Assembly & Constraints

Top-Down vs. Bottom-Up Assembly

There are two fundamental strategies for building assemblies, and most real-world robotics projects use a combination of both:

• Bottom-Up Assembly: You design each part individually in its own file, then insert them into an assembly document and constrain them together. This is the most common approach and gives you maximum reuse — the same motor model can appear in ten different robot designs. Start here for components with well-defined geometry (motors, bearings, brackets, fasteners).
• Top-Down Assembly: You create new parts directly within the assembly environment, referencing the geometry of parts that already exist. This is powerful when parts must conform to each other — for example, designing a custom motor mount that references the motor's bolt pattern and the chassis's mounting surface simultaneously. Use this for integrating and adapting components.
• Hybrid Approach: In practice, you will import standard components (motors, sensors, bearings) bottom-up, then design custom brackets, adapters, and structural members top-down to connect everything. This is the professional robotics workflow.

Grounding the First Component

Every assembly needs an anchor — a single component that is grounded (fixed in space). This component does not move; every other part is positioned relative to it. In robotics, the grounded component is almost always the chassis or base frame.

• The grounded component's origin becomes the assembly origin. Align it thoughtfully — the robot's center-of-footprint or center-of-rotation is a sensible choice.
• In most CAD software, the first component inserted is automatically grounded. You can change which component is grounded at any time.
• Only one component should be grounded. If you accidentally ground multiple parts, your assembly loses the ability to simulate motion correctly.
• Think of grounding as defining your robot's "world frame" — the fixed reference from which all motion is measured.

The Assembly Workflow at a Glance

Before diving deep into each topic, here is the high-level workflow you will follow every time you build an assembly. Each step is covered in detail later in this module:

• 1. Ground the base — Insert and fix your chassis or base frame.
• 2. Insert components — Bring in parts and sub-assemblies from your file system or library.
• 3. Apply joints & constraints — Define how each part connects to and moves relative to its neighbors.
• 4. Test motion — Drag components to verify they move as intended.
• 5. Check interference — Run collision/interference analysis to catch physical overlaps.
• 6. Create exploded views — Generate exploded views and animations for documentation and presentation.

The 6 Degrees of Freedom

Every unconstrained part in your assembly starts with all six DOF. Each constraint you apply removes one or more DOF until the part can only move the way it should in the real mechanism. When all six DOF are removed, the part is fully constrained (rigid). When some DOF remain, the part is under-constrained and retains intentional mobility — like a wheel that must spin.

DOF in Robotics Context

The number of degrees of freedom in a robotic mechanism defines its capability. A 6-DOF industrial robot arm can reach any point in its workspace at any orientation. A 3-DOF planar robot can only move in a flat plane. Understanding DOF is not just a CAD concept — it is the foundation of all robot kinematics.

• Under-constrained parts have remaining DOF. This is correct for moving components (wheels, arms, linkages) but a mistake for structural parts (brackets, frames).
• Fully constrained parts have 0 remaining DOF. Structural members and housings should be fully constrained in your assembly.
• Over-constrained parts have redundant constraints that conflict. This causes solver errors in your CAD software and indicates a problem with your assembly logic. If the software warns you about redundant constraints, investigate and remove the extras.
• Counting DOF: Total mechanism DOF = (6 × number of moving parts) − (DOF removed by all constraints). A well-designed assembly has exactly the DOF you intend — no more, no less.

Standard Joint Types for Robotics

Each joint type allows a specific combination of motion and removes a specific number of degrees of freedom. Choosing the correct joint type is critical — it determines how your mechanism behaves in simulation and motion studies.

Joints vs. Constraints: When to Use Which

• Use joints when you are modeling mechanical connections between parts that may move. Joints carry motion semantics — they define axes of rotation, travel limits, and can drive motion studies and simulations.
• Use constraints when you need precise geometric positioning — aligning faces, setting offsets, or defining angular relationships for parts that do not move relative to each other.
• In practice: Many engineers use joints for all connections (including rigid ones) because joints are easier to modify later. If you constrain a wheel with three separate mates and later realize it should rotate, you must delete and redo. A joint can simply be changed from rigid to revolute.
• Software differences: SolidWorks uses "mates" as the primary system. Fusion 360 and Onshape use "joints." Inventor supports both. The concepts are identical — only the terminology and UI differ.

Why Interference Detection Matters

Robotics assemblies are particularly prone to interference problems because they contain moving parts, tight packaging, and components from multiple designers. Common interference issues include:

• Static interference: Two parts overlap in the home/default position. Often caused by incorrect dimensions, misaligned bolt holes, or components imported at the wrong scale.
• Dynamic interference: Parts collide during motion — an arm swings into the chassis, a gear tooth clips an adjacent mechanism, a cable bundle catches on a rotating shaft. These are harder to find because they only occur at specific positions.
• Fastener interference: Bolt heads, nuts, and washers that protrude into neighboring parts. Easy to miss because fasteners are often hidden or simplified in the model.
• Clearance violations: Parts that technically don't overlap but are too close together — leaving no room for manufacturing tolerances, thermal expansion, or cable routing.

How to Run Interference Analysis

• Step 1: Select the components to check. You can check the entire assembly or select specific pairs/groups of parts. For large assemblies, check sub-assemblies individually first, then check between sub-assemblies.
• Step 2: Run the analysis. The software will compute all volumetric overlaps between selected components and report them as interference volumes with exact measurements.
• Step 3: Review results. Each interference is typically highlighted in the 3D view with a colored volume showing exactly where the overlap occurs. The report lists the interfering part pair, overlap volume, and overlap depth.
• Step 4: For moving assemblies, run interference analysis at multiple positions throughout the range of motion. Drive each joint to its minimum, maximum, and several intermediate positions, checking interference at each.
• Step 5: Resolve every interference. Modify part geometry, adjust constraints, or reposition components until the analysis returns zero interferences at all positions.

Animating Joints

A motion study defines how each joint's position changes over time. You can drive joints with:

• Constant velocity: The joint moves at a fixed speed. Useful for continuous-rotation mechanisms like wheels, conveyors, and spindles.
• Position keyframes: You define the joint's position at specific time points, and the software interpolates the motion between them. Useful for sequenced pick-and-place operations or multi-position mechanisms.
• Input functions: Define joint motion as a mathematical function of time (sinusoidal, trapezoidal velocity profile, etc.). Useful for simulating realistic actuator behavior.
• Driven joints: One joint's motion is defined as a function of another joint's position. This models gear trains, belt-and-pulley systems, and cam-follower mechanisms where one input drives multiple outputs.

Verifying Range of Motion

For every moving joint in your robot, you must verify that it can achieve its full intended range of motion without interference or unexpected behavior:

• Sweep the full range: Drive each joint from its minimum to maximum limit and watch for collisions, binding, or unexpected part behavior.
• Check compound motion: When multiple joints move simultaneously (as they will in the real robot), the combined motion can cause interferences that don't appear when testing joints individually. Test worst-case combinations.
• Measure workspace: For robot arms, trace the end-effector's path through space to visualize the reachable workspace. Verify it covers the required task area.
• Verify clearances at extremes: The minimum clearance between any two moving parts, across the entire range of motion, must exceed your manufacturing tolerance stack-up.
• Export animations: Record motion study animations for design reviews, team communication, and sponsor/client presentations. A 10-second animation is worth more than ten pages of explanation.

Common Motion Study Checks for Robotics

• Drivetrain: Spin all wheels simultaneously. Check that suspension travel doesn't cause wheel-to-frame interference. Verify steering geometry through full lock-to-lock rotation.
• Manipulator arms: Move through the full pick-and-place cycle. Check that the arm can reach all required positions and that no links collide at any point in the trajectory.
• Intake/scoring mechanisms: Cycle through the complete game-piece acquisition and scoring sequence. Verify timing and clearances at full speed.
• Linkages: Animate four-bar and six-bar linkages through their full cycle. Watch for toggle positions, dead points, and unexpected path deviations.