Electronic Design

Instrument Measures Wire Tension

This instrument measures tension on the wires fixed inside the tubes of the proportional wire chambers used to detect charged particles in high-energy physics research. The material is gold-plated tungsten. The wires measure between 3 and 30 cm long, with a diameter of 50 mm.

The nominal value of tension, 200 gmf (grams-force), was to be measured as a quality-control procedure. The two crimped ends of every wire were available for making electrical contacts.

A wire was placed in a strong magnetic field of a permanent magnet. A variable-frequency sinusoidal current was then passed through the wire. The current produced a varying magnetic field around the wire, and the interaction of the two magnetic fields set the wire in vibrations.

The amplitude of vibrations is at its maximum when the frequency of the current passing through the wire equals the wire's natural frequency of oscillation. In turn, the natural frequency of oscillation is a function of tension on the wire, its length, its diameter, and the density of the wire's material.

By knowing all of the parameters, designers can measure the natural frequency of oscillations. Designers also can calculate the tension on a wire by using the relationship:

T = π x ρ (l x f x d)2

where T = wire tension in Newtons, ρ = density of the wire material in kg/m3, l = length of the wire in cm, f = fundamental resonating frequency in Hz, and d = diameter of the wire in meters.

Figure 1 shows the basic block diagram of the instrument. Figure 2 shows the resonance sensing circuit, which forms the heart of this instrument. The voltage-controlled oscillator (VCO) sends a varying frequency signal through the wire under test.

The full-wave active rectifier, which is at the front end of the resonance-sensing circuit, receives the voltage drop across the wire at its input. In most cases, this voltage drop is a fraction of a volt, and it's less than the cut-in voltage of a semiconductor diode.

For rectification, the active rectifier is constructed with op-amps IC1a and IC1b. Opamp IC1c is configured as a noninverting amplifier with a gain of 23. C1 then filters the amplified signal. The time-constant R7-C1 is selected so that the voltage across C1 can change rapidly.

At the same time, the amount of ripple in the dc voltage is small enough for the lowest frequency of operation. The buffer amplifier (IC1d) isolates the dc level from the following differentiator stage (IC2a).

When the wire vibrates in the magnetic field, a small voltage, analogous to the back-EMF generated in the armature conductors of a electric motor, is induced in the wire. At resonance, the oscillations reach their peak, and the induced voltage increases significantly. This raises the dc level at the input of the differentiator, causing the differentiator to output a pulse.

This pulse is used to clamp the ramp amplitude and hence the VCO frequency, which is controlled by the ramp amplitude. The in-built frequency counter reads the VCO frequency at resonance. Tension is then calculated using the relationship described earlier.

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