GB2083663A - Apparatus for sensing and controlling moisture content of granular material - Google Patents

Abstract

An apparatus is provided for sensing the moisture content of granular materials in a drying apparatus and for varying the flowrate of the granular materials in response to the moisture sensed. The apparatus includes apparatus for sensing the moisture content of the granular material as a capacitance value and for producing a signal whose frequency is a function of the capacitance sensed. The apparatus further includes a control apparatus, usually located remote from the sensing and signal generating means, for receiving the frequency signal and varying the flowrate of granular material through a drying apparatus in response to the frequency signal. <IMAGE>

Description

SPECIFICATION Apparatus for sensing and controlling moisture content of granular material The present invention relates to an apparatus for sensing the moisture content of granular material and varying the flow of the granular material in response to the moisture sensed.

It is well known that changes in the moisture content of many materials can be detected by changes in the dielectric constant of the material.

The dielectric constant can be measured as a total capacitance across two conductors placed across or in contact with the material. Capacitance and the dielectric constant are related in the following equation: C = d where C = capacitance of parallel plates in farads e = capacitivity of the dielectric in farads/meter A = area of one plate in meters2 d = distance between the plates in meters Two prior art moisture sensing devices based on this capacitance measurement are described in U.S.

Patents 3,841,610 issued to K. Hanzawa et al., and 3,600,676 issued to Ludwig et al. In both of these devices the capacitance signal produced as an indication of the moisture content is converted to an analog voltage signal, the voltage of which is a function of the moisture content. Voltage signals are often subject to errors caused by temperature, component variations, and analog noise interference. Further, if the analog signal needs to be transmitted over any substantial distance, there is often a problem with signal distortion due to noise voltages.

According to a first aspect of the invention, there is provided apparatus for controlling the moisture content of granular material exiting a drying apparatus by sensing the moisture content of the granular material and varying the flowrate of the granular material in response to the moisture sensed, said apparatus comprising sensing and signal generating means for sensing the moisture content of the granular material as a capacitance value and producing a signal indicative of the capacitance value sensed, said signal having a frequency which is a function of the capacitance sensed, and control means for receiving the frequency signal and varying the flowrate of the granular material through the drying apparatus in response to the frequency signal.

According to a second aspect of the invention, there is provided a drying apparatus for drying granular material and for controlling the moisture content of the granular material exiting therefrom, comprising drying means for exposing the granular material to a drying medium, means for varying the length of time for which the granular material is exposed to the drying medium, sensing and signal generating means for sensing the moisture content of the granular material as a capacitance value and producing a signal indicative of the capacitance value sensed, said signal having a frequency which is a function of the capacitance sensed and control means for receiving the frequency signal and, in response to the frequency signal, controlling the means for varying the length of time for which the granular material is exposed to the drying medium, thereby controlling the moisture content of the granular material.

The following is a more detailed description of one embodiment of the invention, by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a schematic illustration of equipment associated with a grain drier; Figure 2 is a top plan view of a preferred embodiment of a sensing means; Figure 3 is a schematic circuit diagram showing the sensing means of Figure 2 and control means; and Figure 4 is a complete circuit diagram of an apparatus for sensing the moisture content of granular material incorporating the sensing means and control means of Figures 2 and 3.

The apparatus is typically used to control the outflow of grain from a grain drier. Conventional grain drying equipment is shown in Figure 1.Wet grain is conveyed on a conveyor 1 from a wet grain storage bin 2 into the body of a grain drier 3. The grain drier 3 includes an air blower 4 and a source of heat 5 for drying the grain. The dried grain is removed from the drier 3 through rotatable metering bars 6. The speed of rotation of the metering bars 6 and thus the rate of the grain outflow, is controlled by a controlling device 7 (Figures 1,3), for example a d.c. motor or servo. The exiting grain is conveyed by a conveyor 8 into a holding bin 9.

The apparatus is used to control the speed of the controlling device 7 in response to the moisture content of the grain in the drier 3. Since the drain drying rate is a function of the grain output rate, the apparatus controls the moisture content of the exiting grain by controlling the rate of grain output from the drier 3.

The apparatus includes moisture sensing means 10; said means 10 are operative to sense the moisture content of the grain as a capacitance value and produce a signal indicative of the capacitance value sensed, said signal having a frequency which is a function of the capacitance sensed. The apparatus further includes control means 11 operative to compare the period of the frequency signal with an adjustable standard frequency period value and, in response to this comparison, vary the power delivered to the controlling device 7.

With reference to the Figures 2 and 4, the moisture sensing means 10 is seen to include a pair of conductors 12 connected to an oscillator 13. In the preferred embodiment, the oscillator 13 is a 555 timer IC 13a connected as an astable multivibrator.

Each of the conductors 12 is pectinate in shape, having a plurality of parallel spaced conductive fingers 14 interspaced by the fingers 14 of the opposite conductor 12. The multivibrator 13 is connected to a d.c. power supply means 15 as shown; in this case a 12 volt power supply circuit, to be discussed later, is used.

The conductors 12 and the multivibrator components 13 are preferably both mounted on a plate 14a and submerged into the grain in the drier 3. Changes in the moisture content of the grain are detected by changes in the dielectric constant in the grain. The dielectric constant is measured as a resulting capacitance value between the conductors 12. This change in capacitance changes the internal time constant of the multivibrator 13 and thus changes the frequency of the signal output 16 of the multivibrator 13. The sensing means 10 is preferably protected by an epoxy resin coating to protect it from environmental effects.

The range of capacitance values derived from the conductors 12 will vary with the geometry of the conductors 12 and with the dielectric constant of the particular granular material in which it is submerged. When immersed in grains, a useable range of capacitance values has been found to be about 50 - 500 picofarads. Capacitance values in this range can be arrived at using a conductor pattern of 12-3/16" x 6"copperfoil conductor fingers 14 spaced 5/16" apart. A person skilled in this art will be able to vary the conductor pattern accordingly to arrive at a desirable capacitance range. The desired capacitance range will also vary with such factors as the type of granular material, the initial moisture content of the material, and the location of the sensing means 10 in the drying system.

The frequency range of the output of the multivibrator 13 is set by the external resistors 17, 1 7a. An optimum frequency range output is about 10 KHz 100 KHz resulting from the capacitance values usually obtained from the abovedescribed preferred circuitry. A frequency signal in this range can be reliably transmitted over a considerable distance to the control means 11. A capacitor 18 is connected as shown with the vibrator 13 to filter the supply voltage.

The frequency signal 16 is transmitted through a conductive cable 20 to the control means 11, which is usually located exterior of the grain drier 3 in proximity to the controlling device 7.

The control means 11 preferably includes frequency conditioning means 21 for receiving the frequency signal 16 and dividing it down to a frequency value which can be compared to a standard selectable frequency period value. The frequency conditioning means 21 preferably comprises a 12 bit binary counter 22, for example Motorola MC 14040B IC, which is connected so as to divide the frequency signal 16 by 64. The divided signal 23 preferably has a frequency in the range of about 150 to 1500 Hz. A capacitor22a and a resistor22b are connected as shown to form a filter to prevent spurious signals from entering the counter 22.

The control means 11 further includes comparator means 24 which receive the divided signal 23, from the frequency conditioning means 21, compares the period of the signal 23 to an adjustable standard pre-set time period, and, depending on the comparison, produces a constant high or low logic state output signal 25. The comparator means 24 includes first and second retriggerable mono-stable multivibrators 26,27 for example a Motorola MC 14538B IC.

The divided signal 23 is received by the first multivibrator 26. The time constant of the first multivibrator 26 is set by a capacitor 26a, and resistors 26b, 26c and 26d, connected as shown in Figure 4. As long as the time constant of the multivibrator 26 is of a longer time than the period of the frequency signal 23, the output 29 of the multivibrator 26 will stay in a constant high logic state. If the input frequency signal 23 decreases in frequency to a value such that its period becomes less than the time constant, the output 29 of the multivibrator 26 will drop to low for a brief time for each cycle and the input frequency signal 23. Thus either a constant or an alternating logic signal 29 will result. The alternating logic signal is indicative of an increase in the moisture content sensed.

The resistors 26c and 26d are adjustable. Therefore the comparative time constant may be varied to set a desirable frequency range for the frequency signal 23.

The output 29 of the multivibrator 26 is received by the second multivibrator 27. The second muitivibrator has a time constant, typically about one second, set by a capacitor 27a and a resistor 27b connected as shown in Figure 4. If the logic signal 29 is a constant high, the output signal 25 of the multivibrator 27 will be a logic low. Thus a logic low output signal 25 corresponds to a frequency signal 23 having a period less than the time constant set for the multivibrator 26. If the logic signal 29 is alternating high and low, the output signal 25 will be constant logic high. Thus a constant logic high output signal 25 corresponds to a frequency signal 23 having a period greater than the time constant of the multivibrator 26.

The variable control resistor 26d is preferably mounted on a control panel (not shown) and therefore made available to an operator. Thus the logic output 25 will vary according to the setting of the resistor 26d and the frequency of the signal 16 resulting from the moisture sensing means 10. The resistor 26c is preferably an internal pre-set adjustment in the device to vary the range of time constant for the multivibrator 26.

In addition to the logic output signal 25 from the comparator means 24, a second logic output signal 34 is derived from the second multivibrator 26. The logic state of signal 34 is always opposite to the logic state of signal 25. The second logic signal 34 is used with the signal 25 to switch the power control means 30 as hereinafter discussed.

The comparator means 24 can alternatively comprise a frequency to voltage converter. In that case, the output signal from the comparator means would arise from a comparison of this voltage signal to a pre-set voltage signal. The comparator means 24 can alternatively comprise a frequency measurement and comparison using a microcomputer circuit programmed to measure detector frequency and gener ate an output control signal. The output control signal may be a digital on/off signal or an analog voltage corresponding to the magnitude of error detected by the comparator means.

The control means 11 further includes power control means 30, which receives the comparator means output signals 25 and 34 and provides a variable voltage output 31 on the controlling device 7 in response thereto. From the above description of the preferred circuitry of the comparator means 24 it will be understood that an increased moisture content in the grain drier will result in a high logic state output signal 25 and a low logic state output signal 34 from the comparator means 24. Accordingly the power control means 30, in response to this high logic state signal 25 will decrease the power delivered to the controlling device 7, causing it to operate in a low speed mode. The rate of outflow of grain from the drier 3 is thereby reduced.Conversely, the power control means 30, in response to a low logic state signal 25 and a high logic state signal 34 from the comparator means 24, will increase the power delivered to the controlling device 7, causing it to operate in a high speed mode.

The power control means 30 receives the logic output signals 25 and 34 at points 25a and 34a respectively.

The power control means 30, as shown in Figure 4, includes an adjustable voltage regulator IC 40 for example a Motorola LM 317 and a booster pass transistor 42, for example a MJ 4502 from Motorola, connected as shown in Figure 4. A resistor 42a is connected as shown to provide biassing for the transistor 42. The power control means 30 further includes a stabilizing capacitor 44 and protecting diodes 46 and 48. The output 31 a from the IC 40 passes through a diode 50 to reduce the output voltage 31 by about 0.7 V. The minimum output 31a of the IC 40 is about 1.2 V. A diode 52 provides reverse voltage protection when the output 31 is disconnected from an inductive load such as the d.c.

motor 7. A capacitor 54 stabilizes the output 31.

Transistors 56, 58 and 60 are connected to the IC 40 as shown to switch in the various resistances to the voltage regulator 40 and thereby vary the voltage of the output signal 31. Transistor 60 is connected to resistors 62 and 63 and variable resistor 64 as shown. These resistors 62 - 64 set the output voltage 31 and thus the corresponding output rate of the grain drier as controlled by the controlling device 7.

Diodes 65 and 65a are connected as steering diodes.

The transistor 60 is switched on by a logic high signal 25. This logic high, as indicated above, results when the sensing means 10 senses grain wetter than the corresponding frequency period setting in the comparator means 24.

The variable resistor 64 is preferably provided on the control panel (not shown) to enable the operator to adjust the voltage of the output signal 31.

The transistor 58 is connected as shown with resistors 66 and 67 and variable resistor 68. The transistor 58 is switched on when the logic output signals 25 and 34 from the comparator means 24 are low and high respectively. These logic state signals, as explained above, result when the moisture sensing means 10 sense that the grain is dry and thus emits a frequency signal having a period less than the time constant set by the comparator means 24.

Under these circumstances the controlling device will operate at high speed mode. The resistor 68 sets the voltage level of the output signal 31.The resistor 68 is preferably mounted on the control panel (not shown) to enable the operator to vary the actual speed of the high speed mode.

The transistor 56, and the base drive biasing resistor 70 connected therewith, function to short out the voltage regulator IC 40 to produce a new 0 V output signal 31 when so signalled by a mode selection means 80 to be discussed hereinafter.

The power control means 30 may alternatively comprise an on/off control or be modified to vary the power delivered to the controlling device 7 between upper and lower limits in response to the control signal 25 produced by the comparator means 24.

The power control means 30, in an alternate embodiment, could include a conventional UJT phase control circuitry (not shown) connected to an SCR (not shown). The UJT circuitry would provide synchronized pulses to the SCR, which in turn would provide a phase controlled output signal 31 to the controlling device 7.

As indicated above, the control means 11 further preferably includes a mode selection means 80 connected to the power control means 30. The mode selection means 80 includes a three position offauto-manual switch 82 connected as shown. In the off mode of the switch 82 the transistor 56 is turned on by a 12 V signal applied at point 84. The transistor 56 will short out the voltage regulator IC 40 and thus reduce the output signal 31 to near 0 V. This off position therefore stops the controlling device 7.

In the manual mode of the switch 82, a 12 V signal is applied to point 86 to turn on a transistor 88 through a base drive biasing resistor 89. The transistor 88 is connected as shown with the transistor 60.

This transistor 88 when turned on will turn off transistor 60. This manual position of the switch 82 will also provide a 12 V signal at point 90 to turn on the transistor 58 through the steering diode 65a.

Thus in the manual mode, the voltage output signal 31 from the power control means 31 is set by the variable resistor 68, regardless of the logic output signals 24,34 from the comparator means 24. The operator therefore can override the power control means 30, and directly vary the speed of the controlling device 7.

In the auto position of the switch 82 the voltage output signal 31 from the power control means 30 will depend directly on the signals 25 and 34 of the comparator means 24 and the variable settings of the variable resistors 64 and 68 in the power control means 30 and 1 the variable resistors 26c and 26d in the compartor means 24, as previously described.

The controlling device 7 is typically a d.c. motor mechanically connected to the metering bars 6 of the grain drier 3. From the above description it will be understood that the instant apparatus functions to cause grain with a high initial moisture content to be present in the drier 3 for a longer period of time than grain with a low initial moisture content. The power delivered to the controlling device 7 alternates between a low speed and high speed setting as pre-set by the variable resistors 64 and 68. As grain with a higher moisture content is encountered the controlling device 7 will remain the low speed mode for a longer period of time. Conversely, when grain with a lower moisture content is encountered the controlling device 7 will remain in the high speed mode for a longer period of time.

The control means 11 preferably further includes an indicator means 94 to indicate the type of output signal 25,34 emitted from the comparator means 24.

The indicator means 94 includes a 555 timer IC connected as an astable multivibrator 96. The multivibrator 96 receives the logic signal 34 through resistors 97a and 97b and a transistor 97c. These components 97a - c provide the necessary logic signal inversion so as to cause the multivibratorto oscillate when the logic signal 34 is low. Resistors 98 and 100 and capacitor 102 are preferably selected and connected to provide a frequency output signal 104 from the multivibrator 96 of about 2 Hz. The output signal 104 is used to drive a light emitting diode 106, through a resistor 108. The LED 106 is preferably mounted on the control panel (not shown). It will be understood that the LED 106 will be on steady if the logic signal 34 is in a high logic state.

If the logic signal 34 is in a low logic state, the multivibrator 96 will oscillate, causing the LED 106 to flash. Thus a flashing LED indicates a higher moisture content grain is being sensed in the drier 3 and thus the controlling device 7 is in the siow speed, whereas a steady LED indicates a lower moisture content is being sensed and the controlling device 7 is in the high speed.

It should be understood that the components of the sensing means 10 and the control means 11 are connected to d.c. power supply means 15. To that end, the control means 11 further includes a step down transformer 110, a bridge rectifier 112 and a filter capacitor 114. A.C. line voltage is thus stepped down, preferably to about 24 V a.c. (RMS), full wave rectified through the bridge 112 and filtered by the capacitor 114, preferably to about 35 V d.c.. This 35 V d.c. power signal 116 is received by a 12 V d.c. power supply circuit 118 to provide operating power to the moisture sensing means 10 and all logic circuitry in the control means 11. The circuit 118 includes voltage regulator IC 120, for example a Motorola LM 317, which receives the 35 V d.c. signal 116. Resistors 122 and 124 are selected and connected to the IC 120 to provide the desired 12 V output. Diodes 126, 128 are connected to the IC 120 to protect the IC 120.

A capacitor 130 stabilizes the 12 V output voltage 132.

While the present invention has been disclosed in connection with the preferred embodiment thereof, it should be understood that there may be other embodiments which fall within the scope of the invention as defined by the following claims.

Claims (15)

1. Apparatus for controlling the moisture content of granular material exiting a drying apparatus by sensing the moisture content of the granular material and varying the flowrate of the granular material in response to the moisture sensed, said apparatus comprising sensing and signal generating means for sensing the moisture content of the granular material as a capacitance value and producing a signal indicative of the capacitance value sensed, said signal having a frequency which is a function of the capacitance sensed, and control means for receiving the frequency signal and varying the flowrate of the granular material through the drying apparatus in response to the frequency signal.

2. An apparatus according to claim 1 wherein the sensing means includes a pair of conductors, which conductors, when located in proximity to the granu lar material, produce a capacitance, the value of which is a function of the moisture content of the granular material, and wherein the signal generating means is an oscillator which receives the capacitance value and generates a signal having a frequency which is a function of the capacitance sensed.

3. An apparatus according to claim 2, wherein the control means includes a comparator means for comparing the frequency signal with a reference and, in response to this comparison varying the flowrate of the granular material.

4. An apparatus according to claim 3, wherein the control means further comprises power control means which, in response to the comparison, varies the power delivered to a controlling device which controls the flowrate of the granular material.

5. An apparatus according to claim 4, wherein the reference in the comparator means is adjustable.

6. An apparatus according to any one of claims 1, 2 or 4 wherein the control means is remote from the sensing and signal generating means, and the apparatus further includes means for transmitting the frequency signal from the sensing and signal generating means to the control means.

7. A drying apparatus for drying granular material and for controlling the moisture content of the granular material exiting therefrom, comprising drying means for exposing the granular material to a drying medium, means for varying the length of time for which the granular material is exposed to the drying medium, sensing and signal generating means for sensing the moisture content of the granular material as a capacitance value and producing a signal indicative of the capacitance value sensed, said signal having a frequency which is a function of the capacitance sensed, and control means for receiving the frequency signal and, in response to the frequency signal, controlling the means for varying the length of time for which the granular material is exposed to the drying medium, thereby controlling the moisture content of the granular material.

8. A drying apparatus according to claim 7, wherein the drying means for exposing the granular material to the drying medium includes means for continuously moving the material through the drying medium, the means for varying the time for exposing the material includes a controlling device for varying the flowrate of the material through the drying medium, and the control means includes power control means for varying the power delivered to the controlling device in response to the frequency signal to thereby vary the flowrate and moisture content of the granular material.

9. A drying apparatus according to claim 8, wherein the drying medium is air, and the granular material is grain.

10. A drying apparatus according to any one of claims 7 to 9 wherein the control means is remote from the sensing and signal generating means, and the apparatus further includes means for transmitting the frequency signal from the sensing and signal generating means to the control means.

11. A drying apparatus according to any one of claims 7 to 10 wherein the sensing means includes a pair of conductors, which conductors, when located in proximity to the granular material, produce a capacitance, the value of which is a function of the moisture content of the granular material, and wherein the signal generating means is an oscillator which receives the capacitance value and generates a signal having a frequency which is a function of the capacitance sensed.

12. A drying apparatus according to any one of claims 7 to 11 wherein the control means includes a comparator means for comparing the frequency signal with a reference and, in response to this comparison varying the flowrate of the granular material.

13. A drying apparatus according to claim 12 when dependent on claim 8, wherein the power control means receives the signal from the comparator means and varies the power delivered to the controlling device in response to the comparison signal to thereby vary the flowrate and moisture content of the granular material.

14. A drying apparatus according to claim 12 or 13 wherein the reference in the comparator means is adjustable.

15. An apparatus for controlling moistu re con- tent substantially as hereinbefore described with reference to the accompanying drawings.