# Improved Smart Pillow for Remote Health Care System
## 2 Materials and Methods
### Force-Pad Element Concept  
One of the significant tasks of the Remote Healthcare System is to determine if the user is in bed. In the initial version of the smart pillow without a force-pad, position of the head is found by judging the difference of values from three temperature sensors roughly, so when one of the values is higher and close to 37 \({}^{\circ}\)C, the head is near the sensor. This does not account for conditions where the room temperature is close to 37 \({}^{\circ}\)C, such that the position cannot be found. To detect the accurate position and duration of rest, a force-pad is introduced as a pressure sensor for the pillow. The force-pad is a \(4\times 10\) switch matrix, which is shown in Figure 5.  
Each node of the matrix is a Force Sensing Resistor (FSR) [22], which is a material whose resistance changes with an applied pressure, so without a force its resistance is very high, and if a force is applied its resistance decreases. The matrix can be presented as a schematic diagram in Figure 6. When a node is under appropriate pressure, its switch is set to on, which can be expressed as 1 in the force-pad, otherwise it is 0. From the distribution of 1 and 0 values, the position of the head in the pillow is specific to each person. Figure 4: The sectional diagram of the “Smart Magic Pillow”. Based on the principle of the force-pad, the physical interface of the force-pad will contain 14 lines, 10 columns and 4 rows, which is connected to the BT host inside the pillow. To integrate the signal from force-pad to the system, the area of the pad is divided and coded accordingly, which is shown in Figure 7.  
Figure 5: Picture of the force-pad from RoxiFSR Inc., Changzhou, China ([[http://www.roxifsr.com/productinfo/1018397.html](http://www.roxifsr.com/productinfo/1018397.html)]([http://www.roxifsr.com/productinfo/1018397.html](http://www.roxifsr.com/productinfo/1018397.html))). Figure 6: The schematic diagram of the force-pad. Figure 7: The area code of the force-pad. In total, there are 40 pressure switches on the pad, indexed from 1 to 40 and divided into 15 groups. Grouping is based on the principle that more than one switch could be triggered when the head on the pillow. For example, when switch 31 is under pressure, its neighbors 21, 22 and 32 could be under force as well, so they are grouped into group 11. So, for the same reason, the overlap in such a neighbored group can be expected. As a result of the group, only 4 bits are necessary to represent a certain group. When grouping, two obvious features are applied, one is that the third and fourth row of sensors in the far end of pillow are integrated into five groups altogether, not like row one or two, as they are integrated into five groups respectively, because the area of row 1 and 2 are the zone of comfort, which implies that the head is more frequently located on this zoned area than the far end of the pillow. The other feature is overlapping where groups share some common sensors, which is because a group of neighbors are typically under force, not just a single sensor. The overlapping principle makes the grouping more reasonable to determine. For example, without overlapping, if sensors 21, 22 and 31, 32 are grouped into 11, and 23, 24, 33, 34 into group 12, then if 22, 23, 32, 33 are triggered, it will be dilemma to code the group number. So, overlapping can smooth the issue. However, based on the current strategy it could still be problem, so if 21 to 23 and 31 to 33 are triggered, there is the issue of deciding the group segments. Therefore, there are two principles for the decision of grouping, the first is quality of trigger, then the former group is made as a reference. For the previous sample, the quality of trigger in group 11 and 12 are the same, so the first principle is not applicable. So, if the former group number is 11 or 12, the same number is recorded, otherwise one is chosen randomly as they are neighbors.