Nowadays, actuating robotic systems is still one of the biggest challenges. High performances in actuation are needed to enhance behaviours of these systems, while more and more requirements are needed for safety, compliant and human-friendly. Especially, since new generation of robotic systems have to interact with humans and with the environment. This interaction is essential not only in the field of humanoid robotics, it is also applicable to rehabilitation devices, such as prothesis and orthesis. For instance, within the field of humanoid robotics, essential and desirable properties for actuators have to include: 1) high power to mass ratio; 2) ability to produce high torque at low speed; 3) highly integratable (reduction of occupied volume); 4) able to generate smooth human-like movements.
Actuation of robotic system such as humanoid robots is basically based on two major solutions: 1) Electric; and 2) Hydraulic. Electric actuation is typically used for humanoid robots. It is worthy to note that electric actuators have the advantages of reduced cost and their easiness of usage and control. However, a number of disadvantages appear when using electric motors with mechanical reduction device. First of all, due to the quasi-rigid connection between the motor and its payload, without developing a specific control algorithm or adding supplementary mechanical components, it is difficult to produce the stiffness changes needed for safety. Another interesting technology to actuate robotics systems such as humanoids is the use of hydraulic energy. This technology, based on a hydraulic central group, showed exceptional performances in these last years. However, the hydraulic central group solution suffers from several drawbacks. First, and in our opinion the major one, is related to the whole system dimensioning leading to the necessity to satisfy the worst case requirements in terms of flow and pressure needed by all the joints. Another disadvantage, is linked to the the increase of the whole size and the weight of the system, due to, one servo-valve has to be included for each hydraulic actuator. Carrying on the hydraulic central group limits drastically the use of this technology in the case of the development of autonomous systems. The used servo valves to control hydraulic actuators leads also to severe decrease in back-drivability. Further drawback concerns the hydraulic tubes passing through the joints needed to connect the hydraulic motors to the central group. This induces an increase of potential leakage in the connections and pressure drop.
The proposed solution is based on the possibility to merge the above mentioned solutions, in order to take benefit of their advantages. This leads to a technology named Hydrostatic Transmission. The mean idea is to create the hydraulic energy independently for each joint. In this case, hight torque and fluid motion are easy to be produced, pressure can be optimized for each joint, no pipes will passe through the joints. Two possibilities to control the hydrostatic transmission. First one, is to use a fixe displacement pump with electro motors, then the system can be controlled by controlling the motor's speed. the other technology is to used a variable displacement pump with integrated servo-valve, in this case the system can be controlled by the servo-valve or by the motor, which give more possibilities to realize the need motion. for this reason, our hydrostatic transmission is using a integrated servo-valve to control the system.
A new high performance Integrated Electro-Hydraulic Actuator (IEHA) was developed in collaboration between LISV,BIA and ICS. Figure1 show a simplified model of the proposed system. The shaft (1) is connected with electric motor, which is assumed to rotate at high speed. The pistons (2) rotate with the shaft and slide on an inner roller bearing part inserted itself in a carriage (3).
The distance E between the bearing and the shaft centers, called eccentricity, will produce radial movement of the pistons. This allows the pistons to aspirate liquid when they are moving away from the center and to send it out when they are moving toward the shaft axis. Fig (1) Fig(2) The pressure and the flow produced by this phenomenon is a function of the E distance. Increasing E will increase the flow and hence the hydraulic power produced by the pump. Keeping a constant E value will maintain the flow constant but permits the pressure to increase. Changing the direction of the link motion can be obtained by using negative values of E while keeping the same direction of the electric motor rotation. Hence, this will simplify the electric motor control, as one of advantages of the proposed solution. In the proposed solution, the E distance is controlled hydraulically. A micro- valve (4) is built inside the carriage (3). External hydraulic circuit presented by T, P is connected to the micro-valve (4). This micro-valve will be actuated electrically by the integrated induction coil (5). Hence, activating the coil will move the micro valve to the right permitting the high pressure P to be connected to the left side of the carriage (3). The low pressure T will be connected to the right side of the carriage (3). Consequently, the carriage (3) will move to the right, which will change the value of E. Hence the proposed solution can be considered as a continuous transmission ratio varying system – allowing us to take just the amount of power when needed. We developed a IEHA prototype to demonstrate the real performances of the system. Figure (2) shows the CAD scheme of the IEHA first prototype designed. Figure (3) presents the real prototype with Dimensions 40x40x80 mm^3. Preliminary experimental results present the capacity of this prototype, moving approximately 38 kg at 2 cm/s. Fig (3)