Short range electromagnetic proximity detection

A very low-cost, short-range electromagnetic transceiver uses a low frequency oscillator signal as a means of charging a step recovery diode (SRD) which converts the stored charge into a very short impulse to enable very high energy efficiency frequency multiplication. This impulse is coupled to an antenna which radiates the energy and receives reflections from objects in the vicinity. The energy of the received impulse reflections in an indicator of the distance of the object to the sensor. This system may be used in a vehicle to detect obstructions in its path of motion or a parking facility where it would determine the occupancy of each vehicle parking space. In the parking facility embodiment, information is reported to a central office computer which displays spaces which are not occupied and available for assignment. This system may also be used as a proximity detector for other applications, for example, to automatically detect pedestrians approaching a traffic control signal at a street corner or to detect the proximity of any object to another.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The field of the invention is the transmission and receipt of short 
electrical signals for the detection of objects in a zone. More 
particularly, the invention is related to such a detector which has a 
transmitter that employs a step-recovery-diode (SRD) impulse generator and 
a single antenna for both the transmitter and the receiver, wherein a 
positive-intrinsic-negative (PIN) diode is used to control switching of 
the antenna. A preferred application of the invention is to detect the 
location of automobiles in parking stalls in a parking ramp. 
DESCRIPTION OF THE BACKGROUND 
The detection of the presence of a number of objects in a predefined area 
which are capable of reflecting electromagnetic waves has been 
accomplished at short ranges by devices which transmit and receive 
electromagnetic waves over a surface wave transmission line. Such a system 
is shown in U.S. Pat. No. 4,394,640, issued Jul. 19, 1983 in the name of 
Gerald Ross and entitled "Safe Merging System Using Short Pulse Signal 
Reflecting." Switches and a duplexer are used to transition between the 
transmission and receiving stages of the system. 
A comb spectral generator is an electronic device which generates a 
harmonic frequency signal series for a line spectrum of frequencies that 
may extend well into the upper GHz ranges. Comb spectral generation may be 
achieved by use of a snap-back, or step-recovery-diode, (SRD) which is 
biased to a predetermined operating point and has an input coupled to a 
periodic electrical signal generator, such as a sinewave generator. The 
SRD is a microwave diode which has steep doping profiles and narrow 
junctions to maximize charge-storage effects. This leads to fast recovery 
of injected charge and results in a typical transition period of a few 
tenths of a nanosecond and the exceptionally efficient production of 
harmonics of the frequency of the input signal generator. Present day 
versions of the SRD have a relatively low breakdown voltage which limits 
the power capability of the diode, thus making communication systems based 
on the SRD ideal for limited range applications. 
The use of an SRD for harmonic generation is a known technique, as shown in 
U.S. Pat. No. 3,777,271, issued Dec. 4, 1973, in the name of Frederick 
John Telewski, and entitled "Generation of Microwave Frequency Combs with 
Narrow Line Spacing." In the Telewski patent prior art is discussed in 
which a microwave comb generator is constructed by utilizing an SRD and 
associated circuit elements which are driven by a sinewave input 
generator. Such comb generators have been confined to the generation of 
signals that are transmitted over coaxial cables to test the cable or 
receivers that are coupled to the cable. The short range of 
electromagnetic pulses in air has generally been considered as limiting 
their usefulness. 
In operation, SRD's are alternately forward and then reversed biased. When 
they are forward-biased current flows through the diode junction. When 
they are reversed-biased current is swept from the diode junction by 
minority charge carriers. This action produces a transient pulse which is 
very high in harmonics. An SRD produces frequency components which are 
integral multiples of the input signal frequency such that when these 
frequency components are plotted in an amplitude versus frequency plot, 
sharply defined lines representing these frequency components are formed. 
As noted in the Telewski patent, SRD multipliers operate well above 
frequencies above 10 MHz, although signal strength in general decreases 
with increasing harmonic frequency order and becomes too low to be useful 
at about 150-200 lines. It is also noted that the minimum obtainable comb 
line spacing is about one-half to one percent of the frequency of the 
uppermost useful comb line. 
The Telewski patent describes a device in which the conventional comb SRD 
generator is driven simultaneously by two or more signals of different 
frequencies to produce two or more different combs with line spacings that 
correspond to the respective drive signal frequencies. Since the comb 
generator is inherently a non-linear device, it produces the 
intermodulation products of the different combs which results in a 
composite comb that consists of lines that are spaced by the difference 
between the frequencies of the two driving signals. 
Step-recovery diodes, or impulse train generators, are available from 
Hewlett Packard which may be driven at various drive frequencies of 100, 
250, 500 and 1000 MHz. These impulse train generators generate useful 
power at harmonics through 18 GHz. Some types of SRD's require an external 
bias, but the Hewlett Packard models designated by them as 33002A/B, 
33003A/B, 33004A/B and 33005C/D are self-biased. Either an externally 
biased SRD or a self-biased SRD may be employed in the present invention. 
U.S. Pat. No. 3,806,811 entitled "Multiple Carrier Phase Modulated Signal 
Generating Apparatus," issued Apr. 23, 1974 to Wallace T. Thompson, 
employs an SRD in which a "pump" signal is phase-modulated and is used to 
drive the SRD so that all of the spectral lines of the comb have the same 
modulation on them. The addition of phase-modulation of the SRD is a 
feature which may be incorporated into the present invention for various 
applications, if desired. 
High speed switches are also required in the present invention. One type of 
suitable switch is the "positive-intrinsic-negative diode" or PIN diode. 
These devices have a region of intrinsic semiconductor material (equal 
hole and election charge carriers) intermediate P-type and N-type regions. 
When forward bias is applied across the intrinsic region, the diode 
resistance drops very fast allowing PIN diodes to be used as high speed 
switches. Through the application of appropriate voltages, these diodes 
may be made to conduct or inhibit the conduction of r.f. signals. PIN 
switches have a number of desirable features, including that they are 
broadband devices that are fast switching and have high isolation and 
ultra-low power consumption. Consequently, they have been used in a number 
of applications where advantage may be taken of these characteristics. 
Other types of high speed switches that may be employed in the invention 
are Field Effect Transistor (FET) switches, particularly Gallium Arsenide 
switches. 
PIN diodes are also useful in the present invention to provide, if desired, 
amplitude-limiting and phase-modulation of the comb lines. 
U.S. Pat. No. 4,623,856, entitled "Incrementally Tuned R. F. Filter Having 
PIN Diode Switched Lines," issued Nov. 18, 1986 to Robert H. Bickley, at 
al., describes an r.f. filter in which the filter frequency is adjusted 
using PIN diodes. 
U.S. Pat. No. 4,723,306, issued Feb. 2, 1988, entitled "Wide Band 
Transmitter for Short Electro-Magnetic Waves," issued in the name of 
Helmut Fuenfgelder, et al., describes a transmitter in which PIN diodes 
are used as electronic switches. 
U.S. Pat. No. 4,342,008, entitled "Switched Tunable Frequency Multiplier," 
issued Jul. 27, 1982, in the name of Robert E. Hewitt, describes the use 
of a PIN diode for connecting the output coupling loop to a YIG tuned 
frequency multiplier-to-ground. 
U.S. Pat. No. 5,115,215, entitled "PIN Diode Activation Method and 
Apparatus," issued May 19, 1992, to Floyd A. Koontz, shows a device in 
which PIN diodes can be selectively made conductive to shunt the input 
terminals of a device so that the device is thereby effectively bypassed. 
U.S. Pat. No. 5,369,373, issued Nov. 29, 1994, entitled "Comb Data 
Generation" in the names of George F. Nelson and David P. Andersen, and 
now assigned to the assignor of the present invention, shows a comb data 
generator and transmitter which utilizes a step recovery diode (SRD) for 
comb data generator and a PIN diode for signal limiting. 
PIN diode fast-switching switches are commercially available. They may be 
provided also by over-driving variable gain, wideband amplifiers that 
utilize PIN diodes. Representative amplifiers of this type are sold by 
Hewlett Packard under the designations HAMP 4001/4002. 
SUMMARY OF THE INVENTION 
A system having one or more transceivers is used for short range 
electromagnetic proximity detection of objects in one or more areas by 
associating at least one transceiver with each area. Each of the 
transceivers has: 
(a) a comb signal generator that generates line spectrum electromagnetic 
signals in a sequence of groups; 
(b) an antenna coupled to said comb signal generator which transmits the 
electromagnetic signals and receives at least a portion of the 
electromagnetic signals which are reflected from an object; 
(c) a receiver coupled to said antenna means for receiving the line 
spectrum electromagnetic signals from the antenna; and 
(d) a timer coupled to said comb signal generator and to the receiver means 
which determines the proximity distance of an object which reflects the 
transmitted electromagnetic signals by establishing the time that has 
elapsed between the transmission of one group of electromagnetic signals 
and the return of reflected electromagnetic signals from that same group 
of said electromagnetic signals to the antenna by initiating a timing 
action when that group of electromagnetic signals is transmitted from the 
antenna, and by terminating the timing action when that group of 
electromagnetic signals is received by the antenna. 
An excitation source may be integral to each transceiver or may be coupled 
to each of the comb generators to supply a periodic excitation signal. If 
a shared excitation source is employed, the receiver of each of the 
transceivers identifies itself.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention involves the transmission and receipt of electromagnetic 
waves for short range proximity detection. The transmitter portion of the 
invention utilizes the comb generator capabilities of an SRD to generate 
pulsed r.f. signals in response to a very low frequency excitations 
signal. The excitations signal may be sinusoidal or, for example, a 
frequency of 1 MHz or greater. The sinusoidal excitation signal is used as 
a forward bias and produces a current through the SRD. During one-half of 
the cycle of the sinusoidal excitations signal, the forward current 
through the SRD is used to produce a stored charge. During the second 
half-cycle of the excitation signal, the SRD junction is reversed biased. 
As the reverse bias is applied, the junction current stops flowing and the 
junction Is reversed biased. In this portion cycle the minority charge 
carriers in the junction suddenly sweep out the stored charge and produce 
a pico-seconds impulse. This impulse is composed of an extremely large 
number of harmonics of the original excitation signal frequency, which in 
the frequency domain looks like a comb with many harmonics of the input 
excitation signal which extend over many GHz of the spectrum. 
Any of the harmonic frequencies can be selected by an r.f. filter and used 
as an r.f. source with repetition rate controlled by the lower input 
frequency. Output harmonics up to and beyond 18 GHz and output r.f. powers 
up to 10 Watts are available. Different circuit approaches may be used, 
for example, series or shunt, depending on which is the simplest to 
implement in the circuit board for activating the SRD. A series 
implementation couples the pump frequency into the SRD through a 
capacitor. The SRD d.c. bias circuit comprised of an isolation resistor 
(approximately 100 to 1000 Ohms) connected to the SRD in parallel with the 
pump frequency isolation capacitor. Bias current flows through the SRD to 
a termination resistor of 50 Ohms. An L-section filter provides a 
high-pass function out of the SRD to the antenna. This high-pass filter 
permits SRD generated very high frequencies (impulses) to couple to the 
antenna while reflecting the pump frequency back into the SRD. Suitable 
decoupling (bypass) capacitors will be used in the circuit to the bias 
power supply. 
The oscillator may be a sinewave or square-wave or pulse, generator. Each 
of these produce an acceptable pump energy for the impulse generation 
circuit of this sensor. The choice of pump and impulse frequencies are 
determined by the characteristics of the SRD. The transition time 
(t.sub.t) of the SRD is the reciprocal of the highest impulse frequency 
limit for a given diode. For example, the Hewlett Packard SRD Model 
5082-0180 has a transition time of 225 pico-seconds (225.10.sup.-12 
seconds), or at a 4.444 GHz maximum sinusoidal frequency for efficient 
operation. The Minority Carrier Lifetime (.tau.) of the SRD establishes 
the lowest pump frequency for efficient operation. For example, 
.tau.=1/100.sub.ns or 10 MHz. 
The invention may be incorporated into any type of moving vehicle or device 
to enable the operator or a computer to detect the presence of a person, 
an animal or an object in the proximity of the vehicle or device. FIG. 1 
shows the implementation of the invention in which a transceiver 5 may be 
mounted on an automobile, or other vehicle, to detect if any pedestrian or 
object is in the path of the car when it is being backed up. The proximity 
detection transceiver 12 may be mounted in the trunk of the car to enable 
the generated pulses to be transmitted from this area by an antenna 6. If 
a person or object 8 intercepts the transmitted electromagnetic wave 
pulses 7, they will be reflected back to the antenna 6 to enable the 
transceiver to signal that the path of the car is obstructed. 
A schematic drawing of the transceiver 5 of the present invention is shown 
in FIG. 2. The SRD diode 12 is coupled to the excitation source 14, which 
may be provided by a number of sources, including crystal controlled 
stable oscillators, integrated circuit function generators and Voltage 
Controlled Oscillators (VCO's). The magnitude of the output of the 
excitation source may be controlled by the variable resistor 17. The 
excitation source 14 is powered by the voltage V at the terminal 19, which 
is also coupled to other components of the circuit. The variable resistor 
21 is coupled between the voltage terminal and the choke 18 and the 
capacitor 23. The capacitor 23, which is coupled across the choke 18, is 
used to adjust the d.c. bias applied to the SRD to optimize the comb 
generation efficiency for the amplitude of the drive voltage from the 
excitation source 14. 
A radio frequency choke 18 is coupled to the anode of the SRD so that high 
frequency signals generated by the SRD will be reflected back to the SRD 
thereby increasing its efficiency. An example of the power efficiency of a 
typical SRD can be estimated. With an input power of 500 milliWatts at 100 
MHz repetition frequency, the output power is at least 1 milliwatt at all 
harmonics to 2.5 GHz. This means that for 500 mW of input power at 100 
MHz, 1 mW of output power is available at the 1 GHz frequency where 1 GHz 
is an arbitrary choice of a harmonic frequency. The d.c. power supply 
voltage and return signal is provided on a suitable r.f. cable, such as 
coaxial cable, or a twisted pair on which the system receives 100 MHz 
input pulses on. 
The impulse from the SRD 12 is coupled to the antenna 20 through a 
capacitor 22 and a stripline filter 24. This provides isolation of the DC 
bias for the SRD 12 from the antenna. The stripline filter further may be 
etched in the printed circuit card that carries the other components of 
the transceiver. It acts as a harmonic filter and selects the particular 
frequency of transmission. The output is represented by a short r.f. pulse 
which is a harmonic of the input excitation signal and is in the GHz 
range. This short pulse is propagated normal to the circuit and through 
space until it is dissipated or reflected from a target back into the 
antenna. 
Suitable fast-switching selection switches 26,27, which preferably are 
positive-intrinsic-negative PIN diodes or FET diodes, are coupled between 
the antenna and the input of the amplifier section 30 and between the 
input of the amplifier section 30 and ground, respectively. PIN diodes or 
"positive-intrinsic-negative" diodes have a region of intrinsic 
semiconductor material (equal hole and election charge carriers) 
intermediate P-type and N-type regions. When a forward bias is applied 
across the intrinsic region, the diode resistance drops across the 
intrinsic region, the diode resistance drops very fast allowing PIN diodes 
to be used as high speed switches. Through the application of appropriate 
voltages, these diodes may be made to conduct or inhibit the conduction of 
r.f. signals. PIN switches have a number of desirable features, including 
that they are broadband devices that are fast switching and have high 
isolation and ultra-low power consumption. Consequently, they have been 
used in a number of applications where advantage may be taken of these 
characteristics. 
The PIN switches 26 and 27 carry a DC bias circuit which is supplied from 
excitation source 14. The excitation source provides a turn-off d.c. bias 
to the PIN switches 26, 27 on the line 13 to effectively isolate the 
receiver from the transmit energy during the transmit impulse. The SRD 12 
when in its "off" condition between impulses presents a forward drop of 
approximately 0.5 volts for isolation of the impulse transmitter from the 
received signal from the object being detected. With this isolation and 
the diode switches 26, 27, no significant transmit energy will enter the 
receiver and thus no recovery time problems are experienced from 
saturation of the receiver. 
The receiver portion of the invention is shown at the right-hand side of 
FIG. 2. The antenna 20 is used for both transmission and reception of the 
electromagnetic signal. The antenna 20 is coupled to the input of the 
first of the amplifiers 30. The output of the last of these amplifiers is 
connected to a zero-bias Schottky diode 32. The Schottky diode detects the 
reflected r.f. pulse from a target which results in a baseband pulse. The 
estimated return signal into the diode is 10 micro-Watts. With a voltage 
sensitivity of 8 mV per micro-Watt, and a 50% efficiency at 1 GHz, it is 
expected that the resulting signal voltage from the diode will be 40 mV. 
to convert this to a logic signal, it is necessary to amplify by 
approximately 100 times through the amplifier 30. At this point the 
reflected impulse from the object being detected has been reconstructed 
and detected to a pulse whose position in time corresponds to its distance 
from the antenna 20. 
A binary one-shot 34 has one input coupled to the output of the amplifier 
section 30 and the Schottky diode 32. Another input of the one-shot 34 is 
coupled to the stripline filter 24 on the line 35 which is coupled to the 
filter by a directional coupler 33, or signal sampler. This coupler 
samples the transmit pulse and starts the one-shot pulse running. The 
coupler samples the signal by a printed circuit line being in proximity to 
a printed circuit line carrying the transmit pulse current. Some small 
amount of energy from the transmit line is induced into the sampling line. 
This provides a reference "start pulse" signal to the one-shot. A transmit 
pulse drives the flip-flop into a first state until the detected pulse 
from the receiver section triggers it to the opposite state. This 
generates a pulse whose width corresponds to the distance to the object 
from the transmit/receive antenna. This pulse occurs once every cycle of 
the 10 MHz pump frequency and continues until it is shut off due to the 
reflected electromagnetic impulse. 
The pulse from the one-shot 34 is integrated in a resistor 36 and capacitor 
37 RC combination to produce a d.c. voltage on the output line 25 whose 
amplitude is a function of the detected object's distance from the 
antenna. A threshold detector (not shown) may be coupled to the output 
line 25 to determine when the distance of the detected object corresponds 
to one or more preset voltage amplitudes, each of which may represent a 
unique distance from the antenna and each of which can be used to control 
some warning function. 
One application of the present invention is monitoring and improving the 
efficiency of congested automobile parking facilities whereby cars can be 
accommodated more efficiently within the facility so that lost time due to 
driving around looking for an empty stall can be minimized. It also 
permits management personnel to fill a parking slot as soon as it is 
available. The location of empty stalls may be displayed and cars may be 
directed to a specific stall within the facility. A separate transceiver 
may be associated with each stall of the parking ramp. FIG. 3 shows one of 
the printed circuit patch transmitting and receiving antennas 20 which are 
located in the wall 41 of each parking stall for this embodiment. In the 
parking facility embodiment, a low frequency (LF) sinewave generator, for 
example, 100 MHz, may be used to supply the junction charging current of 
the SRD of a transmitter in each parking stall, as shown in FIG. 4. The 
transceivers 40-50 of FIG. 4 represent transceivers in each of n parking 
stalls. 
Each cycle of the LF generator produces a pulsed r.f. output from the SRD 
of each transmitter portion of the transceivers when the junction becomes 
reversed biased by the sinewave from the LF generator. The pulsed r.f. 
signal is transmitted by radiating antenna outward and towards the 
position which a vehicle would occupy if it were in the parking slot. The 
r.f. energy pulse will reflect off a vehicle in the stall and return to 
the antenna. While pulses are received back, it is known that a vehicle 
occupies the parking slot. If no reflected signal is detected it is known 
that no vehicle occupies the parking slot. 
The presence or absence of a car is determined by time division 
multiplexing along the two wire bus 31. This is controlled by the 100 MHz 
gate pulse generator 51. 100 MHz allows time division multiplexing 
resolution of 3 meters, (approximately one car space), 3 meter resolution 
is for 1 wavelength, while a 1/2 wavelength is minimum requirement. The 
detection pulse is sent back to the distribution network 49 on the 
shielded coax or twisted pair bus 31. Power is also supplied on the bus 
31. The return impulse may be of an arbitrary length, determined by the 
width and number of parking stalls. The one-shot 34 produces an output of 
a relatively long duration relative to the pulse received narrow input 
impulse. This pulse width is long enough to be a relatively low frequency 
pulse, and it will propagate back to the controller with minimal losses 
over inexpensive cable. If the original receiver impulse were propagated 
back to the controller, the bus would have to be capable of handling 
nano-second wide pulses of an extremely high bandwidth with little 
degradation. However, this would require expensive cables. 
The transceiver may be located on a wall, adjacent which a car will be 
parked, or, if the cars are in an open space but under a roof, then the 
transceiver may be mounted on the ceiling. The usable range is about 5 to 
10 feet. The master distribution network 57, (or networks if more than one 
is used), is wired to a central computer at the parking facility office 
(not shown), which has a display that may be viewed by an operator. The 
distribution network may be placed in positions which permit all of the 
required stalls to be monitored. The number of networks to be used will be 
determined by the propagation of the timing or clocking pulses from the LF 
generator 47 through the network 49. The gated LF generator 47 of the 
embodiment of FIG. 4 replaces the excitation source 14 of the individual 
unit transceiver of FIG. 2, and is coupled to the transmitter portions of 
the transceivers over the lines 43. The distribution network is connected 
to the receiver portions of the transceivers over the lines 45. The low 
frequency (LF) (i.e., low frequency relative to the radiated frequency) 
generator 47 is a pulse or sinewave generator that functions as a clock 
for the system. The LF generator preferably produces a pulse, although it 
could be a recycle of a sine-wave at a frequency of approximately 100 Mhz, 
so the propagation time of a single event (pulse or sinewave) is no longer 
than a standard car stall width to obtain the proper individual stall 
resolution. In addition, the Minority Carrier Lifetime determines the 
minimum frequency of this LF generator for proper operation of the SRD. 
The LF generator or clock is pulsed or gated at intervals by the gate pulse 
generator which permits one event to propagate the full length of the 
series of parking stalls and to be transmitted on antennas 39.sub.a 
-39.sub.f, and for the reflections corresponding to cars to return to the 
clock and receiver electronics through the antennas 39.sub.a -39.sub.f. 
The receiver gating and detection circuit 53 supplies the individual 
return signals in the transceivers 40-50 to the central computer (not 
shown) in order to determine which parking stalls are filled and not 
filled, so that no unnecessary degradation will occur due to long cable 
path losses to the transceivers 40-50. 
The computer may provide statistical information on each parking space, 
such as the time of occupancy, the time of departure, the average duration 
of a stay, etc. In addition, each stall of the facility may use the wiring 
bus to incorporate emergency buttons which automatically make a call for 
assistance and inform law enforcement personnel where in the facility to 
respond. 
The choice of the lowest possible frequency for the LF generator is 
determined by the width of a typical parking stall (about 3 meters), and 
the time required for the sinewave to propagate through the cable to the 
next stall. This requires at least 83 MHz for the LF generator frequency. 
Each of the stall transducers is wired in parallel to the same cable from 
the LF generator. The addressing of a particular stall is determined by 
the delay time from the LF generator corresponding to the propagation time 
to that stall after the beginning of the LF cycle. If a car is parked in 
every stall, pulses would be received at the LF generator source every 12 
nsec, a non-return-to-zero (NRZ) approach for the pulses. That means that 
if every stall is occupied, the returning pulses would produce a high 
level continuously and only drop to zero if a parking stall was empty. 
This produces a very low data rate when the most critical evaluation of 
the facilities is necessary and the computer control does not have to 
process data for parking slots which are already filled.