SPEED-ADAPTED TRAJECTORIESIN THE CASE OF INSUFFICIENT YDRAULIC RESSURE FOR THE FOUR-LEGGED LARGE-SCALE WALKINGVEHICLE ALDUROABSTRACTWhen operating walking machines, only a coordinated movement of all cylinders and/or motors can lead to safe, stable walking. The large hydraulically driven walking machine ALDURO, which is investigated in this paper, has no external power supply, and therefore the size of the on-board hydraulic power pack and its diesel engine is limited by its weight. When moving several cylinders with high speed, the hydraulic supply can become insufficient and the resulting trajectories of feet and platform can become unpredictable. When the ALDURO is near its stability limit such behaviour can lead to instability and toppling of the system. The proposed solution under discussion here is to observe the position errors and time derivatives for the cylinders and based on this reduce the speed when necessary.1. INTRODUCTIONThe system investigated in this paper is the Anthropomorphically Legged and Wheeled Duisburg Robot (ALDURO). It consists of a platform of 2.0m by 2.2m with a cabin for the operator and four legs, each 1.8m long. The estimated weight is 1200 kg. It can be used as a quadruped walking machine (Fig. 2), and by replacing the two hind feet with wheels, it can also be used as a combined legged and wheeled vehicle (Fig. 1). The latter combines the advantages of a walking machine - high mobility - with the stability and speed of wheeled vehicles 2.When operating in steep and dangerous terrain, safety plays an important role. It must be guaranteed that the cylinders follow the calc ulated trajectories exactly since a wrong movement might cause the robot to become instable. ALDUROs legs are hydraulically driven, with an open hydraulic system. Normally, when planning hydraulic systems, low weight has no high priority 1. For the walking machine ALDURO the ratio power per weight was optimized.Fig. 1: Combined Legged Fig. 2: Walking Machineand Wheeled VehicleWhile actuating several cylinders simultaneously or moving very fast, the volume flow of the hydraulic supply becomes insufficient and the resulting movements uncontrollable.The proposed solution under discussion here is a decrease in speed of all calculated trajectories. This is admissible as long as the robot is statically stable at any moment. By observing the position errors of the cylinders and their time derivatives, a decision is taken on whether to decrease or increase the speed along the trajectories. To ensure that all movements are influenced simultaneously, the model-time, upon which all calculations depend, is slowed down. Thus we can guarantee that legs and platform can and do follow the prescribed trajectories. This strategy is being tested on a virtual model of the ALDURO and is being tested on a virtual model of the ALDURO and a test stand in the laboratory,consisting of a single leg in scale 1:1, giving very good results.Fig. 3: Experimental setup2. EXPERIMENTAL SETUPThe leg of the ALDURO is anthropomorphic, i.e. it is based on the geometry and function of the human leg. The hip joint is a spherical ball joint with three degrees of freedom (d.o.f.) and the knee a revolute joint with one d.o.f. These joints are actuated with hydraulic cylinders (Fig. 4), whereas the two additional d.o.f. of the foot are passive. To make solution of the explicit inverse kinematics possible we lock one of the hip cylinders. The explicit solution of the direct and inverse kinematics is shown in detail in 4. To examine the dynamic behaviour of the leg, and to test different control schemes, an experimental setup for a fully sized leg was developed and built (see Fig. 3). It includes all the hydraulic components that will be used on the first prototype ALDURO and is driven by a stationary hydraulic power pack in the laboratory. The experimental set-up is equipped with an open hydraulic system with a 15kW electrical motor and an axial piston pump producing a volume flow of 40 l/min at 200 bar. This is smaller than what will be used on the real system, which will be powerd by a 27kW diesel engine. For tests with an insufficient hydraulic flow, a second set of four hydraulic cylinders (as used to move one leg) is mounted on the floor next to the test stand.An optical fieldbus for the transfer of the sensor and actuator data between the test stand and the control computer (with real-time operating system) is also installed. The hydraulic cylinders include position and pressure sensors that are used as controller inputs. Thus dynamic tests can be carried out to examine the co-operation between the mechanical and hydraulic components and the electronic control system.The hip plate of the experimental set-up is mounted on a pair of linear bearings, and thus is mobile in vertical direction. When combined with the foot, as shown in Fig. 3, or the passive wheel used on the hind leg of the combi