Microelectronic sensing system enables in-flight arrow ballistics measurement

The selection of components for the prototype and subsequent commercial system was based on the functional requirements identified above and the requirements for small size, high durability, low power consumption, and compatibility with a communication bus for on-board communication.

Microcontroller

The Atmel ATtiny84 was selected by the design team because it offers a relatively large capacity 512 byte EEPROM memory and 512 byte RAM memory, in a small 4x4 mm package size. A self-programmable 8kB Flash program memory allows bootloader implementation via a 1-wire interface. The ATtiny also supports peripheral devices well because it can employ an I²C interface. The rated power consumption of 1 mA in the active mode is higher than some of the alternatives, however, the ATtiny can be programmed to reduce current to 20 µA during periods of inactivity.

Accelerometer

An Analog Devices ADXL346 iMEMS accelerometer was selected based on a combination of its range and resolution. Initial prototypes were built using the ADXL345 in advance of release of the ADXL346. However, upon its release, the Analog Devices’ ADXL346 was included in the follow-on prototypes and the commercial product.

The integral first in first out (FIFO) buffer introduced in the ADXL346 provides a significant advantage for the application. The FIFO stores ballistics data to maximize measurement accuracy by allowing the host processor more available time to write to memory during data collection. At 16g, the maximum range was twice that available from other accelerometers available in the small 3×3 mm package size.

In addition, the relatively high 3.2 ksps sample rate for each of the three axes also provides an advantage. The 10-to-13 bit resolution is also greater than other small form-factor accelerometers. Other features important to the selection include the available I²C digital communication for communicating the ballistic data, and wide range of operating voltage (1.7-3.6 V) suitable for the selected power-source configuration of two coin-cell batteries.

Shock Sensor

A SignalQuest SQ-ASD shock switch with a 150g threshold was selected because of the available activation threshold and a rapid response time of 100µs. The cylindrical shape of the device made component placement a challenge in the surface-mount manufacturing. The shock switch was mounted on the PCB such that it was centrally located on the longitudinal axis of the arrowtip.

Initial life cycle testing identified a need for improved mounting to maintain attachment of the shock switch to the PCB through repeated impacts. As a result of the life-cycle testing, the design team reworked the pad design to increase the robustness of the connections by increasing the size of the pads and adding a via to directly connect the shock-switch contact pads located on opposite sides of the PCB. Subsequent destructive and non-destructive testing demonstrated improved performance more than sufficient to survive hundreds of high-g impacts.

EEPROM

An Atmel AT24C was selected because it offers a large storage capacity and additional features in the smallest package size available. Required features included I²C digital communication, wide operating voltage range of 1.8-5.5 V, low-power sleep mode drawing 1µA of current, drawing a maximum of 2 mA when in an active mode, a power consumption of 5 mA in the write mode and a write-cycle duration of 5 msec. The short write-cycle duration is an advantage due to the sampling speed and amount of data that is collected in-flight.

Conclusion

Increasing performance and decreasing size of today’s components helped the designers achieve the above objectives despite the often competing interests of small size, maximum durability and low power consumption.

Figure 3: Comparison of Velocitip^® Electronic Field Point

and conventional field point

© Full Flight Technology, LLC

Ultimately, the design team delivered a robust design for a commercial product and a first of its kind performance aid in the field of archery, see www.velocitip.com. The complete microelectronic sensing system is housed in an aluminum arrowpoint having an overall mass equal to the mass of the 100 grain steel field point shown in Figure 3.

About the authors

The authors are Robert V. Donahoe^a, John Barton^b, Javier Torres^b, and Jan Vcelak^c, with these affiliations:

a)  Full Flight Technology, LLC, Cambridge, MA;

b) Clarity Centre for Sensor Web Technologies, Tyndall National Institute, University College Cork, Ireland

c) CTU in Prague, FEE, Technicka 2, Prague 6, CZ

Robert Donahoe received his B.S.E.E. degree from Northeastern University in 1985 and a J.D. degree from Boston College Law School in 2001. He is founder of Full Flight Technology, LLC (FFT), Cambridge, MA,  the maker of the Velocitip Ballistic System. His innovations in the area of archery performance measurement form the basis of the Velocitip System and are patented in the U.S. and abroad. He is also a shareholder in the intellectual property law firm of Rhodes Donahoe, P.C. where he continues to assist clients in all areas of intellectual property law.   

John Barton received his M Eng Sc degree from UCC in 2006. He is employed as a Research Scientist in the Wireless Sensor Networks team at Tyndall National Institute, Cork, Ireland where he is currently the Tyndall Project manager for the Clarity Centre for Sensor Web Technologies. John has been the leader of the development of the Tyndall Wireless Sensor Mote platform and worked with FFT on development of the Velocitip System. He has authored or co-authored over 90 peer reviewed papers.

Jan Vcelak received his M.Sc in 2003 and his Ph.D. in 2007 both on Czech Technical University in Prague. From 2000 he worked for Philips CE Czech Rep. 2007-2008 we worked in Ricardo Prague on hybrid/electrical vehicles control electronics, in 2008 he joined Tyndall NI (Ireland) where he worked within Microsystem group in electronics HW/SW and mechanical design. In 2011, he joined CTU and works on the projects related to magnetism and sensor systems for special applications.

Javier Torres Sanchez received his Licenciatura (M.Sc. equivalent) Degree in Physics from the University of Granada (Spain) in 2003 and his Licenciatura (M.Sc. equivalent) Degree in Electronic Engineering from the University of Granada (Spain) in 2007. In July 2006 he joined the Microsystems Group at the Tyndall National Institute where he now works as an Applications Engineer in the area of design and development of wireless sensing system solutions across a wide range of application spaces.

About Full Flight Technology

A pioneer in the application of today’s most advanced sensing technology, Full Flight Technology, LLC (FFT), (Cambridge, Massachusetts, USA) develops powerful, rugged and easy-to-use precision measurement systems for ballistic projectiles and other applications. FFT’s initial customers include best-in-class companies in all product categories in the field of archery.

About Tyndall National Institute

Tyndall National Institute, University College (Cork, Ireland) is one of Europe's leading research centres, specialising in ICT hardware research, with ca 450 staff, students and academic & industrial visiting researchers. Tyndall undertakes internationally-leading research into information and communications technology. Tyndall uses its facilities and expertise to support industry and academia nationally and provides large numbers of highly qualified graduate students, key to the development of Ireland’s national economy. Tyndall has over 200 industry partnerships and customers worldwide. Several start-up companies in Ireland have been based on technology originating at Tyndall.

The Institute’s researchers include 125 PhD and 10 Masters students, and 38 nationalities are represented within the institute at all levels, with its research published in 200 peer reviewed publications last year. Income for 2011 was over €30m. The Wireless Sensor Network (WSN) group at the Tyndall National Institute is developing and deploying sensing systems in a wide variety of application spaces with a wide variety of project partners.

With many years experience in the development and characterisation of micro-sensors and micro-integration technologies, within the WSN group, a technology roadmap has been adopted for the evolution of 3-D autonomous modules incorporating micro sensors, microcontrollers, wireless communications and power conditioning for deployment in the key areas of fitness and health and for monitoring energy and the environment.

About CLARITY

CLARITY is a Science Foundation Ireland (SFI) funded project and is partnership between University College Dublin, Dublin City University and Tyndall National Institute. This ground breaking research centre focuses on the Sensor Web, which captures the intersection between two important research areas: Adaptive Sensing and Information Discovery.