The present invention relates to pen plotters in which a drawing
instrument is carried by a plotting head that is moveable with respect to a chassis
of the plotter so as to print traces on a print medium as a function of graphics
data supplied to the plotter. More specifically, the present invention concerns
pen plotters of the type equipped with a drawing instrument store having a number
of individual receptacles and means for automatically exchanging selected drawing
instruments between the plotting head and the receptacles.
The graphics data are sent to the plotter from an external source,
such as a computer-aided design (CAD) workstation. The data contains information
on the shape of the trace to be plotted, which is interpreted by the plotter's
internal processing unit to control the relative displacements between the plotting
head and the print medium to produce the required trace, which is generally in
the form of successive vectors. The graphics data also include information specifying
the type of drawing instrument to use, depending on the colour and thickness of
the trace, and possibly on the dynamic characteristics of the tracing action, or
the surface of the print medium too. There are two types of command through which
the graphics data can be used to select a drawing instrument :
- either by direct selection of a specific drawing instrument in the store, using
an index number specifying the receptacle of the store assigned to the instrument,
- by selection of a particular type of drawing instrument belonging to a group
of instruments having the same characteristics, the actual receptacles of the store
assigned to instruments of a same group being specified at the level of the plotter's
The first type of command is the simplest to process, but has the
drawback of relying on one and the same drawing intrument for all traces requiring
that type of instrument, which leads to a certain rigidity in management of the
drawing instrument store.
In contrast, the second type of command allows a more flexible management
of the drawing instruments since the selection is based not on a specific instrument,
but rather on a particular type of drawing instrument. The control unit can thus
be pre-programmed to assign not just one, but several receptacles of the store
for each type of drawing instrument, the choice of which actual receptacle to be
employed at any one time for a given instrument group being made independently
according to an arbitrary scheme at the level of the plotter.
This uncoupling between the external command specifying a particular
group of drawing instruments on the one hand, and the selection of a corresponding
instrument in one the store's receptacles assigned to that group on the other,
grants a considerable improvement in the plotter's autonomy in operating cycles
during which worn instruments have to be replaced. Indeed, the control unit can
then command the use of a replacement instrument whenever a current instrument
becomes worn, the wear being determined for each instrument by an internal management
system on the basis of recorded cumulated traces.
Accordingly, when the writing instruments are initially installed
in the instrument store, they are assigned into groups of instruments of the same
type, with each group containing as many instruments as judged necessary to ensure
a maximum of autonomy. If it is considered that the store only needs one drawing
instrument of a certain type, then the latter is nevertheless considered as belonging
to a group, of which it is the sole member.
A group is thus defined as a set of drawing instruments arranged in
the store and considered as mutually interchangeable, being identical or of the
same technology (ball, fibre, ...) and of the same diameter, colour, etc.. The
make-up of a group must be entered into a memory of the control unit before an
operating cycle. It may be redefined at any moment by an operator, for instance
during an intrerruption in the plotting operation.
In prior art pen plotters, the operator can define the groups of drawing
instruments in either of two ways.
The first is by means of a programming language which comprises sub-commands
for assigning a set of receptacles in the writing instrument store to each selected
group, the choice of particular active instrument receptacle within the group being
determined at the level of plotter's control unit.
The second way consists in assigning drawing instrument receptacles
directly through the plotter using a keyboard and display screen connected to the
control unit. In this case, the user defines the groups by assigning each receptacle
with a number designating a particular group.
This last option is however reserved for the more elaborate pen plotters.
Simpler plotters normally have just a very basic control panel with a rather limited
display capacity, if any, making such data input impossible.
In any case, there remains a problem in that the relationship between
the table that assigns the drawing instruments' positions to the different groups
and the actual physical locations of the drawing instruments in the store is little
or not apparent. Accordingly, it is not easy for the operator to ensure that this
relationship is correctly established, and there is thus a risk of error.
Moreover, the input of assignment data for the drawing instrument
positions is a tedious task that complicates the operator's work and adds to the
amount of training required. Finally, this assignment requires a man-machine interface
more complete than otherwise necessary with respect to the display, the control
unit and the input/output circuits, causing extra manufacturing costs of the plotter.
It is an object of the present invention to overcome the above problems
by providing means for defining groups of drawing instruments which are simple
to use and which, in particular, allow the operator to see at a glance which actual
receptacles are assigned to a same group during the process of defining groups
of drawing instruments.
This object is achieved, according to the present invention, by a
pen plotter comprising : a plotting head displaceable relative to a print medium,
a store for drawing instruments, comprising a plurality of
individual drawing instrument receptacles, the receptacles being divided into a
plurality of groups, each group comprising one or a plurality of receptacles and
being assigned to a particular type of drawing instrument,
means for automatically exchanging drawing instruments between
the plotting head and the instrument receptacles of the store, and
a control unit adapted to receive graphics data concerning
traces to be plotted, for controlling relative movements between the plotting head
and the print medium so as to produce a plot thereon corresponding to the information
contained in the received graphics data, and to control a selection of a drawing
instrument as a function both of information contained in the graphics data, which
information identifies a particular type of instrument, and of information identifying
the receptacles of the store that are assigned to a same type of instrument,
wherein the drawing instrument store contains identification
means for identifying the instrument receptacles that belong to a same group of
receptacles, and the plotter comprises detection means operatively cooperating
with the identification means to supply the control unit with information defining
the groups of instrument receptacles formed within the instrument store.
According to a particular embodiment of the invention, the identification
means are associated to intervals between the receptacles, each identification
means being able to adopt one or another of at least two states respectively indicating
that the two receptacles on either side of the corresponding interval belong to
a same group, or that the two receptacles belong to separate groups, the detection
means being capable of detecting the states of the identification means.
The identification means may be in the form of moveable identification
elements that can adopt at least two different positions and cooperate with detection
means capable of detecting the position of each identification element by electro-optical
means or by contact. The identification means may also be made in the form of
magnetic substrates cooperating with detection means comprised of a magnetic read
The invention also relates to a drawing instrument store comprising
receptacles for a pen plotter apparatus, wherein identification means are provided
for identifying the instrument receptacles that belong to a same group of receptacles,
in order to define a distribution of instrument receptacles into different groups.
The present invention also relates to a process for defining groups
of drawing instruments for a drawing instrument store in a pen plotter, the store
comprising a plurality of individual receptacles, the process comprising the steps
of : distributing the instrument receptacles of the store into different receptacle
groups, and sending data corresponding to the distribution to a control unit of
the plotter apparatus,
wherein the process further includes the steps of : identifying
the groups of drawing instrument receptacles using identification means arranged
at the level of the instrument store, the identification means being capable of
adopting at least two different states, and detecting the states of the identification
means to supply the control unit with information defining the groups of instrument
receptacles formed in the store.
The use of identification means according to the present invention
calls for a certain amount of precision in the installation of the store provided
with the identification means with respect to the detection means.
In particular, it is important to ensure that the data collected by
the latter are not liable to be corrupted as a result of vibrations or other spurious
movements transmitted to the store. This precaution applies especially in the case
where the readout of data on the identification means requires a relative displacement
between the latter and the detection means, the readout taking place e.g. when
the store is driven into rotation about its axis, as in carrousel type drawing
Although the above prerogative can be satisfied by some known types
of means for supporting, driving and guiding the drawing instrument store, it is
nevertheless advantageous, according to another aspect of the invention, to provide
a pen plotter having :
a drawing instrument store comprising a plurality of individual
instrument receptacles distributed about an axis of the store, and a crown portion
forming a peripheral path,
means for supporting the store on a chassis of the plotter,
means for driving and guiding the store into rotation about
its axis, comprising a drive element for driving the store by engagement with said
wherein the store guiding means include at least two studs
provided on the store support means for engagement with a guiding ramp formed by
a frusto-conical surface which is provided on said crown portion and which is inclined
with respect to the axis of the store, at least one of said studs engaging with
the crown portion at a location angularly displaced by more than 90&peseta;
with respect to the location of the engagement between the drive element and the
The invention also relates to a drawing instrument store adapted for
such a guiding system.
The present invention shall be more clearly understood upon reading
the description of the preferred embodiments, given below as a non-limiting example
with reference to the appended drawings in which :
- figure 1 is a highly schematic general view of a pen plotter;
- figure 2 is a functional diagram of the control unit for the pen plotter of
- figure 3 is a schematic diagram showing an example of the relationship established
between groups each corresponding to a type of drawing instrument and specific
receptacles of the plotter's instrument store, according to a step of definition
of groups of instrument receptacles;
- figure 4 is a three-quarter view of a drawing instrument store according to
one embodiment of the present invention;
- figure 5 is a simplified side view of the base of the drawing instrument store
shown in figure 4;
- figure 6 shows the drawing instrument store of figure 4, seen along the cross-section
A-A' of figure 7;
- figure 7 is a plan view illustrating very schematically the arrangement of
the means for driving and guiding the instrument store in rotational movement relative
to the chassis of the pen plotter;
- figure 8 is a detailed view showing an identification element mounted on the
base of the store shown in figure 4;
- figure 9 is a schematic illustration of the drawing instrument store showing
the positioning of the identification elements for defining the groups of drawing
instrument receptacles according to the example of figure 3;
- figure 10 is a timing chart for a signal obtained by detection of the identification
elements positioned as shown in figure 9;
- figure 11 is a flow chart indicating the steps for recording into a memory
the group definition data contained in the signals obtained by detection of the
identification elements; and
- figure 12 illustrates group identification elements according to another embodiment
of the invention.
Figure 1 gives a highly' schematic illustration of the general aspect
of a pen plotter.
Pen plotters operate by relative displacements between a print medium
and a drawing instrument along two mutually perpendicular directions X, Y. Typically,
the print medium is displaceable to-and-fro along one of these directions (X),
while the drawing instrument is displaceable to-and-fro along the other direction
In the example, which is based on a particular type of plotter known
as a "drum type plotter", the print medium 2 is in the form of a continuous sheet
that passes over a drum 4 driven into rotation by a motor 6 whose drive shaft may
be coupled directly to the drum 4. The drum 4 has its axis parallel to the Y direction
and rotates so as to displace the sheet along the X direction.
The drawing instrument 10 in active use is held fast by a clamp 12
forming part of a plotting head 14 supported on a carriage 16. The carriage is
displaceable along a sliding rail 18 extending along the Y direction. The carriage
16 is moved along the sliding rail by means of a motor 20 driving a belt attached
to the carriage. The tip of the drawing instrument can be selectively placed in
contact with the sheet 2 by means of an electromagnetic actuator carried by the
plotting head, which can displace the writing instrument along a direction Z between
a raised position and a lowered position.
A drawing instrument store 24, shown here in the form of a carrousel,
is provided for storing a number of instruments in respective receptacles. The
drawing instruments can be exchanged automatically by bringing the plotting head
12 to a transfer position adjacent the carrousel 24 and corresponding to an end
of the travel of the plotting head. The carrousel 24 is rotatably driven by a motor
26 so as to present a selected receptacle against the plotting head 12 in the transfer
position. The receptacle may be empty for receiving the instrument carried by the
plotting head, or it may contain the instrument to be transferred onto the plotting
The process for automatically exchanging drawing instruments is already
well established in the art; more details on that aspect can be found in patent
document FR-A-2 624 794.
The various commands required for controlling the plotter's active
devices, and in particular the different drive motors 6, 20, 26 and the electromagnetic
actuator, are supplied by an internal control unit which shall now be described
with reference to figure 2.
The control unit is organised around a standard type of microprocessor-based
central processing unit (CPU) 30 that sends commands selectively to the different
active devices as a function of the graphics data DG inputted from an external
source. The graphics data DG may originate from graphics processing equipment,
such as a computer aided design (CAD) workstation, and be entered into the CPU
30 via an input interface 32. They include data specifying the traces to be plotted
and the type of drawing instrument to be used for each trace.
The traces to be plotted are usually defined by a succession of two-dimensional
vectors whose respective components determine the displacements of the sheet in
the X direction and of the drawing instrument in the Y direction.
The graphics data relating to the traces to be plotted and the drawing
instruments to be used are interpreted in the CPU 30 to produce commands transmitted
from an output interface 34 to a bus 36 connected to controllers that provide the
electro-mechanical interface for each of the plotter's active devices. For simplification,
the figure only shows the controllers 38a, 38b, 38c corresponding respectively
to the motor 6 for driving the drum 4, the motor 20 for driving the carriage 16
carrying the plotting head 14, and the motor 26 for driving the drawing instrument
store 24. (It will be noted that in this simplified example, a change of drawing
instrument only involves the last two active devices.)
The selection of a specific drawing instrument from among those in
the store's receptacles is determined from the graphics data identifying a type
of instrument to be used, in conjunction with the following two parameters : the
data defining the groups of receptacles, such data being stored in a group memory
unit 35 that can be accessed by the plotter's CPU 30, and the state of wear of
the drawing instruments, this parameter being determined by the CPU 30 on the basis
of data stored in an intrument trace logging memory 37, as shall be explained
The relationship between groups of receptacles in the instrument store
and the types of instruments shall now be explained with reference to the example
shown in figure 3.
The graphics data relative to a type of drawing instrument are coded
in terms of groups GNi, where i is an integer specifying a particular group. When
the plotter is programmed in an initial phase, each group GNi is made to correspond
with one or several receptacles Lk of the drawing instrument store, where k is
an integer specifying a particular receptacle. It will be noted that each receptacle
of the store can be individually placed at a transfer position by means of indexing
devices allowing each receptacle to be identified by its own reference number k.
In the present example, it shall be assumed that the graphics data
employ four groups GN1, GN2, GN3 and GN4, thus allowing four types of drawing
instruments to be specified, respectively OC1, OC2, OC3 and OC4, and which differ
from each other e.g. by their ink colour, linewidth, type of inking system (ball
point, fibre tip, ...), etc..
The operator takes into account the characteristics of the drawing
instruments OCi when programming the plotter in the group definition phase.
As an example, the corresponding information may be in the form of
a specification such as :
Group GN1 : red ink, fibre tip drawing instrument (OC1),
Group GN2 : red ink, ball point drawing instrument (OC2),
Group GN3 : blue ink, fibre tip drawing instrument (OC3),
Group GN4 : blue ink, ball point drawing instrument (OC4).
The operator chooses which receptacles are to go with which type of
drawing instrument OCi, and how many drawing instruments of the same type are
to be kept in the store. This last choice is made in view of maximising the autonomy
of the plotter when the latter is used in conjunction with an internal wear management
program that monitors each drawing instrument and, whenever one of them reaches
the end of its useful life, commands its replacement by an automatic exchange with
another instrument of the same type. Accordingly, for each type of drawing instrument,
the number of instruments to use is chosen as a function of that instrument's lifetime
and an estimation of the total distance to be traced by that type of instrument.
In the example, the number of drawing instruments OC1 to OC4 for
groups GN1 to GN4 are respectively two, one, three and two. The total number of
instruments is made equal to the number of receptacles in the store, to exploit
its full potential.
The drawing instruments of a same group are physically located in
adjacent receptacles of the store, and the group memory block 35 records a table
of correspondance between these groups and the receptacles to which they are assigned.
In the present example, the receptacle identified by L1 is assigned to the first
OC1-type instrument of group GN1, then receptacle L2 is assigned to the second
OC1-type instrument of that group, and so on. The data recorded in the memory thus
contain the following information :
Group GN1 : L1 or L2 (receptacles for OC1-type instruments)
Group GN2: L3 (receptacle for the OC2-type instrument)
Group GN3 : L4 or L5 or L6 (receptacles for OC3-type instruments)
Group GN4 : L7 or L8 (receptacles for OC4-type instruments).
The definition of groups thus consists in loading the group memory
35 with the above table of correspondance between receptacles Lk of the store and
the identification numbers of the groups of instruments GNi.
The information contained in the group memory 35 enables the CPU 30
to select a specific receptacle of the store in response to the graphics data received.
Where a group gives a choice between several receptacles, as is the case with
groups GN1, GN2 and GN3, the choice will depend on the state of wear of the drawing
instruments in those receptacles, and follows a predetermined order, such as a
progression starting from the receptacle Lk having the lowest k value. The state
of wear of the instruments is recorded in an instrument trace logging memory 37,
which totalises the total length traced by the instruments associated with each
of the receptacles Lk. When the totalised value for an instrument associated with
a receptacle Lk exceeds a wear limit that is programmed at the level of the CPU
30 according to the specific wear characteristics of the tool, the CPU 30 replaces
that instrument by another of the same type contained in the next receptacle.
There shall now be explained how the groups of drawing instrument
receptacles in the store can be defined according to different embodiments of the
invention, starting with figure 4 which is a general three-quarter view of an
instrument store equipped with group definition means.
The store 24 is in the form of a carrousel having a peripheral body
40 depending from a central sleeve 42 at the level of a base portion 50. The individual
receptacles for the drawing instruments 10 are uniformly distributed around the
periphery of the body 40 and are defined by recesses 44 opening radially towards
the outside. Each instrument is positioned vertically in its respective receptacle
by means of a rest 46. The latter is in the form of a horizontal stepping surface
formed in the wall 44b of the receptacle and cooperating with a ring 10a on the
writing instrument 10. The instrument is held in place within its receptacle by
means of a tab (not shown) projecting vertically upwards from the outside edge
of the rest 46 and engaging a groove formed in the base of the ring 10a. In an
instrument exchange operation, as described in patent document FR-A-2 624 794,
the plotting head withdraws a selected instrument by effecting a horizontal displacement
such that the clamp 12 engages the instrument's body by resilient deformation of
the two clamp branches, followed by an upward vertical displacement to free the
ring 10a from the tab engaged therewith. An instrument is stowed in its receptacle
by carrying out the same movements in reverse.
The upper end of each drawing instrument 10 (i.e. the end opposite
the tip) rests laterally at the bottom of a recess 44a formed in an upper crown
section 48 of the carrousel. The lower end of each instrument also rests on conical
recess 44c formed in the base 50.
The carrousel is mounted onto the plotter by engagement of its sleeve
42 on a supporting axle.
The group definition means are integrated to the carrousel 40 at the
level of the base 50 and comprise programming elements in the form of toggles 60.
Each interval between two adjacent instrument receptacles is provided with a toggle
60 that can slide along an opening 52 formed in the base 50. The toggle has an
upper portion forming a tip that projects through the base so as to be externally
Depending on whether it is positioned at one side of the opening 52
or the other, the toggle 60 indicates if the adjacent instrument receptacles belong
to the same group or not, with the tips giving the operator a visual indication
of the receptacle groups defined in the carrousel 24, even when the latter is its
working position. The positions of the toggles are detected by electro-optical
As shown in figures 5 and 6, the base 50 has an upper annular horizontal
face 50a in which the openings 52 are formed, and which is prolonged by an outer
vertical rim 51 turned towards the bottom. A crown-shaped vertical inner annular
wall 54 is also formed beneath the upper face 50a, in the region of the openings
The internal face of the outer rim 51 contains a peripheral geared
portion 53 that engages with a pinion 55 coupled to the shaft of the carrousel's
drive motor 26. The latter is a stepper motor for which each step corresponds to
a pulse controlled by the CPU 30.
The inner annular wall 54 has respective cut-outs 56 each corresponding
to a respective opening 52. An additional cut-out 57 is formed in the inner annular
wall 54 to define an angular reference position, which may e.g. correspond to that
of receptacle L1. This cut-out 57 distinguishes from the other cut-outs 56 by its
Each toggle 60 (figures 6 and 8) is connected with a clip-shaped element
62 comprising two parallel wing members 62a, 62b joined together at their lower
ends by a base 64. One of the wing members 62a is taller than the other 62b, so
that its top end projects through the opening 52 to form the aforementioned visible
tip. The top end of the other wing member 62b has a finger or catch 68 turned towards
The clip 62 is fitted on the inner annular wall 54 by passing the
two wing members 62a, 62b on either side from its lower edge. The wing members
62a, 62b spread apart from each other by resilient deformation until the catch
68 comes to press against the upper edge of the inner annular wall 54, inside the
opening 52, as shown in figure 6 (only one clip 62 is shown for the sake of clarity).
As shown in figure 8, the base 64 of the clip 62 is prolonged on either
side of the wing members 62a, 62b -to form a manoeuvering tab that is accessible
for sliding the toggle 60 from one end of the opening 52 to the other. The upper
surface 64a, 64b of the base is designed to slide against the lower edge of the
inner annular wall 54. Each end of the manoeuvering tab comprises a boss 65a, 65b
for blocking the clip 62 in each of its two sliding positions by engagement in
one or the other of two pairs of notches 55a, 55b and 55'a, 55'b respectively,
formed in the lower edge of the inner annular wall 54. The clip 62 is manoeuvered
by the central part of its base, beneath the wing members 62a, 62b. The maintenance
of the toggle in one or the other of its positions is further ensured by the resilient
clamping force exerted by the wing members 62a, 62b on the inner annular wall 54.
The wing members 62a, 62b contain respective aligned windows 66a,
66b that are substantially the same size as the cut-outs 56. The mutual dispositions
of the openings 52, cut-outs 56, toggles 60 and windows 66a, 66b are such that
when a toggle 60 is at a first end of its travel in its corresponding opening 52,
the windows 66a, 66b are aligned with the cut-out 56, while when the toggle is
at the second end of its travel in the opening 52, the cut-out 56 is shut off by
solid portions of the wing members 62a, 62b at one side of the windows 66a, 66b.
An electro-optical detector is fixedly mounted on the plotter and
comprised of a transmitter 70 and detector 72 located at respective sides of the
inner annular wall 54 and the clips 62. In a read mode, the carrousel 24 is driven
into rotation, whereupon each time a clip 62 passes before the detector, the latter
delivers a first or second signal depending on whether the clip is in its first
or second position, namely whether the windows 66a, 66b are aligned with the cut-out
56 and allow the light beam to pass from the transmitter 70 to the receiver 72,
or whether the windows are not aligned with the cut-out 56 so that the light beam
is interrupted by the clip.
The electro-optical detector also detects the passage of cut-out 57,
the signal in this case being longer than that produced in response to passage
of cut-out 56 as seen through the windows 66a, 66b.
Advantageously, the location of each opening 52 relative to the two
drawing instruments on either side thereof is chosen such that when the toggle
60 is at the first position P1, indicating that the two instruments belong to the
same group, its projecting tip is at least partially hidden behind one of the instruments.
Conversely, the projecting tip is chosen to be clearly visible when the toggle
60 is at the position P2 indicating a separation of groups between the two instruments.
This allows an operator to quickly determine the distribution of drawing instruments
in the carrousel by locating the visible projecting tips of the toggles. Such an
arrangement for the openings 52 and drawing instruments 10 can be seen in figure
9, which shows an example of how groups of receptacles are set by the toggles 60
in the carrousel 24.
Figure 9 is a schematic illustration of the toggles 60a to 60h placed
in either the first position (P1) or second position (P2) at intervals between
the receptacles L1 to L8 of the carrousel, so as to define define the groups GN1
to GN4 according to the example of figure 3.
Toggle 60a, which is located between receptacles L1 and L2, is at
position P1 so as to establish that these two receptacles belong to the same group
GN1. This group comprises only two OC1-type instruments. Accordingly, a separation
is established between receptacles L2 and L3 by placing the toggle 60b between
the receptacles L2 and L3 at position P2.
Receptacle L3 is the sole member of the second group GN2. A separation
will therefore be established with respect to its neighbour L4, by placing toggle
60c between receptacles L3 and L4 at position P2.
The same procedure is used to define group GN3, which is composed
of three OC3-type instruments at receptacles L4, L5 and L6, and group GN4, which
is composed of two OC4-type instruments at receptacles L7 and L8.
The data thus encoded by the toggles 60a to 60h are read out by setting
the carrousel into rotation in an arbitrary direction (arrow F), so that each clip
passes across the electro-optical detector 70, 72.
It can be appreciated that a reliable readout of the data encoded
by the toggles 60 will depend, among other things, on the stability of the carrousel
as it is driven into rotation. Indeed, the signals obtained by the electro-optical
detector 70, 72 must correspond precisely with carrousel's displacement steps determined
by motor 26, as explained in more detail with reference to figures 10 and 11.
Accordingly, it is important to make sure that the instrument carrousel
24 is correctly guided and supported during the rotational movements, which often
involve high accelerations and decelerations, as well as when it experiences radially-directed
forces during changes of instruments. The latter forces are produced by the mechanical
contact between the store and the plotting head 14, via the instrument being exchanged.
When an instrument is returned to or withdrawn from its receptacle, it exerts on
the rest 46 a lowering force or lifting force respectively, as well as a transversal
movement. Furthermore, the rotational axis of the carrousel 24 must be correctly
maintained in spite of any mass imbalance resulting from an incomplete loading
of drawing instruments.
There shall now be described a means for guiding the store 24 that
satisfies the above-mentioned requirements while remaining mechanically simple
and economic to manufacture.
As shown in figure 6, the carrousel 24 is received on a central axle
58 attached to the chassis 53 and rotatably mounted around the axle by means of
a bearing 59 that is unitary with the central sleeve 42. The carrousel is guided
by two fixed studs 49 attached to the chassis of the plotter and designed to abut
against a portion of the under surface 51a of the base 50 that forms a frusto-conical
ramp close to the geared portion 53. The contact point of a stud 49 is on a dome-shaped
end surface. The ramp 51a is concentric with the store's axle.
As can be seen from figure 7, which shows very schematically a plan
view of the elements forming the carrousel's drive and guiding system, each of
the two studs 49 is angularly separated from the point where the pinion 55 meshes
with the geared portion 53 by at least 90&peseta; around the axle 58.
Consequently, the contact point of each of the studs presses against the ramp 51a
with a thrust Rp having a horizontal component Rh tending to compensate the force
exerted by the pinion along the radial direction n at the point where it contacts
the geared portion 53.
The above horizontal force component Rh is a function of the angle
of inclination α of the ramp 51a (figure 6, half-angle at the apex of the
cone containing the surface of the ramp), the coefficient of friction at the contact
points between the studs 49 and the ramp 51a , and the weight of the carrousel
24. As a general indication, the optimum angle α is in a range of 30&peseta;
to 60&peseta; for the example considered. Advantageously, the above
parameters can be determined as a whole so as to produce a self-alignment of the
carrousel when the latter is slightly offset in the axial direction. This can be
the case when the carrousel 24 is being received by the axle 58, to allow easy
engagement of the geared portion 53 with the pinion 55. The studs 49 then abut
against the ramp 51a before the carrousel settles into place. The sliding movement
of the ramp 51a against the studs under the weight of the carrousel 24 can thus
take up any play in the pinion-to-gear interface and ensures proper alignment of
the carrousel with respect to its rotational axis, by a balance of forces exerted
by the studs and the pinion.
Such a guiding system can also rapidly damp the vibrations induced
by jittering movements from the plotting head 14 and sheet displacement drum 4.
The vibrations will be all the more limited as the horizontal plane connecting
the thrust points of the studs 49 and the plane passing through the centre of the
pinion 55 are made close to each other.
The studs 49 and the pinion 55 together thus form stable equilibrium
points for supporting the store around the central axle 58. Naturally, it can be
envisaged to provide more than two studs to abut against the ramp 51a. Moreover,
it is clear that the balance of thrust forces exerted by the studs 49 and the pinion
55 can be achieved with just one stud situated at more than 90&peseta;
from the pinion 55 around the axle 58.
The guiding system can be implemented not only with a pinion-and-gear
drive, as in the present example, but more generally with any drive means involving
a contact between a peripheral path on the store and a rotating element, such as
a pressure roller driven by a motor fixed on the plotter's chassis.
There shall now be explained with reference to figures 10 and 11 how
the group programming data are loaded into the group memory 35 of the control unit
from the signals collected by the detector 70, 72.
Figure 10 is a time chart showing the signal S obtained from the detector
after a signal shaping stage. Logic levels 1 and 0 respectively correspond to a
detection and a non-detection of the beam by the receiver 72. In a complete rotation
of the carrousel, the signal S comprises a reference pulse Iref corresponding to
the passage of the reference cut-out 57 before the detector 70, 72, and pulses
l corresponding to the passage of those clips 62 whose ergots are in a position
P1 or in the inverse P2, before the detector.
The steps for the recordal of group definition data are carried out
by a computer program as described below with reference to the flow chart of figure
First, the reference cut-out 57 is searched for by rotating the carrousel
24 using control pulses sent to the stepper motor 26. The carrousel is rotated
until detection of a pulse having a pulsewidth exceeding a predetermined threshold
in the signal S. The widths of the pulses in signal S are measured by counting
the number of clock pulses in the control unit from the detection of a logic level
transition from 0 to 1, to the following opposite transition. Upon detecting that
the pulsewidth threshold is exceeded, the carrousel is stopped at the following
transition from level 1 to level 0, i.e. the falling slope of the Iref pulse. This
position corresponds to an angular reference position of the carrousel with respect
to the plotter. In this case, the reference cut-out 57 is aligned with receptacle
L1, i.e. the vertical centre-line of the latter coincides with the rear edge of
the cut-out 57 in the carrousel's rotation direction. The carrousel's angular reference
position is therefore the one where the optical axis of the detector is substantially
in the median plane of receptacle L1.
Assuming that the angular separation between the position of photoelectric
detector and the instrument transfer position at the end-travel point of the plotting
head are known, as is also the angular position of each drawing instrument receptacle
in the carrousel, it is clear that any receptacle can be brought to the transfer
position by sending a calculated number of control pulses to the drive motor 26.
The recordal of the group definition proper thus begins with the carrousel
at its reference position and the parameters i and k (representing the group and
receptacle numbers respectively) both set to 1 (step 100). There is then recorded
into the memory 35 data establishing that receptacle Lk belongs to group Gi (step
Next, it is checked (test 102) whether i is less than the total number
NL of receptacles in the carrousel (in the present example, NL = 8).
Then, the carrousel drive motor 26 is started (step 103) and the control
pulses sent thereto are counted (step 104).
At each motor control pulse, a sub-routine 105 is performed to detect
the state of signal S. If a logic level 1 is detected, a register S is set to 1.
When the content IM of a motor control pulse counter reaches a value
N corresponding to an angular displacement equal to the pitch between two receptacles
(test 106), the above counter is reset to zero (step 107) and the content of register
S is examined (test 108).
If S is equal to 1, register S is reset to 0 (step 109) and values
i and k are incremented by one unit (step 110), signifiying a passage to the next
group and to the next receptacle, respectively. If S is equal to 0 at test 108,
only the k value is incremented by one unit (step 111).
The process then returns to step 101 for recording the information
specifying that receptacle Lk belongs group Gi.
The automatic data recording process is terminated when the condition
i = NL is detected at test 102.
It will be noted that the above scheme rests on the assumption that
receptacle L1 defines the beginning of a group, and that there is necessarily a
group separation between receptacles L1 and L8. Accordingly, no notice is taken
of the information carried on toggle 60h. However, the program parameters can
easily be modified to accommodate for an additional degree of freedom in the partitioning
between receptacles L1 and L8, by taking into account the position of toggle 60h.
The material aspects of the invention allow for many variants that
are within the reach of the skilled person.
For instance, the aforementioned arrangement based on the use of a
electro-optical detector 70, 72 cooperating with cut-outs associated to intervals
between instrument receptacles can also be implemented with shutters other than
slidable clips 62, such as vertically retractable shutters, or even removable shutters.
This latter variant is illustrated in figure 12, which shows very
schematically a plan view through the upper horizontal face 50a of the carrousel's
base 50. The identification elements are in the form of removeable masks 90 that
can be clipped onto the inner vertical annular wall 54 at the level of an opening
52, so as allow selective closing off of the cut-outs 56.
Each mask 90 is fitted with grippable tab 92 for handling and also
to provide a visual indication of the mask on the carrousel.
In another variant of the present invention, the electro-optical detector
70, 72 are replaced with a feeler type sensor. In this ease, the identification
means take the form of contact elements associated with intervals between the receptacles.
The contact elements can be removable devices having a contact surface
with slight protruberances in relation to the outer surface of the vertical annular
wall 54. The corresponding detector comprises a feeler arranged to follow the outer
surface while the carrousel is made to rotate. The detection of the contact surfaces
is then effected by analysing the transitions in the pressure exerted on the feeler.
Alternatively, the identifying means can be in the form of magnetic
substrates on which are recorded a first item of data indicating that the receptacles
on either side of interval in which it is situated belong the same group, or a
second item of data indicating that these receptacles belong to distinct groups.
In such a variant, the detection means comprise a magnetic reading head fixed to
the plotter chassis and arranged to detect the above first and second items of
information on the magnetic substrates.
It can be noted that all the above variants are also amenable for
the detection of the reference receptacle L1 using similar means.
In the examples described, each identifying element can only adopt
two different states, indicating whether or not the receptacles adjacent thereto
belong to the same group. However, the invention is not restricted to such binary
devices. Indeed, it is relatively simple to design identification elements that
carry not just binary data, but more complex information for sending to the control
unit 30 by any known means. In particular, such elements could be associated with
respective receptacles rather than the interval between the receptacles, in which
case they could identify which specific group the corresponding receptacle belongs
to, as well such information as the order in which the instruments of a particular
group are to be used.
Other means can be envisaged for encoding this more complex information,
such as bar code labels associated with each receptacle and readable by a bar code
reader connected to the CPU 30. The bar codes can also be replaced by colour codes
or embossed coded surfaces readable by a feeler sensor.