Printer having drive and control system for metal band holding print elements

A printer of the type including a platen, a bank of hammers extending in a straight line along and spaced from the platen and a continuous metal band of magnetizable material on which are mounted a plurality of print elements is provided with drive means for moving the band between the platen and the hammer bank and control means for controlling the movement of the band so that the print elements on the band move along the line of hammers. The band drive means includes a drive wheel located near one end of the hammer bank and means for forcing the band into close contact with the peripheral surface of the drive wheel. The means for forcing the band into close contact with the peripheral surface of the drive wheel can be a permanent magnet member located on the surface of the drive wheel. The band movement control means includes guide means for guiding the band so that it extends along the hammer bank and permanent magnet means adapted to force the band into close contact with the guide means. The guide means can be a fixed guide member located near the end of the hammer bank remote from the drive wheel and the permanent magnet means can be a permanent magnet member located on the surface of the guide member and adapted to attract the band to the guide member.

FIELD OF THE INVENTION 
The present invention relates to printers of the kind which have a flexible 
metal band on which print elements are mounted and to systems for driving 
and controlling bands of this kind. 
BACKGROUND OF THE INVENTION 
In one known kind of printer, adapted to print on a print medium as it 
passes over a platen using a plurality of print elements operated 
selectively by hammers, the print elements are mounted on flexible fingers 
forming part of a metal band in the form of a continuous loop, one print 
element being mounted on each flexible finger. The print elements extend 
in a straight line along the band parallel to the longitudinal center line 
of the band. A bank of hammers extends along the platen and is spaced from 
the platen so as to define a print region between the hammer bank and the 
platen. The print medium extends through the print region across the 
platen so that the hammer bank extends across the width of the print 
medium. The metal band on which the print elements are mounted also 
extends through the print region along the platen and across the width of 
the print medium and is located between the hammer bank and the print 
medium. An ink ribbon also is located in the print region between the 
metal band and the print medium. 
The metal band is driven continuously along the platen past the hammer bank 
and across the print medium by a suitable drive system. Operation of any 
one of the hammers of the bank causes the hammer to move towards the metal 
band and to abut against one of the print elements so as to move this 
print element on its flexible finger towards the ink ribbon and the print 
medium. The print element in moving abuts against the ink ribbon and 
forces the ink ribbon into contact with the print medium causing the 
printing of a mark on the print medium of the shape of the print element. 
According to one particular printer of the above kind and illustrated by 
way of example in U.S. Pat. No. 4,428,284, each of the print elements is 
shaped like a dot and operation of each hammer causes the printing of a 
dot on the print medium. As the metal band moves continuously across the 
print medium operation of selected hammers will result in the printing of 
a line of dots in positions on the print medium corresponding to the 
positions of the hammers which are operated. Each hammer is formed with a 
head which has a width in the direction of movement of the band which is 
greater then the width of a single dot. It is therefore possible for each 
hammer to print a dot in any position on the print medium which is covered 
by the hammer by varying the timing of the operation of the hammer 
relative to the movement of the band. As a result each hammer can print a 
dot in a number of positions on the print medium. Therefore, the dots in 
the line printed can occupy many selected positions on the print medium. 
There is only a small gap between each pair of adjacent hammers and the 
hammers can print dots at all required positions along the line printed. 
After one line of dots has been printed the print medium can be moved 
through a small increment transversely to the length of the platen and the 
operation can be repeated resulting in the printing of a second line of 
dots below the first line of dots. By repeating these operations lines of 
dots can be printed as required. 
A character can be printed on the medium by printing dots in selected 
positions in a matrix, for example a matrix of 5 columns and 7 rows. By 
printing dots in selected positions in the lines as described above, 
characters can be printed in selected positions on the medium. In order 
for the characters to be printed correctly it is essential that the 
positions of the dots should be defined accurately. For this to happen it 
is essential that the metal band should move accurately relative to the 
platen and the bank of hammers so that the positions of the print elements 
can be accurately determined. It is therefore necessary to provide a 
suitable means for driving the band along the platen through the print 
region and to control the band as it is driven. 
It is known to drive the metal band by passing the band around the 
periphery of a single drive wheel located at one end of the hammer bank 
and to rotate the drive wheel by means of a suitable motor and thereby 
draw the band along the hammer bank towards the drive wheel. The band as 
it moves past the hammer bank will abut against and be guided by the 
hammer bank. Since the band is continuous in the form of a loop and has an 
inherent stiffness it will extend along the hammer bank from the end of 
the hammer bank at which the drive wheel is located to the other end of 
the hammer bank and will curve back on itself so that it extends back to 
the drive wheel without any further guiding means at the other end of the 
hammer bank. The band can be pressed against the surface of the drive 
wheel by means of a roller on the end of a pivoting arm which is biased 
towards the drive wheel. Such an arrangement is described in the above 
mentioned United States Patent. 
It is essential that the band moves evenly along the platen past the hammer 
bank. The contact between the band and the periphery of the drive wheel 
must be good so that the driving force provided by the rotation of the 
drive wheel is evenly imparted to the band. However, the frictional force 
exerted on the band by the roller pressing the band against the periphery 
of the drive wheel is not always satisfactory for this purpose. It is 
known to provide a drive wheel of the above type with a permanent magnet 
which attracts the band as it passes around the periphery of the wheel as 
an alternative to using the roller pressing the band against the periphery 
of the wheel. 
As an alternative to using only a single drive wheel for the band it is 
possible to provide in addition a freely rotating idler wheel at the end 
of the hammer bank remote from the drive wheel and to curve the band 
around the periphery of this idler wheel. By suitably positioning this 
idler wheel a tension can be exerted in the band. However it is essential 
that the axes of the drive wheel and the idler wheel should be accurately 
aligned in order to ensure that the band moves accurately and evenly past 
the hammer bank and to prevent stresses being exerted in the band and this 
alignment is not always easy to achieve. 
It is also essential that the band be prevented from flexing laterally so 
that it remains flat as it moves along the platen past the hammer bank. 
Without any additional guiding means located at the end of the hammer bank 
remote from the drive wheel there is no tension on the band and as a 
result the band tends to flex as it moves. It is known to guide the band 
as it passes along the hammer bank by suitable guide rollers located at 
the lower edge of the band and against which the band is pressed by means 
of a suitable arm forcing the band downwards towards the rollers. This 
reduces the tendency of the band to flex laterally. However with such an 
arrangement if the force is great enough to ensure good contact between 
the band and the rollers in order to reduce the tendency to flex laterally 
there is a possibility that the band will buckle. This will result in 
uneven movement of the band. 
It is also known to use permanent magnets to control other kinds of print 
element holders as they are moved relative to hammers, for example those 
kinds in which the print elements are connected together to form a chain. 
By way of example such an arrangement is described in U.S. Pat. No. 
3,435,756. 
The object of the present invention is to provide a printer of the type 
including a hammer bank and a metal band on which are mounted print 
elements having improved driving means for moving the band past the hammer 
bank and improved control means for controlling this movement. 
A further object of the invention is to ensure that the driving force is 
fully and evenly imparted to the band so that the movement of the band is 
even. 
A still further object of the invention is to provide control means which 
ensures that the print elements on the band, as the band passes the hammer 
bank, are correctly aligned with the hammers over the whole length of the 
hammer bank. 
SUMMARY OF THE INVENTION 
A drive means for the metal print element band of a printer of the above 
type includes a drive wheel located near one end of the hammer bank of the 
printer, means for rotating the drive wheel and means for forcing the band 
into close contact with the surface of the drive wheel so as to ensure 
that the driving force from the drive wheel is fully imparted to the band. 
The means for forcing the band into close contact with the surface of the 
drive wheel can be a permanent magnet member located on the surface of the 
drive wheel so as to attract the band to the wheel. 
A control means for controlling the movement of the metal print element 
band of the printer includes a guide means for guiding the band so that it 
extends along the hammer bank. Permanent magnet means, which may comprise 
a permanent magnet member located on the guide means, forces the band into 
close contact with the surface of the guide means so that the band is 
guided accurately by the guide means. The guide means may include a fixed 
guide member located at the end of the hammer bank remote from the drive 
wheel with a permanent magnet member on the surface of the guide member. 
The band is attracted by the permanent magnet member to the surface of the 
guide member. As a result the band is accurately guided by the guide 
member and a drag force is exerted on the band tending to tension the 
band. Consequentially the band is under tension as it moves past the 
hammer bank and therefore moves accurately in a straight line. As a result 
the print elements on the band are accurately aligned with the hammers 
along the whole length of the hammer bank. 
The guide means may also include one or more bearing members located 
adjacent to the hammer bank together with permanent magnet members which 
attract one edge of the band against the bearing members thereby ensuring 
the the band is guided accurately by the bearing members.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the printer includes a platen 1 and a bank of hammers 
2 mounted on the frame of the printer (represented diagrammatically at 3) 
and defining a print region 4 which extends between the platen and the 
hammer bank. A flexible metal band 5 in the form of a continuous loop and 
on which are mounted print elements is driven through the print region 4. 
The metal band 5 is made of a magnetizable material, for example 
magnetizable stainless steel and can be the material known by the Trade 
Name Carpenter Custom #455. The band 5 has a thickness of about 4/1000 
inch (0.1 millimeters) and a width of about 11/2 inches (4.00 
millimeters). The metal band 5 passes around the periphery of a drive 
wheel 6 located at one end of the print region and around a fixed guide 
member 7 located at the other end of the print region. 
A print medium 8 and an ink ribbon 9 extend between the platen 1 and the 
metal band 5. The print medium 8, which may be a continuous paper web, is 
driven over the surface of the platen 1 in a transverse direction 
intermittently by a print medium drive means illustrated diagrammatically 
at 11,12 in order to bring selected parts of the print medium into the 
print region. The ink ribbon 9 is driven continuously along the length of 
the platen 1 through the print region 4 between the band 5 and the medium 
8 by a ribbon drive means illustrated diagrammatically at 13,14. The print 
medium drive means 11,12 and the ribbon drive means 13,14 are not 
described in detail because they do not form part of the invention. 
On the periphery of the drive wheel 6 is formed a layer 15 of permanent 
magnet material covered with a layer 15a of polyurethane having a 
thickness of about 2/1000 inch (0.05 millimeters) and on the periphery of 
the guide member 7 is formed a similar layer 16 of permanent magnet 
material covered with a layer 16a of wear resisting material such as high 
molecular weight polyethylene. The permanent magnet layer 15 attracts the 
metal band 5 to the surface of the wheel 6 and the permanent magnet layer 
16 attracts the metal band 5 to the surface of the guide member 7. 
The drive wheel 6 is mounted on a shaft 17 which is rotated at a constant 
speed by a suitable motor 18 so that the drive wheel 6 rotates in an 
anticlockwise direction as seen in FIG. 1 as indicated by the arrow A. The 
attraction of the band 5 to the permanent magnet layer 15 and the 
relatively high friction contact provided by the polyurethane layer 15a 
over the permanent magnet layer ensures that the driving force provided by 
the driving wheel 6 is fully imparted to the band so that the band will be 
pulled evenly through the print region 4 between the platen 1 and the 
hammer mechanism 2. The attraction of the band 5 to the permanent magnet 
layer 16 causes a dragging or retarding force to be exerted on the band. 
This results in the band being tensioned and ensures that the band is flat 
as it passes through the print region 4. This dragging or retarding force 
will be controlled by the relatively low friction contact provided by the 
layer 16a of polyethylene over the permanent magnet layer 16. 
The printer also includes three bearing members 19, 20, 21 for the band 5. 
These bearing members are located respectively at the two ends of the 
print region and in the region of the path of the band from the drive 
member 6 to the guide member 7 and serve to support the lower edge of the 
band 5. One of these bearing members 19 is illustrated more clearly in 
FIG. 2. The bearing member 19 comprises a roller 23 mounted on an axle 24 
the two ends of which are supported in two side members 25 of a 
non-magnetizable material such as aluminum so that the axle 24 extends in 
a direction perpendicular to the plane of the metal band 5. These side 
members 25 are attached to a base member 26 of a magnetizable material 
such as iron. At each end of the base member 26 is located a permanent 
magnet member 27. These permanent magnet members 27 are magnetized so that 
they have poles on their ends and are positioned in the bearing member as 
illustrated so that one has a North pole at its upper end and the other 
has a South pole at its upper end. As a result a flux path is created 
which extends from one magnet 27, through the base member 26, through the 
other magnet 27 and across the gap between the upper pole faces of the 
magnets. The lower edge 28 of the band 5 is supported on the roller 23 so 
that the upper part of this flux path passes through the band. As a result 
the band is strongly attracted towards the roller 23 by the action of the 
permanent magnet members 27. The peripheral bearing surface of the roller 
23 extends parallel to the axle 24 and perpendicular to the plane of the 
band 5. 
A further guide 29 is provided for the band in the region of the path 
between the drive wheel 6 and the guide member 7. This consists of a 
channel member made of a suitable low friction material such as 
polyethylene and formed with a U-shaped channel 30 of width slightly 
greater then the thickness of the band 5. The lower edge 28 of the band 5 
extends along the U-shaped channel 30. 
FIG. 2 also illustrates the band 5 more clearly and is a view of the band 
from the platen 1. The arrow B indicates the direction of movement of the 
band. The band 5, as described above, is made from a thin magnetizable 
material and has holes 31 punched in it to form a plurality of chevron 
shaped fingers 32. At the apex of each chevron shaped finger 32 is 
attached a print element 33 made from stainless steel. As illustrated in 
FIG. 3 each print element 33 consists of a body portion 34 which projects 
through a suitable hole 35 formed in the finger 32. The body portion 34 is 
retained in the hole by a suitable collar 36 which is swaged in position. 
One end of the print element 33 is formed with a relatively large head 37 
and the other end of the print element 33 is formed with a relatively 
small head 38. The element 33 is mounted in the band 5 so that the larger 
head 37 is on the inside of the loop of the band and is therefore adapted 
to be struck by a hammer of the hammer mechanism 2 and the smaller head 38 
is on the outside of the loop of the band and is adapted to strike the ink 
ribbon 9 and cause a mark to be printed on the print medium 8 during 
operation of the printer. The smaller head 38 is shaped as a round dot so 
that in operation each print element will cause a small dot to be printed 
on the print medium 8. Each of the fingers 32 is attached at its two ends 
39 to the band 5. The rest of each finger is spaced from the band by the 
holes 31. 
The hammer bank 2 consists of a plurality of hammers 40 which can be of the 
type described in general terms in U.S. Pat. No. 4,428,284 and as 
illustrated in FIG. 4. Each hammer 40 consists of a resilient flexible arm 
41 attached at one end 42 to a hammer frame member 43 and formed at its 
other end with a hammer head 44. On the frame member 43 are mounted two 
permanent magnets 45, 46 and a pole piece 47 which produce a magnetic 
flux. This flux attracts the free end of the flexible arm 41 into a cocked 
position in which the arm rests against the pole piece 47 and is 
resiliently biased away from this cocked position by its natural 
resilience. A coil 48 surrounds the pole piece 47. When the coil 48 is 
energized it produces another magnetic flux which overcomes the magnetic 
flux produced by the permanent magnets 45, 46 and the pole piece 47 and 
releases the flexible arm 41 from its cocked position. The arm 41 moves 
away from the frame 43 and the hammer head 44 on the free end of the arm 
41 projects through a gap 49 in a guide member 50 attached to the hammer 
bank 2 and strikes the larger head 37 of a print element 33 which is in 
position adjacent to the hammer, as will be described in more detail 
below. The larger head 37 of the print element 33 and the hammer head 44 
on the flexible arm 41 are each made of a suitable material to be able to 
withstand the repeated impacts that occur during operation of the printer. 
The hammers 40 are mounted on a common frame 51 which extends along the 
platen 1 and all the hammer heads 44 are aligned on a line extending along 
the platen. The frame 51 supports the guide member 50 and also includes 
connections to drive circuits for the coils 48 of the hammers which are 
located elsewhere in the printer. The operation of the hammers is 
described in detail in U.S. Pat. No. 4,428,284 and will not be described 
in detail here since it does not form part of the invention. 
The shape of each finger 32 on the band 5 is chosen so that each finger has 
the required flexibility to ensure that when each print element 33 is 
struck by a hammer head 44 it moves accurately at right angles to the band 
and strikes the ink ribbon cleanly. The fingers 32 normally lie in the 
plane of the band 5. If a print element 33 is struck by a hammer head 44 
in the hammer bank 2 the element 33 and the associated finger 32 will move 
out of the plane of the band in order to move into contact with the ink 
ribbon. All the elements 33 are aligned on a line extending along the 
length of the band 5 and parallel to the line of the hammer heads 44. 
As illustrated in FIG. 2, adjacent to the lower edge 28 of the band 5 and 
extending along the length of the band are formed two sets of holes. The 
upper set consists of a plurality of holes 51 each of which corresponds to 
one of the fingers 32 so that the holes of this set extend along the band 
at the same distance apart as the distance between adjacent print elements 
33. The lower set of holes consists of a plurality of holes 52 spaced 
apart by a distance of approximately 1/50 inch (0.5 millimeters). A group 
of 18 holes 52 is provided for each of the upper holes 51. These two sets 
of holes are used for controlling the operation of the hammers 40 as 
described below. 
The larger heads 37 of the print elements 33 project from the inside of the 
band 5. The band passes around the periphery of the drive wheel 6 and over 
the surface of the guide member 7. As described above the surfaces of the 
drive wheel 6 and the guide member 7 are formed with permanent magnet 
layers 15, 16 respectively covered with outer layers 15a, 16a 
respectively. These layers must be formed with a channel at least as wide 
and as deep as the width and height of the larger head 37 of each print 
element 33 in order to accommodate these larger heads and to allow the 
band to lie in close contact with the surfaces of the outer layers of the 
drive wheel 6 and the guide member 7. FIG. 5 illustrates this central 
channel 53 in the layers 15, 15a covering the surfaces of the drive wheel 
6 and how it accommodates the larger head 37 of a print element 33. It 
will be appreciated that the layers over the guide member 7 are formed 
with a similar channel 53. 
The action of the permanent magnet layer 15 on the surface of the drive 
wheel 6 ensures close contact between the band 5 and the drive wheel so 
that the band moves in synchronism with the rotation of the drive wheel 
without any slip. The action of the permanent magnet layer 16 on the 
surface of the guide member 7 increases the frictional force between the 
band 5 and the guide member 7. This results in the exertion of a dragging 
or retarding force on the band which tensions the band as it moves and 
tends to ensure that the band is flat as it passes along the platen 
through the print region of the printer past the hammer bank 2. This 
dragging force on its own is not always adequate to ensure that the band 5 
is flat as it passes the hammer bank 2 and in order to provide another 
force to keep the band flat the printer includes another permanent magnet 
device 61 which extends along the surface of the guide member 50 of the 
hammer bank 2 adjacent to the band 5. 
This permanent magnet device 61 is illustrated in FIGS. 4, 6 and 7. As will 
be seen in FIG. 6 the device 61 is formed from two identical members 61a, 
61b which extend respectively one on each side of the gap 49 in the guide 
member 50. Each member 61a, 61b consists of a plurality of strips 62 of 
permanent magnet material placed side by side and separated by strips 63 
of non-magnetic material. Each permanent magnet strip 62 is magnetized so 
that one surface exhibits a North pole along its full length and the other 
surface exhibits a South pole along its full length. The strips 62, 63 are 
assembled together so that adjacent permanent magnet strips on each side 
of each of the members 61a, 61b exhibit different poles as illustrated in 
FIG. 6. The permanent magnet strips 62 are all the same width with 
opposite sides being parallel and the non-magnetic strips 63 are also all 
the same width with opposite sides being parallel so that the permanent 
magnet strips 62 extend parallel along the length of each member 61a,61b. 
Flux paths are formed as indicated in FIG. 7 extending from the surface of 
each strip 62 to the surface of each adjacent strip 62 exhibiting the 
opposite polarity. One surface of each of the members 61a, 61b is covered 
with an outer wear resisting protective layer 64 of high molecular weight 
polyethylene. The thickness of each permanent magnet member 61a, 61b is of 
the order of 1.25 millimeters and the thickness of each outer protective 
layer 64 is of the order of 0.6 millimeters. 
The members 61a, 61b are attached by any suitable means, for example an 
adhesive, to the surface of the guide member 50 which is adjacent the band 
5 so that the outer layer 64 of each member faces the metal band 5. The 
members 61a,61b are oriented on the guide member 50 so that the strips of 
permanent magnet material 62 extend at an angle to the direction of 
movement of the band 5 past the hammer bank which is represented by the 
arrow C in FIG. 6. As a result the flux paths between the surfaces of 
adjacent strips 6 extend at an angle of less than a right angle to the 
direction of movement of the band 5. 
It is found that, as the band 5 moves past the members 61a,61b in the 
direction of the arrow C, it experiences a downward force in the direction 
of the arrow D (FIG. 6) at right angles to its direction of movement as 
well as a normal sideways force towards the members 61a,61b. The reason 
for the occurrence of this downward force is not clearly understood but it 
is believed to result from the interaction between the edges of the slots 
31 in the band 5 and the flux paths between the surfaces of the strips 62 
illustrated in FIG. 7. It is found that the downward force is reduced if a 
band without any slots 31 is moved over the members 61a,61b and the 
downward force is completely eliminated if the strips 62 are aligned 
accurately parallel or perpendicular to the direction of movement of the 
band 5. 
The permanent magnet device 61 serves three purposes. Firstly, the device 
61 exerts a force sideways on the band 5 which attracts the band onto the 
surface of the guide member 50 and therefore ensures that the band is flat 
as it passes the hammer bank 2. Secondly, the attraction of the band 5 to 
the guide member 50 causes a dragging or retarding force to be exerted on 
the band thereby tensioning the band. Thirdly, the device 61 exerts a 
force downwards on the band in the direction of the arrow D which tends to 
press the lower edge 28 of the band into closer contact with the bearing 
members 19, 20 located at the ends of the hammer bank 2. 
Each hammer 40 includes two permanent magnets 45, 46 and these magnets will 
exert a further sideways force on the band 5 attracting the band towards 
the guide member 50, supplementing the sideways force exerted by the 
device 61. 
The permanent magnet layer 16 on the surface of the guide member 7 may also 
be constructed with permanent magnet strips 62 in the same way as device 
61. If the magnet strips are arranged in parallel lines which are inclined 
to the direction of movement of the band 5, a downward force will be 
exerted on the band as it moves past the guide member 7 and this will 
further tend to press the edge 28 of the band against the adjacent bearing 
member 19. 
In order to provide a means for indicating the position of each of the 
print elements 33 as the band 5 moves through the print region and to 
provide signals for controlling the operation of the hammers 40, an 
emitter device 71 is located in a hole 72 extending within the guide 
member 7 as illustrated diagrammatically in FIG. 1. The emitter device 71 
is illustrated diagrammatically in FIG. 8 and comprises a light source 73 
located inside the loop of the band 5 and two light sensors 74, 75 located 
one above each other outside the loop of the band. The source 73 and the 
sensors 74, 75 are aligned respectively with the two sets of holes 51, 52 
near the lower edge 28 of the band 5 so that light from the source 73 will 
pass through any hole 51, 52 which is aligned with the source and will be 
received by the upper sensor 74 if it passes through the one of the upper 
set of holes 51 and by the lower sensor 75 if it passes through one of the 
lower set of holes 52. As the band moves, successive holes 51,52 will 
allow pulses of light to be received by the sensors 74, 75 which will 
produce corresponding electrical pulses. These electrical pulses can be 
used as timing pulses to control the operation of the hammers 40. 
When any one of the fingers 32 on the band 5 is moved towards the ink 
ribbon 9 by the action of one of the hammer heads 44 striking the print 
element 33 on that finger during the operation of the printer there is a 
possibility that the ink ribbon 9 will remain in contact with the finger 
32. As a result, when the finger 32 moves back into the plane of the band 
5 it may move the ribbon towards the band. FIG. 9 illustrates a further 
device 81 which is used to ensure that the ink ribbon 9 does not continue 
to adhere to the fingers 32 as the band 5 starts to pass onto the surface 
of the drive wheel 6. The device 81 comprises a permanent magnet member 82 
mounted on an extension piece 83 attached to the end of the hammer bank 2. 
The surface of the permanent magnet member 82 is located adjacent to the 
inner surface of the band 5. The permanent magnet member 82 attracts the 
fingers 32 out of the plane of the band 5 onto the side of the band remote 
from the ink ribbon 9 so that if the ribbon is attached to any finger 32 
it will abut against the surface of the band 5 and be removed from the 
finger and will return to its normal position. This ensures that the ink 
ribbon 9 does not remain adhering to the band 5 as the band starts to move 
round the drive wheel 6. The permanent magnet member 82 can be an 
extension of the permanent magnet member 61 and needs to be formed with a 
channel similar to the channel 53 in the surface of the permanent magnet 
layer 15 on the surface of the drive wheel 6 (see FIG. 5). The outer 
surface of the member 82 can be covered with a layer of high molecular 
weight polyethylene. 
In operation the printer is provided with printing data to control the 
printing of characters on the print medium 8. The band 5 is driven and 
controlled by the mechanism described so that it moves evenly through the 
print region 4. The emitter device 71 produces two set of timing pulses in 
synchronism with the movement of the band 5, one set being used to 
indicate the positions of the print elements 33 relative to the hammers 40 
and the other set being used to control the operation of the hammers. 
Printing takes place by selected hammer heads 44 in the hammer bank 2 
moving through the gap 45 in the guide member 50 into contact with 
selected print elements 33. These elements abut against the ribbon 9 and 
press the ribbon 9 into contact with the print medium 8 resulting in the 
printing of dots on the print medium 8 along the line of the hammer heads 
44. By suitably moving the print medium 8 and operating the hammers 40 
dots will be printed in the correct matrix positions to result in the 
printing of characters on the medium 8. Full details of the print 
operation of the printer will not be included here because they do not 
form part of the invention. 
The permanent magnet layer 15 on the surface of the drive wheel 6, the 
permanent magnet layer 16 on the surface of the guide member 7, the 
permanent magnets 27 in the bearing members 19, 20, 21, the permanent 
magnet device 61 on the surface of the guide member 50 on the frame of the 
hammer bank 2 and the permanent magnet member 82 all combine to provide a 
drive and control system for the band 5 which tends to ensure that the 
band is driven evenly through the print region 4 of the printer past the 
hammer bank 2 and is in the correct position relative to the hammer bank 2 
as it moves through the print region so that the print elements on the 
band are accurately aligned with the hammers in the hammer bank along the 
whole length of the hammer bank. 
The permanent magnet layer 15 on the surface of the drive wheel 6 ensures 
that the band 5 remains in close contact with the surface of the drive 
wheel 6. As a result the driving force exerted by the drive wheel is fully 
imparted to the band and therefore the band moves evenly. 
The permanent magnet layer 16 on the surface of the guide member 7 ensures 
that the band 5 remains in close contact with the guide member 7. As a 
result a dragging or retarding force is exerted on the band 5 which tends 
to tension the band and ensure that the band is flat as it passes the 
hammer bank. If this layer 16 is constructed in the same way as the 
permanent magnet device 16 it will also exert a downward force on the band 
5 pressing the lower edge 28 of the band against the bearing member 19. 
The permanent magnets 27 in the bearing members 19, 20, 21 exert a force on 
the band which presses the lower edge 28 of the band into close contact 
with the rollers 23 of the bearing members. As a result the band is 
correctly aligned transversely relative to the hammer bank 2 as it passes 
through the print region. 
The permanent magnet device 61 on the surface of the guide member 50 on the 
frame of the hammer bank 2 exerts three forces on the band 5. The first 
sideways force results in the band 5 being brought into close contact with 
the guide member 50 and therefore being correctly aligned relative to the 
hammer bank in one direction as it passes through the print region and 
causes a second dragging or retarding force to be exerted on the band 
thereby tensioning the band 5. The third downward force results in the 
band being forced downward onto the bearing members 19, 20 so that band 5 
is aligned correctly relative to the hammer bank in another direction as 
it passes through the print region. 
The permanent magnet member 82 ensures that the ribbon 9 does not remain 
adhering to the fingers 32. It therefore reduces the possibility of the 
ribbon being damaged and ensures that the band 5 will pass smoothly to the 
drive wheel 6.