Applikationsnoten

InvenSense : Wireless Motion Sensing Module (WMSM)

Von: InvenSense

Wireless Motion Sensing Module (WMSM)

Wireless Motion Sensing Module (WMSM) is a simple prototype device that acquires and processes an output data from InvenSense dual axis gyroscope, process it and transmit collected information to a remote module taking an advantage of ZigBee communication protocol.

The intention of WMSM is presentation of ability of providing the motion sensing at point and time of need without limiting independence and mobility of system components. WMSM is able to communicate with any other devices that use ZigBee high level network communication protocol. ZigBee 2004 version of network stack is used to achieve bidirectional communication between prototype and remote destination device. For benchmarking the prototype functionality Jennic JN5121-EK010 Evaluation Kit (EVK) was used.

The Jennic Zigbee EVK package consists of one controller board and four sensor boards. For testing WMSM the controller board was used with specially designed software based on Wireless Sensor Network (WSN) project available on Jennic Support Web Portal. During normal operation controller board from EVK is acting as a co-ordinator and on the other side WMSM is acting as Router. Co-ordinator of the ZigBee network is responsible for receiving data from Router sent via the ether and retransmitting it to the PC via serial connection. Co-ordinator is also able to print collected data on its' LCD panel.

WMSM prototype was done in order to show possibility of increasing functionality of many consumer applications. In consumer electronic market there are many applications where rate of rotation (motion) sensing is required. The following example applications can be considered in order to characterize set of demand for prototype device.

GSM car navigation systems require rate of rotation measurements for dead reckoning scenarios. This is essential in case when less than 3 satellites signal is available or very low signal strength is achievable i.e. in high multi path environments like down town locations. An ideal car navigation system should be then equipped in motion sensing module which will allow specifying the movement direction even in case of lack of GPS signal. Because in many cases GPS receivers are mounted independently from LCD panel there is also a need of wireless communication between independent system components. In that case ZigBee communication is an ideal solution. Holter ECG meters require rate of rotation measurements for fall detection situations. In many cases Holter ECGs are used in hospitals allowing the permanent monitoring without need of limiting of patients mobility. In that kind of environments network communication is used to ensure permanent connection with remote telemedicine center. MEMS gyroscopes for emergency detection and ZigBee communication are then very interesting solutions in case of remote monitoring of patients.

Remote wireless game peripherals, TV / DVR remote controllers and handheld TV displays in many cases require the measurement of rate of rotation. This is essential to translate behavior from real world to virtual world in case of remote controllers providing i.e. intensification of gaming experiences. In case of TV displays gyroscopes can be used to allow the user free positioning of display consequently ensuring great freedom for location of display.

Realization of a prototype solution needs careful examination and definition of a set for demands of the proposed system. These should be defined taking into account different aspects that are associated with such the factors as future applications, user expectations and hardware requirements. The WMSM meets the following factors.

Rate of rotation measurement is ensured using InvenSense IDG-300 Dual-Axis Gyroscope Evaluation Board. IDG-300 is an integrated X- and Y-axis angular rate gyroscope on a single chip. It is supplied with 3V voltage. Supply voltage for IDG-300 should meet strict requirements. Power supply Voltage (VDD) rise time must be less than 20ms and gyroscope should be isolated from system power supply noise usually by a combination of an RC filter and Low Drop Out power supply regulator (LDO). The output data of IDG-300 is given in form of analog voltage that is proportional to the angular rate in case when sensor is rotated about X- or Y-axis. All gyro outputs need to be converted to a digital value taking into consideration that minimum 12 bit ADC is needed for best performance. That should result in 0.7mV/bit resolution for 3V supply which is fine in case of 2VmV/ º/s IDG-300's sensitivity.

Analogue data conversion, its processing and wireless transmission to a remote destination is ensured taking an advantage of Jennic JN5121 wireless module enabling developer to implement IEEE802.15.4 or ZigBee compliant system with minimum time to market and at the lowest price. The heart of JN5121 module is JN5121 wireless microcontroller. Each JN5121 chip provides transceiver and microcontroller features with peripherals such as 4 input 12-bit ADC, two 11-bit DACs, temperature sensor, two timer/counters, two 2 UARTs, SPI Port, 2 wire serial interface and GPIOs. The Integrated Peripheral API defines functions that allow simple use of these peripherals without need of accessing registers by the software developer. Jennic J5121 operates with supply voltage equal 2.7V to 3.6V. There are no special external components requirements for JN5121 module what significantly improve simplicity of any prototype design.

Power supply for JN5121 and IDG-300 is ensured using low noise LDO regulator. The Ricoh RP102 voltage regulator guarantee high 3.0V output voltage accuracy, extremely low supply current (50µA) and great ripple rejection (80dB at f=1kHz). These parameters makes RP102 very suitable for the power supply for any battery-powered hand-held equipment like proposed prototype solution. Thanks to low dropout voltage and very good line and load transient response single RP102 IC is an ideal solution to power both components - IDG300 and JN5121. Usage of components form InvenSense, Jennic and Ricoh make the prototype device very simple and cost effective. These elements guarantee developers great functionality connected with simplicity of any prototype and time efficiency of design.

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