Patented Wrist Goniometer, Flexible Zero/Span Controls

Position Measurement & Control - Issue 36

APPLICATION FOCUS

Patented Goniometer Design Gives Accuracy, Flexibility, and Ease of Use

Wrist Angular Displacement Measured By Firstmark Controls Position Transducers

Figure A1 - The wrist goniometer mounted on the right hand, illustrating the hand component and the forearm components. The forearm component includes the wrist band and the housing for the three Firstmark Controls position transducers. The hand component includes the elastic glove and hardware for attachment of the displacement cable ends. Photo credit: Liberty Mutual Research Institute for Safety.

A recently-announced, patented wrist angle measurement tool ("wrist goniometer") uses three Firstmark Controls Series 174 position transducers to sense the complex motions of the human wrist. This device is documented in "A technique for estimation of wrist angular displacement in radial/ulnar deviation and flexion/extension" authored by Raymond W. McGorry, Chien-Chi Chang, and Patrick G. Dempsey of the Liberty Mutual Research Institute for Safety. An abstract of this paper is below.

PAPER ABSTRACT: The development of a device and the derivation of a trigonometric solution that permits direct determination of wrist angular displacement in the flexion/extension and radial/ulnar planes is presented. System performance was quantified during a bench evaluation, and by comparison to an electromagnetic tracking system during a wrist-mounted evaluation with 16 subjects. The wrist-mounted evaluation was repeated with nine subjects to evaluate repeatability of the measurement system. The results indicate that the differences between angular displacement calculated using the goniometer were not signicantly different from the calibration stand, with errors of 0.70 ±0.64° in the flexion/ extension plane and 0.667 ±0.68° in the radial/ulnar deviation plane. In a wrist-mounted evaluation of the goniometer with the electromagnetic tracking system, errors of 0.59 ±0.51°and 0.89 ±0.95°, respectively, were found. Linear regression was used to determine the slope and intercept of the relationship between the goniometer and the electromagnetic tracking system for the nine subjects who repeated the evaluation. Paired t-tests for means found no significant differences (p < 0.05) for the slopes and intercepts of the repeated measures. The results suggest that the proposed device and algorithm provide an accurate measurement of wrist angular displacement in two planes, with a minimum of preparation or calibration.

To obtain the complete journal article, visit the International Journal of Industrial Ergonomics.

PRODUCT FOCUS

Electrical Output Benefits of the Series 6 Position Transducers

Broad Operating Temperature and Flexible Zero/Span Controls

The Series 6 position transducers with a maximum range of 85 inches (2159 mm) are generating significant interest and sales. Why? A key reason is the unique benefits of the Series 6 voltage conditioner and the 4-20 mA electrical outputs. Some of these benefits include:

• Voltage Conditioner Output
• Unlike signal conditioned transducers that allow only one output range, users can adjust outputs to be differential (±5 VDC, ±10 VDC) or single-ended (0-5 VDC, 0-10 VDC).
• Maximum resolution can be achieved during analog to digital conversion. Example: A user's 16-bit system is set to have a 0-10 VDC input range with a 1080-mm Series 6 position transducer. However, the application uses only a 700-mm range. Without the signal conditioner, the user would be getting an output from 0 to 6.48 VDC (with a 10 VDC input) and would not be using the entire 0-10 VDC range to get maximum resolution (65536 counts). With the signal conditioner, the output can be set to exactly match the range the user's system is set for allowing maximum resolution (to the limitation of the zero and span controls). For the above example, the user will see a resolution of 0.0165 mm without the signal conditioner and a resolution of 0.0107 mm with the signal conditioner. This equates to a 35% improvement in effective resolution with the signal conditioner.
• 4-20 mA (Current Loop) Output
• Simple electrical hook-up (2-wire configuration carries both power and signal)
• Robust signal: the high noise immunity compared to voltage signals allows the 4-20 mA type signal to be used for long distance signal travel
• Live zero: 4 mA distinguishes between zero (4 mA) and no information (0 mA)

The Series 6 position transducers with the voltage conditioner and 4-20 mA outputs share these benefits:

• -40° to +85° C operating temperature range
• reverse polarity protected
• low temperature drift (±0.01%/°C max)
• Accepts unregulated power up to 40 VDC that allows the product to be functional with battery power

APPLICATION CORNER

Errors and Temperature Changes

Q. I have a Model 150-0121 position transducer. What errors will I see with changing temperature?

A. All standard analog-output Firstmark Controls position transducers use conductive plastic or hybrid potentiometers that should be used as voltage dividers (voltage measurement) and not rheostats (resistance measurement). As such, because the changing temperature is changing the material characteristics of the entire potentiometric element, there will be no errors due to temperature change from the potentiometer. However, thermal expansion will slightly affect the transducers mechanical parts. The largest component of this thermal change effect is typically the displacement cable. To learn more about this effect, which is generally negligible for most applications, see the Temperature Effect on Displacement Cable Length Calculator.

Figure C1 - Metal displacement cable (wire rope) is typically the largest contributor to temperature-change error. This error is generally insignificant.

NEWS YOU CAN USE

• A Look at Analog Signal Types
• Power Rating Explained
• What Is the Accuracy of Our Products?
• Latest Poll: What is your preferred term?

ISSN 1527-5108 • Document Number S050V(040623)