Vibrating Wire Explained
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The SFT II exploits the dependency of a vibrating wire's resonant frequency on its tension to measure applied loads. Through mechanical means the applied load is transmitted to the wire causing a change in its resonant frequency from which the value of the applied load is computed by an on-board 32-bit floating point microprocessor. A fully calibrated (linearized, spanned and temperature compensated) signal is transmitted via RS 485 serial communication to the K-Tron controller.
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Up to fifteen SFT IIs can be handled on the same serial bus, transferring weight data at Baud rates from 2400 to 76800. To maintain the sensing wire in constant resonant motion, necessary excitation circuits are provided. Their purpose is to drive the wire to resonance and to restore the energy lost in vibration without influencing the vibration frequency.
Driving at Resonant Frequency
This is accomplished by flanking the fine, electrically conductive wire with a pair of permanent magnets which immerse the wire in a constant magnetic field. Realizing that a current-carrying wire in a magnetic field will displace itself perpendicularly to the lines of magnetic flux, start-up and feedback circuits provide precisely regulated pulses of current which first drive the wire to its resonant frequency, and then maintain it in vibration regardless of tension. (The wire possesses a rectangular cross section to constrain vibration to a single plane. And because a high vibration frequency is desirable to avoid signal contamination, the wire is prestressed and forced to vibrate at its second harmonic -two anti-nodes - at all times.) The excitation circuitry, in performing its primary function of wire-driving, is very similar to the circuitry that drives the quartz crystal in a common digital watch.
Frequency Count Yields True Digital Weight
The output of this circuit also provides the frequency signal which is further processed. As a discretely countable, frequency-based measure, this technique results in a true, digital determination of weight, directly at the point of measurement. The wire's resonant frequency is not a linear function of applied tension but, in practice, is almost parabolic. To calculate the applied load from the measured frequency, mathematically-based regression techniques are used. To assure the utmost precision, linearization coefficients for each SFT II are individually established at the factory and reside in an on-board EEPROM.
Few Environmental Effects
Of the possible environmental sources of error, only temperature variations require compensation. The effects of ambient magnetic field, changes in atmospheric pressure, relative humidity, etc. have been found to be negligible. To adjust for changes in ambient temperature, a thermal sensor is installed within the SFT II. Each SFT II's thermal response characteristics are determined through testing at manufacture, and appropriate coefficients are registered in the on-board EEPROM for continuous electronic compensation.
Interference Free Digital Communication
Using a single RS 422/485 serial interface, weight data from as many as fifteen SFT IIs can be transmitted reliably in interference free digital format over distances of up to 500 meters (1,640 ft), significantly cutting cabling costs.
Smart Sensors Reduce Controller Overhead
And because each SFT II has its own microprocessor, operations such as linearization, temperature compensation and other signal processing can be accomplished within the sensor itself, unburdening the controller from such tasks, and resulting in faster, more responsive control. Addressability enables SFT IIs to be individually polled by the controller to provide measurement data from a single SFT II installation or from simultaneous measurements in multiple SFT II weighing systems such as in a batching application where a single hopper is weighed by multiple SFT IIs. The SFT II's communication capability also enables it to time stamp its data transmission, report status information, accept set-up programming from a remote source, and provide automatic error checking.