1994

Lehtinen, Hannu

Force control of legs of evolving walking machines is assumed to be essential in natural soft and uneven terrain. The main duties of a leg are to support and propel the vehicle in co-operation with the other legs. Force control fills the support requirement also when a leg or many legs penetrate the terrain. It also prevents rolling of the body due to lift-off or placement of legs. Due to the relatively large mass and inertia of the body, dynamic equations of a free object are a basis for "computed-torque" based calculations of desired body forces to move or participate in the applications as desired. The large and varying amount of friction in the leg mechanisms while the body is carried and the practical delays and saturation in the hydraulic system increase the complexity of the process to be controlled. The main contributions in this thesis are as follows. The body forces are transformed to the supporting legs in two phases: sets of minimum forces perpendicular to the resultant body force and forces parallel to the same resultant. This method minimizes the possibility of slippage with walking machines, where the desired body force is often close to vertical due to the weight of the body. A load adaptive PI force control method for the hydraulic actuation system of MECANT I consisting of an asymmetric cylinder and a symmetrical valve has been developed. The I term of the controller is changed according to the desired load. A rule based altitude controller and a dead-zone and saturation based attitude controller have been designed. The first tests with force controlled vertical actuators show the usability of the method and fast responses to deviations in body orientation. The deviations are usually corrected within 1 second.