Design of a Robotic System to Measure Propulsion Work of Over-Ground Wheelchair Maneuvers.

A wheelchair-propelling robot has been developed to measure the efficiency of manual wheelchairs. The use of a robot has certain advantages compared to the use of human operators with respect to repeatability of measurements and the ability to compare many more wheelchair configurations than possible with human operators. Its design and implementation required significant engineering and validation of hardware and control systems. The robot can propel a wheelchair according to pre-programmed accelerations and velocities and measures the forces required to achieve these maneuvers. Wheel velocities were within 0.1 m/s of programmed values and coefficients of variation . Torque measurements were also repeatable with . By determining the propulsion torque required to propel the wheelchair through a series of canonical maneuvers, task-dependent input work for various wheelchairs and configurations can be compared. This metric would serve to quantify the combined inertial and frictional resistance of the mechanical system.

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.

Validation of an accelerometer-based method to measure the use of manual wheelchairs.

The goal of this project was to develop and validate a methodology for measuring manual wheelchair movement. The ability to study wheelchair movement is necessary across a number of clinical and research topics in rehabilitation, including the outcomes of rehabilitation interventions, the long-term effects of wheelchair propulsion on shoulder health, and improved wheelchair prescription and design. This study used a wheel-mounted accelerometer to continuously measure distance wheeled, and to continuously determine if the wheelchair is moving. Validation of the system and algorithm was tested across typical mobility-related activities of daily living, which included short slow movements with frequent starts, stops, and turns, and straight, steady state propulsion. Accuracy was found to be greater than 90% across wheelchair and wheel types (spoke and mag), propulsion techniques (manual and foot), speeds, and everyday mobility-related activities of daily living. Although a number of approaches for wheelchair monitoring are currently present in the literature, many are limited in the data they provide. The methodology presented in this paper can be applied to a variety of commercially available products that record bi-axial accelerations, and used to answer many research questions in wheeled mobility.

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.

Assessment of an ultrasonic dermal scanner for skin thickness measurements.

The measurement precision of a 20 MHz ultrasound dermal scanning system, Episcan, manufactured by Longport Inc. was investigated for its use in the measurement of skin thickness. Results from measurements of relatively ideal homogeneous, uniformly thick plastic plates indicate that the scanner can accurately measure to depths of 8-34 mm from the surface of the transducer or approximately 18 mm from the surface of the latex probe-cover with a high level of precision, less than 1% of the mean thickness. The precision was determined to be dependent on the depth of the scan. Using the measured optimal gain of 45%, a sample rate of 0.013 micros, and an A-scan record length of 1024 points, we determined the thickness resolution of the device is on the order of 0.003 mm for 2 mm thick layers and 0.01 mm for 5 mm thick layers. We conclude that variations greater than this value for clinical dermal thickness measurements are not due to instrument precision, but must result from the limitations of analyzing the data from real tissue.