U.S. patent application number 13/487304 was filed with the patent office on 2012-12-06 for systems and methods for determining stock quantities using a capacitive inventory sensor.
Invention is credited to Gordon E. Hardman, Gary L. Overhultz, John W. Pyne.
Application Number | 20120310570 13/487304 |
Document ID | / |
Family ID | 47262313 |
Filed Date | 2012-12-06 |
United States Patent
Application |
20120310570 |
Kind Code |
A1 |
Pyne; John W. ; et
al. |
December 6, 2012 |
Systems and Methods for Determining Stock Quantities Using a
Capacitive Inventory Sensor
Abstract
Systems and methods are provided for a capacitive inventory
sensor. A system includes a track configured for retaining items. A
first conducting plate is positioned along a portion of the track,
and a second conducting plate is positioned in parallel with the
first conducting plate along a portion of the track. The second
conducting plate is positioned a distance from the first conducting
plate, and the second plate is configured to have the items placed
on top of the second plate. The system further includes a
capacitance sensor configured for connection to the first and
second conducting plates, where the capacitance sensor is
configured to measure a capacitance between the first and second
conducting plates, and where the capacitance varies based on a
number of items positioned on the sensor track.
Inventors: |
Pyne; John W.; (Erie,
CO) ; Overhultz; Gary L.; (River Forest, IL) ;
Hardman; Gordon E.; (Boulder, CO) |
Family ID: |
47262313 |
Appl. No.: |
13/487304 |
Filed: |
June 4, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61493190 |
Jun 3, 2011 |
|
|
|
Current U.S.
Class: |
702/65 ;
324/679 |
Current CPC
Class: |
G06Q 10/087 20130101;
G01G 19/42 20130101; G01G 7/06 20130101; G07F 11/00 20130101 |
Class at
Publication: |
702/65 ;
324/679 |
International
Class: |
G01R 27/26 20060101
G01R027/26; G06F 19/00 20110101 G06F019/00 |
Claims
1. A capacitive inventory sensor, comprising: a track configured
for retaining items; a first conducting plate positioned along a
bottom portion of the track; a second conducting plate positioned
in parallel with the first conducting plate along the bottom
portion of the track, wherein the second conducting plate is
positioned a distance from the first conducting plate, and wherein
the second plate is configured to have the items placed on top of
the second plate; and a capacitance sensor configured for
connection to the first and second conducting plates, wherein the
capacitance sensor is configured to measure a capacitance between
the first and second conducting plates, wherein the capacitance
varies based on a number of items positioned on top of the second
plate.
2. The sensor of claim 1, further comprising a transmitter, wherein
the transmitter is configured to transmit an alert indicative of a
number of items retained by the track.
3. The sensor of claim 2, further comprising a data processor
configured to determine a number of items retained by the track
based on the measured capacitance.
4. The sensor of claim 3, wherein the processor is configured to
cause the transmitter to transmit the alert when the number of
items meets one or more threshold criteria.
5. The sensor of claim 3, wherein the processor is configured to
cause the transmitter to transmit the alert when the number of
items is equal to zero.
6. The sensor of claim 1, wherein a face of the first place is
positioned opposite a face of the second face plate.
7. The sensor of claim 1, further comprising a single serving
beverage dispensing machine.
8. The sensor of claim 1, wherein the track includes a front
portion, and wherein the track is configured to travel the items
toward the front portion of the track as items are removed from the
track.
9. A capacitive inventory sensor, comprising: a track configured
for retaining items, wherein the track includes a front portion,
and wherein the track includes a pusher that is configured to move
along the track and to push the items toward the front portion of
the track as items are removed from the track; a stationary first
conducting plate positioned along a length of the track; a
stationary second conducting plate positioned along a length of the
track; a moveable third conducting plate connected to the pusher,
wherein the a face of the third conducting plate is positioned
opposite a face of the first conducting plate and a face of the
second conducting plate such that the moveable third conducting
plate overlaps a portion of the first conducting plate and a
portion of the second conducting plate; a capacitance sensor
configured to measure a combined capacitance formed among the first
conducting plate, the second conducting plate, and the third
conducting plate, wherein the measured capacitance varies based on
a position of the pusher along the track.
10. The sensor of claim 9, further comprising a data processor
configured to determine a number of items present on the track
based on the measured capacitance.
11. The sensor of claim 10, further comprising a data processor
configured to determine the position of the pusher based on the
measured capacitance.
12. The sensor of claim 11, wherein the position of the pusher is
determined according to: x=2(C.sub.m-C.sub.1)/m, where x is the
position of the pusher, C.sub.m is the measured capacitance,
C.sub.1 is the capacitance between the first conducting plate and
the second conducting plate, and m is a constant.
13. The sensor of claim 12, wherein m=k*a/d, where k is a constant,
a is an area of a face of the first conducting plate overlapped by
the third conducting plate.
14. The sensor of claim 9, further comprising a force mechanism,
wherein the force mechanism is configured to force the pusher
toward the front portion of the track as items are removed from the
track.
15. The sensor of claim 14, wherein the force mechanism comprises a
spring.
16. The sensor of claim 9, wherein the face of the first conducting
plate and face of the second conducting plate are substantially
rectangular.
17. The sensor of claim 9, wherein the face of the first conducting
plate varies in width along the length of the track.
18. The sensor of claim 9, wherein the second conducting plate is
discontinuous along the length of the track.
19. The sensor of claim 18, further comprising: a stationary fourth
conducting plate positioned along the length of the track, wherein
the third conducting plate is discontinuous along the length of the
track, wherein the capacitance sensor further measures a second
combined capacitance formed among the first conducting plate, the
third conducting plate, and the fourth conducting plate, wherein
the position of the pusher is determined based upon the measured
capacitance and the measured second capacitance.
20. A system for identifying a presence of an item along a
transmission line, comprising: a transmission line responsive to
one or more items positioned along the transmission line; an
impulse generator configured to transmit an impulse at a known
frequency from a first end of the transmission line; a terminator
configured to receive the impulse at a second end of the
transmission line; an impulse detector positioned at the first end
of the transmission line, wherein the presence or absence of an
item along the transmission line is determined based upon a signal
detected by the impulse detector.
21. The system of claim 20, wherein the transmission line is
embedded in a shelf of a product display.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The application claims priority to U.S. Provisional Patent
Application No. 61/493,190, filed Jun. 3, 2011, and entitled
"Determining Stock Quantities Using a Capacitive Inventory Sensor,"
the entirety of which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The technology described herein relates generally to
inventory detection and more specifically to inventory detection
based on capacitance variation.
BACKGROUND
[0003] In retail environments, there are a variety of systems that
mechanically move products so that they may be more easily seen and
accessed by customers. These may be gravity fed, or have some sort
of stored energy, such as a spring, which pushes product to the
front of the display; hence these devices are frequently
generically referred to as "pushers". Though more expensive than
simply depositing product on the bare shelves and allowing
customers to move items at their discretion, pushers are rapidly
cost-justified for certain products by the resulting lift in sales
and reduced labor costs associated with restoring order to product
that has been "shopped." Retailers are rapidly adopting pusher
systems and expanding their use within stores to include more
product lines.
SUMMARY
[0004] In accordance with the teachings herein,
computer-implemented systems and methods are provided for a
capacitive inventory sensor. A system includes a track configured
for retaining items. A first conducting plate is positioned along a
bottom portion of the track, and a second conducting plate is
positioned in parallel with the first conducting plate along the
bottom portion of the track. The second conducting plate is
positioned a distance from the first conducting plate, and the
second plate is configured to have the items placed on top of the
second plate. The system further includes a capacitance sensor
configured for connection to the first and second conducting
plates, where the capacitance sensor is configured to measure a
capacitance between the first and second conducting plates, and
where the capacitance varies based on a number of items positioned
on top of the second plate.
[0005] As another example, a capacitive inventory sensor includes a
track configured for retaining items, where the track includes a
pusher that is configured to move along the track and to push the
items toward the front portion of the track as items are removed
from the track. A stationary first conducting plate is positioned
along a length of the track, and a stationary second conducting
plate is also positioned along a length of the track. A moveable
third conducting plate is connected to the pusher. The a face of
the third conducting plate is positioned opposite a face of the
first conducting plate and a face of the second conducting plate
such that the moveable third conducting plate overlaps a portion of
the first conducting plate and a portion of the second conducting
plate. A capacitance sensor is configured to measure a combined
capacitance formed among the first conducting plate, the second
conducting plate, and the third conducting plate, wherein the
measured capacitance varies based on a position of the pusher along
the track.
[0006] As a further example, a system for identifying a presence of
an item along a transmission line includes a transmission line
responsive to one or more items positioned along the transmission
line. An impulse generator is configured to transmit an impulse at
a known frequency from a first end of the transmission line. A
terminator is configured to receive the impulse at a second end of
the transmission line. An impulse detector is positioned at the
first end of the transmission line, and the presence or absence of
an item along the transmission line is determined based upon a
signal detected by the impulse detector.
BRIEF DESCRIPTION OF DRAWINGS
[0007] FIG. 1 is a block diagram depicting an environment that
includes an inventory management system and capacitive inventory
sensors.
[0008] FIG. 2 is a block diagram depicting an example environment
for tracking product inventory using capacitive inventory sensors
activated by a switch at a transmitter.
[0009] FIG. 3 depicts a side view of an example configuration of a
capacitive inventory sensor.
[0010] FIG. 4 depicts a side view of an example configuration of a
capacitive inventory sensor that does not include a pusher.
[0011] FIG. 5 depicts an example capacitive inventory sensor.
[0012] FIG. 6 depicts an example capacitive inventory sensor with
bottles loaded onto the sensor.
[0013] FIG. 7 is a circuit diagram depicting an example
configuration for a capacitive inventory sensor.
[0014] FIG. 8 is a graph depicting an example relationship between
an A/D converter range and a first capacitive inventory sensor.
[0015] FIG. 9 is a circuit diagram depicting an example
configuration for a capacitive inventory sensor that includes an
add-on capacitor.
[0016] FIG. 10 is a graph depicting an example relationship between
A/D converter values and a number of bottles of beverage present on
a capacitive inventory sensor.
[0017] FIG. 11 is a graph depicting a relationship between an A/D
converter range and capacitance values of a capacitive inventory
sensor that could be achieved using a differential sampling
technique.
[0018] FIG. 12 is a graph depicting a sample advantage that could
be realized using the differential sampling technique.
[0019] FIG. 13 depicts an alternative form for a capacitive
inventory sensor that utilizes a moveable capacitor plate.
[0020] FIG. 14 is a circuit diagram depicting an equivalent circuit
to the configuration depicted in FIG. 13.
[0021] FIG. 15 is a graph depicting an example expected and
measured variation in capacitance based on a position of the
moveable plate.
[0022] FIG. 16 depicts an example three-plate capacitive inventory
sensor.
[0023] FIG. 17 depicts a configuration where the stationary first
plate and second plate extend the length of the capacitive
inventory sensor and vary in width along that length.
[0024] FIG. 18 depicts a coded digital capacitive inventory sensor
configuration.
[0025] FIG. 19 depicts an example implementation of a coded digital
capacitive inventory sensor.
[0026] FIG. 20 is a graph depicting measured data over the first
four portions in 0.25'' increments.
[0027] FIG. 21 is a block diagram depicting a transmission line
reflection sensing counter.
DETAILED DESCRIPTION
[0028] FIG. 1 is a block diagram depicting an environment that
includes an inventory management system and capacitive inventory
sensors. Retailers know that much of the time some fraction of
pushers or other inventory holders are depleted or void of product
and that this results in a shortfall in sales compared to what
would have been possible with more ample stock. Some fraction of
the empty pushers is due to the fact that re-stocking personnel are
unaware of the fact that the pushers are empty. Often, facilities
upstream in the distribution system--and even neighboring
facilities in the same chain--have plenty of product but no
awareness that a given store is void or nearly void. It is commonly
estimated that out-of-stocks (OOS) average around 10%, and
fast-moving high-profile products suffer most. When a customer
encounters out-of-stock conditions on a key item, not only do sales
of that item suffer, many times the customer will shop elsewhere
for that item, so sales of other items the customer would have
bought in the original store are also lost. If OOS conditions occur
too frequently, store loyalty may erode.
[0029] In the absence of more timely, granular information about
low-stock status, many retailers have minimized OOS probability by
overstocking. This needlessly ties up valuable working capital,
stocking space, shelf space, and employee hours for over-inventory
management. In cases where product shelf life is short,
overstocking results in expensive returns or deep consumer
discounts to incentivize quick sale. Extra facings of a single
product come at the expense of inventory variety that could
otherwise appeal to additional customers or potentially out-sell
the marginal stock of the original brand.
[0030] Promotional or seasonal periods exacerbate the delicate
balance between out-of-stocks and over-stocks. A successful
promotion can double or triple sales within a store. Some retailers
resort to secondary locations during a peak or promoted season.
This can improve sales by allowing impulse purchases; however it
can also depress sales if retailers are unaware that the "home"
location for a product is low or empty and transfer inventory from
the secondary location to the place customers usually go to find
the product. Because promoted and seasonal products make up a
sizeable portion of retailer sales and profits, the concept of
"right-sized" inventory is elusive and transient.
[0031] Lost sales due to out-of-stocks are estimated in billions of
dollars annually. In fast-moving categories where consumers
typically purchase multiples of a given item, having only one item
on the shelf is almost as consequential. When low-stock and
over-stock situations are factored in, the economic motivation to
address replenishment deficiencies more efficiently and immediately
soars. A means to automatically alert responsive personnel within
and beyond the store has been unavailable due to expense and lack
of reliability. Many methods that have been attempted also
negatively impact the shopping experience or the ease of
restocking. To be effective, stock-monitoring mechanisms can should
be inexpensive, rugged, easy to retrofit to existing installations,
unobtrusive, extensible to a variety of stock-monitoring
conditions, reliable/accurate, and easy to integrate into the end
users existing IT infrastructure.
[0032] With reference to FIG. 1, an inventory management system 102
is in communication with a receiver 104 and a data store 106 for
managing and receiving data about inventory distributed throughout
an environment. For example, the inventory may be distributed
throughout a factory, a distribution center, a store, or a single
serving distribution machine. Based on the data received by the
inventory management system 102 from the receiver 104 and the data
store 106, the inventory management system 102 may perform certain
actions to maintain sufficient levels of inventory throughout the
environment. For example, when the inventory management system 102
receives data that indicates that an inventory level at a
particular location in the environment has run out or is running
low on product, an alert can be issued that instructs and employee
to restock the particular location. The inventory management system
102 may also receive data that indicates that the environment is
running low on product and that more should be ordered. Such a
decision to order more product may be based on an amount of product
detected, an ordering threshold, an expected rate of product
transfer/sales, a detected rate of product transfer/sales, etc.
[0033] The data store 106 for storing amounts of product detected
and the inventory management system 102 for making inventory
management decisions based on amounts of product detected are
responsive to the receiver 104, which is configured to wirelessly
communicate with a transmitter 108. The transmitter is responsive
to a plurality of capacitive inventory sensors (CIS) 110 that are
configured to detect an amount of product present in an area based
on a capacitance measurement. For example, a capacitor may be
implemented such that the capacitance between its constituent
plates varies based on an amount of weight placed on top of the
capacitor. In another configuration, one plate of a capacitor may
be placed on a pusher with another one or more stationary plates
being positioned along the length of the track, where the
capacitance among the plates varies based on an overlap between the
stationary plates and the pusher plate.
[0034] Each of the capacitive inventory sensors 110 includes a
capacitance measurement device and a capacitor in some form. The
capacitor has a base capacitance when an associated shelf or
storage area is empty. That capacitance changes when items are
added or removed. In one example, the capacitance changes in a
linear fashion as items are added and removed. The capacitance
measurement device may take the form of a capacitive sensor module
(CSM), such as ones commonly utilized in MP3 players, smart phones,
computer pads, computer displays, and lab test equipment. A
capacitive sensor module detects when a finger, tap pen, or other
items are touched to a section of a screen. A capacitive sensor
module can be programmed to detect the change in capacitance above
a threshold level. When a threshold is exceeded, the module
activates, sending a signal (e.g., a signal indicating the presence
of absence of items). In another implementation, a micro-controller
with an analog to digital converter (e.g., a 10-bit A/D converter),
can be used to detect changes in capacitance with more granularity
than the on-off detection provided by the capacitive sensor
module.
[0035] The selection of the capacitor configuration to use for a
particular product or location may be based on a number of factors.
While pushers work well with many products, there are also several
instances where it is desirable to monitor the presence or absence
of shelf stock when there are no pusher paddles present. It is
common, for example, to find single-serve beverages or other items
in a retail cooler that rely on gravity to pull them along a track
or to the front of a shelf for easy consumer access. In such cases,
adding a pusher paddle may be prohibitively expensive or
unreliable, as tracks often become sticky from product spills. Ease
of restocking has also been found to be impeded, in some cases, by
the presence of pusher paddles. Lastly, there are many areas of a
retail environment that do not utilize pusher paddles due to
awkward product size or unmet sales volume criteria that would
justify their use. It is also common for a given consumer product
to be located in several places throughout a store. While one or
more of those locations may have pusher paddles associated with
them, it is often the case that not all locations of a given
product have pusher paddles. Not only may it be important to note
the stocking level of each location, it may often be desirable to
know the particular location that is selling the most or least of a
given item so allocated retail space can be adjusted to derive more
sales within a given area or department.
[0036] FIG. 2 is a block diagram depicting an example environment
for tracking product inventory using capacitive inventory sensors
activated by a switch at a transmitter. A plurality of capacitive
inventory sensors 202 are responsive to a transmitter 204 that
selectively provides power to the capacitive inventory sensors 202
from a battery 206 via a switch 208. In response to a command
(e.g., from the inventory management system 210, in response to a
processor at the transmitter 204, from a clock signal), a voltage
measurement is taken to determine a capacitance of one of the
capacitive inventory sensors 202. The measured voltage, indicative
of a capacitance at one of the capacitive inventory sensors 202 and
thus indicative of the amount of product present at the capacitive
inventory sensor 202 of interest, is transmitted via the antenna
214 of the transmitter 204 to the receiver 216, where such data is
forwarded to the inventory management system 210 and the data store
218. The capacitive inventory sensors 202 may be individually
connected to the antenna, or the sensors 202 may be connected to
the antenna via a serial communication line, where sensors 202 that
are currently not powered via the switch 208 are in a high
impedance state and will not interfere with the currently powered
sensor 202.
[0037] FIG. 3 depicts a side view of an example configuration of a
capacitive inventory sensor that includes a pusher. The capacitive
inventory sensor includes a capacitor that is formed from a top
plate 302, a bottom plate 304, and a dielectric layer 306, which
could be formed from air or another dielectric material. Units of
product 308 are positioned on top of the top plate 302 and are
forced toward a front portion of the capacitive inventory sensor on
a shelf 310 by a pusher 312. A capacitance measuring device 314 is
connected to the top plate 302 and the bottom plate 304 to measure
the capacitance formed by the top plate 302, bottom plate 304, and
dielectric 306. The capacitance varies based on an amount of
product 308 that is placed upon the top plate 302, as certain
characteristics of the capacitor 302, 304, 306 vary based upon
weight being positioned on top of the top plate 302 (e.g., the
distance between the top plate 302 and bottom plate 304 may
decrease as more weight is added). The capacitance varies in a
predictable way, such that an amount of product 308 positioned on
top of the top plate 302 can be determined based on the magnitude
of the capacitance change. Such data is forwarded by the
capacitance measuring device 314 to the inventory management system
and data store for consideration and appropriate action.
[0038] FIG. 4 depicts a side view of an example configuration of a
capacitive inventory sensor that does not include a pusher. The
capacitive inventory sensor includes a capacitor that is formed
from a top plate 402, a bottom plate 404, and a dielectric layer
406. Units of product 408 are positioned on top of the top plate
402 and are forced toward a front portion of the capacitive
inventory sensor by gravity based on the forward tilt of the top
plate 402. A capacitance measuring device 414 is connected to the
top plate 402 and the bottom plate 404 to measure the capacitance
formed by the top plate 402, bottom plate 404, and dielectric 406.
The capacitance varies based on an amount of product 408 that is
placed upon the top plate 402, as certain characteristics of the
capacitor 402, 404, 406 vary based upon weight being positioned on
top of the top plate 402 (e.g., the distance between the top plate
402 and bottom plate 404 may decrease as more weight is added).
Data is forwarded by the capacitance measuring device 410 to the
inventory management system and data store for consideration and
appropriate action.
[0039] FIG. 5 depicts an example capacitive inventory sensor. The
example capacitive inventory sensor is configured for holding
plastic or glass bottles. The capacitive inventory sensor includes
a top plate 502 on which the bottles are placed. As bottles are
placed on the top plate 502, the capacitance formed by the top
plate 502 and a bottom plate positioned under the top plate 502
changes. A capacitance measuring device may be connected at a back
portion 504 of the capacitive inventory sensor. The capacitive
inventory sensor may be place on an incline, such as on a shelf in
a convenience or grocery store cooler or within a single serving
food/beverage machine (e.g., a pop machine) so that bottles placed
on the capacitive inventory sensor will tend to slide toward a
front portion 506 of the capacitive inventory sensor. Molding 508
(e.g., plastic, metal, composite) may be incorporated with the
capacitive inventory sensor to hold the bottles within the
capacitive inventory sensor on the top plate 502.
[0040] FIG. 6 depicts an example capacitive inventory sensor with
bottles loaded onto the sensor. In the example of FIG. 6, a number
of bottles 602 are loaded into the capacitive inventory sensor on
top of a top plate 604 that forms a capacitor with a bottom plate.
A capacitance measuring device is connected at a back portion 606
of the capacitive inventory sensor. Based on the capacitance
measured, an inventory management system may be able to sense
whether any product is present on the displayed capacitive
inventory sensor and may further be able to tell a number of units
of product that are on the displayed capacitive inventory sensor.
Stocking and reordering operations may then be commanded
accordingly.
[0041] FIG. 7 is a circuit diagram depicting an example
configuration for a capacitive inventory sensor. The capacitive
inventory sensor 702 includes a capacitor 702 whose capacitance
varies based on a condition. For example, the capacitance may vary
as weight is added on top of one of the plates of the capacitor 702
as described above. As a further example, the capacitance may vary
based on a position of one of the plates of the capacitor 702, as
will be described in further detail herein below. The capacitance
of the capacitor 702 is measured via a microcontroller with a 10
bit A/D converter 704. To perform such a measurement, the
microcontroller 704 uses a switch 706 to charge a sample and hold
capacitor 708 to a voltage, V.sub.DD. The switch then transitions
to a second position, connecting the capacitive inventory sensor
702 in parallel with the sample and hold capacitor 708,
distributing the charge between the sample and hold capacitor 708
and the capacitive inventory sensor 702. The voltage then present
at the A/D converter 710 is representative of the capacitance of
the capacitive inventory sensor 702, where a larger value of
capacitance of the capacitive inventory sensor 702 will result in a
lower voltage sensed at the A/D converter 710.
[0042] The voltage measured by the A/D converter 710 is related to
the capacitance of the capacitive inventory sensor 702 according to
the following formula:
V.sub.CIS=(C.sub.r*V.sub.DD)/(C.sub.CIS+C.sub.r),
where V.sub.CIS is the voltage of the capacitive inventory sensor
702 measured by the A/D converter 710, C.sub.r is the capacitance
of the internal sample and hold capacitor and any add-on capacitors
(none included in FIG. 7), V.sub.DD is the voltage used to charge
the sample and hold capacitor 708, and C.sub.CIS is the capacitance
of the capacitive inventory sensor 702.
[0043] FIG. 8 is a graph depicting an example relationship between
an A/D converter's range and a first capacitive inventory sensor
constructed similarly to the example of FIG. 7. In the present
example, the effective range of the capacitive inventory sensor is
between 65 pF when empty up to 145 pF when full. This range of
capacitances is low on the A/D converter scale, which could result
in difficulty in discerning the number of items present because the
range of operation of the capacitive inventory sensor uses such a
small portion of the A/D converter range. It may be desirable shift
the response curve to the right, such that a more dynamic range of
A/D converter values is present in the 65 pF-145 pF range.
[0044] Such shifting can be accomplished in a variety of ways, such
as by incorporating an add-on capacitor in parallel with the sample
and hold capacitor. FIG. 9 is a circuit diagram depicting an
example configuration for a capacitive inventory sensor that
includes an add-on capacitor. The capacitive inventory sensor 902
includes a capacitor 902 whose capacitance varies based on a
condition. The capacitance of the capacitor 902 is measured via a
microcontroller with a 10 bit A/D converter 904. To perform such a
measurement, the microcontroller 904 uses a switch 906 to charge a
sample and hold capacitor 908 and an add-on capacitor 910 to a
voltage, V.sub.DD. The switch then transitions to a second
position, connecting the capacitive inventory sensor 902 in
parallel with the sample and hold capacitor 908, distributing the
charge between the sample and hold capacitor 908 and the capacitive
inventory sensor 902. The voltage then present at the A/D converter
912 is representative of the capacitance of the capacitive
inventory sensor 902, where a larger value of capacitance of the
capacitive inventory sensor 902 will result in a lower voltage
sensed at the A/D converter 912. The presence of the add-on
capacitor 910 shifts the relationship shown in FIG. 8 to the right
according to the relationship of the formula described above with
respect to FIG. 7.
[0045] FIG. 10 is a graph depicting an example relationship between
A/D converter values and a number of bottles of beverage present on
a capacitive inventory sensor. The A/D converter 912 measures a
voltage that varies according to the capacitance of the capacitive
inventory sensor 902. That capacitance varies in a predictable
manner based on a condition, such as the number of bottles of
beverage of a known weight present on one plate of the capacitive
inventory sensor 902. That relationship can be used to analytically
or experimentally develop the graph of FIG. 10, which maps voltages
measured by the A/D converter 912 to an amount of product present
at the capacitive inventory sensor 902. Amounts of product or
changes in the amounts of product can be transmitted to an
inventory management system, which can analyze the values to
provide data displays or make appropriate stocking/reordering
decisions accordingly.
[0046] Other variations may also be made in a capacitive inventory
sensor circuit. In one example, the capacitive inventory sensor may
be designed so achieve a linear response to a change in capacitance
versus the change in the number of items counted by the capacitive
inventory sensor. The capacitive inventory sensor may be buffered
from neighboring sensors so that a minimal reaction to items
present at neighboring sensors is detected by the capacitive
inventory sensor. It may further be desirable to shield the
capacitive inventory sensor from an expected environment such as to
compensate for metal shelving, radio frequency interference,
etc.
[0047] FIG. 11 is a graph depicting a relationship between an A/D
converter range and capacitance values of a capacitive inventory
sensor that could be achieved using a differential sampling
technique. This approach repeats the steps described above, where a
filtered value is acquired and stored for final calculations. This
voltage sample is defined as V1. The process is then reversed,
where the capacitive inventory capacitor is fully charged and the
sample and hold or reference capacitor is fully discharged. The
reference capacitor is enabled and connected to the fully charged
capacitive inventory sensor, resulting in another voltage to be
sampled by the A/D converter. This voltage is defined as V2. The
final value is determined by computing the delta of the two samples
(V2-V1). A graph of this technique is plotted in FIG. 11. FIG. 12
is a graph depicting a sample advantage that could be realized
using the differential sampling technique. As shown in FIG. 12, the
differential sampling technique enables use of a broader portion of
the A/D converter range through the effective range of the
capacitive inventory sensor capacitance value, as indicated by the
steeper slope of the differential sampling technique plot. This
steeper slope translates to an increase in resolution and accuracy
of the capacitive inventory sensor.
[0048] In one example, the capacitive inventory sensor used for
this data has a sensor section comprising two plates of a capacitor
running down a middle portion of the sensor along the length of the
sensor. The middle portion width dimension is 1.5''. Ground plates
are positioned on each side of the sensor separated by 1/8'' gap. A
low cost micro-controller is positioned on the back section of the
capacitive inventory sensor. Other components may include the
external add-on capacitor and an interface connector. Several of
these capacitive inventory sensors can be connected to a central
collector which will control when each sensor is powered up. Data
can be collected and transmitted wirelessly (or in a wired fashion)
to a common node. The node will periodically transmit the data to
the inventory management system data servers for analysis and
report generation. A capacitive inventory sensor can be
manufactured using a variety of materials, such as low cost
plastic. The conductive grounds and sensor section can be made by
accurately spraying conductive paint. Other materials could include
flex circuit or even cardboard.
[0049] FIG. 13 depicts an alternative form for a capacitive
inventory sensor that utilizes a moveable capacitor plate. The
capacitor inventory sensor is formed using three parallel plates.
Two of the plates 1302, 1304 are fixed and stationary. The third
plate 1306 is moveable, and may be connected to a moveable member
in a capacitive inventory sensor, such as on a pusher. As indicated
in FIG. 13, as the moveable plate 1306 moves, the moveable plate
1306 overlaps differing portions 1308 of the stationary plates
1302, 1304. The differing amounts of overlap result in differing
levels of capacitance among the three plates 1302, 1304, 1306.
Measurement of such capacitances can indicate the position of the
moveable plate 1306, which can indicate the position of an object,
such as a pusher, to which the moveable plate 1306 is connected.
The pusher position can in turn be translated into an amount of
product present in the pusher assembly.
[0050] FIG. 14 is a circuit diagram depicting an equivalent circuit
to the configuration depicted in FIG. 13. The two stationary plates
can be arranged edge to edge so that the capacitance 1402 between
the two stationary plates is small. The moveable plate is
positioned opposite the two stationary plates so that a face of the
moveable plate is opposite the faces of the two stationary plates.
Such a configuration forms two capacitors in series, as shown at
1404, 1406. A capacitance measuring device 1408 is configured to
measure the capacitances 1402, 1404, 1406 formed by the three
plates.
[0051] The configuration shown in FIGS. 13 and 14 enables
estimation of the capacitance, C.sub.2, between stationary plate 1
and moveable plate 3 according to:
C.sub.2=k*A.sub.1/d,
where k is a constant, A.sub.1 is the area of overlap between
plates 1 and 3, and d is the distance of separation between
stationary plates 1 and 2 and moveable plate 3. Similarly, the
capacitance between stationary plate 2 and moveable plate 3 can be
estimated as:
C.sub.3=k*A.sub.2/d,
where k is a constant, A.sub.2 is the area of overlap between
plates 2 and 3, and d is the distance of separation between
stationary plates 1 and 2 and moveable plate 3. A position for x=0
(where x is representative of the position of the moveable third
plate, and the structure (e.g., the pusher) to which the third
plate is attached) can be selected such that:
A=a*x,
where a is based upon by the widths of the plates and can be
determined using a geometric calculation. Such selection results
in:
C.sub.2=C.sub.3=k*a*x/d.
Simplifying k*a/d into a single constant m,
C.sub.2=C.sub.3=m*x.
Thus, the total capacitance, C.sub.m, seen by the measuring device
408 is:
C.sub.m=C.sub.1+m*x/2.
This is an equation of a straight line, where C.sub.m is linearly
proportional to the displacement x with a known offset of C.sub.1.
This equation can be rearranged to identify the position, x as:
x=2(C.sub.m-C.sub.1)/m.
Thus, the measured capacitance can be translated into a position
estimate.
[0052] FIG. 15 is a graph depicting an example expected and
measured variation in capacitance based on a position of the
moveable plate. The straight line 1502 is based on the above
described linear equation. The non-straight but linearly trending
line 1504 is based on actual measurements taken as a moveable third
plate was translated with respect to a pair of stationary plates.
The real-life measured capacitances 1504 are sufficient for
identifying an approximate position of the moveable third plate.
This position is related to the position of the structure, such as
the pusher, to which the third plate is connected. Knowing the
dimensions of product to be positioned in the capacitive inventory
sensor, the position of the pusher can be translated to a number of
items present in the capacitive inventory sensor.
[0053] FIG. 16 depicts an example three-plate capacitive inventory
sensor. Two of the plates 1602, 1604 are fixed and stationary. The
third plate 1606 is moveable, and may be connected to a moveable
member in a capacitive inventory sensor, such as on a pusher. As
the moveable plate 1606 moves, the moveable plate 1606 overlaps
differing portions of the stationary plates 1602, 1604. The
differing amounts of overlap result in differing levels of
capacitance among the three plates 1602, 1604, 1606. Measurement of
such capacitances can indicate the position of the moveable plate
1606, which can indicate the position of an object, such as a
pusher, to which the moveable plate 1606 is connected. The pusher
position can in turn be translated into an amount of product
present in the pusher assembly.
[0054] The measured capacitance is based on the amount of overlap
present between the stationary first plate and second plate and the
moveable third plate. The amount of overlap among the plates can be
varied in several different ways. As depicted in FIG. 13, the
amount of overlap can be varied where the moveable third plate
extends beyond the length of the stationary first plate and the
stationary second plate.
[0055] FIG. 17 depicts a configuration where the stationary first
plate and second plate extend the length of the capacitive
inventory sensor and vary in width along that length. A stationary
first plate 1702 and stationary second plate 1704 are positioned
along a length of the capacitive inventory sensor track. The first
and second plates 1702, 1704 are largest at a back portion 1706 of
the track and taper toward the front portion 1708 of the track. The
moveable third plate 1710 is configured to be attached to the
pusher 1712, such that the third plate 1710 traverses the length of
the track with the pusher 1712. The capacitance between the
stationary plates 1702, 1704 and the moveable plate 1710 increases
as the third plate 1710 moves toward the back portion 1706 of the
track because a larger area of first and second plate conductors is
directly opposite the face of the third plate 1710 in the back
portion position 1706. By measuring the capacitances among the
three plates 1702, 1704, 1710 in a similar manner as described with
respect to FIG. 14, a position of the third plate 1710, and thus
the pusher 1712 can be determined. The shape of the stationary
first and second plates 1702, 1704 could be varied in other ways
(e.g., in a stepped fashion, in a curved fashion) to provide
different levels of discrimination, to provide greater resolution
at a particular portion of the track, etc.
[0056] A position of the moveable third plate can also be
determined using a digital coding configuration. FIG. 18 depicts a
coded digital capacitive inventory sensor configuration. A first
stationary plate 1802 is provided at a consistent thickness along
the length of a capacitive inventory sensor track. A plurality of
additional stationary plates 1804, 1806, 1808, 1810 have varying,
stepped widths along the length of the track. A moveable third
plate 1812 is configured to move along the length of the track.
When the moveable third plate 1812 is positioned opposite a wide
portion of one of the additional stationary plates 1804, 1806,
1808, 1810, a capacitance of known magnitude is formed between the
moveable third plate and that stationary plate. When the moveable
third plate 1812 is positioned opposite a narrow portion of one of
the additional stationary plates 1804, 1806, 1808, 1810, a
capacitance of smaller magnitude is formed between the moveable
third plate and that stationary plate. A capacitance measuring
device may be connected among the stationary first plate 1802, the
moveable third plate 1812, and each of the additional stationary
plates 1804, 1806, 1808, 1810 in sequence, resulting in four
capacitance measurements that, in combination, are indicative of
the position of the moveable third plate 1812. Based on the on-off
indication associated with each of the additional stationary plates
1804, 1806, 1808, 1810, the position of the moveable third plate
1812 can be decoded. In the example of FIG. 18, the wide and narrow
portions of the additional stationary plates 1804, 1806, 1808,
1810, are arranged in a Gray code, where only one additional
stationary plate 1804, 1806, 1808, 1810 has a bit (portion) that
changes in each position along the track. In some configurations,
the bits (portions) may be unequally spaced to provide higher
resolution at different portions of the track.
[0057] FIG. 19 depicts an example implementation of a coded digital
capacitive inventory sensor. The example includes a rectangular
piece of single-sided copper-laminated board from which copper has
been selectively removed to leave a reference track and four code
tracks. The code areas are one inch long. The moveable capacitor
plate is made from a rectangle of the same material, with the
copper covered with Kapton tape to form a dielectric. FIG. 20 is a
graph depicting measured data over the first four portions in
0.25'' increments. The changes from "0" to "1" in the code can be
seen.
[0058] FIG. 21 is a block diagram depicting a transmission line
reflection sensing counter. When a short (in time) electrical
impulse is applied to the terminals of a transmission line using a
pulse generator 2102, the pulse will travel along a transmission
line 2104 at a speed equal to the speed of light multiplied by the
"velocity factor" of the transmission line 2104. If the line 2104
is of uniform characteristic impedance, this impulse looks the same
at each point along the transmission line 2104, apart from
attenuation due to the characteristics of the materials that make
up the transmission line 2104, and a time delay. If the impulse
encounters a portion of the line 2104 where there is an impedance
different from the so-called "characteristic impedance of the
line", the impulse splits into two components. One continues on
down the line 2104, slightly diminished in amplitude (called the
forward pulse), and the other is reflected back toward the input
end of the transmission line (the reflected pulse). If the input
end of the cable is observed, say with an oscilloscope or impulse
detector 2106, the source pulse and the reflected pulse may be
observed, separated in time. The time difference between them is
equal to twice the travel time from the input end of the
transmission line 2104 to the source of the reflection. This
approach has been used for decades to locate the source of problems
on transmission lines 2104, cables etc. Under appropriate
conditions, a transmission line 2104 having multiple impedance
discontinuities will show reflections from all of these. A
transmission line 2104 which is infinitely long or one that is
terminated with an impedance 2108 equal to its characteristic
impedance will have no reflections. The approach works well in all
types of transmission lines, such as coaxial cable, parallel wire
transmission line, microstrip line, and twisted pairs. Coaxial
cable confines the fields associated with transmission of signals
entirely within its structure. Some types of transmission line,
such as parallel wire line and microstrip line have fields which
extend significant distances beyond the physical extents of the
transmission line. Objects 2110, such as units of product on a
shelf 2112, nearby the line 2104 will interact with the field and
cause the impedance to change, generating a source for a potential
reflection of a pulse.
[0059] This gives rise to the notion of using such a line 2104 to
monitor the presence or absence of objects 2110 in its vicinity.
For instance, a suitable transmission line 2104 could be embedded
in the shelf 2112 of a display on which products 2110 are to be
placed, and fitted at one end with a pulse generator 2102 and a
means of detecting reflections 2106 and at the other end with a
termination 2108. With no stock on the shelf 2112, the pulses are
all absorbed by the termination 2108. As product 2110 is placed on
the shelf, reflections will begin to appear. The implementation
issues in doing this are mostly to do with the requirement for very
precise resolution in time. Using the rule of thumb that in one
nanosecond an electromagnetic wave travels a foot in free space,
one can see that if the objects 2110 to be monitored are inches in
diameter, resolution of better than one nanosecond is required.
This is no problem for lab equipment, particularly if a human is
interpreting, say, an oscilloscope display. Trying to do this
automatically and inexpensively using limited electrical power is
not an easy task. Apart from the pulse generator 2102, there is the
need for a so-called directional coupler connected between the
pulse generator 2102 and the input of the transmission line 2104,
and a high-speed high-resolution analog-to-digital convertor (ADC)
2106. After the pulse is generated, the ADC 2106 samples the
reflected signal arm of the directional coupler until all
reflections have died away. Successive samples are stored in
computer memory, and can then be processed by a computer program to
reveal the impedance discontinuities that reveal the presence of
product 2110 on the shelf 2112.
[0060] In one example, the signal at the reflected arm of the
directional coupler can be passed to a simple diode detector. All
the reflected signals can be rectified and averaged. Under
conditions of no stock, the output from the detector 2106 was low.
As stock was added, the signal level rose. Such detection may work
best as a "none or some" monitor. If the output of coupler is
digitized with sufficient resolution, identification of the
location of individual items can be achieved.
[0061] Another use of impulse functions is in determining the
frequency response of networks. This can be accomplished by using
mathematical properties of the Fourier Transform. Applying the
Fourier Transform (actually the so-called Fast Fourier Transform,
or FFT) to a time-sequence of samples yields a frequency-sequence
of results. The time response and frequency response form a
so-called Fourier Transform pair. In general, the more compressed a
signal is in time, the more spread out it is in frequency, and vice
versa. An infinitely narrow impulse has a uniform spectrum from DC
to infinite frequency. A narrow pulse in the nanosecond range will
have a spectrum stretching out beyond 1 GHz. Hence a network, such
as a filter, can be subjected to a single impulse, and then its
frequency response is simply the FFT of the output of the network.
In this way, the frequency response can be obtained very
conveniently, without having to use a swept source to measure at
every frequency.
[0062] The present approach is to do this the other way round.
First the transmission line is swept, recording the output of the
directional couplers reflected arm over a wide range of
frequencies. The amplitude and phase of the steady state response
is recorded. The results are, in fact, the same coefficients that
would have been determined using the impulse method. By applying an
FFT to the steady state coefficients then, one obtains the impulse
response of the transmission line--a sequence of values stretching
over a time interval. This set of values will look just like the
TDR results, but obtained in a quite different way. If a low power,
wide frequency range network analyzer is built, it can be used to
obtain the required frequency response values. With recent advances
in Direct Digital Synthesis (DDS) integrated circuits, building a
simple vector network analyzer that uses modest amounts of power is
becoming easier and less expensive.
[0063] TDR is a promising way to not only count products on a self,
but also determine where on the shelf they are. As discussed here,
it can be done either in the traditional way using an impulse or by
using a vector network analyzer and applying an FFT to the
results.
[0064] It should be understood that as used in the description
herein and throughout the claims that follow, the meaning of "a,"
"an," and "the" includes plural reference unless the context
clearly dictates otherwise. Also, as used in the description herein
and throughout the claims that follow, the meaning of "in" includes
"in" and "on" unless the context clearly dictates otherwise.
Finally, as used in the description herein and throughout the
claims that follow, the meanings of "and" and "or" include both the
conjunctive and disjunctive and may be used interchangeably unless
the context expressly dictates otherwise; the phrase "exclusive or"
may be used to indicate situation where only the disjunctive
meaning may apply.
* * * * *