The operator’s safety is a very crucial issue in heavy duty machines. The dangerousness of the working environment forces the operator to consider several parameters at the same time, and a large amount of practice and concentration are usually required to operate the machine safely. In fact, man-machine interaction is often very poor in these kind of machines. With the aim of improving this interaction and enhancing stability, a stability control system is proposed, which is based on a simplified model of the machine and on a 2-degrees-of-freedom force-feedback joystick, and combines the use of a passive ad an active actuator. Active drives can generate high-fidelity sensations but, in general, they have a poor torque/size ratio, they are expensive and can become dangerous for the operator. On the other hand, passive devices (like brakes) can generate higher torques, are cheaper and safer, however they cannot provide energy to the system but only dissipate it; this feature reduces the range of sensations that can be rendered to the operator. In order to partially overcome this limitation, an elastic element is integrated in series with the brake. In this way, it is possible to store some potential energy that can be returned to the operator to give a better sensation of the proximity of an instability working condition for the machine. In the paper, the stability control system is introduced, and the design of the haptic joystick is presented. First experimental results on a simulator are finally illustrated.

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