ON THE SELECTION OF A DRIVE FOR THE MEMORY UNIT OF A RADIATION (ISOTOPE) FLAW DETECTOR

K. M. BOCHKAREV

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

In radiation (isotope) flaw detectors, magnetic-memory units are used both to reduce the probability of missing a defect by implementing the principle of statistical summation of information from \(n\)-detectors [1], and to stabilize detector parameters in a multichannel flaw detector [2].

This paper considers a drive device for the magnetic-memory unit of a six-channel scintillation flaw detector with statistical summation. The maximum signal delay time in the sixth channel is 60 sec and decreases accordingly as the channel number decreases. For statistical summation of the signals arriving from the detectors, a necessary condition during readout is that their phases coincide; for this purpose the speed of motion of the tape must be synchronized with the speed of motion of the item being inspected, i.e., the dependence of the tape speed on the control signal must be linear. This is achieved by means of an automatic-control system, whose functional diagram is shown in Fig. 1.

Fig. 1. Functional diagram of automatic control, where Z.O. is the setting element; E.S. is the comparison element; \(<\) is the amplifier; I.E. is the actuating element; F is an external disturbing action; O.S. is feedback.

According to calculations and preliminary experiments on a model, for the tape-transport mechanism a separately excited direct-current motor of the type...

MI-11LT with a built-in tachogenerator, possessing linear mechanical and adjustment characteristics.

From the theory of direct-current electrical machines, the basic equations are known that describe the physical processes in electric motors; from these it follows that regulation of the rotational speed of the motor armature can be accomplished in three ways: by changing the active resistance in the armature circuit \(R_{\text{a}}\), by changing the magnetic flux, or by changing the voltage on the armature [3].

Figure 2 shows the basic circuit diagram of the drive for the magnetic-memory unit, in which control of the number of revolutions is effected by changing the supply voltage on the motor armature by means of a control voltage. This method of regulation is the most optimal from the standpoint of ensuring high reliability of the motor in operation.

Fig. 2. Basic circuit diagram of the drive of the magnetic-memory unit of a radiation (isotope) flaw detector

Maintenance of the specified value of the motor speed is carried out by an automatic-control system containing a bridge that compares the control voltage taken from the tachogenerator, which is connected to the motor shaft of the inspected product and to the tachogenerator of the magnetic-memory-unit motor; an amplifier (as the circuit solution, a transistor version is used, embodying the positive properties of both magnetic and tube amplifiers: sufficient mechanical strength, durability, inertia-free operation, small dimensions, and ensuring the instantaneous readiness of the amplifier for action, etc. [4, 5]); and the tachogenerator \(TG_1\), connected to the shaft of the motor of the tape-transport mechanism. As \(TG_1\), a small-size electric motor of type SL-221 is used.

A semiconductor triode \(T_2\) (PTSB9) is connected in series in the motor armature circuit. By changing the emitter–collector voltage drop of transistor \(T_2\), the voltage on the motor armature winding changes, which leads to an increase or decrease in the number of revolutions of the motor. Transistor \(T_2\) is controlled by an amplifier assembled on transistor \(T_1\) (PTSB9), in whose base circuit a bridge is connected that compares the voltage \(U_{\text{у}}\), taken from

with the tachogenerator of the inspected article, and TG\(_1\) (the tachogenerator of the motor) of the magnetic-memory unit.

The operating point on the characteristic of transistor T\(_1\) is selected by means of resistors \(R_4\) and \(R_5\), connected in a bridge. The mismatch voltage controls transistor T\(_1\), which, in turn, controls transistor T\(_2\).

To protect the transistors at the instant of motor starting from the starting current, which in the armature winding reaches a considerable value exceeding its nominal value by a factor of 10–20 [3], a time relay RV is provided; it has two normally closed contacts RV\(_1\) and RV\(_3\) and a normally open contact RV\(_2\), which disconnects the regulating system during starting.

Starting through RV\(_1\) leads to a sharp increase in the number of revolutions of the motor, since the entire supply voltage is applied completely to the motor armature winding, which may lead to rupture of the magnetic tape; this drawback is eliminated by means of an additional resistance \(R_9\), connected in series with the armature winding.

After 1 sec (the operating time of RV, determined by the time constant of the circuit \(R_{10}; R_{11}; C_2; R_{\text{winding RV}}\); the internal resistance of the power source), relay RV operates, contacts RV\(_3\), RV\(_1\) open and contacts RV\(_2\), which connect the regulating circuit, close.

A relay TR with normally closed RT\(_1\) and normally open RT\(_2\) contacts is included in the collector circuit of T\(_2\); it is provided to protect T\(_2\) from motor overloads. The operating current can be varied by means of resistance \(R\), connected in parallel with the RT winding. For technical reasons the operating current of RT was chosen as 1.5 A. When the motor is overloaded above the permissible value, RT operates, contacts RT\(_1\) open, RT\(_2\) close, and the current flowing through the blocking winding O.B., relay RT, and resistances \(R_7\), \(R_8\) disconnects the motor and the regulating circuit. Thus, double protection of transistor T\(_1\) from the starting current and the overload current is provided, which increases the reliability and service life of the regulating system. With the aid of switch P, the system can be returned to its initial state.

Experimental data were obtained and are given in Table 1. From these experimental data, the graph of the dependence \(n = f(U_y)\), shown in Fig. 3, was plotted. It is evident from the graph that the regulating circuit shown in Fig. 2 makes it possible to set various values of the motor rotation speed and to maintain the set value with high accuracy.

Fig. 3. Dependence of the number of revolutions of the drive motor of the magnetic-memory unit on the control voltage.

When the mains voltage was varied by \(\pm 10\%\) of nominal, the number of revolutions of the motor of the tape-drive mechanism changed, for various values of \(U_y\), by no more than 0.3%.

As the experiments showed, the reaction time of the tracking system is 0.01–0.015 sec. This is an order of magnitude less than the time constant of the mechanism for moving the inspected article; therefore the tracking system is stable in operation.

It was found experimentally that the total noise of the delay unit and the adder does not exceed 1%. Since in the flaw detector only 5 percent of the signal from each detector is subject to conversion and summation, the increase in the overall instrumental error of the measurement of the flaw detector is only 0.05%, while the statistical measurement error is reduced by a factor of \(\sqrt{6}\).

Table 1

\[ U_{\mathrm{exc}} = 110\ \mathrm{V} \]

Thus, the drive device of the magnetic-memory unit satisfies the assigned task. The device is reliable in operation. Experimental studies of the system presented correspond to the calculations given. The device was developed with a view to its introduction at one of the enterprises as part of a flaw detector.

References

1. Report on the research topic “Development of an isotope gamma flaw detector.” Research Institute of Electronic Introscopes, Tomsk, 1968.

2. V. D. Kleiner, V. S. Ezhov. Application for an inventor’s certificate No. 1328074. Radiation thickness (density) meter for material. 1969.

3. M. G. Chilin. General Course of Electric Drive. Energiya, 1965.

4. V. K. Zakharov. Electronic Elements of Automation. Energiya, 1967.

5. T. A. Glazenko. Pulse Semiconductor Amplifiers in Electric Drives. Energiya, 1965.