TECHNICAL FIELD This invention relates to the control of burners and has particular although by no means exclusive application to the control of burners adapted to produce heated aggregate mixes, especially bituminous mixes

BACKGROUND ART The present applicant's prior Australian patent application 20761/76 describes a reactor for the production of bituminous mixes of the kind in which a cold aggregate mix and a bituminous binder are fed to a _. reactor in the form of an elongate hollow rotatable drum. The feed is heated in the drum and thereby converted to a bituminous mix in which the binder adheringly coats the aggregate particles. The reactor includes a burner which may, for example, be an air-atomised burner of a liquid fuel-atomised burner, although the latter is preferred.

A preferred feature of the reactor disclosed in co-pending application 20761/76 is the control of the burner in dependence upon the temperature of the product mix retrieved from the reactor. It has been found that the application of simple techniques to this control of the burner does not result in reliable operation to the extent that a product of required specification

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- is continuously and reliably retrieved from the reactor. It is therefore an objective of this invention to provide a burner reactor incorporating a control which allows more reliable operation than hitherto achieved. DISCLOSURE OF THE INVENTION

The present invention derives from a realization that the ob served .unreliable operation of the burner is primarily due to a long time delay between a burner adjustment and its resulting effect on the produc The invention provides a reactor having a treatment chamber, a burner for heating reactants in the chamber, control elements operatively associated with the burner, means for retrieving product from the treatment chamber and a burner controller; characterized in that said burner controller comprises:- sensor means to continuously monitor a para¬ meter of the product and output a first electrical signal representative of the ongoing value of that parameter; comparator means responsive to said first electrical signal to output a second electrical signal representative of the deviation of said parameter from a reference value therefor; sampling means to sample the amplitude of said second signal at prescribed intervals of time and to latch a digital representation of the last such sampled amplitude; and command means to operate said control elements and thereby adjust the burner in dependence upon the presently latched digital representation and upon the direction of the deviation represented thereby.

Advantageously, first clock means is operatively coupled . . _

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to said sampling means for adjustably setting said sampling intervals (which are preferably uniform) , so as to be consistent with the known delay time for an adjustment of said burner to be reflected in the

5 monitored product parameter.

Advantageously, said command means is arranged to operate the burner control elements for a period proportional to said presently latched digital representation of the desired parameter deviation.

10 The proportionality factor involved may be variable by second clock means independently of the variation of the sampling interval; in any event, the control signal period should preferably be . sufficient to trigger the adjustment required but less than said

15 sampling intervals.

Preferably, said command means comprises electrical circuitry including an up/down counter and a pair of gates associated with respective said control elements, the inputs of each gate being connected to

20 respectively receive signals representing a count by ^" said counter of said digital representation and the most significant bit (m.s.b.) of said digital representation, diode means being provided on the m.s.b. input of one of said gates. The sensor means may comprise a temperature

25 sensor.

The invention also provides a method of adjusting the burner of a burner reactor in dependence upon the monitored value of a parameter of a product retrieved from the reactor, which method is characterized by:-

30. continuously monitoring said product parameter

• and obtaining a first electrical signal representative of the ongoing value of the parameter; comparing said first signal with a reference signal to thereby obtain a second electrical signal

representative of the deviation of said parameter from the reference signal; sampling the amplitude of said second signal at prescribed intervals of time and latching a digital representation of the last such sampled value; and adjusting the burner in dependence upon the presently latched digital representation and upon the direction of the deviation represented thereby. BRIEF DESCRIPTION OF DRAWING

The invention will be further described, by way of example only.-with reference to the accompanying drawing, which is a circuit diagram of a reactor with controller in accordance with the invention. BEST MODE OF CARRYING OUT THE INVENTION and INDUSTRIAL APPLICABILITY The controller represented by the attached circuit diagram includes a thermocouple temperature sensor 10 positioned to continuously monitor the temperature of the product mix 12 retrieved at gate 14 of a rotary reactor or kiln 16. Reactor 16 receives feed in the form of aggregate mix and bituminous binder. A liquid fuel atomised burner 18 heats the feed and thereby converts it to the bituminous mix 12 in which the binder adheringly coats the aggregate particles.

Operatively associated with burner 18 are a pair of control elements in the guise of relays 20, 21 which respectively increase and decrease the fuel and air intake of the burner. The purpose of the controller is to operate these relays, and thereby adjust the burner, in a manner to sustain a uniform or nearly uniform value for a selected parameter of the product mix, in this instance temperature. Sensor 10 outputs a signal with a current

proportional to temperature and a span 4 - 20 mA repre¬ senting a temperature range 0-200°C. The signal is fed to a current-to-voltage amplifier/converter 22 which removes the 4 mA offset .and converts the 16 mA current

5 span to a corresponding voltage span V. This latter signal constitutes one input to comparison means, being differential amplifier 24, the other input being a uniform voltage V determining a reference temperature value set at potentiometer 25.

10 The output of differential amplifier 24 is a deviation signal directly proportional to the deviation between V and V , but with an applied bias voltage one-half the span voltage of the succeeding analogue-to- digital (A/D) converter 26 to which the deviation signal

15 is now fed.

Converter 26 samples the amplitude of the deviation signal at a uniform sampling interval determined by an adjustable oscillator 28, and latches a digital representation of the sampled value. The sampling

2.0 interval is set to be consistent with the known delay time for a burner adjustment to be reflected in the product temperature. For the application at hand, the interval is variable by adjustment of oscillator 28 over the range 20 seconds to 4 minutes.

25 A/D converter 26 indicates when conversion is completed and data latched by applying a logical high ("1") from its "end of conversion" output 26a. This "end of conversion" is used to load the lesser bits of the conversion at data output 26b into an up/down

30 counter 30. The most significant bit (M.S.B.) of the conversion at output line 26c is used to command the counter as to whether it should count "up" or "down", since the logical value of the M.S.B. determines which side of the bias voltage the digitally represented

deviation value lies.

The up/down counter has a terminal count output 30a which goes to logical low whenever the count is at zero or its maximum binary count, and logical high for any intermediate count. Just prior to any A/D conversion the counter is reset to zero at input 30c. At completion of a conversion the counter, having received a command at lead input 30b from converter output 26a, will be loaded with the read value. If the deviation represented thereby is zero or very small the conversion will yield a binary zero result and hence the counter output 30a will remain at zero and the cir - cuit will take no corrective action. However, if the conversion yields a non-zero result (i.e. a substantial deviation) the counter will be loaded and output 30a will output a logical high ("1") .

The signal for counter output 30a is applied via AND gate 32 to a pair of further AND gates 34, 35 which respectively enable the appropriate relay 20 or 21. The direction of adjustment, and thus the relay to be switched on, is determined by the direction of the deviation represented by the current latched digital count in A/D converter 26. As discussed, this informat¬ ion is also contained in the M.S.B. output line 26c of the converter and thus the logical value of the M.S.B. is also applied by line 33 to gates 34, 35, in one case via diode 36. If the M.S.B. is low (O) - deviation negative, temperature too low - only the raise temperature relay is allowed to operate. Similarly if the M.S.B. is high (1) - deviation positive, temperature - too high - then only the lower temperature relay will operate.

Once a relay 20 or 21 is closed, counter 30 will

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count and the output at 30a will remain at logical high until 0 or maximum count is attained. Thus, the. duration of the burner adjustment is directly proportion¬ al to the magnitude of the currently latched count value and thereby to the sampled temperature deviation represented thereby.

Once the corrective pulse has been applied, the output of AND gate 32, which inputs from converter output 26a and from counter output 30a, will inhibit any further action until the next scan pulse, whereupon the procedure will repeat itself.

Counter 30 is clocked via triple AND gate 38, which is subject to converter "end of conversion" output 26a, to counter output 30a post AND gate 32, and to unit period oscillator 40. In this way, the relays are set to be operative only for integer multiples of the period of oscillator 40, with the integer limited to the range 0 to the maximum count of the binary up/ down counter. ^' For the application at hand, this period can be varied in the range 0.125 to 4.0 seconds, which determines a realy ON band time of 0.125 seconds to four times the maximum count in seconds. The real relay ON time should always be sufficient to trigger actual adjustment but is desirably somewhat less than the sampling interval determined by oscillator 28.

It will be appreciated that the described circuit affords a simple yet effective controller which utilises basic components and permits wholly independent adjustment of sampling interval and adjustment interval. The circuit is found to be especially well adapted to burner control on the basis of a monitored product parameter. The conversion to and latching of a digital

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representation of a periodically sampled value of the parameter permits much-larger sampling intervals than hitherto obtainable, a fact which enhances the uniformity and proper control of product specification.

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