Fig. 7. Set up of load cell calibration. (a) Load cell calibration method in laboratory; (b) Calibration curve of one of the used load cells to determine the carriage motion resistance. working at the frequency of 80 Hz. The duty of this device is to receive transducers' output data, dividing time to each one, adjusting desired period for automatic data acquisition, and transferring data to a laptop. After preparing the conditions for starting each test, to achieve the desired velocity and slip, both inverters of the wheel electromotor and the wheel carriage were started simultaneously and 1.5 meters ahead of the starting point. The wheel carriage began to move towards the stress measurement system, and the data acquisition process was started 75 cm after the first stress sensing element. An automated circuit was used to achieve high precision in data acquisition and similar conditions for all tests as well as to eliminate possible human error. This circuit consisted of two magnetic relays, one in the