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Study Reviews Control Strategies for Active Lower Limb Assistive Devices

Prostheses and orthoses have become increasingly advanced, enabling mobility for users. However, challenges still remain in the development of innate control between the device and the user’s neuromusculoskeletal system.

Michael R. Tucker, doctoral student at the Rehabilitation Engineering Laboratory and Swiss Federal Institute of Technology, reviewed the latest techniques for controlling robotic lower limb devices, as well as the challenges and opportunities surrounding them.

Initial review

The review, recently published in the Journal of Neuroengineering and Rehabilitation, examined control methods for wearable hip, knee and ankle devices.

While power output characteristics vary substantially among these devices, and between prostheses and orthoses overall, there are some concepts that can be applied universally, the researchers found.

This research presents a classification scheme and framework, which offer guidelines for accelerating prosthetic development and increasing activities of daily living in users. The framework accounts for physical and informatic interactions between the controller, the user, the environment and the device. Each element also considers locomotion and input to the controller.

“First, we took a look at the user in order to understand the native control structure and how it can be tapped into for control outputs and sensory feedback,” Tucker told O&P News. “Then, we looked at how the device can translate that information, along with sensed information about the environment, into a trajectory for the robotic prosthesis or orthosis to follow.”

Levels of control

Researchers found that the controller of a prosthetic or orthotic device can be subdivided into three parts: high-, mid- and low-level.

A high-level controller must perceive locomotive intent of the user through a combination of activity mode detection and direct volitional control.

Activity mode detection identifies the current locomotive task, such as standing, level walking or stair descent. Direct volitional control allows the user to manipulate the device’s state, joint positions, velocities and torques. This shared control approach limits the cognitive burden on the user.

“In essence, very complex high-level motion intentions, when combined with sensory information, are converted into low-level commands for the individual joints to follow,” Tucker said. “This task is facilitated by the distributive nature of the controller, which is why most able-bodied subjects rarely put much thought into how they walk.”

At the mid-level, locomotive intent is translated from the high-level controller to a desired device state for the low-level controller to track. The user’s state within the gait cycle is also determined at this level.

The mid-level controller is responsible for coordinating control between actuated joints across one or multiple devices.

Additionally, a control law is applied in the form of position, velocity, torque or impedance. The input and output control laws impact the device’s ability to interact with the user and the environment.

At the low-level, the controller drives the actuator, calculating and reducing any error between a device’s current and desired states. This is achieved through feedforward or feedback control, and typically accounts for kinematic and kinetic properties of the device.

Further considerations

Mechanical design and implementation of O&P devices were beyond the scope of the review, but findings suggest that the hardware still be considered as it could influence control possibilities.

Since most devices are in close physical contact with the user and generate substantial output forces, passive and active safety mechanisms should be of high importance, Tucker said. They should also take into account individual user capabilities and cognition in order to achieve better functional outcomes.

“There are many issues related to the safety and efficacy of active lower limb prostheses and orthoses,” Tucker said. “Such devices are capable of generating substantial, potentially injurious forces, and the penalties for misclassifying a user’s motion intentions can be serious.”

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But these issues can be addressed through safety mechanisms that are unique to active devices relative to their passive counterparts, he added. They can also be addressed through further refinement of sensing and control systems, incorporation of fail-safe mechanisms and additional user training on how to react and recover from a device failure.

“It is proposed that for the optimal controller, the communication between the user and the prosthesis or orthosis must be bidirectional. The device should not only recognize and respond to the user’s motion intentions, but provide intuitive sensory feedback that will allow the user to react and respond to the dynamic conditions underlying human gait.”

Another area of concern is performance limitations and saturation effects of the device. Even state-of-the-art portable devices must be driven beyond operating range to achieve power outputs of demanding activities.

While this is generally acceptable for short periods of time, it may be necessary for the controller to de-rate the actuator if conditions are sustained over longer periods.

Energy efficiency could optimize device performance, the study found. In addition to minimizing energy expenditure of the user, the device should minimize its own energy consumption to extend its range of operation.

Future initiatives

“[While there are many methods for controlling assistive devices], it is impossible to say at this point whether there is a best technique for controlling a particular joint or class of device,” Tucker said.

“What is required is a set of benchmarks that can be used to evaluate the performance of these devices, especially in the context of everyday locomotion during activities of daily living.”

The overarching design goal is seamless integration of the device with the user’s sensory-motor control loops, residual musculoskeletal and central nervous systems.

A well-designed prosthetic and orthotic controller must have an understanding of the user. It must be robust enough to accommodate gait patterns that are far-removed from the nominal condition, Tucker said.

“[More advanced orthotic and prosthetic devices] are coming, and when they do, it will be necessary for those involved with prescribing, maintaining and insuring such devices to have enough of an understanding of the underlying technology to effectively put them into service.”

This research serves as a template for that, he said, and offers a basis for developing future multifunctional controllers for lower limb devices.

“We hope this review provides a vocabulary that can be used in between practitioners and engineers for future development of prosthesis and orthoses.

“We are drawing ever closer to the ultimate goal of providing actuated devices for the assistance of locomotion…and researchers around the globe are contributing pieces to the puzzle. We hope this study can provide structure to that puzzle.” – by Shawn M. Carter

References:

Tucker M. J Neuroeng Rehabil. 2015;doi:10.1186/1743-0003-12-1

Control strategies for active lower extremity prosthetics and orthotics: a review. Available at www.jneuroengrehab.com. Accessed Feb. 2, 2015.

Disclosure: Tucker reports that his research was supported by the Swiss National Center of Competence in Research on Robotics.

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