Field of the Invention;

This invention relates to linear actuators. The invention is particularly useful in positioning the print head mechanism of a high speed printer although it would also have utility in many other situations wherein linear, or even rotary, movement is required. Description of the Prior Art;

Electromagnetic actuators capable of providing a direct linear output movement have been known for many years; however, the commercial applications of such actuators have been restricted because of various inherent deficiencies. Typically, where linear movement is re¬ quired, standard rotary electromagnetic motors are used in combination with mechanical devices which convert the rotary motion of the prime mover to linear motion. For example, in the case of modern high speed printers, a carriage containing a printer head is moved rapidly back and forth by a rotary motor and a belt, pulley or lead screw mechanism.

There are advantages in using linear actuators rather than rotary motors to drive linearly movable devices such as printer heads. In the first place, a linear actuator typically has fewer moving parts since no devices are required to convert rotary motion to linear motion. Secojdly, the linear actuator can inherently provide a higher rate of movement (and thus a higher printing rate) since there is no need to convert from rotary to linear motion. In cases where it may not be necessary to drive the actuated device at a constant speed, less power may be required in the case of a linear actuator since a sinusoidal input signal can be used.

Known linear actuators comprise solenoid-type devices and so-called "voice coil" actuators. In a solenoid-type device, a travelling magnetic field "pulls" an armature from one end of a magnet to the other. These travelling magnetic field actuators require relatively

complex construction and control circuitry arrangements, and constant velocity is difficult to achieve.

"Voice coil" actuators have gained acceptance in the computer field for positioning magnetic heads relative to rotatable storage discs. These devices, as shown in Gillum Patent No. 3,723,780, may comprise a cylindrical magnet establishing a radial magnetic field to a central (axial) core. An armature, comprising coils enveloping the core, moves linearly when a current is passed through it.

In conventional "voice coil" type linear actu¬ ators, the force applied to the armature (for a given armature current) varies as the armature moves relative to the magnet. This is because the flux in the core due to the armature current tends to aid the field of the magnet at one end of its travel and and to oppose the field of the magnet at the other end of its travel. This change in force causes a variation in volocity as a function of armature position; as a result, the use of such actuators has been limited to short stroke operations in which the armature is moved a distance less than its length.

Griffing Patent No. 3,899,699 discloses a linear actuator intended to overcome at least in part the foregoing deficiencies of voice coil-type actuators. In Griffing, a movable magnetic return path is incorporated in the armature so that the flux strength remains rela¬ tively constant over the full length of travel of the armature. However, the incorporation of the magnetic return path into the coil construction requires a cumber- some mechanical configuration, the weight of which tends partially to lessen the inherent advantages of a voice coil-type linear actuator. Objects of the Invention;

A principal object of this invention is to provide a linear actuator which can provide relatively long linear movements without incorporating into the armature heavy ferrous members to complete the return path.

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A more specific object is to provide a linear actuator of particular utility as the driving means for a printer head assembly in a high speed printer.

Another object of the invention is to provide a linear actuator wherein two or more armatures cooperating with a single stator construction may be independently controlled.

A still further object is to provide a high speed printer having two or more printer head assemblies each of which may be independently controlled by means of a single actuator. Summary of the Invention

Briefly, in accordance with the invention, a linear actuator comprises an elongated magnet, C-shaped in crosssection, through which a ferrous core extends axially. A magnetic path between the magnet and core is completed by end pieces at the respective ends of the assembly. An armature enveloping the core moves linearly in the air gap between the magnet and core with the armature being movable a distance which is large relative to its width. For example, if the linear actuator is used to drive the print head assembly of a high speed printer, the magnet and core may extend the width of the printer with the print head assembly (e.g., matrix or daisy wheel) physi- cally secured to the armature so that it can be positioned at any point across the paper. If desired, more than one armature may be associated with a single magnet and core. The Drawings

Figure 1 is a perspective view of part of a printer showing how the linear actuator of the invention might be used with two independent print head assemblies; Figure 2 is a cross-sectional side view of the linear actuator and one of the print head assemblies; Figure 3 is a cross-sectional view of a pre- ferred armature construction;

Figure 4 is a diagram used for explanatory purposes showing the flux fields which exist in the actuator;

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-4- Figure 5 is a block diagram of the control system which actuates the print head mechanism; and

Figure 6 is a block diagram of a preferred velocity control system. Detailed Description

A linear actuator in accordance with the inven¬ tion would have utility in many situations where linear motion is required or desirable. It may even be used to provide rotary motion. In explaining the construction and operation of the invention, a preferred embodiment is described in combination with a high speed printer and, in particular, a standard matrixtype printer. The invention, in its basic form, is not restricted to use with a printer or any other particular mechanism. Referring now to Figs. 1 and 2, the printer may include a platen 14 (not shown in Fig. 1 for purposes of clarity) suitably supported within the printer frame (not shown). A pair of standard matrix heads 16 are supported on respective printer carriages 18 which move laterally on two guide rods 20 and 22, each of which is supported at its opposite ends in the printer frame. The printer heads 16, their carriages 18, and the respective driving mechanisms are shown as being identical for simplicity of explanation; however, the print heads may be of different constructions. For example, one may be a "matrix" type and the other a "daisy" wheel. Other types may also be used and more than two heads (as shown) may be controlled. A printing ribbon 24 (shown diagrammatically in Fig. 1 in phantom lines) passes between the platen 14 and the forward ends of matrix heads 16 to enable imprinting on the paper. The ribbon 24 may be supported and moved by conventional means.

The printer operates by moving the print head carriages 18 from left to right (or right to left) with matrix heads 16 being actuated at appropriate intervals to cause selected characters to be printed on paper (not shown) which is fed by conventional means past platen 14.

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An optical encoder strip 19 extends across the printer for the purpose of encoding the printer head position used to actuate the printer head. This printing operation is described in further detail below with reference to Fig. 5. In accordance with the invention, a linear actuator comprises a cylindrically shaped magnet 34, C-shaped in cross-section, enveloping a ferrous core 36. The magnet 34 and core 36 extend substantially across the entire width of the printer with the magnetic circuit ^" being closed at opposite ends by annular end pieces 38 and 40. In accordance with the preferred embodiment, magnet 34 comprises a plurality of axially aligned magnets 34A, 34B, 34C, etc. and each of these magnets, in turn, com¬ prises complementary half sections. For example, the magnet 34A includes upper half sections 34A ¹ and lower half sections 34A' ^• (Fig. 2).

The entire assembly as so far described is retained within an outer ferrous shell 42 which also consists of upper and lower sections 42A and 42B as shown in Fig. 2 secured together by bolt 43 and nut 45. The fact that the magnet 34 and shell 42 comprise complemen¬ tary sections is important from a production viewpoint since each half section (top and bottom) may be assembled without great difficulty and the two sections bolted together. This minimizes problems caused by the forces of the magnets which otherwise would make manufacture very difficult. Moreover, the preferred construction facilitates production of the relatively simpler ferrous "halves" 42A and 42B which (for example) may be stampings of constant cross-section or cold-drawn steel sections.

The C-shaped permanent magnets 34A, B, C etc. may be made of barium ferrite or other suitable material having polarity as indicated in Fig. 4. The core 36, annular members 38 and 40, and shell 42 may be made of soft steel. The actual cross-sectional shape of these parts is not critical although for obvious reasons a cylindrical shape is preferred. The term "C-shaped" as

used herein is only intended to indicate that the magnet includes an opening or slot and not that it be shaped in a particular way.

An armature 60, comprising a series of turns of insulated copper wire 60A wound on a bobbin 60B (Fig. 3), preferably made of a light plastic material, is adapted to move in the air gap between the core 36 and magnet 34. The armature is secured to a web member 61 which extends through the opening in the magnet 34 to mechanically couple the armature to the mechanism to be driven. In this case, web 61 is secured to the printer head carriages 18 by fasteners 62. The armature is attached to web member 61 by an adhesive having a high thermoconductivity such as ECCOBOND 281. This enables the conduction of heat directly from the copper windings of the armature to the web member for dissipation within the body of the printer. It is desirable that the web member 61 be made of a light metal having good thermoconductivity, such as aluminum or magnesium. A major benefit of the invention is that the armature construction, which is capable of moving the entire length of the printer, includes no heavy ferrous parts and, therefore, is capable of quick starting and stopping motions as required to drive a printer head assembly. The carriage 18 includes bearings 63 and 64 which ride on guide rails 20 and 22, respectively; fric¬ tion should be minimized. The guide rails 20 and 22 also serve the important function of supporting the armature in a fixed position within the air gap between magnet 34 and core 36 for the full length of travel.

A small light source 65 and a photosensor 66 may be mounted on a base 67 attached to the bottom of carriage 18 with optical encoding strip 19 passing between the light source 65 and photosensor. As explained below, the photosensor 66 generates a series of pulses as the printer head moves relative to the stationary strip 19. These pulses are used to control the printer head speed and to actuate the printer head at the correct positions.

The operation of the linear actuator is explain¬ ed with reference to Fig. 4. Each magnet 34A, B, etc. produces a magnetic flux field which crosses the air gap and returns to the magnet through the core 36, end piece 38 or 40, and shell 42. Since the flux tends to take the shortest path, the predominant flux paths 70 for the "left" hand magnets is opposite the predominant flux paths 72 for the "right" hand magnets.

If a current is fed to armature coil 60A, the coil will develop a magnetic field which interacts with the stator field and causes the armature to move, for example, from right to left. If the armature current is reversed, the armature will then move from left to right because the magnetic field caused by the armature current is also reversed.

The reason for the non-linear force vs. distance relationship is explained with reference to Fig. 4 wherein an armature coil 60A is shown in right and left hand positions. If the current direction is as shown (clock- wise, if viewed from the right hand side), the typical flux path for the coil will be as shown at 74 and 76 at the right side, and 78 and 80 at the left side. The flux 74, 76 (right) is in opposition to the predominant stator flux 72; the flux 78, 80 (left) aids stator flux 70, tend- ing to saturate the left side of the actuator. Hence, the force applied to the armature on the right side of the magnet is stronger than the force applied on the left side and the armature therefore tends to slow down as it moves from right to left. If the current flow in the armature is reversed, the armature flux is also reversed and the force decreases as the armature moves from left to right.

In the illustrated embodiment of the invention, a standard matrix printer head assembly is used. These devices comprise a vertical column of nine pins (shown at 82 in Fig. 1) each of which is solenoid actuated. By

actuating selected pins as the vertical column of pins moves from left to right (or right to left) individual characters can be formed as a matrix of discrete dots. Typically, each character "cell" may occupy .1 inch of which 70% (i.e., seven dots) is used to form a character, the remaining space (equivalent to three dots) forming the space between letters. This requires that a maximum of seven of the nine vertical pins be actuable every .01 inch as the carriage moves across the paper. The maximum operating speed of such devices is generally in the order of 180 characters per second, i.e., based on the figures presented above, about eighteen inches per second.

The electronic circuitry and computer program- ming used to actuate the matrix head assembly may be conventional and, therefore, is shown only in general diagrammatic form in Fig. 5. Customarily, the input data is stored in a line buffer 90 capable of storing data corresponding to an entire line of characters. The output of the line buffer is in the form of a six bit ASCII code which is coupled to a Read Only Memory (ROM) 92 where it is decoded and stored in the form of a 9 x 7 matrix. If 128 separate characters are to be reproduceable, then ROM 92 must be able to produce at least 128 separate 9 x 7 matrices. The optical encoder strip 19 comprises a series of opaque and transparent stripes, each .01 inches wide. Thus, as the printer head moves, the photosensor 66 generates a pulse every .01 inch. These pulses are fed to a counter 94 which causes the ROM 92 to read out the selected 9 x 7 matrix a column at a time. The output of the ROM therefore comprises nine separate lines which are coupled to the respective solenoids of the print head to actuate the nine pins so that each column (of a selected character) is printed at each .01 inch interval. The device as so far described will print even though the speed of the carriage will vary as it moves

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across the paper. However, where speed is a consideration,- it is desirable to move the printer head at a constant maximum speed within the constraints of the printer mechanism, i.e., in the case of a typical matrix head assembly, at about eighteen inches per second. This (or any other) desired velocity relationship can be maintained with an appropriate velocity feedback system wherein the current applied to the armature coil is varied as a function of velocity which, in turn, varies with position. Such a velocity feedback system is shown in Fig. 6.

The velocity control signals are derived from the optical encoder strip which, as mentioned above, causes photosensor 66 to generate a pulse every .01 inch of carriage movement. These pulses are amplified by amplifier 96 and converted into a square wave by square wave generator 98. A monostable multivibrator (one-shot) 100 produces a series of pulses at the leading and trailing edges of each of the square waves and the output of the multivibrator is integrated by integrator 102. Thus, as the velocity of the printer head increases, the frequency of the pulses applied to amplifier 96 increases and, consequently, the direct voltage output of integrator 102 also increases.

A reference voltage generator 104 produces a reference voltage (the waveform of which is represented in solid lines beneath generator 104) which increases quickly to a voltage which corresponds to the maximum speed of the printer (e.g., eighteen inches per second). If the actual velocity of the printer head assembly, as represented at the output of integrator 102 differs from the velocity as represented by the corresponding reference voltage, an error voltage is generated by a comparator 106. This error voltage is used to increase (decrease) the current applied to the armature coil (by a power amplifier 108) to thereby correct the armature velocity. Inherently, any change in velocity will be similarly corrected as the

printer moves across the paper, decelerating at the end of its travel to follow the deceleration "ramp" of the reference voltage.

If it is desired to drive the printer head assembly at a constant velocity it is necessary that a relatively large accelerating and decelerating current be applied at the start and end of the printer travel. This requirement is reflected in the current capacity of the armature coil as well as the thermal properties of the ^" coil insulation. The higher the capacity required, the larger the armature and thus the greater the inertia and the lower the efficiency. With the linear actuator of the invention, it is not necessary in all cases to drive the printer head assemblies at a constant speed. Particularly, in the case where there are two or more printer head assemblies cooperating with a single magnet, it may be highly desir¬ able to drive the printer head at a variable velocity the curve of which (as a function of distance) approximates a sine wave. This would reduce the magnitude of the accele¬ rating and decelerating currents, as well as the average velocity, but, in the case of multiple printer heads, the gain achieved in terms of reduced power and thermal capacity would more than offset this. To provide a sinusoidal (or other) velocity control, the velocity feedback system of Fig. 6 would be used but the reference voltage produced by generator 104 would approximate a sinusoidal signal as shown in dotted lines. In the case of full character printing, e.g. if a daisy wheel is used as the printing element, the print wheel (daisy) must be at rest before printing takes place. In such a case, the armature must move in a stepping mode from character to character. One of the major benefits of the invention is tha two or more independently controlled armatures may cooperat

with a single stator construction. In the case of a printer, this can lead to greatly increased speeds approach¬ ing speeds now attainable with so-called line printers. For example, if six print heads were used, printing speeds in the order of six lines (of 132 characters) per second may be obtainable with each head printing only one sixth of a line. By analogy, in Figure 1, each of the two print heads 16 would print one half of each line with care being taken, of course, to ensure that the two assemblies do not mechanically interfere with each other.