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United States Patent 3,596,143
Gruetzmacher ,   et al. July 27, 1971

MAGNETIC PARTICLE INSPECTION APPARATUS

Abstract

Magnetic particle inspection apparatus in which a coil around a part or contacts engaged with a part are connected through one or more silicon diodes to the secondary of a stepdown transformer the primary of which is connected through a silicon controlled rectifier to an AC source, a control signal being applied to the gate of the silicon controlled rectifier, preferably through a second silicon controlled rectifier controlled by a unijunction transistor oscillator the timing of which is controlled to control magnetizing current. The diodes and the rectifier conduct during the same half cycles of the alternating current and magnetizing current is always discontinued at the end of a half-cycle of one polarity of the alternating current with a consistent steep wave shape such that the remnant magnetization from one part to another is uniform and the degree of concentration of particles over a defect provides an accurate indication of the character of the defect. Another important feature is in the provision of means to prevent saturation of the transformer core, preferably by providing an air gap therein.


Inventors: Gruetzmacher; Ralph W. (River Grove, IL), Schroeder; Kenneth W. (Arlington Heights, IL)
Assignee: Magnaflux Corporation (Chicago, IL)
Appl. No.: 04/718,437
Filed: April 3, 1968


Current U.S. Class: 361/143 ; 324/216
Current International Class: G01N 27/84 (20060101); G01N 27/82 (20060101); G01r 033/12 ()
Field of Search: 317/123,148.5,157.5 324/38

References Cited

U.S. Patent Documents
3315098 April 1967 Eckl
3292004 December 1966 Heiberger
3401313 September 1968 Littwin
3346778 October 1967 Schroeder
3324354 June 1967 Schroeder et al.
3417295 December 1968 Littwin
3378762 April 1968 Parker
3274452 September 1966 Landes
2973470 February 1961 Kohn
3146392 August 1964 Sylvan
3358218 December 1967 Halpin
Primary Examiner: Miller; J. D.
Assistant Examiner: Moose, Jr.; Harry E.

Claims



We claim:

1. In magnetic particle inspection apparatus wherein magnetic particles are applied to the surface of a magnetized part to be concentrated over defects in the part and to indicate the location and character of defects in the part, magnetizing means comprising: magnetic field-developing means adapted to be associated with the part to develop a magnetic field therein, a transformer having primary and secondary windings, means including unidirectional conduction means for coupling said secondary winding to said magnetic field-developing means, means for coupling said primary winding to a source of alternating current including a controlled rectifier having a control electrode, control means for applying a control signal to said control electrode, and switch means connected in circuit with said control means and operable to a first condition to effect application of said control signal to said control electrode to initiate conduction of said rectifier at a point of a half-cycle of one polarity of the alternating current to remain conductive until the end of the half-cycle of said one polarity, said switch means being operable to a second condition at any time subsequent to said point and prior to the corresponding point of the next subsequent half-cycle of said one polarity to prevent application of said control signal to said control electrode and conduction of said rectifier during said next subsequent half-cycle of said one polarity, such that conduction of current to said field-developing means is consistently discontinued with a steep wave shape, whereby the remnant magnetization from one part to another is uniform and whereby the degree of concentration of magnetic particles over a defect provides an accurate indication of the character of the defect.

2. In magnetic particle inspection apparatus as defined in claim 1, said unidirectional conduction means and said controlled rectifier means being arranged to conduct in half-cycles of the alternating current of the same polarity.

3. In magnetic particle inspection apparatus as defined in claim 1, said transformer having a high ratio of primary to secondary turns whereby the current conducted through said controlled rectifier is relatively small in comparison to the current conducted through said unidirectional conduction means.

4. In magnetic particle inspection apparatus as defined in claim 1, said controlled rectifier being a silicon controlled rectifier.

5. In magnetic particle inspection apparatus as defined in claim 1, said unidirectional conduction means being in the form of silicon diodes.

6. In magnetic particle inspection apparatus as defined in claim 1, said control means including means for controlling the point of application of said control signal during a half-cycle of said one polarity of the alternating current to control the effective magnitude of the current applied to said field-developing means.

7. In magnetic particle inspection apparatus as defined in claim 6, said point being controllable through a range extending from a time within a few degrees after the start of a half-cycle of said one polarity to a time to within a few degrees before the end of a half-cycle of said one polarity.

8. In magnetic particle inspection apparatus as defined in claim 7, said control means including a unijunction transistor relaxation oscillator arranged to operate during said half-cycles of said one polarity, and adjustable resistance means for varying the rate of operation of said oscillator, said switch means being arranged for selectively rendering said oscillator operative or inoperative.

9. In magnetic particle inspection apparatus as defined in claim 7, said control means including a second controlled rectifier coupled to said control electrode of the first controlled rectifier and having a control electrode, and circuit means controlled by said switch means for applying a firing pulse to said control electrode of said second controlled rectifier.

10. In magnetic particle inspection apparatus as defined in claim 9, means for applying a relatively high supply voltage to said second controlled rectifier continuing to the end of half-cycles of said one polarity to insure application of a firing pulse of adequate amplitude to said control electrode of the first controlled rectifier and to allow firing of the first controlled rectifier at a time to within a few degrees before the end of a half-cycle of one polarity.

11. In magnetic particle inspection apparatus as defined in claim 10, said voltage supply means for said second controlled rectifier including a capacitor, and diode means for charging said capacitor from said source of alternating current during half-cycles of said one polarity.

12. In magnetic particle inspection apparatus as defined in claim 1, said transformer having a core of saturable material, said core having means for preventing saturation of said core.

13. In magnetic particle inspection apparatus as defined in claim 12, said means for preventing saturation of said core comprising means defining an air gap in said core having a length sufficient to prevent saturation thereof.
Description



This invention relates to magnetic particle inspection apparatus and more particularly to magnetic particle inspection apparatus in which the magnetizing current can be readily and accurately controlled over a wide range and in which magnetizing current is interrupted with a steep wave shape which is consistent and uniform from one part to another, such that the degree of concentration of particles over a defect provides an accurate indication of the character of the defect. The apparatus is relatively small in size and inexpensive in construction while being very stable and reliable in operation.

Magnetic particle inspection systems have heretofore been proposed in which a coil around a part or contact engaged with a part are connected through rectifier devices to a transformer secondary winding to apply a half-wave magnetizing field to the part and to leave the part residually magnetized when the current applied to the transformer primary winding is interrupted. With such systems, difficulties have been encountered in obtaining a uniform and reliable indication of the character of defects. It has been found that such difficulties are due to a very erratic current waveform at the break of the primary current. On some occasions, a relatively slowly decaying current is obtained at the break and on other occasions, the last half-cycle is of reduced amplitude, both of which result in a substantial change in the remnant magnetization of the part.

To overcome such difficulties, silicon controlled rectifiers have been used in the secondary circuit to conduct the end of the last half-cycle in which a control signal is applied and so as to obtain a consistent steep wave shape. Such silicon controlled rectifiers also permit control of the magnitude of the current. However, the amperage which can be handled by presently available silicon controlled rectifiers is restricted and difficulties are encountered in paralleling silicon controlled rectifiers and in connection with firing circuits for a plurality of silicon controlled rectifiers.

According to this invention, a controlled rectifier, preferably a silicon controlled rectifier, is connected between a primary winding of a transformer and an alternating current source and a secondary winding of the transformer is connected through one or more silicon diodes or other unidirectional conduction means to a coil around a part or to contacts engaged with a part. A control signal is applied to the gate or control electrode of the controlled rectifier, with the conduction of the rectifier being discontinued at a predetermined point of a cycle of the alternating current such that the conduction of magnetizing current is consistently discontinued with a certain wave shape. Although requiring semiconductor devices in both the primary and secondary circuits, it is found that this arrangement is substantially less expensive than one in which the silicon controlled rectifiers are in the secondary circuit because the diodes required in the secondary circuit are relatively inexpensive and the controlled rectifier required in the primary circuit is also relatively inexpensive since the required current therethrough is reduced in proportion to the turns ratio of the transformer. Further, it is easier to operate well within the range of the devices to obtain a high degree of reliability.

In accordance with a specific feature of the invention, the secondary diodes and the controlled rectifier are so poled as to conduct in the half-cycles of the same polarity and a consistent steep waveform at the termination of the current is obtained.

Another feature of the invention is in the application of a control signal to the controlled rectifier at a controllable point during half-cycles of one polarity of the alternating current to control the effective magnitude of the magnetizing current, the point being preferably controlled through a range extending from a time within a few degrees after the start of a half-cycle to within a few degrees before the end of a half-cycle.

The required control circuit is comparatively simple. Preferably, a unijunction transistor relaxation oscillator is used, the rate of operation of which is controlled by adjustable resistance means, with on-off switch means being provided for selectively rendering the oscillator operative or inoperative. In addition, the control circuit preferably includes a second controlled rectifier to which a firing voltage is applied through an isolation transformer from the unijunction transistor oscillator.

It is found that problems can be encountered in the system with respect to overheating of the transformer and insufficient output current. It is found that such problems arise because of saturation of the transformer due to the application of the magnetic field in one direction only, and another important feature of the invention is in the provision of means for preventing saturation of the core of the transformer. Preferably, an air gap is provided in the core of the transformer having a length sufficient to prevent substantial saturation of the core thereof. With this feature, the size of the transformer can be decreased while undue heating is avoided and while a high output current is obtained.

This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate a preferred embodiment and in which:

FIG. 1 is a schematic diagram of magnetic particle inspection apparatus constructed according to the invention; and

FIG. 2 is a view illustrating waveforms, for the purpose of explaining the operation of circuits shown in FIG. 1.

Reference numeral 10 generally designates magnetic particle inspection apparatus constructed in accordance with the principles of this invention. In the operation of the apparatus 10, a part 11 to be tested may be placed between a pair of contacts 13 and 14 for passage of current through the part to develop a magnetic field therein. After such magnetization, magnetic particles are applied to the surface of the part 11, either in dry form or in a fluid medium, to be attracted over and concentrated by localized magnetic fields produced by cracks or inhomogeneities in the part. In the alternative, a part 15 may be magnetized by placing it within a coil 16.

To develop the field in part 11 or in part 15, a secondary winding 17 of a transformer 18 is selectively connected through ganged selector switch contacts 19 and 20 either to the contacts 13 and 14 or to the coil 16, with a pair of parallel-connected silicon diodes 21 and 22 being connected in series between the winding 17 and the selector switch contact 19. The transformer 18 has a primary winding 23 which is connected to the cathode of a silicon controlled rectifier 24 and to one terminal of a fuse 25, the anode of the silicon controlled rectifier 24 and the other terminal of the fuse 25 being connected to terminals 27 and 28. Terminals 27 and 28 are connected to a suitable source of alternating current such as, for example, a source of 470 volt, 60 Hz. current.

A control signal is applied to the gate of the silicon controlled rectifier 24 at a point in a positive half cycle, or a number of successive positive half-cycles, in which the terminal 27 is of positive polarity, to cause conduction of the silicon controlled rectifier 24 until the end of each such positive half-cycle. Preferably, the diodes 21 and 22 are so poled as to be conductive during the same positive half-cycles, to cause magnetizing current to flow. When the control signal is terminated, the current will not flow during the next positive half-cycle. Accordingly, the flow of magnetizing current is always terminated with a uniform steep waveform and with any given setting of the control circuitry, the remnant magnetization of successive parts placed between the contacts 13 and 14 or in the field of the coil 16 will be uniform and consistent. This is highly advantageous in that the degree of concentration of magnetic particles over a defect provides a reliable indication of the character of the defect, when the magnetization is uniform.

To apply the control signal to the c control electrode or gate of the silicon controlled rectifier, it is connected through a resistor 30 and a capacitor 31 in parallel to the cathode of a silicon controlled rectifier 32 having an anode connected through a diode 33 to one terminal of a secondary winding 34 of a transformer 35, the anode of the silicon controlled rectifier 32 being also connected through a capacitor 36 to the other terminal of the secondary winding 34 which is connected to the cathode of the silicon controlled rectifier 24. To limit the firing voltage applied to the silicon controlled rectifier 24, a Zener diode 37 is connected between the gate and cathode thereof.

The transformer 35 has a primary winding 38 which is connected to circuit points 39 and 40, connected to the secondary winding 41 of a stepdown transformer 42 having a primary winding connected to terminals 27 and 28.

To control conduction of the silicon controlled rectifier 32, and to thereby control conduction of the silicon controlled rectifier 24, the gate and cathode of the silicon controlled rectifier 32 are connected to the secondary winding 45 of a pulse transformer 46, a resistor 47 being connected across the secondary winding 45. A primary winding 48 of the pulse transformer 46 is connected to a first base electrode of a unijunction transistor 50 and also to the circuit point 40. A second base electrode of the unijunction transistor 50 is connected through a resistor 51 and a diode 52 to the circuit point 39 and is also connected through an adjustable resistor 53 to the emitter of the transistor 50 which is connected through a capacitor 54 to the circuit point 40 and also through a pushbutton switch 55 to the circuit point 40.

With regard to the general operation of the circuit, switch 55 is connected in circuit with control means which function to apply a control signal to the control electrode or gate of the silicon controlled rectifier 24, such control means being formed by the combination of the unijunction transistor 50 and associated circuit components and the silicon controlled rectifier 32 and associated circuit components. When switch 55 is opened, i.e. operated to a first condition, the unijunction transistor 50 and controlled rectifier 32 so operate as to effect application of a control signal to the control electrode or gate of the rectifier 24 to initiate conduction of the rectifier 24 at a point of a half cycle of one polarity of the alternating current to remain conductive until the end of the half cycle of such polarity. The switch 55 is operable to a closed position, i.e. a second condition, at any time subsequent to the point at which the controlled rectifier 24 was rendered conductive and application of the control signal to the gate of the rectifier 24 during the next subsequent half-cycle of such one polarity is prevented. With this operation, conduction of current to the field-developing means is consistently discontinued with a steep wave shape and the remnant magnetization from one part to another is uniform, so that the degree of concentration of magnetic particles over a defect provides an accurate indication of the character of the defect.

The operation will now be described in detail with reference to FIG. 2 which shows the waveforms produced at various points of the circuit. Reference numeral 57 indicates a form of the supplied AC voltage, supplied to the terminals 27 and 28 and also developed with reduced amplitude between circuit points 39 and 40, and with further reduced amplitude across the secondary winding 34 of transformer 35. When the switch 55 is opened, the next positive half-cycle of voltage will result in charging of the capacitor 54 through the resistor 53, resistor 51 and diode 52 until the firing point of the unijunction transistor 50 is reached, whereupon the transistor 50 will conduct to discharge the capacitor 54 and to develop a pulse of current through the primary winding 48 of the transformer 46, to thereby apply a pulse of current from the secondary winding 45 to the gate-cathode circuit of the silicon controlled rectifier 32. When silicon controlled rectifier 32 conducts, a firing pulse is applied to the gate-cathode circuit of the silicon controlled rectifier 24 to cause it to conduct until the end of the positive half cycle in which conduction is initiated. When the adjustable resistor 53 has a comparatively low value, the unijunction transistor 50 may fire a number of times during each positive half-cycle, the waveform of the voltage applied to the primary 48 of the pulse transformer 46 being indicated by reference numeral 58 in FIG. 2, under such conditions. However, only the first pulse during each positive half-cycle will fire the silicon controlled rectifier 32 to fire the silicon controlled rectifier 24, after which the rectifier 24 will remain conductive until the end of the positive half-cycle. Thus reference numeral 59 indicates the waveform of current through the rectifier 24 under conditions where the timing resistor 53 has a low value. It is noted that each positive pulse of current through the controlled rectifier 24, under such conditions, is only a few degrees less than a full half-cycle. With the diodes 21 and 22 being poled to conduct during the same half-cycles, the magnetizing current applied through the part 11 or through the coil 16 will have substantially the same wave shape.

When the resistor 53 is adjusted to have a relatively high value, the voltage across the timing capacitor 54 will not reach the firing voltage of the unijunction transistor 50 until close to the end of a positive half-cycle. Reference numeral 60 indicates the waveform of the voltage applied to the pulse transformer 46 under such conditions. Reference numeral 61 indicates the waveform of the current through the rectifier 24. under such conditions, it being noted that current pulses are developed of much shorter duration and of reduced amplitude. The effective value of the magnetizing current under such conditions is only a very small fraction of the effective value of the magnetizing current obtained when the timing resistor 53 has a low value.

Thus with the circuit, the effective value of the magnetizing current can be controlled over a wide range.

At any given position of adjustment of the resistor 53, the final pulse of current will have the same wave shape, regardless of when the switch 55 is closed. Thus if switch 55 is closed before the instant of time at which the unijunction transistor 50 would otherwise produce the firing pulse, the firing pulse will not be developed. If switch 55 is closed after the development of a firing pulse, it will have no effect upon the continued flow of current through the rectifier 24 during the remainder of the positive half-cycle.

To allow adjustment of the effective value of the magnetizing current down to very low values, it is important that sufficient voltage be applied to the rectifier 32 at the end portion of each positive half-cycle. This is accomplished through the use of the diode 33 and capacitor 36, the capacitor 36 being charged through the diode 33 to nearly the peak value of the voltage developed across the secondary winding 34 so that a relatively high voltage is available for firing the rectifier 32 even though the voltage of the secondary winding 34 may be relatively low at such time. The diode 33, of course, isolates the capacitor 36 from the winding 34 to prevent discharge of the capacitor 36 when the voltage across the winding 34 drops to a low value near the end of a positive half-cycle. The capacitor 31 is important in producing a rapid turning on of the rectifier 24 in response to a current pulse developed by the rectifier 32.

It has been found that a problem can be encountered with the circuit as thus far described with respect to overheating of the transformer 18 and with respect to insufficient output current. It has also been found that such problems arise because of saturation of the transformer, the magnetizing field being applied on one direction only. An important feature of the invention is in the provision of means for preventing saturation of the transformer. Preferably, this is accomplished by providing at least one air gap in the core of the transformer. As illustrated diagrammatically, the transformer 18 has a core formed by two C-shaped sections 63 and 64 which may be formed from suitable laminations of a saturable magnetic material, and a pair of shims 65 and 66 of nonmagnetic material are provided between abutting ends of the C-shaped sections 63 and 64, to provide in effect air gaps.

By way of illustrative example, and not by way of limitation, the following values of components may be used:

REFERENCE NUMERAL VALUE __________________________________________________________________________ 30 22 ohms 31 0.1 microfarads 36 1 microfarad 47 47 ohms 51 15,000 ohms 53 100,000 ohms 54 0.1 microfarads __________________________________________________________________________

The silicon controlled rectifier may be a type C50N, the silicon controlled Rectifier 32 may be a type 2N1595, the unijunction transistor 50 may be a type 2N2647. The Zener diode 37 may have a voltage rating of 5.6 volts. The AC voltage between terminals 39 and 40 may be 115 volts.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

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