Carriage positioning system

A system for positioning an X-axis carriage and a Y-axis carriage comprises two flexible coupling elements for the X-axis carriage, and a single flexible coupling element for the Y-axis carriage. Providing two such coupling elements for the X-axis carriage enables shorter-length elements to be used, thereby providing higher dimensional stability under load or temperature variations, and also higher-load capabilities than when a single flexible coupling element is used for the respective carriage. Further, several preferred embodiments are described wherein each of the flexible coupling elements occupies a single plane, thereby enabling belts, rather than cables, to be used, further enhancing the dimensional stability and strength of the system. Still further, in all the described arrangements, the transmission ratio of the two X-axis coupling elements and of the single Y-axis coupling element are all the same, thereby obviating the need for different transmission elements to equalize the transmission ratios.

BACKGROUND OF THE INVENTION 
The present invention relates to a carriage positioning system. The 
invention is particularly useful in a system for positioning a carriage 
along either the X- or Y-axis, and is therefore described below 
particularly with respect to such a system. 
Two-axis carriage positioning systems are widely used in a number of 
applications where orthogonal positioning is required, for example in data 
plotters for plotting curves or preparing drawings. As a rule, the 
carriages are driven to their specified positions by X-axis and Y-axis 
drive motors. Other systems are known wherein the carriages are manually 
driven but are coupled to shaft encoders to read-out their positions. The 
coupling between the respective motors or encoders and carriages is 
usually effected by flexible coupling elements, such as cables, belts or 
chains. Usually a single cable, belt or chain is trained over a plurality 
of pulleys for driving the X-axis carriage; and a separate cable, belt or 
chain is trained over another group of pulleys for driving the Y-axis 
carriage. 
In the known systems using a single flexible coupling element (e.g., cable, 
belt or chain) for the X-axis carriage, the coupling element must 
necessarily be very long, and therefore dimensional changes in the element 
due to strain or temperature-variations greatly affect the precision of 
the system in positioning the respective carriage. In addition, the use of 
a single such flexible coupling element for the X-axis places a great load 
on the couling element which tends to increase dimensional changes due to 
strain, and also limits the load capability of the positioning system. 
Further, many of the known systems require the flexible coupling elements 
to change planes, and therefore, are either restricted to the use of 
cables, or include complicated constructions to accommodate the change in 
planes. Still further, the pulley arrangement in many of the known systems 
produce a transmission ratio or the X-axis coupling element which is not 
identical to that of the Y-axis coupling element, and therefore require 
different gearing or the like in order to accommodate or equate the 
different transmission ratios. 
BRIEF SUMMARY OF THE PRESENT INVENTION 
An object of the present invention is to provide a carriage positioning 
system having advantages in one or all of the above respects. 
According to a broad aspect of the present invention, there is provided a 
carriage positioning system including two flexible coupling elements each 
coupling one end of the carriage for movement with respect to the frame. 
Each of the two flexible coupling elements has its opposite ends fixed to 
the frame on the opposite sides of the respective end of the carriage and 
is trained over a pulley carried by the carriage on each of the two 
opposite sides of the carriage, and over a further pulley carried by the 
frame on each of the two opposite sides of the carriage. Each of the 
flexible coupling elements is coupled to a member (e.g. motor) for driving 
the carriage or to a member (e.g. encoder) for being driven by the 
carriage. 
In the preferred embodiments of the invention described below, the 
positioning system further includes a second carriage (e.g. the Y-axis 
carriage) carried by and movable with respect to said first-mentioned 
carriage (e.g., the X-axis carriage) along an axis perpendicular to the 
first carriage. The second carriage includes a single flexible coupling 
element fixed at its two ends to the frame at one side of the two 
carriages, which coupling element is trained over two pulleys carried by 
each end of the first-mentioned (e.g. X-axis) carriage, a further pulley 
carried by each end of the second (e.g. Y-axis) carriage between the 
associated last mentioned two pulleys, and a still further pulley carried 
by the frame at the side thereof opposite to that to which the two ends of 
the single flexible coupling elements are fixed. The single flexible 
coupling element is also coupled to a member (e.g. motor or encoder) for 
driving or for being driven thereby. 
Several embodiments of the invention are described below, wherein the 
complete length of each of the flexible coupling elements occupies a 
single plane. In such embodiments, the flexible coupling elements may each 
be in the form of a belt, which is far preferable in many applications 
over a cable (normally required in arrangements involving a change in 
plane) because of the dimensional integrity and the strength of belts. 
The flexible coupling elements in most applications would normally be 
coupled to X-axis and Y-axis motors for driving the carriages along their 
respective axes according to the positional data supplied to the motors. 
However, there are many applications wherein the carriages are manually, 
or otherwise, driven, and the flexible coupling elements are coupled to 
shaft encoders for reading-out the positions of the carriages. 
Further features and advantages of the invention will be apparent from the 
description below.

DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference first to FIG. 1, there is illustrated one form of two axes 
positioning system, which may be a plotter or the like for moving pen, 
paper, or other device with respect to a frame, schematically designated 
2. Thus, the system illustrated in FIG. 1 includes an X-carriage, 
generally designated 10, movable along the X-axis by a first flexible 
coupling element 20 at one end of the carriage driven by an X-axis motor 
30. The X-axis carriage 10 is also driven by a second flexible coupling 
element 40 coupled to the opposite end of the carriage and driven by a 
second X-axis motor 50. 
Mounted on the X-axis carriage 10 is a Y-axis carriage 60 which is driven 
along the Y-axis by a flexible coupling element 70 driven by a Y-axis 
motor 80. It will thus be seen that by controlling the two X-axis motors 
30 and 50 synchronously, and the Y-axis motor 80, carriage 60 may be 
positioned at any desired point along the X-axis and Y-axis. 
Because each of the flexible coupling elements 20, 40 and 70 in FIG. 1 
occupies a single plane (i.e., none requires a change in planes) they are 
all shown as belts, which is an important advantage of the illustrated 
arrangement because of the strength and dimensional stability provided by 
belts, as distinguished from cables for example. 
With respect to belt 20, it will be seen that its opposite ends are fixed 
to the frame 2, and the intermediate portion of the belt is trained over a 
pair of pulleys 11, 12 carried at one end of the X-axis carriage 10, and 
also over a pair of pulleys 13, 14 carried by the frame 2. All the pulleys 
11, 12, 13 and 14, as well as the complete length of belt 20, are in a 
common plane, which is perpendicular to the X-Y plane of movement of the 
two carriages 10 and 60. Motor 30 is coupled to belt 20 in any suitable 
manner, the coupling being schematically illustrated in FIG. 1 as 
including a drive wheel 31 coupled to the motor shaft and in turn driving, 
via belt 32, another wheel 33 coupled by shaft 34 to pulley 13 for belt 
20. 
Belt 40 at the other side of the X-axis carriage 10 is driven in a similar 
manner by its motor 50, namely via pulleys 15 and 16 carried by that end 
of the carriage, and pulleys 17 and 18 carried by the frame 2. It will be 
noted that all of these pulleys are in the same plane, parallel to the 
plane of pulleys 11-14 at the opposite end of the X-carriage 10, and 
perpendicular to the X-Y plane of movement of the carriages. 
The Y-carriage 60 is mounted for movement on the X-axis carriage 10, and is 
moved by a single belt 70 along the Y-axis. Thus, belt 70 is fixed at its 
opposite ends to frame 2 at one side of the two carriages. The 
intermediate portion of belt 70 is trained over a series of pulleys, 
including pulley 71 carried by the X-carriage 10, pulley 72 carried by the 
Y-axis carriage 60, pulley 73 carried by the Y-axis carriage 10, pulleys 
74 and 75 carried by the fixed frame 2, and finally pulleys 76, 77 and 78 
corresponding to pulleys 71, 72 and 73 at the opposite end of the Y-axis 
carriage 60. 
The two X-axes motors 30 and 50 would have to be synchronously operated. 
For this purpose, FIG. 1 illustrates a control, schematically indicated by 
block 52, for operating the two motors 30 and 50 in synchronism. 
The Y-axis motor 80 for driving the Y-axis belt 70 may be coupled to the 
latter cable in any suitable manner. This coupling is schematically 
illustrated in FIG. 1 as including a wheel 81 driven by the Y-axis motor 
80 via a belt 82, which wheel 81 is coupled via shaft 83 to pulley 74 on 
which is trained the Y-axis cable 70. 
The provision of two belts (or other flexible coupling elements) 20 and 40, 
respectively, for driving the X-axis carriage 10 provides a number of 
important advantages. First, it substantially shortens the length of the 
X-axis belts, which would otherwise be required if a single common belt 
were used, and therefore enables a high degree of dimensional stability to 
be maintained even under high load or temperature variations. Further, by 
dividing the load for moving the X-axis carriage 10 between two belts, 
each driven by a separate motor, the load-capability of the system is 
substantially increased. Further, since the two X-axis belts 20 and 40, 
and their respective pulleys, are each in common planes (perpendicular to 
the X-Y plane) this arrangement permits the use of belts, rather than 
cables, thereby further increasing the dimensional stability and the load 
capability of the system. Finally, the transmission ratio of the two 
X-axis belts 20 and 40, and of the Y-axis belt 70, are all the same, 
thereby obviating the need for different transmission elements to equalize 
the transmission ratios. 
FIG. 2 illustrates an arrangement similar to that of FIG. 1, except that 
instead of providing two separate motors 30, 50 synchronously driven by 
control unit 52, for driving the two X-axes belts 20 and 40, a single 
X-axis motor is used. Such a motor is shown at 130 in FIG. 2, wherein it 
will be seen that it drives a shaft 134, coupling the two pulleys 113 and 
117, of the two X-axis drive belts 120 and 140, corresponding to elements 
13, 17, 20 and 40 in FIG. 1. The X-axis drive belts 120 and 140 are 
otherwise the same as in FIG. 1 for driving the X-axis carriage 110. 
The Y-axis carriage 160 in FIG. 2 is driven in the same manner as in FIG. 1 
by a single Y-axis flexible coupling belt 170 driven by a Y-axis motor 180 
via a wheel 181 and another belt 182. 
The arrangement illustrated in FIG. 3 is similar to that of FIG. 2, in that 
it also includes a single X-axis motor 230 driving two X-axis flexible 
coupling elements 220 and 240 of the X-axis carriage 210, except that in 
the FIG. 3 arrangement, all the flexible coupling elements are cables, and 
the coupling of the X-axis drive motor is somewhat different than in FIG. 
2. Thus, in FIG. 3 the two X-axis coupling elements 220, 240 are cables, 
rather than belts because they change planes. Thus, the end portion of the 
cables trained over the two end-face pulleys (e.g., 211, 212) are in the 
same plane (perpendicular to the X-Y plane) as in FIGS. 1 and 2; but the 
intermediate portions of the cables are trained over the two pulleys 213 
and 214 carried by the frame 202 and are disposed in the X-Y plane, i.e., 
perpendicular to the plane of pulleys 211 and 212. In addition, two 
further pulleys 215 and 216 are provided on the frame 202, also in the 
same plane as pulleys 213 and 214, for engaging the intermediate portion 
of the X-axis cable 220. The other X-axis cable 240 is similarly disposed 
at the opposite side of the X-axis carriage 210. 
Both of the X-axis cables 220, 240 are driven by a single X-axis motor 230 
via a drum 231 engaging the intermediate portions of the cables. 
The Y-axis carriage 260 is driven by means of a single Y-axis cable 270 
which in turn is driven by a Y-axis motor 280 via a drum 282 engaging the 
Y-axis cable 270. 
The arrangement illustrated in FIG. 4 is very similar to that of FIG. 3 
except that the two X-axis flexible coupling elements 320, 340 are 
disposed so as to occupy a single plane, namely the X-Y plane, and 
therefore, although shown as cables, they could also be belts. In this 
case, the two pulleys 311, 312 carried by the X-axis carriage 310 are 
mounted on the underface of the carriage so as to be disposed in the X-Y 
plane, which is the same plane as occupied by the four pulleys 313, 314, 
315 and 316 carried by the frame 302. 
The Y-axis carriage 360 in the FIG. 4 arrangement is driven in exactly the 
same manner as in FIG. 3, namely by the Y-axis motor 380 driving drum 382 
coupled to the single Y-axis cable or belt 370. 
It will be seen that in all the above-described embodiments, two X-axis 
belts (or cables, viz FIG. 3) are used for driving the X-axis carriage, 
thereby enabling each such belts (or cables) to be relatively short. As 
described above, this maintains the dimensions of the belt under stress 
and varying temperature conditions and increases the load capable of being 
moved. In addition, in all of the disclosed embodiments, except that of 
FIG. 3, the X-axis cables or belts occupy a single plane, thereby 
permitting the use of belts, which provide even greater dimensional 
stability and strength than cables. Further, in all the disclosed 
arrangements the transmission ratio between the X-axis carriage and its 
drive motor (or motors) is the same as between the Y-axis carriage and its 
drive motor, thereby obviating the need for different 
transmission-ratio-changing mechanism for the two axes. 
While the invention has been described with respect to four preferred 
embodiments, it will be appreciated that many variations, modifications 
and other applications of the invention may be made. For example, instead 
of motors driving the belts (or cables), there could be used encoders 
which read-out the position of the carriages when driven manually or 
otherwise. Many other variations, modifications and applications of the 
invention will be apparent.