US2840763A - Stroboscopic lamp control arrangements - Google Patents

Description

June 24, 1958 s. l. RAMBO EI'AL 2,840,763

STROBOSCOPIC LAMP CONTROL ARRANGEMENTS Filed March 8, 1954 2 Sheets-Sheet 1 Fig. i V

C2 3 l I M e 1 4 EGI E= 300v. V

Fig. 2 v E Fig. 5.

WITNESSES INVENTORS Sheldon I. Rambo a W Milton P. Vore I BY 21% Ma" W ATTORNEY June 24, 1958 s. 1. RAMBO ETAL 9,

STROBOSCOPIC LAMP CONTROL ARRANGEMENTS Filed March 8, 1954 2 Sheets-Sheet 2 Fig. 7.

Fig.8.

United States Patent" STROBOSCOPIC LAMP CONTROL ARRANGEMENTS Sheldon I. Rambo, Baltimore, and Milton P. Vore, Catonsvlll e, Md., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvama Application March 8, 1954, Serial No. 414,768

. 2 Claims. (Cl. 315233) This invention relates, generally, to control arrangements embodying gas-filled tubes of the type commonly employed as stroboscopic lamps.

Commercially available tubes in this category are generally filled with neon gas, examples being the cold cathode triode GL-ZOO, the cold cathode tetrode 1D2l and the hot cathode thyratron KU610. These tubes are designed for triggering by a grid voltage pulse of a few microseconds duration, and emit a burst of light of short duration, usually slightly longer than the triggering pulse, toilluminate a rotating part, or one moving in translation, synchronously with the motion in order to eifectively stop the motion. In some devices, the manual adjustment of an oscillator circuit controlling a grid of the stroboscopic lamp, or tube, functions as a measure of the vibration frequency. In a balancing machine, the stroboscopic lamp, which is triggered at the running frequency of a part being checked for unbalance, is used to illuminate the rotating part. By means of suitable indicia on the part, or on some body rotating with the part, the heavy or light spot on the rotating part may be illuminated and made to appear stationary to indicate the angle of unbalance with respect to a reference, whereby corrections for reducing unbalance in a chosen axial correction plane may be made. i

In many applications ofstroboscopic lamps, or tubes, and with particular reference to balancing machines, it has been noticed that the inside of the glass envelope begins to darken after a few hours of operation, and the darkening increases with continued use. This darkening, or blackening, greatly reduces the light output long before the lamp reaches a reasonable service life, and the cause of this discoloration causes premature failure of the lamp.

' The reason for this phenomenon has been the subject of exhaustive study for a number of years, and we have found that the apparent cause is due to the application of a reverse voltage across the anode and cathode of the tube during the interval of tube discharge because of electrical oscillation in the circuits connected to and supplying the anode-cathode voltage for the tube or lamp.

Accordingly, one object of this invention is to provide improved circuitry embodying a stroboscopic lamp which affords increased life of the lamp over prior arrangements.

Another object of this invention is to provide im-J proved circuitry embodying a stroboscopic lamp in which sidered in conjunction with the accompanying drawings, in which:

Figure 1 diagrammatically illustrates a prior art circuit arrangement for controlling a gas-filled stroboscopic lamp;

Fig. 2 graphically illustrates the dynamic characteristic of such a lamp relating the grid control voltage thereto;

Fig. 3 illustrates the theoretical or ideal plate voltage characteristic with respect to time of the supply circuit of Fig. 1;

Figs. 4 and 5 illustrate the respective grid voltages applied to the respective grids of the stroboscopic lamp of Fig. 1;

Fig. 6 graphically illustrates the characteristics of the actual plate voltage of a circuit of the type shown in Fig. 1;

Figs. 7, 8 and 9 illustrate respective stroboscopic lampcircuit arrangements embodying the principles of this invention; and

Fig. 10 diagrammatically illustrates the application of this invention to a balancing machine.

In the application of strobosco-pic lamps in arrangements for measuring the frequency of vibration of vibrating parts, and in the application to balancing machines wherein the stroboscopic lamp comprises part of an arrangement for measuring the angle of unbalance of a rotating part with respect to a fixed machine reference,

- 2 and respective grids g1 and g2. The tube is supplied with a-suitable anode-cathode voltage by a capacitor C which is charged from a source of direct current voltage generally designated B through a resistor R The ca'pacitor is'connected across the plate 1 and cathode 2 of the tube by means of two flexible lengths of conductors having inductive properties represented by inductors L and L It is to be understood that these are not coils which are placed in the circuit, but merely represent the inductive characteristics of the lengths of flexible con,- ductors connecting the capacitor C in energizing relation with the stroboscopic lamp. In this circuit, the cathode 2 of the stroboscopic lamp is grounded. Grid g2 is controlled by a voltage which is taken from a suitable tap on a voltage divider circuit comprising resistors R and R, which are connected across the supply circuit for the tube. Grid g1 is capacitor-coupled by means of a capacitor C to a supply of triggering voltage, the characteristics of which are depicted adjacent grid terminal 3 and identified as grid voltage e Resistor R completes the grid circuit to ground.

Assume for the purposes of this discussion that the tube in the perimeter of the curve, the tube will not ionize sufficiently to cause anode-cathode gas breakdown. the voltages applied to grids g1 and g2 pass outside the curve, ionization takes place and a flash of light results? This light flash persists until the anode-cathode voltage Patented June 24, 1958 In this tion of the grid voltage pulse a is shown in Fig. 2 "to illustrate how breakdown occurs.

Returning to Fig. l, a voltage E of approximately 300 volts D.-'C. charges capacitor C exponentially towards the value of the supply voltage B through the resistor R Neglecting the inductive properties of the flexible conducto'rs between the capacitor and the tube represented in inductors L and L the anode-cathode voltage before ionization of the gas takes place is The second grid g2 follows at a voltage of A voltage pulse e is applied to the grid g1 through capacitor C The various voltage wave forms applied, respectively, to the anode 1, to the grid e and to grid e are represented in Figs. 3, 4 and 5. In this discussion, the inductive properties of the flexible conductors between the capacitor and the stroboscopic lamp are neglected, and in Fig. 3 and in the equation described above, the characteristic e represents the plate-to-cathode voltage of the tube.

However, in practice, conductor inductances L and L cannot be neglected. They may have an appreciable value, since it is usually desirable to locate the stroboscopic lamp some distance from the other components in order to permit its positioning close to the moving object under study. A study of the circuit shows that the energy supplying the discharge in the lamp, and hence the illumination of the lamp, is obtained from the capacitor C Hence, the current in this discharge must flow through the flexible conductors having the inductive properties represented by inductors L and L Therefore, the discharge will be oscillatory in character with a frequency of approximately That is, the wave form of the anode-cathode voltage will not be as shown in Fig. 3, but rather as shown in Fig. 6 This has been experimentally determined by the use of an oscilloscope. It has been proven by extensive experiment that whenever the anode is allowed to become negative with respect to the cathode during ionization and breakdown, as in Fig. 6, tube envelope discoloration will result. The time required for this phenomenon to become evident seems to be a function of, the duration of the negative anode-cathode voltage in addition to the energy in the discharge.

The systems herein disclosed are directed to preventing the anode from becoming negative with respect to the cathode during tube discharge or conduction, and hence function to prevent tube blackening, or discoloration, with .its attendant loss of illumination and shortening of the tube life. The circuits are of three specific types.

(1) Those which reduce L and L to a minimum,

(2) Those which damp out oscillation quickly, and

(3) Those which short circuit the negative voltage. In general, these may be referred to as circuit arrangements including the capacitor C of the supply circuit for preventing the application of a voltage to the anode which is negative with respect to the cathode of the stroboscopic lamp. It has been found experimentally that if the inductive properties of the flexible conductors represented in inductors .L and L are made very small by shortening the conductors between C and the anode and cathode of the stroboscopic lamp to 1" or less, the energy .in the reversed voltage portion of the wave 'is too small to cause detectable discoloration of the tube envelope. Hence, a simple expedient for preventing tube envelope discoloration is that of positioning the capacitor physically 'close'to the stroboscopic lamp so that the lead lengths between the capacitor and the lamp may be kept at a minimum value. The flexible conductors between the capacitor and the supply voltage source may be any convenient length and will yet have inductive properties such as L and L but this inductive property has been shifted by this expedient to a position in series with the charging circuit for capacitor C rather than the discharging circuit. This circuit arrangement is illustrated in Fig. 7, wherein the capacitor C is mounted inside the housing of the stroboscopic lamp. This housing is diagrammatically depicted in dot-dash outline.

While the arrangement of Fig. 7 is probably electrically the simplest way of overcoming the problem of reversed anode voltage during discharge, since it involves no change in component sizes or involves no additional components,'it is not always possible to employ this corrective means, since such means will of necessity increase the size of the lamp housing. Often it is desired to keep the lamp housing as small as physically possible in order to increase its usefulness. Inasmuch as capacitor C will frequently occupy a volume at least half that of the lamp itself, the housing must be made considerably larger to accommodate the capacitor.

This ditficulty is overcome as shown in Fig. 8 by inserting a damping resistor r in series in the capacitor discharge loop through the tube V. This resistor may be mounted either at the lamp or at the capacitor C whichever is convenient, since it need only be placed in series with the capacitors discharge path to damp out oscillations and prevent the anode-cathode voltage from reversing or becoming negative in an amount sufficient to cause tube discoloration. Considerable experimental evidence has been obtained to prove that this circuit prevents tube discoloration. It should be observed that this effect is not due simply to the discharge current limiting action of the damping resistor r, but rather to the damping of oscillation of the anodecathode voltage which drives the anode negative with respect to the cathode in the absence of such a damping resistor. This conclusion has been verified by increasing the size of capacitor C when resistor r is included in the circuit until the discharge current magnitude is made the same as with a smaller capacitor, and no damping resistor. Increasing the size of capacitor C does not produce envelope discoloration as long as the damping resistor is used.

The third basic circuit for preventing reversed or negative anode-cathode voltage during conduction, and hence preventing tube discoloration is shown in Fig. 9. In this circuit, a rectifier designated X is connected between the anode and cathode of the tube and is poled in such sense that when the lower terminal of the capacitor becomes positive and the upper terminal negative, which applies a reversed voltage to the stroboscopic lamp, the rectifier conducts and effectively short circuits the tube from the capacitor. Thus, the negative portions of the oscillator voltage in the supply circuit for the tube is shunted from the tube, and the anode is prevented from becoming appreciably negative with respect to the cathode. Alter-- natively a rectifier such as X may be inserted across capacitor C to directly shunt the capacitor. When connected across the capacitor, the rectifier again tends to' the upper edges of which appear in the drawings. As viewed, the vertical dimension of these springs is perpendicular to the plane of the paper. Their bottom ends are securely anchored to the base of the balancing machine and their upper ends mount the respective bearings 6 and 7. In this way, the bearings are mounted for translational movement in a single plane; namely, the plane of the paper. The rotor is adapted to be driven by a motor such as M, which is connected by means of a flexible coupling represented herein by the dotted line 9 connecting the motor shaft to the shaft 10 of the rotor 5. The motor M drives the rotor at some suitable speed for balancing purposes. As the rotor rotates, the mass unbalance due both to radial and axial mass asymmetry, causes angular oscillation of the rotor in the two degrees of freedom in the plane of the paper deflecting the bearings 6 and 7 in their flexible mounts. This bearing deflection is detected by respective electrical pickups 11 and 12, which are equipped with mechanical prods which ride against the respective bearings, and are spring-loaded thereagainst. This is only one of several ways of detecting vibration at the bearings. The vibratory movement of the prods of the electrical pickup following the vibration of the respective bearings is utilized to actuate suitable electrical means such as a coil movable in a magnetic field to produce a voltage indicative of the vibratory movement, and in suitable time phase relation therewith. Such a voltage, which is a sine wave voltage having a frequency corresponding to running frequency as produced by one type of electrodynamic pickup, is modified in the network 13 by suitable squaring and clipping circuitry (not shown in the interest of simplicity) to produce voltage pulses synchronized with running frequency such as the voltage i, shown in Fig. l.

The balancing procedure herein employed is generally known as the two-plane balancing method, wherein unbalance determinations are to be made in respective axially displaced rotor planes identified as A and B in the drawings. In this arrangement, assuming unbalance determinations are to be made in plane A, provision is made in the network to eliminate from the unbalance determinations in plane A, the effect of unbalanced conditions relating to plane B. Alternatively by suitable mechanical arrangements, movement in plane B may be restrained while permitting the bearing 6 to oscillate in the single plane of freedom. Arrangements of both of the above types are well known to the art and are therefore not discussed in detail.

The output of the network represented in the pulse type grid control voltage, together with the energizing voltage for the stroboscopic lamp identified again, in this Fig. 10, as V, is applied over flexible conductors, herein identified by the single line 14, to the stroboscopic lamp. The lengths of the conductors are determined by the requirements of the specific machine, and being of appreciable length have an appreciable value of inductance; as herein before explained. In fact, the inductance is suflicient to induce the described oscillation in the plate voltage for the tube. In such applications as this, therefore, any of the three expedients for eliminating such oscillation in the plate supply voltage for the stroboscopic lamp may be practiced to improve the life and useful service of the tube.

The grid voltage pulses applied from the network to the tube V are synchronized with the running frequency of the rotor 5. Consequently, the tube flashes synchronously with the rotor being balanced and, depending upon the circuit arrangement, will effectively illuminate the same spot on the rotor each time the rotor makes one revolution. By placing suitable markings on the rotor or on some part which rotates with the rotor, and which need not necessarily vibrate with the rotor, a particular angular position of the rotor with respect to a suitable reference angular position on the machine frame may be determined, at which weight is to be added or removed in order to achieve running, or dynamic balancing, of the rotor with respect to plane A. When this correction is made, the above-described operation is repeated with respect to plane B. Usually these two operations are sufiicient to achieve the required degree of dynamic balance of the rotor.

Machines of the type illustrated in Fig. 10 are capable of relatively high production, and for eflicient operation, place relatively stringent demands on the components relating to system operation. Hence, malfunction of a stroboscopic lamp such as V, through poor illumination of the part being balanced, and through short service life, respectively, impair the quality of balance and the productivity of the machine. Tests which have been conducted with the improved circuits herein illustrated, conservatively indicate about a 300% improvement in tube life.

The foregoing disclosure and the showings made in the drawings are merely illustrative of the principles of this invention, and are not to be considered in a limiting sense.

We claim as our invention:

1. A control circuit for a portable stroboscopic lamp of the type which discolors and darkens with the application of reverse voltages across its anode and cathode comprising, a direct current supply circuit having an output capacitor, a stroboscopic lamp having an anode, a cathode and a control grid and adapted for operation in an area remote from said supply circuit; an electrical cable connecting said capacitor across said anode and said cathode to supply energizing voltage over its length to said remotely disposed lamp, said electrical cable by reason of its length having sufiicient inductance to form an oscillator circuit with said capacitor producing an oscillatory voltage across said anode and cathode and driving said anode negative with respect to said cathode, means for applying an intermittent voltage to said control grid to trigger said tube, and resistance means comprising a part of said oscillator circuit providing resistance damping to prevent said oscillator circuit from oscillating.

2. A control circuit for a stroboscopic lamp of the type which discolors and darkens with the application of reverse voltages across its anode and cathode comprising, a direct current supply circuit having an output capacitor, a stroboscopic lamp having an anode, a cathode and a control grid and adapted for operation in an area remote from said supply circuit; a length of electrical cable connecting said capacitor across said anode and said cathode to supply energizing voltage over its length to said remotely disposed lamp, said electrical cable by reason of its length having sufficient inductance to form an oscillator circuit with said capacitor producing an oscillatory voltage across said anode and cathode and driving said anode negative with respect 7 to said cathode, means for applying an intermittent voltage to said control grid to trigger said tube, and a resistor connected in series in said oscillator circuit to damp electrical oscillation and prevent said anode from being driven negative with respect to said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 1,904,124 Cockrell Apr. 18, 1933 2,073,247 Miller Mar. 9, 1937 2,092,096 Swedlund Sept. 7, 1937 2,325,385 Rava Mar. 18, 1941 2,331,317 Germeshausen Oct. 12, 1943 2,342,257 Edgerton Feb. 22, 1944 2,383,405 Merrill et al. Aug. 21, 1945 2,516,326 Knowles et a1. July 25, 1950 2,521,141 Allen Sept. 5, 1950 FOREIGN PATENTS 649,320 Great Britain Mar. 3, 1949

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