Adaptive Control of Quadruped Locomotion Through Variable Compliance of Revolute Spiral Feet

Abstract

In this article we present a novel mechanical design of a robot leg that possesses active and variable passive compliance properties. The hip and knee joints provide active compliance, while the variable passive compliance comes from the spiral foot spring, mounted on the ankle joint, which changes its stiffness by rotating and changing contact angle with the ground. The stiffness of the foot for various contact angles was identified experimentally by using the strength tester measurement system. The method for damping coefficient identification, based on the observation of energy losses during the stance phase of leg hopping motion, is described and used to obtain the foot damping model. The adaptation of spiral foot stiffness to varying ground stiffness is achieved by extracting a leg contact time from a feedback signal provided by a flex sensor mounted on the foot. The experiments on a single leg and quadruped platforms have confirmed that the presented spiral foot design provides stiffness adaptability, partial recovery of the energy from the previous hop and restriction of stance contact time, which are all necessary conditions to obtain more efficient quadruped locomotion.

New Variable Passive-Compliant Element Design for Quadruped Adaptation to Stiffness-Varying Terrain

“…In order to evaluate the beneficial aspect of the newly designed quadruped legs with VPC elements, we performed a set of experiments to compare the previous [26] and current design of the VPC element. The benefit of the new VPC design is that it provides a more stable hopping cycle by eliminating the spiral spring flexion in nondesirable directions, as shown in Figure 3, allowing the robot to hop continuously in one place.…”

“…During experiments with a single leg in the previous work [26], an additional problem was identified. The previous VPC element was unable to produce constant hopping in one place since the contact surface of the revolute spring was slightly curved.…”

“…In this work, we extend the concept of the variable passivecompliant leg design described in [26] with additional beneficial extensions. The VPC element design in [26] proved to be an adequate option for the simple integration of VPC behaviour into a robot leg.…”

“…Under the known leg stiffness, the change in terrain characteristics will influence the change in the stance time t s , which can be used to calculate the α position needed in order to compensate for the disturbance more quickly. An example is presented in [26,39], but the controller design for the purposes of terrain adaptation goes beyond the scope of this work. Besides energy storage characteristics, the additional benefit of the newly designed VPC element is the ability to vary and adapt the quadruped hopping frequency which is otherwise affected by changes to ground surface stiffness.…”

“…The VPC element design in [26] proved to be an adequate option for the simple integration of VPC behaviour into a robot leg. The design relies on structural compliance.…”