1.4 min where the PID algorithm was clearly not up to the demands made on it. Nevertheless, over most of the heating phase, the maximum deviation was Oe8’C and generally the deviation was less than O-1‘C. Figure 6 also shows the overall calibration accuracy of the temperature measuring instrumentation. In Table 1 is presented an analysis of the overall accuracy of a typical industrial thermometer. Figure 6 shows
:‘I
200
13( TI C
P/
tPa 160
12( 120
30
lot 40
(
10
20
40 t
0
/MEutes
Fig. 6. Retort performance plotted as temperature, T, versus time, t: -, set point; &_&; mean and +/- standard error on the mean for 10 thermocouples distributed throughout the retort; . . ... . PRT temperature; O--+--O, pressure.
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TABLE 1 Overall Precision of an Industrial Digital Thermometer PRT
Thermocouple Type T (Cu/CuhQ)
Measurement
accuracy
Digitising error Characterisation accuracy Cold junction compensation Temperature coefficient
(at 2O’C) (at 100°C)
20-25°C (a) 15-30°C (b) (a)
(b)
k 0.09”c +o.o1”c zt 0~005 “c + O*Ol”C _ _ II - a
Long term stability (I year) RSS accuracy Maximum error
+ 0.15”c * 0.025”c f 0.05”c + 0.05”c _ f 0.5”c _ a _ (2 ~o~ool”c
+ 0*09”c r 0.09”c +0*12%-z ztro.12”c
+ 0.17”c f 053°C f 0.28”C + 0.78”C
a Negligible.
that the retort performance lies well within the accuracy that can be expected from this sort of instrumentation. The error bars on the thermocouples are the standard error on the mean. In the plateau at 130°C between 30 and 35 min the PRT registered 130.05 +O.O5”C and the thermocouples registered 130.20 +O.O5’C, where the errors are standard errors on the mean. The mercury-in-glass thermometer registered 130.20 +O.O5’C and the mercury stem oscillated by +O.O5”C. So the thermocouples and the PRT had drifted apart by 0.15”C over 6 months and the thermocouples and the mercury-in-glass thermometer had not drifted apart at all. Since the thermocouples were distributed evenly throughout the retort the standard error will contain a contribution from temperature variations throughout the retort, and because an error of + 0.05”C will not show on Fig. 6, the error bars on the thermocouple data represent the variation in temperature through the retort. Not surprisingly, this variation is greatest during the cooling phase. In this and other experiments the temperature control was better than O.O5”C, for a constant set point. After 6 months use it was
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B. P. Lappo, M. J. W. Povey
possible to rely on 10 thermocouples operating out of 16 connected. No attempt was made to repair the faulty thermocouples apart from the normal check on the connections. Four out of the 6 faulty thermocouples operated intermittently in the steam but not in water or air. The influence of temperature errors Some trouble was taken to ensure that the temperature measurement system was accurate. In order to estimate the overall accuracy which it is sensible to demand of a temperature measurement system, an estimate has been made of the influence of temperature errors on the predicted sterility of a canned product in the absence of all other sources of error. The estimate is based on a temperature profile at the geometric centre of a can whose exterior has been exposed to a typical retort temperature/time profile. The can centre temperature has been predicted from a finite difference solution of the unsteady-state heat
t/mins Fig. 7. Log(TmtO, - Tcentre) plotted versus t for a conduction pack with Tretort= 110°C. In this and Fig. 5 r is can radius, d is can height and a is the thermal diffusivity of the food.
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Fig. 8.
LogVAt
-Tcentre
) P lotted versus t for a conduction 120°C.
pack with Treto,, =
transfer equation computed on the development facility. Figures 7 and 8 show the results of this theoretical investigation into the effect of an error in the measurement of the temperature exterior to the can on the F,-, predicted for the geometric centre of the can. The accuracy of the finite difference solution of the unsteady state heat transfer equations was checked against within-can thermocouple measurements made at the geometric centre of the can. The results of the theoretical model were well within the accuracy of the temperature measurements and, since the theoretical investigation was much more convenient to carry out than the experimental one, the theoretical approach has been used in preference. Both Figs 7 and 8 refer to conduction packs: Fig. 7 to a standard, Al can and in Fig. 8 to a small can undergoing a high-temperature short-time process. The heating curves in both cases were chosen whereby the external temperature fed into the program was such
50
B. P. Lappo, M. J. W.Povey
that a 5D process would be achieved at the can centre for Bacillus stearothermophilus if the external temperature was + l°C + 0.1% in error. Due to the difficulty of achieving this exactly, an F,, just greater than 5D was settled on. If the temperature error now becomes - 1°C - 0.1% then an additional 150 min of processing is required to achieve the required 5D process. Thus, if a temperature measurement system is accurately specified as + 1°C f O-l%, 75 min of processing will be required over and above that which would be predicted by the on-line retort temperature measurement system, in order to guarantee adequate sterility. By reducing the error to + 0.3 f 0.1% we can reduce the overprocess time needed to account for this error to 25 min, saving 50 min or 12% of total process time. Another way of considering the effect of temperature errors can be seen in Fig. 8. At 120°C for a small conduction pack, a rt 1“C +_0.1% error will produce a possible range in F0 of 20 min in a 28 min process. This treatment ignores all sources of error apart from that of the temperature measurement. For instance, when computing F0 values achieved at the can centre from the temperature external to the can, account needs to be taken of the variation in the thermal properties of the food and the displacement of the thermal centre of the can from the geometric centre. Present within-can temperature measurement techniques are not a satisfactory substitute for retort temperature measurement because of difficulties in centering the probe, probe conduction errors and uncertainty as to where the coldest part of the can lies. The ideal solution to the problem of on-line control of the sterilisation of canned food would involve a combination of improved within-can temperature measurement and improved retort temperature measurement. This information, together with the finite difference prediction of can temperatures, would enable more accurate control of the thermal sterilisation of canned foods. Treatment of this subject may be found in Lenz and Lund (1977). Teixeira et al. (1975) and Meffert (1983).
CONCLUSION As a result of the theoretical investigation, it is concluded that an overall accuracy of *0.3”C f 0.1% in the retort temperature measurement would produce worthwhile savings in process time referred to a
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heat-resistant organism such as B. stearothermophilus. An accuracy of f 0. l°C + 0,176 produces little additional savings in terms of either process time or overprocessing consequences. For the high temperature process (Fig. 8) the gains from reduced error arise more in terms of reduction in overprocessing rather than from significant savings in process time. The developement facility described above has been working in the authors’ laboratory for 2 years and has been used in teaching for 1 year. It has been shown that completely automatic control of the retort heating profile is possible along with pressure control during cooling, that on-line prediction and control of F0 value is feasible and also that a significant reduction in process time can be achieved by improving the precision of the measurement of retort temperatures.
FURTHER
WORK
Applications of the development facility at present include fully automatic thermal processing of canned food, teaching retort operating procedures, demonstrating control theory to students and the assessment of thermal processes for canning. Research applications include the study of the optimisation of nutritional and sensory attributes in the thermal processing of food and a study of control strategies in retorting, particularly during the cooling process. The optimisation studies involve detailed computer modelling of the effects of thermal processing of canned food components, including ascorbic acid, ferrous/ ferric iron ratio, thiamine and colour, using the external heating profile as the input variable. Optimum profiles so obtained are then tested in the retort. This aspect of the work is being conducted jointly with Janice Ryley of this Department and H. J. Heinz Ltd, Hayes Park, Middlesex, UK.
ACKNOWLEDGEMENTS We wish to thank Mr J. Lamb of the Procter Department for his help in the design of the layout of the services to the retort and Mr G. Bown of the Campden Food Preservation Research Association for valuable discussions. We also thank Messrs Boyes, McCaw and Wilson of the
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B. P. Lappo, M. J. W. Povey
Procter Department technical staff for much help and advice during the construction of the development facility. REFERENCES Bown, G. (1982). Application
of microprocessor technologies in the food and allied industries. Measurement and Control, 15 (1 I), 409-12. Hayakawa, K. L. (1977). Mathematical methods for estimating proper thermal processes and their computer implementation. Advances in Food Research, 23, 75-141. Holdsworth, D. (1974). Instrumentation developments for heat processing. Food Manufacture, 49 (1 I), 35-8,58. Holdsworth, D. (1983). Developments in the control of sterilising retorts. Process Biochemistry, 16 (5), 24-8. Lenz, M. K. and Lund, D. B. (1977). The lethality-Fourier number method: confidence intervals for calculated lethality and mass average retention of conduction heating, canned foods. J. Food Sci., 42 (4) 1002-7. Meffert, H. F. Th. (1983). In: Physical Properties of Foods, ed. R. Jowitt et al., Elsevier Applied Science Publishers Ltd, London, pp. 246-7. Ohlsson, T. (1980~). Optimal sterilisation temperatures for flat containers. J. Food Sci., 45,848-52. Ohlsson, T. (1980b). Optimal sterilisation termperatures for sensory quality in cylindrical containers. J. Food Sci., 45, 15 17-2 I. Olsen, F. C. W. and Jackson, J. M. (1942). Heating curves, theory and practical applications. Ind. Eng. Chem., 34,337. Saguy, I. and Karel, M. (1979). Optimal retort temperature profile in optimising thiamine retention in conduction type heating of canned foods. J. Food Sci., 44, 1485-90. Steele, D. J. (1980a). Microprocessor applied to retort control. Food Engineering International, 5 (12) 28-32. Steele, D. J. (1980b). Microprocessors and their application to the control of a horizontal batch retort. IFSTProceedings, 13 (3) 183-93. Teixeira, A. A. and Manson, I. E. (1982). Computer control of batch retort operations with on-line correction of process deviations. Food Technol., 36,85-90. Teixeira, A. A., Dixon, J. R., Zahradnik, J. W. and Zinmeister, G. E. (1969). Computer optimisation of nutrient retention in the thermal processing of conduction heated foods. Food Technol., 23 (6), 137-42. Teixeira, A. A., Stumbo, C. R. and Zahradnik, J. W. (1975). Experimental evaluation of mathematical and computer models for thermal process evaluation. J. Food Sci., 40,653-5. van Boxtel, L. B. J. and de Fielliettaz Goethart, R. L. (1982). Optimisation of the sterilisation process. Voedingsmiddelentecnologie, 15 (Supplement 24) 2-6.
53
APPENDIX: STERILISATION MONITOR As an example of the use of the development facility, a self-contained sterilisation monitor, capable of monitoring 10 thermocouples with an overall accuracy of + 0.3’C has been constructed. The monitor uses a COMARK 6000 digital thermometer which interfaces with a Creative Microsystems single-board computer (SBC) via BCD outputs. The digital thermometer is itself a programmable instrument of great flexibility and can operate independently of the monitor. The calculation of the overall accuracy is shown in Table 1. The SBC contains 6 kbytes of Read Only Memory (ROM), or Random Access Memory (RAM), two serial interfaces and 32 digital inputs/outputs. The complete system consists of the SBC, 32 kbyte memory board, a power supply, a low cost matrix printer and cabling. Up to 10 thermocouples can be scanned at any rate from one per 1 s to one per 99 999 s; the corresponding sterilisation or cook value is then computed and the result printed out. Individual values of Tref and z can be assigned to each channel so that sterilisation and cook values can be computed simultaneously. Between each scan of the thermocouples, each thermocouple can be examined individually and the digital thermometer reprogrammed if required during monitoring. The thermometer can accept all standard thermocouple types as well as microvolt inputs from a PRT, for example. To reduce costs, no keyboard is used; instead, the keyboard and display of the digital thermometer are used to feed the initial data to the monitor. The monitor shares a great deal of software with the development system and it is planned to include in the monitor thef,,, j computation which was developed for the retort.
5 (1986)
31-53
A Microprocessor Control System for Thermal Sterilisation Operations B. P. Lappo and M. J. W. Povey* Procter Department
of Food Science, The University, Leeds LS2 9JT, UK
ABSTRACT
A development facility, comprising a steam sterilising retort, a microprocessor development system and associated instrumentation and control equipment, is described. Features of the system are its modular design and the use of high level language software. The performance of the facility is reported and the influence of instrument accuracy on the control of sterilisation is explored. A sterilisation monitor capable of scanning IO thermocouples and computing individual sterilisation or ‘cook’ values for each channel is described.
NOMENCLATURE Algorithm A set of instructions for the execution of a calculation Analogue signal A continuous signal which may vary in magnitude Analogue to digital (A to D) Conversion of an analogue signal into a digital signal Assembler An aid to programming in machine code Basic A programming language, very widely used on small computers Decimal reduction time (II) The time during which the bacterial count decreases ten-fold Differential A signal which depends on the difference between two signals Digital signal A signal consisting of a stream of pulses representing numbers *To whom correspondence
should be addressed. 31
of Food Engineering 0260-8774/86/$03.50 - 0 Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain.
Journal
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B. P. Lappo, M. J. W. Povey
Digital to analogue (D to A) Conversion of a digital signal into an analogue signal F The time, in minutes, of a hypothetical process which is brought instantaneously to a reference temperature after which it is instantaneously cooled and which is equivalent in lethality to the actual process under consideration F0 Fis designatedF,,ifz = 10 and Tref= 121.1”C f’, j In Fig. 1 the h ea t penetration factor fi, is the time for the straight line plot to traverse one log cycle of temperature deficit and j is the ratio of the pseudo-initial deficit (p.i.d.) to the initial temperature deficit (r). .fr, is related to the thermal diffusivity (a) in conduction packs through the expression fi, = 6.63 X 1O-“/[ (l/ Y’ + 1 .708/h2)c\l] where f,, is in minutes, r and h are the radius and height of the can in metres and (x is in m2 s-’ Hardware The physical components which comprise the equipment Interface Connection between the computer and the rest of the equipment Lethal rate (L) If the destruction rate for microorganisms is arbitrarily defined as 1 at cef then, from the definition of z, it follows that the rate at any other temperature is given by L = 10 -(T=f-W Machine code The digital instructions used by the microprocessor itself. All instructions to the computer ultimately take this form Mainframe A computer capable of supporting many users simultaneously Microprocessor A microelectronic circuit capable, in response to externally supplied machine code, of rapidly executing the addition and subtraction of digital signals Multiplexer Apparatus which selects any one of many signals Optical isolation Electrical isolation by means of light signals PID Proportional, integral and derivative - an algorithm frequently used to control process inputs and outputs to equipment via valves RAM Random access memory ~- erasable electronic memory ROM Read only memory - non-erasable electronic memory Serial port A computer connection to other equipment admitting a stream of digital signals, one at a time Set point A selected value of a variable which it is desired to achieve Software That part of the computing process which can be altered without any apparent change in its physical form. Information in
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machine code is software as are the instruction manuals for the equipment Source code The code in which the software was originally written. If the source code is in a high level language such as Basic it will require translation into machine code Tref The temperature at which sterilisation processes are compared. It is 12 1.1 “C in this paper z The temperature rise required to reduce the decimal reduction time ten-fold
fH
__ :
j :
p.i.d. I
I ; 20 Process
40 time,
min
Fig. 1. A typical heat penetration curve plotted as the logarithm of the temperature deficit versus process time. The positions of the initial temperature deficit (I) and the pseudo initial deficit (p.i.d.) are indicated on the graph.
34
B. P. Lappo, M. J. W. Povey
INTRODUCTION
A number of authors have drawn attention to the benefits which may accrue through optimisation and automatic control of the thermal sterilisation operation in food preservation. These potential advantages include minimising process deviations (Teixeira and Manson, 1982) on-line computation of the degree of thermal sterilisation (Holdsworth, 1974; Hayakawa, 1977) and maximising nutrient retention (Teixeira et al., 1969; Saguy and Karel, 1979). Other potential benefits include automatic documentation of the process (Holdsworth, 1983) optimisation of textural, flavour and colour characteristics and on-line measurement of heat penetration. The evidence for an optimum process for nutrient retention (Saguy and Karel, 1979) is repeated in perhaps more convincing terms for textural and flavour characteristics in the ‘cook value’ (Ohlsson, 198Oa, b; van Boxtel and de Fielliettaz Goethart, 1982). However, the great majority of these contributions have been theoretical in content. Examples of experimental verification of the theory include work by Teixeira ef al. (1975) and Ohlsson (1980b). However, nutrient or cook value optimisation is only a part of the story so far as automatic control of the sterilisation process is concerned. For instance, considerable reduction in process times might be achieved through improved instrumentation, with consequent increases in product throughput in batch retorting operations. In the present authors’ view it is necessary to approach the problem of improving the thermal sterilisation step differently. All the advantages detailed above imply the use of an automatic control system for a thermal steriliser and if the best control strategy is to be found then a development system is required which allows all aspects of the strategy to be employed. Aspects to be considered include the factory-installed capital cost of the system, interaction between operators and instruments, maintenance of hardware and software, reliability, instrumentation accuracy, the effects of fluctuations in services on operation and the interaction between management and the system. A number of general features, desirable in any control system, should be noted. For ease of maintenance, both the hardware and the software should be modular. Thus, if a fault develops in any module it can be traced easily, the module removed and returned to its manufacturer and replaced with a spare. In the case of software, modularity implies a high degree
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of documentation and, ideally, source code written in a high level language. In this way many people can work on the software and changes can be incorporated in individual modules, leaving other modules unaffected. Consequently, one of the main benefits of a software-based system, ease of modification, can be achieved. In addition, both hardware and software can be tailored to particular requirements, without rewriting all the software or rebuilding the hardware. An earlier approach to the problem of building an automatic retort system is described by Steele (19804b) and a more recently developed system is described by Bown (1982). For a large-scale system, consisting of two or more steam retorts, high level language software becomes essential. In addition, the option of distributed control should be available, as opposed to real-time control of every operation in every element of the system by one computer. A distributed control system may consist of a number of local controllers each sufficient to control automatically an individual retort. Communication with the central controller would be by a pair of wires with consequent dramatic reduction in cabling costs. Communication would consist of initiation by the central computer of the required process, probably following a start command from those responsible for loading the retort. The local controller would then carry out the required process without any further intervention from the central computer, so that if any fault developed centrally the retorting operation would continue to completion. The local controller would report progress back to the central computer, drive a local mimic diagram display and control local operator intervention. Such a system would also allow each retort to be driven in the conventional, semiautomatic manner. A development system should permit exploration of a number of available control strategies and allow the accumulation of considerable experience with the particular hardware/software combination chosen. The system described below is capable of following a prescribed temperature-time profile and of altering its profile according to an Fb value computed from any of 16 within-can temperature measurements or by an FO value derived from retort temperature. Control of pressure is available during the cooling process in order to reduce stresses on the container. The hardware is modular and the software is written almost entirely in an advanced form of Basic, allowing user documentation of the program and a modular construction.
36
B. P. Lappo, M. J. W. Povey
Experimental data is presented to characterise the performance of the retort and its instrumentation. The importance of instrumentation accuracy is discussed and a computer calculation of heat penetration into a can is used to investigate the significance of errors in temperature sensing with regard to process times.
EXPERIMENTAL The development system is outlined in the form of a block diagram in Fig. 2. The facility was designed and assembled by the authors. Computer system The computer is the Motorola Exorset 30 microprocessor development system for which a wide range of interfaces and software are available. The hardware consists of a standard typewriter keyboard together with 16 function keys, a visual display unit with graphics capability, a dot matrix printer for printing of screen images, 32 digital outputs, 32 digital inputs, 16 analogue inputs and two analogue outputs. The 16 analogue inputs are multiplexed into a dual input, 12 bit analogue-todigital converter with a conversion time of 25 ps maximum and a sensitivity of 10 mV full scale. A serial and a tape port are also available. Permanent data storage is achieved through two floppy disc units and programs and experimental data are also stored on one of the University mainframe computers through a serial link between the Exorset and the mainframe. Retort and instrumentation The retort is a vertical, steam-heated retort containing two crates. The pipe layout, services and control valves are indicated in Fig. 3, and a visual impression of the entire system appears in Fig. 4. The retort itself is provided with two over-pressure relief valves and a safety device which prevents lid release until the retort is vented. The lid is opened and closed pneumatically. The retort maximum operating pressure is 350 kPa, the steam pressure 275 kPa and the supply line is 4 in diameter. Compressed air, available at 550 kPa, is reduced to 275 kPa, entering the retort via a 4 in line. Water enters at 400 kPa via a 1 in line. Con-
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38
B. P. Lappo, M. J. W. Povey
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Fig. 4. Arrangement of the complete development facility. Key: 1, high quality compressed air for modulating valves; 2, safety release valve; 3, blow-off valve; 4, main vent; 5, water level detector; 6, emergency vent; 7, water top; 8, blow-off valve; 9, vent; 10, vent; 11, steam; 12, water; 13, compressed air steam top; 14, NPL calibrated mercury-in-glass thermometer, IOO-150°C; 15, main drain; 16, drain; 17, water/bottom; 18, compressed air steam bottom; 19, mains control box; 20, computer interface; 21, computer; 22, thermocouples; 23, vent; 24, printer; 25, platinum resistance thermometer; 26, Honeywell Versapak controllers; 27, Honeywell chart recorder; 28, syphon; 29, top drain; 30, pressure transducer; 3 1, compressed air.
40
B. P. Lappo, M. J. W. Povey
densate is removed from the base of the retort by a steam trap and the retort is vented through a 1 in line and a ) in petcock on the instrument pocket. Instrumentation includes a platinum resistance thermometer (PRT), a diffused silicon pressure transducer, 16 thermocouple connectors of the Ecklund type, suitable for attachment to within-can thermocouples, a mercury-in-glass thermometer calibrated as a secondary standard to +O-05°C and a Bourdon-type pressure gauge. The thermocouples are multiplexed into one analogue input, the signal first having been amplified and linearised in hardware. The radio-frequency filtered, linearised and amplified differential output from each thermocouple is presented to the A-to-D converter and read by the computer. For each thermocouple, this is done every O-5 s and 20 readings are taken and averaged before being accepted as an authentic temperature reading. Thus the data from 16 thermocouples are available, every 10 s, based on the average of 20 measurements per thermocouple. The thermocouple wire runs in a grounded sheath and detection is by a differential amplifier. Cold junction compensation is carried out by the platinum resistance thermometer inside the connector block to which the thermocouples are attached. The thermocouple wire runs unbroken from the amplifier connection block to the connector at the can, entering the retort through a compression gland with silicone rope packing. Each wire is insulated with PTFE sheath and the two wires for each thermocouple run within a PTFE sheath also. The sheath is cut just outside the retort to prevent water being forced back to the thermocouple connector block by the pressure within the retort. The PRT communicates with the computer via a 4-20 mA current loop running through a standard 2.50 s2 resistor, the voltage drop across which is fed in differential form to the analogue input to the computer. The sensing element of the pressure tranducer directly controls the current loop running through it, and this loop passes through another 250 a standard resistor for transmission of the signal to the computer in a similar manner to that of the PRT. Both current loops pass through a standard circular chart recorder providing a permanent record of temperature and pressure within the retort. Control equipment Two controllers supply a 4-20 mA current converters on the two pneumatic positioning
to electro-pneumatic valves, which in their
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turn control the steam and air inputs and the main vent (see Figs 2 and 3). These controllers will operate with either an internally generated set point or an external set-point generated by the computer. The controllers include PI control which can be replaced by PID control generated by the computer. The electro-pneumatic converters are of the pneumatic amplifier type and therefore require a clean, dry air supply. Either conventional, semi-automatic control is provided by manual setting of the controllers, or fully automatic control is available from the computer. The controllers can operate independently of the computer if necessary. It should be noted that it is not necessary to control both temperature and pressure since controlling the temperature of saturated steam will automatically control the pressure, and vice versa. However, having separate controllers gives much greater flexibility in controlling the process when the heat transfer medium is not saturated steam. Seven solenoid valves (see Fig. 2) are switched using 240 V AC which is controlled by the computer’s digital outputs through optically isolated solid state relays. The solenoid valves are arranged so that, in the case of a failure in the power supply, the retort can be operated manually. Great care was taken to isolate the mains supply from the analogue and digital circuitry. RF suppression was included in the mains supply to all circuitry. In order to permit the controller to be operated without switching on the computer, the analogue circuitry power supply is separate from that of the computer. This is necessary in order to avoid earth loops. Earthing is further complicated by the need to keep digital and analogue earths separate and by the need to earth the current loops at the analogue input so that the voltage of the loop was within range of the analogue input circuitry. Power-on detection is provided by an optically isolated connection between mains and a digital input to the computer. Detection of low water level and high water level is by ACexcited conductivity sensors, which trigger relays when water is detected. Relay closure is detected by a 5 to 0 V transition at a digital input to the computer. A 24 V AC supply provides lights on two small viewing ports set into the retort. Software
All of the software is written in an advanced form of Basic called BasicM which is compiled into machine code prior to loading into the machine, ready for execution. This approach allowed us to write clear,
42
B. P. Lappo, M. J. W. Povey
well-documented code which could easily be understood by other programmers and which employed the resources of the computer very efficiently. Three software packages have been written for the retort development system so far. The first, called CMIMIC and written to instruct students in the operation of the retort, leads them through the initial operations of powering up the retort, connecting thermocouples, turning on the steam supply, etc. Once all the services have been activated, a mimic diagram is displayed showing the location and state of each solenoid valve. Each solenoid valve is also associated with one of 16 function keys on the computer keyboard whereby the valve can be opened or closed. Retort temperature and pressure are displayed at the top of the screen, together with the set points for temperature and pressure. The set points may be altered at any time by pressing the appropriate function key and typing in the new set points. Elapsed time is displayed, starting from the time the steam is let into the retort. Thermocouple temperatures are printed out, for up to 16 thermocouples, every 10 s. Because Simpsons rule is used to integrate the lethal rate, the respective F,, values are printed out every 20 s. Retort temperature and pressure are printed every 10 s. An estimated time to the end of the process is also given based on an extrapolation of the rate of accumulation of F, to the target FO. This enables the process to be controlled to achieve a target F. instead of the more usual predetermined temperature-time profile. The process is brought to a halt by another function key. The second program is called PROFILE and provides totally automatic control for the retort. The operator is asked for information about the product and the thermal process required or alternatively this information can be obtained automatically from disc, together with a predetermined temperature-time profile. At present the information required of the user includes 2 and &, target F,, value, heating time. total cooling and pressure cooling times, thermocouples required and retort temperature. Once this information has been supplied a mimic diagram is displayed, similar to that described for CMIMIC, as is the printout from PROFILE and unless automatic operation is initiated PROFILE is in all respects similar to CMIMIC. However, once automatic operation is initiated by depression of the appropriate function key, operator intervention is unnecessary and all solenoid valve sequencing, etc., is carried out automatically. This includes the pressure
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cooling phase in which the exterior pressure is balanced against the estimated internal pressure of the can. In the event of any difficulties, the operator can assert manual control to greater or lesser degree, depending on the problem, optionally returning to automatic operation once the problem has been solved. With a simple modification, this program can be run for a specified product, the only operator intervention being loading and closing the retort, inserting the disc into the disc drive and typing a single command on the keyboard. The third program, called RTP, extends the capabilities of CMIMIC to function key control of proportional, integral and derivative constants and the integral term itself. An on-line plot of temperature and pressure against time is provided and this plot can be printed if required. RTP is used to determine the proportional, integral and derivative terms required for optimum retort control and for demonstrating control theory to students. The fourth program, BATH, assists in the calibration of the thermocouples, the pressure transducer and the PRT by first logging the readings from these sensors, requesting the operator to input a reading from the secondary standard mercury-in-glass thermometer and, after repeating the process at a series of temperatures, applying a linear regression fit to determine the calibration constants for the sensors. Whilst logging the data it controls the retort in the manner described for CMIMIC. Analysis of data from retort runs is carried out on the retort computer or two similar computers. Alternatively, the data can be transmitted to the University’s central computer. Other software available on the retort development system include a program which aids the computation of fh and j by employing linear regression and computer graphics, a finite difference model of unsteady-state heat transfer to and from finite cylinders and a program which predicts nutrient retention in canned foods subjected to any temperature-time profile. All the software is in the form of modules, mostly written in BasicM with a very few, short, time-critical routines written in ASSEMBLER. All the programs share common elements so that the development of new routines amounts to selection of the appropriate modules, together with new code as necessary. For example, the same linear regression routines are used in two different programs. All the software is stored on the University Amdahl mainframe computer as well as locally, on disc.
B. P. Lappo, M. J. W. Povey
44
Experimental procedure
In order to determine the performance of the retort and its instrumentation, the following procedure was followed. Six months before taking the results described in this paper, the thermocouples and the PRT were calibrated against the secondary standard mercury-in-glass thermometer. This was done by immersing the thermocouples and the PRT, together with the thermometer, in an oil bath with a temperature controller capable of holding the temperature steady to within 0.05’C. Using the calibration program BATH, already described, the sensors were calibrated over the range 100-l 50°C. The pressure transducer was calibrated in the retort by measuring its output as a function of temperature as determined by the calibrated PRT, and then using the steam tables to obtain the pressure. Athough the calibration was regularly checked over the following 6 months, the calibration constants in the software were not changed so that a check on long-term drift in calibration could be made. For the retort performance trial, Ecklund needle thermocouples were attached to the thermocouple plugs but were not placed in cans. Instead, the thermocouples were distributed throughout the retort according to the plan shown in Fig. 5. Thus a check could be made on the temperature distribution within the retort. The retort was filled with 50 cans, each containing water so that the temperature distribution observed was typical of a loaded retort.
RESULTS AND DISCUSSION Retort performance
The temperature/time and pressure/time profiles for one retort run are plotted in Fig. 6 and the data presented are for the worst case of 10 runs, each employing a different temperature profile. The retort is controlled so that it follows a predetermined temperature profile over the whole process. The profile is supplied as data off disc and the required profile is plotted in the figure. The mean and standard error of the mean of 10 thermocouples is also plotted. The temperature is determined from the PRT and pressure is determined from the
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T-
7 00 Iim
m-any-in-glass
thermmeter
PRT
txktmlrasket
Instrument psket
f
L
f _I
Fig. 5. Retort dimension and arrangement of thermocouples and $ represent the positions of thermocouples 90” behind, principal plane of the retort, respectively.
in the retort: 0, X in front and in the
transducer. The retort was controlled by the PROFILE program employing PID control for which the control constants had been determined for a step input from room temperature to 121’C. For a constant set point, the temperature was maintained to within O.l’C of the set point. With a variable set point, the maximum deviation was 3°C. This deviation occurred during the venting phase when sequencing of the venting process overrode the temperature required by the profile pressure
B. P. Lappo, M. J. W. Povey
46
which had been predetermined. Apart from this, the worst deviation occurred at the sudden gradient change at 2 1.4 min where the PID algorithm was clearly not up to the demands made on it. Nevertheless, over most of the heating phase, the maximum deviation was Oe8’C and generally the deviation was less than O-1‘C. Figure 6 also shows the overall calibration accuracy of the temperature measuring instrumentation. In Table 1 is presented an analysis of the overall accuracy of a typical industrial thermometer. Figure 6 shows
:‘I
200
13( TI C
P/
tPa 160
12( 120
30
lot 40
(
10
20
40 t
0
/MEutes
Fig. 6. Retort performance plotted as temperature, T, versus time, t: -, set point; &_&; mean and +/- standard error on the mean for 10 thermocouples distributed throughout the retort; . . ... . PRT temperature; O--+--O, pressure.
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TABLE 1 Overall Precision of an Industrial Digital Thermometer PRT
Thermocouple Type T (Cu/CuhQ)
Measurement
accuracy
Digitising error Characterisation accuracy Cold junction compensation Temperature coefficient
(at 2O’C) (at 100°C)
20-25°C (a) 15-30°C (b) (a)
(b)
k 0.09”c +o.o1”c zt 0~005 “c + O*Ol”C _ _ II - a
Long term stability (I year) RSS accuracy Maximum error
+ 0.15”c * 0.025”c f 0.05”c + 0.05”c _ f 0.5”c _ a _ (2 ~o~ool”c
+ 0*09”c r 0.09”c +0*12%-z ztro.12”c
+ 0.17”c f 053°C f 0.28”C + 0.78”C
a Negligible.
that the retort performance lies well within the accuracy that can be expected from this sort of instrumentation. The error bars on the thermocouples are the standard error on the mean. In the plateau at 130°C between 30 and 35 min the PRT registered 130.05 +O.O5”C and the thermocouples registered 130.20 +O.O5’C, where the errors are standard errors on the mean. The mercury-in-glass thermometer registered 130.20 +O.O5’C and the mercury stem oscillated by +O.O5”C. So the thermocouples and the PRT had drifted apart by 0.15”C over 6 months and the thermocouples and the mercury-in-glass thermometer had not drifted apart at all. Since the thermocouples were distributed evenly throughout the retort the standard error will contain a contribution from temperature variations throughout the retort, and because an error of + 0.05”C will not show on Fig. 6, the error bars on the thermocouple data represent the variation in temperature through the retort. Not surprisingly, this variation is greatest during the cooling phase. In this and other experiments the temperature control was better than O.O5”C, for a constant set point. After 6 months use it was
48
B. P. Lappo, M. J. W. Povey
possible to rely on 10 thermocouples operating out of 16 connected. No attempt was made to repair the faulty thermocouples apart from the normal check on the connections. Four out of the 6 faulty thermocouples operated intermittently in the steam but not in water or air. The influence of temperature errors Some trouble was taken to ensure that the temperature measurement system was accurate. In order to estimate the overall accuracy which it is sensible to demand of a temperature measurement system, an estimate has been made of the influence of temperature errors on the predicted sterility of a canned product in the absence of all other sources of error. The estimate is based on a temperature profile at the geometric centre of a can whose exterior has been exposed to a typical retort temperature/time profile. The can centre temperature has been predicted from a finite difference solution of the unsteady-state heat
t/mins Fig. 7. Log(TmtO, - Tcentre) plotted versus t for a conduction pack with Tretort= 110°C. In this and Fig. 5 r is can radius, d is can height and a is the thermal diffusivity of the food.
Microprocessor
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control for sterilisation
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Fig. 8.
LogVAt
-Tcentre
) P lotted versus t for a conduction 120°C.
pack with Treto,, =
transfer equation computed on the development facility. Figures 7 and 8 show the results of this theoretical investigation into the effect of an error in the measurement of the temperature exterior to the can on the F,-, predicted for the geometric centre of the can. The accuracy of the finite difference solution of the unsteady state heat transfer equations was checked against within-can thermocouple measurements made at the geometric centre of the can. The results of the theoretical model were well within the accuracy of the temperature measurements and, since the theoretical investigation was much more convenient to carry out than the experimental one, the theoretical approach has been used in preference. Both Figs 7 and 8 refer to conduction packs: Fig. 7 to a standard, Al can and in Fig. 8 to a small can undergoing a high-temperature short-time process. The heating curves in both cases were chosen whereby the external temperature fed into the program was such
50
B. P. Lappo, M. J. W.Povey
that a 5D process would be achieved at the can centre for Bacillus stearothermophilus if the external temperature was + l°C + 0.1% in error. Due to the difficulty of achieving this exactly, an F,, just greater than 5D was settled on. If the temperature error now becomes - 1°C - 0.1% then an additional 150 min of processing is required to achieve the required 5D process. Thus, if a temperature measurement system is accurately specified as + 1°C f O-l%, 75 min of processing will be required over and above that which would be predicted by the on-line retort temperature measurement system, in order to guarantee adequate sterility. By reducing the error to + 0.3 f 0.1% we can reduce the overprocess time needed to account for this error to 25 min, saving 50 min or 12% of total process time. Another way of considering the effect of temperature errors can be seen in Fig. 8. At 120°C for a small conduction pack, a rt 1“C +_0.1% error will produce a possible range in F0 of 20 min in a 28 min process. This treatment ignores all sources of error apart from that of the temperature measurement. For instance, when computing F0 values achieved at the can centre from the temperature external to the can, account needs to be taken of the variation in the thermal properties of the food and the displacement of the thermal centre of the can from the geometric centre. Present within-can temperature measurement techniques are not a satisfactory substitute for retort temperature measurement because of difficulties in centering the probe, probe conduction errors and uncertainty as to where the coldest part of the can lies. The ideal solution to the problem of on-line control of the sterilisation of canned food would involve a combination of improved within-can temperature measurement and improved retort temperature measurement. This information, together with the finite difference prediction of can temperatures, would enable more accurate control of the thermal sterilisation of canned foods. Treatment of this subject may be found in Lenz and Lund (1977). Teixeira et al. (1975) and Meffert (1983).
CONCLUSION As a result of the theoretical investigation, it is concluded that an overall accuracy of *0.3”C f 0.1% in the retort temperature measurement would produce worthwhile savings in process time referred to a
Microprocessor
control for sterilisation
51
heat-resistant organism such as B. stearothermophilus. An accuracy of f 0. l°C + 0,176 produces little additional savings in terms of either process time or overprocessing consequences. For the high temperature process (Fig. 8) the gains from reduced error arise more in terms of reduction in overprocessing rather than from significant savings in process time. The developement facility described above has been working in the authors’ laboratory for 2 years and has been used in teaching for 1 year. It has been shown that completely automatic control of the retort heating profile is possible along with pressure control during cooling, that on-line prediction and control of F0 value is feasible and also that a significant reduction in process time can be achieved by improving the precision of the measurement of retort temperatures.
FURTHER
WORK
Applications of the development facility at present include fully automatic thermal processing of canned food, teaching retort operating procedures, demonstrating control theory to students and the assessment of thermal processes for canning. Research applications include the study of the optimisation of nutritional and sensory attributes in the thermal processing of food and a study of control strategies in retorting, particularly during the cooling process. The optimisation studies involve detailed computer modelling of the effects of thermal processing of canned food components, including ascorbic acid, ferrous/ ferric iron ratio, thiamine and colour, using the external heating profile as the input variable. Optimum profiles so obtained are then tested in the retort. This aspect of the work is being conducted jointly with Janice Ryley of this Department and H. J. Heinz Ltd, Hayes Park, Middlesex, UK.
ACKNOWLEDGEMENTS We wish to thank Mr J. Lamb of the Procter Department for his help in the design of the layout of the services to the retort and Mr G. Bown of the Campden Food Preservation Research Association for valuable discussions. We also thank Messrs Boyes, McCaw and Wilson of the
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Procter Department technical staff for much help and advice during the construction of the development facility. REFERENCES Bown, G. (1982). Application
of microprocessor technologies in the food and allied industries. Measurement and Control, 15 (1 I), 409-12. Hayakawa, K. L. (1977). Mathematical methods for estimating proper thermal processes and their computer implementation. Advances in Food Research, 23, 75-141. Holdsworth, D. (1974). Instrumentation developments for heat processing. Food Manufacture, 49 (1 I), 35-8,58. Holdsworth, D. (1983). Developments in the control of sterilising retorts. Process Biochemistry, 16 (5), 24-8. Lenz, M. K. and Lund, D. B. (1977). The lethality-Fourier number method: confidence intervals for calculated lethality and mass average retention of conduction heating, canned foods. J. Food Sci., 42 (4) 1002-7. Meffert, H. F. Th. (1983). In: Physical Properties of Foods, ed. R. Jowitt et al., Elsevier Applied Science Publishers Ltd, London, pp. 246-7. Ohlsson, T. (1980~). Optimal sterilisation temperatures for flat containers. J. Food Sci., 45,848-52. Ohlsson, T. (1980b). Optimal sterilisation termperatures for sensory quality in cylindrical containers. J. Food Sci., 45, 15 17-2 I. Olsen, F. C. W. and Jackson, J. M. (1942). Heating curves, theory and practical applications. Ind. Eng. Chem., 34,337. Saguy, I. and Karel, M. (1979). Optimal retort temperature profile in optimising thiamine retention in conduction type heating of canned foods. J. Food Sci., 44, 1485-90. Steele, D. J. (1980a). Microprocessor applied to retort control. Food Engineering International, 5 (12) 28-32. Steele, D. J. (1980b). Microprocessors and their application to the control of a horizontal batch retort. IFSTProceedings, 13 (3) 183-93. Teixeira, A. A. and Manson, I. E. (1982). Computer control of batch retort operations with on-line correction of process deviations. Food Technol., 36,85-90. Teixeira, A. A., Dixon, J. R., Zahradnik, J. W. and Zinmeister, G. E. (1969). Computer optimisation of nutrient retention in the thermal processing of conduction heated foods. Food Technol., 23 (6), 137-42. Teixeira, A. A., Stumbo, C. R. and Zahradnik, J. W. (1975). Experimental evaluation of mathematical and computer models for thermal process evaluation. J. Food Sci., 40,653-5. van Boxtel, L. B. J. and de Fielliettaz Goethart, R. L. (1982). Optimisation of the sterilisation process. Voedingsmiddelentecnologie, 15 (Supplement 24) 2-6.
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APPENDIX: STERILISATION MONITOR As an example of the use of the development facility, a self-contained sterilisation monitor, capable of monitoring 10 thermocouples with an overall accuracy of + 0.3’C has been constructed. The monitor uses a COMARK 6000 digital thermometer which interfaces with a Creative Microsystems single-board computer (SBC) via BCD outputs. The digital thermometer is itself a programmable instrument of great flexibility and can operate independently of the monitor. The calculation of the overall accuracy is shown in Table 1. The SBC contains 6 kbytes of Read Only Memory (ROM), or Random Access Memory (RAM), two serial interfaces and 32 digital inputs/outputs. The complete system consists of the SBC, 32 kbyte memory board, a power supply, a low cost matrix printer and cabling. Up to 10 thermocouples can be scanned at any rate from one per 1 s to one per 99 999 s; the corresponding sterilisation or cook value is then computed and the result printed out. Individual values of Tref and z can be assigned to each channel so that sterilisation and cook values can be computed simultaneously. Between each scan of the thermocouples, each thermocouple can be examined individually and the digital thermometer reprogrammed if required during monitoring. The thermometer can accept all standard thermocouple types as well as microvolt inputs from a PRT, for example. To reduce costs, no keyboard is used; instead, the keyboard and display of the digital thermometer are used to feed the initial data to the monitor. The monitor shares a great deal of software with the development system and it is planned to include in the monitor thef,,, j computation which was developed for the retort.