Robert G. Radwin

Repetitive trauma disorders: job evaluation and design

Repetitive trauma disorders of the upper extremity are a major cause of lost work in many hand-intensive industries. Reported risk factors include repetitive and forceful exertions, certain postures, mechanical stress, low temperatures, gloves, and vibration. Risk factors can be identified with job analysis procedures based on traditional work-methods analysis. Risk factors can be controlled through reallocation of work, balancing of tools, selection of alternative tool designs, work relocation, selection of suitable hand protection, and elimination of hand exposure to low temperatures and vibration. Drawing-board manikins are used with computer-aided design systems to estimate the best work location for a given task.

External finger forces in submaximal five-finger static pinch prehension.

Small conductive polymer force sensors were attached to the distal phalangeal pads for measuring individual finger forces exerted during submaximal static pinch. A linear force summing strain gauge dynamometer for measuring resultant five-finger pinch force was grasped vertically using a neutral wrist posture. Individual finger forces were measured at fixed total pinch force levels of 10%, 20%, and 30% of maximum voluntary exertion using pinch spans of 45 mm and 65 mm. Total pinch force and individual finger forces were also measured while similarly grasping the dynamometer and supporting fixed weights for 1.0 kg, 1.5 kg, and 2.0 kg loads using pinch spans of 45 mm and 65 mm. The index and middle fingers exerted more than 3 N greater average force than the ring and small fingers for the fixed total pinch force task. No significant individual finger force differences were observed at the 10% maximum voluntary exertion level, however both the index and middle fingers exerted more than 5 N greater force than the ring and small fingers at the 30% maximum voluntary exertion level. The average contribution of the index, middle, ring, and small fingers were 33%, 33%, 17%, and 15%, respectfully, for the fixed total pinch force task. As exertion level increased from 10% to 30%, the contribution of the middle finger was not constant increasing from 25% to 38%. Total pinch force increased from 15 N to 30 N when the load weight increased from 1.0 kg to 2.0 kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Grip force vectors for varying handle diameters and hand sizes.

Grip force was measured along two orthogonal axes and vector summed. Sixtyone participants recruited from a manufacturing facility (29 men and 32 women) grasped instrumented cylinders (2.54, 3.81, 5.08, 6.35, and 7.62 cm diameter) using a maximal voluntary power grip. Two orthogonal force measurements relative to the third metacarpal were resolved into a magnitude and corresponding angle. On average, magnitude increased 34.8 N as handle diameter increased from 2.54 cm to 3.81 cm, and then monotonically declined 103.8 N as the handle diameter increased to 7.62 cm. The average direction monotonically decreased from 59.2° to 37.7° as handle diameter decreased from the largest to the smallest. When the diameter was smallest, the greatest force component, Fx (168.6 N), was in the direction where the fingertips opposed the palm. Conversely, when the diameter was largest, the smallest component, Fx(77.7 N), was in the same direction. These values are averaged for the left and right hand. The angle for the largest diameter increased with increasing hand size. These relationships should be useful for the design of handles that require gripping in specific directions, such as for hand tools and controls. Actual or potential applications of this research include the design of handles that require gripping in specific directions, such as for hand tools and controls, that reduce effort, and that prevent fatigue and overexertion.

Power hand tool vibration effects on grip exertions

Operation of vibrating power hand tools can result in excessive grip force, which may increase the risk of cumulative trauma disorders in the upper extremities. An experiment was performed to study grip force exerted by 14 subjects operating a simulated hand tool vibrating at 9.8 m/s2 and 49 m/s2 acceleration magnitudes, at 40 Hz and 160 Hz frequencies, with vibration delivered in three orthogonal directions, and with 1.5kg and 3.0kg load weights. Average grip force increased from 25.3 N without vibration to 32.1 N (27%) for vibration at 40 Hz, and to 27.1N (7%) for vibration at 160 Hz. Average grip force also increased from 27.4 N at 9.8 m/s2 acceleration to 31.8 N (16%) at 49m/s2. Significant interactions between acceleration x frequency, and frequency x direction were also found. The largest average grip force increase was from 25.3N without vibration to 35.8N (42%) for 40 Hz and 49 m/s2 vibration. The magnitude of this increase was of the same order as for a two-fold increase in load weight, where ave...

A method for evaluating head-controlled computer input devices using Fitts law

The discrete movement task employed in this study consisted of moving a cursor from the center of a computer display screen to circular targets located 24.4 and 110.9 mm in eight radial directions. The target diameters were 2.7, 8.1, and 24.2 mm. Performance measures included movement time, cursor path distance, and root-mean-square cursor deviation. Ten subjects with no movement disabilities were studied using a conventional mouse and a lightweight ultrasonic headcontrolled computer input pointing device. Average movement time was 306 ms greater (63%) for the head-controlled pointer than for the mouse. The effect of direction on movement time for the mouse was relatively small compared with the head-controlled pointer, which was lowest at 90 and 270 deg, corresponding to head extension and head flexion, respectively. Average path distance and root mean square displacement was lowest at off-diagonal directions (0, 90, 180, and 270 deg). This methodology was also shown to be useful for evaluating performance using an alternative head-controlled input device for two subjects having cerebral palsy, and measured subtle performance improvements after providing a disabled subject with lateral torso support.

Comparison of impedance and inductance ventilation sensors on adults during breathing, motion, and simulated airway obstruction

The goal of this study was to compare the relative performance of two noninvasive ventilation sensing technologies on adults during artifacts. The authors recorded changes in transthoracic impedance and cross-sectional area of the abdomen (abd) and ribcage (rc) using impedance pneumography (IP) and respiratory inductance plethysmography (RIP) on ten adult subjects during natural breathing, motion artifact, simulated airway obstruction, yawning, snoring, apnea, and coughing. The authors used a pneumotachometer to measure air flow and tidal volume as the standard. They calibrated all sensors during natural breathing, and performed measurements during all maneuvers without changing the calibration parameters. No sensor provided the most-accurate measure of tidal volume for all maneuvers. Overall, the combination of inductance sensors [RIP(sum)] calibrated during an isovolume maneuver had a bias (weighted mean difference) as low or lower than all individual sensors and all combinations of sensors. The IP(rc) sensor had a bias as low or lower than any individual sensor. The cross-correlation coefficient between sensors was high during natural breathing, but decreased during artifacts. The cross correlation between sensor pairs was lower during artifacts without breathing than it was during maneuvers with breathing for four different sensor combinations. The authors tested a simple breath-detection algorithm on all sensors and found that RIP(sum) resulted in the fewest number of false breath detections, with sensitivity of 90.8% and positive predictivity of 93.6%.

Activation Force and Travel Effects on Overexertion in Repetitive Key Tapping

Key switch design parameters, including make force, make travel, and over travel, were investigated for minimizing operator-exerted force while maximizing key-tapping speed. A mechanical apparatus was designed, constructed, and used for independently controlling key switch parameters and for directly measuring finger exertions during repetitive key tapping using strain gauge load cells. The task for the 25 participants involved using the index finger of the dominant hand to repeatedly depress a single key as rapidly as possible. Participants received visual and auditory feedback upon a successful keystroke. Peak force exerted decreased 24% and key-tapping rate increased 2% when over travel was distended from 0.0 to 3.0 mm. Although peak force exerted was not significantly affected by make point travel, key-tapping rate increased 2% when make point travel was reduced from 4.0 to 1.0 mm. These results indicate that key switch mechanisms that provide adequate over travel might enable operators to exert less force during repetitive key tapping without inhibiting performance

A silicon-based tactile sensor for finger-mounted applications

Presents a silicon-based force sensor packaged in a flexible package and describes the sensors performance on human subjects. The sensing element consists of a circular silicon diaphragm (200-/spl mu/m thick with a 2-mm radius) over a 10-/spl mu/m sealed cavity with a solid Torlon dome providing force-to-pressure transduction to the diaphragm. Two dome heights (0.5 and 1.5 mm) were compared. The sensor with the taller dome showed improved sensitivity. Dynamic calibration and tracking experiments are performed with the sensor mounted on the dominant thumb of 5 human subjects. Both force and loading direction are statistically significant (P<0.05). Subject variability accounted for 8.7% of the variance, while loading direction accounted for 1.9% of the variance. Average errors for the tracking experiment range from -2.8 to 1.0 N and are subject dependent. Three out of 4 subjects showed increasing negative error with increasing load.

A conductive polymer sensor for measuring external finger forces

This paper describes the construction and use of a durable and thin force sensor that can be attached to the palmar surface of the fingers and hands for studying the biomechanics of grasp and for use in hand injury rehabilitation. These force sensors were constructed using a modified commercially available conductive polymer pressure sensing element and installing an epoxy dome for directing applied forces through a 12 mm diameter active sensing area. The installation of an epoxy dome was effective for making the sensors insensitive to contact surfaces varying from 25 to 1100 mm2 and a 16 mm radius surface curved convex towards the finger. The completed sensors were only 1.8 mm thick and capable of being taped to the distal phalangeal finger pads. They were calibrated on the hand by pinching a strain gage dynamometer. The useful range was between 0 and 30 N with an accuracy of 1 N for both static loading and normal dynamic grasp activities. The sensor time constant was 0.54 ms for a step force input. Because of varying offset voltages every time the sensors were attached, these sensors should be calibrated on the hand before each use. The sensors were used for measuring finger forces during controlled pinching and lifting tasks, and during ordinary grasping activities, such as picking up a book or a box, where the useful force range and response for these sensors were adequate.

A silicon force sensor for robotics and medicine

Abstract This paper describes the development of a silicon-based force sensor packaged in a flexible polyimide-based package. The fabrication process is compatible with standard integrated circuit processes and produces a flexible package that sandwiches the metal leads between protective polyimide layers. Silicon direct bonding and bulk micromachining (both isotropic and anisotropic) are utilized to fabricate the silicon sensing element. The sensing element consists of a circular diaphragm (200 μm thick with a 200 μm radius) over a 10 μm deep sealed cavity. The shallow capacity depth provides built-in overforce protection. The diaphragm is instrumented with piezoresistors in a Wheatstone bridge configuration. Sensitivity to force is realized via the addition of a solid dome over the silicon diaphragm. The dome transmits the applied force to the diaphragm. Torlon and epoxy domes are bench tested. The epoxy dome produces significant hysteresis, while the Torlon dome shows low hysteresis (2.4% of the mean output) and low nonrepeatability ( −1 N −1 are typical. The response is linear for low forces (