本研究の目的は，アホウドリなど海鳥の給餌行動を調べるための巣箱の中に設置可能なヒナドリ用の体重計を実現することである．そのために，提案するセンサ構造を試作し，体重に相当する力を印加した際の応答を計測しその特性を評価した．
オオミズナギドリを計測対象とし体重計の仕様を決定した．ヒナドリは最大で600 g程度となり，巣箱の中に落ち葉や泥などが堆積することを考慮すると，具体的な計測域の要求仕様として1.5 kg程度でかつ分解能として10 g以下と定義した．また巣箱の寸法としては，50 cm x 30 cm x 30 cm程度を想定し実験時には地中に埋めることを想定した．さらにヒナドリが誕生してから巣立ちまでの期間をバイオロギングシステムで記録することを想定し，最大で半年間，サンプリングレートとして10分に1回程度で精度よく体重変化を計測できるデバイスを目指した．
フォースプレートの構成として，センサ素子と機械的ばね構造を分けて設計した．ばね構造とセンサ素子がそれぞれ独立しているため，ばね構造の配置やばね定数の調整が可能であり，体重計の形状や力レンジ，分解能などの仕様を容易に変更することができる．フォースプレートの底面と天板はアクリル，補強材はアルミ二ウム，ばね部と柱はステンレスで製作した．ばねは十分な強度を確保し，なおかつばねのばね定数と面積を小さくするためスパイラル状のばねとした．計測環境は高湿度な地中であるため，耐環境性に優れたセンサを用いる必要がある．このため，耐環境性に優れた渦電流式の変位センサを使用した．
製作した体重計の特性を見積もるため，加えた力とセンサ出力の関係を評価した．加えた力が20 N以下のとき，加えた力とセンサ出力の比例関係が成り立っていた．また，体重計に力を加えていないときのセンサ出力を計測してノイズレベルを求め，分解能を評価した結果，ノイズレベルは1.1 mVで分解能は4.2 gであることが確認された．
The purpose of this study is to realize a scale for shorebirds that can be installed in a nest box to study the feeding behavior of seabirds such as albatrosses. For this purpose, a prototype of the proposed sensor structure was fabricated, and its characteristics were evaluated by measuring its response when a force equivalent to body weight was applied.
The specifications of the scale were determined using a long-eared bulbul as the measurement target. Considering that the maximum weight of a brown-headed cowbird is about 600 g and that fallen leaves and mud accumulate in the nest box, we defined the specific measurement range requirements as about 1.5 kg and resolution as less than 10 g. The maximum weight of a brown-headed cowbird is about 600 g. The maximum weight of a brown-headed cowbird is about 1.5 kg, and the resolution is less than 10 g. The dimensions of the nest box were assumed to be 50 cm x 30 cm x 30 cm, and it was assumed that the nest box would be buried in the ground during the experiment. We also aimed to create a device that can accurately measure weight change at a sampling rate of about once every 10 minutes for up to six months, assuming that the period from birth to departure from the nest is recorded by the biologging system.
The force plate was designed with a separate sensor element and mechanical spring structure. Because the spring structure and sensor elements are independent of each other, the arrangement of the spring structure and spring constant can be adjusted, and specifications such as scale shape, force range, and resolution can be easily changed. The bottom and top of the force plate are made of acrylic, the reinforcement is made of aluminum, and the spring and column are made of stainless steel. The springs are spiral springs to ensure sufficient strength and to reduce the spring constant and area of the spring. Because the measurement environment is underground with high humidity, it is necessary to use a sensor with excellent environmental resistance. For this reason, an eddy-current displacement sensor with excellent environmental resistance was used.
To estimate the characteristics of the scales, the relationship between the force applied and the sensor output was evaluated. When the applied force was less than 20 N, a proportional relationship between the applied force and the sensor output was established. The sensor output when no force was applied to the scale was measured to obtain the noise level, and the resolution was evaluated.