A Simple and Economical 3-D Force Plate for Gait Analysis

Three-dimensional force plates are used to measure the ground reaction forces/moments and the center of pressure (COP) during the stance phase of walking. Force plates consist of three essential components, that is, lower platform, upper platform (typically with square or rectangular shape), and some load cells between both platforms. The design of the load cells, their location in the force plate, and the type of sensors used in the load cells determine the main differences. Two types of sensors are usually used in load cells: resistive strain gauges and piezoelectric crystals. Commercial force plates usually have four 3-D load cells located at the corners of the plate. The number of resistive strain gauges used commonly in each load cell of force plates based on resistive strain gauges is 24, whereas piezoelectric force plates have three orthogonal piezoelectric sensors packaged in each load cell. Although the operational performance of commercial force plates is very good, they are very expensive, and many research groups cannot afford to buy them. A novel 3-D force plate for gait analysis is proposed in this study with a simple design that is easy and economical compared to commercial force plates (approximately one-tenth of the cost of commercial force plates). The foot-ground reaction forces/moments and the COP are estimated from only eight force sensors strategically distributed on the force plate. Each force sensor consists of a uniaxial cantilever beam and a strain sensor attached at its fixed end. The design and manufacture of cantilever beam force sensors are simpler and more economical than 3-D load cells used in commercial force plates. The cantilever beam force sensors are designed to maximize the signal-force relation and minimize vibration modes. The performance of the proposed force plate (linearity, ±1.6%FSO—full-scale output—hysteresis ±0.5%FSO, accuracy error ±2%AL—applied load— crosstalk ±5%AL, and COP accuracy error, 14.5 mm) has been compared with the performance of other commercial force plates. In general, the deviation of the specifications in our system are slightly greater than in commercial force plates. These deviations might be acceptable if we consider that we used manufacturing methods and machines which are not as precise as those used by companies that manufacture this type of force plate. In addition, two types of strain sensors, resistive strain gauges, and piezoelectric paint sensors were compared in this study. Experimental tests showed similar results with both strain sensors. The relative mean difference in the estimation of the forces/moments with both strain sensors was lower than 5% in all cases except the vertical component of the force, Fy, which was about 9%, and the mean difference in the estimation of the COP with both sensors was 11 mm. Therefore, they might be used to design this type of force plate.