Design of a Portable Pneumatic Power Source with High Output Pressure for Wearable Robotic Applications
ABSTRACT
Despite the rapid advances in soft robotic actuation technologies, the main energy source that powers most wearable systems remains the conventional tethered stationary air compressor that greatly limits these systems’ applicability. Several portable pneumatic energy sources have been introduced; however, the limited maximum output pressure and flow rate, size and weight, large operational noise, and potential safety hazards must still be resolved before being applied to current wearable applications. In this study, we propose the design of a portable double-piston crank microcompressor that can generate up to a maximum gauge output pressure of 986 [kPa] and a maximum flow rate of 9.78 [LPM], while maintaining a simple structure, static mass of 1.5 [kg] and not generating any safety hazards. The design requirements in terms of maximum pressure and flow rate were optimized based on wearable robotic applications. The sound intensity level generated by the developed microcompressor was approximately 65 [dB], which can be used for long-term usage, at maximum flow rate when measured from a 0.5 [m] distance.
EXISTING SYSTEM :
The field of soft pneumatic robotics has been rapidly evolving throughout the past few decades, specifically in the form of mobile and wearable robots due to the desirable characteristics of their soft driving fluids and structural versatility . The inherent compliance introduced by the compressibility of fluids allows safe interaction between the robot and environment or between the user and wearable robot. This nature is important in many wearable applications such as for augmentation , rehabilitation , and orthotic/prosthetic applications and in mobile robots applications such as for locomotion and manipulation tasks in unprecedented conditions. Despite the fact that the soft robotic technology has greatly advanced, the most commonly used energy source in pneumatically actuated robots remains a conventional tethered stationary air compressor, which greatly limits the applicability of the technologies. There were a number of efforts to develop portable pneumatic energy sources . A the technologies that have been introduced for mobile and wearable soft robots, the most commonly used mobile energy sources are the use of highly compressed gas or liquid vessels, sources that rely on direct chemical reactions and electric motor based microcompressors. Highly compressed fluid vessels can provide pneumatic energy at high pressure and flow rate and can be stored for extended periods prior to opening . The most commonly used fluids are compressed air, carbon dioxide (CO2) and nitrogen gas (N2) which can be stored at 20 ~ 30 [MPa] in gas form or 5.6 [MPa] in liquid form . These fluids have the advantage of not producing harmful chemical by-products, which is especially important in wearable devices. Compressed CO2 or air was used for a portable powered ankle-foot orthosis and for an augmented lumbar support . However, the use of gas or fluid vessels has limited duration of operation, therefore, requiring vessel replacements or increased size and weight of the vessels that is undesirable for wearable applications. Furthermore, for applications that require continuous high flow rates, blockage or freezing due to the endothermic expansion of highly pressurized gases must be considered. The operating pressure of actuators for most wearable applications are much lower than that of the compressed pressure, and thus little energy can be transformed to the available pneumatic pressure due to its lower conversion efficiency . Another widely used method to generate mobile pneumatic energy is sources that rely on direct chemical reactions. Explosive combustion of fuels such as butane and methane and the decomposition of peroxide monopropellants have been introduced throughout the literature . In case of combustion based energy sources, the high energy density and high reaction speed is a desirable advantage for wearable applications. However, the handling of potential hazards such as high local temperature, heat and steam, loud noise, chemical by-products and possibilities of explosions must be resolved before the technology can be applied as a mobile energy source. Moreover, the rapid reaction makes it difficult to model the energy generation for relatively smaller soft actuators. Decomposition based energy generation, most commonly with hydrogen peroxide (H2O2), shares similar advantages with combustion based methods (high energy density). Moreover, as there is no need of an ignition source, the technology is relatively safer in case of hydrogen peroxide with concentrations less than 70% . However, the local temperature rises up to approximately 100 [℃] and the limited response time should be improved for applications in mobile and wearable robots. Electric motor based microcompressors that can continuously compress ambient air using reciprocating positive displacement have been used in many soft robotic applications. The displacement of a diaphragm generates pressure differentials within an air chamber and the check valves at the inlet and outlet of the chamber allows the compression of air. Since all robotic systems require electric power for actuation and sensing, battery based microcompressors has the advantage of not requiring additional energy sources compared to fluid vessels and chemical based sources. However, the insufficient air flow and limited maximum pressure (max. output pressure is 400 [kPa]) of current commercial reciprocating microcompressors limit the usability and applicability to current wearable applications that require high output force/torque . Promising results were proposed to improve the maximum output pressure and air flow performance of commercial microcompressors by connecting multiple units in different configurations . Series configuration allowed to improve the maximum output pressure and parallel configurations can be used for increased maximum flow rate. Although the performance was improved, the entire system becomes bulky and the loud noise generated during the operation of each microcompressors restricts the use in everyday environments and quiet operations.
PROPOSED SYSTEM :
In this paper, we propose the design process and implementation of a simple reciprocating positive displacement type portable double-piston crank microcompressor that has a maximum output pressure and flow rate adequate for wearable robotic applications and that emits reasonable operational noise (less than 65 [dB] [33]) for long-term usage. The high maximum output pressure was achieved by utilizing an EC motor with a piston crank-shaft mechanism that can momentarily generate high forces. Since the force generated by the piston is inversely proportional to rotational velocity, the limited flow rate was compensated using the double-piston crank mechanism. For this study, the EC motor and kinematic variables of the piston crank-shaft mechanism were designed to generate an maximum output pressure and flow rate (1000 [kPa] and 10 [LPM]) to power an ankle foot orthosis that we have previously developed.
Fig. 1 Example of a portable pneumatic power source for wearable robotic applications.
Fig. 2 Kinematic model of a piston-crank mechanism for compressing ambient air.
In our future work, additional piston crank structures will be added to increase the flow rate, and friction between pistons and the cylinder will be modeled to optimize the electric motor size and kinematics of the piston-crank mechanism. Additionally, the effect of different kinematic variables on energy consumption will be studied. Finally, the proposed pneumatic power source will be used to power an ankle foot orthosis.
CONCLUSION
A double-piston crank based microcompressor was proposed in this study. The main goal of this study was to provide the design process of the kinematics of the piston crank mechanism so that it can be used to generate adequate amount of maximum pressure, flow rate and operational noise for an ankle foot orthosis applications. Since, maximum output pressure is inversely proportional to the flow rate the design of the kinematics of the piston-crank mechanism and selection of electric motor can be optimized depending on the application. In this study, the proposed double-piston microcompressor had a maximum output pressure of 986 [kPa], a flow rate of 9.78 [LPM] and an operational noise of 65 [dB]. These values are adequate to power our previously developed AFO for every day usage. In our future work, additional piston crank structures will be added to increase the flow rate, and friction between pistons and the cylinder will be modeled to optimize the electric motor size and kinematics of the piston-crank mechanism. Additionally, the effect of different kinematic variables on energy consumption will be studied. Finally, the proposed pneumatic power source will be used to power an ankle foot orthosis.