The invention relates to a method and apparatus for simulating the
presence or passage of grain in or through a monitor of the type used to measure
and the moisture content of grain and determine crop yield. The invention permits
all elements of the monitor to be exercised for demonstration, test/maintenance
or calibration purposes without requiring the presence of grain in the sensor
region of the monitor.
It is well known to provide grain harvesters with monitors for measuring
the internal moisture content and bulk density of grain as it is harvested. In
one type of monitor, transmit and receive antennas are disposed on opposite sides
of a chute or tube through which the harvested grain flows. An RF microwave measurement
signal is applied to the transmit antenna and the resulting signal at the receive
antenna is analyzed to determine the attenuation and phase shift of the measurement
signal. Knowing the phase shift and attenuation, the moisture content and bulk
density of the grain may be calculated. Monitors of this type are described in
IEEE Transactions On Instrumentation and Measurement, Vol. 41, No.1, pages 111-115,
February 1992; and in Sensors, September 1992, pages 68-74.
A second type of monitor, generally not as accurate as the microwave
monitor, employs the variation in capacitance between two plates to determine the
moisture content of grain between the plates. A system of this type is disclosed
Both types of monitors described above require the presence or passage
of grain between the sensor electrodes (antennas or capacitor plates) in order
to verify the performance of the monitors. This requirement makes it inconvenient
to operate the monitors for test/maintenance, calibration and/or demonstration
purposes, particularly when they are installed on harvesters. Even when used in
a laboratory for experimental purposes as described in the above-cited IEEE publication
it would be more convenient to be able to carry out experiments with the monitors
without having to use grain in them.
An object of the invention is to provide a method and apparatus for
simulating the presence of grain in the sensor region of a grain moisture monitor
thereby permitting exercise of the monitor for maintenance/test, calibration or
demonstration purposes without the presence of grain.
According to one aspect of the present invention there is provided
a monitor for measuring the moisture content of grain, the monitor having a measurement
signal path including a sensor region in which grain may selectively be present
or not present, means for transmitting a measurement signal along said path through
said sensor region, and analyzer means being connected to said path and including
means for determining the moisture content of crop material present in said sensor
region from the attenuation of said measurement signal in said path.
Said monitor is characterized in that it further includes simulator
means for simulating the presence in said sensor region of moist crop material,
whereby said transmitting means and said analyzer means may be exercised to produce
an indication of crop moisture content even though no crop material is present
in said sensor region.
According to another aspect of the present invention there is provided
a method of simulating the presence of crop material in a sensor region of a crop
moisture monitor having a signal source for transmitting a measurement signal
along a measurement signal path through said sensor region and analyzer means for
producing an indication of the moisture content of crop material from attenuation
of the measurement signal as it is transmitted along said path,
said method being characterized in that it comprises:
- electrically inserting an attenuator into the measurement signal path; and
- controlling the attenuator so as to attenuate the measurement signal to the
same degree crop material would attenuate the measurement signal if crop material
were present in the sensor region.
Advantageously, the grain simulator is obtained by inserting a variable
attenuator into the measurement signal path and using means for controlling said
attenuator in order to attenuate the measurement signal to the same degree that
the moisture content of grain would attenuate the signal.
The attenuator may be a PIN diode, preferably precisely calibrated
to a known standard such as the National Institute of Standards and Technology,
or it may be a calibrated programmable capacitor. The diode may be used in circuitry
involving transmitting and receiving RF waves by means of antennas. The capacitor
may be used in a monitor measuring variations in capacitance between two capacitor
plates installed on opposite sides of the sensor region.
An embodiment of the present invention will now be described in further
detail, with reference to the accompanying drawings, in which:
- Figure 1 schematically illustrates a moisture content monitor having a grain
- Figure 2 illustrates a first embodiment of the invention adapted for use in
a microwave type monitor for measuring the moisture content of grain in a harvester;
- Figure 3 illustrates a second embodiment of the invention adapted for use in
a capacitive type monitor for measuring the moisture content of grain.
Figure 1 is a schematic diagram illustrating a grain simulator according
to the invention in combination with a conventional monitor for measuring grain
moisture content, the monitor being shown in generic form. The monitor includes
an AC signal source 10 for transmitting a measurement signal along a measurement
signal path 12 which includes a first sensor electrode 14, a sensor region 16
and a second sensor electrode 18.
The monitor may be a laboratory instrument for analysis of grain
or it may be mounted on a grain harvester to measure the moisture content of grain
as the grain is harvested. When used as a laboratory instrument, a container of
grain is placed in sensor region 16. When the monitor is used on a harvester, the
grain being harvested is pushed upwardly through the sensor region 16 by a broken
flyte auger, as is known in the art.
As the measurement signal produced by signal source 10 is applied
to electrode 14, it passes through the sensor region 16 and is attenuated and shifted
in phase to a degree which depends on the moisture content and bulk density of
the grain. The attenuation is almost a straight line function of the moisture content.
The attenuated and phase shifted measurement signal is sensed by the second electrode
18 and applied to an analyzer 20. Analyzer 20 analyzes the measurement signal to
determine the attenuation (and possibly the phase shift) imposed on the measurement
signal in traversing the measurement signal path, and produces a signal MC or other
indication of the moisture content of the grain.
When there is no grain present in the sensor region 16, and assuming
the monitor is properly calibrated, the indication of moisture content MC is always
zero. Therefore, the entire monitor cannot be exercised for maintenance/test,
calibration or demonstration purposes unless grain is available to fill the sensor
region. This is inconvenient and wastes grain, particularly when the monitor is
mounted in a harvester and the sensor region 16 comprises a grain feed tube within
In accordance with the present invention, the monitor is provided
with a simulator means for simulating the presence of grain in the sensor region
16 or the passage of grain through the sensor region. The simulator means comprises
a programmable attenuator 22 for electrically inserting a known impedance into
the measurement signal path 12 and an attenuation control means 24 for applying
a control voltage to the attenuator to control the impedance. The attenuator is
precisely calibrated to a standard hence it is possible to predict what the magnitude
of the signal MC should be when a control voltage of known magnitude is applied
to the attenuator.
The control means 24 includes circuit means for producing control
signals of known magnitude for controlling the attenuator to thereby control attenuation
of the measurement signal. When the monitor is performing its normal measurement
function, the control means controls the attenuator 22 so that it does not attenuate
the measurement signal in path 12. The analyzer 20 thus produces an output signal
MC indicative of the moisture content of grain present in sensor region 16.
The attenuator 22 and attenuation control means 24 permit the monitor
to be exercised for maintenance/test, calibration or demonstration purposes even
though no grain is present in the sensor region 16. For example, a calibration
check may be made on all components of the monitor, including the signal source
10, by applying a fixed magnitude control voltage of known magnitude to the attenuator
22 to cause an expected magnitude of the output signal MC, measuring the actual
magnitude of the output signal, and determining how closely the actual magnitude
matches the expected magnitude.
The monitor may be exercised for demonstration purposes by applying
to attenuator 22 a control voltage of slowly varying magnitude while displaying
the output signal MC. The output signal MC will slowly vary in magnitude thus
simulating the changes in grain moisture content encountered when actually harvesting
The programmable attenuator 22, attenuation control circuit 24 an
analyzer 20 may take any one of several forms as subsequently described. The programmable
attenuator 22 may be located in the measurement signal path 12 downstream of the
electrode 18 rather than upstream of the electrode 14.
Figure 2 illustrates a preferred embodiment of the grain simulator
adapted for use with a conventional microwave type monitor for monitoring the moisture
content of grain in a harvesting machine. The monitor comprises an RF source 110
for producing a measurement signal in the frequency range of about 1-10 GHz. The
measurement signal is transmitted over a measurement signal path 112 that includes
a programmable attenuator 122, a transmit antenna 114, a receive antenna 118 and
a controllable gain amplifier or gain control means 126. The output of amplifier
126 is connected to one input of a quadrature demodulator 130 and the outputs of
the demodulator are connected to a programmable computer or processor 132 through
an analog to digital converter (ADC) 134.
Antennas 114 and 118 are sensor electrodes disposed on opposite sides
of a sensor region 116. A grain feed tube 128 of the harvester extends through
the sensor region and a broken flyte auger (not shown) pushes grain upwardly through
tube 128 during crop harvesting. Tube 128 is made of a plastic or other material
that is transparent to microwaves in the 1-10 GHz range, or is provided with windows
of such a material, so that the measurement signal traversing path 112 is not affected
by the tube.
The demodulator 130, processor 132 and ADC 134 comprise an analyzer
for analyzing the measurement signal output from amplifier 126 and producing an
output signal indicating the moisture content of grain flowing through tube 128.
The measurement signal output from RF source 110 is applied as a reference signal
to one input of demodulator 130 which compares the reference signal with the measurement
signal output from amplifier 126 and produces two time varying output voltages
I and Q, where I and Q represent the magnitude of the output signal from amplifier
126 measured at 0° and 90° of the reference signal. The voltages I and Q are converted
to digital values by ADC 134 and processor 132 executes a stored program to first
compute the attenuation A and phase shift &phis; imparted to the measurement signal
as it traverses the measurement signal path 112. The attenuation
and the phase shift
&phis; = tan-1(Q/I)
degrees. The processor 132 then calculates the percent grain moisture content
(wet basis) MC from the values A, &phis; and stored constant values, known in the
prior art, which are specific for each type of grain (e.g. wheat, barley) and each
variety of grain of a given type (e.g. red winter wheat). A display 140 may be
provided on an operator's console for displaying the moisture content of grain
passing through tube 128.
For the purpose of simulating grain flow through the monitor, an
attenuator 122 in the form of a programmable PIN diode is provided in the measurement
signal path 112 at any point upstream of amplifier 126. The diode is calibrated
to an accuracy traceable to the National Institute of Standards and Technology.
Such diodes are commercially available from Hewlett-Packard Company. At RF frequencies,
and with a forward bias voltage being applied, the diode acts as a pure resistance,
the magnitude of the resistance being inversely proportional to the forward bias
current. With reverse or zero bias the diode exhibits a very high resistance.
The processor 132 and a digital-to-analog converter (DAC) 136 comprise an attenuator
control means for controlling the attenuator 122. The processor applies digital
values one at a time to DAC 136 which converts each value to an analog bias voltage.
The bias voltage is applied to the PIN diode attenuator 122 via leads 142 and
an RF isolation circuit 144. The purpose of isolation circuit 144 is to isolate
DAC 136 from the RF signal present on the measurement signal path 112.
The processor 132 is programmed to apply a high forward bias to the
PIN diode attenuator 122 during normal harvesting operation. This causes the diode
resistance to drop to practically zero, in effect electrically removing the diode
from the measurement signal path so that the only attenuation of the measurement
signal is that resulting from its passage through the grain.
To simulate the flow of grain through tube 128, say for test/maintenance
or demonstration purposes, processor 132 accesses a stored table of digital values.
These values may have been previously stored in a memory of the processor by operation
of keys on an operator's control panel 138, or they may be values stored in a ROM
in the processor. The digital values are chosen such that, when converted to bias
voltages by DAC 136 and applied to PIN diode attenuator 122, the processor 132
will cause display 140 to display a time varying value of moisture content, just
as it does when grain of varying moisture content is being harvested. The digital
values are applied to DAC 136 to generate a time varying bias voltage that is applied
to the PIN diode attenuator 122 to vary its resistance. As the attenuation of
the measurement signal varies, demodulator 130 detects the variations and processor
132 computes values of moisture content which are successively displayed on display
To simulate the flow of grain through the monitor for calibration
purposes, the processor 132 applies a digital value to DAC 136 to cause a bias
voltage of known value to be applied to PIN diode attenuator 122. The bias voltage
causes the resistance of the attenuator to assume a known value simulating the
attenuation imposed on the measurement signal by the moisture content of grain
if grain was passing through tube 128. Demodulator 130 detects the attenuated
measurement signal and processor 132 computes the simulated moisture content. Processor
132 then compares the computed simulated moisture content with a stored value
representing the expected moisture content. If the computed moisture content is
equal to the expected moisture content then the monitor is properly calibrated.
If the computed moisture content and the expected moisture content are not equal,
the processor generates a digital correction factor that is converted to an analog
control voltage by DAC 136 and applied via lead 146 to the control input of the
controllable gain amplifier 126.
In simulating the presence of grain, no phase shift is imposed on
the measurement signal other than the inherent phase shift resulting from passage
of the measurement signal over the path 112. This does not significantly alter
the accuracy of the calibration or the simulation because moisture content is
essentially a straight line function of attenuation. Phase shift, which is a function
of grain density, has little effect in the calculation of moisture content.
Although the present invention provides for calibration of the monitor
by simulating the presence of grain in the tube 128, the monitor shown in Figure
2 may be calibrated on-the-fly. That is, the monitor may also be calibrated while
harvesting is taking place and grain is flowing through the tube 128.
Figure 3 illustrates the invention in the environment of a capacitance
type monitor such as might be used in a laboratory, at a grain elevator, or even
by a farmer. The monitor comprises a signal source 210 for generating a measurement
signal, a bridge circuit 250, an analyzer 220 and a display 252.
Signal source 210 may be any AC source for generating a signal in
the frequency range suitable for capacitance measurements and preferably generates
a measurement signal in the low radio frequency range. One output of signal source
210 and an input node 256 of the bridge circuit are connected to ground and the
second output of the signal source is transmitted via a measurement signal path
212 to input node 258 of the bridge circuit.
The bridge circuit 250 comprises first and second inductances L and
L' and a capacitor C disposed in first, second and third legs, respectively. The
fourth leg is provided with first and second sensor electrodes or plates 214 and
218 spaced apart to define a sensor region 216 for receiving a sample receptacle
254. The receptacle is made of a non-conductive material, preferably plastic, that
is transparent to the measurement signal. The plates 214 and 218 and grain held
in the receptacle 254 act as a variable capacitor having a capacitance determined
by the particular grain in the receptacle and the grain moisture content. Capacitor
C and inductance L' are preferably adjustable to permit balancing the bridge and
remove any parasitic stray capacitance, thereby increasing the accuracy of grain
The output nodes 260 and 262 of the bridge circuit are connected
to inputs of the analyzer 220. The analyzer may be a known balance detector for
determining the difference in magnitude between voltages present at nodes 260 and
262. The analyzer may be, for example, a Hewlett-Packard HP8510 network analyzer
in which case the phase shift of the measurement signal may be determined in addition
to its attenuation.
To determine the moisture content of a sample of grain, the bridge
is first balanced with receptacle 254 empty. When properly balanced, the measurement
signal applied to nodes 256 and 258 causes equal voltages to appear at nodes 260
and 262 and the analyzer output produces a zero output. After the bridge is balanced,
the receptacle 254 is filled with the sample and positioned in the sensor region
216 between plates 214 and 218. The grain in receptacle 254 attenuates the measurement
signal between nodes 258 and 260 but the voltage at node 262 remains unchanged.
Analyzer 220 determines the difference in amplitude between the voltages at nodes
260 and 262 and applies an output signal representing this difference to display
252. The magnitude of this signal is almost a straight line function of the moisture
content of the grain sample in receptacle 254, hence the display may be calibrated
so as to indicate the percent moisture content of the grain.
The monitor is provided with a precisely calibrated programmable
attenuator in the form of a variable capacitor 222 for simulating the presence
of grain in the sensor region 216. Capacitor 222 is connected in series with a
switch 224 between the output of measurement signal source 210 and bridge node
260. This places the switch and capacitor in parallel with the sensor region 216.
Capacitor 222 may be a silicon planar variable capacitance diode
of the type sold commercially by Zetex Corp. Switch 224 may be a manually operated
switch or a switch controlled from the processor 232 in which case it may be an
electro-mechanical or electronic switch or even a PIN diode.
A control means for controlling capacitor 222 comprises a processor
232, which may be a conventional personal computer and a DAC 236 responsive to
digital values applied thereto by the computer for generating analog control signals
for controlling the capacitor. The control signals are applied to capacitor 222
through an isolation circuit 244 which isolates the DAC from the measurement signal
produced by signal source 210.
The presence of grain in the sensor region 216 may be simulated by
closing switch 224 and applying a digital signal to DAC 236 to produce a control
voltage of known magnitude for controlling capacitor 222. This electrically inserts
into measurement signal path 212 a known capacitance corresponding to the capacitance
which would be introduced into the path by grain having a given moisture content.
When the receptacle 254 is empty, or is removed from sensor region
216, the sensor region acts as an open circuit hence the magnitude of the measurement
signal at node 260 is dependent only on the attenuation introduced by capacitor
222. Analyzer 220 compares the voltages at nodes 260 and 262 and applies a signal
corresponding to the difference in amplitude between the voltages to display 252
to indicate a moisture content. This moisture content is known or predictable
from the value of the digital signal applied to DAC 236 hence calibration of the
monitor may be checked.
The flow of grain of varying moisture content through sensor region
216 may be simulated by storing a table of values in the processor memory and applying
them one at a time to DAC 236 so that capacitor 222 causes a varying attenuation
of the measurement signal.
Although preferred embodiments have been described in detail to illustrate
the principles of the invention, it will be understood that various modifications
and substitutions may be made in the described embodiments without departing from
the spirit and scope of the invention as defined by the appended claims.