Device for sensing the number of items stored in a compartment

A device for sensing the number of items (14) consecutively disposed in a compartment (12), preferable bottles or cans disposed in a beverage magazine (13) in a refrigerator (10). At each item (14) at opposite sides of said item electric conducting areas (20a,b,c;21a,b,c) are provided which form a capacitor (22a,b,c). The capacitors formed in each compartment are interconnected to form a circuit (C.sub.k) the capacitance of which depends on the number of items (14) in the compartment (12).

Often, hotel rooms are equipped with refrigerators containing a selection 
of beverages to be used at the discretion of the room guest. As a rule, 
the beverages taken out are debited manually upon information given by the 
guest as to the number of beverages taken out. 
There are also computer-based systems which are connected to the 
refrigerators in the hotel rooms. In such a system there is a demand for 
sensors which indicate, in some way, that a beverage has been taken out 
and which one has been selected. 
In a known computer-based registering system for hotel refrigerators a 
number of compartments have been provided by lying bottles and cans. In 
each compartment a number of bottles or cans are arranged which contain 
the same kind of beverage. The compartments are blocked by gates and by 
contacts an indication is given each time the gate is opened and a bottle 
or can taken out. 
A disadvantage related to the use of contacts in the way described for 
indicating of withdrawals is that one only gets an idea about the number 
of withdrawals made but not about the actual contents of the refrigerator. 
Therefore, this kind of indication presupposes that the computer system be 
regularly updated with information as to the actual contents of the 
refrigerators, for example when the refrigerators are being refilled. 
The object of the invention is, in particular in hotel refrigerators, to 
simplify the registering of withdrawals made in such a way that at the 
same time it will be possible to get a clear picture of the actual 
contents of the different refrigerators.

In the following a few examples will be given of practical calculations of 
the capacitance of the capacitors 22a, b, c which together form the 
capacitance C.sub.k. In the magazine certain dimensional magnitudes cannot 
be influenced, among these the distance d between the capacitor plates and 
the width of the plates=the length of the can plus a certain play. The 
length l of the magazine depends on the number of packages and in the 
example the length equals to three times the diameter of the can. In case 
of a can d.apprxeq.0.072 m, b=0.11 m and 1=0.2 m. A calculation of the 
capacitance in the traditional way gives for the empty magazine the value 
2.7 pF. 
When a conducting cylinder (the can) is put between the plates the 
capacitance of the capacitor increases. The increase depends on the 
diameter of the cylinder and the position thereof between the plates. A 
symmetric positioning gives the lowest increase of the capacitance whereas 
the highest increase will be achieved when the can rests on one of the 
capacitor plates with an intermediate layer of a thin insulating tape of a 
thickness 0.05 mm. The increase achieved with the can position as shown 
equals to 4.5 pF. 
A magazine for bottles has a greater width b=0.22 m. The remaining 
dimensions are the same. The capacitance of an empty magazine is 5.329 pF 
(calculated value). The contents of the bottle is considered conducting 
which is an approximation quite acceptable. The inner diameter of the 
bottle is smaller than the outer diameter of the can causing a smaller 
increase of the capacitance. With the bottle positioned as in the can case 
at a distance of 0.05 mm from the bottom plate the maximum increase will 
be about 3.5 pF. Especially in connection with bottles the appearance of 
labels can distort the measurement result. This is caused by the 
capacitance between the plates taking different values depending on the 
label being turned downwards or to any other direction. In order to 
decrease the influence of the label the thickness of the insulating tape 
should be least equal the thickness of the label. 
In connection with the practical calculations it has been assumed that the 
capacitor plates extend along the whole surface of the walls 15, 16 in 
FIG. 4. However, advantageously the capacitor plates can instead by 
provided as strips positioned on the walls 15 and 16, respectively, and 
only in the area of those positions in the compartment where cans or 
bottles are to be found. The increase of the capacitance will be the same 
but the capacitance of the empty magazine will be lower, in the can case 
0.8 pF and for the bottle magazine 1.6 pF. In addition to the reduction of 
the capacitance of the empty magazine the described design of the 
capacitor plates results in a lower value of the capacitance 30. The 
advantage connected therewith will be indicated below. 
According to the invention the measuring of the capacitance C.sub.k of the 
magazine takes place by applying a low-frequency sine voltage U.sub.in to 
the terminal 26 causing an output voltage U.sub.out on the terminal 28 the 
amplitude of which depends on the number of cans or bottles in the 
magazine. The output voltage is heavily attenuated by a high-pass filter 
formed by the capacitance C.sub.k and a resistance R. As a result even 
small changes in the capacitance C.sub.k will result in large changes in 
the impedance of this capacitance. The resistance value is chosen so that 
the output signal becomes as large as possible. this will be achieved if R 
is chosen large at the same time as the capacitance 30 is small. In order 
to remedy the influence of variations in the capacitance 30 due to 
constructional defects a higher value of this capacitance is chosen than 
the optimum lowest one resulting in a reduction of the output voltage. For 
magazine of cans the following values of the output voltage have been 
measured: empty magazine--0.10 volts, one can --0.27 volts, two cans--0.43 
volts and three cans--0.56 volts. Corresponding values for a bottle 
magazine have been measured as 0.09 volts, 0.17 volts, 0.26 volts and 0.32 
volts, respectively. 
In FIG. 6 a block diagram is shown of a device 31 for measuring of a number 
of magazines 13, in the example six magazines. The measuring device 
generates analogue measurement values in the form of DC voltages which in 
case of a computer-based system are converted into digital form. However, 
this conversion falls outside of the scope of the invention and will not 
be described in detail. The measuring circuit shown in FIG. 5 can be 
connected to the measuring device in any known way. In the embodiment 
shown in FIGS. 1-3 the back of the magazine is provided with two metal 
plates 32, 33, which cooperate with two plates 34, 35 disposed in each 
compartment. The plate 32 corresponds to the terminal 26 of FIG. 4, 
whereas the plate 33 corresponds to the terminal 28. Via plates 34, 35 the 
inserted magazine is capacitively connected to the measuring device 31. In 
the circuit diagram of FIG. 3 the plates 32, 34; 33, 35 are represented by 
capacitors 36, 37. These are selected to have a large capacitance in order 
not to influence the measurement. 
The input voltage to the measuring circuit of FIG. 5 is generated by a 
function generator 38 emitting a sine voltage. After amplification in an 
amplifier 39 the signal is transferred to one of the magazines by a 
multiplexer 40. The multiplexer is controlled by control signals on a data 
bus 41 so as to consecutively connect the six magazines to the measuring 
device. From the amplifier 39 the signal is also transferred to an 
attenuating device 42 for bottle magazines and to an attenuating device 43 
for can magazines, respectively. Via a multiplexer 44 and an impedance 
converter 45 the signal from the connected magazine is transferred to the 
positive input of a differential amplifier 46. The negative input of this 
amplifier is connected to the outputs of the attenuating devices 42, 43 
via a multiplexer 47. Via a multiplexer 48 and an additional multiplexer 
49 the output of the differential amplifier is connected to a rectifier 50 
via an amplifier 51 and an impedance converter 52. The rectifier is 
connected to an A/D-converter 53 which converts the analogue voltage 
values into digital form to be transferred to a computer, not shown, via a 
data bus 54. A conductor 55 directly interconnects the two multiplexer 
units 48 and 49. These units are also interconnected by an amplifier 56. 
Because the signals from a bottle magazine are at a lower level than those 
from a can magazine amplification must be used if the same measuring chain 
is to be used. Therefore, a bottle magazine signal is led via the 
amplifier 56 whereas a can magazine signal is led via the conductor 55. 
The function of the two attenuating devices 42, 43 is to emit a signal to 
the differential amplifier which with a signal from the magazine in case 
of empty magazine results in 0 volts out from the amplifier thereby 
calibrating the same. The different multiplexer units 40, 44, 47, 48, 49 
are controlled by control signals on the bus 41 to consecutively connect 
the six different magazines 13 to the measuring device. The control 
signals select the correct attenuating device 42 or 43 and control whether 
or not the amplifier 55 needs to be connected.