Partitioning kinetic energy during freewheeling wheelchair maneuvers.

This paper describes a systematic method to partition the kinetic energy (KE) of a free-wheeling wheelchair. An ultralightweight rigid frame wheelchair was instrumented with two axle-mounted encoders and data acquisition equipment to accurately measure the velocity of the drive wheels. A mathematical model was created combining physical specifications and geometry of the wheelchair and its components. Two able-bodied subjects propelled the wheelchair over four courses that involved straight and turning maneuvers at differing speeds. The KE of the wheelchair was divided into three components: translational, rotational, and turning energy. This technique was sensitive to the changing contributions of the three energy components across maneuvers. Translational energy represented the major component of total KE in all maneuvers except a zero radius turn in which turning energy was dominant. Both translational and rotational energies are directly related to wheelchair speed. Partitioning KE offers a useful means of investigating the dynamics of a moving wheelchair. The described technique permits analysis of KE imparted to the wheelchair during maneuvers involving changes in speed and direction, which are most representative of mobility in everyday life. This technique can be used to study the effort required to maneuver different types and configurations of wheelchairs.

Changes in inertia and effect on turning effort across different wheelchair configurations.

When executing turning maneuvers, manual wheelchair users must overcome the rotational inertia of the wheelchair system. Differences in wheelchair rotational inertia can result in increases in torque required to maneuver, resulting in greater propulsion effort and stress on the shoulder joints. The inertias of various configurations of an ultralightweight wheelchair were measured using a rotational inertia-measuring device. Adjustments in axle position, changes in wheel and tire type, and the addition of several accessories had various effects on rotational inertias. The configuration with the highest rotational inertia (solid tires, mag wheels with rearward axle) exceeded the configuration with the lowest (pneumatic tires, spoke wheels with forward axle) by 28%. The greater inertia requires increased torque to accelerate the wheelchair during turning. At a representative maximum acceleration, the reactive torque spanned the range of 11.7 to 15.0 N-m across the wheelchair configurations. At higher accelerations, these torques exceeded that required to overcome caster scrub during turning. These results indicate that a wheelchair's rotational inertia can significantly influence the torque required during turning and that this influence will affect active users who turn at higher speeds. Categorizing wheelchairs using both mass and rotational inertia would better represent differences in effort during wheelchair maneuvers.

Test method for empirically determining inertial properties of manual wheelchairs.

The iMachine is a spring-loaded turntable used to measure inertial properties of irregularly shaped rigid bodies, specifically manual wheelchairs. We used a Newton-Euler approach to calculate wheelchair mass and center of mass (CM) location from static force measurements using load cells. We determined the moment of inertia about the vertical axis from the natural frequency of the system in simple harmonic motion. The device was calibrated to eliminate the effects of platform components on measurement error. For objects with known inertial properties, the average relative error of the mass and the CM coordinates (x and y) were 0.76%, 0.89%, and 1.99%, respectively. The resolution of the moment of inertia calculation depends on the ratio of test piece inertia to system inertia, such that the higher the ratio, the more accurate the measurements. We conducted a Gage Repeatability and Reproducibility (Gage R&R) test using three manual wheelchairs measured three times by three operators; the results showed that over 90% of the variance in inertia was caused by differences in the wheelchairs being measured. Gage R&R analysis indicated that measurement system operation was acceptable using criteria from the Automobile Industry Action Group for both inertia and mass measurements.