Method of interrogating a plurality of transponders arranged in the transmission range of an interrogating device and transponders for use in the said method

A single interrogation device interrogates a plurality of transponders arranged within the range of transmission of the interrogation device and identifies them without any mutual interference. The interference free operation is obtained by the interrogation device sequentially transmitting a plurality of RF interrogation pulses, separated from each other in time and whose energy changes from one pulse to the next one, to transponders which have an energy storage element and which respond to the interrogation pulses with an answer signal in dependence upon the condition that, at the end of each interrogation pulse, the charge voltage present at the energy storage element falls within a predetermined voltage range.

FIELD OF THE INVENTION 
The invention relates to a method for the interrogation of a plurality of 
batteryless transponders arranged in the transmission range of an 
interrogation device. Furthermore the invention relates to a transponder 
for use in such a method. 
BACKGROUND OF THE INVENTION 
A transponder system is described, in which with the aid of an 
interrogation device batteryless transponders may be caused to transmit an 
answer signal, which may contain encoded information as regards the 
identity of the respectively reacting transponder and possibly further 
information as well. The particular feature of the transponder utilized is 
that it does not comprise any batteries to supply its operating power. The 
operating voltage which the transponder requires in order to transmit its 
response signal is obtained from a RF pulse interrogation which is 
transmitted by the interrogation device of the transponder system. In the 
transponder this RF interrogation pulse is rectified and the voltage then 
produced is utilized for charging a capacitor constituting an energy 
storing means. As soon as circuit unit in the transponder detects the end 
of this RF interrogation pulse and sufficient energy has been stored in 
the energy storing element, the transponder transmits the above noted 
answer signal. This answer signal may then be received and processed by 
the interrogation device. 
Such transponders of the type described may, for instance be implanted in 
animals or by arranged on articles so that with the aid of the 
interrogation device, the animals or the articles may be identified on the 
basis of the encoded information in the answer signals. 
One problem in conjunction with such a transponder system occurs if a 
plurality of transponders are present in the transmission range of the 
interrogation device. Such transponders then namely simultaneously receive 
one transmitted RF interrogation pulse and will then also transmit their 
answer signal back at the same time as well, if after the end of the RF 
interrogation pulse there is sufficient energy stored in their energy 
storing means. The simultaneously produced answer signals render 
unambiguous identification of the respective transponder by the 
interrogation device impossible. 
In addition, if instead of the interrogator transmitting one high power 
interrogation pulse, which would provide at least the furthest transponder 
with enough energy to respond, the interrogator transmitted a successive 
series of low power to high power pulses, wherein the low power pulse 
would be enough energy to charge-up the closest transponder and the high 
power pulse would be enough energy to charge-up the furthest transponder, 
transponders will still simultaneously respond. To illustrate an example, 
assume that the transponder has a discharge function such that if the 
transponder does not receive adequate power to transmit an entire response 
telegram upon the termination of the interrogation pulse, the transponder 
discharges. Then, when the interrogator transmits a minimum power pulse, 
only the closest transponder is adequately charged with enough energy to 
respond, so there is no interference from other transponders trying to 
respond simultaneously. However, when the interrogator transmits a higher 
power interrogation pulse, not only are the further transponders 
charged-up adequately to respond, but the closer transponders are also 
charged up adequately to respond, thereby causing interference in the 
reception of either answer signal. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a method of the type 
initially mentioned such that with the aid of one interrogation device it 
is also possible to interrogate a plurality of transponders arranged 
within the range of transmission and to identify them without any mutual 
interference. 
In accordance with the invention, this object is to be attained by the 
interrogation device sequentially transmitting a plurality of RF 
interrogation pulses, separated from each other in time and whose energy 
changes from one pulse to the next one, to transponders which have an 
energy storage element and which respond to the interrogation pulses with 
an answer signal in dependence upon the condition that, at the end of each 
interrogation pulse, the charge voltage present at the energy storage 
element falls within a predetermined voltage range. 
On using the method in accordance with the invention, the RF interrogation 
device transmits a series of various power level RF interrogation pulses. 
Several transponders, for example Transponders 1, 2, and 3 are located 
within the transmission range of the interrogator, wherein Transponder 1 
is located closer to the interrogator than Transponder 2, and Transponder 
3 is located further from the interrogator than Transponders 1 or 2. In 
order to transmit an answer signal in response to the interrogation 
signal, each of the Transponders require the charge voltage present at the 
energy storage element to fall within the same predetermined voltage level 
range. However, because they are located at different distances from the 
interrogator, Transponders 1, 2 and 3 each receive varying amounts of 
energy from any of the transmitted interrogation pulses. Therefore, only 
the transponder which receives an amount of interrogation signal energy, 
present at the energy storage element, that falls within a predetermined 
voltage range, transmits an answer signal. For example, the interrogator 
transmits an interrogation pulse and at the end of the interrogation 
pulse, the charge voltage present at the energy storage element of 
Transponder 2 falls within the predetermined voltage level range, thereby 
enabling the transmission of an answer signal from Transponder 2. 
Transponder 1, however, receives much more energy from the same 
interrogation signal, therefore, the charge voltage present at the energy 
storage element of Transponder 1 does not fall within the predetermined 
voltage range, thus Transponder 1 does not transmit the answer signal. 
Likewise, Transponder 3 receives less energy from the same interrogation 
pulse than the other transponders because it is located further from the 
interrogator, and, again, the charge voltage present at the energy storage 
element does not fall within the predetermined voltage range, thus 
Transponder 3 does not transmit the answer signal. As a consequence of the 
use of the method in accordance with the invention, the probability that 
after the transmission of a respective RF interrogation pulse with a 
predetermined energy level only one transponder will respond, is 
considerably increased, more particularly if the predetermined range of 
the charge voltage, which has to be reached as a condition for the 
enablement for the return of an answer signal, is made relatively narrow. 
In fact the narrower range, the greater the probability that the answer 
return condition is fulfilled for only respectively one of the 
transponders in the transmission range of the interrogation device. 
Further advantageous developments of the invention are recited in the 
following paragraphs. 
A transponder for use in the device of the present invention may be 
characterized by a window comparator to whose input the charge voltage of 
the energy storage element is fed and at whose output an enable signal 
appears, when the charge voltage is within a predetermined range, the 
enable signal functioning to cause the return of the transponder answer 
signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With the aid of the interrogation device, which is diagrammatically 
illustrated in FIG. 1, it is possible to transmit RF interrogation pulses 
whose energy increases from one pulse to the next one. The interrogation 
device comprises a transmitting end stage 12, which may be controlled by a 
control unit and by means of a D/A converter 16. The control unit 14 
provides the D/A converter 16 with progressively increasing digital 
values, which are converted into analog voltage values by the D/A 
converter. These voltage values function as a supply or power voltage for 
the transmitter end stage 12. The control unit 14 furthermore sends enable 
pulses to the transmitter end stage 12, which are synchronized to time 
respectively with the digital values sent to the D/A converter 16. This 
means that every time the D/A converter applies a voltage with a certain 
values as a power voltage to the transmitter end stage 12, the transmitter 
end stage is enabled for the transmission of a RF interrogation pulse by 
means of an antenna 18. The transmitter end stage 12 then provides a RF 
interrogation pulse, whose amplitude is respectively dependent on the 
supply voltage supplied at the point in time in question by the D/A 
converter 16. 
The FIG. 2 shows time graphs of the signals at the points A, B and C of 
FIG. 1. It will be seen from these figures that the RF interrogation 
pulses, which are supplied by the transmitter end stage 12 to the antenna 
18 and are radiated by the latter, increase in amplitude from one pulse to 
the next one so that these pulses respectively have a larger energy level. 
This ever increasing amplitude is achieved because the D/A converter 16 
supplies progressively increasing supply voltages U.sub.0, U.sub.1, 
U.sub.2 and U.sub.3 to the transmitter end stage 12. The number of pulses 
with an increasing energy radiated by the antenna 18 will be dependent on 
the individual case of application. 
In FIG. 1 only those modifications are illustrated which are necessary in 
this known interrogation device in order to render it possible to radiate 
pulses with changing energy in the design of FIG. 1 so that the energy 
increases from one pulse to the next one. 
FIG. 2 shows an embodiment of the invention as an interrogation device 20, 
with the aid of which it is possible to radiate RF interrogation pulses 
whose duration increases from one pulse to the next one so that the 
radiated energy as well increases form pulse to pulse as well. The 
interrogation device 20 comprises a transmitter end stage 22 which is 
connected with a set supply voltage U equal to U.sub.v. With the aid of a 
control unit 24 it is possible to enable pulses with a progressively 
increasing duration to be supplied to the transmitter 22 so that 
accordingly the transmitter end stage 22 as well is supplied with RF 
interrogation enable pulses with a progressively increasing length and to 
radiate such pulses with an increasing length via an antenna 26. 
FIG. 4 contains time graphs of the signal occurring at the circuit points 
A, B and C of FIG. 3. As will be seen the duration of the enable signals 
fed to the transmitter end stage 22 by the control unit 24 becomes larger 
and larger from signal to signal (.DELTA.t.sub.0 &lt;.DELTA.t.sub.1) so that 
the antenna 26 as well will radiate signals with an ever increasing 
duration. 
It will now be explained with reference to FIG. 5 how using an 
interrogation device with the design illustrated in FIG. 1, in a 
transponder system two transponders 30 and 32 may be interrogated, which 
are arranged within the range of the interrogation device 10. At the start 
of an interrogation cycle the control unit 14 supplies a digital signal to 
the D/A converter, and such signal is converted by the converter 16 into a 
voltage U.sub.0 as a supply voltage for the transmitter end stage 12. 
Simultaneously the control unit 14 feeds the enable signal to the 
transmitter end stage, such signal commencing at the point in time t.sub.0 
and having the duration of .DELTA.t. During this duration the transmitter 
end stage 12 will produce a first RF interrogation pulse, which is 
radiated by the antenna. 
As stated in the introduction hereof the transponders 30 and 32 are not 
provided with a power supply in the form of a battery: they derive their 
driving power form the respectively received RF interrogation pulse. This 
involves the rectification of this pulse and charging of a capacitor by 
means of the voltage produced by rectification. The two transponders 30 
and 32 simultaneously receive the RF interrogation pulse radiated by the 
antenna 18 so that in both transponders the charging of the capacitor, 
functioning as a power source, starts at the point in time t.sub.0. Since 
the transponder 30 is at a shorter distance from the interrogation device 
10 than the transponder 32, the transponder 30 receives the RF 
interrogation pulse with a greater field strength so that accordingly 
furthermore the voltage produced by rectification has a higher value as 
well than that in the transponder 32. The consequence of this is that the 
capacitor utilized as a power source in the transponder 32 charges up to a 
higher value than that in the transponder 32. In the time graphs of FIG. 5 
the capacitor voltages U.sub.30 and U.sub.32 are shown and it is to be 
seen that in the transponder 30 the capacitor voltage will, after the 
expiry of the pulse duration .DELTA.t, have a voltage value which falls 
within the predetermined voltage range, for example, between two voltage 
values S.sub.1 and S.sub.2, thereby enabling transponder 30 to transmit an 
answer signal. Owing to the greater distance of the transponder 32 from 
the interrogation device 10 and the accordingly lower field strength, in 
the transponder 32, the voltage U.sub.32 will only reach a value lower 
than the voltage value S.sub.1, which does not fall within the 
predetermined voltage range of S.sub.1 to S.sub.2, thereby prohibiting the 
transmission of the answer signal by transponder 32. 
After the end of the time period .DELTA.t the first RF interrogation pulse 
ends and after a pause of a predetermined duration under the control of 
the control unit 14, the transmitter end stage will start transmitting a 
further RF interrogation pulse whose amplitude is however greater. For a 
description of the present situation it is assumed that the capacitors in 
the transponder 30 and 32 are discharged so that the charging thereof by 
the voltage, which is produced by rectification of the RF interrogation 
pulse, starts at the voltage value 0 again. The details of the 
transponder, with which this is rendered possible, will be described later 
with reference to FIG. 7. 
As the graphs of FIG. 5 show, the capacitor utilized in the transponder 30 
as the power source is charged up to a significantly higher voltage owing 
to the greater amplitude of the second RF interrogation pulse, the voltage 
value, existing at the end of the second RF interrogation pulse, being 
above the voltage value S.sub.2, or outside the predetermined voltage 
range. In the second transponder 32 however a voltage value is reached, 
which is between the voltage values S.sub.1 and S.sub.2, thereby enabling 
the transmission of an answer signal by transponder 32. 
As will be explained below in detail, the fact that the charge voltage 
present at the end of a RF interrogation pulse at the capacitor in a 
transponder 30 or 32 falls within the predetermined range of between the 
voltage values S.sub.1 and S.sub.2, is utilized for the interrogation of 
individual transponders, even if a plurality of transponders are 
simultaneously located within transmission range of the respective 
interrogation device. 
With reference to FIG. 6 it is to be noted that in the case of the use of 
an interrogation device of the type illustrated in FIG. 3, it is possible 
for two transponders to be interrogated which are in the transmission 
range of the interrogation device. The interrogation device 20 begins at 
the point t.sub.0 in time to transmit a first RF interrogation pulse, 
which has a predetermined duration .DELTA.t.sub.0. This RF interrogation 
pulse is received by both transponders 34 and 36. Since the transponder 34 
is at a shorter distance from the interrogation device 20 than the 
transponder 36, the greater field strength at the position of the 
transponder 34 will lead to a greater charge of the capacitor utilized as 
the power source so that accordingly, as illustrated in graphs of FIG. 6, 
at the end of the first RF interrogation pulse, that is to say after the 
time t.sub.0 +.DELTA.t.sub.0, a charge voltage will be reached which falls 
within the predetermined range of between the voltage values S.sub.1 and 
S.sub.2, thereby enabling the transmission of the answer signal from 
transponder 34. However, on the other hand, owing to the lower field 
strength in the transponder 36, the voltage U.sub.36 will only reach a 
value lower than the voltage value S.sub.1, which does not fall within the 
predetermined voltage range of S.sub.1 to S.sub.2, thereby prohibiting the 
transmission of the answer signal by transponder 36. The second RF 
interrogation pulse transmitted after a predetermined pause by the 
interrogation device 20, has longer duration .DELTA.t.sub.1 so that 
accordingly more energy will be fed to the transponders 34 and 36. At the 
end of the second RF interrogation pulse, the charge voltage of the energy 
storage element of transponder 34 will reach a value above the voltage 
value S.sub.2, which is outside the predetermined range of between the 
voltage values of S.sub.1 and S.sub.2, and which prohibits transmission of 
an answer signal by transponder 34. While, on the other hand, the charge 
voltage in the transponder 36 will reach a value which falls within the 
predetermined range of between the voltage values S.sub.1 and S.sub.2. 
As in the example of FIG. 5, it is possible to use this fact for the 
interrogation of the two transponders 34 and 36 which are arranged within 
transmission range of the interrogation device 20. 
The main features of the transponder design are illustrated in FIG. 7, the 
circuit diagram of FIG. 7 showing the features of the circuitry, with the 
aid of which processing of the charge voltages occurring at the capacitor 
functioning as a power source is rendered possible. 
The transponder 30 of FIG. 7 comprises an antenna 38, with which the RF 
interrogation pulses may be received. Together with a capacitor 40 this 
antenna 38 constitutes an oscillating circuit, which is tuned to the 
frequency of the RF interrogation pulses. By means of a diode 42 the 
respectively received RF interrogation pulse is rectified and the 
rectified voltage causes the charging of a capacitor 44, whose charge 
voltage constitutes the supply voltage of the transponder 30. By means of 
a window comparator 46 it is possible to respectively ascertain whether or 
not the charge voltage at the capacitor 44 has a value which is between 
the two threshold values S.sub.1 and S.sub.2, which are referred to in the 
FIG. 5 and 6. The transponder 30 comprises furthermore a RF threshold 
detector 48, which has the function of ascertaining whether the amplitude 
of the RF oscillation at the oscillating circuit consisting of the antenna 
38 and the capacitor 40 has gone below a predetermined threshold. Dropping 
below this value in fact signifies the end of a received RF interrogation 
pulse. 
The transponder 30 furthermore comprises a control logic system 50, which 
in a way dependent on the signals from the window comparator 46 and the RF 
threshold value detector 48 initiates the different various control 
operations in the transponder 30. 
The transponder 30 operates as follows on receiving a RF interrogation 
pulse: 
On the reception of a RF interrogation pulse the oscillating circuit 
constituted by the antenna 38 and the capacitor 40 is caused to start 
oscillating its resonant frequency corresponding to the frequency of the 
RF interrogation pulse. The direct voltage produced by rectification using 
the diode 42 causes charging of the capacitor 44. After the end of the RF 
interrogation pulse the oscillation in the said resonant circuit also dies 
down and the RF threshold detector 48 sends a signal to the control logic 
system 50 via its output 54, when the Rf oscillation has sunk below 
predetermined threshold value. Simultaneously the RF threshold detector 48 
sends a signal to the window comparator via its output 56, such signal 
causing the window comparator 46 to check the charge voltage at the 
capacitor 44 to see if it has a value between the threshold values S.sub.1 
and S.sub.2. If this is the case, the window comparator 46 will feed a 
signal to the control logic system indicating the fulfillment of this 
condition. The control logic then produces an information signal at its 
output 58 containing a code group representing the identity of the 
transponder 30, such information signal being transmitted via the antenna 
38 so that it may be received by the interrogation device. 
After the end of the information signal the control logic system 50 will 
provide a further signal at its output 60, such signal functioning to 
discharge the capacitor 44. 
If on the other hand the window comparator 46 ascertains that the charge 
voltage at the capacitor 44 does not, at the end of the RF interrogation 
pulse, have a value between the threshold values S.sub.1 and S.sub.2, no 
signal is sent via the output 52 to the control logic system so that 
accordingly the transponder 30 does not send any information signal to the 
interrogation device. In this case as well the control logic system 50 
produces a signal at the output 60 to however cause the discharge of the 
capacitor 44 so that the transponder 30 is again ready to receive further 
RF interrogation pulse and to commence recharging the capacitor 44 
starting at the voltage value 0. 
For the window comparator 46 it is possible for instance to use the circuit 
which is described in the book "Halbleiter-Schaltungstechnik" by Tietze 
and Schenk, page 182. This circuit comprises two comparator modules having 
respectively a positive and a negative input. The negative input of the 
one comparator and the positive input of the other comparator are 
connected together and receive the unknown input voltage, while the 
positive input of the one comparator receives the reference voltage 
constituting the upper threshold value S.sub.2 and the negative input of 
the other comparator receives the reference voltage constituting the lower 
threshold value S.sub.1. The outputs of the two comparators are connected 
with the inputs of an AND circuit, which provides a signal at the output, 
when voltage applied to the connected input of the comparators has reached 
a value between the reference voltages. 
In the case of the application of the method in accordance with the 
invention it is assumed that the transponders arranged within the 
transmission range of the interrogation device are at different distances 
form the same. If this applies, at the end of each RF interrogation pulse 
there will be different charge voltages at the capacitor 44 so that 
respectively only one transponder responds, in the case of which charge 
voltage is between the two threshold values S.sub.1 and S.sub.2. The 
nearer the two threshold values S.sub.1 and S.sub.2 are to each other, the 
greater the certainty that only one of a plurality of transponders will 
respond, since the probability will decrease that in the case of two 
transponders the said condition is fulfilled. In the case of a smaller 
distance between the threshold values S.sub.1 and S.sub.2 it is naturally 
also necessary for the differences in energy between sequentially 
transmitted RF interrogation pulses to be made smaller taking into account 
the threshold value difference. 
The number of RF interrogation pulses sequentially transmitted during an 
interrogation cycle will be dependent on the number of distance ranges 
into which the transmission range of the interrogation device is divided. 
In the embodiments of the invention illustrated in FIGS. 1, 2 and 3, 4 
operation is with respectively four RF interrogation pulses, this meaning 
that four distance ranges are set so that within one interrogation cycle 
at the most four transponders are able to respond, which are in the 
respective distance ranges. 
In practice the interrogation device operates with a frequency of the Rf 
interrogation pulses of 134 KHz. In the case of the use of the embodiment 
of the invention illustrated in FIGS. 2 and 3, that is to say with RF 
interrogation pulse, whose duration increases from one pulse to the next 
one, the pulse duration was incremented in steps of 1 ms. It was possible 
to show that in this method the transponders were able to separately 
respond providing their distances from the interrogation device only 
differed by amounts in the order of mm.