Proportional myoelectric control can be used to (among other purposes) activate robotic lower limb exoskeletons. A proportional myoelectric control system utilizes a microcontroller or computer that inputs electromyography (EMG) signals from sensors on the leg muscle(s) and then activates the corresponding joint actuator(s) proportionally to the EMG signal.
A robotic exoskeleton is a type of orthosis that uses actuators to either assist or resist the movement of a joint of an intact limb; this is not to be confused with a powered prosthesis, which replaces a missing limb. There are four purposes that robotic lower limb exoskeletons can accomplish: [2]
Robotic lower-limb exoskeletons can be controlled by several methods, including a footswitch (a pressure sensor attached to the bottom of the foot), gait-phase estimation (using joint angles to determine the current phase of walking), and myoelectric control (using electromyography). [2] [3] This article focuses on myoelectric control.
Sensors on the skin detect electromyography (EMG) signals from the muscles of the wearer's leg(s). EMG signals can be measured from just one muscle or many, depending on the type of the exoskeleton and how many joints are actuated. Each signal measured is then sent to a controller, which is either an onboard microcontroller (mounted to the exoskeleton) or to a nearby computer. Onboard microcontrollers are used for long-term assistive devices since the wearer must be able to walk in different locations while wearing the exoskeleton, whereas computers not carried by the exoskeleton can be used for therapeutic or research purposes since the wearer does not have to walk very far in a clinical or lab environment.
The controller filters out noise from the EMG signals and then normalizes them so as to better analyze the muscle activation pattern. The normalized EMG value of a muscle represents its activation percentage, since the EMG signal is normalized by dividing it by the maximum possible EMG reading for the muscle it came from. The maximum EMG reading is generated when a muscle is fully contracted. An alternative method to normalization is to proportionally match the actuator power to the EMG signal between a minimum activation threshold and an upper saturation level.
With a proportional myoelectric controller, the power sent to an actuator is proportional to the amplitude of the normalized EMG signal from a muscle. [4] When the muscle is inactive, the actuator receives no power from the controller, and when the muscle is fully contracted, the actuator produces maximum torque about the joint it controls. For example, a powered ankle-foot orthosis (AFO) could employ a pneumatic artificial muscle to provide plantar flexion torque proportional to the activation level of the soleus (one of the calf muscles). This control method enables the exoskeleton to be controlled by the same neural pathways as the wearer's biological muscles and has been shown to allow individuals to walk with a more normal gait than other control methods, such as using a footswitch. [5] Proportional myoelectric control of robotic lower limb exoskeletons has advantages over other control methods, such as:
However, proportional myoelectric control also has disadvantages compared to other control methods, including:
Direct proportional control works well when each joint of the exoskeleton is actuated in one direction (uni-directional actuation), such as a pneumatic piston only bending the knee, but is less effective when two joint actuators work in opposition (bi-directional actuation). An example of this would be ankle exoskeleton using one pneumatic artificial muscle for dorsiflexion based on tibialis anterior (shin muscle) EMG and another pneumatic artificial muscle for plantar flexion based on soleus (calf muscle) EMG. This could result in a large degree of co-activation of the two actuators and make walking more difficult. [11] To correct for this unwanted co-activation, a rule can be added to the control scheme so that artificial dorsiflexor activation is inhibited when soleus EMG is above a set threshold. Proportional control with flexor inhibition allows for a more natural gait than with direct proportional control; flexor inhibition also allows subjects to walk much more easily with combined knee and ankle exoskeletons with bi-directional actuators at each joint. [7]
Performance enhancement deals with increasing typical human capabilities, such as strength or endurance. Many full-body robotic exoskeletons currently in development use controllers based on joint torques and angles instead of electromyography. See Powered exoskeletons.
One application of a robot lower limb exoskeleton is to assist in the movement of a disabled individual in order to walk. Individuals with spinal cord injury, weakened leg muscles, poor neuromuscular control, or who have suffered a stroke could benefit from wearing such a device. The exoskeleton provides torque about a joint in the same direction that EMG data indicate the joint is rotating. For example, high EMG signals in the vastus medialis (a quadriceps muscle) and low EMG signals in the biceps femoris (a hamstring muscle) would indicate that the user is extending his/her leg, therefore the exoskeleton would provide torque on the knee to help straighten the leg.
Proportional myoelectric control and robotic exoskeletons have been used in upper limb devices for decades, but engineers have only recently begun using them for lower-limb devices to better understand human biomechanics and neural control of locomotion. [12] [13] By using an exoskeleton with a proportional myoelectric controller, scientists can use a non-invasive means of studying the neural plasticity associated with modifying a muscle's force (biological +/- artificial force), as well as how motor memories for locomotor control are formed. [11]
Robotic lower limb exoskeletons have the potential to help an individual recover from an injury such as a stroke, spinal cord injury, or other neurological disabilities. Neurological motor disorders often result in reduced volitional muscle activation amplitude, impaired proprioception, and disordered muscle coordination; a robotic exoskeleton with proportional myoelectric control can improve all three of these by amplifying the relationship between muscle activation and proprioceptive feedback. By increasing the consequences of muscle activation, an exoskeleton can improve sensory feedback in a physiological way, which in turn can improve motor control [2] Individuals with spinal cord injury or who have had a stroke can improve their motor capabilities through intense gait rehabilitation, [14] which can require up to three physical therapists to help partially support the body weight of the individual. [15] Robotic lower limb exoskeletons could help in both of these areas.
The neuromuscular system has targeted joint torques it tries to generate while walking. Assistive exoskeletons produce some of the torque needed to move one or more leg joints while walking, which allows a healthy individual to generate less muscle torque in those joints and use less metabolic energy. The muscle torque is reduced enough to keep the net torque about each joint approximately the same as when walking without an exoskeleton. [16] The net torque about each joint is the muscular torque plus the actuator torque. Disabled individuals do not see much of a decrease, if any, in muscular torque while walking with an exoskeleton because their muscles are not strong enough to walk with a normal gait, or at all; the exoskeleton provides the remaining torque needed for them to walk.
In medicine, a prosthesis, or a prosthetic implant, is an artificial device that replaces a missing body part, which may be lost through trauma, disease, or a condition present at birth. Prostheses are intended to restore the normal functions of the missing body part. Amputee rehabilitation is primarily coordinated by a physiatrist as part of an inter-disciplinary team consisting of physiatrists, prosthetists, nurses, physical therapists, and occupational therapists. Prostheses can be created by hand or with computer-aided design (CAD), a software interface that helps creators design and analyze the creation with computer-generated 2-D and 3-D graphics as well as analysis and optimization tools.
Functional electrical stimulation (FES) is a technique that uses low-energy electrical pulses to artificially generate body movements in individuals who have been paralyzed due to injury to the central nervous system. More specifically, FES can be used to generate muscle contraction in otherwise paralyzed limbs to produce functions such as grasping, walking, bladder voiding and standing. This technology was originally used to develop neuroprostheses that were implemented to permanently substitute impaired functions in individuals with spinal cord injury (SCI), head injury, stroke and other neurological disorders. In other words, a person would use the device each time he or she wanted to generate a desired function. FES is sometimes also referred to as neuromuscular electrical stimulation (NMES).
A gait is a manner of limb movements made during locomotion. Human gaits are the various ways in which humans can move, either naturally or as a result of specialized training. Human gait is defined as bipedal forward propulsion of the center of gravity of the human body, in which there are sinuous movements of different segments of the body with little energy spent. Varied gaits are characterized by differences such as limb movement patterns, overall velocity, forces, kinetic and potential energy cycles, and changes in contact with the ground.
Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyograph detects the electric potential generated by muscle cells when these cells are electrically or neurologically activated. The signals can be analyzed to detect abnormalities, activation level, or recruitment order, or to analyze the biomechanics of human or animal movement. Needle EMG is an electrodiagnostic medicine technique commonly used by neurologists. Surface EMG is a non-medical procedure used to assess muscle activation by several professionals, including physiotherapists, kinesiologists and biomedical engineers. In computer science, EMG is also used as middleware in gesture recognition towards allowing the input of physical action to a computer as a form of human-computer interaction.
The Hybrid Assistive Limb is a powered exoskeleton suit developed by Japan's Tsukuba University and the robotics company Cyberdyne. It is designed to support and expand the physical capabilities of its users, particularly people with physical disabilities. There are two primary versions of the system: HAL 3, which only provides leg function, and HAL 5, which is a full-body exoskeleton for the arms, legs, and torso.
Bio-mechatronics is an applied interdisciplinary science that aims to integrate biology and mechatronics. It also encompasses the fields of robotics and neuroscience. Biomechatronic devices cover a wide range of applications, from developing prosthetic limbs to engineering solutions concerning respiration, vision, and the cardiovascular system.
In physiology, motor coordination is the orchestrated movement of multiple body parts as required to accomplish intended actions, like walking. This coordination is achieved by adjusting kinematic and kinetic parameters associated with each body part involved in the intended movement. The modifications of these parameters typically relies on sensory feedback from one or more sensory modalities, such as proprioception and vision.
The goal of the LOPES project is to design and implement a gait rehabilitation robot for treadmill training. The target group consists of people who have had a stroke and have impaired motor control. The main goals of LOPES are:
Gait training or gait rehabilitation is the act of learning how to walk, either as a child, or, more frequently, after sustaining an injury or disability. Normal human gait is a complex process, which happens due to co-ordinated movements of the whole of the body, requiring the whole of Central Nervous System - the brain and spinal cord, to function properly. Any disease process affecting the brain, spinal cord, peripheral nerves emerging from them supplying the muscles, or the muscles itself can cause deviations of gait. The process of relearning how to walk is generally facilitated by Physiatrists or Rehabilitation medicine (PM&R) consultants, physical therapists or physiotherapists, along with occupational therapists and other allied specialists. The most common cause for gait impairment is due to an injury of one or both legs. Gait training is not simply re-educating a patient on how to walk, but also includes an initial assessment of their gait cycle - Gait analysis, creation of a plan to address the problem, as well as teaching the patient on how to walk on different surfaces. Assistive devices and splints (orthosis) are often used in gait training, especially with those who have had surgery or an injury on their legs, but also with those who have balance or strength impairments as well.
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots. Robotics is related to the sciences of electronics, engineering, mechanics, and software.
Orthotics is a medical specialty that focuses on the design and application of orthoses, sometimes known as braces or calipers. An orthosis is "an externally applied device used to influence the structural and functional characteristics of the neuromuscular and skeletal systems." Orthotists are professionals who specialize in designing these braces.
Locomotor effects of shoes are the way in which the physical characteristics or components of shoes influence the locomotion neuromechanics of a person. Depending on the characteristics of the shoes, the effects are various, ranging from alteration in balance and posture, muscle activity of different muscles as measured by electromyography (EMG), and the impact force. There are many different types of shoes that exist, such as running, walking, loafers, high heels, sandals, slippers, work boots, dress shoes, and many more. However, a typical shoe will be composed of an insole, midsole, outsole, and heels, if any. In an unshod condition, where one is without any shoes, the locomotor effects are primarily observed in the heel strike patterns and resulting impact forces generated on the ground.
Spinal locomotion results from intricate dynamic interactions between a central program in lower thoracolumbar spine and proprioceptive feedback from body in the absence of central control by brain as in complete spinal cord injury (SCI). Following SCI, the spinal circuitry below the lesion site does not become silent; rather, it continues to maintain active and functional neuronal properties, although in a modified manner.
A powered exoskeleton is a mobile machine that is wearable over all or part of the human body, providing ergonomic structural support and powered by a system of electric motors, pneumatics, levers, hydraulics or a combination of cybernetic technologies, while allowing for sufficient limb movement with increased strength and endurance. The exoskeleton is designed to provide better mechanical load tolerance, and its control system aims to sense and synchronize with the user's intended motion and relay the signal to motors which manage the gears. The exoskeleton also protects the user's shoulder, waist, back and thigh against overload, and stabilizes movements when lifting and holding heavy items.
Terrestrial locomotion by means of a running gait can be accomplished on level surfaces. However, in most outdoor environments an individual will experience terrain undulations requiring uphill running. Similar conditions can be mimicked in a controlled environment on a treadmill also. Additionally, running on inclines is used by runners, both distance and sprinter, to improve cardiovascular conditioning and lower limb strength.
Neuromechanics of orthoses refers to how the human body interacts with orthoses. Millions of people in the U.S. suffer from stroke, multiple sclerosis, postpolio, spinal cord injuries, or various other ailments that benefit from the use of orthoses. Insofar as active orthoses and powered exoskeletons are concerned, the technology to build these devices is improving rapidly, but little research has been done on the human side of these human-machine interfaces.
Neuromechanics is an interdisciplinary field that combines biomechanics and neuroscience to understand how the nervous system interacts with the skeletal and muscular systems to enable animals to move. In a motor task, like reaching for an object, neural commands are sent to motor neurons to activate a set of muscles, called muscle synergies. Given which muscles are activated and how they are connected to the skeleton, there will be a corresponding and specific movement of the body. In addition to participating in reflexes, neuromechanical process may also be shaped through motor adaptation and learning.
As humans move through their environment, they must change the stiffness of their joints in order to effectively interact with their surroundings. Stiffness is the degree to a which an object resists deformation when subjected to a known force. This idea is also referred to as impedance, however, sometimes the idea of deformation under a given load is discussed under the term "compliance" which is the opposite of stiffness . In order to effectively interact with their environment, humans must adjust the stiffness of their limbs. This is accomplished via the co-contraction of antagonistic muscle groups.
Cutaneous, superficial, or skin reflexes, are activated by skin receptors and play a valuable role in locomotion, providing quick responses to unexpected environmental challenges. They have been shown to be important in responses to obstacles or stumbling, in preparing for visually challenging terrain, and for assistance in making adjustments when instability is introduced. In addition to the role in normal locomotion, cutaneous reflexes are being studied for their potential in enhancing rehabilitation therapy (physiotherapy) for people with gait abnormalities.
Robotic prosthesis control is a method for controlling a prosthesis in such a way that the controlled robotic prosthesis restores a biologically accurate gait to a person with a loss of limb. This is a special branch of control that has an emphasis on the interaction between humans and robotics.
