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Compliance control is highly relevant to human safety in human–robot interaction (HRI). This paper presents a review of various compliance control techniques. The paper is aimed to provide a good background knowledge for new researchers and highlight the current hot issues in compliance control research. Active compliance, passive compliance, adaptive and reinforcement learning-based compliance control techniques are discussed. This paper provides a comprehensive literature survey of compliance control keeping in view physical human robot interaction (pHRI) e.g., passing an object, such as a cup, between a human and a robot. Compliance control may eventually provide an immediate and effective layer of safety by avoiding pushing, pulling or clamping in pHRI. Emerging areas such as soft robotics, which exploit the deformability of biomaterial as well as hybrid approaches which combine active and passive compliance are also highlighted.

Most robots are today controlled as being entirely rigid. But often, as for HRP-2 robot, there are flexible parts, intended for example to absorb impacts. The deformation of this flexibility modifies the orientation of the robot and endangers balance. Nevertheless, robots have usually inertial sensors inertial measurement units (IMUs) to reconstruct their orientation based on gravity and inertial effects. Moreover, humanoids have usually to ensure a firm contact with the ground, which provides reliable information on surrounding environment. We show in this study how important it is to take into account these information to improve IMU-based position/orientation reconstruction. We use an extended Kalman filter to rebuild the deformation, making the fusion between IMU and contact information, and without making any assumption on the dynamics of the flexibility. We show how, with this simple setting, we are able to compensate for perturbations and to stabilize the end-effector's position/orientation in the world reference frame. We show also that this estimation is reliable enough to enable a closed-loop stabilization of the flexibility and control of the center of mass (CoM) position with the simplest possible model.

Joints’ backdrivability is desired for robots that perform tasks contacting the environment, in addition to the high torque and fast response property. The electro-hydrostatic actuator (EHA) is an approach to realize force-sensitive robots. To experimentally confirm the performance of a biped robot driven by EHAs, we developed the fully electro-hydrostatically driven humanoid robot Hydra. In this paper, we evaluate the whole-body control performance realized by integrating encoders, pressure sensors, and IMU through a high-speed communication bus to the distributed whole-body control system. We report the first example of bipedal locomotion by an EHA-driven robot in both position-controlled and torque-controlled approaches. The robot could keep the balance even when the ground condition was changing impulsively and utilize its high joint backdrivability to absorb a disturbance by the null space compliance. We also report practical challenges in implementing compliant control in real hardware with limitations in parameter accuracy, torque, and response. We experimentally confirmed that the resolved viscoelasticity control (RVC), which has indirect feedback of operational space tasks by projecting the operational space feedback gain to the joint space one, was effective to tune a proper gain to stabilize the center-of-mass motion while avoiding joint-level oscillation invoked by the control bandwidth limitation. The attached multimedia file includes the video of all experiments presented in the paper.

Biomechanics research shows that the ability of the human locomotor system depends on the functionality of a highly compliant motor system that enables a variety of different motions (such as walking and running) and control paradigms (such as flexible combination of feedforward and feedback controls strategies) and reliance on stabilizing properties of compliant gaits. As a new approach of transferring this knowledge into a humanoid robot, the design and implementation of the first of a planned series of biologically inspired, compliant, and musculoskeletal robots is presented in this paper. Its three-segmented legs are actuated by compliant mono- and biarticular structures, which mimic the main nine human leg muscle groups, by applying series elastic actuation consisting of cables and springs in combination with electrical actuators. By means of this platform, we aim to transfer versatile human locomotion abilities, namely running and later on walking, into one humanoid robot design. First experimental results for passive rebound, as well as push-off with active knee and ankle joints, and synchronous and alternate hopping are described and discussed. BioBiped1 will serve for further evaluation of the validity of biomechanical concepts for humanoid locomotion.

A mechanical mock of the cardiovascular system was used to simulate different conditions of ventricular-arterial mechanical matching. Four ventricles and four aortas with different elastances were set up, and all possible connection combinations tested by sampling ventricular and aortic pressures and flows. The mechanical energy produced by the simulated ventricles and the amount transferred to the aorta in the different connection conditions were calculated. The results demonstrate a clear dependence between the mechanical work production of the ventricles and the ventricular elastance (slope of the end-systolic pressure–volume relation) (from 20.42 ± 0.02 mJ/beat to 12.10 ± 0.02 mJ/beat), and an efficiency of energy transfer to the aorta strongly dependent on ventricular-aortic mechanical matching (from 56% to 20.7%). These results show that, even in optimal simulated conditions, only 56% of the energy produced is transferred to the load; and highlight the important role of mechanical aspects in conditions of very limited cardiovascular performance (i.e. final stages of heart failure), where energy transfer efficiency may be as low as 20.7%. This evidence emphasizes that the mechanical aspects must also be entertained in the evolution of complex cardiovascular pathologies, evaluating the possibility of combining mechanical and pharmaceutical interventions.

This paper contains an adjustment of the stiffness and control algorithm of an ankle joint to maintain the posture of a humanoid biped walking robot using an IMU (Inertial measurement Unit) in single support phase. One of the difficulties in maintaining the balance of a robot in single support phase involves the sole of the robot, which is easy to separate from the ground. This phenomenon is caused by an external disturbance or when the sole lands on uneven terrain. A method of adjusting the compliance in the ankle joint is introduced to reduce the tendency of this type of occurrence. In order to maintain the posture of a robot with a compliant ankle joint, a posture controller composed of a body balancing controller and a vibration reduction controller is adopted. A walking experiment is implemented using a compliant ankle joint and the posture controller. The proposed method is shown to enable rapid walking as well as walking on uneven terrain.

Compliance control is highly relevant to human safety in human–robot interaction (HRI). This paper presents multi-dimensional compliance control of a humanoid robot arm. A dynamic model-free adaptive controller with an anti-windup compensator is implemented on four degrees of freedom (DOF) of a humanoid robot arm. The paper is aimed to compliment the associated review paper on compliance control. This is a model reference adaptive compliance scheme which employs end-effector forces (measured via joint torque sensors) as a feedback. The robot's body-own torques are separated from external torques via a simple but effective algorithm. In addition, an experiment of physical human robot interaction is conducted employing the above mentioned adaptive compliance control along with a speech interface. The experiment is focused on passing an object (a cup) between a human and a robot. Compliance is providing an immediate layer of safety for this HRI scenario by avoiding pushing, pulling or clamping and minimizing the effect of collisions with the environment.

When executing tasks, robots are required to demonstrate compliance to unexpected external disturbances or human–robot interactions, and return to the demanded posture when the disturbances or contacts are removed. Traditional Virtual Model Control (VMC) requires precise gravitational compensation to accurately control the posture of a robot. Hence, load variations or other uncertain unmodeled factors in the robot will result in offsets to its balance posture, which makes the robot deviate from the demanded posture when it is in a static state. To reject this offset without sacrificing the compliance of the robot, an adaptive controller is proposed in this paper to implement adaptive compliance on the robot, which makes the robot robust to the variations in gravitational loads in the double leg support phase. The adaptive controller is a combination of the VMC controller and an online gravitational loads estimator, in which the estimator is derived in the double leg support phase to estimate the values of these parameters and obtains an online updating law based on a Kalman optimal estimator. Then, a Lyapunov function is designed to modify and combine the controller and the online gravitational loads estimator. The experiments are conducted on a 4 DoF bipedal robot in the sagittal plane to validate the effectiveness of the controller and show that, by estimating the gravitational loads of the robot, the effects of load variations on balance posture are rejected without sacrificing compliance.

Using unique data on chemical manufacturing facilities regulated under the Clean Water Act (CWA), this paper analyzes the effect of one particular environmental management practice — internal monitoring — on regulated facilities’ compliance with wastewater discharge limits. To deepen our understanding of the role played by internal monitoring, we extend our analysis by exploring the interaction between internal monitoring and self-audits, which should share a similar purpose to internal monitoring by providing systematic information to regulated facilities on their pollution abatement efforts. Baseline empirical results robustly demonstrate that increases in the extent of internal monitoring significantly improve facilities’ environmental compliance. The extended empirical results offer moderate evidence that audits complement internal monitoring (and vice versa): as facilities audit more frequently, the productive effect of increased internal monitoring grows.

Traditional arterial tonometry permits noninvasive and continuous recording of the arterial pressure waveform, by applanating a superficial artery supported by a bone. In the paper, we present an arterial tonometer to simultaneously register the blood pressure waveform and the arterial time-varying volume. The tonometer consisted mainly of a chamber filled with a conductive fluid, a flexible diphragm in touch with an artery, and a pressure sensor used to detect the underlying arterial pressure. In addition, four electrodes were in parallel diposed in the chamber, two of them were triggered with a constant-current source, and the voltage difference between the other two inner electrodes was assocated with the amount of change in the arterial volume. The pressure calibration curve performed with a mercury sphygmomanometer showed a fairly linear relationship (r = 0.998) between the tonometer's chamber pressure and the voltage output of the pressure-sensing circuit. The volume calibration was carried out with vessel-like cylinders of various diameters and a linear relationship (r = 0.884) of the change in vessel volume to the voltage output of the volume-sensing circuit was obtained. Clinical testing results revealed that the noninvasive blood pressure measurement with the tonometer was appreciably consistent with the invasive measurement with the catheter-tipped pressure transducer. In summary, the arterial applanation tonometer developed may be used to reliably determine the full arterial blood pressure waveform and the change in the arterial volume, and to make the wall compliance assessment of a superficial artery possible.

The energetic effects of knee locking and addition of linear elastic members to different joints of a seven-link fully actuated planner bipedal robot were studied. The focus was on the reduction of energy consumption during walking. An impactless walking gait was studied and the energetic cost of walking was determined without joint stiffness and knee locking as a baseline for comparison. The gait trajectory was then optimized by adding spring to different joints, energetic cost of walk was then calculated at different walking speeds. Support knee was then mechanically locked and gait was optimized to find the walking cost. The energetic cost of walking determined for the above two cases was then compared to the baseline cost. It was observed that addition of torsional springs at both hips can reduce the walking cost up to 38%, support hip up to 40% with spring stiffness as an optimization parameter for both cases while mechanically locking the support knee can reduce the cost of walking up to 92% at slow walking speeds with gait and knee locking angle optimized.