Tamper-proof bracelet for home arrest system

A personnel monitoring apparatus has a transmitter for transmitting a data encoded signal. A strap is provided for coupling the transmitter to a person to be monitored. An emitter is attached to the transmitter for emitting an optical signal. A detector is attached to the transmitter for detecting the optical signal. An optical fiber is incorporated with the strap for optically coupling the emitter and the detector. A comparator compares the optical signal and a reference signal, and the result of the comparison sets the value of a preselected data bit in the data encoded signal.

FIELD OF THE INVENTION 
The present invention relates to personnel monitoring systems, and more 
particularly, to a tamper-proof transmitter apparatus which is utilizable 
in a home arrest system. 
BACKGROUND OF THE INVENTION 
The increase in prison population coupled with the shortage of adequate 
prison space has increased the interest in using personnel monitoring 
devices as part of a home arrest system. 
Various personnel monitoring devices have been proposed in the past. For 
example, U.S. Pat. No. 4,973,944 discloses a proximity device which is 
strapped by a band to a person's leg or wrist. The device transmits 
encoded RF signals which are received by a receiving station. When the 
device is taken out of range or the band is severed, the receiving station 
does not receive the transmitted signals, and an alert condition is 
initiated. The band is made of plastic and has electrically conductive 
material adhered to its outer surface such that a resistive DC circuit 
path exists when both ends of the band are brought into contact with one 
another. Likewise, U.S. Pat. No. 4,980,671 discloses a transmitter secured 
to the body of a person by a conductive mounting strap. An electronic 
latch outputs a signal indicative of whether or not the strap has been 
broken. 
These known personnel monitoring devices have the disadvantage that 
conductive elements are employed to maintain continuity through the strap. 
However, conductive elements may be effectively bypassed through the use 
of appropriate jumpers. Thus, one can remove the strap while bypassing the 
conductive element, thereby rendering the device ineffective for its 
intended purpose. 
Fiber optic technology has been used as a substitute for conductive 
elements in certain theft detection devices. For example, U.S. Pat. No. 
5,055,827 discloses an optical fiber attached to an appliance and 
connected to a control box. An alarm is activated if the detected light 
signal is attenuated, such as would occur if the optical fiber were bent 
or broken in an attempt to steal the appliance. However, no one has 
heretofore employed fiber optic technology in a personnel monitoring 
device to provide a signal indicative of a tamper condition. 
According to the present invention, fiber optic technology is utilized in a 
personnel monitoring device to avoid the disadvantage described above. 
SUMMARY OF THE INVENTION 
A tamper-proof personnel monitoring apparatus is disclosed. A housing is 
suited for attachment to the ankle of a person by a strap. The housing 
contains a transmitter, an optical emitter, an optical detector, and a 
comparator. The transmitter transmits data encoded signals to be received 
by a receiver so as to verify the location of the person. 
The strap has a fiber optic cable embedded therein. One end of the strap is 
secured to the housing so that the fiber optic cable is aligned with the 
emitter. The other end of the strap is secured to the housing so that the 
fiber optic cable is aligned with the detector. The emitter is 
periodically pulsed, and the detector is periodically sampled. The sampled 
signal is compared to a reference signal, and a preselected one of the 
data bits of the data encoded signal is controlled by the comparison. When 
the strap is bent or broken, the amount of light received by the detector 
is attenuated and the sampled signal drops below the reference signal, 
thus changing the state of the preselected bit.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred embodiment of a tamper-proof bracelet 10 according to the 
present invention is illustrated in FIG. 1. The tamper-proof bracelet 10 
is useful, for example, to secure a transmitter housing 12 to a person's 
ankle with strap 14 so that the location of the person may be monitored. 
The housing 12 is a sealed unit made from hard plastic and measuring 
approximately 2.5 inches wide by 2 inches long by 1 inch thick. Slots 22, 
24 are provided in the housing 12 for receiving the strap 14. Clear 
plastic windows 23, 25 are provided in the center of slots 22, 24, 
respectively. A printed circuit board 16 is mounted inside the housing 12. 
The circuit board 16 includes a transmitter (not shown) for transmitting 
signals, preferably RF signals, to a remote receiving station (not shown) 
according to known techniques. See, for example, U.S. Pat. No. 4,747,120. 
The housing 12 also has a battery compartment 17 provided therein. 
The circuit board 16 also has an optical emitter 18 and an optical detector 
20 mounted thereon. The optical emitter 18 is preferably a conventional 
light emitting diode of the type which emits infrared energy, for example, 
type SFH487-3. The emitter 18 is fixed in position proximate to the window 
23. The optical detector 20 is preferably a conventional photo diode, for 
example, type BPW-34. The detector 20 is fixed in position proximate to 
the window 25. 
Referring now to FIGS. 2A-2D, the strap 14 measures approximately 13 inches 
in length by 1.250 inches in width by 0.150 inches in thickness. The strap 
14 is preferably a cast urethane elastomer, but may be any type of 
flexible plastic or rubber. Embedded within the strap 14 along its center 
line is a fiber optic cable 26. The preferred fiber optic cable 26 is 
Mitsubishi Rayon SHV 4001 or its equivalent. At one end 14E of the strap 
14, a pair of holes 27 having a 0.150 inch diameter are provided for 
securing that end of the cable within slot 22. At the other end 14D of the 
cable 26, additional pairs of holes 28 are provided over some length of 
the cable, for example, twenty five pairs of holes are illustrated in FIG. 
2, so that the length of the strap may adjusted by cutting the strap so 
that it fits snugly around the ankle of the person to be monitored. FIG. 
2B-2C provide side and end views of the strap 14 showing dimensions of the 
preferred embodiment. FIG. 2D provides a detailed view of the end 14E of 
the strap showing a conical taper to facilitate optical alignment of the 
cable 26 with the emitter 18 through window 23. 
In order to ensure practical application of this tamper detection scheme, 
the strap material must be carefully constructed to ensure that tampers 
cannot be falsely generated by bending or pinching the strap. Seals 
disposed at the ends of the strap are designed to prevent fluids from 
penetrating the area between the fiber optic ends and the clear plastic 
windows. Otherwise, the presence of fluid may block the light path and 
cause false tamper signals to be transmitted. 
FIGS. 3 and 4 show a cross-section of the transmitter housing 12 to more 
clearly illustrate connection of the strap 14 to the housing 12. The 
emitter end 14E of strap 14 may be secured into slot 22 at the time of 
manufacture by affixing clip 30 to the housing 12 such that pins 31 on the 
clip 30 insert through holes 27. The detector end 14D of the strap 14 is 
cut to fit, then wrapped around the person's ankle and secured in slot 24 
by clip 32 such that pins 33 on clip 32 insert through holes 28. 
Once the strap 14 is secured in place, an optical path is created between 
the optical emitter 18 and the optical detector 20 via fiber optic cable 
26. The optical emitter 18 is mounted such that emitted energy will pass 
through the window 23 in slot 22. Infrared energy passing through the 
window enters the fiber optic cable 26 and passes through the cable to 
impinge on optical detector 20 through window 25. Thus, for normal 
operation, the emitter will emit a known quantity of infrared energy and 
the detector will detect a known quantity of infrared energy. 
If the strap is cut or otherwise disconnected, the optical circuit is 
broken and a controller sends a tamper indication as a part of the 
transmitted message. The receiver (not shown), located nearby within 
person's home, receives the transmitted message including a tamper 
indication and notifies a monitoring center via a telephone line that a 
tamper has occurred, thus prompting fast action on the part of the 
monitoring service. 
FIGS. 5A-5C are circuit diagrams for the preferred embodiment of a tamper 
proof bracelet 10 according to the present invention. Identification and 
values for conventional circuit elements, including IC chips, are provided 
on these drawings, and the interconnection of same is preferably as shown. 
In FIG. 5A, a microprocessor 40, such as the Motorola MC68HC705C8FB 
microprocessor, controls the transmission of data from the bracelet 10. 
Tamper status is provided to the microprocessor 40 by the TAMPER DET 
signal, generated by optical detector 20 as noted below. 
The optical emitter 18 includes an LED 42 that is cycled on and off by 
output PC3 from microprocessor 40. The optical detector 20 includes a 
photodiode 44 that receives infrared energy from the optical emitter 18 on 
a duty cycled basis and generates a signal V.sub.S. The comparator 46 
compares the signal V.sub.S received by the photodiode 44 to a reference 
signal V.sub.R derived from signal V.sub.X by resistors R34 and R35. If 
the received signal V.sub.S drops below the reference signal V.sub.R, then 
a tamper condition exists and the state of signal TAMPER DET changes. The 
microprocessor latches this input and sets a tamper status bit 
accordingly. 
Pins PC0 and PC1 of microprocessor 40 are alternately enabled to drive 
signals ANT1 and ANT2, respectively. Referring to FIG. 5B, signals ANT1 
and ANT2 thus alternately drive data transmissions via first antenna 50 
and second antenna 52. The microprocessor 40 is preferably preprogrammed 
to transmit a predetermined number of data bits in bursts approximately 
every 20 seconds, including the tamper status bit. Therefore, if a tamper 
condition of the bracelet 10 is detected, the state of the tamper status 
bit is changed and the remote receiving station will be alerted to the 
condition. 
The intensity of the light output from the emitter and the sensitivity of 
the detector should be chosen to allow easy detection of actual tampers 
while precluding false alarms. Also, the strap should be designed to be 
easily cut to length without polishing or finishing the ends of the fiber 
optic cable 26. 
In order to conserve transmitter battery life, illumination of the emitter 
and subsequent sampling of the detector can be done on a duty-cycled 
basis. In the preferred embodiment, the optical detector 20 is first 
sampled to verify a "dark" condition signal level. Then, the emitter 18 is 
turned on for ten microseconds and the optical detector 20 is sampled to 
verify a "light" condition signal level. Finally, the emitter 18 is turned 
off. Preferably, this sequence takes place every 500 milliseconds. 
A personnel monitoring device has been disclosed which utilizes an optical 
fiber to enhance the reliability of a tamper detection scheme for the 
device, thereby virtually eliminating the possibility of by-passing the 
tamper circuit. 
It should be understood that the invention is not intended to be limited by 
the specifics of the above-described embodiment, but rather defined by the 
accompanying claims.