ON THE SELECTION OF THE 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, and to stabilize detector parameters in a multichannel flaw detector.

This paper considers the 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 correspondingly as the channel number decreases. For statistical summation of the signals arriving from the detectors, a necessary condition during readout is their phase coincidence. For this purpose, the speed of motion of the tape must be synchronized with the speed of motion of the inspected item; 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, the functional diagram of which is shown in Fig. 1.

Fig. 1. Functional diagram of the tracking system.

According to calculations and preliminary experiments on a model, a separately excited DC motor of the MI-11LT type with a built-in tachogenerator, possessing linear mechanical and control characteristics, was selected as the controlled object for the tape-transport mechanism.

From the theory of DC electrical machines, the basic equations are known that describe the physical processes in electric motors; from these equations, regulation of the rotational speed of the motor armature can be carried out in three ways: by changing the active resistance in the armature circuit \(R_{\text{я}}\), by changing the magnetic flux, and by changing the armature voltage [1].

Figure 2 presents the schematic diagram of the drive for the memory unit.

magnetic memory, in which control of the number of revolutions is accomplished by varying 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. Schematic diagram of the tracking system.

Maintaining the specified value of the motor speed is carried out by an automatic control system containing: a bridge comparing the control voltage taken from the tachogenerator, which is connected to the shaft of the motor of the inspected product, and the tachogenerator—the motor of the magnetic-memory unit; an amplifier (as the circuit solution, a transistor version is used, combining the positive properties of both magnetic and tube amplifiers: sufficient mechanical strength, durability, inertia-free operation, small dimensions, ensuring the instantaneous readiness of the amplifier for action, etc.) [2, 3]; and 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\(_2\) (PChBE) is connected in series into the armature circuit of the motor. 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 motor revolutions. Transistor T\(_2\) is controlled by an amplifier assembled on transistor T\(_1\) (PChBE), into whose base circuit a bridge is connected that compares the voltage taken from the product tachogenerator and TG\(_1\) (the tachogenerator-motor) of the magnetic-memory unit.

The operating point on the characteristic of transistor T\(_1\) is selected with the aid of resistances R\(_4\) and R\(_5\), connected into the bridge.

The mismatch voltage controls transistor T\(_1\), which, in turn, controls transistor T\(_2\).

To protect the transistors at the moment of motor start-up from the starting current, which in the armature winding reaches a considerable value exceeding its nominal value by 10–20 times [2], a time relay RV is provided, which has two normally closed contacts RV\(_1\) and RV\(_3\) and a normally open RV\(_2\), disconnecting the control system during start-up.

Starting through RV\(_1\) leads to a sharp increase in the number of motor revolutions, since all supply voltages are applied fully to the motor armature winding, which may lead to tearing 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 relay RV, determined by the time constant of the circuit \(R_{10}; R_{11}; C_2; R_{\text{ob RV}}\); internal resistance of the power supply), relay RV operates; contacts \(RV_3, RV_1\) open and contacts \(RV_2\), which connect the regulating circuit, close.

Relay RT, with normally closed \(RT_1\) and normally open \(RT_2\) contacts, is connected in the collector circuit of \(T_2\) and is provided to protect \(T_2\) against motor overloads. The operating current can be changed by means of the 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 level, RT operates, contacts \(RT_1\) open, and \(RT_2\) close. Current flows through the blocking winding O. B, the RT relay, and resistors \(R_7R_8\), disconnecting the motor and the regulating circuit. Thus, double protection of transistor \(T_1\) is provided against starting current and overload current, which increases the reliability and durability of the regulating system.

Using switch 2, the system can be returned to its initial state.

Table 1

\(U_{\text{exc}} = 110\) V

Experimental data were obtained, which are given in Table 1. From these experimental data, the graph of the dependence \(n=f(U_y)\), shown in Fig. 3, was constructed.

Fig. 3. Dependence of the number of revolutions on the control voltage.

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.

When the line voltage changed by \(\pm 10\%\) from the nominal value, the number of revolutions of the motor of the tape-transport mechanism changed, for different values of \(U_y\), by no more than 0.3%.

As experiments have shown, the response time of the tracking system is \(0.01 \div 0.015\) s. This is an order of magnitude less than the time constant of the mechanism for moving the inspected product; 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 \(5\%\) of the signal from each detector is subject to conversion and summation, the increase in the overall instrumental error of measurement of the flaw detector is only \(0.05\%\), while the statistical measurement error is reduced by \(\sqrt{6}\) times.

Thus, the drive device of the magnetic-memory unit satisfies the assigned task.

The device is reliable in operation. Experimental studies of the system described correspond to the calculations presented. The device was developed for the purpose of introducing it at one of the enterprises as part of a flaw detector.

REFERENCES

1. M. G. Chilin. General Course of Electric Drive. Moscow, “Energiya,” 1965.
2. T. A. Glazenko. Pulse Semiconductor Amplifiers in Electric Drives. Moscow, “Energiya,” 1965.
3. V. K. Zakharov. Electronic Elements of Automation. Moscow, “Energiya,” 1967.