Measurement method of a microspring-supported force plate with an external laser displacement meter

Abstract

Ground reaction force (GRF) is a significant factor for the evaluation of animal locomotion. Recently, micro force plates have been implemented as the GRF measurement method for tiny insects. Previous micro force plates were highly sensitive but fragile and laborious to fabricate because of the use of strain-sensing elements. Here, we applied high-resolution 3D printing and a noncontact displacement meter to a micro force plate for a fruit fly. A force plate structure with 3D supporting microsprings is fabricated with easy and reproducible micro 3D printing, enabling both high sensitivity and toughness. By detecting the displacement of the plate centre externally, the vertical GRF when a fruit fly lands on the plate surface is calculated via the spring constant as the whole device. The spring constant of the microspring is designed to be approximately 5.98 N/m so that a high-resolution external laser displacement meter realizes a force resolution less than 1/50 of the body weight of a fruit fly. Providing that the four springs have the same spring constant and the displacement meter aligns at the plate centre, there is no positional error when converting from displacement to force in principle. However, the fabrication error leads to the spring constant difference. Here, we theoretically and experimentally determined the measurement point of the distance sensor where the positional error caused by the difference in spring constant of the four microsprings is compensated. It was confirmed that the calibration process improved the position error to be within ±1.5% in the experiment. Finally, we demonstrated the GRF measurement of a fruit fly. The average GRF was 6.5 μN, which was equal to the weight of a fruit fly. Our proposed device can help evaluate the biomechanics of tiny insects.

A Polyimide Film-Based Simple Force Plate for Measuring the Body Mass of Tiny Insects

“…However, there is a limit to realizing a small spring constant so that the force plate deforms under its own weight, similar to our developed device. Compared to the spring constant of previously 3D-printed highly sensitive force plate with a spring constant of 5.98 N/m [ 19 ], that of our developed device was approximately 30 times lower.…”

“…Theoretically, the displacement of the plate center remains the same regardless of where the force is applied. In practice, displacement may vary depending on the location of the applied force [ 19 ]. We experimentally evaluated the distribution of the spring-constant error of the force plate.…”

“…From Equation (6), when the central plate is rigid, the total force applied to the plate can be calculated from the center displacement and the spring constant alone [ 19 ].…”

MEMS highly sensitive and large-area force plate for total ground reaction force measurement of running ant

“…In these cases, the GRF vibration in the direction of gravity was several tens of the percent of body weight. In contrast, GRF measurements of a fruit fly, which was similar in weight to ants, indicated little vibration during walking [21]. Therefore, it is suggested that the slight vibration in the direction of gravity is a characteristic of the mg-order insect's gait.…”

“…Recently, force plates using different measurement principles have been developed for fruit flies. These plates consist of a plate structure supported by mechanical springs and a laser displacement sensor [21][22][23]. The plate has a sufficiently large area to measure the total GRF while the fly is walking.…”