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ACES - Technical description: limits and resolution of measurement

This section discusses the limits and resolution of measurements on the ARIEL CES machines. In addition, a discussion of system checkout for the purpose of maintaining system operation at peak accuracy will be presented. Sample test results for both the Multi-Function and Arm-Leg machines are also included.

LIMITS AND RESOLUTION OF MEASUREMENT

Each exercise station has separate transducers which are individually calibrated. The pressure transducer is used for calibrating force while the potentiometer is used for calibrating position and velocity.

SYSTEM ACCURACY

The analog measurement of transducer values on the Ariel systems have an accuracy limitation by the finite representation in a digital computer. Because data values must change by one bit (binary digit) when represented digitally, any given transducer measurement could have a possible error approaching one bit value.

In practical terms, the absolute accuracy depends on the resolution of the conversion from analog values to digital values. On the Ariel CES, one bit value typically represents 0.3 pounds of force, and about 0.01 degree of bar movement.

These accuracy levels are entirely sufficient for the vast majority of resistive training and rehabilitation uses.

SYSTEM RESOLUTION

The approximate resolution for the pressure transducer is a 13 bit (1/4096), and for the potentiometer, the resolution is up to 14 bit (approximately 1/8192). These resolution values represent a fixed percent of the value as given below:
       Force = 0.0244%
       Position = 0.0122%

For a Multi-Function system with a maximum force set for 1200 pounds, a bar range of 65 degrees and a maximum speed set to 200 degrees per second, the approximate resolution is:
       Force = 0.293 pounds
       Position = 0.008 degrees

As you can see, the CES offers a wide range of force and velocity measurements, and at the same time the system maintains very high resolution, providing the clinician with high precision measurements. Thus, the same exercise systems can be used for precision diagnostics in rehabilitation applications, and on the other hand as a �heavy-duty� training device utilized by elite athletes.

SYSTEM CHECKOUT

The force and velocity transducers are easy to check and calibrate. To insure that your exercise system performs accurately, it is important to check the force and velocity calibration periodically. If the calibration is found to differ appreciably from the system resolution given above, the appropriate transducer should be re-calibrated. The system should also be re-calibrated if there is any hardware change to the exercise station external to the computer.

FORCE calibration is checked using the CALIBRATION CHECK option of the calibrate program. When performing this check, a known weight will be placed on the bar and the bar will fall through a selected range of motion. The system will record the observed force level vs. bar position. This level should be consistent with the weight.

The VELOCITY calibration can be checked by selecting a constant velocity exercise with one repetition. A weight is used to cause the bar to fall through the range of motion. The observed velocity should be consistent with the selected velocity. The velocity measurement can be performed in a number of ways. The time for the bar to travel the range of motion can be compared to the expected time knowing the bar range and selected velocity. This time can be measured directly with a stopwatch.

Audio instructions

If your Ariel computer is configured with the optional sound board and speakers, audio instructions will guide the user through the exercise session. Instructional commands are stored as Microsoft compatible waveform (*.WAV) files and can be customized for each facility if desired.

There are many methods and programs that can be used to create *.WAV files. The instructions below are written for the Sound Recorder program included in Microsoft Windows 95/98/2000.

To Record A Waveform (WAV) File

1.     Select START, PROGRAMS, ACCESSORIES, ENTERTAINMENT (or MULTIMEDIA) and then SOUND RECORDER to open the Sound Recorder program.

2.     Make certain an audio input device is connected to the computer.

3.     Select FILE, NEW to create a new WAV file.

4.     Click the RECORD icon to begin recording.

5.     Click the STOP icon to stop recording.

6.     Select FILE, SAVE AS to save the recording to a waveform (WAV) file. NOTE: the file must have the same name as the one being replaced in order for the CES program to play it at the appropriate time.

LIST OF CES WAV FILES

Ariel CES customers can record �customized� audio instructions to guide subjects through the exercise routines. In order for the CES software to recognize the customized WAV files, they must be created using the same names and copied to the appropriate directory (usually the C:\PROGRAM FILES\ARIEL DYNAMICS\CESW directory). It is also recommended to make a backup of the original WAV files prior to customizing.

Listed below are the current file names and text contents of the currently used WAV files.  

See also

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