Hartmut Geyer

• PhD
• Professor (Assistant) at Carnegie Mellon University

In a recent human study, we found that adaptive changes in step length asymmetry (SLA) are correlated with similar changes in the H-reflex gains of the leg muscles during split-belt treadmill locomotion. While this observation indicated a closer link between gait asymmetry and spinal reflex adaptation, it did not reveal their causal relationship. T...

Behaviors of animal bipedal locomotion can be described, in a simplified form, by the bipedal spring-mass model. The model provides predictive power, and helps us understand this complex dynamical behavior. In this paper, we analyzed a range of gaits generated by the bipedal spring-mass model during walking, and proposed a stabilizing touch-down co...

Policy decomposition is a novel framework for approximating optimal control policies of complex dynamical systems with a hierarchy of policies derived from smaller but tractable subsystems. It stands out amongst the class of hierarchical control methods by estimating a priori how well the closed-loop behavior of different control hierarchies matc...

Walking is unstable and requires active control. Foot placement is the primary strategy to maintain frontal-plane balance with contributions from lateral ankle torques, ankle push-off and trunk postural adjustments. Because these strategies interact, their individual contributions are difficult to study. Here, we used computational modelling to und...

In a recent human study, we found that adaptive changes in step length asymmetry (SLA) are correlated with similar changes in the H-reflex gains of the leg muscles during split-belt treadmill locomotion. While this observation indicated a closer link between gait asymmetry and spinal reflex adaptation, it did not reveal their causal relationship. T...

The dynamics and interaction of spinal and suparspinal centers during locomotor adaptation remain vaguely understood. In this work, we use Hoffmann reflex measurements to investigate changes of spinal reflex gains during split-belt locomotor adaptation. We show that spinal reflex gains are dynamically modulated during split-belt locomotor adaptatio...

Policy Decomposition (PoDec) is a framework that lessens the curse of dimensionality when deriving policies to optimal control problems. For a given system representation, i.e. the state variables and control inputs describing a system, PoDec generates strategies to decompose the joint optimization of policies for all control inputs. Thereby, polic...

Spinal animals can regain locomotor function through gait training. However, the neural processes involved in this recovery are poorly understood. Here we use computer simulation to address if the reorganization of spinal circuits associated with the functional recovery leads to meaningful, stable circuitry function. Specifically, we develop a neur...

Existing models of human walking use low-level reflexes or neural oscillators to generate movement. While appropriate to generate the stable, rhythmic movement patterns of steady-state walking, these models lack the ability to change their movement patterns or spontaneously generate new movements in the specific, goal-directed way characteristic of...

Computing optimal control policies for complex dynamical systems requires approximation methods to remain computationally tractable. Several approximation methods have been developed to tackle this problem. However, these methods do not reason about the suboptimality induced in the resulting control policies due to these approximations. We introduc...

Existing models of human walking use low-level reflexes or neural oscillators to generate movement. While appropriate to generate the stable, rhythmic movement patterns of steady-state walking, these models lack the ability to change their movement patterns or spontaneously generate new movements in the specific, goal-directed way characteristic of...

Over the past two decades, a number of control strategies have been developed for powered transfemoral prostheses. These strategies have in common that they help restore gait in amputee locomotion. However, it remains unclear how well they support balance recovery after disturbances. Here, we first present a comparison of balance recovery performan...

Numerically computing global policies to optimal control problems for complex dynamical systems is mostly intractable. In consequence, a number of approximation methods have been developed. However, none of the current methods can quantify by how much the resulting control underperforms the elusive globally optimal solution. Here we propose policy...

The spinal cord is essential to the control of locomotion in legged animals and humans. However, the actual circuitry of the spinal controller remains only vaguely understood. Here we approach this problem from the viewpoint of learning. More precisely, we assume the circuitry evolves through the transfer of control from the brain to the spinal cor...

Maintaining balance during walking is a continuous sensorimotor control problem. Throughout the movement, the central nervous system has to collect sensory data about the current state of the body in space, use this information to detect possible threats to balance and adapt the movement pattern to ensure stability. Failure of this sensorimotor loo...

We present a control strategy for powered prostheses based on a robust estimate of the gait phase that is used to determine appropriate control actions. We use an extended Kalman filter (EKF) that fuses joint angle and velocity measurements to estimate the gait phase, which we define in this work to be a variable that progresses continuously during...

Despite enhancements in the development of robotic systems, the energy economy of today's robots lags far behind that of biological systems. This is in particular critical for untethered legged robot locomotion. To elucidate the current stage of energy efficiency in legged robotic systems, this paper provides an overview on recent advancements in d...

It is often assumed that the spinal control of human locomotion combines feed-forward central pattern generation with sensory feedback via muscle reflexes. However, the actual contribution of each component to the generation and stabilization of gait is not well understood, as direct experimental evidence for either is difficult to obtain. We here...

Learning controllers for bipedal robots is a challenging problem, often requiring expert knowledge and extensive tuning of parameters that vary in different situations. Recently, deep reinforcement learning has shown promise at automatically learning controllers for complex systems in simulation. This has been followed by a push towards learning co...

Owing to their morphology and mechanical design, bipedal robots have the ability to traverse over a wide range of terrain including those with discrete footholds such as stepping stones. This paper addresses the challenge of planar dynamic robotic walking over stochastically generated stepping stones with significant variations in step length and s...

This paper presents a method based on a human upper limb model that assesses the severity of spasticity in patients with stroke objectively. The kinematic model consists of four moving segments connected by four joints. The joint torques are computed using inverse dynamics with measurements from three inertial measurement units (IMUs) attached to t...

While neuro-musculo-skeletal models are a common tool in theoretical studies on human gait, they are rarely used for studying human motor control of standing balance. As a result, it is difficult to assess whether proposed control strategies of standing balance can be realized by the human neuromuscular structure. Nor is it clear how the human cont...

Biological bipeds have long been thought to take advantage of compliance and passive dynamics to walk and run, but realizing robotic locomotion in this fashion has been difficult in practice. Assume The Robot Is A Sphere (ATRIAS) is a bipedal robot designed to take advantage of the inherent stabilizing effects that emerge as a result of tuned mecha...

We study the control of human gait termination with a simple bipedal locomotion model. Several control strategies have been proposed for gait termination. However, the relative importance of these strategies has not been evaluated in models of human gait. Here we extend the bipedal spring mass walking model in a least parameter fashion and study th...

Key points: Although the natural decline in walking performance with ageing affects the quality of life of a growing elderly population, its physiological origins remain unknown. By using predictive neuromechanical simulations of human walking with age-related neuro-musculo-skeletal changes, we find evidence that the loss of muscle strength and mu...

Nature has solved the problem of controlling legged locomotion many thousands of times in animals exhibiting an enormous variety of neuromechanical structures. Control systems features that are common to nearly all species include feedback control of limb displacement and force and feedforward generation of motor patterns by neural networks within...

We seek to improve balance recovery in amputee gait by taking advantage of the advent of active leg prostheses. Toward this goal, we use inspiration from biology to identify reflex-like control strategies that stabilize gait, refine these strategies in simulations of amputee locomotion, and evaluate the resulting controllers in experiments with hum...

Controllers in robotics often consist of expert-designed heuristics, which can be hard to tune in higher dimensions. It is typical to use simulation to learn these parameters, but controllers learned in simulation often don't transfer to hardware. This necessitates optimization directly on hardware. However, collecting data on hardware can be expen...

Neuromuscular control models describe human locomotion by combining kinematic chain representations of the human skeleton with models of muscle-tendon actuators and control architectures mimicking neural circuits. The models have originally been developed to better understand human motor control. However, over the past two decades, they have reache...

Neuromechanical simulations have been used to study the spinal control of human locomotion which involves complex mechanical dynamics. So far, most neuromechanical simulation studies have focused on demonstrating the capability of a proposed control model in generating normal walking. As many of these models with competing control hypotheses can ge...

Spinal-reflex-based neuromuscular model of human locomotion.

Simulation replications of the human disturbance experiments.

Theoretical spring mass models with deadbeat foot placement policies reveal very robust running in the presence of large and frequent, unexpected gait disturbances. Although this performance goes beyond what has been demonstrated on running machines, a transfer of this theory has only been investigated for simplified monopod systems. Here we invest...

We present a new algorithm for optimizing control policies for human-in-the-loop systems based on qualitative preference feedback. This method is especially applicable to systems such as lower-limb prostheses and exoskeletons for which it is difficult to define an objective function, hard to identify a model, and costly to repeat hardware experimen...

This chapter emphasizes that spring-like leg function is a commonly observed Feature during movements, which can be described by characteristic leg parameters. It first describes the leg function during the take-off phase in the long jump, to explain the interplay between leg stiffness and leg angle at touch-down in optimizing jumping distance. The...

Series elastic actuators primarily use linear springs in their drivetrains, which introduces a design tradeoff: soft springs provide higher torque resolution at the cost of system bandwidth, whereas stiff springs provide a fast response but lower torque resolution. Nonlinear springs (NLSs) potentially incorporate the benefits of both soft and stiff...

Animals and robots balance dynamically by plac ing their feet into proper ground targets. While foot placement controls exist for both fully robotic systems and powered prostheses, none enable the dynamism and reactiveness of able- bodied humans. A control approach was recently developed for an ideal double pendulum dynamical system that places fee...

Lower limb amputees are at high risk of falling as current prosthetic legs provide only limited functionality for recovering balance after unexpected disturbances. For instance, the most established control method used on powered leg prostheses tracks local joint impedance functions without taking the global function of the leg in balance recovery...

In this paper we propose the fastest converging control policy (also known as deadbeat control) for walking with the bipedal spring-mass model, which serves as an abstraction of a robot on compliant legs. To fully leverage the passive dynamics of the system, the touchdown angle of the swing-leg is assigned as the only control input of the system. W...

The analysis of the conceptual spring mass model for running reveals swing-leg placement policies that generate very robust locomotion in unobserved terrain with large changes in ground height. However, while this theoretical result suggests a potential for large improvements on the robustness of running machines, it has so far not been demonstrate...

Neural networks along the spinal cord contribute substantially to generating locomotion behaviours in humans and other legged animals. However, the neural circuitry involved in this spinal control remains unclear. We here propose a specific circuitry that emphasizes feedback integration over central pattern generation. The circuitry is based on neu...

Motivated by an interest in human-like controllers for humanoids to increase their social acceptance, we investigate lateral balancing for artistic performances on challenging surfaces. Control design for lateral balancing in humanoids has primarily focused on optimal control techniques. While these techniques generate balancing controllers, it rem...

Transfemoral amputees often suffer from falls and a fear of falling that leads to a decreased quality of life. Existing control strategies for powered knee-ankle prostheses demonstrate only limited ability to react to disturbances that induce falls such as trips, slips, and obstacles. In contrast, prior work on neuromuscular modeling of human locom...

A model-based neuromechanical controller for a robotic limb having at least one joint includes a finite state machine configured to receive feedback data relating to the state of the robotic limb and to determine the state of the robotic limb, a muscle model processor configured to receive state information from the finite state machine and, using...

Control design of running robots is often based on mapping the behavior of lower order models onto the robotic systems, and the robustness of running is largely determined by the robustness of these underlying models. However, existing implementations do not take full advantage of the stability that the low order models can provide. In particular,...

Series elastic actuators often use linear metal springs in their drivetrains, which requires design compromises between torque resolution and actuation bandwidth. Nonlinear springs (NLSs), with variable stiffness, overcome this limitation, enabling both high torque resolution and high bandwidth. Current NLS designs combine variable cam structures w...

Over the past three decades, the spring-mass model has developed into the basic behavior model to study running in animals and robots. In the planar version, this model has helped to reveal and understand the passive stabilization of running in the horizontal and sagittal planes, and to derive from this knowledge control strategies for running robo...

Papers from a flagship conference reflect the latest developments in the field, including work in such rapidly advancing areas as human-robot interaction and formal methods. Robotics: Science and Systems VIII spans a wide spectrum of robotics, bringing together contributions from researchers working on the mathematical foundations of robotics, robo...

The human foot, which is the part of the body that interacts with the environment during locomotion, consists of rich biomechanical design. One of the unique designs of human feet is the windlass mechanism. In a previous simulation study, we found that the windlass mechanism seems to improve the energy efficiency of walking. To better understand th...

Understanding the neuromuscular control underlying human locomotion has the potential to deliver practical controllers for humanoid and prosthetic robots. However, neurocontrollers developed in forward dynamic simulations are seldom applied as practical controllers due to their lack of robustness and adaptability. A key element for robust and adapt...

The neural controller that generates human locomotion can currently not be measured directly, and researchers often resort to forward dynamic simulations of the human neuromuscular system to propose and test different controller architectures. However, most of these models are restricted to locomotion in the sagittal plane, which limits the ability...

Swing leg placement is vital to dynamic stability in legged robots and animals. The most common approaches to generating swing leg motions in robotics use either position or impedance tracking of defined joint trajectories. While these approaches suffice in humanoids, they severely limit swing leg placement under large disturbances in prosthetic li...

Swing leg placement is vital to dynamic stability in legged robots and animals. The most common approaches to generating swing leg motions in robotics use either position or impedance tracking of defined joint trajectories. While these approaches suffice in humanoids, they severely limit swing leg placement under large disturbances in prosthetic li...

Several recent studies on the control of legged locomotion in animal and robot running focus on the influence of different leg parameters on gait stability. In a preceding investigation self-stability controls showing deadbeat behavior could be obtained by studying the dynamics of the system in dependence of the leg orientation carefully adjusted d...

Current control approaches to robotic legged loco-motion rely on centralized planning and tracking or motion pat-tern matching. Central control is not available to robotic assistive devices that integrate with humans, and matching predefined patterns severely limits user dexterity. By contrast, biological systems show substantial legged dexterity e...

Parallel passive-elastic elements can reduce the energy consumption and torque requirements for motors in powered legged systems. However, the hardware design for such combined actuators is challenged by the need to engage and disengage the parallel elasticity depending on the gait phase. Although clutches in the drive train are often proposed, com...

Although current humanoid controllers can rely on inverse kinematics or dynamics of the full humanoid system, powered prosthetic legs or assistive devices cannot, because they do not have access to the full states of the human system. This limitation creates the need for alternative control strategies. One strategy is to embed fundamental knowledge...

While humanoid feet are made of rigid plates, human feet have evolved into highly articulated and flexible elements. This adaptiveness provides key advantages. It absorbs impacts and secures grip when interacting with the environment. However, the human foot design potentially increases the energetic cost, because it features actuators and provides...

Dynamic balance depends on proper foot placement in legged locomotion and corresponding placement strategies have mainly been developed using the linear inverted pendulum model as theoretical framework. While this model can identify single leg strategies for balance control, it does not consider the double support that is common to bipedal locomoti...

Obstacle avoidance in bipedal robots is achieved with the help of sensory feedback and closed loop control. Although computational power increased exponentially during the last years it is still the limiting factor for dynamic locomotion in uneven terrain. We in-troduce a simple robot architecture based on compliant leg behavior. With minimal senso...

Control schemes for powered ankle-foot prostheses rely upon fixed torque-ankle state relationships obtained from measurements of intact humans walking at target speeds and across known terrains. Although effective at their intended gait speed and terrain, these controllers do not allow for adaptation to environmental disturbances such as speed tran...

In legged systems, springy legs facilitate gaits with subsequent contact and flight phases. Here, we test whether electrical motors can generate leg behaviors suitable for stable hopping. We built a vertically operating sledge actuated by a motor-driven leg. The motor torque simulates either a linear leg spring or a muscle-reflex system. For stable...

Research on the biomechanics of animal and human locomotion provides insight into basic principles of locomotion and respective implications for construction and control. Nearly elastic operation of the leg is necessary to reproduce the basic dynamics in walking and running. Elastic leg operation can be modelled with a spring-mass model. This model...

The basic mechanics of human locomotion are associated with vaulting over stiff legs in walking and rebounding on compliant legs in running. However, while rebounding legs well explain the stance dynamics of running, stiff legs cannot reproduce that of walking. With a simple bipedal spring-mass model, we show that not stiff but compliant legs are e...

To walk or run, most legged robots feedback-control predefined joint trajectories. Although this approach works in the laboratory, it hardly succeeds on natural ground, because in such an uncertain and irregular terrain this control requires a sensory precision and an actuator bandwidth that are beyond current technologies. No such requirement is f...