Legged locomotion in biological systems is a complex and not fully understood problem. A great progress to simplify understanding locomotion dynamics and control was made by introducing simple models, coined “templates”, able to represent the overall dynamics of animal (including human) gaits.

Before describing template based modeling we first summarize the challenges on robotics specially legged robots. See the following video for entering the discussion.

Inspired from what Russ said the challneges of the current technologies in robotics can be summarized in Fig. 1. Expensive robots reauiring highe precision sensors and actuators, with complex and accurate controller which needs precise design and manufacturing requiring huge amunt of energy are too far from biological locomotors.

Figure 1. Challenges of the current technologies in robotics.

The big question is how to resolve these issues. Shall we target more advanced sensors and actuators or more efficient batteries? Our solution is given in Fig. 2 for the next generation of robots. Low precision of the sensors and actuators can be compensated by bioinspired approaches which are based on simplification and counting on system dynamics. Therefore, our basic principles are simplification and bioinspiration.

Figure 3. Design and control of the next generation of legged robots.

Our understanding of legged locomotion is based on two fundamental ingredients shown in Fig. 3: Mechanics and control. The locomotor body (mechanics) need to be optimized to simplify and fit to control. The neural system in human body plays the role of the controller which matches the body properties.This combination needs to be robust against any uncertainties, perturbations, and disturbance. Such a robustness is achievable with simple design and the minimum dependency to operational tools like sensors and actuators.

Fig. 3. Basic elements in legged locomotion

In order to design a legged system, we use the trilogy shown in Fig. 4. First, we learn from nature (biology) to extract basic principles. The next step is building the models to verify the basic concepts and develop new methods. Finally, the design and control approaches need to be implemented on the hardware. One of the most useful applications of studying legged locomotion from a biomechanical point of view is implementing the ideas on new assistive devices like prostheses and orthoses.

Different steps in designing a legged system from the biomechanical point of view and its application in daily life.

Here, we focus on modeling as the second step. The goal is presenting an overview on advanced conceptual models and their extension. In this part, we describe template anchor concept, three locomotion sub-function concept and few template models for each sub-function.

Modeling of legged systems can be performed at two levels: 1- conceptual models 2- detailed models.

In the first group, simple models are used to describe selected basic features of legged locomotion. Such models are not considering the details of dynamics and control of the systems. Basic elements in physics like mass, spring, damper, and pendulum are employed to represent the fundamental behavior of the locomotor. Spring Loaded Inverted Pendulum (SLIP) and Inverted Pendulum (IP) are two of such template models.

The second group comprises of detailed dynamic models including body mechanics, actuators, sensors, and controller. For example neuromuscular models [Geyer10] or the human motion models in OPENSIM which includes EMG signal, muscle models, segemented mechanisms including details of length, mass and inertia are of the second group. In [Full99], the concept of template & anchor was introduced which connected these two groups (See Fig. 1)

Figure 1. Template & anchor concept from [Full99].

Inspired from template models explaining biological locomotory systems and Robert's pioneering legged robots, locomotion can be realized by basic sub-functions: (i) stance leg function, (ii) leg swinging and (iii) balancing. Combinations of these three sub-functions can generate different gaits with diverse properties. Using the template models, we can investigate how locomotor sub-functions contribute to stabilizing different gaits (hopping, running and walking) in different conditions (e.g., speeds). Such basic analysis on human locomotion using conceptual models can result in developing new methods in design and control of legged systems like humanoid robots and assistive devices (exoskeletons, orthoses, and prostheses).

Figure 5. Three locomotor subfunctions: Stance, swing, and balance.

With this approach, we can identify the relation between different locomotor sub-functions e.g., between balance and stance (using stance force for tuning balance control) or balance and swing (two joint hip muscles can support the swing leg control relating it to the upper body posture) and implement the concept of modular control based on locomotor sub-functions with a limited exchange of sensory information on several hardware platforms (legged robots, exoskeleton).

1. Explain the template & Anchor concept in 5 sentences with your words.
2. Another template model LLS (called lateral leg spring) mentioned in [Full99] from [Schmitt00]. Find this model and explain it in one paragraph.
3. What are the three basic locomotor subfunctions? Can you introduce physical template model (e.g. spring mass) for each of them?

[Geyer10] Geyer, H., & Herr, H. (2010). A muscle-reflex model that encodes principles of legged mechanics produces human walking dynamics and muscle activities. IEEE Transactions on neural systems and rehabilitation engineering, 18(3), 263-273.

[Full99] RJ Full and DE Koditschek. Templates and anchors: neuromechanical hypotheses of legged locomotion on land. Journal of Experimental Biology, 202(23):3325–3332, 1999.

[Schmitt00] Schmitt, J., & Holmes, P. (2000). Mechanical models for insect locomotion: dynamics and stability in the horizontal plane I. Theory. Biological cybernetics, 83(6), 501-515.