Plotter, Sound Ranging Mk 1.
                  
CONTENTS
                           CHAPTER I

                                                                          PAGE
Theory ..    ..   ..    ..   ..    ..    ..    ..    ..    ..    3

                           CHAPTER II 
Description-
  Grid table      ..    ..   ..    ..    ..    ..    ..    ..    5
  Traversing arm pivoting bracket  ..    ..    ..     ..   ..    5
  Traversing arm  ..    ..   ..   ..     ..    ..    ..   ..     6
  Setting drums   ..    ..   ..   ..     ..    ..    ..   ..     6
  Spring drums    ..    ..   ..    ..    ..    ..    ..    ..    6
  Microphone table and standards   ..    ..    ..     ..   ..    6 

                           CHAPTER III
Method of use     ..    ..   ..    ..    ..    ..    ..    ..    6
  Adjustment for uniformity of length of connecting wires     .. 7
  Plotting a gun location     ..   ..    ..     ..    ..   ..    7
  Removal and replacement of wires from the microphone pulleys   8
  Instrument setting microphone standards     ..     ..   ..     8
 

                         LIST OF PLATES
                                                                                PLATE

Plotter, S.R., Mark 1-
Left view ..   ..   ..   ..   ..   ..     ..   ..    ..   ..     1
Left view, showing calibration pulleys and standard   ..   ..    2
Front view     ..   ..   ..   ..    ..    ..   ..    ..   ..     3
Right view, showing wires from microphone pulley standards to
spring drums   ..   ..   ..   ..   ..     ..   ..    ..   ..     4
View of spring and setting drums   ..    ..   ..     ..   ..     5
Instrument, setting, microphone standards     ..    ..    ..     6
Instrument, setting, microphone standards-in position on table   7
Elevation and plan views ..   ..   ..    ..    ..    ..   ..     8
Case, No. 1    ..   ..   ..   ..    ..    ..   ..    ..   ..     9
Case, No. 2    ..   ..   ..   ..    ..    ..   ..    ..   ..    10
Case, No. 3    ..   ..   ..   ..    ..    ..   ..    ..   ..    11
Accessories    ..   ..   ..   ..    ..    ..   ..    ..   ..    12 

The sound ranging plotter has been designed as a location plotting device used primarily when the microphones constituting the sound ranging base are irregularly spaced and aligned. It is, in effect, however, a general purpose plotter for straight or irregular bases, with the limitation that the ratio of the maximum range of location to the total length of base is as 3 is to 2.

The complete plotter is essentially a small model of the microphone lay-out and gun position in true geometrical relation to one another. It will be seen from plate I that M₁, M₂, M₃, M₄, etc. are microphone positions and G is the gun position on the surface of the earth represented by the base plate of the instrument.

The principle of the plotter is illustrated in Fig. I. M₁, M₂ and M₃ represent microphone positions and G the gun position. Attached to each microphone and passing freely through the gun position, imagine inextensible cords of the same length and with the free ends drawn tightly and disposed in a common direction. Clearly the difference of the distances GM₃-GM₂ or T₂ and of the distances GM₂-GM₁ or T₁, are equal to the intervals of separation of the free ends, N₃-N₂ and N₂-N₁.

If the values of t₂ and t₁ are known and the free ends of the cords N₁, N₂ and N₃ are adjusted to the corresponding values as shown and are clamped together at the position of the shortest free end (in this case N₃) then the inextensible cord system can only be drawn tight when the point G occupies one particular position relative to M₁, M₂ and M₃ in which the value of the differences t₂ and t₁ are those between the distances from G of the pairs of microphones M₃-M₂ and M₂-M₁.

It is obvious that an instrument made exactly as in Figs.1 and 2 would be impracticable mechanically owing to the difficulty of collecting the inextensible cords at a single centre. However, the problem is not altered in any way if the microphone positions and gun position are replaced by circular drums of the same diameter upon which the cords will roll or run, since the arcs of contact of the cords on the drum G shown as a, b and g in Fig. 2 are equal to the arcs as a, b and g of the drums M₁, M₂ and M₃ from which the cords may be supposed to be unwound.

Carrying the development further, imagine the drum at the microphone points replaced by free running pulleys and the drum at the gun position are replaced by pairs of pulleys of the same diameter as the single pulleys of the microphone positions and mounted on a shaft normal to the surface of the paper. The gun pulleys are very thin and each pair is mounted as closely as possible in the plane of its corresponding microphone pulley.

Now imagine inextensible cords of the same length and lying in the planes of their respective microphone pulleys anchored at one end over the point A, Fig. 3. Each cord passes over one of a pair of gun pulleys, out to its microphone pulley, back to the other pulley of the pair, and is drawn away in the direction B. Clearly the separation of the free ends N₁ and N₉ in the direction of B is now 2t₁, and that of N₂ and N₃ is 2t₂, so that the scale of the setting up device is doubled.

The above scheme, as shown in Fig. 3, is the scheme of the actual instrument, in effect.

The ends of the cords shown anchored at A (Fig. 3) are attached to separate spring-loaded drums while the free ends N₁, N₂ and N₃ are attached to another bank of separate drums which carry divided scales and are adjusted by gear and pinions to the differences 2t₁ and 2t₂ as required.

                                                                       CHAPTER II

                                                                        Description

Plate 8 represents a general view of the plotter. It has three essentially separate parts (a) the semicircular base plate or grid table, which carries the table on which the microphone pulley standards are mounted; (b) a traversing arm pivoting bracket, which supports the socket of a ball and socket joint perpendicularly over the centre of the semicircular arc of the grid table, and (c) the traversing arm, hinged at one end by means of the ball and socket joint, mentioned above, to the traversing arm pivoting bracket, and supported at the free end on two traversing rollers which roll on the edge of the grid table. This arrangement prevents rotation of the arm in any other plane than one parallel to the plane of the grid table.

The grid table, with its microphone table and standards in position, is the scale model of the surface of the earth and microphone stations, and the scale adopted is 1/25,000.

Five duralumin plates, made to fit upon studs in the surface of the grid table, are engraved with kilometre grids having the centre of the microphone table as centre, and orientation differences of 10 degrees. This provides that a grid may be selected to make the line of the selected microphone base lie within ±5 degrees of the line AB (Plate 8), and the centre of the base within ±½ km. of the centre C of the table. One of these grids is shown in position partly cut away.

The traversing arm pivoting bracket is simply a support, as described.

The traversing arm, supports a slide on its under side which is arranged along its length and parallel to the surface of the grid table and microphone table.

A carriage lead screw of large pitch, ½ inch, operated from the free end of the traversing arm by a hand wheel moves a traversing arm carriage on which the gun pulleys are mounted. These are carried on a spindle which is truly perpendicular to the microphone and grid tables together with two banks of guide pulleys necessary to guide the wires to the banks of setting drums and spring drums (Plate 5) carried on the free end of the traversing arm.

The six setting drums have circumferences of 24 cm. or 6,000/25,000 metres

and scales subdivided to intervals corresponding to 20/25,000 metres on the periphery of the drum or 10/25,000 relative movement of the microphone pulley and gun pulley. The scales are numbered to indicate intervals of 100 metres and kilometres and have separate counter scales to indicate kilometres up to 10. Each scale is read from a line engraved upon a plate attached to the frame and adjustable through a small arc of the scale to correct for small differences in the lengths of the wires.

The six spring drums of the same diameter as the setting drums have scales subdivided to intervals corresponding to 100 metres relative movement of the microphone pulley and gun pulley. These drums rotate upon carefully selected ball bearings, which, while accurate, are free running. The scales are read from cursor lines engraved upon a transparent cursor surrounding the drums, a cylindrical lens being fitted to assist in obtaining an accurate setting.

Both setting drums and spring drums are screw-threaded to take the steel piano wires which connect them together and otherwise pass round the corresponding gun pulleys and microphone pulleys.

The microphone table and standards. The microphone table represents an area 11 km. long and 6 km. wide, and is detachable from the grid table to which it is secured by large screws with wing heads. The surface is accurately machined and has slots for T-headed bolts to which microphone pulley standards can be secured.

The six microphone pulley standards decrease in height from No. 1 to No. 6 (No.1 standard is arranged on the right-hand side of the microphone table looking from the table across the grid table) so that each pulley lies in the plane of the corresponding gun pulleys, setting drums and spring drums. This can be seen in Plate 8 where two standards, Nos. 1 and 6 are shown.

CHAPTER III

METHOD OF USE

The above general description is sufficient to make the method of operation of the plotter intelligible.

Having laid out a sound ranging base and surveyed the microphone positions, the traversing arm should be removed with the plotting wires secured to their respective anchor pulleys and replaced pro tem. in its packing case. Then, with the microphone table in position on the plotter base, the Microphone Standards Setting Instrument (Plate 6) is placed on the grid table and located in a position, at an angle relative to the microphone base line most suited to convenience, by the use of three ground pins locating in holes in the grid table drilled at intervals of 10 degrees of rotation. By applying the co-ordinates, found by survey, to the two

arms of the instrument, the microphone pulleys may be positioned with extreme accuracy.

Apart from purposes of checking, provided the position of the actual microphones is not altered, it will not be necessary to make use of this instrument again while situated at any given centre and any number of enemy gun positions may be plotted without alteration to the settings.

Having set the positions of the microphone pulleys, a grid sheet is selected to make the general line of the base correspond with the length of the microphone table as closely as possible and this grid sheet is pressed securely into position on the surface of the grid table, being located by three dowels.

This done, the traversing arm may now he replaced with care and the next steps taken.

Adjustment for uniformity of length of the connecting wires

A calibration pulley standard is provided (Plate 2) with six calibration pulleys with separate spindles arranged co-axially at the intervals apart of the gun pulleys and perpendicular to the base. This test or calibration standard can be secured to the microphone table or to the grid table at the left side of the microphone table. When this has been done, the piano steel wires are removed from their retaining pulleys in the traversing arm and are transferred to the corresponding pulleys of the calibration standard.

The setting drum scales are then set to their zero positions and all the spring drum readings should be exactly alike whatever the position of the gun pulley or traversing arm on the grid table. If the spring drum readings are not alike, the setting drum zero plates can be adjusted to make them alike, and, when this has been effected, the wires are placed upon the pulleys of the corresponding microphone standards on the microphone table and the plotter is ready for use.

Plotting a gun location

When the readings of a sound ranging record have been corrected for time correction and temperature and wind corrections, the readings can be applied directly to the setting drums of the plotter, selecting an arbitrary zero. When this is done the traversing arm is rotated upon the grid table and the gun pulleys are traversed along the slide by means of the feed screw until all the spring drums read as nearly as possible alike. Only an accurate record will allow these drums to be adjusted with their reading exactly alike however, when this comparison has been effected, the position of the gun pulleys can be read upon the grid by means of the romer provided and the gun location is known.

If the location is more or less normal to the centre of the microphone base, the line and range adjustments of the gun pulleys are almost independent of one another, i.e. the traversing arm is swung for the line adjustment and the lead screw is rotated for the range adjustment. However, if the location is to a flank, these adjustments are not independent and range corrections will call for line correction to correspond.

Looking at the cursor of the spring drum, if the readings of the six scales from the top downwards read from right to left, then the traversing arm requires moving to the right and vice versa.

Again, when the line adjustment has been made approximately and the readings of the centre microphones are left of those at the flanks, then the traversing screw is rotated right to bring the readings into line, and vice versa.

A little practice with calculated readings for grid intersection positions will soon familiarize an operator with this part of the technique.

Removal and replacement of wires front the microphone pulleys

A special tool is provided to facilitate the removal and replacement of wires from the microphone pulleys. This tool can be made to grip the spindle of one of the free pulleys of the calibration standard in such a way that the pulley can be manoeuvred behind the pulley from which the wire is to be removed and transferred to its new pulley without risk of damage to the wire or pulleys. A firm hand is required for this operation and care should be exercised to see that the wire is entirely in the groove of the pulley to which it is transferred.

Instrument setting microphone standards (Plates 6 and 7)

This instrument consists essentially of a rectangular base casting carrying two accurately ground dovetail slides on which slide bearers carrying calibrated arms at right angles to the slides and each having at its inner end a micrometer touch dial.

In the base are three accurately spaced holes with which the instrument is located on the plotter base. Since the map grid used will be a N-S. E-W grid, the angle chosen will be as near to the inclination of the microphone base to the E-W direction as the 10-degree divisions on the grid table permit.

The dovetail slides form the axes of reference, and the sliding arms determine the position of the microphone pulley standards with respect to these axes. The arms may be moved along the slides to the required position and then clamped. They may then be moved along their length at fight angles to their bearers so that their ends locate the microphone pulley standard accurately by aid of the micrometer dials. The arms are graduated every 1,000, 100 and 20 metres, and with a vernier fixed to each arm, adjustment can be read to single metres. Clamps are provided for both the vernier and arm so that these adjustments can be made independently. The touch gauge at the end of each arm reads to twenty-five thousandths of an inch and two divisions are equivalent to 1 25 metres. With the aid of these touch gauges the position of the microphone pulley standards can be fixed with great accuracy.

The centres of the two slides are at a distance equivalent to 6,000 metres from the mid point of the microphone table. When setting up the microphone base, it is essential to fix the centre microphone as near to the centre of the table as possible. For example, when using a N-E grid, the origin of co-ordinates is arranged so that the centre microphone is as near 6,000 E-6,000 N as possible.

In a particular case, say that the centre microphones co-ordinates are : -

                                            N                                                                         E
                                     22863l.0 m.                                                        130712.0 m.

Take the new co-ordinates : -

                                             N                                                                        E
                                        5631.0                                                                5712.0

(the nearest convenient co-ordinates to 6,000 N and E) and subtract 223000 and 125000 from the North and East co-ordinates, respectively, of each of the other microphones.

The procedure to be used in setting up the microphone pulley base on the microphone table is as follows

With a protractor, the angle between the microphone base line and the E-W grid line is found to the nearest 10 degrees. The microphone table, having been previously clamped on the plotter grid table and securely held by the clamping screws, the setting instrument is oriented to the angle with respect to one of the bearers. This bearer will now correspond to the E-W grid line.

The pulley is removed from No. 6 standard by loosening the screw which tightens the clamping bar across the V-groove. The aligning rod provided is then placed in the V-grooves at the top and bottom of the standard and the clamping bars tightened up. No excessive force is required, and care should be taken that the aligning rod does not project below the base. It should be about 1/32 of an inch above the base.

The co-ordinate arms are set to the appropriate co-ordinates for No. 6 microphone (i.e. the lowest standard) and moved along their slides until the ends of the dial] gauge rods are nearly touching.

The T-headed clamping bolt is put in the slot nearest the point of intersection of the dial gauge rods and the microphone standard moved until the aligning rod enters the angle formed by the ends of these rods. It is then moved about gently until both dial gauges read somewhere near zero and lightly clamped, using the box spanner provided. The microphone standard can now be gently tapped into its final position where the dial gauge readings are within one division of zero, and the nut tightened up.

This procedure is repeated for the other standards in order. It is not essential to start with No. 6 standard, but the manipulation is slightly easier if the setting up is done in this order. The aligning rod is, of course, removed from the standard after setting up, and the pulley replaced.

The greatest care must be taken to ensure accurate positioning of the microphone pulley standards, as, in effect, a map of the microphone system is being set up, and upon this accuracy depends the accuracy of subsequent sound location.

When tightening the nuts in the standards, care must be taken not to twist the standards out of position.

The microphone standards having been correctly positioned on the microphone table, the traversing arm should be replaced on the plotter base, and the zero readings checked by transferring the plotting wires to the calibration standard and observing the readings of the spring and setting drums, which should all read the same.

The zero readings having been found to be correct, the plotting wires should be transferred to their correct microphone pulleys, respectively, using the tool provided (Plate 12) and the time differences set on the setting drum from the figures furnished by the sound ranging recorder.

This being done, upon adjusting the traversing arm for range and line, a position should be found when all the figures on the reading drums will read the same. The point then indicated being the position of the enemy gun, etc., and the accurate co-ordinates may be measured by the aid of the romers furnished in the case (Plate 12).

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