Typically this interface is custom designed for individual applications, though this paper will suggest a ... Figure 1: Sensor and interface to end user application.
James, Daniel A., Neil P. Davey, and Leon Gourdeas. "A modular integrated platform for microsensor applications." Microelectronics, MEMS, and Nanotechnology. International Society for Optics and Photonics, 2004. Find us at Queensland Sports Technology Cluster
A modular integrated platform for micro sensor applications Daniel A. James, Neil Davey and Leon Gourdeas CRC for Microtechnology, Griffith University, Brisbane, Australia ABSTRACT A modular self-contained modular platform is described, for easy integration with micro sensors and other sensor elements. The platform is designed to be physically robust and suitable for harsh environments. The platform features switch able power modes, signal processing capabilities and extensive I/O for sensor and external device communications, data download and transmission. The modular design allows flexible implementation of required functionality depending on the particular application and also provides flexibility for packaging solutions. Two practical applications of the platform are presented to demonstrate its use. Firstly a variety of human exercise activities are investigated using accelerometers. Secondly a weather station made up of environmental sensors using off the shelf and prototype sensors is described. Both of these applications differ greatly in their operational requirements. These implementations demonstrate the adaptability of the platform for different applications. Keywords: sensors, data acquisition, signal processing, low power electronics, system design
1. INTRODUCTION Microsensor development today is a well established field; however the complexity and connectivity requirements of such sensors can vary widely. Sensors, in general, require some form of interface to facilitate their intended function. The nature of this interface varies widely depending on the type of sensor as well as its intended application. Without such an interface sensors are not so useful. Hence there exists a need for an interface platform that can meet the varied requirements of differing sensors, for both prototype development and for end product commercialization. Figure 1 describes the process of sensing. As illustrated an interface platform can form an integral part of the sensing process. Typically this interface is custom designed for individual applications, though this paper will suggest a more modular, flexible approach. Sensor and sensing applications generally begin with a need to measure some definable physical event. Following this a sensor concept is formed or developed by incorporating some ideas about how we might measure this event and some scientific principle to do this. Design, engineering and fabrication is then required to construct this sensor. Finally an interface is required to fit between the sensor and the intended application. For some sensors this can be a trivial step, however for many there are a lot of steps involved. This interface involves powering the sensor and communication of the sensors output to the application via a variety of protocols. Additionally the sensors output usually require manipulation before it is usable. This includes processes such as calibration, calculation of desired derivative quantities (e.g. %RH), SNR improvement and the cancellation of hysteresis effects, thus satisfying the user (application) requirements.
Physical event Sensor concept Sensor
Interface Platform Application ready Figure 1: Sensor and interface to end user application
2.
REQUIREMENTS
The design of the platform can be obtained by first determining the generalized requirements of the sensor and that of the application. 1.1 Sensor requirements Basic sensors generally require only a few connections, power, ground and an output of some kind. More complicated sensors however require additional connections for multiple outputs and often have control lines as well. Hot wire anemometry for example requires switching power and analogue sampling lines for calculation of wind speed and direction1. Sensor outputs also vary depending on the physical principle of the sensors active component. Some common methods include resistance and capacitive based sensors, as well as current and voltage sources. Increasing the use of MEMS sensors (such as accelerometers) provides derivative information output from on chip conditioning and so outputs such as PWM need to be catered for as well. 1.2 Application requirements Application requirements are widely varied. The most basic form of output is an analogue or digital signal ready for coupling to a more complex system. The required output can be a direct sensor output or some derivative measure of it. Further many end user applications require processed outputs that can include filtering, summary and/or calibrated outputs, periodic and continuous communications. The form of the output may also require specific encoding of the data and adherence to a myriad of communications protocols. 1.3 Interface Platform Thus the interface platform is required to reconcile the requirements of both the sensor and end user application. Additionally the platform should maintain a flexibility to cater to a wide range of requirements, and be easily adaptable for different applications. It is particularly important to maintain this flexibility so as to allow more options for platform packaging. Desirable features include low-power consumption, multiple analogue and digital I/O for sensing and control, sufficient power for the demands of the sensor and long-term operation. Variable power modes of operation are desirable are they the conservation of power between sensor samples in extreme applications. Processing requirements of the sensor inputs
are widely varied and dependant on the sensing application, hardware and protocols for connectivity to user applications, and rate of sensor acquisition required. Short to long term storage of sensor data is also desirable.
3. DESIGN Design of the platform required careful consideration of the above constraints. It was determined that the platform would have to make certain compromises in order to maintain flexibility for a variety of applications. These compromises included a layout for flexibility and a number of I/O pins, rather than a specific design optimized for size. The platform layout was designed as a series of modules rather than a single board so that specific features could be included as required per application. The final platform design consists of up to 5 modules:• Microprocessor • Power supply • Battery • IR communications • RF communications Here the modular structure of the design allows for easy technology updates to the platform without the need for complete redesign. Of these, the microprocessor module is central to the platforms function as it controls the other modules as well as the sensor data acquisition. 3.1 Microprocessor module This module controls the entire platform, including data acquisition from sensors, management of the power supply, data storage and communications via the RF or IR communications modules. The features of this module were:• Hitachi H8/300H family microprocessor • On-board sensors (ADXL202 accelerometers) • LED indicator • 16MB+ on board memory • I/O header to Power module • I/O header to communications module(s) • I/O headers for external devices (10bit ADC and digital I/O) The processor is controlled by a high and low priority scheduler that has been detailed previously2. The high priority scheduler controls time critical tasks like data acquisition and communications, whilst the low priority scheduler handles background house keeping tasks like data storage and processing. This module uses two clock crystals to switch from active mode to sleep mode. Thus the platform can power down between measuring events, or measurement epochs to conserve power. A multimedia card (MMC) is used for data storage, after the data is first packed by the micro processor. The memory cards are also used for the storage of important information for platform and sensor calibration. These cards are readily available and the memory capacity is increasing. Utilization of the microprocessor for these tasks is low, thus allowing the microprocessor to be used for more complex tasks, such as data compression and signal conditioning2. 3.2 Power supply module The power supply module regulates the voltage from the battery supplies, thus providing power to the core and other modules. This module is also responsible for the charging of these batteries.
The Power module consists of • Inductive coupling ring for charging • Charging circuit • Power supply and regulation circuits • I/O header to microprocessor module
Figure 2: Inductive charging section
Inductive charging was selected as the recharging mechanism so that the platform, once fabricated, can be hermetically sealed. Thus allowing the platform to operate in harsh conditions, without the need to compromise its structural integrity (for charging purposes). Figure 2 shows the inductive charging section of the module. L1, The primary coil is fed 0.5A(12V) at 19kHz, this is coupled to L2, the receiving coil of the platform using a ferrite core providing 7Vdc at 150mA for charging and 15mA for trickle charging the nickel metal hydride cells (640mAhr). A step up transformer and regulator ensures a regulated supply of 3VDC is provided by the module. The power module is monitored by the processing module using a ‘coulomb counter’ to accurately determine operating time, to be reported to the user if required. For low powered applications it is possible to have the unit operating for months between charges on only modest battery capacity. 3.3 Communications modules Two communications modules are provided for either Radio Frequency (RF) or Infra-Red (IR) communications to the platform. Both of these modules are “add on”, depending on the required application and allow the structural integrity and hermatic sealing of the platform to be maintained, whilst providing communications. The RF module operates in the ISM (Industrial, Scientific & Medical) band providing a typical range of approximately 100m. The module is based on the Nordic nRF0433 transceiver module which operates at bit rates of 20 Kbits/sec with 10mW(10dBm). The range can be altered by appropriate choice of transmitter power (programmable) and antenna selection. Tested configurations include monopole, helical monopole and patch antennas. Each of these antennae has associated advantages and disadvantages and is selected depending on the application. Typically the RF communications module is used in a ‘live’ environment where constant sensor data flow is required. The platform can operate either on its own or as part of an adhoc network2 containing a number of such platforms. The IR module is used for short distance communications and is the preferred solution for periodic download of data, as many PDA’s and laptops now have IR capability. This module is based on the Vishay TFDS4500 IR head along with the Microchip MCP2120 encoder/decoder chip. The firmware and memory on the microprocessor module can also be updated using IR for platform configuration, sensor calibration and other updates without a need for return to base. 3.4 Implementation With these designs in mind the platform was implemented using a 16bit 10MHz microprocessor. Figure 3 shows an overview of the described modules, various interconnects and their dimensions. Whilst the physical size of the device is not optimized for size it represents a good compromise of flexibility of applications and usability for different sensors.
Exte rn a l D e vic e s Vc c , G nd , 4 a n a lo g 4 d ig ita l
Vc c , G nd , Tx Rx 3 I/O En a b le
Vc c , G nd ,
Po we r Un it
Vc c , G nd , BATT
Mic ro p ro c e sso r
~ 30m m
UP
C o m m s (IR) ~ 35m m
8 p in p ro g He a d e r (1 u nu se d )
Plu g in R F re p la c e m e n t Fo r wire le ss a p p lic a tio ns
~ 30m m
Ba tte ry Un it
C h a rg e r
BATT
In d u c tive c o u p lin g
~ 30m m
PSU BATT
~ 15m m
IR c o m m s
~ 33m m
~ 35m m ~ 35m m
C o m m s (RF)
Figure 3 Integrated platform schematic, showing modules
In the following section two applications of the platform are described together with some results. Other applications of the platform have been previously described3.
4. RESULTS Two micro sensor applications are shown, providing example implementations of the developed platform. These applications were chosen for their differing requirements and to demonstrate the flexibility of the platform. In the first application human athlete activity is of interest. The primary sensors for this application are accelerometers, though other sensors have also been added in related works3. For this application high speed sampling across a number of channels is required for short periods of time on a mobile subject. The packaging required the modules to be arranged for subject comfort, and was placed on the subject’s sacrum using a belt clip arrangement. The second application is a weather station4 consisting of a number of micro sensors suitable for weather monitoring. For this application, stand alone operation for long periods of time is required with only a slow sample rate required. Table 1 shows the differing requirements of the two applications that the platform must meet.
Requirement Category Sensors
Human Motion Acceleromete rs (ADXL202)
Weather Station Humidity, Wind speed Wind dir’n Temperature Light
Sensor Output
Analogue
Sample Rate Interface complexity Communications
150Hz Simple
Digital Freq., Analogue ~1hr-1 Complex
Data handling
Continuous Tx, little preprocessing
Current Consumption
15mA@3V
Operating time Storage Packaging
4hrs nil Biomechanica l-ly neutral Splash proof
RF (real time)
RS232 (offline or real time) Store and dump, Calibrated sensed data on-board ~20uA(Sleep mode) 20mA (Active Sampling) Months 16Mb Weatherproof
Table 1: Implementation requirements for human motion and weather station applications
Figure 4(a) shows the completed weather station, able to operate independently for long periods of time. In this implementation the two communications modules have not been used though an RS232 driver has been installed for either a radio modem or direct periodic connection to a PC. This facilitates periodic (or continuous) download of data on dynamic configuration of the platform. A software client Figure 4(b) was also developed for this application to calibrate the sensors, set individual sample rates and for periodic data download.
(a)
(b) Figure 4: Weather sensing station incorporating the platform4
Figure 5 shows a completed human motion monitor, encapsulated in a small plastic box. This implementation uses two dual axis ADXL202 accelerometers to measure human activity for sporting and health applications. The RF communications is facilitated by a small internal helical antenna and the device is recharged in a manner similar to a cordless toothbrush with the illustrated charger using the inductive charging in the power module. This platform is worn in the small of the back and is biomechanically neutral to the wearer.
Figure 5: Human motion(R) and inductive charging unit (L)
Figure 6 shows raw acceleration data obtained from treadmill tests of a runners gait. Here stride characteristics are clearly visible in the vertical(Z) and lateral axis(X), a detailed analysis of this work will be published at a later date.
Figure 6: Tri-axial acceleration stride data collected from an elite athlete
5.
CONCLUSIONS
The inherent flexibility of a standardized platform provides a number of advantages for research. Development time is vastly reduced by having common hardware and firmware code base available for configuration to the intended application. Further with efforts devoted to a common solution the platform can continue to evolve, by reducing overall size and applying technology updates as required. This platform has been used in a number of other applications beyond those described in this work, which have yet to be published. This paper has demonstrated the value in utilizing a standardized approach for a sensing platform and described its operation. By utilizing this platform, the development of two widely differing applications was able to be completed in a comparatively short space of time.
ACKNOWLEDGEMENTS This work was supported by the CRC for Microtechnology and the School of Microelectronics, Griffith University.
REFERENCES 1
Richard J. Adamec, David V. Thiel and Philip Tanner, “Modelling and fabrication of a planar thin film airflow sensor”, Proceedings of SPIE Vol. 4593 (2001), pp.156-163
2
Andrew J. Wixted, Daniel A. James and David V. Thiel, “Low Power Operating System and Wireless Networking for a Real Time Sensor Network”, ICITA, Bathurst, Australia, Nov 2002
3
D. C. Billing, V. Filipou, J.P. Hayes, C.R.Nagarajah, D.A. James, “Development and Application of a Wireless Insole Device for the Measurement of Human Gait Kinematics”, World Congress on Medical Physics and Biomedical Engineering, August 24-29, 2003
4
Liam Brennan, “Weather sensing and analysis system “, Thesis, June 2003, unpublished.
