Description of the Head-movement Sensor

Một phần của tài liệu Bayesian recursive algorithms for estimating free space and user intentions in a semi autonomous wheelchair with a stereoscopic camera system (Trang 240 - 245)

Chapter 6. Conclusion and Future Work

A. 4 Description of the Head-movement Sensor

As shown in Figure A.8, the ‘ZSTAR’ head movement sensor includes two transmitter and receiver boards. The transmitter is installed on the top of the user’s cap. The user can move the head so that the ZSTAR sensor provides head commands to control the wheelchair. Head signals are transferred to the receiver which is connected with the Apple Mac mini computer.

The demo consists of the two boards, which communicate over the RF medium utilizing the freely available software stack SMAC:

- Sensor Board (or remote board) containing the MMA7260QT 3-axis accelerometer, S08 family MC9S08QG8 8-bit microcontroller and the 2.4GHz RF chip MC13191 for wireless communication.

- USB stick, again with the MC13191 RF front-end, and the HC08 family MCHC908JW32 for the USB communication.

The ‘ZSTAR’ system is designed to allow visualization of key accelerometer applications the cost-effective 2.4 GHz wireless solution based on the MC13191 transceiver. The ZSTAR is a two-board design in which an MMA7260Q triple-axis accelerometer is controlled by an 8-bit MCU MC9S08QG8 and connected via a wireless link to a computer as shown in Figure A.9. The USB stick connects via the computer’s USB port. For the USB communication, a USB 2.0 Full-Speed 8-bit microcontroller MC68HC908JW32 is implemented as shown in Figure A.10. The tool contains all of the hardware, software.

The accelerometer measurements can be grouped into 6 sensing functions such as Fall, Tilt, Motion, Positioning, Shock and Vibration.

The RD3152MMA7260Q development tool offers robust wireless communication using the powerful, easy-to-use 2.4GHz frequency MC13191 transceiver. Minor changes can be made with pin to pin compatibility allowing implementation of the MC13192 and MC13193 for ZigBeeTM wireless applications.

Figure A.8: The ‘ZSTAR’ transmitter and receiver system of the head-movement

Figure A.9: ZSTAR sensor board software overview

Figure A.10: ZSTAR USB stick software overview

The ZSTAR design provides a small portable board with the capability to demonstrate and evaluate various accelerometer applications that accommodate the cost-effective low-power wireless connection. Benefits include:

- A multi-axis acceleration sensor that enables multiple detection situations for potential applications and includes a g-select feature with a 1.5g–10g range.

- The highly integrated and versatile MC9S08QG8 MCU includes a background debugging system and on-chip in-circuit emulation (ICE) with real-time bus capture—

providing single-wire debugging and emulation interface.

- The MC13191/MC13192/MC13193 2.4 GHz low-power transceivers provide a cost-effective solution for short-range data links and networks.

- The MC68HC908JW32 MCU is in the family of the enhanced M68HC08 CPU (CPU08) and is available with a variety of memory sizes and types, modules and package types.

- Battery life extension through low-power, low-current operation.

- Software support for easy development.

The features of the ZSTAR Board system

• Accelerometer: MMA7260Q (MMA7261Q/ MMA6270Q/ MMA6271Q/

MMA6280Q/ MMA6281Q)

- Package: Quad Flat No-Lead (QFN) 6 x 6 x 1.45 mm - Power: Low Voltage 2.2V–3.6V

- Low power consumption: 500 μA (3 μA in standby mode) - Selective g-range:

1.5g, 2g, 4g, 6g (MMA7260Q/MMA6270Q/MMA6280Q) 2.5g, 3.3g, 6.7g, 10g (MMA7261Q/MMA6271Q/MMA6281Q) - Response time: 1 ms

• Microprocessor: MC9S08QG8

• Wireless connectivity: ZigBeeTM transceivers (MC1319x)

• Microprocessor: MC68HC908JW32 (USB 2.0 Full Speed)

• Software that collects sensor data from the accelerometer and creates a data packet to send over the SMAC (Simple Media Access Controller) driver using the MC13191 RF transceiver

• Software within the USB stick to create a bridge between the RF link and USB connection

Figure A.11: Sensor Board Overview

Figure A.12: Sensor Board Block Diagram

The Sensor Board utilizes a small footprint size dual-layer printed circuit board (PCB) containing all the necessary circuitry for MMA7260QT accelerometer sensing and transferring data over a radio frequency (RF) as shown in Figure A.11.

The board is powered by a Lithium coin-sized CR2032 battery with provisions also made for the larger capacity CR2477 size. The block diagram of the board is described in Figure A.12.

Figure A.13 shows in more detail, how different software and hardware modules co- operate with each other. The main task of the Sensor Board is to:

- periodically wake-up from power saving mode

- measure all three XYZ acceleration values from the Sensor - compose a data frame using simple ZSTAR RF Protocol

- use SMAC (Simple Media Access Controller) to send this data frame over the RF link

- wait for an acknowledgment from the other end (here, the USB stick) - go to sleep

This basic loop repeats roughly 30 times per second providing nearly a real-time response from the Sensor.

Figure A.13: ZSTAR Sensor Board Software Overview

Một phần của tài liệu Bayesian recursive algorithms for estimating free space and user intentions in a semi autonomous wheelchair with a stereoscopic camera system (Trang 240 - 245)

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