Grain is required to have moisture content in a specific range for storage and processing purposes. During and after harvest very large volumes of grain have to be handled quickly and brought to the required moisture content. Too high a moisture content must be avoided to prevent spoilage by moulds in storage. Too low a moisture content must be avoided for three reasons. Firstly loss of more moisture than required lowers the mass of dried material available for sale and therefore the value of a given initial amount of grain. Secondly grain drying is energy-intensive and excessive drying is therefore a waste of money. Thirdly if the grain 1s kept an unnecessarily long time in a drier the drier itself is not being used economically. It is therefore most important that the large volume of grain in the driers now used for economic efficiency is brought to as uniform a moisture content as possible. In addition not only must the average moisture content of a batch be 1n the required range but the extremes in the batch must also be in the range, for the reasons above.

It is an object of the invention to improve the drying of grain and particularly the control of moisture content and range. According to one aspect of the invention there is provided a method of controlling the drying of grain in a recirculating batch drier including recirculating the grain in the drier, monitoring the moisture content of grain in an overburden zone and in a ventilated zone to provide time-based moisture content information for each zone, comparing on a time basis the moisture content to determine the convergence of moisture content in the zones, and

providing a signal on said convergence to indicate a uniformity of moisture content of the grain in the drier.

The interval between successive said convergences may be controlled by adjusting the grain recirculation rate. The required interval and recirculation rate or other control factor may be determined by automatic means responsive to the information from the zones and to the initial and required moisture content. Preferably the monitoring of moisture content is at the centre of a zone. According to another aspect of the invention there is provided a recirculating batch grain drier and control arrangement including a drier body in which in operation the grain forms a ventilation zone and an overburden zone for the ventilation zone and means to recirculate grain through said body, grain moisture content sensors responsive to the respective grain moisture content in said zones to produce moisture content information outputs, control means responsive to the variation of respective moisture content information with time to compare said information to identify convergence of respective moisture content information and determine the interval between said convergences, the control means further including means to relate said interval to the time to achieve a required moisture content of grain in recirculation and to effect a control action on the recirculation of grain so that a said convergence occurs at the time to achieve said required moisture content whereby the grain in recirculation is at said required moisture content for discharge.

A grain moisture content sensor may include a capacitative measuring cell having a first localised electrode and a second surrounding electrode, means to energise the cell with an alternating potential and means responsive to variation in the alternating potential across the cell when grain is placed between the first and second electrodes to indicate the moisture content of said grain and produce moisture content information.

The grain may be supplied to the sensor on a batch basis.

Embodiments of the Invention will now be described with reference to the accompanying drawings in which

Figure 1 shows in schematic outline a recirculation grain drier, and

Figure 2 shows a graph useful 1n understanding the invention

Figure 3 shows a schematic outline of a grain sensor in accordance with the invention.

Figure 1 shows in outline a recirculation grain drier and Figure 2 a graph relating to grain moisture content typical of such driers.

Essentially such driers include an upright vessel having a lower ventilation zone VZ and an upper overburden zone 0Z. The inlet IN is at the top of the vessel to the overburden zone and the outlet OUT from the bottom of the ventilation zone. A recirculation conduit RC links the outlet to the inlet to recirculate grain under the action of pump P or like device.

Typically some 60% of the grain is in the ventilation zone and

40% is in the overburden zone. In operation both zones are filled with grain to be dried and the pump operated to move grain from the outlet to the inlet so that grain moves down the vessel. Drying is carried out in the ventilation zone in any suitable manner (as is well-known in the art) while the overburden zone provides a seal for the ventilation zone. For example ventilation may be by cross-flow with the grain confined between perforated metal sheets. Another technique is to use inverted V-shape metal ducts to produce a mixture of co-current, counter-current and cross-flow.

In accordance with the invention two grain moisture content sensors SI and S2 are provided. These sensors are required to determine from time to time the moisture content of grain respectively in the overburden zone and the ventilation zone and preferably at the centres of the respective zones.

As the grain recirculates the initial volume of overburden zone grain progressively moves into the ventilation zone past sensor SI. As long as this movement 1s of the initial overburden- grain the sensor SI is in the grain of the initial moisture content (assumed to be substantially uniform). Over this time the sensor S2 is in grain which has longer and longer time of ventilation and thus of reducing moisture content. However when the overburden zone grain begins to reach the position of sensor S2 all the grain in turn has been ventilated for the same time so moisture content will be substantially constant (curve S2 in Figure 2). From the start of drying grain of constant moisture content surrounds the overburden zone sensor until the original overburden grain reaches the overburden zone sensor again (curve SI in Figure 2) when the reducing moisture content is sensed. After a relatively short time the conditions "stabilize" into a rhythmic pattern of sensor output shown in the graph of Figure 2. This is a graph of moisture content (MC) against drying time (T). The pattern is formed by the repeated passage of the original overburden zone grain providing the distinctive periods of substantially constant readings from the moisture content sensors.

The value of moisture content from the two sensors SI, S2 is information which can be supplied to an information processing device of suitable form to be processed to produce a control signal for the drier. From Figure 2 it is seen that the time intervals, t\ , between successive occasions when the two sensors indicate the same value of moisture content are substantially equal. By adjusting the rate at which grain circulates the time interval t] can be altered. From the slope of the line joining the points of substantially equal moisture content the time tz for a particular quantity of grain in the drier to become of a required moisture content can be predicted.

This predicted time can be compared with the value of tj_ then the drier can continue to be operated until the required uniform moisture content is achieved and the grain then discharged. If the predicted time is not an integral multiple of ti then the rate of grain circulation can be adjusted to achieve the required integral multiple with respect to the Interval time so that again drying can be stopped with a uniform moisture content.

The necessary calculations can be performed in various ways. Approximate estimates may even be calculable by an operator. However for more precise results a suitable programmed control device or a set of tables may be used. The drier may be controlled 1n an open-loop or closed-loop manner, the latter providing the operation with the minimum of attention.

It may be necessary to make more than one adjustment to the drier to achieve the required integral value of the interval with respect to the time remaining to achieve required moisture content. The appropriate control techniques will be readily apparent to those skilled in the art from the requirements set out above and will not be described further. In particular 1t should be noted that the number of 'cycles" shown in Figure 2 is by way of example only.

A suitable regime for execution by a programmable control device would start with the selection of drier air temperature and final grain temperature, continue with the filling of the drier with grain and then the recirculation of the grain controlled in accordance with the invention to complete a recirculation at the end of the calculated drying time for a required moisture content. The grain could then be recirculated without heating until the required final grain temperature is reached, and then discharged.

Typical "cycles" are expected to be some tens of minutes in length. To reduce moisture from 25% to 17% in a typical drier three to eight cycles might be needed, over say one to three hours.

A form of moisture content sensor particularly suitable for use with grain driers will now be described. Such sensors have hitherto been expensive and slow in operation. There is a need for a sensor to give a quick, and accurate value of moisture content during the drying of grain in a grain drier. This will assist in efficient use of the drier.

Figure 3 shows a sensor embodying an aspect of the invention. A capacitative measuring cell, CMC, is formed by an outer, generally tubular, electrode 0E and an inner, rod-like, electrode IE. The inner electrode is supported to be along the axis of the outer electrode by insulating rods IR. Two are shown but more may be used if needed. An insulated conductive connection to the inner electrode is conveniently provided through one of the rods IR and to emerge as terminal Tl . Terminal T2 is provided on the outer electrode. Other insulation and connections, which may include guard ring or like connection arrangements, may be provided as will be apparent to those skilled in the art.

The cell CMC is arranged to be included in a grain flow path. In one form this may be a grain duct of generally the same size as the outer electrode 0E, the duct possibly forming the outer electrode, or the sensor may be in a side branch from a grain duct to which branch grain is allowed entry and discharge by suitable controls. In particular the sensor can be supported in a grain drier, with the inner electrode- axis vertical, to be immersed in the grain and measure moisture content. Again suitable entry and discharge means can be provided. The sensor is conveniently mounted on a panel in the side of a duct or drier to be swingable out for service. Entry and discharge is then controllable by a plate movable to open or close an aperture at the lower, smaller, end of a conical outlet adaptor itself at the lower end of the outer electrode or outer part of the sensor. The plate may be controlled by an electro-pneumatic actuator and the aperture may be offset from the centre-line of the sensor. No closure is needed at the top, inlet, end.

The cell provides a capacitor in which the dielectric is formed by the moist grain. The value of this capacitor can be measured by suitable a.c. bridge techniques and converted to a moisture content based on a calibration of the cell, for example with grain of known moisture, as indicated by a.c. bridge device ACB. Typically the bridge operates at a medium to high radio frequency, values of 1% to 2 MHz being typical. However the drying process is fairly slow, when considered in electronic signal processing terms, so the output of sensor information can be at a much lower frequency, say once a second, and still be adequately frequent for the computation of the control actions. In this way the high frequency signals are used only in the vicinity of the sensor, where they can be screened, and only low frequency signals need be sent any great distance in the electrically difficult environment of a grain drier and associated conveyors etc using large electric motors and similar sources of interference.

In general it is preferred that the cell be filled and the grain at rest before a measurement be taken but apart from this constraint very rapid readings can be taken. The control plate mentioned above is effective to permit frequent readings of fresh samples of grain at rest.

The above techniques provide improved measurement and control of grain moisture content and grain drying to improve the efficiency of the procedure. in particular despite the wide variations in moisture content in different parts of the drier at different time a control technique using only two sensors can produce consistent grain moisture content in a finished batch of dried grain. The above techniques provide improved measurement and control of grain moisture content and grain drying to improve the efficiency of the procedure.