Accuracy Versus Precision, Cable Fundamental Frequency Calculator

Position Measurement & Control - Issue 31

ACCURACY VERSUS PRECISION

What is the Difference?

An accurate measurement is one that is very close to the true value of the phenomenon you are observing. A precise measurement is one that has very little scatter: repeat measurements give more or less the same value. If the measured data have high precision but poor accuracy, one may suspect that a systematic bias has been introduced: you are using an instrument where the zero position has not been set properly.

Figure A1 - Precision is a measure of repeatability while accuracy refers to how close the average value is to the "true" value.

If you do not know the expected value of a phenomenon but are trying to determine just that, it is obviously better to have accurate observations with poor precision than very precise, but inaccurate values. Why? The former will give a correct, but imprecise estimate while the latter will give a wrong, but very precise result!

A few other notes on the subject:

• precision is the limit on accuracy
• with the removal of mistakes and systematic errors, precision equals accuracy

Source: Cecily Wolfe, University of Hawaii

CABLE FUNDAMENTAL FREQUENCY CALCULATOR

Determine Resonances Caused by Vibration

Draw wire transducers (also referred to as wire sensors and cable displacement transducers) are subject to the same laws of physics as musical instruments. As such, the cables on these sensors have fundamental frequencies that may cause the cable to resonate. While such resonance is very rare and difficult to achieve, it is possible. The Cable Fundamental Frequency Calculator is a tool to estimate the fundamental frequency of Firstmark Controls position transducer cables:

• calcfund.htm

The fundamental frequency of a position transducer cable can change based on the amount of cable exposed to the excitation source and the cable tension which varies according to the displacement of the cable. The fundamental frequency of most Firstmark Controls position transducer cables is rather high due to 3 factors:

• small mass of the cable per unit length
• relatively short length of cable exposed to the excitation source
• relatively high cable tension

Figure B1 - Fundamental frequency or 1st harmonic.

BIKING WITH SYMVIONICS

Dynamic Response on a Mountain Bike

The Interactive Analysis and Display System (IADS) is a real-time and post-test software package that provides engineers with data monitoring and analysis capabilities. IADS provides the engineer with advanced data organization, processing and display capabilities. These capabilities offer enhanced safety-of-test monitoring (in a mission control room) and advanced near real-time analysis capabilities to support test-point clearance decisions. IADS is developed by Symvionics, Inc.(an engineering company)of Arcadia, California USA.

Symvionics used Series 160 position transducers with h bases to flexibly measure the displacement of the front and rear shocks on a mountain bike. The bike was instrumented to demonstrate the viability of the recently developed real time IADS interface to National Instruments E Series data acquisition units. Specific real time parameters obtained were steering angle, front and rear shock displacement, rear wheel speed and crankshaft speed. The acquisition was so precise and successful that the users were able to animate a mountain bike simulation on a terrain map (analogous to a flight simulator) in real time.

Figure C1 - Series 160 position transducer monitoring rear shock displacement on mountain bike.

For more information on IADS, contact:

• Patrick S. Mattingly
• Symvionics, Inc. - http://www.symvionics.com/iads/
• 661-277-4785 - pmattingly@symvionics.com

NEWS YOU CAN USE

Firstmark Controls representatives are located in virtually ever corner of the world to assist you with your position measurement challenges. These representatives have extensive sensor, data acquisition, and engineering skills and can assist you in making the best position measurement for a given application. To find your representative, visit reps.htm

Figure D1 - Firstmark Controls representatives are available worldwide.

• Position Transducer Solution Guide
• Coated Versus Uncoated Displacement Cable
• Analog Position Transducer Offer
• 4-20 mA Users Sought
• Poll Update: In which industries do you use position transducers?

ISSN 1527-5108 • Document Number S050Q(031105)