Miniature pocket-worn motion sensor with DCF77 clock


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Type: Semester thesis
Students: Mario Schweizer
Advisor: Daniel Roggen, Marc Bächlin
Project: DAPHNet

Recording motion over long periods of time can provide useful information about the evolution of the user's state (e.g. change in user behavior) useful e.g. for medical applications.

Within this project a battery powered motion recorder board that is able to acquire movement data of a person from an on board accelerometer and store them on a multimedia card for up to seven days has been develop.

The block diagram of the system is shown in the following figure. The system consists of an Atmel Atmega64L microcontroller with its interfaces to the accelerometer ADXL330, the MMC, some output LEDs and input buttons. The data transmission to the MMC goes over SPI and the analog signals from the ADXL330 accelerometer are connected to the analog-to-digital converter inputs of the controller. A peripheral interface to USB over a UART-USB transceiver is realized. An external 32786 Hz quartz crystal is used to set up a clock as a reference for timers.

Furthermore the motion recorder sets up the base to use and work with the "DCF77" radio clock signal from Frankfurt, Germany. A DCF77 clock is connected to an interrupt. This timestamp from the Central European Time (CET) can be applied to improve time triggered applications concerning accuracy and on the other hand to synchronize several sensor nodes together in a further work.
The powering is realized over 2 AA batteries or USB depending if the board is connected to a PC.

Block schematic of the hardware
Block schematic of the hardware

This projects motion recorder has a power consumption of about 31 mW as a maximum. The size (with integrated DCF77 module and batteries) is 58 times 50 times 18 millimeters and the weight is 87 gram.

DCF-Board with dimensions
DCF-Board with dimensions

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