[0001] This invention relates to plotters/drawing aids and like devices.
[0002] The invention is particularly, though not exclusively, concerned with chart plotters
for maritime navigation.
[0003] In maritime navigation most chartwork involves the transfer of bearings and courses,
usually with reference to a compass rose printed on the chart. Such transfer is commonly
carried out using a parallel ruler or parallelogram - plotter. Both of these have
disadvantages. In particular the ruler requires two-handed operation and must be walked
painstakingly across the chart; it is very difficult to do this in rough weather without
slipping and losing accuracy. The disadvantages of the parallel ruler can be readily
overcome using a parallelogram-plotter instead, but the conventional form of such
a plotter requires space extending beyond the draughting or chart area itself; this
is of especial disadvantage in small boats where the space available is limited and
chart space often doubles as a food-preparation area or cabin table.
[0004] British Patent Specification No. 155,153 describes apparatus for facilitating drawing
which is intended for application to a drawing board so as to displace a set square
in a parallel manner over the board. This apparatus utilises a parallelogram arrangement
and comprises a travelling member (on which the set square is pivoted) to which two
flexible tension members are connected at equal distances,one each side of the pivot
point for the set square. The tension members lead,via a pair of guides, to a tension/equalisation
arrangement in the form of a pulley system carried on a fixed element on the apparatus,
the guides being so spaced that the two tension members are drawn out and retracted
to vary their length together in a parallel manner by manipulation of the travelling
member over the board, against the action of the tension/equalisation arrangement.
Because of the need to provide a pulley system, the fixed element is sufficiently
large to require space extending beyond the draughting area of the drawing board as
clearly shown in the Figures of the drawings attached to the Patent Specification.
Thus, even if the described apparatus was modified for use as a chart plotter, it
would be no more suitable for small boats than the conventional parallel plotter referred
to above
[0005] It is an object of the present invention to provide an improved form of plotter/drawing
aid which is readily applicable where space is at a premium and which is simple and
relatively inexpensive to produce.
[0006] According to one aspect of the present invention there is provided a plotter/drawing
aid of the kind comprising a pair of flexible tension members which are connected
at respective common outer ends to a travelling member and which lead back over a
pair of guides to a tension/equalisation arrangement, the spacing between the tension
members where they connect to the travelling member and where they pass over their
respective guides being such that the tension members maintain a parallel relationship
during movement of the travelling member, this movement causing the tension members
to vary their length together, characterised in that the tension/equalisation arrangement
comprises a single rotatable member to which the inner ends of both tension members
are attached at angularly spaced positions, in that said guides are located with respect
to the rotatable member such that the tension members approach the rotatable member
from opposite directions, and in that said rotatable member is biased in one rotational
direction whereby both the tension members are wound onto said rotatable member.
[0007] Preferably, the tension members are in the form of tapes, the inner ends of which
are so connected to the rotatable member that they wind upon one another in bifilar
fashion.
[0008] This and other aspects and features of the present invention will be apparent from
the following description with reference to the accompanying drawings, in which :-
Figure 1 illustrates the principle of a conventional parallelogram-plotter;
Figure 2 illustrates the principle of a parallelogram-plotter in accordance with the
present invention;
Figure 3 is a plan view, with cover-plate removed, of an example of parallelogram-plotter
in accordance with the present invention;
Figure 4 is an end elevation of the parallelogram-plotter of Figure 3;
Figure 5 is a perspective view of the parallelogram-plotter of Figure 3; and
Figures illustrate details of construction of 6 and 7 the parallelogram-plotter of
Figure 3.
[0009] Referring to Figure 1, the conventional form of parallelogram-plotting or draughting
device utilizes two intercoupled parallelograms with fixed-length sides one of which,
side AA', is normally fixed to the chart or draughting table T. The sides are pivoted
to one another, but wherever side BB' is placed within the limits of extension of
the intercoupled parallelograms, the angle of the side BB' with respect to the side
AA' remains constant. A straightedge SE is pivoted to side BB' and may be locked to
it so that once the straightedge SE has been aligned with a vector or(otherwise set
to a desired angle) and then locked to the side BB', it may be transferred from one
place to any other of the table T to lay down a parallel vector or line accurately
without difficulty. However there is the disadvantage with this form of plotter that
for some positions of the straightedge SE the arms extend well beyond the boundaries
of the table T. Furthermore parts of the table area are obscured from view by the
arms, and the general construction of the device is bulky and cumbersome.
[0010] The parallelogram-plotter of the present invention as illustrated in Figure 2, seeks
to overcome the above disadvantages of the conventional parallelogram plotter by applying
the parallelogram principle in a different way. Instead of two coupled parallelograms,
only one having a pair of opposite sides CD and C'D' of variable length, is used.
In this way a straightedge S'E' pivoted on the side DD' opposite the fixed side CC',
may be positioned anywhere on the table T' to preset a vector or angle. When the straightedge
S'E' is locked to side DD', it may be moved to any other position-on the table T'
and will maintain the same vector angle provided that the lengths of the adjustable
sides CD and C'D' remain equal to one another.
[0011] A plotter incorporating the above-described principle of the present invention is
illustrated in Figures 3 to 7 and will now be described.
[0012] Referring to Figures 3 to 5, a straightedge 1 is pivoted on a travelling member in
the form of an arm 2 to which are attached flexible tension members in the form of
tapes 3 and 4. The tapes 3 and 4 feed round a pair of guides in the form of posts
5 and 6 and thence to wind upon one another in bifilar fashion round a rotatable member
in the form of a drum 7. The drum 7 is rotated to tension the tapes 3 and 4 under
the action of a helical torsion spring 8 inside the drum, so as to draw the arm 2
in, up towards the posts 5 and 6. Post 5 and 6 are moulded integrally with a base
9 (for example of glass-filled polypropylene) which may be fixed to any convenient
point around the plotting table or draughting surface. The drum 7 (for example of
nylon) is rotatably mounted (plain bearing) on a stud 10 that projects upwardly from
the base 9, between the base 9 and a cover-plate 11 (omitted from Figure 3 ,and for
example of glass-filled polypropylene) that is clamped onto the two posts 5 and 6
and a third post 12 of the base 9.
[0013] The posts 5 and 6 are provided with respective outer sleeves 13 and 14 (for example
of polytetrafluorethylene) on which the tapes 3 and 4 run; this eases passage of the
tapes 3 and 4 round the posts 5 and 6 as the arm 2 is moved in and out, and avoid
any need for use of special pulleys. The outer diameters of the sleeves 13 and 14
are the same as those of posts 15 and 16 to which the outer ends of the tapes 3 and
4 are respectively anchored on the arm 2. This ensures that an accurate parallelogram
is maintained irrespective of the position of the arm 2 relative to the base 9.
[0014] The tapes 3 and 4 are anchored at their inner ends to the periphery of the drum 7
and their lengths set so that the arm 2 is parallel to a line joining the centres
of the two posts 5 and 6. More particularly, the tapes 3 and 4 are anchored to the
drum 7 at diametrically opposite points so that equal tape lengths to the arm 2 are
accurately maintained; due to the positioning of the posts 5 and 6;there is an merement
of half a revolution in the winding diameter (i.e. draw-off positions) of the tapes
3 and 4, but since the tape anchorage points on the drum are diametrically opposite
one another, the same increment is experienced simultaneously by both tapes. Because
the tapes 3 and 4 are wound on the same drum and are interleaved helically, they always
wind and unwind in unison and by equal amounts, so maintaining the parallelogram.
[0015] The tapes 3 and 4.must be very flexible, but at the same time must exhibit minimal
stretch; they may advantageously in this respect be of thin (e.g. 0.19 mm thickness)
polyester material such as sold as 'Melinex' 226 ('Melinex' being a trade mark). Furthermore,
the tapes 3 and 4 may be formed by opposite-end lengths of a single tape that extends
across the inside of the drum 7 to emerge through diametrically- opposed anchorage
slots in the drum-periphery. On the other hand, and as provided in the specific plotter
illustrated, separate lengths may be used with their inner ends anchored to the drum
periphery; this enables a more compact drum 7 to be utilized. The separate-length
tapes 3 and 4 may be enlarged at both ends (formed by subjecting the ends to heat)
to facilitate their anchorage both to the drum 7 and the posts 15 and 16. In the latter
respect, the outer enlarged ends of the tapes 3 and 4 engage to be retained respectively
in parallel slots 17 and .18 of the posts 15 and 16, whereas, as illustrated in detail
in Figures 6 and 7, their inner enlarged ends engage in respective diammetrically-opposed
slots 19 and 20 in the drum periphery.to effect their firm retention to the drum 7.
The slot 19 also provides a convenient outer anchorage for the spring 8 within the
drum 7, as illustrated in Figure 6. The spring 8, which is preferably a constant tension
stainless-steel spring with a torque selected in the range of 750 to 1200 gm cm(such
as sold under the trade mark "Tensator"),is anchored at its inner end in a slot 21
of a downwardly-projecting hollow arbor 22 that is formed integrally with the cover
plate 11-and engages upon the stud 10.
[0016] Clamping pressure between the straightedge 1 and the arm 2 can be slackened off to
enable the straightedge 1 to be angularly oriented as desired with respect to the
arm 2 by rotating a knob 23 atop the arm 2. Once the desired orientation of the straightedge
1 (which may be pivoted centrally of its length,vor otherwise, to the arm 2) has been
achieved,it may again be clamped tightly to the arm 2 to retain that relative orientation,
simply by tightening-up the knob 23. Thus in order to transfer a vector using the
plotter of Figures 3 to 7 it is simply necessary first to loosen the knob 23'and orientate
the straightedge 1 into alignment with, or to be otherwise parallel to, the required
vector. The straightedge 1 is then locked by tightening of the knob 23, and may then
be moved to a new position to transfer that vector. The tapes 3 and 4 automatically
retract onto the drum 7 to take up any slack under the tension of the spring 8 as
the straightedge is moved, and can be readily extended against the action of the spring
8 as desired to enable unrestricted movement of the straightedge 1 anywhere over the
chart.
[0017] Because of its compactness and portability, the extending-parallelogram plotter of
Figures 3 to 7 offers advantage over the conventional fixed-parallelogram plotter.
Furthermore it provides minimal obscuration within the chart area, and does not need
to extend outside the boundaries of that area in use. The base 9 is preferably mounted
in one corner of the chart table so as to avoid inaccuracies that can arise where
an overflattened configuration of parallelogram is utilized. Mounting by means of
two screws may be facili-.. tated by provision of a slot 24 opening from the base
periphery,and an aperture 25 of key-hole shape in the base 9, as illustrated in Figure
3.
[0018] Although the principle of the extending-parallelogram device of the present invention
has been described above more especially in regard to maritime plotting, it is readily
applicable also to aviation and other navigation. Moreover it is applicable beyond
the field of navigation to draughting aids where transfers of parallel lines and vectors
are to be achieved.
1. A plotter/drawing aid of the kind comprising a pair of flexible tension members
which are connected at respective common outer ends to a travelling member and which
lead back over a pair of guides to a tension/equalisation arrangement, the spacing
between the tension members where they connect to the travelling member and where
they pass over their respective guides being such that the tension members maintain
a parallel relationship during movement of the travelling member this movement causing
the tension members to vary their length together, characterised in that the tension/equalisation
arrangement comprises a single rotatable member to which the inner ends of both tension
members are attached at angularly spaced positions,in that said guides are located
with respect to the rotatable member such that the tension members approach the rotatable
member from opposite directionsYand in that said rotatable member is biased in one rotational direction whereby both
the tension members are wound onto said rotatable member.
2. A plotter/drawing aid according to Claim 1, characterised in that the tension members
are in the form of tapes, and in that the inner ends of said tapes are so connected
to the rotatable member that they wind upon one another in bifilar fashion.
3. A plotter/drawing aid according to Claim 2, characterised in that the inner ends
of the tape tension members areconnected to the rotatable member at diametrically
opposite positions, and in that the spacing of the guides provides an increment of
half a revolution in the draw-off positions of said tapes from the rotatable member
for accurately maintaining equal tape lengths in use.
4. A plotter/drawing aid according to Claim 2 or 3, characterised in that the tapes
are connected to the travelling member via parts which are of the same diameter as
the said guides to ensure that an accurate parallelagram is always maintained in use.
5. A plotter/drawing aid according to any one of Claims 1 to 4, characterised in that
the rotatable member is in the form of a drum, and in that said drum is biased by
means of a helical torsion spring mounted internally of the drum.
6. A plotter/drawing aid according to any one of Claims 1 to 5, characterised in that
said torsion spring is a constant tensionspring with a torque selected in the range
750 to 1200 gm cm.
7. A plotter/drawing aid according to any one of Claims 2 to 6, characterised in that
the tape tension members are of a material such as a polyester whereby they may be
made thin and flexible, whilst exhibiting minimal stretch.
8. A Plotter/drawing aid according to any one of Claims 1 to 7, characterised in that
the tension members are formed by opposite end lengths of a single elongate member
which extends across and is attached to the rotatable member at the appropriate angularly
spaced positions.
9. A plotter/drawing aid according to any one of Claims 1 to 8, characterised in that
said rotatable member and said guides are supported on a stud and two posts respectively,which
are formed integrally at appropriate, relatively spaced positions on a base of moulded
plastics material, and in that a covering plate is clamped onto the posts to retain
said rotatable member and guides.
10. A chart plotter incorporating features according to any one of the preceding claims,
characterised in that a straightedge is pivoted to said travelling member and is arranged
to be clamped in a desired angular orientation by releasable clamping means.