Mechanical Design
Overview of Drosophibot II's mechanical design, taken from Clarus Goldsmith's dissertation.
CAD files for Drosophibot II can be found at this public GitHub repo, in addition to the lab Google Drive:
Overview
Drosophibot II is a hexapod robot with 22 actuated degrees of freedom. Each leg is segmented similarly to the leg segments of Drosophila, with a varying number of joints actuated within each leg pair. The middle and hind pairs of legs each have three DoF, corresponding to the CTr, TrF, and FTi joints (Fig. 2 & 3, respectively). The front legs have an additional two DoF for the ThC1 and ThC3 to enable the more complex motions Drosophila’s front limbs undergo during walking (Fig. 4). DoF that were omitted from the robot’s legs were fixed at their average position during the animal kinematic analysis presented in Goldsmith et al. 2022. The majority of Drosphibot II’s components are 3-D printed out of Onyx composite nylon. Each joint is actuated by a Dynamixel MX-series smart servo; MX-28s are used for most joints, while MX-64s are used for the CTr due to the increased torque requirement at this joint. Using a single rotary actuator centered on the DoF axis differs from biological systems, which use antagonistic muscle pairs anchored on more proximal segments via tendons and ligaments to actuate leg DoF. However, previous work has shown that the servomotors' internal controller can be tuned to behave dynamically similarly to an insect leg joint (Szczecinski et al. 2019). As such, this actuation scheme can be used without detracting from the biomimicry of the robot.
Table 1 presents the upper and lower limits of each joint in the robot's legs. For joints such as the TrF where a full 360 degrees of rotation in either direction is theoretically possible, the limits were instead functionally considered as -120o and +120o to prevent actuator and sensing cables twisting around the leg segment.
Because 45% of Drosophila’s mass is in its abdomen (which the robot does not need), the CoM of Drosophibot II’s thorax and legs is much farther forward than in the insect. Previous investigations have calculated the anterior-posterior location of the insect’s CoM as between the middle and hind legs (Szczecinski et al. 2018), while Drosophibot II’s CoM naturally falls between the middle and front legs. To shift the CoM to a more biological location, I have included an abdominal segment on Drosophibot II with slots for additional weight. I have found that including 1kg of mass approximately 125mm from the end of the thorax is enough to shift the CoM to an animal-like position. While adding mass does increase the load on the actuators, particularly in the hind legs, the robot’s strength-to-weight ratio is such that the required joint torques are still well within the operating limit of the servos. This weight is presently provided by a 1kg lab weight, but could be replaced by more functional ballast (e.g., batteries, sensors, control boards) in the future.
The following sections will discuss each of the components I designed for Drosophibot II in more detail. Each leg segment design is discussed, in addition to a section for the combination thorax and abdomen.
Leg designs
ThC Joint
As the front legs include two DoF in the ThC, mounting hardware was necessary in these legs for the two MX-28 servomotors that actuate theseDoF. Figure 5 shows the "ThC joint" segment in the robot's front legs. It is comprised of two MX-28 servomotors connected via a U-shaped Onyx nylon bracket. The bracket, referred to as the "ThC bridge" (Fig. 5B), has an internal width of 30.5 mm to fit around the body of the MX-28 servo. The bottom face includes four M2 clearance holes in the pattern of the MX-28's horn mounting holes. The bracket also features four M2.5 clearance holes on its vertical faces for attachment to the MX-28 actuating the ThC1. These holes are countersunk into 1.5 mm deep pockets to provide the maximum amount of vertical clearance between the rest of the leg and the ThC3 servomotor during rotations. The vertical side walls each have a thickness of 3.8mm to maximize the distance between their edge and the servo horn's face for similar reasons. The bottom edge has a larger thickness of 4.78mm to create additional strength in the part.
Coxa
Only the front pair of legs include a mobile coxa due to their inclusion of ThC DoF. The middle and hind legs have their coxa segments accounted for in their attachment points on the thorax, with the mobile segments beginning distal to the CTr joint. Figure 6 shows renderings and drawings of the front leg coxa segment. The segment is almost entirely comprised of the MX-64 servomotor that actuates the CTr joint. A small U-shaped bracket made of Onyx nylon attaches the segment to the horn of the servomotor for the ThC3 joint (Fig.~6B). The bracket has an internal width of 34.5 mm to fit around the body of the MX-64 servo. It is between 4-4.5 mm thick throughout, with four holes for M2 bolts on its top face in the pattern of the MX-28's horn mounting holes. The top face also includes a 23 mm diameter, 1 mm deep pocket that accommodates the MX-28 horn to minimize the distance between the segment and the horn. The bracket additionally has clearance holes for four M2.5 bolts on its vertical faces for attachment to the MX-64 servo.
Trochanter
Each leg's trochanter segment is made up of a single bracket composed of Onyx nylon. Two different designs were created; one for the middle and hind legs (Fig. 7A) and one for the front legs (Fig.7B). The major dimensions of the two designs are the same; the only significant difference is that the front leg bracket has lowered walls on the distal edge to allow further levation of the leg before collision with the front leg's coxa segment. The inner width of each bracket is 48 mm, allowing it to freely fit around the horns of the Dynamixel MX-64 servo actuating the CTr joint. The bracket attaches to these horns with six M2.5 screws, three per side.
The front face of the bracket is angled 116o from the horizontal, corresponding to the fixed average angle of the TrF2 joint from Goldsmith et al. 2022. This face includes additional carbon fiber reinforcement to better resist twisting and four countersunk M2 clearance holes for attaching to the horn of the Dynamixel MX-28 servo that actuates the TrF joint. The holes are positioned on the distal face such that the point around which they are centered vertically aligns with the CTr joint axis. The distance between these two points is 25.32 mm, the minimum value that maintained the bracket's ability to fully rotate around the edge of the MX-64.
In the middle and hind leg version of the bracket, the distal edge is lengthened slightly in the center to allow for attachment of a strain gauge rosette. As the distal edge is shortened in the front leg version, an indentation in the center was instead made to accommodate the strain rosette.
In total, each bracket is 34.66 mm long, 30 mm tall, and 58 mm wide (Fig. 7C). It allows for 47o of total angular motion (15o of depression and 32o of levation) in the middle and hind legs and 65o of total motion (43o of depression and 22o of levation) in the front legs.
Femur
Each of Drosophibot II's femur segments consists of two Dynamixel MX-28 servos on the proximal and distal ends and an intermediary piece made of Onyx nylon (Fig. 8A, B). The proximal MX-28 actuates the TrF joint, while the distal MX-28 actuates the FTi joint. The intermediary plastic piece bridges the two servos and adds length of the femur to achieve the necessary leg segment proportions. Figure 8C shows drawings of the plastic femur component for the middle and hind legs with major dimensions labeled. The component is designed to place the TrF and FTi axes at the same height while also allowing as much movement of the FTi joint as possible. As such, the component is angled upward to attach into the top four bolt holes of the FTi servo in its required orientation. This angling allows the tibia to flex further inward compared to a purely horizontal component, increasing the RoM of the joint. The component attaches onto the FTi and TrF servos with four M2.5 bolts each. The length between the two servo mounting areas is hollow, with a wall thickness of 4.76 mm. This design reduces component weight and allows for internal routing of the power and signal cables for the FTi servo.
As shown in Figure 8B, the same plastic femur component in the front legs is 20.8 mm shorter to account for the shorter overall length of the fly's front femurs. However, the component has the same mounting design for the two servomotors, as well as the same vertical height; the length is the only dimension changed.
Tibia
Figure 9 shows renderings and drawings of the tibia segment for each of Drosophibot II's legs. The segment is 3D printed entirely out of Onyx nylon. As with the trochanter and femur, there are two versions of the component: one for the middle and hind legs (Fig. 9A), and one for the front legs (Fig. 9B). The only difference between the two designs is the front leg version is 20 mm shorter than the version for the middle/hind legs. Otherwise the two feature the same overall structure.
The proximal end of the tibia includes a branched bracket structure with an inner width of 42 mm. This bracket shape facilitates attachment to the MX-28 servomotor actuating the FTi joint in the femur via six M2 bolts. The overall depth of the bracket is 23.50 mm (Fig. 9C), corresponding to the minimum distance necessary for the piece to freely rotate around the corners of the FTi servo. Across all legs, this tibia design allows for 80o of rotation in the FTi joint (40o each for flexion and extension). The main length of the tibia has a 15 mm hollow square profile with 4.5 mm wall thickness (Fig. 9C, section view). In the middle/hind leg version of the part, this length extends 72.74 mm outward from the servo bracket section. In the front leg version, this length is shortened to 52.74 mm. The tibia is hollow to reduce the overall weight of the segment.
The distal end of the tibia includes two mounting holes for the tarsal segment. Both holes are 3.5 mm in diameter, and are meant for inserting M2.5 heat-set brass inserts into the plastic. Inserts were chosen over directly tapping the Onyx to decrease the likelihood of stripping threads over time. The first hole is in the ventral half of the segment running parallel to the major axis of the part (Fig. 9C, bottom right). The second hole lies at a 45o angle from the part's major axis on the dorsal face. As the tarsus is set at a 45o angle from the tibia, these two mounting holes allow for securing the two segments along both of their respective major axes.
Tarsus
Figure 10 shows the tarsal segment designs used in Drosophibot II. Each tarsal segment is 3D printed out of Onyx nylon. Each leg uses the same overarching tarsus design, with each leg pair having a different overall segment length (Fig. 10A-C). Each segment begins with a 15 mm square profile on its proximal end, tapering to a 15 mm diameter circle at the distal end.
The proximal end of each segment is angled at 45o from the horizontal to align with the angling of the tibia. A countersunk 2.7 mm clearance hole originating from the ventral face runs perpendicular to the distal face for the bottom M2.5 mounting screw into the tibia (Fig. 10D, top view and section view). A tab is included on the proximal edge of the dorsal face with an additional 2.7 mm clearance hole for the upper tibia-tarsus M2.5 mounting screw.
The distal end of each segment includes a pocket with the same dimensions as an M4 hex nut, allowing such a component to be embedded in the part during a programmed pause in the 3D printing. A 4.3mm diameter thru hole is also included on the distal face running through the center of the pocket to enable access to the nut's threads. The nut can then be used to mounting a modular tarsus tip for better traction on a variety of surfaces.
Similar to the femur and tibia, each segment is hollow (Fig. 10D, section view) with a wall thickness of 3.5 mm. Unlike the other two segments, however, the proximal and distal faces are not fully open; the proximal face is fully enclosed while the distal face includes only the aforementioned M4 clearance hole. This difference means any interior support material added by the 3D printing software cannot be removed post-print like for the other segments. However, it was experimentally determined during development that the tarsal segment's width is small enough that the dorsal face can successfully print without the support material, so this material was instead removed during the programmed pause for fitting in the M4 hex nut.
The middle leg tarsi have a functional length (measured from the center of the angled proximal edge to the distal edge) of 88.80 mm (Fig. 10A,D), while the front tarsi have a functional length of 56.58 mm. The hind leg tarsal segments are the longest, with a functional length of 107.83 mm.
Tarsus Tip
As mentioned in the previous section, the distal end of each tarsal segment includes a captive nut to facilitate the inclusion of modular tarsus tips. This design allows for the tarsal tips to be unthreaded from the tarsus and swapped for other designs to best suit the robot’s present terrain (e.g., linoleum, gravel, etc.), increasing the capacity for the robot to walk across a variety of test surfaces. For the data collection in this work, two different tarsal tips were used: a 3-D printed Onyx core coated with a layer of Dragon Skin 10 silicone rubber (Smooth-On Inc., Macungie, PA) (i.e., "silicone tip", Fig. 11A) , and a Flexible 80A Resin (Formlabs, Somerville, MA) tip with an Onyx "cap" (i.e., "resin tip", Fig. 11D).
The Onyx core of the silicone tip is 15mm long and 11mm in diameter. It consists of a bullet-shaped bottom piece and a top ``cap" piece, with an M4 hex bolt sandwiched between the two pieces (Fig. 11B). The bottom piece has a hexagonal pocket on its top face sized to loosely hold the head of the bolt, as well as two tabs on either side of the pocket. The top and bottom pieces attach together by press-fitting these tabs into corresponding pockets on the cap piece. Once the core is assembled, the outer silicone layer can then be added using a custom injection mold. The overall length and diameter of the finished silicon tip is 17mm and 15mm, respectively (Fig. 11C).
The resin tip is designed similarly to the silicon tip's core. The tip includes a bottom piece 3D printed out of flexible resin and a top cap piece printed out of Onyx nylon with an M4 hex bolt fit between the two (Fig. 11E). The bottom piece similarly includes a hexagonal pocket to encapsulate the head of an M4 hex bolt. However, the resin bottom piece additionally includes three cylindrical holes that correspond to three circular posts on the cap piece. Due to the tolerances of the available resin printer, the two pieces were not able to be press-fit together. Instead, J-B Weld Plastic Bonder epoxy adhesive (J-B Weld, Sulphur Springs, TX) was used to adhere the two pieces together. An assembled resin tip measures 15mm tall and 15mm in diameter (Fig. 11F).
Thorax and abdomen designs
Figure 12 shows renderings and drawings of the thorax and abdomen subassembly of Drosophibot II. All together, the thorax and abdomen combination is 388.5mm long and 100 mm wide. This subsystem's primarily design aim was to provide rigid mounting locations for the following: all six legs at their desired distances to each other; electronic circuit boards; hardware for the robot to hang from a hook; and an adjustable abdominal weight. All custom-designed components in the system are 3D printed out of Onyx nylon. The following paragraphs will discuss how the design achieves each of these objectives.
The thorax segment is comprised of four major pieces, numbered 1-4 in Figure 12B: A mounting bracket for the middle legs, a mounting bracket for the hind legs, a rear thorax plate, and a front thorax plate, respectively. The two leg mounting brackets are designed to hold the MX-64 servomotors actuating each leg's CTr joint at the fixed ThC angles found during the kinematic analysis in Goldsmith et al. 2022, as well as ensuring the legs are the appropriate distance from each other laterally and anteroposteriorly. Each servo secures to the bracket with four M2.5 bolts. The two brackets mount into the rear thorax plate via four M4 countersunk bolts each, with two 5.7 mm square posts on each to ensure proper orientation for mounting.
The rear thorax plate (3 in Fig. 12B) is composed of a horizontal section of length 121.48 mm that facilitates mounting of the middle and hind leg brackets and an angled section extending 93.64 mm horizontally and 21.8 mm vertically before terminating in three horizontal tabs. This angling spans the necessary horizontal and vertical distance between the front and middle leg pairs. Due to size constraints of the available print bed, the thorax attachment plate for the front legs (4 in Fig. 12B) was printed separately and attached to the rear thorax plate via three M4 bolts. An additional brace bar (5 in Fig. 12B) attaches between the middle leg bracket and the front thorax plate to provide rigidity between the rear and front plates. To further increase rigidity, the horizontal face and tabs were printed with two groups of four layers of carbon fiber reinforcement; one dorsal and one ventral.
The top face of the rear thorax plate has a variety of mounting structures for the various circuit boards needed for power and data collection, the bracket for hanging the robot on a winch (7 in Fig. 12B), and the abdominal segment (8 in Fig. 12B). There are 24 M2.5 clearance holes on the rear thorax plate in total to accommodate these components; four each for the two main circuit boards, winch bracket, and abdomen, and an additional eight to accommodate future components. The eight holes for the circuit boards are spaced to fit the mounting holes for the OpenCM microcontroller and the custom power board detailed in the Electronics Design section, and are included in the center of 2.5 mm tall posts to provide clearance for components on the bottom sides of the boards. The threads for each of these mounting holes are provided by a series of embedded M2.5 nuts press-fit into pockets below the dorsal surface during the 3D-printing process.
As mentioned in the preceding paragraphs, the front thorax plate's primary function is providing mounting for the front legs, as well as achieving sufficient horizontal and vertical distance between the front and middle leg pairs for the robot's desired proportions. Two Dynamixel FR07-S101 brackets (6 in Fig. 12B) are mounted to the front plate with four M2.5 bolts to provide mounting for the MX-28 servos actuating the ThC3 joints in the front legs. These servos mount to the brackets via four M2.5 bolts. The plate also includes two additional M2.5 clearance holes toward the front for mounting the brace bar between the front thorax plate and the middle leg bracket that increases rigidity between the rear and front thorax plates. Similar to the rear thorax plate, the front plate was also printed with two four-layer groups of carbon fiber reinforcement.
The abdominal segment (8 in Fig. 12B) attaches to the thorax section via four M2.5 bolts into the rear thorax plate and two M2.5 bolts into the hind legs bracket. The mounting threads for these latter bolts are achieved via heat-set M2.5 threaded inserts fit into the hind legs bracket. The abdomen extends 173.38 mm posterior to the edge of the rear thorax plate. Three channels run the length of the piece. The inner, middle channel is wide enough to accommodate a cylindrical lab weight to be supported laying on its side in the channel. The weight can be adjusted along the channel to shift the overall center-of-mass of the robot anteroposteriorly. It is held in place by a bracket (9 in Fig. 12B) that bolts into the outer two channels with two M2.5 bolts. An M4 set screw is included at the top of the bracket to secure the lab weight in place. The abdominal segment was also printed with two layers of carbon fiber reinforcement.
References
C. A. Goldsmith, M. Haustein, T. Bockemühl, A. Büschges, and N. S. Szczecinski, “Analyzing 3D Limb Kinematics of Drosophila Melanogaster for Robotic Platform Development,” in Biomimetic and Biohybrid Systems, A. Hunt, V. Vouloutsi, K. Moses, R. Quinn, A. Mura, T. Prescott, and P. F. M. J. Verschure, Eds., Cham: Springer International Publishing, 2022, pp. 111–122.
N. S. Szczecinski, T. Bockemühl, A. S. Chockley, and A. Büschges, “Static stability predicts the continuum of interleg coordination patterns in Drosophila,” The Journal of Experimental Biology, vol. 221, p. jeb.189142, Jan. 2018, doi: 10.1242/jeb.189142.
N. S. Szczecinski, C. A. Goldsmith, F. R. Young, and R. D. Quinn, “Tuning a Robot Servomotor to Exhibit Muscle-Like Dynamics,” in Biomimetic and Biohybrid Systems, U. Martinez-Hernandez, V. Vouloutsi, A. Mura, M. Mangan, M. Asada, T. J. Prescott, and P. F. M. J. Verschure, Eds., Cham: Springer International Publishing, 2019, pp. 254–265.
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