Considered Concepts

This section will focus on the determination of the most suitable sensor to meet the design metrics. The remaining two components of the design are the base unit (information hub) and the remote receiver for the parents room. Although these components are important, their basic designs can easily be modeled after pre existing devices. The concept design phase was focused primarily on finding the best way to gather information about the infants' breathing patterns and send said information to the base unit.

Secondary research was first conducted to identify a range of sensors that could be used for the design. Pulse oximetry, respiratory impedance, CO₂ detection, heart rate detection, movement monitors and thermal imaging were the found results from this research.

Pulse oximetry is a method for determining the oxygen saturation of blood. It functions by using three different LED's along with signal processing techniques. Two LED's - One which is Red (~660nm) and one which is Infrared(~910nm) are shown through a site with oxygenated blood from arteries. The blood will absorb differing amounts of light depending on it's oxygenation at the time. Concerns with using pulse oximetry at home were expressed by the staff of the Gritman Medical Sleep Center as the following. Many false alarms are due to this sensor merely because it must attach to the infants' foot and therefore is often pulled out of place or kicked off completely.

A respiratory impedance sensor detects the changing impedance of the thoractic cavity of the chest. This impedance is a function of volume of air in the chest. In general, cavity impedance will increase during inhalation and decrease during exhalation, leading to a cyclic pattern that is present during normal breathing. This principle could be used to determine when an infant stops breathing by employing a small high frequency (in the range of 50 kHz to 500 KHz) current between two surface electrodes to detect when the impedance stops changing, and thus breathing has stopped. The cost of the electrodes for this monitoring would be around $70. The invasiveness of this sensor raises concern since electrical current would be conducted directly through the infant.

There are two methods of CO₂ detection, nondispersive infared (NDIR) and by chemical reaction. NDIR sensors measure the absorbtion of the characteristic wavelength of infared light as it passes through a light tube. As CO₂ filters into the tube, the specific change in measured absorbtion signifies the presence of CO₂. These sensors typically have sensitivities of 20-50 parts per million and usually cost between $100 and $1,000 in the (US). NIDR sensors are currently the most reliable method for detecting CO₂

Chemical sensors utilize thin layers of polymer or heteropolysiloxane to detect CO₂. Though smaller and size and power consumption, these sensors typically have a shorter life span than NDIR sensors. Unfortunately, each of these sensor types are limited to detect the presence of a gas and not the gas flow. If breathing stops there will be a delay period in which the sensor will continue to detect CO₂. Therefore, CO₂ sensors should not be relied upon to provide second-to-second updates on infant breathing patterns.

Heart rate monitors are typically used in two different applications; medical and recreational. Both of which utilize a different method of detecting the heart rate. For the purposes of the design, the heart rate of an infant can provide important information about breathing patterns.

The 12-lead electrocardiograph (ECG) monitor is the standard monitor used by hospitals around the world to measure a patient's heartbeat. These machines record from 12 different points of view the electrical activity that results when the muscle cells in the heart contract. The monitor then displays these readings as a waveform. Measuring from 12 different points of view makes these monitors very accurate because no single point on the heart will give the same reading as another. However, for a reliable, accurate monitor the price range varies from $1,500 to $5,000. A cheaper alternative also utilized in medical practice detects the heart beat by observing the change in impedance of the chest cavity similar to the respiratory impedance sensor. The same concern of conducting current through the infant is also present with this heart rate monitor.

The sensors used in recreation track the heart rate by monitoring the pulse. Heart rate monitors that use pulse meters measure the mechanical pulse of blood flow through the capillaries. These tend to be much less accurate than monitors using electrical activity. However, these monitors can be found in strapless forms (no chest-strap) and can be used in wristwatches and other attachment methods. Although generally cheap and effective, these monitors are designed for adults. This is a problem because a newborn infant's heart rate is much larger than that of an adult; 100-160 beats per minute for a newborn compared to 60-100 beats per minute for an adult. However, the same principles used in these devices can still be used in our device to monitor an infant's heart rate.

Three types of movement sensors were researched: motion sensors, strain gauges and weight scales. For the purposes of this design, the sensors would focus on the movement of the infants' chest as it breathes.

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 the inability to see motion going on under sheets or blankets so the infants' motion could be blocked by its beading, giving a false positive to the alarm.

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. 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 sensor approximately $5.00.

Weight Scales simply determine the force being applied by a certain object. 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. Sub categories include: spring, hydraulic or pneumatic and balance. The most appealing weight scale, because of size, price, accuracy (1/100 grams) 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.

Thermal imaging equipment is sensitive to only infared light which is emitted from any source of heat. The thermal imaging camera could be used to observe the motion of chest movements from the sleeping infant similar to the motion sensor. Although thermal imaging would reduce the issue of not being able to see through sheets, camera prices starting at $2,000 ultimately keep this sensor option from being cost effective.

The second phase of sensor determination was to conduct primary research on as many of the aforementioned sensor options as possible. Each sensor was to be tested for sensitivity when subjected to the expected conditions of monitor operation.

A new sensor concept was developed by the team around this time and was included in the second phase. This concept is based on using a chest strap integrated with a zig-zag pattern of wire which can act as a variable inductor. As the chest expands, the distance between the loops of wire will increase, resulting in a decrease of inductance. This variable inductance strap can be connected as a frequency determinant component within an oscillator circuit. A change in inductance (L₁) will translate to a change in generated frequency as seen in the following characteristic equation for a standard Colpitts oscillator circuit.