The chapters following the introduction are describing our domain-specific development towards Serval, our newest robot, which integrates its predecessors’ features. To facilitate the understanding of our design-choices, we highlight in this subsection the purpose of the constructed robots.
Lynx
Lynx is a compliant quadruped robot with the focus on three modular spine designs and a pantograph leg design. It was mainly built out of milled carbon- and glass fiber plates as well as 3D-printed ABS-pieces. The actuation is realized with RC-Servomotors (Kondo KRS2350 ICS, stall torque 2 Nm at 6 V) that are controlled by an RB110-electronics board with integrated Linux-OS. The robot has 9 actuated degrees of freedom (DOF), two per leg and one in the spine. It consists of two trunk segments of that the front one is slightly heavier (about 40 g) caused by the location of the RB110, the legs and an active spine that connects the trunk elements. The spine-versions (SV) are all actively actuated but differ in their use of the compliant elements as well as a "single point of rotation" (the strongest abstraction from nature) vs. "multiple points of rotation" (a less strong abstraction from nature). The design is completed by a passive tail-like structure, that acts like a 5th-leg-stabilizer of the system in case of high pitching motion induced by bad gaits (it prevents the robot from falling backward). In these cases, the compliant elements in the structure will push the robot in the opposite pitch-direction. This results in the establishment of ground contact with all four legs. This tail-like structure represents a non-bio-inspired part, as animals (expect the Kangaroo and some small mammals) seem not to use their tails for active pitch support during ground locomotion (ongoing research).
Cheetah-Cub-S
Cheetah-Cub-S is a hybrid robot that combines the pantographic leg design of Cheetah-Cub with a flexible spine. It thus gains the ability to steer. The spine can bend laterally much like a spine of lizards and is actuated by a single motor located in the middle between fore and hind trunk segments. Cheetah-Cub-S can turn with a radius of 0.5 m and a speed of 0.35 ms−1with
4.1. State-of-the-Art: A General and non-exhaustive Overview
a non-optimized gait, taken from Cheetah-Cub. Higher turning-speeds should be achievable with the optimization of the slipping behavior. The steering is not limited to one fixed radius at the time, that was confirmed by letting the robot run a slalom with different turning radii. Additional experiments classified the payload capacity of the robot. Another comparison study with Cheetah-Cub was done in which steering without a spine and abduction/adduction was investigated.
Cheetah-Cub-AL
Cheetah-Cub was not fundamentally altered from its early development days. Some major changes are introduced with Cheetah-Cub-AL. The leg was redesigned and features now a (to the saggital plane of the leg) symmetric diagonal spring, canceling unwanted bending behavior present in previous Cheetah-Cub-versions. Additionally, making use of classical CNC-manufacturing techniques with aluminum in combination with ball-bearings in every joint, friction was reduced, alignment of the axis and repeatability of experiments were impro- ved. The changes to the trunk are little but feature now easy access to the control board for development purposes. Another major change is the switch to a new operating system, Jokto, that improves stability and ease of use. Tuleu implemented inverse-kinematics of the legs for control purposes. This allowed to tune gaits much faster and more intuitively. The robot was featured recently in Prof. Ijspeert’s talk in TED Global Geneva.
Serval
Serval, the last in a line of robot iterations, is meant to serve as a quadruped for agile movement. We use the previously researched mechanisms, control structures and gained knowledge in the electronics development to build a combined and hopefully higher performing robot. Serval consists of an active 3-DOF spine (combining advantages from Lynx and Cheetah-Cub-S), leg units with adduction/abduction mechanism and a scaled ASLP-version of Cheetah-Cub-AL. All motors (Dynamixel MX64R and MX28R) are combined with in-series elastics to protect the somewhat sensitive gear-boxes from harm in different load scenarios. The robot is equipped only with a minimal sensor set, consisting of a low-cost, medium-grade IMU. Collaborations started close to the end of this thesis will provide contact and GRF sensing with capacitive sensors as well as a sensitive skin for physical guidance. Control is realized through inverse kinematics for the legs, (for now) offsets in the spine and an underlying CPG-network for pattern generation. Reflexes, like in Oncilla, were not yet implemented, but are ongoing and future work.
Chapter 4. Introduction
Table 4.3 – Characteristic values of quadruped in BIOROB; Robots built prior to this thesis: Cheetah-Cub (CC), Bobcat; built prior and in the first months of this thesis by the author: Lynx; Robots built in collaboration with major contribution from the author: Oncilla, Cheetah- Cub-S (CCS); Robot built solely by the author: Serval, Cheetah-Cub-AL (CCAL); Geometric measures extracted from CAD, additional information extracted form publications and data- sheets; DS-Diagonal Spring, PS-Parallel spring,FS-Foot spring, PR-Protraction/Retraction, FE-Flexion/Extension, AA-Adduction/Abduction, SBC-Single Board Computer; Iterations- Iterations until the final design, BT-Blue-tooth, G-Gear, Ko-Kondo, Dx-Dynamixel, Ma-Maxon, AJE-Absolute joint encoder; geometric measures rounded to the [mm], hanging in air
Unit CC CCAL CCS Lynx Bobcat Oncilla Serval
Height: Max [mm] 233 264 217 288 (?) 357 390 Height: Ground-Hip [mm] 166 164 166 160 125 201 228 Width: Max [mm] 124 128 132 129 (?) 245 247 Width: Leg-leg [mm] 89 91 96 101 97-127 138 211 Length: Max [mm] 246 248 271 438 (?) 468 563 Length: Hip-Hip [mm] 207 206 206 226 166 223 378 Mass: Total [g] 1100 1200 1160 1200 1030 5050 3560 Mass: Electronics [g] 560 560 608 608 608 2845 2167 Mass: Mechanics [g] 540 640 552 592 422 2205 1393 Stiffness: DS [N/mm] 2.33 3.6 2.33 2.33 2.33 5.8 7.76 Stiffness: PS [N/mm] 4.8/ 2.33 (?) 7.4 9.06 Stiffness: FS [N/mm] 1.98 (?) Sensor 1.98 (x2) Stiffness: AA [Nm/rad] 253.2 Stiffness: Spine [N/mm] (?) 8.4/ 52 DOF: Actuated 8 8 9 9 9 12 15 ROM: PR fore [°] +122/-40 (?) ±34 +76/-50 ROM: PR hind [°] +70/-90 (?) ±34 +84/-64 ROM: FE [mm] 69 (?) 70 93 ROM: AA [°] ±8 +90/-70 ROM: Spine [°] ±10 ±30/ -15 ±35 ±90/±30
Motor: Servo Ko KRS2350 ICS Dx MX28R/64R
Voltage: Servo [V] 9-12 10-14.8
Stall torgue: Servo [Nm] 2 (6V) 2.5/ 6 (12V)
No load speed: Servo [°/s] 375 (6V) 330/ 378 (12V)
Gear ratio: Servo 200:1 193:1/ 200:1
Motor: EC Ma 323218
Voltage: EC [V] 24
Stall torgue: EC [Nm] 0,639 (45,5A)
Gear box: G Ma 370687
Gear ratio: G+Cus 84:1/ 56:1
Stall torgue: EC+G [Nm] 7.1/ 4.7 (6A)
No load speed: EC+G [°/s] 1164/ 499
SBC RoBoard RB-110 Odroid XU4
Connectivity WiFi BT, Wifi
Sensors None AJE, 3D-GRF, IMU IMU, (GRF, Skin)
Untethered No Yes
LiPo-Battery No 3S-4.5Ah-45C 3S-3.3Ah-25C
Iterations >2 2 1.5 2 1 >3 1.5
5
Domain Specific Design I: Mechanics
The mechanics chapter, as the first domain-specific chapter, will start by summarizing the basic and advanced methods used in lightweight prototyping. Consequently, a general over- view of production processes and materials will be presented and their usefulness analyzed. Further on, we highlight the mechanical development of different robots towards our last robot, Serval, that combines mechanisms tested in earlier systems.
5.1 Materials and Methods for Lightweight Structures
For the success of a mechanical design of a legged robot, the use of different materials in the right combination is imperative. Each material has its properties and special difficulties as well as advantages, that are construction-relevant. Based on the construction-boundaries such as:
• Lightweight construction • Robustness
• Flexibility through compliance • Higher stiffness for skeleton-parts • Ease of assembly (modular structure) • Fast production (prototyping)
A broad range of materials can be chosen to build our robots. To connect the designed parts classical methods like gluing, soldering, screwing with or without inserts and fitting may be
Chapter 5. Domain Specific Design I: Mechanics
selected, depending on the situation at hand. These connection methods are not specially discussed but may play a part in the material selection.
This chapter deals with the basics of lightweight construction, its elements, materials and functional principles. Also, a brief insight into the theory of engineering mechanics is given. These points form the basis for the design of our systems and are therefore essential for this work.