Goals of the project                

Research on Hybrid Actuation 

Within the HADE framework, new actuators based on hybrid technologies (smart materials and conventional technologies) are being studied and some prototypes have been tested and characterized. First a comparative analysis of novel technologies for actuators has been performed. The comparison has been based on an experimental characterization of actuators in terms of specific power, force and speed. The characterization has been performed using two ad-hoc test benches shown in Fig. 1.

(a)                                                 (b)
Figure 1: Actuator test benches developed in the HADE project; (a) General test bench; (b) Small actuator test bench

Following the idea of combining technologies, the HADE project studies the use of Series Elastic Actuation (SEAs) with Magneto-rheological fluid (MRF) active dampers in new biomimetic designs for robotic legs to achieve energy-efficient adaptable damping at series elastic actuation; and the combined use of Magnetic Shape Memory Alloys (MSMA) with piezo-acoustic assistance for improving the power of piezohydraulic actuators. The new developments are being characterized using two versatile test benches designed and developed for this project purposes shown in Fig.1: a general-actuator testbench, devised for analysis and characterization of any type actuator (i.e. hydraulic, pneumatic, electromechanical, linear, rotary drive) of medium size (between 2 and 50 cm long) (see Fig.1(a)) and a MSM actuator testbench, specifically designed for small actuators (below 2 cm long) of short stroke (below 1 cm) (see Fig.1(b))

The HADE Leg

Besides designing and developing new actuators for empowering legged robots, the HADE project intends to use them in a real robot prototype. For this purpose, a real prototype of a leg for agile locomotion has been developed. The leg is also intended to be a testbench for the new actuators being developed, in order to analyse their combined performance to fulfil the leg kinematic and dynamic requirements. The target is to achieve an average robot speed of 1.5 m/s carrying a payload twice the robot's weight. Therefore, the HADE leg has been designed to easily place and remove actuators from its joints and to measure overall performance. Figure 2 shows the second prototype of the HADE leg: its biomimetic concept, its design consisting of two Series Elastic Actuators at knee and hip, a MRF damper at the knee to control knee damping and a Superficial Digital Flexor tendon tha passively drives the ankle joint and provides elastic recoil to the  gait cycle.

         
(a)                                                   (b)                                                    (c)
Figure 2: The HADE2 Leg for Agile Locomotion: (a) biomimetic concep; (b) mechanical design; (c) real prototype

In order to design a leg mechanism able to provide the robot with agile features, nature could be the best source for inspiration. Horse’s legs have been evolved to provide speed, endurance, and strength superior to any other animal of equal size. However, in the process of copying from nature a desired system performance, one has to be careful in what issues must be extracted and translated to a technological design. The job of the biomimeticist is to identify those elements responsible for producing the desired characteristics on biological systems and to extract the key principles underlying their biological function and then translate them to a technological instantiation that is limited by its own human engineering. 

 The adaptation of horses legs towards an agile performance is based on longer legs than similar quadrupeds relative to the body size, which provide longer stride lengths. The horse’s legs are relatively lightweight, featuring a mass distribution which improves its oscillation frequency. The leg kinematic structure has evolved to optimize the use of its joints for load bearing. Added to relevant muscle power and with enough economy of effort to provide endurance, which is achieved by means of elastic energy storage in tendons during certain phases of the locomotion cycle and the later return of this energy to the more exigent phases.

These principles underlying horse legs power capabilities have been extracted in this work and translated to a biomimetic leg concept for the final technological instantiation of a leg prototype with the future goal of developing an agile quadruped.

Experiments carried out with the leg prototype have been conducted to test the effectiveness of using MRF dampers at the knee to reduce energy consumption. Video 1 below shows knee motion at 1Hz frequency with the leg in support phase. Video 2 shows the same knee motion with an added payload of 13 Kg which simulated the body weight supported by the leg. The Magneto-rheological Fluid Damper at the knee is shown in the video.

Video 1: Knee motion at 1Hz frequency combining SEA and MRF Brake

Video 2: Knee cycle at 1 Hz with MRF damping and 13kg body weight

See more videos of the locomotion of the HADE2 leg walking at 1.2 m/s in Section videos below.

EMG Command of Series Elastic Actuators

Some experiments were conducted to test the EMG-commanded motion of Series Elastic Actuators. These SEAs are controlled using a compliance-control scheme. The force reference to the controller is based on the user’s intention to move by means of Electromyographic (EMG) signals. Video 3 shows the experimental parameter identification for the EMG-force model, and Video 4 shows an experiment of the SEA actuating the ankle of the HADE leg reproducing the motion of the human ankle.

Video 3: Experimental identification of the EMG-based user/robot force model

2.- Garcia, E., Arevalo, J.C., Muñoz, G. and Gonzalez de Santos, P. "Combining series-elastic actuation and magneto-rheological damping for the control of agile locomotion" Robotics and Autonomous Systems, Vol. 59, No. 10, pp. 827–839, 2011