Mimicking the working principles of biological organisms has been a major trend in modern robotics research. Leveraging the underlying biological principles, a biorobotic system enjoys similar advantages of its biological counterpart, and thus enables it to better serve human purposes. When constructing a biorobotic system, a major technological challenge is the development of a compact and powerful actuation system within an energetically autonomous package. Such actuation system is a key component to enable the self-powered biorobotic system in practical daily use. Our research in this area is focused on chemo-fluidic muscle actuation, a fully self-contained high-performance actuation system that may potentially serve as a fundamental building block of future wearable robotic devices and mobile robots. Further, we are also exploring fundamental innovations in the muscle actuator structure and functioning mechanism. The sleeve muscle, a novel type of muscle actuator created by our group, not only provides significant increase in actuation performance, but also enables a unique design philosophy with greater efficiency and higher level of biomimetics.
Chemo-Fluidic Muscle Actuation
Among the various types of muscle actuators, the pneumatic muscle actuator (also known as pneumatic artificial muscle, fluidic muscle, or McKibben muscle) possesses multiple unique advantages, including:
1) Simple structure. Pneumatic muscle simulates the shortening of biological muscle through the pressurizing of a flexible tube. The simple structure results in a minimum level of mechanical complexity, which reduces the cost and improves the reliability in the operation.
2) High power density. The reported values of power density range from 3 to 10 kW/kg, one to two orders of magnitude higher than the typical values for electric motors (0.1 kW/kg) and pneumatic cylinder-type actuators (0.4 kW/kg).
3) Potential of simulating the elastic behavior of biological skeletal muscles. The muscle force decreases with the shortening, and through the proper choice of parameters, the pneumatic muscle may simulate the force-length relationship of skeletal muscles.
Despite the excellent performances, pneumatic muscle has not gained extensive use in portable biorobotic systems such as robotic prostheses. The fundamental reason, presumably, is the lack of a compact pneumatic supply to complement the muscle actuator when constructing a portable actuation system. To address this challenging issue, we have developed a new chemo-fluidic muscle actuation system, in which a unique class of liquid fuel, namely monopropellant, is introduced as a compact and efficient energy storage medium. In general, liquid fuels possess significantly higher energy densities than electrochemical batteries, the dominant form of energy source in mobile robotics. However, for the majority of liquid fuels, the traditional means of converting the chemical energy into useable work often involves heat release through combustion, a process that is hard to control, and involves complex hardware. Unlike other types of liquid fuels, a monopropellant (such as hydrogen peroxide) releases energy through catalytic decomposition:
2H2O2 —–> 2H2O + O2 + Heat
The reaction occurs instantaneously, and the energy release is highly controllable through the modulation of liquid flow. Furthermore, the whole process is highly efficient, as a result of the direct conversion from chemical energy into mechanical work, and generates safe reaction products (oxygen and water). Combining monopropellant with pneumatic muscle, the new chemo-fluidic muscle actuation system leverages the high energy density provided by the liquid fuel and the high power density provided by the pneumatic muscle, and offers a potential of significantly enhancing the performance of biorobotic systems such as humanoid robots and robotic prostheses/ orthoses.
Sleeve Muscle – Substantial Performance Increase through Structural Innovation
In its basic form, a pneumatic muscle actuator generates power output through the shortening of the flexible membrane under the internal pressure. Applying the principle of virtual work, the force output can be expressed as:
F = (-dV/dL) * Pgauge
where Pgauge is the air gauge pressure, and V and L are the chamber internal volume and length, respectively. This equation indicates that, to generate a positive force output (i.e., +ve F), the internal volume must expand when the muscle length shortens. Based on this conclusion, further analysis on the forces applied to the end of the actuator indicates that the membrane-generated pulling force F2 needs to overcome the pushing force applied to the end plane F1 to generate the overall actuator output force F: F = F2
– F1. Inspired by this observation, we came up with the concept of sleeve muscle, which incorporates a solid insert at the center to largely eliminate the negative contribution from the pushing force F2. Compared with the traditional pneumatic muscle, a sleeve muscle provides a constant increase of output force over the entire range of contraction (exceeding 100% in the large-contraction region). Furthermore, with the reduction of internal volume, the sleeve muscle consumes less energy and provides a faster dynamic response in operation. The overall increase in performance is substantial.
Integrated Design of Joint Actuation and Structural Load Bearing
In addition to the advantages in actuation performance, the sleeve muscle also offers a unique opportunity for the integrated design of the actuator and the load-bearing structure. Specifically, the cylindrical insert in the sleeve muscle can be used to serve the dual roles of the structural insert of the sleeve muscle as well as the load-bearing tube (i.e., the robotic counterpart of the bone). As such, the muscle actuator completely surrounds the load-bearing tube (i.e., the insert), with the moving end of the actuator sliding on the surface of the tube. This unique integrated actuation-load bearing structure forms a highly compact robotic limb design, which conceptually mimicking the biological muscle-bone structure while avoiding the excessive complexity of utilizing more than one actuator for each joint.
This unique design philosophy is being explored in our robotic lower-limb prosthesis research. To demonstrate its performance and investigate the modeling and control methods, we have constructed a demonstration apparatus in the form of a robotic elbow. As shown in the figure, the robotic elbow’s load-bearing “bone” also serves as the insert of the sleeve muscle. The sleeve muscle essentially functions as an artificial bicep that slides over the “bone” surface, driving the joint motion through an artificial tendon. As the requirement for joint extension is less demanding, a torsional spring embedded in the joint serves this purpose. Experiments on this sleeve muscle-powered elbow demonstrated the feasibility of this concept as well as its capability of generating highly controllable motion required for a robotic system.
Double-Acting Sleeve Muscle: A Unique Bi-Directional Muscle Actuator
Developed by simulating the functionality of skeletal muscles, the majority of artificial muscle actuators are uni-directional (i.e., generating a pulling force only). As such, bi-directional actuation of a robotic joint usually requires two actuators in an antagonistic configuration, making the system bulky and complex. Our group developed a new bi-directional muscle actuator to address this problem. On the basis of the single-acting sleeve muscle, the new design incorporates an additional chamber at the center of the actuator to generate a pushing force when pressurized.
Developed by simulating the functionality of skeletal muscles, the majority of artificial muscle actuators are uni-directional (i.e., generating a pulling force only). As such, bi-directional actuation of a robotic joint usually requires two actuators in an antagonistic configuration, making the system bulky and complex. Our group developed a new bi-directional muscle actuator to address this problem. On the basis of the single-acting sleeve muscle, the new design incorporates an additional chamber at the center of the actuator to generate a pushing force when pressurized.