Multiplexed surface acoustical wave apparatus

An apparatus is featured for controlling remote devices. The status or one or more switches associated with remote devices may be transmitted along a single transmission line. One end of the transmission line contains a surface acoustical wave (SAW) delay line. The SAW delay line responds to an interrogation pulse of radio frequency electromagnetic energy with a series of delayed pulses, each of which transmits the status of a switch by means of the presence or absence of the pulse. The SAW delay line and switches are completely passive, requiring no power source. The transmission line may optionally be interrupted and coupled by means of a pair of inductive coils. This offers the potential for rotation, as within a steering column of an automobile, without wear or damage to the transmission line. The other end of the transmission line contains electronic circuitry to provide the interrogation pulse, to decode the return delayed pulses, and to drive other circuitry as required by the switch positions.

Related Applications 
This application is related to co-pending U.S. patent applications, as 
follows: U.S. patent application Ser. No. 07/483,349 (now allowed), filed 
FEb. 20, 1990, titled CODED SURFACE ACOUSTICAL WAVE (SAW) MOTOR VEHICLE 
SECURITY DEVICE; U.S. patent application Ser. No. 07/276,072, filed Nov. 
25, 1988 (Attorney Docket No. 88-3-515), titled SPURIOUS SIGNAL CORRECTION 
FOR SURFACE ACOUSTIC WAVE (SAW) SECURITY DEVICES, now U.S. Pat. No. 
4,945,354; and U.S. patent application Ser. No. 07/399,126, filed Aug. 28, 
1989 (Attorney Docket No. 89-2-381), titled EASILY ENCODABLE SURFACE 
ACOUSTIC WAVE, (SAW) SECURITY DEVICES; and hereby incorporates by way of 
reference all of the teachings and description therein. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to apparatus for digitally multiplexing a 
plurality of remote, electronically operative devices, and more 
particularly to a multiplexed apparatus utilizing a surface acoustical 
wave system. 
In an electrical system featuring a number of diverse electronically 
operated devices, it has been found necessary to locate the switch 
controls at a remote location. Such a system is frequently to be found in 
automobiles, wherein the horn, directional signals, lights, wipers, etc., 
are all controllable from a wand mounted upon the steering wheel column. 
Other automotive functions such as oil pressure, water temperature and 
vehicular speed are usually mounted upon a single panel or dash. 
In such cases, it may be expensive or inconvenient to provide an individual 
transmission line to each device or switch. In the aforementioned motor 
vehicle steering wheel example, manufacturing and reliability problems are 
posed by passing a great number of wires up a steering column to a 
rotating member. In such cases, it can be desirable to employ a 
multiplexing technique whereby the status of more than one switch can be 
transmitted using a single transmission line. Means such as time division 
multiplexing, frequency division multiplexing, and packet transmission are 
well known to those skilled in the art of electrical communications. These 
well known multiplexing methods require that sophisticated electronic 
circuits requiring power be installed at both ends of the transmission 
line. 
It would be desirable to replace the electronic circuitry at one end of the 
transmission line with a passive component not requiring power. Using the 
steering wheel and tire pressure sensor switch as examples, it would also 
be desirable to have a simple and reliable means to transmit such data 
using a noncontacting connector with freedom to rotate. 
The present invention has developed a system that can control and/or 
interrogate remote, electronically operative devices. A single 
transmission line can carry multiplexed digital information to a pair of 
coupling inductive coils. One of the coils is associated with a 
programmable surface acoustical wave device. 
A surface acoustical wave transponder that responds to an interrogating 
pulse with a delayed sequence of pulses, and a circuit for comparing the 
proper pulse sequence is described in the, aforementioned U.S. patent 
application Ser. No. 07/483,349. The circuit described therein (see FIG. 
8) can be utilized in the present invention to differentiate between 
switching signals for the remote devices, which switching signals are 
propagated as reflected surface acoustical waves from the programmable 
transducers of the SAW device located within the steering wheel column. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to obviate the disadvantages of 
the prior art. 
It is another object of this invention to provide a multiplexed SAW 
apparatus for interrogating and/or controlling a number of remote devices. 
It is another object of the invention to provide a passive SAW system for 
controlling or interrogating a number of remote devices. 
It is a further object of this invention to control or interrogate remote 
devices without direct electrical 
e connection, utilizing a coil coupled SAW device. 
These objects are accomplished, in one aspect of the invention, by the 
provision of a multiplexed apparatus for controlling and/or interrogating 
a number of remote devices. A SAW transponder contains an interface 
transducer and a plurality of programmable SAW reflectors disposed upon a 
piezoelectric substrate. 
An interrogating signal is imparted to the SAW transducer either by direct 
connection to a signal source by means of a cable or by means of inductive 
couping coils. The electrical interrogating signal is converted to SAW 
energy by means of the SAW transducer and travels along the surface of a 
peizoelectric substrate such as lithium niobate. Upon encountering a 
programmable reflector, the SAW energy either passes unimpeded or a 
portion of its energy is redirected back towards the SAW transducer, 
depending on which function has been programmed into the reflector. 
The programming of the transducer is accomplished by setting an electrical 
switch to the open or closed position. The reflected SAW energy reaches 
the SAW transducer after a delay time and is reconverted to electrical 
energy which can then be interpreted by electronic circuitry. The 
individual reflectors are distinguished by the unique time of the 
transponded pulse produced by each reflector.

BEST MODE FOR CARRYING OUT THE INVENTION 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims taken in conjunction 
with the above-described drawings. 
Generally speaking, the invention relates to a multiplexed apparatus for 
interrogating the status of a number of switches and/or a number of remote 
devices. The apparatus features a SAW device having a number of 
programmable or switchable transducers. By means of opening or closing a 
respective switch associated with each one of the programmable 
transducers, the transducers are caused to change their acoustical wave 
state between a first normally non-reflective surface acoustical wave 
state, and a second reflective surface acoustical wave state. 
Remote devices respectively associated with each transducer can thus be 
controlled and/or interrogated by programming the transducers to be in 
either one of the two operative states. 
For the purpose of brevity like elements will have the same numerical 
designation throughout the figures. 
Now referring to FIG. 1 an example of a SAW transponder 100 is shown 
comprising a piezoelectric substrate 110, in accordance with the present 
invention. In the figure, transducer 130 disposed on substrate 110 is 
connected to an inductive coil 230, preferably designed so that its 
inductance resonates with the capacitance of the transducer in accordance 
with well known principles. A so-called "split electrode" transducer 
design is shown in FIG. 1, with electrodes spaced at quarter 
acoustic-wavelengths of the fundamental SAW frequency. It is also possible 
to employ a so-called "single electrode" transducer in which each split 
electrode is replaced with a single unsplit electrode spaced at half 
acoustic-wavelengths of the fundamental SAW frequency. When excited by an 
interrogation pulse, transducer 130 launches SAW 210 towards the 
switchable reflector 160. 
The switchable reflector 160 disposed on substrate 110 in FIG. 1 is also a 
"split electrode" transducer. It may or may not have the same number of 
electrodes as transducer 130. The switchable reflector 160 has a property 
by which it is able to transmit SAW's without reflection, when it is 
shorted by a closed switch 240. However, if the switch is opened, the 
switchable reflector becomes partially reflecting, creating a reflected 
SAW 220 which can be transduced to electromagnetic energy and 
retransmitted by transducer 130 to coil 230. The switchable reflector may 
also be constructed from a "single electrode" transducer, but the role of 
the switch may be reversed; that is, shorting the switch 240 may lead to 
reflection and vice versa. The "split electrode" configuration is 
preferred for the switchable reflector. 
Another embodiment of the present invention is illustrated in FIG. 2. Here 
is shown a transponder 120 with one transducer 180 and six switchable 
reflectors 170, 171, 172, 173, 174 and 175. Each switchable reflector is 
connected to a switch, not shown, by a twisted pair of plastic insulated 
#22 wire of length 7.5 cm (Experimentation has shown that twisted pairs up 
to 15 cm length may be used without degradation of signals.) The substrate 
11 is Y-cut Z-propagating lithium niobate. Lithium niobate was chosen for 
its high coupling coefficient; the YZ cut was chosen for its ruggedness. 
The electrode spacings were chosen so that the fundamental frequency of 
the transducers was 44.3 MHz. The use of low frequencies below 100 MHz and 
preferably below 50 MHz is preferred since low cost electronic circuitry 
and switches can be used. Moreover, simple photolithographic techniques 
can be used to manufacture the SAW transponders (typical linewidths=10 
microns). 
Sample voltages from the pickup coil, not shown, coupled to coil 230 
attached to transducer 180 in FIG. 2 are shown in FIGS. 3 and 4. In FIG. 
3, all switches are closed. In FIG. 4, four switches (connected to 
switchable reflectors 170, 171, 173 and 175) are open, yielding four 
readily detectable pulses. These pulses can be detected, and decoded by 
electronic circuitry 310 shown in FIG. 8 of, and described in U.S. patent 
application, Ser. No. 07/483,349. 
In the preferred embodiment electrodes of the transducers are spaced at 
quarter acoustic-wavelengths, so that reflection from individual 
electrodes is subject to destructive interference and is completely 
cancelled. When the reflector transducer is open circuited, the incoming 
interrogating SAW causes buildup of voltage across the transducer, which 
in turn causes re-radiation of SAW's in both directions; the re-radiation 
towards the interrogating transducer constitutes partial reflection. This 
phenomenon is known as regeneration. When the reflector-transducer is 
shorted, this voltage cannot build up and no reflection occurs. 
The switches, not shown, which respectively open or close the opposite line 
ends of each transducer 170 through 175 are associated with the remote 
devices, and thus provide the means to control or interrogate these 
devices. These switches modulate the reflective nature of the transducers 
which can be sensed by the electronic circuit. 
Since other modifications and changes varied to fit particular operating 
requirements and environments will be apparent to those skilled in the 
art, the invention is not considered limited to the example chosen for 
purposes of disclosure, and covers all changes and modifications which do 
not constitute departures from the true spirit and scope of this 
invention. 
Having thus described the present invention, what is desired to be 
protected by Letters Patent is presented by the following appended claims.