Personal event monitor with linear omnidirectional response

The invention described herein may be manufactured, used, and licensed by or for the U.S. Government for government purposes.

The present invention relates to a monitor for alerting those responsible for the care of individuals to the onset of anomalous physical activity. More particularly the invention relates to a monitor responsive to anomalous activity while distinguishing the same from casual activity associated with normal quiet daytime activities.

The problem of alerting caregivers to the onset of distress or to a medical disorder such as a seizure has always existed. The specific problem is to alert those responsible for the care of individuals to the onset of anomalous physical activity and to distinguish this type of activity from normal casual motion. The anomalous physical activity ranges from that associated from distress while sleeping to that associated with certain medical disorders such as epilepsy. The individual may suffer from a condition where prompt detection and enhanced accuracy in documentation of medical episodes is required to assist in improving care. The device would have great benefit for parents, teachers and other caregivers who cannot continuously and visually monitor the individual in their care. It would be a great benefit of having a monitor useful in the home or in group environments such as a classroom of students with special needs.

Caregivers of individuals afflicted with certain illnesses or conditions such as epilepsy are required to closely and continuously monitor those under their care. This is also the case for parents concerned with restless activity in their children during sleeping hours. U.S. Pat. Nos. 5,523,742, 5,610,590, and 5,879,309 describe monitors developed to address these problems.

Continuous visual monitoring of individuals is usually impossible and periodic monitoring is often insufficient. The monitors described in U.S. Pat. Nos. 5,523,742 and 5,610,590 provide relief during sleeping hours, but are inappropriate for reliably discriminating seizures from the casual activity associated with normal quiet daytime activities. The use of either of these monitors would produce an unacceptably high false alarm rate resulting in undue anxiety and, perhaps, even a loss of faith in the device. The monitor described in U.S. Pat. No. 5,879,309 employs a custom made sensor that is not 5 commercially available and is costly to produce. The sensor used in U.S. Pat. No. 5,879,309 also has a highly nonlinear response that does not allow optimal adjustments of monitor sensitivity. It also lacks the sensitivity to respond to low amplitude motion that is characteristic of certain types of disorders.

Accordingly, one object of the present invention is to provide a sensor that is sensitive to respond to low amplitude motion.

Another object of this invention is to provide a monitor device that minimizes false alarms due to reading, walking or related casual activities.

A specific object of this invention is to provide a monitor device that uses a new, commercially available sensor having a linear, omnidirectional response with high sensitivity, ultra-low power consumption, low cost, and small size.

Other objects will appear hereinafter.

It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner.

Specifically, the present invention comprises a monitor device for monitoring the activity of an individual to provide an alarm for anomalous by the individual. The device incorporates a new, commercially available sensor having desired features of linear, omnidirectional response, high sensitivity, ultra-low power consumption, low cost and small size.

Also included in the monitor device of the present invention is interface electronics for receiving the voltage outputs and buffering the voltage outputs to generate a first reference voltage for each axis of the accelerometer. Each voltage output is amplified and compared to a first reference voltage to produce a digital signal.

A microcontroller compares the digital signal to an adjustable second reference voltage. The microcontroller discriminates between normal activity and anomalous activity by identifying sensor activity within sequences of preselected time intervals and sending an alarm signal upon detection of the anomalous activity. An alarm for receiving the alarm signal and signaling an alarm completes the device.

For a more complete understanding of the invention, reference is hereby made to the drawings, in which:

FIG. 1 is a circuit diagram illustrating the monitor electronics connected to the sensor; and

FIG. 2 is a circuit diagram of the electronics in conjunction with the microprocessor and alarm.

The present invention has many advantages. It provides a means for alerting caregivers to anomalous physical activity while minimizing false alarms due to reading, walking, or related casual activities. The monitor described in detail below is small, lightweight, portable, simple to use, and inexpensive to produce.

Two separate monitors have been developed under the present invention. One is for daytime use and one for use during the night. The monitors are conceptually identical, differing only in the type of alarm and power supply used. Shown in FIG. 1 is the electronics used in the daytime monitor. The monitor device employs a small, inexpensive, low-power, triaxial accelerometer, 1 , which is also designated U 3 in FIG. 1 . Accelerometer 1 is used to detect anomalous physical activity in an individual, where the anomalous activity may, for example, be associated with a seizure episode. Accelerometer 1 simultaneously measures acceleration in three orthogonal axes, labeled x, y and z. Upon activation by physical motion, the accelerometer 1 generates a voltage output V that provides a measure of the amplitude and frequency of the motion. A preferred accelerometer for use in the present invention is known as Accelerometer ACH-04-08-05, manufactured by Measurement Specialties Incorporated, located in Norristown, Penn. The ACH-04-08-05 operates over a temperature range of from −40° C. to 85° C., much larger than the normal environment for humans, and nominal sensitivity will typically changes less than 2 dB over that range of temperature.

The accelerometer outputs are buffered by internal JFET's which require external biasing. The interface electronics, shown in dashed oval 2 , are identical for all three axes, so only the y-axis electronics will be described. The JFETs are biased using a source-resistor bias in a source-follower configuration. V ref1 is generated using a buffered voltage divider, shown in FIG. 1 as 3 generally, and is set to 1.64 VDC using a 3 VDC supply. A 0.6 DC voltage drop across the JFET gate results in approximately 2.24 volts at the JFET's source. The 2.24 volts across R 1 to ground biases the JFET using 2.24 μA of current. Therefore, 7.72 μA are required to bias all three axes of accelerometer 1 . C 1 blocks the DC bias offset and the R 2 forms a high pass filter with a pole at 0.16 Hz. R 3 and R 4 define the system gain of 294. R 4 and C 3 form another high pass filter with a pole at 2.12 Hz and minimize DC offset at the output by setting the DC gain at 1.

The amplified signal is then compared with a reference voltage V ref2 that is set by a user adjustable buffered divider shown in FIG. 1 generally as 4 . R 30 and R 31 provide 30 mv of hysteresis. The output of the comparator circuit is a digital signal that is analyzed by the microprocessor, shown generally as 5 and also as U 4 in FIG. 2 . The microcontroller discriminates normal casual activity from anomalous activity by identifying sensor activity or lack of activity within sequences of preselected time intervals. The hex rotary switch 6 in FIG. 2 sets the time interval and total sampling time used by microprocessor 5 . Switch settings are given below in Table I.

The values of V ref2 sets the sensitivity of the monitor device of this invention. This combination of user selectable parameters permit an unlimited range of settings to accommodate a wide variety of conditions. If the microprocessor 5 detects anomalous activity or if the battery monitor, shown as 7 in FIG. 2, signals a low-battery condition, an alarm is activated. The listing of the microcode for microprocessor 5 of the monitor device are given below in Table II. This code is the microcode listing for the microprocessor used in the daytime implementation, using Microchip PIIC16LC585.

The alarm for the daytime monitor is an acoustic transducer 8 in FIG. 2 . The audible signal for anomalous physical activity is distinct from that for a low battery condition. If unusual activity is detected, switch 6 setting, shown in FIG. 2, determines whether the alarm is active until the monitor is manually reset, or if it automatically resets after a preselected period of time. Power for the daytime monitor is derived from a standard coin cell battery, shown in FIG. 2 as 3VDC. Battery life is difficult to estimate since the oscillator circuitry of the processor is disabled to conserve power until activity is detected. Battery life is also a function of the number of alarms. A battery life of approximately 2700 hours is expected using a Panasonic® 2032 coin cell lithium battery.

The nighttime monitor transmits a FCC compliant radio frequency signal to a remote, compatible receiver when the alarm criteria are satisfied. The receiver then activates the desired alarm mechanism, such as a radio, lamp or buzzer. The signal is retransmitted periodically until the monitor is manually reset. In addition to the remote alarm, a LED on the monitor continuously flashes at a rate that indicates if the alarm is the result of a potential seizure or low battery.

Power for the nighttime monitor is derived from a standard miniature 12 volt battery. A Maxim MAX874 low-dropout precision voltage reference is utilized to supply 4 volts to the monitor circuitry.

The daytime monitor is small and easily attached to an individual. The power switch is preferably recessed on the side of the monitor and is the only means of resetting the monitor. There are internal adjustments that are available to permit optimization of the monitor response for each application. In an effort to conserve battery life, no indicator is used in the daytime mode to notify the user that the device is operational. Instead, the monitor beeps twice in the daytime mode or the LED flashes twice for the nighttime mode, when it is first turned on. This notifies the user that the battery voltage is adequate and the unit is operating properly. If this signal does not occur, the battery needs to be replaced. Obviously, if the signal does not occur when a fresh battery is installed, the unit is malfunctioning and should not be used.

While particular embodiments of the present invention have been illustrated and described herein, it is not intended that these illustrations and descriptions limit the invention. Changes and modifications may be made herein without departing from the scope and spirit of the following claims.