Microelectronic sensing system enables in-flight arrow ballistics measurement

(Editor's note: this is the latest in our on-going Extreme Design series, where a single design or test objective—not low cost—dominates the engineering challenge. In this story, we see how small, high-performance MEMS transducers for acceleration, coupled with other low-power ICs,  are enabling data acquisition in applications that would have been considered impossible just a few years ago. To see all the previous entries in the Extreme Design series, click here.)

Microelectronic sensing systems now appear in all fields of endeavor. Today, such systems are often applied to measure athletic performance by including the sensing system on the athlete in wearable sensing systems, or in athletic equipment itself.

These applications provide significant challenges to the designer because the systems must be highly robust due to the dynamic nature of athletics. For example, such systems must precisely measure the movement of athletes and equipment through rapid starts/stops, impact shocks, changes in direction and other more subtle movements.

Further, the systems are generally self-contained and must include the necessary sensors, power supply and communication hardware for external data communication. All these elements must be housed to allow the sensing system to be easily integrated into the sport while providing protection through the robust motion of the athlete or athletic equipment.

The preceding requirements are challenge enough without more. In addition, however, many of these applications also place additional constraints on the designer because of the small form factor required of the microelectronic sensing system. The challenge is even more extreme in the field of ballistics where the requirements of robust design and small form factor are increased due to the extremes in acceleration and deceleration of the projectile.

The Unique Application

The authors set out to develop a microelectronic sensing system integrated into an arrow to record ballistic data while the arrow is in flight. The system is designed as a sports aid used to precisely measure the ballistic motion of the arrow. The performance of the archer and archery equipment is evaluated based on the aerodynamic efficiency of the arrow in-flight as determined with the recorded flight-data.

Aerodynamic efficiency (or arrow drag) is directly related to the construction of the arrow, for example, the arrow shaft diameter, and the type of vanes or fletching, among other factors. It is also a measure of how efficiently the energy stored in the bow is transferred to the arrow, because an inefficient release of the arrow results in instability in flight.

For example, aerodynamic efficiency is affected by the amount of oscillation of the arrow shaft in flight and the rate at which the oscillation decays during flight. Arrow oscillation and damping can be improved by proper technique, and proper selection and adjustment of both the bow and the arrow.

Ideally, the microelectronic sensing system is not an add-on component but is instead integrated in a conventional component of the arrow in a form factor and weight, allowing the otherwise-conventional component with embedded sensing system to be used as a direct replacement for the conventional archery component. Achieving the preceding goal maintains the true ballistic performance of the arrow with an integrated sensing system.  

Because many archers use a 100 grain (6.48 gram) removable arrow point (known as a “field point”), the authors elected to package a self-contained microelectronic sensing system in an electronic field point, while replicating the form factor of a conventional field point as closely as possible, Figure 1. The electronic field point is also configured to plug into a docking station. The arrow point communicates the ballistic data to the docking station using a hardwired serial communication bus after recording ballistic data for one or more shots.

Figure 1: Electronic arrowtip construction

© Full Flight Technology, LLC

Functional Requirements

Due to the form-factor requirements and limits of available power sources suitable for inclusion in the required form factor, the components (Figure 2) must all employ the smallest package size and lowest-power consumption available in their respective device family. In addition, they must also be robust enough for continued reliable performance through the repeated hi-g launch as well as impact forces of the arrow which can be greater than 1200 g and 4000 g, respectively.

Microcontroller

The microcontroller performs a variety of tasks in the sensing system including: detection of wake-up events; performing total flight time T measurement with timers included in the microcontroller; writing and reading data to/from internal and external EEPROM via an internal serial communication bus; reading data from the accelerometer, storing the embedded software to implement a control algorithm and bus communication protocols; providing RAM capacity to accommodate variables and an EEPROM page data stack.

The microcontroller provides the system with three modes of operation: a sleep (or pre-flight) mode; a flight mode and a communication mode. In flight mode, the system records launch and impact events and stores the accelerometer output data. The system operates in the communication mode when the arrowtip is plugged into the docking station. The wake-up event detection allows the microcontroller to remain in sleep mode when not in use to reduce power consumption to as low as 0.1uA during periods of inactivity.

Accelerometer

The accelerometer measures the arrow’s deceleration in flight. In addition, the accelerometer detects the occurrence of the launch and impact events used to establish the time of flight. Due to the forces involved, a relatively hi-g range is desired. A multi-axis accelerometer is also desirable because of the possibility of processing the acceleration forces orthogonal to the axis of the arrow shaft in order to evaluate the deflection/oscillation of the arrow shaft.

Shock Sensor

The shock sensor detects the start of the arrow’s flight (when the bow string is released by the archer) and the moment the arrow impacts the face of the target. The shock sensor has a minimum acceleration value at which it activates to insure that it does not falsely trigger when the arrow tip is handled during normal use before or after a shot. Since an arrow’s flight begins with a launch phase measured from the moment the archer releases the string lasting until the arrow disengages from the bow string, the shock sensor can detect the start of the launch-phase.

The start of the free-flight phase and target impact are also detected by the shock sensor to provide another approach for measuring the time of flight. The initial shock-sensor output triggered at the start of the launch phase is also used as the wake-up event that activates the microcontroller from the sleep mode to enter the flight mode.

Figure 2: Block diagram of electronic arrowtip circuit

© Full Flight Technology, LLC

EEPROM

The system includes an EEPROM external to the microcontroller to store the ballistic data recorded during each shot. The memory must have a capacity to store this data for up to four shots, each having a maximum flight time of 800 ms. The memory must also include a serial communication interface compatible with the microcontroller.

In addition, the memory must be capable of being employed in a page write operation to reduce power consumption and maintain a high rate of data recording in the flight mode. A maximum page write time of 5 ms is desired because of the rate at which ballistic data is recorded by the system when in flight mode. Power consumption of 5 mA or less is also desired for write and read cycles.