Clinical Context
SickKids rheumatology uses sit-to-stand (STS) tests to evaluate pediatric patients over time. The problem was that standard chairs are fixed height, which means the test conditions change as a child grows. A 6-year-old and a 12-year-old doing the same STS test with the same chair are not doing the same test. The measurement is inconsistent.
The chair had to solve this: adjust seat height precisely enough that every patient gets a standardized posture, regardless of leg length. That meant a continuous adjustment range wide enough to cover the pediatric population, a mechanism that was stable under load, and accuracy within clinical tolerance.
SEAT RANGE 12.7 — 59.5 cm // 130 kg CAPACITY
Design Constraints
Clinical hardware has constraints that a normal build doesn't. The device sits in a hospital. It needs to be cleaned. It needs to be operated by clinical staff with no engineering background. It cannot have sharp edges, pinch points, or mechanisms that could hurt a child. And it needs to be built and validated within an academic semester.
Mechanism
The design is a scissor-lift frame driven by a single electric linear actuator. The scissor geometry provides vertical lift from horizontal actuator travel, which made it possible to mount the actuator in a low-profile position that doesn't interfere with the seating area or the patient's legs.
Material selection was driven by the load requirement and the need for a rigid-enough frame that compliance in the mechanism didn't eat into the accuracy budget. Geometry was modeled in Fusion 360 and checked against the load cases before anything was ordered.
Validation Results
Validation runs measured actual seat height vs. commanded height across the full range. The target was to stay within 2.00 cm on average. The prototype came in at 0.07 cm mean error.
What I Took From It
The 0.07 cm number sounds good on a slide but it came from a lot of iteration on the actuator mounting position and the guide rail tolerance. Early prototypes were closer to 0.6–0.8 cm because compliance in the joint geometry was compounding. The accuracy gain came from tightening the mechanical stack, not from anything clever.
Working in a clinical constraint set also changed how I thought about design. The mechanism is not the product. The user experience is the product. If a nurse can't operate it cleanly in a 10-second window between patient movements, the spec numbers don't matter.
