Daniel M. Vogt

Embedded 3D Printing of Strain Sensors within Highly Stretchable Elastomers

A new method, embedded-3D printing (e-3DP), is reported for fabricating strain sensors within highly conformal and extensible elastomeric matrices. e-3DP allows soft sensors to be created in nearly arbitrary planar and 3D motifs in a highly programmable and seamless manner. Several embodiments are demonstrated and sensor performance is characterized.

Design and Characterization of a Soft Multi-Axis Force Sensor Using Embedded Microfluidic Channels

Thin, highly compliant sensing skins could provide valuable information for a host of grasping and locomotion tasks with minimal impact on the host system. We describe the design, fabrication, and characterization of a novel soft multi-axis force sensor made of highly deformable materials. The sensor is capable of measuring normal and in-plane shear forces. This soft sensor is composed of an elastomer (modulus: 69 kPa) with embedded microchannels filled with a conductive liquid. Depending on the magnitude and the direction of an applied force, all or part of the microchannels will be compressed, changing their electrical resistance. The two designs presented in this paper differ in their flexibility and channel configurations. The channel dimensions are approximately 200 × 200 μm and 300 × 700 μm for the two prototypes, respectively. The overall size of each sensor is 50 × 60 × 7 mm. The first prototype demonstrated force sensitivities along the two principal in-plane axes of 37.0 and -28.6 mV/N. The second prototype demonstrated the capability to detecting and differentiating normal and in-plane forces. In addition, this paper presents the results of a parameter study for different design configurations.

Capacitive Soft Strain Sensors via Multicore–Shell Fiber Printing

A new method for fabricating textile integrable capacitive soft strain sensors is reported, based on multicore-shell fiber printing. The fiber sensors consist of four concentric, alternating layers of conductor and dielectric, respectively. These wearable sensors provide accurate and hysteresis-free strain measurements under both static and dynamic conditions.

Wearable soft sensing suit for human gait measurement

Wearable robots based on soft materials will augment mobility and performance of the host without restricting natural kinematics. Such wearable robots will need soft sensors to monitor the movement of the wearer and robot outside the lab. Until now wearable soft sensors have not demonstrated significant mechanical robustness nor been systematically characterized for human motion studies of walking and running. Here, we present the design and systematic characterization of a soft sensing suit for monitoring hip, knee, and ankle sagittal plane joint angles. We used hyper-elastic strain sensors based on microchannels of liquid metal embedded within elastomer, but refined their design with the use of discretized stiffness gradients to improve mechanical durability. We found that these robust sensors could stretch up to 396% of their original lengths, would restrict the wearer by less than 0.17% of any given joint’s torque, had gauge factor sensitivities of greater than 2.2, and exhibited less than 2% change in electromechanical specifications through 1500 cycles of loading–unloading. We also evaluated the accuracy and variability of the soft sensing suit by comparing it with joint angle data obtained through optical motion capture. The sensing suit had root mean square (RMS) errors of less than 5° for a walking speed of 0.89 m/s and reached a maximum RMS error of 15° for a running speed of 2.7 m/s. Despite the deviation of absolute measure, the relative repeatability of the sensing suit’s joint angle measurements were statistically equivalent to that of optical motion capture at all speeds. We anticipate that wearable soft sensing will also have applications beyond wearable robotics, such as in medical diagnostics and in human–computer interaction.

Flexible, stretchable tactile arrays from MEMS barometers

Many applications for tactile sensors require a flexible, stretchable array to allow installation on curved surfaces or to measure forces on deformable objects. This paper presents a sensor array created with barometers and flexible printed circuit boards that delivers high sensitivity on a flexible, stretchable package using commercial off-the-shelf (COTS) components: MEMS barometers and commercially-compatible flexible printed circuit boards. The array is demonstrated on the surface of a jamming gripper, where it provides the ability to sense grasping events and detect object shape.

Fluid-driven origami-inspired artificial muscles

Significance Artificial muscles are flexible actuators with capabilities similar to, or even beyond, natural muscles. They have been widely used in many applications as alternatives to more traditional rigid electromagnetic motors. Numerous studies focus on rapid design and low-cost fabrication of artificial muscles with customized performances. Here, we present an architecture for fluidic artificial muscles with unprecedented performance-to-cost ratio. These artificial muscles can be programed to produce not only a single contraction but also complex multiaxial actuation, and even controllable motion with multiple degrees of freedom. Moreover, a wide variety of materials and fabrication processes can be used to build the artificial muscles with other functions beyond basic actuation. Artificial muscles hold promise for safe and powerful actuation for myriad common machines and robots. However, the design, fabrication, and implementation of artificial muscles are often limited by their material costs, operating principle, scalability, and single-degree-of-freedom contractile actuation motions. Here we propose an architecture for fluid-driven origami-inspired artificial muscles. This concept requires only a compressible skeleton, a flexible skin, and a fluid medium. A mechanical model is developed to explain the interaction of the three components. A fabrication method is introduced to rapidly manufacture low-cost artificial muscles using various materials and at multiple scales. The artificial muscles can be programed to achieve multiaxial motions including contraction, bending, and torsion. These motions can be aggregated into systems with multiple degrees of freedom, which are able to produce controllable motions at different rates. Our artificial muscles can be driven by fluids at negative pressures (relative to ambient). This feature makes actuation safer than most other fluidic artificial muscles that operate with positive pressures. Experiments reveal that these muscles can contract over 90% of their initial lengths, generate stresses of ∼600 kPa, and produce peak power densities over 2 kW/kg—all equal to, or in excess of, natural muscle. This architecture for artificial muscles opens the door to rapid design and low-cost fabrication of actuation systems for numerous applications at multiple scales, ranging from miniature medical devices to wearable robotic exoskeletons to large deployable structures for space exploration.

A soft multi-axis force sensor

Thin, highly compliant sensing skins could provide valuable information for a host of grasping and locomotion tasks with minimal impact on the host system. We describe the design, fabrication and characterization of a novel soft multi-axis force sensor made of highly deformable materials. The sensor is capable of not only measuring normal force and but also 2D in-plane forces. This soft sensor is composed of an elastomer (modulus: 69 kPa) embedded with microchannels filled with conductive liquid. Depending on the intensity and the direction of an applied force, the cross-section of some of the channels will be compressed, changing its electrical resistance. The channel dimensions of the current prototype are 200 μm × 200 μm and the overall size of the sensor is 50 mm × 60 mm × 7 mm. Characterization results showed sensitivities in the two principal in-plane directions of 37.0 mV/N and -28.6 mV/N, respectively.

Soft Somatosensitive Actuators via Embedded 3D Printing

Humans possess manual dexterity, motor skills, and other physical abilities that rely on feedback provided by the somatosensory system. Herein, a method is reported for creating soft somatosensitive actuators (SSAs) via embedded 3D printing, which are innervated with multiple conductive features that simultaneously enable haptic, proprioceptive, and thermoceptive sensing. This novel manufacturing approach enables the seamless integration of multiple ionically conductive and fluidic features within elastomeric matrices to produce SSAs with the desired bioinspired sensing and actuation capabilities. Each printed sensor is composed of an ionically conductive gel that exhibits both long-term stability and hysteresis-free performance. As an exemplar, multiple SSAs are combined into a soft robotic gripper that provides proprioceptive and haptic feedback via embedded curvature, inflation, and contact sensors, including deep and fine touch contact sensors. The multimaterial manufacturing platform enables complex sensing motifs to be easily integrated into soft actuating systems, which is a necessary step toward closed-loop feedback control of soft robots, machines, and haptic devices.

Aerodynamic evaluation of four butterfly species for the design of flapping-gliding robotic insects

Alternating gliding and active propulsion is a potentially energy saving strategy for small-scale flight. With the goal of finding optimal wing shapes for flapping-gliding robots we evaluate the quasi-steady aerodynamic performance of four butterfly species (Monarch (Danaus plexippus), the Orange Aeroplane (Pantoporia consimilis), the Glasswing (Acraea andromacha) and the Four-barred Swordtail (Protographium Ieosthenes)). We fabricate at-scale wing models based on measured wing shapes and vary the forewing angle in nine steps to account for the ability of the butterfly to change the relative orientation of its forewing and hindwing during flight. For comparison we include twelve non-biological planforms as performance benchmarks for the butterfly wing shapes. We then test these 48 wing models at 2m/s, 3.5m/s and 5m/s (Reynolds number between 2597 and 12632) in a low speed wind tunnel which allows lift and drag force measurements of centimeter-size wings. The results indicate that the forewing orientation which maximizes the wing span offers the best gliding performance and that overall the gliding ratios are highest at 3.5m/s. The wing shapes with the best gliding ratio are found in the Glasswing butterfly with a maximum of 6.26 which is very high compared to the gliding performance of similarly sized flying robots. The results from this study are important for the development of novel biologically-inspired flying micro robots as well as for biomechanics studies in biology.

A Soft Combustion-driven Pump for Soft Robots

This paper describes the design and manufacture of a monolithic high-pressure diaphragm pump made entirely of soft elastomer material and driven by a combustion chamber incorporated within the soft pump structure. The pump can deliver pressures up to 60 kPa and can reach output flows up to 40 ml/min. Methane (CH4) combustion is used as the actuation source. The pump uses two soft flap-structured check valves for directing the flow. Pumping pressure and frequency dependence were measured and analyzed. Results show that controlled and repeatable combustion of methane is possible without damaging the soft structure. Experimentally, 6–10% methane is identified as the ideal air-fuel ratio for combustion. With continuous delivery of reactants, a 1 Hz pumping frequency was achieved. The volume of the combustion chamber and the material stiffness are identified to be major determinants of the stroke volume.Copyright