Mechanical Design and Materials

A robot's structure is its skeleton, its armor, and its kinematic framework all at once. It must be strong enough to bear loads without failing, stiff enough to keep sensors and actuators in their correct relative positions, light enough to be moved by available actuators, and manufacturable within budget. These requirements are in tension — stronger structures tend to be heavier; stiffer structures tend to be more expensive to produce. Mechanical design is the art of finding the best compromise, guided by quantitative analysis of forces and material properties.

Forces, Loads, and Load Paths

Before selecting any material or geometry, an engineer must understand the forces the structure must carry. Loads on a robot structure include: Static loads: the weight of the robot's own components (gravity acting on mass). A robot arm joint must support the weight of every link and payload outboard of it. Dynamic loads: forces produced by acceleration. When a robot arm accelerates a payload at 5 m/s^2, the joint experiences a force equal to mass x acceleration in addition to the static weight. Impact loads: sudden forces from collisions or drops. A field robot landing after a jump experiences a peak impact force many times its static weight, concentrated in fractions of a second. Fatigue loads: repeated cycling. A robot that makes 10,000 arm cycles per day experiences cumulative fatigue in its structural members; a metal that is strong under a single load can fail at much lower stress levels after many repeated cycles. A load path is the route through the structure by which a force travels from its point of application to the structural supports. Good mechanical design ensures that load paths pass through material efficiently (straight, with minimal bends) and that no single joint or member becomes a bottleneck that concentrates stress. Stress concentration at notches, holes, and sharp corners is responsible for the majority of structural fatigue failures in mechanical systems.

Materials for Robot Structures

Material selection is driven by the specific stiffness (stiffness per unit mass) and specific strength (strength per unit mass) of the material, as well as its cost, machinability, and compatibility with the manufacturing process. Aluminum alloys (6061, 7075) are the workhorse of robot structures. They have a good strength-to-weight ratio (7075-T6 aluminum has a yield strength of 503 MPa at 2.81 g/cm^3 density), are easily machined on CNC mills, and are inexpensive. Most drone frames, robot arm links, and wheeled robot chassis use aluminum. Carbon fiber reinforced polymer (CFRP) offers exceptional specific stiffness — a carbon fiber tube is stiffer per unit mass than any common metal. CFRP is used where minimum weight is critical: racing drone frames, high-performance robot arms, and the body panels of Boston Dynamics' Spot. The trade-offs are high cost, difficulty in joining pieces together, and brittleness under impact — CFRP shatters rather than bending plastically. Steel has the highest absolute strength and stiffness of common structural metals but is three times denser than aluminum. It is reserved for high-force applications where space is limited: hydraulic cylinder bodies, gear teeth, and safety-critical fasteners. FDM 3D-printed polymers (PLA, PETG, nylon, polycarbonate) have enabled rapid prototyping of complex structural shapes that would be expensive to machine. Print-in-place flexures, hollow organic structures, and complex brackets can be produced overnight. However, FDM-printed parts have anisotropic strength (much weaker in the direction perpendicular to print layers) and poor fatigue resistance — suitable for prototyping and low-load production parts, not for primary load-bearing structure.

Kinematics: Joints and Workspaces

A robot manipulator's mechanical design determines its kinematics — the geometric relationship between joint positions and the position and orientation of the end-effector (gripper or tool). Two concepts are essential. Degrees of freedom (DOF) is the number of independent variables needed to fully specify a configuration. A free rigid body in 3-D space has 6 DOF (3 translational, 3 rotational). A robot arm needs at least 6 joints to achieve arbitrary position and orientation of its end-effector in 3-D space; arms with fewer DOF have constrained workspaces. Forward kinematics computes the end-effector position given joint angles — it is algebraically straightforward. Inverse kinematics computes the joint angles needed to achieve a desired end-effector position — it can have zero, one, or multiple solutions, and for most arm geometries requires iterative numerical methods. The workspace of a manipulator is the set of positions the end-effector can reach. Workspace shape and size depend entirely on link lengths and joint ranges. A Cartesian (gantry) robot has a rectangular workspace; a SCARA robot has a cylindrical workspace; a 6-DOF articulated arm has a roughly spherical workspace with a dead zone near the base. Robot placement in a work cell must ensure the entire required task space falls within the workspace — a placement error discovered during commissioning is expensive.