Movement MonitorsDuring normal behavior of any human, there is at least some movement due to many factors. One such factor is always present during life. Breathing moves the chest cavity and back in and out. This movement, however slight, in an infant can be measured by many means. Three of the most practical for our needs are motion sensors, strain gauges, and weight scales. 1. Motion Sensors use cameras to take successive picture of an area and then compare them to a reference image, pixel by pixel. If the number of pixels that are different exceeds some threshold, motion is occurring and the device will act on that motion. Processing is needed to distinguish the degree of change to determine whether motion is occurring because of false-positives associated with varying light, CCD dark currents and other minute factors. These devices are very cheap ranging from $20.00 and up and have a wide variety of sensitivity. These devices could be used to see the babies slight change in movement during inhaling and exhaling. The down side of these devices is that they have no ability to determine whether air is in fact getting into the lungs and the inability to see motion going on under sheets or blankets so the babies motion could be blocked by its beading, giving a false positive to the alarm. 2. Strain Gauges use the principal of electrical resistance to detect whether there is deformation on the object the strain gauge is attached hence sensing a force. Resistance is equal to (L*P)/A. L is the length in the conductor, A is the cross-sectional area and P is the electrical resistivity of the material. The strain gauge is composed of an insulating flexible backing, which supports a metallic foil pattern. The strain gauge is attached to the object using any suitable means. When a force strong enough to cause any deformation in the material is supplied, it deforms the strain gauge as well. This deformation of the strain gauge alters the cross-sectional area and length of the conductor in it causing an overall change in resistance. This change in resistance can be measured by many devices by simply running a very small current through it to determine the resistivity of the device. The down side of strain gauges are their change in resistance due to temperature R=Ro(1+aT), which is accurate at and around room temperature. For this equation a is the thermal resistance coefficient and T is the temperature in celcious. This problem can be accounted for using a strain gauge set up called a Wheatstone Bridge. Another possible problem that I can foresee would be the deterioration and possible fault of the strain gauge over time. This measuring device could be used around the chest of the infant, or preferably on the surface where the baby is sleeping. The price of this type of measuring device is very cheap around $5.00. Strain Gauge Trial. We experimented with this measuring device in a lab setting, applying the strain gauges to aluminum foil then to the out sides edges of an air mattress and slightly pressed on the air mattress. We then attached wires to the strain gauge and hooked it up to an ohm meter and we found a very slight change in resistance around one hundredth of an ohm. Next, we attached the gauges directly to the mattress and placed them in the center near the location of the force. This gave us a change in resistance around a few tenths of an ohm, a substantial improvement. This trial was very promising in that it gave us a measurable change when force was applied and was extremely steady, not fluctuating more than a hundredth of an ohm when there was no force applied. 3. Weight Scales simply determine the force being applied by a certain object. Force equal to m*a where m is the mass and a is the acceleration. When the baby inhales there will be a change in force on the mattress due to the acceleration of the chest cavity up and down. The forces can easily be measured by a variety of different weight scales having two main categories digital and analog and sub categories being spring, hydraulic or pneumatic and balance. The most appealing weight scale, because of size, price, accuracy and ease of use, would be an electronic mass balance scale. The price of the needed balance off the shelf would be $60.00 and up, depending on the exact quality and accuracy wanted. | 