10 Instrumenting a Rowing Ergometer for Improved Training 95 The coordinates of the point of application of the normal force on the stretcher can be calculated knowing the total vertical force Ftot (the sum of the four vertical load cells) and the moment of the vertical forces with respect to the transversal and longitudinal axes of the ergometer (x andy) as ( Px D My Ftot Py D Mx Ftot (10.1) where Mx D.F1 F2 F3 CF4/b My D.F1 CF2 F3 F4/a (10.2) while a andb represent the distance between the vertical load cells along the x andy directions; F1, F2, F3 andF4 represent the measurements of the front-left, rear-left, rear-right and front-right load cells respectively. The knowledge of the centre of forces allows the trainer to correct the athlete posture, in order to avoid unwanted asymmetries in his rowing dynamics. 10.3 The Seat Due to the changes on the stretcher, also the seat has been modified. In particular, since the stretcher height increased, the vertical position of the seat has been increased of the same quantity, in order to keep the athlete position unchanged. Figure 10.2 show the design of the new seat. Four vertical single-axis load cells have been mounted under the seat to measure the vertical forces applied by the athlete. In this case, both the lateral and longitudinal forces are negligible, due to the rowing symmetry and to the longitudinal sliding of the seat respectively. For this reason no longitudinal nor lateral load cells have been mounted on the seat. Anyway, an horizontal rod has been placed between the seat and its chassis. As explained in Sect. 10.2 for the stretcher (equations 10.1 and 10.2), the centre of forces has been calculated also for the seat. The seat position with respect to the rowing ergometer chassis is measured by means of a linear encoder. Since the seat slides with respect to the ground, in order to simplify the wiring all its sensors are measured using a new National Instruments wireless acquisition board (NI 9191). 10.4 The Handlebar The handlebar of the rowing ergometer has been completely re-designed in order to insert a force and displacement measurement. The force is measured through a single-axis load cell, while the displacement is measured through a linear Fig. 10.2 The re-designed seat
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