The problem
Automated charging helps fleets, disabled drivers and autonomous vehicles, but connecting a heavy, cable-laden connector to an inlet whose position varies with parking accuracy is hard. The arm must reach a range of positions, align the connector, push it in with the right force and fail safe. This project focuses on the mechanical design and its kinematic and structural checks.
Objectives
- Define the parking misalignment range, inlet envelope and connector insertion force.
- Choose an arm architecture and size it for workspace and payload, including cable weight.
- Use Motion studies to check reach, insertion trajectory and joint torques.
- Size the structure for stiffness and strength, and design a passive compliance element for alignment.
What you need
Tools and toolboxes
- SOLIDWORKS Part and Assembly — Base, linear axes or links, wrist, compliant connector holder and cable carrier, with mates for each joint.
- SOLIDWORKS Motion Study (Motion Analysis) — Motors on each joint driven by segments or expressions, contact between connector and inlet, and torque and reaction plots; needs SOLIDWORKS Premium or a Simulation Professional or Premium licence.
- SOLIDWORKS Simulation (Static and Frequency) — Arm deflection and stress at full reach with payload, and natural frequencies to avoid vibration during motion.
- SOLIDWORKS Weldments — Frame for the base and gantry members.
- Mass Properties — Link masses and inertias for torque estimates.
- Excel or MATLAB — Workspace mapping, inverse kinematics for chosen poses and actuator sizing.
Software
- SOLIDWORKS 2023 or later (Premium for Motion Analysis)
- SOLIDWORKS Simulation
- Excel or MATLAB
Extra parts and data
- Connector and inlet dimensional data — Insertion force, alignment chamfers and envelope come from the connector standard and maker data, which should be checked for the chosen type.
Key concepts
- Workspace
- The set of positions and orientations the arm can reach. It must cover every inlet position allowed by parking misalignment.
- Passive compliance
- A spring-loaded or remote-centre compliance wrist that lets the connector shift slightly to align with the inlet chamfers during insertion, so the arm does not need perfect accuracy.
- Joint torque
- The torque each joint motor must supply, from gravity on links, payload and cable, plus acceleration and insertion force.
- Fail-safe behaviour
- If power is lost or a fault occurs, the arm must stop without harming people or the car and allow the connector to be released.
Step-by-step guide
- 1
Define requirements
Set an example parking misalignment range: ±150 mm side to side, ±200 mm fore-aft and ±5 degrees yaw. Define the inlet height range across example vehicle types. Set connector mass, cable mass per metre for the hanging length and an example insertion force from connector data, to be confirmed. Set cycle-time and safety requirements, including speed limits near people and a manual release.
Checkpoint: A requirements table with misalignment ranges, inlet heights, payload, insertion force and safety rules.
- 2
Choose the architecture
Compare a wall-mounted Cartesian gantry, a SCARA-type arm and an articulated arm for reach, stiffness, footprint and cost. A Cartesian design is easy to model and control and stiff for its weight, while an articulated arm is more flexible. Score options and choose one, justifying the choice. Sketch the chosen kinematic chain with joint types and limits. Include maintenance access and the risk of the arm striking a person in the scoring.
Checkpoint: A concept matrix with a chosen architecture and its joint list.
- 3
Map the workspace
In Excel or MATLAB, compute inlet positions over the misalignment grid, then compute joint positions for each with inverse kinematics. Check every pose is within joint limits and the arm clears the vehicle body. Build a 2D layout sketch of the arm to confirm the extreme poses graphically. Adjust link lengths or axis strokes until all poses are reachable. Include the extreme yaw cases, which usually set the wrist rotation range.
Checkpoint: A workspace map in which every inlet position in the misalignment grid is reachable within joint limits.
- 4
Model the arm
Model the base frame as a weldment, linear axes with guides and carriages or links with bearing housings, a wrist and a compliant connector holder. Add a cable carrier or balancer for the charging cable. Use aluminium extrusions or steel from the material library. Record each link's mass and inertia with Mass Properties for torque calculations. Add covers over guides and screws to keep dust and rain out in an outdoor bay.
Checkpoint: A full arm assembly with masses and inertias recorded for every moving part.
- 5
Design the compliance wrist
Design a holder that allows a few millimetres of lateral float and a few degrees of angular float, centred by springs, so the connector can follow inlet chamfers. Use a remote-centre compliance idea or a simple spring-centred plate. Define spring rates so the connector stays centred under its own weight but moves under the side force from the chamfer. Add a sensor to detect when float is used up.
Checkpoint: A wrist with stated float range and spring rates, holding the connector centred under gravity.
- 6
Simulate approach and insertion
In a Motion Analysis study, drive each joint with motors using segment profiles to approach the inlet at a stated misalignment, then insert along the inlet axis. Add contact between connector and a simplified inlet with chamfers. Turn on gravity and include cable weight as a force or a distributed mass. Plot joint torques, insertion force and wrist float. Repeat for the four corner cases of the misalignment grid.
Checkpoint: The connector inserts fully at the worst misalignment with wrist float within its range and insertion force near the target.
- 7
Size actuators
From the motion results, take peak and RMS torque or force for each joint, add margins for friction and wear and convert through a gear or screw ratio. Select example motors and gearboxes or ball screws whose ratings exceed the requirements. Check holding torque with power off and whether brakes are needed on vertical axes so the arm cannot fall.
Checkpoint: An actuator table with required and selected ratings, including brakes on gravity-loaded axes.
- 8
Check stiffness and strength
In a static study, put the arm at full reach with payload, cable load and insertion force reaction, and fix the base. Check deflection at the connector against the wrist's float range and stress against yield. Run a frequency study at full reach and make sure the first natural frequency is well above the motion's excitation frequency. Refine mesh and confirm convergence.
Checkpoint: Deflection well within wrist float, factor of safety above an example 2 and first natural frequency above a stated limit.
- 9
Plan safety and cycle time
From the motion profiles, compute cycle time from vehicle stop to connector inserted, and from charge end to arm parked. Identify pinch points and set guarding, speed limits and force limits for human proximity. List sensors for presence detection and insertion confirmation. Describe the fail-safe state if power fails during insertion. Show where time can be saved by moving axes together, and the safety limits that prevent it near people. Note which sensor confirms each step before the next begins.
Checkpoint: A cycle-time breakdown and a safety concept with pinch points and responses listed.
- 10
Drawings and report
Make a general arrangement drawing with workspace envelope, key link drawings and an actuator list. Write up requirements, architecture choice, workspace map, compliance design, motion results, actuator sizing, structural checks and safety concept. State limitations: simplified cable, rigid links, no control system. Recommend a scaled prototype test of insertion with an inlet mock-up. Include a sequence diagram of approach, insertion, charging and retraction, and the parked position of the arm when idle.
Checkpoint: Drawings and a report with workspace, torques, stiffness and safety concept.
Testing & validation
- Every inlet position in the misalignment grid is reachable.
- Insertion succeeds at worst-case misalignment with wrist float in range.
- Actuator ratings exceed required torques with margin.
- Structural deflection, stress and natural frequency meet targets.
Expected results
An automatic charging arm design with a workspace map, a compliance wrist, insertion simulation, sized actuators, structural checks and a safety concept with cycle times.
Research angle for your M.Tech thesis
The research angle is the trade-off between arm accuracy, passive compliance and insertion success over a realistic parking misalignment distribution. A study could vary wrist compliance and arm stiffness, measure insertion success and forces in simulation and on a scale rig, and define design rules for compliant connectors. Comparing architectures on cost, footprint and cycle time adds practical value.
Take it further
- Add a vision-based inlet locating concept and its sensor mount.
- Design an underbody conductive connector as an alternative.
- Run a fatigue study for many daily cycles.
- Build a 3D-printed scale model to test insertion with the compliance wrist.
Viva and review questions
Why add passive compliance instead of a more accurate arm?
A compliant wrist lets the connector self-align on the inlet chamfers, which tolerates position errors and avoids the cost and complexity of a very accurate arm.
How did you include the cable?
As a force or distributed mass at the connector for the hanging length, and through a cable carrier on the arm. A detailed cable model would need nonlinear analysis.
Why check natural frequency?
If the arm's natural frequency is close to the frequencies of its motion, it can vibrate and miss the inlet or overload joints.
What happens if power fails?
Brakes hold gravity-loaded axes, the arm stops in place, and the connector can be released manually, which is part of the safety concept.
Learn it at DIYguru
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