DEVICE FOR MEASURING THE DIAMETER OF THIN WIRES

V. K. ZHUKOV, V. E. DREIZIN, I. G. LESHCHENKO

(Presented by the scientific seminar of the Faculty of Automation and Computer Engineering)

The device is intended for calibrating the holes of drawing dies by measuring the diameter of wires drawn through them. The sensitive element of the device is a slotted inductive transducer [1], made on a ferrite ring with an outer diameter of 10 mm and a magnetic permeability of 1000.

The transducer winding contains 8 turns wound on both halves of the ring. The position of the wire in the slot of the transducer is fixed by means of guide devices installed on the sides. The width of the slot is 0.6 mm.

To reduce the influence of ambient conditions on the accuracy of control, two identical transducers are used in the device, one of which is the measuring transducer \(D_i\), and the other the reference transducer \(D_e\) (Fig. 1). By means of an electronic switch \(K\), the transducers are alternately connected to the measuring generator \(G_1\) as an element of its oscillatory circuit. In this case the generator produces packets of high-frequency voltages, the frequencies of which are proportional to the inductances of the transducers \(D_i\) and \(D_e\) and are respectively equal to \(f_{1i}\) and \(f_{1e}\). The repetition frequency of the packets is determined by the switching frequency. The voltage of the measuring

Fig. 1. Block diagram of the device

from the generator is fed to mixer \(C\), to which a voltage of frequency \(f_2\) from the auxiliary generator \(G_2\) is simultaneously applied. The voltages of the difference frequencies

\[ f_{\mathrm{rn}}=f_{1\mathrm{n}}-f_2 \tag{1} \]

and

\[ f_{\mathrm{re}}=f_{1\mathrm{e}}-f_2 \tag{2} \]

are selected in the mixer load and are fed to the frequency detector F.D., whose transition frequency during adjustment is set equal to \(f_{\mathrm{d0}}\), and whose transfer coefficient is equal to \(S_{\mathrm{d}}\). The voltage at the output of the frequency detector will be

\[ U_{\mathrm{dn}}=S_{\mathrm{d}}(f_{\mathrm{d0}}-f_{\mathrm{rn}}) =S_{\mathrm{d}}(f_{\mathrm{d0}}-f_{1\mathrm{n}}+f_2) \tag{3} \]

or

\[ U_{\mathrm{de}}=S_{\mathrm{d}}(f_{\mathrm{d0}}-f_{\mathrm{re}}) =S_{\mathrm{d}}(f_{\mathrm{d0}}-f_{1\mathrm{e}}+f_2) \tag{4} \]

depending on which sensor is connected to the measuring generator. The useful signal of the detector is determined by the alternating component of its output voltage,

\[ U_{\mathrm{d}\sim}=\frac{U_{\mathrm{dn}}-U_{\mathrm{de}}}{2} =\frac{1}{2}S_{\mathrm{d}}(f_{1\mathrm{e}}-f_{1\mathrm{n}}), \tag{5} \]

whereas the constant component

\[ U_{\mathrm{d}=}=\frac{U_{\mathrm{dn}}+U_{\mathrm{de}}}{2} =\frac{1}{2}S_{\mathrm{d}}(2f_{\mathrm{d0}}+2f_2-f_{1\mathrm{n}}-f_{1\mathrm{e}}), \tag{6} \]

as will be shown below, is undesirable. The alternating voltage of the detector, through switch \(P_1\) (in the “generator” position), is amplified by amplifier \(U\) and fed to the amplitude detector A.D., whose load is the indicating instrument.

The readings of the indicator will be determined, in accordance with formula (5), by the difference in the reactive resistances of the sensors. Without a wire, the resistances of the sensors are balanced by the “generator setting 0” capacitor, connected in parallel with the reference sensor. When a wire is inserted into the measuring sensor, its reactance changes. This change is compensated by the “reading” capacitor, with whose rotor the scale of the reading device, graduated in microns, is mechanically connected. The moment of compensation is marked by the zero readings of the indicator.

Instability of the frequencies of generators \(G_1\) and \(G_2\), as well as of the transition frequency of the frequency detector, leads to the appearance of a constant component in the detector output voltage (even if the reactive resistances of the sensors are equal).

In this case the influence of the switching intervals corresponding to the instant of sensor switching begins to appear. This influence is expressed in the appearance of an additional alternating voltage, which violates the correspondence between the zero readings of the indicator and the condition of equality of the reactive resistances of the sensor circuits, leading to measurement error. To prevent this, provision is made for the possibility of changing the frequency of generator \(G_2\) by means of the “diskr. setting 0” capacitor. In the “diskr” position of switch \(P_1\), the constant component of the frequency-detector voltage is modulated and, through the amplifier, is fed to the amplitude detector of the indicator. By changing the frequency of generator \(G_2\), the deflection of the indicator pointer is reduced to zero. This will indicate the absence of a constant voltage across the load of the frequency detector.

Figure 2. Schematic circuit diagram of the instrument

The circuit diagram of the instrument is shown in Fig. 2. The electronic commutator (3) is made with diodes \(D_3\) and \(D_4\), connected into the collector circuits of transistors \(T_2\) and \(T_3\), which, together with resistors \(R_4 \div R_{12}\), form a trigger circuit. The introduction into the circuit of resistors \(R_6\), \(R_7\), and \(R_9\) made it possible to obtain a blocking voltage for the switching diodes. When a diode, for example \(D_3\), is open, a current of about \(50\ \mathrm{mA}\) flows through it, and sensor \(D_i\) is connected to the generator circuit through the small resistance of the diode. At this time a blocking voltage is applied to the other diode, \(D_4\), and sensor \(D_e\) is connected to the generator circuit through the capacitance of the diode’s \(p\)—\(n\) junction, which does not exceed \(1\) picofarad.

Therefore, in this state of the commutator, the inductance of sensor \(D_e\) will have practically no effect on the frequency of the oscillations generated by the measuring generator. The trigger is started from the mains through the shaping stage \(T_1\), the differentiating circuit \(C_1 R_3\), and diodes \(D_1\), \(D_2\).

The measuring generator (\(T_4\)) is assembled according to the capacitive three-point circuit. The decoupling chokes \(L_1\), \(L_2\), \(L_3\) eliminate the influence of the commutator circuits on the generator frequency. The mixer, made on triode \(T_5\), is supplied with voltage from the measuring generator and from an auxiliary generator, assembled on triode \(T_{15}\) according to the capacitive three-point circuit. The frequency-detector circuit includes triode \(T_6\) and diodes \(D_5\), \(D_6\). Silicon diodes \(D_7\), \(D_8\) are connected in parallel with the detector load (\(R_{24}\), \(R_{25}\)); they serve to limit the output voltage, which prevents overload of the indicating instrument.

Depending on the position of switch \(P_1\), the output of the frequency detector is connected either directly to the amplifier (triodes \(T_9\), \(T_{10}\)) or through a modulator comprising resistor \(R_{27}\) and triodes \(T_7\) and \(T_8\). The circuit \(R_{26} C_{15}\) is used to filter the alternating component of the frequency-detector voltage. The amplitude detector (\(D_9\), \(C_{17}\)) is made according to a parallel circuit. The instrument is powered from a \(220\ \mathrm{V}\) mains supply. A bridge circuit (diodes \(D_{11} \div D_{14}\)) is used for rectification. In order to eliminate the influence of mains-voltage instability on the accuracy of monitoring, a voltage stabilizer on triodes \(T_{11} \div T_{14}\) and reference diode \(D_{10}\) is used.

The instrument has a measuring range from \(0.15\ \mathrm{mm}\) to \(0.39\ \mathrm{mm}\), with small overall dimensions and weight. The frequency of the measuring generator is \(12\ \mathrm{MHz}\), and that of the auxiliary generator is \(10\ \mathrm{MHz}\).

Factory tests showed that the measurement error over the entire range does not exceed \(\pm 2\ \mu\mathrm{m}\).

If the instrument is equipped with a remote sensor, it can be used to measure the diameter of wires during the drawing process.

REFERENCES

1. V. K. Zhukov, V. E. Dreizin, I. G. Leshchenko. A slotted inductive sensor (published in the present collection).

2. V. K. Zhukov, V. E. Dreizin, M. S. Roitman, I. G. Leshchenko. Measuring circuits for monitoring the diameter of thin wires by the eddy-current method. Proceedings of TPI, vol. 141, Tomsk, 1965.

3. V. K. Zhukov. Electronic commutator. Proceedings of TPI, vol. 141, Tomsk, 1965.