There are several system requirements that the proposed activity monitor must meet in order to satisfy the customer’s needs. First, the device must be able to record activity data consisting of the user’s acceleration and the ambient light in their environment, sampled at a rate of once per second when the user is active. The occurrence of this data will be tracked by a real time clock, and this information will be saved to the device’s internal flash memory, which must be large enough to store measurements for three consecutive 12 hour days. After data collection is completed, the user information will be transferred via USB in a format compatible with iCeNSA’s software. The device will be powered by a rechargeable poly lithium ion cell battery which will provide enough power to allow the device to operate for a minimum of 12 hours on a single charge; the battery will be recharged via USB. In order to achieve this battery lifetime, the device will enter a low-power “sleep” mode when activity falls below a predetermined threshold. When the device is active, both sensors and the memory device will be configured to operate with minimal power consumption while maintaining appropriate data accuracy. A microcontroller will be utilized to communicate and coordinate the multiple subsystems located within the device.
The mechanical constraints on the monitor consist of the device’s size, weight, and durability. The device must be small and light enough to be wearable on the user’s body, ideally the arm or wrist, without inhibiting the user’s normal activity. As such, the final device should be roughly the size of a credit card. Due to the physical nature of the data being recorded, the device must also be able to withstand significant shock, be water resistant, and have a case that is composed of an environmentally friendly material.
In addition to meeting the above constraints, all device components should be of minimal cost, as the customer wishes to produce several devices. Therefore, the target price for the end produce is below $50. If possible, the construction process of the device should be simple enough that it can be assembled by students in South Bend schools, further integrating the community into this project.
The overall operation of this activity monitor is relatively straightforward. Two sensors—an accelerometer and a light sensor—will be used to track the user’s movement and environment, with the accelerometer calculating acceleration in the x, y, and z axes and the light sensor providing UV light readings in the test subject’s environment. This data will polled once a second when the user is active and time stamped using a real time clock. When the user’s activity is below a threshold value, the device components will be placed in sleep mode to conserve power. All information will be stored to flash memory via the microcontroller, and after data collection has occurred, the information stored on the device can be accessed through a USB connection. During this process, the microcontroller will transfer data to the FTDI chip, which acts as an interface between the UART transfer utilized by the microcontroller and the USB transfer used by the computer on the user end. Data can be accessed through the computer’s COM port, and this information will be formatted to the specifications of existing iCeNSA software. Powering the device is a polymer lithium ion battery, with the output voltage stepped down from 3.7 to 3.3 volts through a linear regulator. A durable, water-resistant case will cover all device components. As the primary users of these devices will be children, no external buttons will be on the device; the only alterations to the solid casing will be a light well for the light sensor and an LED to indicate when the device is charging.