This protocol provides a detailed list of steps to be performed for the manufacturing, control and evaluation of the climbing performance of a gecko-inspired soft robot.
This protocol presents a method for manufacturing, control, and evaluation of the performance of a soft robot that can climb inclined flat surfaces with slopes of up to 84°. The manufacturing method is valid for the fast pneunet bending actuators in general and might, therefore, be interesting for newcomers to the field of actuator manufacturing. The control of the robot is achieved by means of a pneumatic control box that can provide arbitrary pressures and can be built by only using purchased components, a laser cutter, and a soldering iron. For the walking performance of the robot, the pressure-angle calibration plays a crucial role. Therefore, a semi-automated method for the pressure-angle calibration is presented. At high inclines (> 70°), the robot can no longer reliably fix itself to the walking plane. Therefore, the gait pattern is modified to ensure that the feet can be fixed on the walking plane.
The interaction between humans and machines is becoming constantly closer. The increasing robot density in companies and households poses new challenges for the robot technology. Frequently, dangers are excluded by separation methods, but in many areas, especially in households, this is not a satisfactory solution. Soft robotics tackles this problem by using properties of soft materials and structures to develop new types of machines that behave like living organisms1, which is why soft robots are often inspired by biological models2. Most soft robots can be classified into two different types: mobile robots and robots designed for gripping and manipulation3. For soft mobile robots, typical locomotion principles are crawling, walking, running, jumping, flying, and swimming4. Another interesting field of application for soft robots is climbing – a combination of locomotion and adhesion5. Soft machines are very robust and cannot damage their surroundings due to their softness. This characteristic predestines this robot class for climbing, as they can easily survive a fall. Consequently, the literature offers several examples of soft robots capable of climbing6,7,8.
The goal of this protocol is to provide a method to manufacture, control, and evaluate the performance of a gecko-inspired, climbing soft robot9. Its design is based on the use of fast pneunet soft bending actuators10 made of elastomer. However, another soft actuator design and/or material could also be used. The literature offers a wide range of different designs of soft actuators11 and suitable materials12. The presented manufacturing method is similar to existing methods13 but includes some modifications that result in increased repeatability and robustness, at least in the case of the soft climbing robot9. The method is valid for fast pneunet bending actuators in general and might, therefore, be interesting for newcomers to the field of actuator manufacturing.
For controlling pneumatic actuated soft robots, the literature provides different solutions. It ranges from low-cost and easy-to-replicate control boards13 to powerful but more complex boards14, which cannot be rebuilt without special tools. Here, a brief description is provided for building a pneumatic control box by only using a laser cutter and a soldering iron. The control box allows the supply of any pressure and offers real-time sensory feedback, which is especially important for robotics applications. However, it can also be used for many other applications.
1. Printing of molds
2. Preparing the elastomer
3. Manufacturing of upper part (base part)
4. Manufacturing of lower part (bottom part)
5. Joining the base and bottom part
6. Joining of all limbs
7. Mounting of supply tube inlets
8. Building the control box
9. Building a test bench with embedded measurement system
10. Setting up the entire system
11. Running the control box
12. Calibrating the robot
13. Creating a gait pattern
14. Carrying out the climbing experiment
15. Evaluating the experiment
The presented protocol results in three things: a soft climbing robot, a universally applicable control box, and a control strategy for the robot's straight motion that increases its ability to climb and at the same time decreases its consumed energy. The control box described in Section 8 enables a continuous supply of any desired pressure level on up to six channels (expandable to eight) and additionally on four channels the supply of vacuum (expandable as required). The "User Interface Unit" enables the user to easily operate the control box at runtime and the interface to the monitor allows the measured data to be directly viewed and saved as a csv-file. The pattern-reference mode of the control box provides the user with an intuitive interface to loop predefined patterns. This can be the gait pattern of the robot, as in this protocol, or it can be used for actuator fatigue testing, or any other application that requires cyclic loading. Figure 1 depicts all hardware components assembled in the control box and the measurement system and how they are connected.
The gait pattern for the robot's straight motion is formulated in angular references8. To operate the robot, those angular references must be converted into pressure references. The control strategy used in this protocol is based on a prior angle-pressure calibration. Each method of calibration results in a different alpha-pressure curve. Therefore, it is necessary to adapt the calibration procedure to the real operating conditions as far as possible. When changing the inclination angle of the walking plane, the operating conditions change as well. Therefore, the angle-pressure curve must be re-calibrated for each inclination. Figure 2A shows the velocity of the robot for various inclines with an unchanged calibration and a re-calibrated angle-pressure curve. The experiment clearly shows the effectiveness of the re-calibration. The re-calibrated robot is not only way faster, it is also able to climb steeper inclines (84° instead of 76°) while consuming less energy9 as depicted in Figure 2B. In Figure 3, a series of photographs of the robot’s motion is shown for an inclination of 48°. The figure clearly illustrates that the climbing performance with re-calibration shown in Figure 3B is much better than with unchanged calibration shown in Figure 3A as the shift in position within the same time interval is almost twice as large. This robot can move very fast compared to other soft robots. Qin et al.7 summarize the forward velocities of various soft robots. Without payload and in the horizontal plane, the robot described in this protocol is five times faster in relation to the body length than the fastest robot in Ref.7.
Figure 1: Diagram of hardware components assembled in the control box. Therein denotes the pressure reference for the i-th channel, ui the control signal of the i-th proportional valve, the vector containing the angular references, α the vector containing the angle measurements, x the vector containing the position measurements, and ƒ the vector containing the control signals for the direct-acting solenoid valves, i.e., the fixation states of the feet. UI is short for “User Interface Unit”, BBB is an abbreviation for BeagleBone Black, i.e., the single-board computer used in the control box, and RPi is short for Raspberry Pi, i.e., the single-board computer used in the measurement system. Please click here to view a larger version of this figure.
Figure 2: Evaluation of the climbing performance. Dashed curves show the values for constant and solid curves for re-calibrated pressure references. (A) Forward velocity of the robot for various inclination angles. (B) Energy consumption for various inclination angles. This figure is adapted from Ref.9. Please click here to view a larger version of this figure.
Figure 3: Series of photos of the robot’s motion at an inclination of 48°. The time elapsed between each photo is 1.2 s. (A) Motion for constant pressure references and (B) the motion for recalibrated pressure references. Please click here to view a larger version of this figure.
Supplementary Figure 1: Preparation of the elastomer. Please click here to download this figure.
Supplementary Figure 2: Comparison of air bubble formation during evacuation before and after casting. (A) Evacuation of the elastomer is performed only before casting. Trapped air bubbles stay in place, but they are more in the area of the bumps, which does not greatly affect the actuator’s functionality. (B) Evacuation is performed before and after casting. Trapped air bubbles rise but get stuck again on the upper side of the struts and create holes in the actuator which can affect the functionality. Please click here to download this figure.
Supplementary Figure 3: Examples of successful and unsuccessful cured castings. Upper row shows successful examples and lower row unsuccessful examples. If the defect is not clearly recognizable, it is marked with a green circle. Please click here to download this figure.
Supplementary Figure 4: Manufacturing of the base part. Please click here to download this figure.
Supplementary Figure 5: Scheme for manufacturing the bottom part. A tube (which is later used as the supply tube for the suction cup) is clamped into the mold before casting. Then, the mold is filled with liquid elastomer. Please click here to download this figure.
Supplementary Figure 6: Joining of base and bottom part. Please click here to download this figure.
Supplementary Figure 7: Lamination casting of a soft bending actuator. Liquid elastomer is represented in red, cured elastomer in light red, and the strain-limiting layer as well as the molds in black. (A) Mixed elastomer is poured into two separate molds – one for the base part and one for the bottom part. Thereby, the bottom part is only half filled. A strain-limiting layer (supply tube) is then inserted into the bottom part mold. (B) The parts are cured and the base part is demolded. (C) The bottom part mold is filled to the top with liquid elastomer. (D) The base part is dipped into this mold. (E) The two parts are cured together. (F) The actuator is demolded. This figure is based on Ref.13. Please click here to download this figure.
Supplementary Figure 8: Joining of all limbs. (A) Covering the surfaces to be joined with fluid elastomer. (B) Rendered view of the complete assembly. Please click here to download this figure.
Supplementary Figure 9: Mounting the supply tube inlets. Please click here to download this figure.
Supplementary Figure 10: Photographs of the control box. (A) Front view of the User Interface Unit for enabling the user to interact with the robot. (B) Detail view of a Valve Unit. (C) Top view of the entire control box. Please click here to download this figure.
Supplementary Figure 11: Circuit diagram of the User Interface Unit. Please click here to download this figure.
Supplementary Figure 12: Circuit diagram of the Valve Unit. Please click here to download this figure.
Supplementary Figure 13: Simplified circuit diagram of the entire control box. Please click here to download this figure.
Supplementary Figure 14: Diagram of used pins of the single-board computers embedded in the control box. (A) Used pins of the board needed for user communication. (B) Used pins of the board needed for robot control. Please click here to download this figure.
Supplementary Figure 15: Rendered view of the walking plane with installed measurement system. Please click here to download this figure.
Supplementary Figure 16: Visualization of the lifting effect. Pin needles with 6 mm heads are inserted into both ends of the torso. This minimizes friction during walking and causes the suction cups to have full contact with the walking plane. Please click here to download this figure.
Supplementary Figure 17: Assembly of the visual markers. The markers are mounted on the robot by using pin needles. Marker 0 is mounted at the front left foot, marker 1 at the torso’s front, marker 2 at the front right foot, marker 3 at the rear left foot, marker 4 at the torso’s back, and marker 5 at the rear right foot. For the assembly of marker 4, three pin needles are used This figure is adapted from Ref.9. Please click here to download this figure.
Supplementary Figure 18: Legend of buttons of the control box. Please click here to download this figure.
Supplementary Figure 19: Legend of buttons of the Graphical User Interface. Please click here to download this figure.
Supplementary Figure 20: Gait patterns for straight movement of the robot. Fixed feet are indicated by filled circles and unfixed feet by unfilled circles. (A) Gait pattern for low and moderate inclination angles (< 70°). (B) Gait pattern for high inclinations (> 70°). Vacuum is applied to red and black filled feet. Black filled feet are fixed to the ground, whereas red feet do not necessarily have to be. In order to secure the fixation, the foot to be fixed is swinged back and forth once. This figure is adapted from Ref.9. Please click here to download this figure.
Supplementary Figure 21: Rendered explosion view of the soft climbing robot. Dovetails are located at the legs and corresponding keyways at the torso’s ends. This makes the joining process much more precise. This figure is adapted from Ref.9. Please click here to download this figure.
Supplementary Figure 22: Different calibration procedures for the determination of the pressure-angle curve. Each subfigure shows the qualitative pressure course and snapshots of the corresponding robot pose. (A) Each actuator is inflated continuously beginning from 0 bar up to 1 bar, while all others remain pressureless. (B) A pressure plateau is applied to a single actuator for 3 s; then, it is deflated completely for 2 s. In the next round, the level of the pressure plateau is increased by the increment until the plateau reaches 1 bar. This is done for each actuator individually. (C) Same procedure as in mode 2, but here, the same plateau is applied to actuators (0,3,4), respectively actuators (1,2,5), at the same time. (D) Same procedure as in mode 3, but plateaus for actuators (0,3) are starting at 0 bar (like before) and ending at 1.2 bar (instead of 1 bar). Basically, the increment for actuators (0,3) is slightly increased, while the increments for the other actuators remain the same. Please click here to download this figure.
Supplementary Figure 23: Angle-pressure curves for different calibration procedures. Please click here to download this figure.
Supplementary Animation 1: Animation of the robot’s straight gait. Please click here to download this file.
Supplementary Animation 2: Animation of the robot’s climbing gait. Please click here to download this file.
Supplementary File 1: Instructions for configuring the single-board computers. Please click here to download this file.
Supplementary File 2: Print template for the visual markers. Please click here to download this file.
Supplementary Data 1: CAD files. This zip-compressed folder contains the *.stl-files for printing the molds, the *.dxf-files for laser cutting the housing of the control box, the *.stl-files for printing the clamps used for the measurement system, and the *.dxf-file for laser cutting the frame of the measurement system. Please click here to download this file.
Supplementary Data 2: Code to run on the single-board computers. This zip-compressed folder contains the programs and their sources running on the board used for the “User Interface Unit”, the board used for robot control, and the board used for image processing. Upload the complete folder to all three boards. Please click here to download this file.
Supplementary Data 3: Exemplary measurement data. This zip-compressed folder contains two *.csv files generated during the calibration procedure. Please click here to download this file.
Supplementary Data 4: Calibration script. This zip-compressed folder contains the python script and its sources for evaluating the measurement data generated during the calibration procedure. Please click here to download this file.
Supplementary Data 5: Evaluation script. This zip-compressed folder contains two python scripts and their sources for evaluating the measurement data generated during the climbing experiment. In addition, it contains all the measurement data used for the generation of Figure 2. Please click here to download this file.
The presented protocol includes many different aspects related to the climbing soft robot from Ref.9, including manufacturing, control, calibration, and performance evaluation. In the following, the pros and cons resulting from the protocol are discussed and structured according to the aspects mentioned above.
The presented manufacturing method is strongly based on the existing literature10,13. A substantial difference is the design of the actuator. To join the individual limbs, dovetail guides are inserted at appropriate points, as shown in Supplementary Figure 21. This results in a much more precise and robust connection between the limbs compared to the previous design of the robot8. Furthermore, the supply tubes are embedded in the bottom part of the actuators. This integrated design allows the suction cups to be supplied with vacuum and at the same time makes the bottom layer no longer stretchable, which significantly increases the performance of the actuator. Another difference to the procedure described in the literature is that the mixed elastomer is evacuated only once (immediately after mixing). Many sources recommend evacuating the elastomer twice: once after mixing and once after it has been filled into the mold. It may happen that air remains trapped in very small spaces. In the vacuum chamber, this air expands and in the best case rises to the surface. Often enough, however, these air bubbles get stuck on their way, creating unpleasant holes in the finished casting. Here, a decision must be made as to what is more important: perfect contours on the bottom side of the base part or as little risk as possible of producing a non-functional actuator (cf. Supplementary Figure 2). In this protocol, no second evacuation is performed. In the procedure presented, the height of the bottom part may vary as it is filled manually, and, unlike for the base part, there is no possibility of cutting it to a uniform height after curing. To ensure that the height of the bottom part is as uniform as possible, it is recommended to use a syringe when filling the mold of the bottom part and to measure the volume poured in. However, depending on how much time has elapsed since mixing, the flow properties of the elastomer change significantly. Therefore, it is recommended to always use freshly mixed elastomer. Joining the base and the bottom part of the actuator involves the largest process uncertainty. If the elastomer bath is too high, the air channel between the chambers will most likely be covered as well. Then, the actuator is no longer usable. If the elastomer bath is too low, the sealing lip may not be covered in its entire circumference and the actuator would leak. Therefore, it takes a certain amount of practice to dose the elastomer bath correctly. Important for joining in general is a fat-free joining surface. If the joining surface is too contaminated, the finished actuator may delaminate. Therefore, it is essential to ensure that the parts are only touched on surfaces that are not to be joined. A major limitation of the manufacturing method is the number of pieces to be realized. The production of a single actuator takes at least two hours in total. Although it is possible to work with several molds in parallel, more than four is not recommendable due to time constraints. The pot life of the elastomer is too short to be able to fill even more molds. In addition, the 3D-printed molds only withstand a limited number of production cycles (approx. 10–20) before they become very deformed or break. A further limitation is the process uncertainties already discussed. Since almost all process steps are performed manually, each actuator is a little different. This can lead to two robots that are identical in construction but show two very different behaviors.
With the control box, a method is provided to control the robot. Nevertheless, for each pneumatic system, the control gains of the script "Code/arduino_p_ctr.ino" must be determined individually. This is not covered in the protocol. However, the "pressure reference mode" of the control box allows a playful handling of the robot, so that controller tuning can be made without writing several scripts. Another limitation of the control box is its cost as the material costs about 7000 US$ in total. The literature11 offers a building instruction for a control box that costs only about 900 US$ and with some upgrades could also be used to operate the robot.
Critical for the calibration of the individual actuators is the choice of the calibration procedure. Supplementary Figure 22 shows the qualitative course of the pressure references over time for four different procedures and Supplementary Figure 23 shows the resulting angle pressure curves. As can be seen in the latter, each method of calibration results in a different angle-pressure curve. This shows that the relationship between pressure and angle is highly dependent on the load acting on the actuator. Therefore, the calibration procedure must reflect the real load case as best as possible. Consequently, it is necessary to adapt the calibration procedure to the real operating conditions as far as possible. The best walking performance is obtained with calibration procedure 4. However, as can be seen in Figure 3B, the subsequent poses in the series are not completely symmetrical, which is an indicator for the potential of improvement in calibration.
Critical to the measuring system is the assembly of the visual markers15 in Section 10. Since they cannot be mounted directly at the desired points (because the tubes interfere), the measured points must be shifted artificially. Special care must be taken when determining this offset vector (in pixel coordinates of the camera); otherwise, the entire measurement will have significant systematic errors. It must also be ensured that the tags do not displace with time. If this happens, e.g., due to a downfall of the robot, the corresponding tag must be remounted in the exact same place. In any case, it should be checked regularly whether the measuring system still produces reliable output.
The limiting factor in the experiment is the fixation of the feet. In order to be able to climb even steeper inclinations, the fixation mechanism must be reconsidered. Currently, the robot is not able to actively push its feet against the walking plane, and for high inclines, the normal force caused by gravity is too small to bring the suction cups close enough to the walking plane to ensure reliable suction.
The presented manufacturing method can be transferred to any fluidic elastomer actuator and could, therefore, be interesting for future applications. The presented control box enables the control of any pneumatic system consisting of six individual actuators (expandable up to eight), including robotic platforms as they require fast sensory feedback. Therefore, it could be used as a universal platform for testing and control future robots. Finally, the presented calibration method can be, in principle, to any feed-forward controlled pneumatic system. In summary, all presented methods are universal within the discussed scope.
The authors have nothing to disclose.
The authors like to thank Fynn Knudsen, Aravinda Bhari, and Jacob Muchynski for helpful discussions and the inspiration.
3D Printer | Formlabs | Form 2 | |
acrylic glass plate with two holes | – | for casting, see Supplementary | |
acrylic glass back panel | – | see Supplementary | |
acrylic glass bottom panel | – | see Supplementary | |
acrylic glass front panel | – | see Supplementary | |
acrylic glass side panel | – | see Supplementary | |
acrylic glass top panel | – | see Supplementary | |
Arduino Nano | Arduino | A000005 | |
Allan Key 1mm | available in every workshop | ||
BeagleBone Black | beagleboard | BBB01-SC-505 | |
butterfly cannula | B. Braun Melsungen AG | 5039573 | |
clamp 1 for measurement system | – | see Supplementary | |
Clamp 2 for measurement system | – | see Supplementary | |
cutter knife | available in every workshop | ||
direct acting solenoid valves | Norgren | EXCEL22 DM/49/MDZ83J/T4 | |
elastomer | Wacker Chemie | ELASTOSIL M4601 | |
frame measurement system part 1 | – | see Supplementary | |
frame measurement system part 2 | – | see Supplementary | |
laser cutter | Trotec | SP500 | |
LED | RND COMPONENTS | RND 210-00013 | |
LCD | JOY-IT | SBC-LCD16X2 | |
mould bottom part leg | – | see Supplementary | |
mould bottom part torso 1 | – | see Supplementary | |
mould bottom part torso 2 | – | see Supplementary | |
mould leg 1 | – | see Supplementary | |
mould leg 2 | – | see Supplementary | |
mould torso 1 | – | see Supplementary | |
mould torso 2 | – | see Supplementary | |
oven | Binder | ED 115 | |
Plastic Cup | available in every supermarket | ||
Plastic syringe | available in every pharmacy | ||
poster panel | Net-xpress.de (distributor) | 10620232 | as walking plane |
Potentiometer | VISHAY | P16NM103MAB15 | |
Power Supply | Pulse Dimension | CPS20.241-C1 | |
pressure sensor | Honeywell | SSCDANN150PG2A5 | |
Pressure Source | EINHELL | 4020600 | |
proportional valves | Festo | MPYE-5-1/8-LF-010-B | 6x |
Raspberry Pi | RASPBERRY PI | RASPBERRY PI 3B+ | |
Raspberry Pi Cam | RASPBERRY PI | RASPBERRY PI CAMERA V2.1 | |
resin | formlabs | grey resin 1l | |
screw clamps | VELLEMAN | 3935-12 | |
silicon tube 2mm | Festo | PUN-H-2X0,4-NT | for connecting robot to control box |
silicone Tube 2.5mm | Schlauch24 | n/a | for supply tube inlet (https://www.ebay.de/itm/281761715815) |
Switches | MIYAMA | MS 165 | |
ultrasonic bath | RND LAB | 605-00034 | |
UV chamber | formlabs | Form Cure | |
Vacuum chamber + pump | COPALTEC | PURE PERFEKTION | |
weight scale | KERN-SOHN | PCB 2500-2 | min. resolution 1g |