The Problem
Chronic lower back pain affects hundreds of millions of people worldwide. Most mechanical therapy devices are either static (the patient stays fixed while a module acts on them) or require the patient to actively move. Chirobot takes a different approach: the robot itself moves freely beneath a supine patient, delivering dynamic massage patterns across the back without requiring the patient to reposition.
The Design Challenge
The core constraint was holonomic motion under patient load. A standard differential drive can only move forward/backward and rotate; to change direction, it must first reorient. That is unusable for a therapy robot that needs to trace arbitrary patterns under a lying patient.
We chose a 3-wheeled holonomic base with omnidirectional wheels arranged at 120° symmetry. This gives full 3-DOF motion (translation in any direction and simultaneous rotation) with no need to reorient. The 120° symmetric layout also optimizes weight distribution and stability under the patient's body weight.
Mechanical Architecture
The chassis was designed from scratch in SolidWorks, with modularity as a core principle: every component can be swapped without disassembling the full base. This was critical during early lab iterations where wheel geometry and motor mounting angles changed frequently.
Key decisions:
- Low-profile chassis to minimize the ground clearance needed for a patient platform above
- Recessed wheel wells to protect the omnidirectional wheels while keeping the overall footprint compact
- Motor-to-frame interfaces using clamping collars and vibration-resistant fasteners (important given the continuous vibration load during operation)
Fabrication and Iteration
The chassis was fabricated using FDM 3D printing, which allowed rapid iteration between design revisions. Early prints used organic, support-heavy geometries that were structurally sound but difficult to assemble. Later iterations moved to cleaner low-profile geometries with integrated motor mounting channels, reducing print time and improving assembly speed.
Actuation is handled by high-torque servos with custom shaft couplings, powered by 2200 mAh LiPo packs. ESCs are recessed within the chassis volume to protect the electronics and keep the center of mass low.
Where We Are
The current focus is finalizing the base geometry, which is the hardest part of the project. Once the base form factor is fixed, the mechanical assembly, electronics integration, and closed-loop control implementation follow in sequence. Experimental protocols are being developed to evaluate biomechanical performance under patient loads, with data analyzed in Python to drive iterative design improvements.