US2178471A - Device for oscillographing - Google Patents

Description

Oct. 3l, 1939. s,- DE BRU|N 2,178,471

DEVICE FDR OSCILLOGRAPHING Filed April 20, 1938 l f? ,6h/Pang@ Peter/FAG@ Arta/mfr Patented Oct. 3 1, 1939 UNITED STATES DEVICE FOR OSCILLOGRAPHING Sake Leendert de Bruin, Eindhoven, Netherlands, assigner to N. V. Philips Gloellampenfabrieken, Eindhoven, Netherlands Application April 20, 1938, Serial No. 203,187 In Germany May 24, 1937 Claims.

My invention relates to a device for oscillographing variable electric or mechanical quantities.

The U.V S. patent application Serial No. 182,028, iiled December 27, 1937, of which I am a co-inventor, describes a method and device for oscillographing electric or mechanical -quantities in which the variations in these quantities modulate a high-frequency generator, for instance by producing changes in an impedance. In a particularly advantageous form of construction of such a device, the variable impedance forms part of a bridge having one diagonal connected tothe highfrequency generator, and another diagonal from which modulated high-frequency oscillations are taken oii, ampliiied and rectified to produce rectiiied currents which are used to control a measuring device or oscillograph. The variable -impedance, which is preferably a capacity, is located y at the measuring point, i. e. the point at which the quantities to be measured occur, and is connected through one or more connecting leads to the indicating point, i. e. the point at which the high-frequency generator and the remainder of the bridge are located.

With such devices4 it is possible to compare two quantities which are in time-relation with each other. For example, the indicator-diagram of a steam engine can be obtained by using the pressure variations in the cylinder to vary a capacity whose value controls the ordinates, and by periodically varying a second capacity in accordance with the displacement of the piston to produce the abscissae. Although the variable capacities may be very small and the bridge arrangement makes it possible to use extremely small condensers for this purpose, the device described in -the above application has the drawback that the connecting leads between the variable capacities and the remainder of the apparatus have capacities which are comparatively high and in many cases are variable. This reduces the sensi- The variable impedance may be connected to the remainder of thebridge through very short conductors, and a supply conductor of any desired length may be used between this bridge and the parts at the indicating point without deleteriously alecting the operation of the device. This supply conductor for the high-frequency oscillations is preferably constructed as a two-conductor cable, and it is preferably inductively coupled to the high-frequency generator or to the 'measuring bridge, or to both.

pendent-as regards the electric magnitude del termined by time-of the disturbing capacity ei'- fect of the connecting leads.

In order that the invention may be clearly understood and readily carried into effect, I-shall describe the same in more detail with reference to the accompanying drawing, in which:

Figure 1 is a schematic wiring diagram of an oscillograpln'ng device according to the invention, and

Fig. 2 is a sectionized and partly-schematic view of a measuring-bridgedevice.

The device shown in Fig. 1 comprises a shielded high-frequency generator I, two measuring bridges 2 and 3, two amplier-rectifier units 4 and 5 which may be of one or more stages, and a Braun-tube B having two pairs of deecting plates 30-63 and Ill- 6L The required supply devices for the anode voltage, grid bias, and the heating current, as well as the structural details of units 4 and 5, have been omitted from the drawing for clarity.

High-frequency generato:1 I comprises a triode l having an incandescible cathode I0, a control grid i3, and an anode I5, and an oscillatory circuit comprising a coil 8 and a condenser 9. Cathode IB is connected through a condenser II to a point S6 on coil 8 and to a lead I2 to which the anode voltage is applied, and grid I3 is connected through a condenser iB to one end of coil 8 whose other end is connected to the anode l5. A resistance i6 connected between grid I3 and cathode l0 serves to determine the average grid potential.

Inductively coupled with coil 8 are two highfrequency coils il and I8. Coil I'I has its ends connected through' condensers 45 and 46, and supply conductors dl and 42 to a winding I9 of a high-frequency transformer A having a second in series across winding 20. Connected in parall lel with the series connection of condensers 23 and 24 are two variable condensers 25 and 2S whose connecting point $5 has a fixed potential,

preferably. grounded as shown. Condenser 25 serves as the impedance which is influenced by the variations to be recorded, and condenser 26 is made variable in order to allow the working point of the bridge to be adjusted. The connecting point 6| between condensers 23 and 24 is connected to amplifier-rectifier 4 through a conductor 28, which is electrostatically shielded by a sleeve 21 having a fixed potential. As shown, it is grounded.

In the state of rest, i, e. when the capacity of condenser 25 is not being varied, a high-frequency current, which has larger or smaller value depending upon the adjustment of condenser 26, flows through conductor 28 to amplifier-rectifier 4. When the capacity of condenser 25 is altered by the quantities to be measured, for instance by the variations in the pressure within an internal combustion engine or by vibrations in a building construction, the change in capacity modulates the high-frequency current flowing in the conductor 28 and these modulated oscillations are amplified and rectified by unit 4. The resulting pulsatory direct current passes through a resistance 29, which has one end connected to deflecting plate 30 of the Braun-tube 6 and a tap 62 connected to the opposite plate G3 to thereby apply across these plates a Voltage which varies in accordance with the variations in the capacity of condenser 25.

To the other pair of plates 3| and 64 may be applied any desired saw tooth voltage, or an alternating voltage in synchronism with the quantity to be measured .as is the casein the present instance. The measuring bridge 3 serves this purpose and consists of two fixed condensers 32 and 33 connected in series across winding 22, and two variable condensers 34 and 35. The connecting point 66 between condensers 32 and 33 is connected to the amplier-rectier 5 through a conductor 3`| provided with a grounded sleeve 36. This bridge operates similarly to bridge 2, and the capacity of condenser 34 is controlled, for instance, by the shaft of the motor whose pressure is being measured, in such a manner that it varies sinusoidally with time between two denite values. As a result the amplitude of the high-frequency voltage set up between ground and conductor 31 is varied depending on the position of the rotating shaft of the motor under test, and amplifier-rectifier 5 produces a pulsatory direct current which flows through a resistance 38 whose ends are connected through condensers 39 and 40 to plates 3l and 64 of tube 6. As a result there is applied across plates 3| and 64 an alternating voltage which varies in accordance with the capacity of condenser 34.

When there is no potential difference applied across plates 30 and 63, this alternating voltage causes the luminous spot on the uorescent screen of the Braun-tube to reciprocate in synchronism with the rotation of the motor being tested. When the potential variations are applied across plates 30 and 63, which variations are functions of the pressure in the cylinder, the spot is caused to also move between these plates, and as a result the desired indicator diagram is produced on the luminous screen of the VBrauntube 6.

In accordance with the present invention, variable condenser 25, together with the remainder of the measuring bridge 2, is formed as 4a unit and is located at the measuring point, l. e. at the engine, when recording an indicator diagram.` The condenser 25 may, for instance, be

constructed as a diaphragm-system of a pressure cartridge, and the remainder of the bridge together with the high-frequency transformer A, is assembled together with this pressurecartridge to form a unit which is connected through supply leads 4| and 42 to generator I.

Supply leads 4| and 42 may be of any desired length so that the indicating point may be arranged at some distance from the measuring point, for instance on a control board. As the capacity of these leads with respect to ground, or their mutual capacity is not in parallel with the variable capacity of the condenser 25, the percentage-variations of the capacity. of the condenser 25 are much larger than they would be if this condenser were connected through long leads to a bridge located at the indicating point, because in the latter case the capacity of the leads would be in parallel with the capacity of the modulating condenser. Furthermore, the accuracy of the oscillographs can no longer be disturbed by variations in the capacity of these leads. The same applies for the ordinates determined by time, if the measuring bridge 3 is formed as a'. unit located at the driving point and is connected through supply leads 43 and 44 to the generator The Working of the unit 3 on the amplitude of the ordinates on the screen of tube 6 does not depend on the length of supply leads 43 and 44.

Condensers 45, 46, 4`| and 48 have impedances which are so related to that of the leads 4I, 42, 43, and 44 and of the coupling coils Il, I8, i9 and 2| that the system obtained is tuned to the oscillations to be transmitted. Because of this any harmonic oscillations produced by generator areprevented from interfering with the operation of the Braun-tube.

As the tuned system of the double conductor 4|-42 is inductively coupled to bridge 2 and generator I, the bridge is as independent as possible of the capacity of these conductors and of the generator parts. For the same purpose, and to insure that oscillations can only be transmitted inductively to the bridge, electro-static screen-ing means 52 and 61, which are maintained at fixed potentials, are interposed between windings |920 and 2|-22 respectively of the highfrequency transformers A and B. This entirely avoids a capacitative coupling which would have a disturbing influence and facilitate the passage of higher harmonics, and as a result the zeropoint of the bridge can be more sharply adjusted.

Furthermore, it is advantageous not to provide the tuning capacity of the systems comprising conductors 4I-42 and conductors 43-44 in only one, of the conductors of each system, but to distribute them symmetrically in the two conductors. In this manner the current in the two conductors of each system is equal, and the double conductors 4|-42 and 43-44 will'not have an inductive efect on the measuring leads 28 and 31 to thereby affect the accuracy of the indication of the measuring device.

The tuning condensers 26 and 35, which as stated are made variablevin order that the zeropoint of the bridges may be adjusted at will, should preferably be given a capacity which is small withl respect to the self-capacity of the leads 4|, 42, 43 and 44, so that the latter will affect the tuning as little as possible, and the amplitude of the high-frequency transmitted will be substantially independent of random capacity variations of these leads. When conductor 28 is currentless in the position of rest, the deviation of the luminous spot on the screen of the Brauntube always has the same direction. When the bridge does not operate at the zero-point the amplitude of the high-frequency current caused by variations in the capacity of the condenser 25 increases in one direction and decreases in the opposite direction so that the deviation on the luminous screen may occur in two directions, which is desirable for recording pressurediagrams including vacuum measurements.

Fig. 2 illustrated the construction of a pressurecartridge device for recording indicator-diagrams by means of the apparatus shown in Fig. 1. This device comprises a metal member 55 screwed onto the end of a grounded housing 54Y and having a cavitied end adapted to be screwed into an opening in the cylinder head of an engine (not shown). A flexible diaphragm 56 is secured within a cavity 53 in the member 55 and constitutes one condenser plate, which may be moved by the pressure of the gas within the cylinder relative to a fixed plate 51 mounted in an insulated manner at the bottom of cavity 53. Plates 56 and 51 act as the condenser 25 of Fig. 1, and the remaining parts of the bridge 2 of Fig. 1 are indicated in Fig. 2 by the reference numerals used in Fig. 1. It will be noted that condensers 23, 24, and 26, as well as the high-frequency transformer A are located within the housing 50 to form a unit with condenser 56-51. leads 4|, 42, and 28 of Fig. 1 pass through a grounded flexible metal tube 5I which leads to the indicating point.

The wall of'housing '54 is provided with an aperture 50 which serves to allow the zero-point of condenser 26 to be adjusted by means of a key or a screw driver.

The measuring bridge 3 of Fig. 1 may be constructed in a unit in a similar manner with the leads 43, 44 and 31 passing through a grounded flexible metal tube to the indicating point.

While I have described my invention in connection with specific examples and embodiments, I do not wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

1. A device for oscillographing variable quantities occurring at a measuring point comprising means disposed at some distance from the measuring point for generating high-frequency current, means disposed at the measuring point for modulating said current in accordance with the quantities to be measured and comprising an A. C. bridge arrangement having an impedance adapted to be varied in accordance with said quantities, a two-conductor cable of considerable length connecting one diagonal of said bridge to said generating means, means to tune said cable to the high-frequency oscillations to be transmitted, an oscillograph, and demodulating means connected between the. other diagonal of said bridge arrangement and the oscillograph to apply to said oscillograph a controlling voltage which varies in accordance with the quantities being measured.

2. A device for oscillographing variable quantities occurring at a measuring point comprising means disposed at some distance from the measuring point for generating high-frequency current, means disposed at the measuring point for modulating said current in accordance with the quantities to be measured and comprising an A. C. bridge arrangement having an impedance The three adapted to be varied in accordance with said quantities, a two-conductor cable of considerable ylength connected between one diagonal of said bridge and said generating means, means to inductively couple `said cable to the generator, means to tune the cable to the frequency of the oscillations to be transmitted, an oscillograph, and modulating means connected between the other diagonal of the bridge and said oscillograph to apply to the oscillograph a controlling voltage which varies in accordance with the quantities being measured.

3. A device for oscillographing variable quantities occurring at a measuring point comprising means disposed at some distance from the measuring point for generating high-frequency current, means disposed at the measuring point for modulating said current in accordance with the quantities to be measured and comprising an A. C. bridge arrangement having an impedance to be varied in accordance with said quantities, a high-frequency transformer having a winding connected across one diagonal of the bridge and a second winding, means electrostatically shielding said windings from each other, a two-conductor cable of considerable length connected between the second winding and said generating means, means to tune said cable to the frequency to be transmitted, an oscillograph, and demodulating means connected between the other diagonal of the bridge and said oscillograph to apply to the oscillograph a controlling voltage which varies in accordance with said quantities.

4. A device for oscillographing variable quantities occurring at a measuring point comprising means disposed at some distance from the measuring point for generating high-frequency current, means disposed at the measuring point for modulating said current in accordance with the quantities to b'e measured and comprising an A. C. bridge arrangement having an impedance adapted to be varied in accordance with said quantities, a two-conductor cable of considerable length connected between one diagonal of said bridge and said generating means, means to tune said cable to the frequency to be transmitted comprising two condensers of equal capacity, one in each conductor, an oscillograph, and modulating means connected between the other diagonal of the bridge and said oscillograph to apply to said oscillograph a controlling voltage which varies in accordance with the quantities being measured.

5. A device for oscillographing variable quantities occurring at a measuring point comprising means disposed at some distance from the measuring point for generating high-frequency current, means disposed at said measuring point for modulating said current in accordance with the quantities to be measured and comprising an A. C. bridge arrangement having an impedance adapted to be varied in accordance with said quantities, a two conductor cable of considerable length connecting one diagonal of said bridge to said generating means, means to tune said cable to the frequency of the oscillations to be transmitted including an impedance in each conductor, said impedances having a value which is small with respect to the self-capacity of the cable, an oscillograph, and modulating means connected between the other diagonal of said bridge and said oscillograph to apply ltto the oscillograph a controlling voltage whichr varies in accordance with said quantities.

SAKE LEENDERT mi BRUIN.

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