Hardware as well as algorithmic issues and results that show the feasibility of the proposal are presented.2.?Hardware of the Tactile Based Human-Machine Driving InterfaceThe basic operation of the developed system is illustrated in Figure 1. The person who drives the wheelchair or trolley grasps the handlebar and the resulting force map is registered by the tactile sensor. This information is processed by a microcontroller and different patterns are extracted from the force map. These patterns are associated with user intention. For instance: accelerate, decelerate, turn to the left, etc. According to the intention detected (and other parameters which will be explained in Section 3), the control electronics generates the appropriate signals in order to activate the wheel motors and make the chair move.
The system provides output signals similar to those generated by the joystick which is incorporated by power wheelchairs.Figure 1.System scheme.2.1. Tactile SensorThe tactile sensor can be a discrete array of force sensors or can be made as a single sensor matrix using many different technologies (piezoresistive, capacitive, optical, etc.) [12]. A common and low cost realization consists of an array of electrodes with a conductive rubber or polymer placed atop [13].In the case of the first prototype of this paper, an array of piezoresistive sensors has been used (see Figure 2). Its size is 6 �� 12 elements (two sub-arrays of 6 �� 6 elements, one per hand). Each tactile element (tactel) works as a variable resistor so that the higher the force applied, the lower its electrical resistance.
The signal conditioning electronics scans the array and provides a force value for every tactel, so the force map can be built.Figure 2.Piezoresistive matrix of the first prototype of tactile sensor.Figure 3 shows the implementation of the matrix of Figure 2. It is composed of one rigid Printed Circuit Board (PCB) per row in each sub-array. Then the PCBs are joined together with soldered flexible tinned bridges that make the columns of the matrix (see Figure 3a). Dacomitinib The tactels are commercial FSR 402 force sensors [14] from Interlinks Electronics (Camarillo, CA, USA), soldered on the upper side of the PCBs. They must lie on a flat surface as they are sensitive to folds that cause undesired interferences [10]. The assembled structure is mounted, embracing the wheelchair handlebar, as can be seen in Figure 3b.
Figure 3.First prototype of the proposed device. (a) Raw tactile sensor prior to embracing the handlebar; (b) resulting implementation.This first prototype was used to carry out several experiments that focused on knowing how the force maps evolve while the chair is being driven by an attendant. However, the tinned bridges that join the PCBs are fragile and the structure had to be frequently taken apart for repair.