TRANSPORT and ROAD RESEARCH LABORATORY

Department of the Environment

TRRL REPORT LR 578

INSTRUMENTATION FOR A FULLY

AUTOMATIC ASPHALT MIXING PLANT

by

R. WEEKS

Construction Methods Division Highways Department

Transport and Road Research Laboratory Crnwthorne, Berkshire

1973

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 St April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1. Introduction

2. Instrumentation requirements

2.1 Measurements for process control

2.2 Sensors for plant supervision

3. Measurement of temperature

3.1 Thermocouples

3.2 Infra-red pyrometers

4. Weighing equipment

5. Measurement of moisture in aggregates

5.1 General considerations

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7.

5.1.1

5.1.2

5.1.3

5.1.4

5.1.5

5.1.6

Electrical-resistance measuring devices

Capacitance measuring devices

Infra-red reflectance measuring devices

Neutron moisture meters

Nuclear magnetic resonance

Microwave moisture meters

5.2 The microwave moisture meter

5.2.1 General description

5.2.2 Installation

5.2.3 Performance

Contents measurement and level indication

Motion, position and flow sensing

7.1 General

7.2 The motion-sensing circuit

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8. Summary of current position

9. Acknowledgement

10. References

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(• CROWN COPYRIGHT 1973

Extracts f rom the text may be reproduced

provided the source is acknowledged

INSTRUMENTATION FOR A FULLY

AUTOMATIC ASPHALT MIXING PLANT

ABSTRACT

The operation of a pilot-scale asphalt plant is being put under the complete control of a small on-line computer with the aim of improving the accuracy and uniformity of both the composition and temperature of the materials produced by the plant. The computer control unit requires accurate information about the plant and the process; equipment for weighing, for moisture and temperature measurement and for sensing contents, level, motion and position has been installed on the plant for this purpose. The Report discusses the factors that influenced the selection of the various instruments and outlines the progress made to date in the evaluation of those installed on the plant.

1. INTRODUCTION

Because changes in aggregate state and plant behaviour inevitably occur under normal operating conditions, bituminous road materials that are truly uniform in both composition and temperature can be produced only in plants that are capable of compensating for these changes.

There are many mixing plants in operation today that are automatically controlled in the limited sense that operations such as proportioning of aggregate feed, batching and mixing are repetitively sequenced without frequent human intervention. Such plants are capable of providing really uniform materials only when all the incoming material is completely uniform and the behaviour of the plant is completely consistent.

In practice, therefore, a fully automatic operating system, capable of detecting changes and using the information to take corrective action, is essential if a mixing plant is to produce consistently uniform material. The form such a system might take has been considered by Kirkham and Mathews 1 .

This Report is concerned with the instrumentation required for effective fully automatic control of a mixing plant. Detailed descriptions of the experimental plant and the digital process controller that is being used to assess the performance of the instruments will be given in other Reports. The schematic diagrams shown in Figs 1 and 2 are sufficient to support the present discussion; these show the materials flow in the mixing plant in relation to the aspects of instrumentation discussed and the general layout of the computer-control system to which the instruments are connected.

2. I N S T R U M E N T A T I O N REQUIREMENTS

Instrumentat ion requirements for the plant can be divided into two categories: those dealing with information about the state of the process and those dealing with the state of the plant itself.

2.1 Measurements for process control

The first category includes measurements of temperature in the aggregate dryer and in the mixer and measurement of the moisture content o f the cold aggregate. These are essential if a correct and uniform final temperature of mixed material is to be achieved, which in turn is needed if a satisfactory and uniform level of compaction is to be obtained on the road. Also included in this category is the weighing of the mix components as this determines the quality of the product in terms of composition.

A fourth process-variable is the gradation of the aggregate in the feed and in the hot bins. No instrument exists at present that will give on-line size-analysis for the range of sizes of aggregate used in road materials.

2.2 Sensors for plant supervision

The second category includes measurements of contents and sensing of level, motion, position and flow. These are variables which, although not necessarily affecting the quality of the product directly, must be either correct or within Set limits if the process is to continue uninterrupted. Bitumen temperature is also included in this category at present, although it does have some effect on product quality, because it is controlled independently.

Contents measurement is required in storage tanks and bins for stock control and level sensing is required to give high- and low-level alarms for the control of quantities in feed hoppers, hot bins, etc. A variety of equipment is available for these applications except for contents measurement in aggregate bins. For this, the only accurate method available is to weigh the bins. An alternative approach to stock control might use level sensors in conjunction with continuous input-output records to estimate the weight o f aggregate at any time. This would be suitable for systems that include a digital computer because the current estimate of contents would be instantly available to the process controller and to the plant operator.

Motion, position and flow sensors are required to ensure that the plant is functioning correctly; that is that conveyors move, shafts turn, doors open, material flows along the system, etc. Instruments for these purposes must obviously be robust, cheap and effective. Many commercially produced devices do not fully satisfy these criteria; in particular they are often expensive when considered in terms of total plant instrumentation costs because quite large numbers of them are necessary, even on small plants.

3. M E A S U R E M E N T OF TEMPERATURE

3.1 Thermocouples

Base-metal thermocouples are cheap, reliable, sensitive and reasonably accurate when used in the range 0 to 250 degrees C 2. They are also robust and can be replaced without recalibration of transmitters or indicators. It is, therefore, convenient to use them in control systems. They are used as basic instruments for temperature measurement on the experimental plant. Iron-constantan thermocouples are used at present but corrosion problems with iron make nickel/chromium - nickel[aluminium a better choice. Copper - constantan could also be used but is less linear as shown by the calibration curve in Fig.3. Both combinations are however less sensitive than iron-constantan.

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Many transmitters capable of amplifying thermocouple signals and compensating for cold-junction temperature changes are manufactured. Servo-potentiometer devices for this purpose are accurate and quite cheap. They usually operate an indicator directly and because of this are useful where an accurate display of temperature is required as well as a signal for process control. Where large visual indicators are not required, solid-state transmitters may be used; these are more expensive but have no moving parts and can be installed near to the thermocouples, thus eliminating the difficulties associated with transmitting low-level signals for long distances in compensating cable.

The major disadvantage of thermocouples is that they are contact devices. They are subject to damage and severe wear when used in a stream of fast-moving aggregate. They are also inaccurate under these conditions because of poor thermal contact with the hot material. Heavy shielding will prolong the life of a probe but will also reduce the speed of response dramatically. A device, which is based on a design developed some years ago by Russell, 3, is used on the experimental plant to ensure that the thermocouple is always immersed in freshly heated, slow-moving aggregate. In such conditions problems of excessive wear and poor thermal contact are minimized.

The 'Russell' device is fitted in the continuous-dryer output-chute as shown in Fig.4. and Plates 1 and 2, and consists of a fixed thermocouple within a catch-pot formed by a gate placed between the sides o f the chute. This gate is closed periodically to interrupt the flow of material in order to build up aggregate around the thermocouple. After a few seconds the gate is opened to release the aggregate and then closed again to hold fresh aggregate around the probe. With optimum timing, this system gives a good indication of the bulk temperature of the aggregate directly, in so far as such a parameter exists for a inhomogeneous material recently subjected to rapid heating. The performance of the device will be checked by comparing • its readings with the mean bulk temperatures of samples of hot aggregate. These temperatures will be found by allowing the samples to come to thermal equilibrium in a well insulated container fitted with a thermocouple. This is also shown in Plate 1.

3.2 Infra-red Pyrometers

An alternative approach to the problem of measuring aggregate temperature with reasonable sensitivity but avoiding excessive wear is to measure the radiant heat energy using an infra-red radiation pyrometer 4 (Fig 5.). This is a non-contacting device consisting essentially of a lens, to focus the radiation, and an infra- red detector. Although i t eliminates some of the drawbacks of thermocouples, the pyrometer has disadvantages of its own:

a. Because of the physical laws governing heat radiation, the pyrometer output signal is non-linear with temperature and the accuracy of the instrument deteriorates rapidly at temperatures below 100°C.

b. For any real material, the energy emitted at a given temperature is a fraction of that from a theoretical full, or 'black-body', radiator. This fraction, known as the emissivity of the surface, is dependent on the properties of the material, in particular the surface texture and thus varies with grading as well as with the type of stone and mixture used. It must be known if accurate results are to be obtained from the pyrometer but can be measured only with special equipment.

C. The pyrometer is less robust with respect to mechanical or thermal shock than is a thermocouple. It is however, easy, if expensive, to replace when damaged and it does not require on-plant calibration as the instrument comes calibrated by the manufacturer. On-site calibration checks are virtually impossible withoutspecial equipment, because

the pyrometer measures the surface temperature and any simple checking thermometer

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measures some local bulk temperature which may be several degrees different. Consequently a pyrometer must, in general, be returned to the manufacturer when its performance is suspect.

d. Dust in the atmosphere will affect the readings by absorbing some of the radiation from the aggregate and emitting radiation characteristic of its own temperature. Dust can be kept out of the field of view and off the lens to a large extent by using a compressed-air purge. The effects of dust in the air can be minimised by siting the pyrometer close to the aggregate surface. These comments apply equally to water vapour if it is present in large quantities.

Two infra-red pyrometers have been fitted to the experimental plant for evaluation, one at the output of the continuous dryer (Plate 1) and one at the mixer (Plate 3). Attempts have been made to minimise the disadvantages outlined above.

The digit~ computer in the process-control unit is'used to overcome the problem of non-!inearity and satisfactory sensitivity is maintained by restricting the measurements of temperature to above 50°C.

Emissivity measurements have been carried out for the Laboratory on a range of aggregates, binders and mixtures. The results are encouraging. They suggest that, for the range of materials tested, the variations in emissivity are sufficiently low for one average value to be used for dry aggregates and another for mixtures containing between four and ten per cent by weight of binder. The emissivity values used on the experimental plant are 0.91 for the dryer pyrometer and 0.94 for that on the mixer. The maximum errors resulting from the use of these values are expected to be less than 3 degrees Celsius.

At the output of the dryer, the surface temperature of the aggregate is likely to be different from its bulk temperature because the material is freshly heated and has therefore not reached steady-state conditions. The extent of this effect will be measured by comparison of the pyrometer readings with those from the Russell device. The problem should not arise at the mixer where the temperature of the material is much more uniform and a fresh surface is continually presented to the pyrometer.

In this application, however, careful siting of the pyrometer is necessary to avoid frequent cleaning of the lens as the air purge cannot remove splashes of binder.

4. WEIGHING EQUIPMENT

The choice o f weighing equipment is primarily between mechanical lever devices and the various forms of 'load cell'. Lever devices are bulky and have moving parts, knife edges, etc, which are susceptible to wear. Their use also involves quite largeweigh-hopper deflections which can give clearance problems. Load cells are relatively small, low-deflection devices. There are many types; the most common use inductance, pneumatic or hydraulic pressure, or strain gauges to give a direct, analogue output. Load cells of the strain- gauge type were chosen for the asphalt-plant system (Plate 3) because they deflect very lithe when loaded and were the most readily available at the time the experimental plant was designed. The load cells give an electrical output which is fed to indicators of the servo-potentiometer type. These indicators are accurate and electronically stable b.ut, having moving parts, they suffer from severe wear brought about by plant vibration transmitted to them as noise in the signal from the load cells. The wear problem could be eliminated by using solid-state transmitters but the vibration also gives unacceptable fluctuations in the readings which simple filtering cannot remove without, at the same time, lengthening the response-time of the system. Problems also arise from sideways loading of the cells by the tiebars that hold the weigh hoppers in position; the current performance o f the system might be improved by redesign of the load-cell mountings to isolate the cells from vibration and from lateral loads.

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5. MEASUREMENT OF MOISTURE IN AGGREGATES

5.1 General considerations

Batch-heater coating plants are generally fed with pre-dried aggregates or with material of low moisture content. Under these conditions, moisture content is of little importance in the operation of batch-heater plants, particularly where the period of heating is controlled by temperature measurement of the aggregate rather than being of fixed length as has been customary in the past.

The situation is entirely different on a continuous-dryer asphalt plant, where the aggregate feed is often taken from stockpiles in the open air. The rotary dryer performs the double function of drying and heating the material and, in the absence of changes in burner setting or feed-rate, the output-temperature is very dependent on the moisture content of the feed material. To obtain a correct and uniform temperature in the final product, close control of the dryer output temperature is essential. However, rapid changes in the moisture content of the feed material combined with the slow temperature response of the dryer to changes in burner-setting make it difficult, if not impossible, to control the output- temperature by using this temperature alone to control the burner setting*. Some form of moisture measurement on the incoming aggregate is necessary to enable the control system to anticipate changes in the moisture content of the material entering the dryer.

Instruments for on-line measurement of the moisture content of solids have been a continual source of difficulty to, and have been largely avoided by, automation engineers in many industries. Equipment is available for moisture measurement in gases, usually measuring dew point directly 6r using hygroscopic sensors. This equipment can be used to measure moisture in fibrous or granular materials where the moisture content approaches an equilibrium level with the ambient air but is not suitable for use on wet aggregates which are generally over-saturated in this respect.

A wide variety of instruments is produced commercially for moisture measurement in solids; many o f these are not suitable for use with road-making aggregates because their readings are not specific to water but also depend on other properties of the subject material which may vary considerably 5' 6. For this reason a feasibility study has been made in order to determine which of the available instruments may be useful for the measurement of moisture in road-making aggregates.

5.1.1 Electrical-resistance measuring devices

These give readings which are dependent on the aggregate material, its size range, its packing density and in particular on solutes in the water 6. Such devices are useful only for consistent materials under controlled conditions.

5.1.2 Capacitance measuring devices

These are also dependent on packing density, because the readings depend on the total quantity, of water in the range of the sensor rather than on the water/aggregate ratio 6. The readings are also affected by build-up of solid material on the probes, by the composition of the aggregate, by temperature, and by the presence of any electrolytes in the water which will effectively short-circuit the capacitor plates. Under favourable conditions, accuracies around 1 per cent of water by weight can be achieved. This is not sufficient for automatic control of asphalt plant.

5.1.3 Infra-red reflectance measuring devices

These are very suitable for moisture-content measurements of materials in continuous processes because they have a single, non-contacting sensing head 5,7. In some industries, notably textiles, they

It is known that several unsuccessful attempts have been made to do this but detailed results of the trials have not been published.

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can give very accurate results. The infra-red method is one of the few where readings do not appear to depend on packing density. Unfortunately, the readings are severely affected by the particle size and by the colour o f aggregates. This was shown in a short series of tests on asphalt sands. The results showed that the calibration curves required for several similar materials were quite different and highly non-linear.

5.1.4 Neutron moisture meters

These measure the intensity of slow neutrons scattered back from the subject material when fast neutrons from a radio-isotope source are directed on to it 5 6 The readings are directly related to the quanti ty of hydrogen in the material, provided that other very light atoms are absent, because the slowing- down process results mainly from elastic collisions, the greatest energy transfer occurring between particles of equal mass. Hydrogenous material and other light elements are not usually found in dry aggregates and thus the measurements are likely to be dependent only on the quantity of water in the range of the sensor; for this reason the measured moisture contents will be dependent on the packing density of the aggregate. There are two important disadvantages of this method. One is that the readings must be averaged over a relatively long period (perhaps 1 O0 seconds) to eliminate the essential randomness of the nuclear process; the other is that any elements that absorb slow neutrons will give falsely low readings of moisture content. iron and chlorine are two such elements that are found in aggregates. Although their concentrations in any one material would be relatively constant, separate calibrations might well be necessary for each aggregate and mixture used. The overall accuracy of the method can be around 0.5 per cent of moisture by weight which is barely adequate for dryer control 8.

5.1.5 Nuclear Magnetic Resonance

The apparatus measures the radio-frequency power absorbed in nuclei and can be tuned to 'see' only free protons so that hydrogen in the dry solid and in bound water is ignored. The readings will change with the water-absorption properties of the aggregate so separate calibrations might be required for each material. Chemical composition of the aggregate is not important but metallic conductors tend to damp out the resonance and, in sufficient concentration, make readings impossible. Generally accuracies of about 0.5 per cent of water by weight are obtainable; for on-line use on an asphalt plant the equipment would be very expensive 5 9

5.1.6 Microwave moisture meters

These measure the attenuation of a microwave beam passed through the sample material. Their readings depend on the moisture content of the sample and are largely independent of aggregate composition and o f dissolved chemicals; they are seriously affected only by metallic conductors 10. "the wavelengths used are long enough to avoid problems due to grading changes, at least in the smaller aggregate fractions. Effects due to absorbed and bound water are small and readings, accurate to about 0.2 per cent of moisture by weight, can be obtained with very short respo*nse times.

The readings are affected by temperature and by packing density but temperature compensation has been found to be relatively easy in most materials and some form of aggregate conditioning 'should be possible to achieve constant density in the sample material. Alternatively integration of the signal to give the total mass flow of water into the dryer would give readings which are independent of the mass or packing density of the aggregate.

From these considerations it would appear that a suitably designed microwave-absorption moisture meter might give sufficiently fast, accurate and reliable readings for continuous dryer control. It was for this reason that such an instrument was purchased for use on the experimental plant. The total cost of installation of the instrument was about £1,500 including the cost of providing protection from the weather. This cost is comparable with that for neutron and some infra-red devices, cheaper than that for nuclear- magnetic-resonance and most infra-red devices, and more expensive than that for capacitance and resistance devices.

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5.2 The Microwave Moisture Meter

5.2.1 General description

The meter that is being evaluated is a Microwave Instruments' MX 100; Fig.6 shows the principle of operation of the device. It operates on a 'double beam' principle to eliminate drift problems associated with the microwave source and detector. X-band microwaves (9.4 GHz; wavelength about 32 mm) from the Klystron source are passed alternately to 'sample' and 'reference' beams by the switch which is driven at 400 Hz by the oscillator. The two beams are reunited by the coupler and fed to the detector which senses the 400 Hz 'difference' signal caused by attenuation in the sample. This signal is amplified and used to drive a piston attenuator in the reference beam until the difference signal is reduced to zero. The attenuation caused by the sample is then read-off from the position of the piston attenuator. A slave potentiometer on the piston attenuator enables signals, which are linear with piston movement, to be transmitted to a chart recorder and to the process controller.

5.2.2. Installation

The instrument has been installed adjacent to the main feed conveyor to the continuous dryer (Fig. 7). The sample beam is passed through the belt and through a 50 mm layer of material.

Idlers have been fitted on either side of the head to steady the conveyor belt because the degree of attenuation of the beam depends on the thickness of the layer of material and on microwave reflection effects in the sensing head. A funnel-shaped aggregate-conditioning 'plough' has been installed over the belt to induce a reasonably uniform density of material; the sensing head is fitted with a flat 'plough' to trim off the layer to constant depth.

5.2.3. Performance

The belt gives an attenuation of 11 dB when stationary and 11 + 1 dB when running. A vulcanised joint in the belt gives a peak to 13.5 dB as shown in Fig. 8a. The variations in the attenuation result from absorption in the belt material and from reflection effects caused by vertical movement of the belt which is not pressed against the idlers when unloaded. The 'ears' on the ends of the signal given by a batch of wet aggregate on Fig. 8b are caused by the reflection of part of the beam out of the system by the sloping ends of the batch.

To date, tests on the performance of the MX 100 have been confined to obtaining calibration curves and to the assessment of temperature effects on the attenuation signal. The aggregates used have included several sands and crushed rocks in sizes below 3 mm nominal diameter; particle-size effects will be investigated later.

Calibration. The calibration curves were obtained by pouring samples of the materials loosely onto the conveyor belt and running the belt so that the materials passed first through the conditioning plough, then through the sensing head. In this way the test conditions closely resembled actual production conditions with approximately 5-second batches of feed material. Oven-dried moisture contents were obtained on samples of the aggregate taken from the belt and plotted against attenuation readings from the recorder chart.

The curves resulting from the various aggregates shown in Fig.9 were all similar both in shape and in level of attenuation. The shape is essentially parabolic; the linear broken line in Fig. 10, derived by taking the square roots of the attenuations in the full line, demonstrates this.

However, the experimental curve does not go smoothly to the origin but turns suddenly downwards as the moisture content falls below about 1 per cent. At this point, the surface of the aggregate ceases to give a strong reflection and the reflection pattern in the sensing head becomes that caused by the empty

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belt alone. Because of this effect, moisture contents below 1 per cent cannot be measured easily and dry sand cannot be used as a calibration standard.

The variation between the curves of Fig. 9 is likely to be due either to variation of the attenuation by water with temperature or to variation of the zero moisture-content level of the curve, caused by absorption in the dry material or reflection changes with change of grading. Because a temperature effect was expected, this was investigated first.

Effects o f temperature. Samples of the various aggregates were prepared with different moisture contents and then sealed in polythene bags (polythene does not absorb microwaves). Each sample was cooled in a refrigerator, then placed in the sensing head on the belt and allowed to come to ambient temperature while measurements of temperature and attenuation were taken. The sample was then heated in a warm room and the procedure repeated for above-ambient temperatures.

Fig.! ! . shows a typical curve. The placing of the samples in the sensing head by hand did not give consistent sample thickness or packing density and therefore the attenuation levels of the curves do not relate exactly to the moisture contents of the samples. For this reason, the cooling and heating parts of each curve did not always coincide and were adjusted by eye to give a smooth curve. The problem had been expected; the tests were chosen each day according to the ambient temperature, cold tests on hot days and vice versa, to give overlapping curves.

Some of the samples were inadvertently frozen solid; ice does not absorb the microwaves because the molecules are tightly bound in the solid and cannot respond at X-band frequencies; therefore the initial signal was very low but rose rapidly as the samples thawed, as illustrated by the dashed line in Fig . l t . In general, the results were all similar in form, the attenuation being linear with temperature below about 15°C and becoming constant above that temperature with a zero slope at about 20°C. Plotting the slopes of the linear parts of all the graphs against attenuation at 15°C provides the data shown in Fig. 12; these imply that a single temperature coefficient can be used for all the aggregates tested (0.017 per °C).

Other considerations and results obtained. Temperature-correction reduced the variation between the curves of Fig. 9, to some extent but a more marked improvement was seen (Fig. 13) when the corrected curves were adjusted using zero-moisture-content levels calculated on the assumption that the temperature corrected curves were symmetrical parabolas. The significance of this is not clear; it may imply a dependence on aggregate type or grading, in which case tests on each aggregate used might be required to achieve the ultimate accuracy; or it may indicate a day-to-day variation in the experimental conditions. Further investigations will be carried out. Computer programs are being developed to process the signal auto- matically and to provide direct read-out of moisture content corrected for temperature and for any temporal variations in zero-moisture-content level.

The results of an initial trial are shown in Fig. 14. Batches of sand each thirty seconds long, were dispensed from the plant feeder unit and passed through the microwave sensing head. Samples were taken from each batch for oven-drying and the moisture-meter signal was fed to a chart recorder and to the computer . The chart record was processed by hand to obtain moisture-content values and the values calculated by the computer were corrected for temperature by hand before plotting. Although this was only one isolated trial the result supports all the other data obtained at the Laboratory to date, from which it is clear that the microwave instrument is capable of giving moisture-content readings, for sands and fine crushed rocks, that are sufficiently accurate for dryer control.

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6. CONTENTS MEASUREMENT A N D LEVEL I N D I C A T I O N

For stock control, production planning, and feed-rate control in continuous systems, it would be advantageous to have measurements of the exact mass or volume contents of almost every bin, hopper or tank of a mixing plant. Burner fuel can be gauged easily using conventional, reliable, float-actuated meters but binders and aggregates present problems.

Binders, because of their high and temperature-dependent viscosity, cannot be gauged reliably by float or probe techniques; hence, some method which operates from outside the tank is to be preferred. Two methods are available: gamma-ray absorption and weighing I 1 Gamma-ray devices measure the mass of material in the radiation beam between a source and a detector and, for material o f constant density, the depth in a tank can be inferred. The instruments are fairly accurate and reliable but are very expensive compared with weighing equipment. Weighing is a simple, direct, way of measuring the mass of material in a container and the equipment is relatively cheap and easy to install. This method is being used on the experimental plant. Each binder tank is supported at one end by pivot rollers and at the other by one or two strain-gauge load-cells, according to tank size. Small potentiometric indicators are used to give visual contents readings and to provide a read-out to the process controller.

Most aggregates develop considerable internal friction and bins and hoppers tend to be shallow in relation to their cross-sectional area. The free surface of the mass of aggregate is therefore of considerable area and irregular as shown for example in Plate 4. A pure level measurement of the surface at some point is meaningless as a method of defining the total mass of aggregate present. Weighing is, therefore, the only reliable and universally applicable method of contents measurement in aggregate bins and hoppers and could be used with advantage on newly designed plant. On existing plant both the cost and complexity of installation would be very great and could not, in general, be justified. For this reason no bin-contents measurements will be made on the experimental plant; only high-and low-level sensors will be used.

Several techniques can be used for level sensing 11. Ultrasonic, photo-electric and nuclear beam devices are simple in concept but difficulties arise over dust, heat and cost. Pressure-diaphragm devices have been found to be insensitive, inaccurate and unreliable with aggregates. The 'stalled-paddle' type of level sensor (Fig. 15) works quite well in aggregates and is relatively cheap (at about £50 each) but it has moving parts and bearings which are subject to wear and its sensing level is essentially fixed. Capacitance devices (sensitive to changes in dielectric constant near the probe), cost about £70 each but have no moving parts and can be provided with very robust probes (Fig. 16). The switching level can be varied to some extent, so that probe length and siting is not so critical as with the paddle devices.

To compare their performance in service, both paddle and capacitance types of level sensor have been installed on the plant in aggregate bins (Plate 4) and in the filler silo. If it were considered necessary, it might be possible to make reasonably accurate contents (rather then level) measurements in filler silos using a capacitance device. The angle of repose is much lower for filler than for coarse or fine aggregates and silos tend to have a more regular shape than hoppers and bins. Some problems could be expected from changes in packing density and moisture-content and from 'rat-holing' around the probes. On the experimental plant, the measurement of the quantity of material in the filler silo is not contemplated.

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7. MOTION, POSITION AND FLOW SENSING

7.1 General

A great deal o f reliance is placed on the operator o f a manually controlled or semi-automatic plant to ensure that the machinery functions correctly and to take appropriate action when a failure occurs. Even on such plants, sensors to give immediate indication of failure would be valuable, but in fully automatic systems sensors are absolutely necessary if the process controller is to detect fault conditions successfully.

On a mixing plant there are three basic requirements for sensing; motion sensors to check that belts move, mixer-paddles turn, etc; position sensors to check that doors open and close fully; and flow sensors to check for blockage or leakage of material. These applications involve quite a large total number of devices, even on a small plant, so some thought is required to eliminate non-essential applications and to choose robust, effective and cheap instruments.

For flow and position sensing, simple switches actuated by flaps and levers are sufficient in most cases. The motion-sensing function necessitates specially designed circuits; several devices are produced commercially, most ly using inductive pickups or cam- or magnet-operated switches; prices range from £30 upwards for the former type and from £20 upwards for the latter. Thus, if one sensor is used on e.ach independent moving component of a plant, total costs can be very high. For this reason a very simple, cheap and effective circuit has been designed at the Laboratory that can be used to detect the motion of any mechanical part which has a relatively constant speed or cyclic frequency.

7.2 The motion-sensing circuit

The circuit shown in Fig. 17 consists of a relay ($2), controlled by a capacitor (C2) and fed from a d.c. supply by a changeover switch (SI) and a capacitance- resistance net work which eliminates anomalous readings. The switch (S1) is operated periodically by the moving part and passes a train of current pulses to the capacitors C1 and C2. S1 can be a microswitch operated by a cam, a change-over reed-switch operated by a permanent magnet fixed to the part, or a relay on the circuit board operated remotely by a single-contact switch on the plant ($3 in Fig. 17). This last arrangement permits the use of very small, sensitive, reed-switches as sensors on the plant and minimises setting-up problems and the effect of long-term degradation in the magnets.

When the mechanical part in question is moving, the relay ($2) is energised and the capacitor (C2) is charged from the power supply. The component values are chosen so that C2 smooths out the voltage on the relay coil, supplying sufficient current to hold in the relay between pulses and to provide a short delay when the motion stops. This eliminates spurious alarms generated by short-term oveiioading, power failure, etc. The delay time of the circuit can be adjusted to some extent by changing the value of the resistor (R2), which also serves to limit the current drawn from the supply through S1 (if S1 is a reed-switch the maximum current allowed would be of the order o f 200 mA).

The principle of using a capacitor to introduce delay into a relay circuit is well known and in common use for a wide variety of applications. It is used alone by some manufacturers for motion sensing but this has a serious, inherent disadvantage because it does not detect current pulses but merely the presence or absence of some minimum mean current drawn from the power supply. This means that if the motion stops in such a position that S1 is held closed, then the power supply is connected directly across the relay which remains energised indefinitely, indicating normal motion when in fact a fault has occurred. The probabili ty of this happening can be reduced by shortening the pulse time but only to a limited extent, because the pulses must be sufficiently long to maintain the charge on the capacitor.

10

The capacitor (C1), the resistor (RI) and the normally closed contacts of S1 are used to eliminate this problem. The value of C1 is sufficiently large to pass the full current required by the delay circuit, for the period when the supply is connected, during normal motion. After each pulse, it is discharged through RI which acts only as a current limiter. Thus the circuit behaves as a simple delayed relay during normal motion, and, when the motion stops with the power supply isolated by S1, the relay drops out after the set delay time. However, when the motion stops with the power supply connected, C1 becomes fully charged and blocks any further supply of current to the system. The relay then drops out after a total delay only slightly longer than in the normal case. The discharge circuit for C1 is necessary to prevent its becoming fully charged during normal motion and causing the circuit to give a false alarm.

The circuit, consisting of an encapsulated reed-switch (for S1), two resistors, two capacitors and a relay, can be built for a component cost of about £5.

8. SUMMARY OF CURRENT POSITION

Instruments for moisture content and temperature measurement and for weighing have been installed on the experimental plant. Contents-measuring instruments and sensors for level, motion, position and flow have also been fitted. Testing of the instruments under service conditions has begun, and programming of the plant operating system is in progress. Some preliminary trials have been completed to provide information for use in the development of the dryer-temperature control system. Details of these trials will be published in a separate Report 12. The asphalt plant is expected to begin operating under a simple fully automatic system during 1973.

9. ACKNOWLEDGEMENT

This report was prepared in the Construction Methods Division (Division Head: Mr. N.W. Lister) of the Highways Department.

10. REFERENCES

.

.

.

.

KIRKHAM, R.H.H. and D.H. MATHEWS. Research on the automatic control of manufacture of pavement materials. Department of the Environment, RRL Report LR 401. Crowthorne, 1971 (Road Research Laboratory).

BRITISH STANDARDS INSTITUTION. British Standard 1041: Part 4: 1966. Thermocouples. London, 1966 (British Standards Institution).

RUSSELL, W.L. A new device for measuring the temperature of the aggregate on asphalt plants. Rds Rd Constr. 1961, 39, (457), 12-14.

LAND, T. Practical aspects of radiation pyrometry. Trans. Soc. Instrum. Technol., 1959, 11 (March), 10-18.

5. ANON. Engineering Outline 85, Moisture Measurement. Engng, 1967, 204, (5286), 229-232.

11

6. GODDING, R.G. and D. BIRD. An apparatus for continuous measurement of water content of foundry sands. British Cast Iron Research Association Journal, 1963, 11, (5), 641-61.

7. MATHEWS, C.N.G. A remote reading moisture gauge. Ind. Electronics, 1967, 5, (11), 486-9.

8. BURN, K.N. Design and calibration of a neutron moisture meter. ASTM spec. tech. publ. No. 293, 14-26. Philadelphia, 1960 (American Society for Testing Materials).

9. SIMPSON, R.J. The use of nuclear magnetic resonance for the determination of moisture and liquid fat content. Measmt Control, 1968, 1, (March), 82-3.

10. SUMMERHILL, S. Microwaves in the measurement of moisture. Instrum. Rev., 1967, 14, (190), 419-21.

11. ANON. SIRA SURVEY 1 : LEVEL MEASUREMENT. Instrum. Control. Engng, 1970, 8, (Apr.-Ju!.), parts !-4.

12. JORDON, P.G., B.W. FERNE and R. WEEKS. Temperature distributions in the rotary dryer of an asphalt plant. Department of the Environment, TRRL Report (In preparation).

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ABSTRACT

Instrumentation for a fully automatic asphalt mixing plant: R WEEKS: Depar tment o f the Environment, T R R L Report LR 578: Crowthorne, 1973 (Transport and Road Research Laboratory). The operat ion of a pilot-scale asphalt plant is being put under the comple te control of a small on-line computer with the aim of improving the accuracy and un i formi ty of both the composi t ion and temperature of the materials p roduced by the plant. The computer control unit requires accurate information about the plant and the process; equipment for weighing, for moisture and temperature measurement and for sensing contents , level, motion and position has been installed on the plant for this purpose. The Repor t dis- cusses the factors that influenced the selection of the various ins t ruments and outl ines the progress made to date in the evaluation of those installed on the plant.

ABSTRACT

Instrumentation for a fully automatic asphalt mixing plant: R WEEKS: Depar tment o f the Environment, TRRL Report LR 578: Crowthorne, 1973 (Transport and Road Research Laboratory). The operation of a pilot-scale asphalt plant is being put under the comple te control of a small on-line computer with the aim of improving the accuracy and un i formi ty of both the composit ion and temperature of the materials p roduced by the plant. The computer control unit requires accurate information about the plant and the process; equipment for weighing, for moisture and temperature measurement and for sensing contents , level, mot ion and position has been installed on the plant for this purpose. The Repor t dis- cusses the factors that influenced the selection of the various ins t ruments and outl ines the progress made to date in the evaluation of those installed on the plant.