Digital Fine-coarse Servomechanism For A Single Element Printer Control System

Abstract

A single element printer selection servomechanism for selectively positioning a spherical printing head to one of a plurality of printing positions arrayed about the printing element. Each printing position comprises a type character disposed within a sector on the surface of the printing element. An error signal representative of the difference between actual and commanded position of the printing element energizes a motor to roughly position the sector of the printing element containing the selected type character. Fine positioning means provide a signal to precisely position the midpoint of the sector of a selected type character after the error signal has been reduced to zero and rough positioning taken place. Circuit means are also provided for braking the motor when its speed reaches a predetermined amount relative to the amplitude of the error signal.

Description

This invention relates to a single element printer and more particularly to a servomechanism for selectively positioning the printing element of a single element printer or typewriter.

A spherical-shaped printing element is connected for rotational movement to a DC torque motor through a universal coupling. The torque motor is energized with an analog error voltage representative of the position of the printing element relative to its commanded position. The position of the printing element is digitally encoded by means of a code wheel attached to the motor shaft for movement with the printing element. An electronic keyboard provides a discrete digital code for each character key depressed. Alternately the print character selection code may be the output of a computer or similar device. These digital codes are converted to analog voltages and summed to produce the analog error voltage for driving the torque motor.

The printing element has 22 type characters in each of four rows. Each type character is equally spaced from adjacent type characters and occupies a printing position within approximately a 16.degree. sector on a circumference of the printing element. When a portion of a 16.degree. sector of a selected type character is positioned according to a command the error signal has become zero. Since it is necessary to precisely position the type character which is at the midpoint of the sector, fine positioning means automatically take over when the analog error signal becomes zero. This fine positioning is also accomplished by means of the DC torque motor 12 which receives positive or negative voltages provided in response to the least significant bits provided by the Gray to Binary code converter 17.

The input to the torque motor during coarse positioning is changed in accordance with the velocity of the motor. That is, when the motor's velocity reaches a predetermined amount relative to the amplitude of the error signal, a braking signal is applied to the motor.

The primary object of the present invention, therefore, is to provide a single element printer selection system wherein an electronically controlled servomechanism accurately positions the printing element of a single element printer or typewriter within minimum time intervals. Since the character selection input to the present system is a digital code, it may serve as a print out or terminal element of an electronic data processing system or the like.

Other objects of the present invention will become apparent with the reading of the following description wherein:

FIG. 1 is a block diagram illustrating the selection servomechanism of the present invention;

FIG. 2 is a representation of the printing element drive arrangement;

FIG. 3 is a flat view of the coded disc shown in FIG. 2;

FIG. 4 is a block diagram of the logic of the fine positioning control;

FIG. 5 is a schematic representation of the power amplifier of the present invention and,

FIG. 6 is a diagram of the voltage inputs to the motor drive circuit.

Referring more particularly to FIGS. 1 and 2 there is shown a spherical printing element 11 similar to the one used on the IBM Selectric typewriter. As best seen in FIG. 2 the printing element may have four rows of 22 type characters each. Each type character occupies the midpoint of a sector or arc of somewhat more than 16.degree.. Thus, during printing impact the printing element 11 must present the midpoint of the 16.degree. sector of the selected character.

FIG. 1 illustrates the block diagram of a system for accomplishing type character selection with respect to a single row by rotation of the printing element. The manner in which rows are selected by tilting the printing element 11 is not a part of the present invention but will be explained hereinafter to the extent necessary to fully explain the present invention.

A DC torque motor 12, which is of a type commercially available, is connected to the printing element 11 through a universal coupling 13. The universal coupling 13 transmits the rotational drive from motor 12 as well as permits printing movement of the printing element i.e., in the direction perpendicular to the plane of FIG. 2.

Mounted for rotation with the output shaft of the torque motor 12 is a code disc 14. The code disc 14 is shown in FIG. 3 and has concentric slotted and non-slotted rings which are arranged to provide 44 discrete combinations of slots and no-slots along as many radial sectors, Thus, an elongated source of light 15 so disposed as to project a line of light extending along a radius of the code disc 14 will project through the disc 44 different combinations of light-dark areas for each revolution of the code disc 14.

The light code passing through the code disc 14 is converted to electrical form by means of an optical pick-up 16. The optical pick-up, a commercially available item, comprises six photo transistors (one for each ring of code disc 14) arranged opposite from the light source on the other side of the code disc 14. Thus, the optical pick-up 16 which may include an amplifier provides as an output a six bit digital code capable of 64 variations. In the present case only 44 variations are used with each two being indicative of a segment on the disc 14. In this way the instantaneous position of the printing element 11 has two codes for each of its 22 type character printing positions. Since 64 code variations are actually available a 32 position printing element is possible without any additional circuitry.

The output via the code disc 14 is in the Gray Code. FIG. 3 shows the slot arrangement of the disc 14. Each radial line on the disc has a code e.g., the dotted line shown has code 001010. This and 001110 e.g., may encode the position for the letter h on the printing element since each position is represented by two discrete codes on the code disc 14. The upper case of each letter is disposed 180.degree. away from the lower case. The code of an upper case letter is found by adding 11 bits to the code of the lower case number. The Gray Code is used for the actual position encoding to diminish possibility of error since in the Gray Code there is never more than a change of one bit for each adjacent position on the code disc 14.

The converter 17 converts from Gray to binary code which as will be seen is the code in which character selection is made from the keyboard 19. However, it should be noted there is no necessity that there be exact identity of the bit positions between a selected character and its encoded position.

Printing element position decoder 18 is connected to receive the five most significant bits from the converter 17. The least significant bit from the converter is provided as an input to fine positioning control 24 where it is converted into a positive or negative drive voltage.

Block 19 represents an electronic keyboard which has a conventionally arranged type character selection keyboard which is not shown. Depression of a type character selection key causes the keyboard 19 to provide a discrete binary code indicative of the type character commanded to be printed.

The coded output from the electronic keyboard is fed to a position command decoder 20 where it is converted into an analog voltage.

Print element position decoder 18 and position command decoder 20 are conventional digital to analog converters wherein each binary code input is converted to a discrete current output level.

As aforesaid, the electrical form of the Gray Code is converted to the binary code in converter 17 and then converted into an analog signal current Ie where Ie=I encoder. For example, for lower case z may be:

Binary Weight 2.sup.4 2.sup.3 2.sup.2 2.sup.1 2.sup.0 Position Code 0 1 0 1 0 Signal Current (ma) 0 .8 0 .2 0

Since a position weighing factor of 0.1 ma per position is used, the position of z given by -1.0 ma is 10 positions in a negative direction from a given reference point. In practice a fixed current of +0.5 ma is added to give, in the present instance, -0.6 ma as the position of z.

Since there are 22 type characters in a row of the printing element 11 and each has two encoded positions, there are 22 discrete analog currents.

The only difference between the two binary codes representing one position is in the least significant bit. This bit is used in the fine positioning mode as will be discussed further on in this specification.

The keyboard 19 provides a discrete seven bit binary code for each character selection key depressed. However, two of these bits are used for row selection i.e., tilting of the printing element 11 via a solenoid arrangement. Five bit words are more than sufficient to encode each of the characters on the keyboard since each five bit code can be used once for each of the four rows i.e., for each of the four rows the same five bit word may stand for a different type character.

For keyboard selection of the character z the output from the keyboard 19 is converted into an analog current Ik in the position command decoder 20 e.g., in a manner similar to:

Binary Weight 2.sup.4 2.sup.3 2.sup.2 2.sup.1 2.sup.0 Position Code 0 0 1 1 0 Signal Current (ma) 0 0 .4 .2 0

The current Ik generated would then be 0.6 ma.

The analog signals from printing element position decoder 18 and position command decoder 20 are fed into a summing amplifier 21 to form an error voltage proportional to the absolute value of Ik+Ie proportional in magnitude and direction to the distance the printing element 11 is away from the commanded position. This error voltage which has a scale factor of .+-.1 volt/position is used to drive the servo motor 12 to position the printing element in the coarse mode.

Thus, an error signal voltage having a magnitude and sense dependent on the character key depressed and the position of the printing element 11 is provided on the output terminal of the summing amplifier 21. This error signal is fed as the driving voltage to the motor 12 via normally conductive transistor 23, servo amplifier 25 and gate 27. The motor 12 turns the printing element 11 to the commanded position. As the motor turns the error signal is diminished until it becomes zero. At this time the sector containing the selected character is positioned but the midpoint of the sector where the type character is actually located may not be accurately positioned. This is accomplished in the fine positioning mode to be described subsequently.

Block 27 represents a gate which may be an electronic switch such as a transistor. It receives an enabling signal from the keyboard each time a key is depressed and serves as an enabling gate such that the selection motor 12 is operative for a sufficient amount of time to complete the selection process.

The servo amplifier 25 is conventional. It runs saturated current mode i.e., fully "on" regardless of the magnitude of the input. Its output is positive or negative depending on the sign of the input. Thus, the motor 12 always receives full current i.e., the energizing current doesn't diminish as the error signal voltage is reduced. Thus, the motor acceleration is maximized. The high angular velocity as well as the lack of resolution of the position encoding of only 44 positions in the 360.degree. predicate the use of rate damping of the motor 12. A rate sensing network 26 which may be a C.E.M.F. Sensor measures applied motor voltage and actual motor current is used to compute the counter EMF of the motor 12, this value being proportional to rate of speed of the motor 12. This voltage is fed into the servo amplifier 25 along with the error voltage i.e., each voltage is fed to a common junction P on the input terminal of the servo amplifier 25 where they are summed.

During character selection as the printing element 11 approaches its commanded position, the error voltage A approaches zero while the rate sensing voltage B increases due to motor acceleration. As best seen in the timing diagram of FIG. 6, motor 12 begins braking when the rate sensing voltage relative to the error voltage becomes greater. FIG. 6 shows this in curve C which corresponds to the voltage at point C in FIG. 2. As seen in FIG. 6 the voltage at point P passes through zero when the rate sensing voltage B becomes greater in magnitude than the error voltage A. A negative current applied to the motor 12 then causes it to slow down. During this time the voltage B is diminishing because the motor 12 is decelerating. Error voltage A continues to be reduced until voltage C again passes through zero. At this point motor 12 is roughly positioned although voltage C may oscillate temporarily about point zero. The error voltage A is then zero.

It should be remembered that depending on the character selecting key depressed, the error voltage could have been negative. The rate sensing voltage would then have been positive since the motor 12 would have been driven in the opposite direction. The effect is still the same i.e., the motor is driven to roughly position the printing element 11.

The rate sensing circuit is conventional having parameters chosen so that its voltage is opposite in sign to the error voltage while keeping the same order of magnitude as the error voltage. When the motor is roughly positioned, the fine positioning control 24 described more fully hereinbelow in reference to FIG. 4 becomes operative.

Fine positioning control 24 includes a circuit which provides a voltage in response to a signal from zero voltage detector 22 to turn off the transistor 23 when the error voltage has been reduced to zero.

The fine positioning control 24 also provides positive or negative driving voltages to the motor 12. The same signal from the zero voltage detector 22 which initiates the turn off voltage to the transistor 23 also enables logic circuitry within the fine positioning control to respond to the least significant bits of the position codes. The code converter 17 provides an input to the fine positioning control 24 for the purpose of feeding to it the least significant or homing bits. These homing bits are used to position the center of the character position or sector precisely. The homing bits are the least significant bits obtained after conversion from the Gray to binary code. The high and low bits from the code converter 17 cause the motor 12 to oscillate and therefore the midpoint of the sector of the selected character on the printing element 11 to oscillate about the commanded position. This occurs because the code disc 14 is oscillating about a corresponding slot, no-slot point. The oscillations become smaller in duration until the printing element 11 zeros in on and is hovering closely about its commanded position at which time it is locked into position e.g., by a detent arrangement (not shown). Somewhat in advance of this the printing element has been propelled on its forward path to impact.

The physical environment of a single element printer and particularly a single element typewriter imposes physical restrictions on the servo motor used. For example, the motor must be relatively short along its longitudinal dimension to prevent interference with the interworkings of the printer or typewriter. It must have high torque e.g., 20-25 oz/in. Typical dc motors with this high torque are 2 to 3 inches long since the permanent magnets which they require must be long to provide the torque. Therefore, a motor built by Inland Motor Company and designated by NT 1368 which is only 0.697 inch in length is used. However to provide high torque output its permanent magnet must be magnetized to its maximum. In this state the permanent magnet is subject to demagnetization if its overdriven with a consequent reduction of torque. Since in the present invention it is necessary to operate the motor at absolute maximum, it is necessary at all times to provide it with full driving current. To avoid demagnetization, however, the driving current may never exceed the rated limit.

As previously pointed out the polarity of the current applied to the torque motor is abruptly changed to start braking it when the error voltage and the rate sensing voltage reach a predetermined relationship. The back EMF of the motor then becomes additive to the applied voltage and the motor would receive a driving current above the rated limit and be so damaged that the field magnet would have to be replaced or remagnetized.

Therefore, in order to use the selected motor without damaging it is necessary to provide a power amplifier having symmetrical current limiting which will always provide peak current. Such a power amplifier must provide a constant output current unaffected by load resistance. Thus, variations in the resistance of the servo motor armature winding whether due to variations in temperature or some other cause will not affect the output current. For the given supply voltage of .+-.25 volts the load resistance may vary from 8 ohms all the way down to 0 ohms without changing the output current.

FIG. 5 illustrates the power amplifier 27 of the present invention which accomplishes the foregoing. A transistor T1 of the NPN type has its emitter connected to ground through a resistance Rx. Resistance Rx is the only resistance which ever needs to be adjusted or changed to control the reference currents as will be described further on in this specification.

The base of transistor T1 is connected to a reference voltage source e.g., 5v and its collector is connected to the base of PNP transistor T2. Resistance R1 and R2 are connected respectively to the collector of transistor T1 and the emitter of transistor T2 and in common to a positive voltage source B+ through a third resistance R3. The collector of transistor T2 is connected to a negative voltage power source B- through resistance R4 and R5. In a practical embodiment the positive and negative voltage sources were 25 volts in magnitude.

The transistor T1 and T2 generate equal reference voltages in resistance R3 and R5 controllable from a common point i.e., resistance Rx and neither of which is affected by line variation.

The foregoing is accomplished in the following manner:

The transistor T1 generates a constant current through resistance R1 which provides a constant voltage across R1. This voltage causes the transistor T2 to generate a constant current in its collector circuit. The constant currents in resistances R1 and R2 are summed in resistance R3 providing a constant reference voltage across resistance R3. Inasmuch as transistor T2 is driven via transistor T1, its current is proportional to the current in transistor T1. Therefore, the value of resistance R5 may be chosen so that the voltage thereacross is equal to the voltage across resistance R3.

Thus, if the value of resistance of Rx is changed, it will equally affect the voltages across resistance R3 and R5. Therefore, the voltages across each of resistances R3 and R5 are constant reference sources controllable from a common point with neither voltage able to be affected by the external reference voltage variation without the other being equally affected. This eliminates need for external control of the voltages B+ and B-.

The transistor T3, T4 and T5 together form a conventional three stage amplifier for amplifying positive going signals applied to the base of the transistor T3 via input terminal 28. The voltage on the input terminal 28 is limited by means of Zener diodes Z1 and Z2 from overdriving the amplifier.

Transistors T7, T8 and T9 likewise form a three stage amplifier for amplifying negative going signals applied to the input terminal 28.

The biasing parameters of the amplifier comprising the transistors T3, T4 and T5 are chosen such that any positive voltage (limited to 5 volts as aforesaid) applied to the input terminal 28 fully saturates the amplifier and causes full load current to flow in the load resistance R6 which is connected between the positive power source B+ and the emitter of the transistor T5.

In a similar manner full current flow is provided in the resistance R7 connected between the negative power source B- and the emitter of the transistor T9 when a negative signal is applied to the input terminal 28.

The collectors of the transistors T5 and T9 are commonly connected to output terminal 29 from which the motor 12 is energized. As should be clear the current through load resistances R6 and R7 is positive or negative depending on the sign of the input voltage. Since either one or the other transistor T5 or T9 is conductive (in a practical embodiment this is .+-.3 amperes), the motor 12 receives full driving or braking current during coarse positioning.

As previously pointed out while it is necessary to drive the motor 12 at its rated maximum, any overdrive will cause its permanent magnet to demagnetize and it will lose its torque. Therefore, the power amplifier 27 limits the current in the load resistances R6 and R7 which otherwise might increase beyond the prescribed limit due to the back EMF of the motor 12 and its additive effect when the motor 12 is abruptly braked as explained heretofore.

The constant reference voltages of resistances R3 and R5 provide the current limiting feature as explained hereinbelow.

Transistors T10 and T11 which are of the PNP type form a differential amplifier in the positive signal section of the amplifier and transistors T12 and T13 which are of the NPN type form a differential amplifier in the negative signal section of the amplifier.

The emitters of the transistors T10 and T11 are connected to the positive power source B+ through resistance R8 and the emitters of the transistors T12 and T13 are connected to the negative power source B- via resistance R9.

The collectors of transistors T11 and T13 are connected to ground through resistances R10 and R11, respectively. The collector of transistor T10 is connected to the emitter of the transistor T3 via a resistance R12. The collector of the transistor T12 is connected to the emitter of the transistor T7 via a resistance R13.

The voltage across the resistance R3 biases the transistor T11 normally on. When, however, the current in the resistance R6 reaches the limit, the voltage across resistance R6 becomes equal to the voltage across R3 causing the transistors T10 and T11 to begin to conduct current equally. This new current in the transistor T10 flows through the resistances R12 and R14 to ground causing the transistor T3 to become back biased and accept less input which in turn limits the drive also to the transistors T4 and T5. Accordingly, the load current in resistance R6 is limited and the motor 12 is not overdriven.

In a similar manner for the negative section of the power amplifier current is made to flow equally in the transistors T12 and T13 when the voltage (now negative) in R7 equals the reference voltage in resistance R5. Then current flows to ground through resistance R13 and R15 to make the transistors T7, T8 and T9 less conductive to limit the current in resistance R7 to its maximum negative value to prevent overdrive of the motor 12.

The resistances R16 through R21 are the ordinary resistances found in conventional amplifiers of this type. A capacitor (not shown) may be connected between the collectors of the transistors T3 and T4 and transistors T7 and T8 if necessary to prevent oscillation.

The output current from the power amplifier is, in addition to not being a function of load resistance, also not a function of supply voltages as should now be clear.

The power amplifier 27, therefore, insures maximum drive for the motor at all times but limits current symmetrically to prevent damage. The reference voltages are adjustable from the common point Rx which may be done at the factory.

FIG. 4 discloses the detail of the logic of fine positioning control 24 which centers the printing element 11 about the midpoint of the 16.degree. sector. In the binary code (i.e., after Gray to binary decoding) this point may be considered as located where the transition from high to low or low to high takes place in the least significant bit.

Logic elements L1, L2, L3, L7 and L8 are inverter circuits which convert high inputs to lows and low inputs to highs.

Logic elements L4, L5, L6, L9 and L10 are NAND circuits which provide a low output when both inputs are high and a high output when both inputs are unalike, or both low.

Logic element Lx is an exclusive OR circuit i.e., it has a high output when one or the other inputs are high. When both inputs are alike, the output of OR circuit Lx is low.

The output terminal A of the zero voltage detector 22 is connected to the input terminal of inverter circuit L1 and to one of the input terminals of the NAND circuits L9 and L10. The zero voltage detector provides a high on its output terminal A when coarse positioning is complete i.e., when the error voltage has been reduced to zero. Before coarse positioning has been attained, the zero voltage detector 22 provides a low on its output terminal.

The low on the output terminal A causes the output of inverter circuit L1 to be released (i.e., able to go high providing the input to L3 is low).

The OR circuit Lx has input terminals B and C. The input terminal B is connected to the keyboard 19 which provides a high (1) when the character to be selected is upper case and a low (0) when the character to be selected is lower case.

The second input terminal C of the OR circuit Lx is coupled to the Gray encoded output of the optical pick-up 16 which provides a high on the input terminal C when the position encoded is in the upper case section of i.e., the first 180.degree. of the printing element 11. A low is provided on the input terminal C when the position encoded is in the lower case section or the second 180.degree. of the printing element. Therefore, when the servo motor 12 is selecting within the upper or lower case area of the printing element alone, the inputs to the OR circuit Lx are both highs or both lows causing a low at its output. This low provided to the inverted L3 along with the low provided at the input of inverter L1 forces a low from the output of the inverter circuit L2.

The output terminal of the inverter L2 is connected to the base of the transistor Q1 whose collector in turn is connected to the base of the switching transistor 23. The low on the output of the inverter L2 maintains transistors Q1 and 23 conductive. This causes the error voltage output from the current summing amplifier 21 to be available at the servo amplifier 25 through the transistor 23.

Once coarse positioning is attained the transistor 23 is turned off since the output terminal A goes high causing a low at the output of the inverter L1 and therefore a high at the output of the inverter L2. This high causes transistors Q1 and 23 to become non-conductive, thus disconnecting the current summing amplifier 21 from the servo amplifier 25 to prevent backloading of the fine positioning signal during fine positioning by the current summing amplifier 21.

Simultaneous with this event the fine positioning mode is initiated. The high on the output terminal A is provided as one of the inputs to each of NAND circuits L9 and L10. The input terminal F provides the least significant bit (the homing bit) from the converter 17 as the second input to the NAND circuit L9 via inverter circuit L8 and the second input to the NAND circuit L10.

The output terminal of the NAND circuit L10 is connected to the base of transistor Q3. The collector of the transistor Q3 is connected to a positive source (+15v) via a load resistance while its emitter is grounded. The collector of the transistor Q3 is connected to the servo amplifier 25 via resistance R25.

The output terminal of the NAND circuit L9 is connected to the base of a transistor Q4. The collector of transistor Q4 is connected to the base of a transistor Q5. The collector of the transistor Q5 is connected to a negative voltage source (-15v) through a voltage divider. The junction of the voltage divider is connected to the servo amplifier 25 via resistance R26.

Since the outputs of NAND circuits L4, L5, L9 and L10 are high during coarse positioning, the transistors Q3, Q4 and Q5 are in the conductive state during this time so that no positioning signal is available through resistances R25 and R26. When transistors Q3 or Q4 and Q5 are turned off, a positive or negative positioning signal is made available through resistances R25 or R26, respectively. This may occur in two situations, i.e., in the fine positioning mode or when an overshoot of the motor occurs as will be more fully explained hereinbelow.

As aforesaid after coarse positioning has been completed, i.e., the printing element 11, motor 12 and code disc 14 have come within .+-.8.degree. of the selected position, the signal on output terminal A goes high (1).

The high on output terminal A is one input to each of NAND circuit L9 and L10. Thus, when a high appears on the other input terminal of the NAND circuit L10, its output goes low. This turns off transistor Q3 causing a positive signal to be supplied to the servo amplifier 25. Alternately, when the NAND circuit L9 receives a high on its other input terminal via inverter circuit L8, its output goes low. This turns off the transistors Q4 and Q5 thus making available a negative signal to the servo amplifier 25 via the resistance R26.

It can now readily be seen that the signals from resistances R25 and R26 supply directional fine positioning information to the motor 12.

In the fine positioning mode the least significant bit of the encoded information from converter 17 is made available on the terminal F. This bit alternates between high (1) and low (0) as the code disc 14 varies in position about midpoint of the selected 16.degree. sector. In this way the power amplifier 27 may be fed bidirectional control information. Thus, when the least significant bit is a high, transistor Q3 is turned off and a positive signal is supplied to drive the motor 12. When the least significant bit is a low, the output of inverter L9 is low and transistor Q5 goes off causing a negative signal to be supplied to the motor 12. Thus, the motor oscillates about this high low transition point in the code (the center of the 16.degree. sector) until it is fine positioned and homing is complete.

As previously pointed out the foregoing logic circuitry may be used to compensate for overshoot of the servo system. Overshoot is caused by the discontinuity on the code disc 14 (i.e., the 360.degree./0.degree. transition point on the code disc 14) defined where code position 22 is contiguous with code position 1. As the code disc passes this position the voltage from decoder 18 would abruptly drop or rise depending on the direction of rotation.

Code positions 1 and 22 may be visualized by considering the printing element 11 which is divided into 22 equal character positions. The code disc which is attached to and moves with the drive shaft of the printing element 11 is, therefore, also divided into 22 positions. At the 360.degree./0.degree. point on the code disc 14 is the juncture of positions 1 and 22. This may be physically located by observing at what point on the code disc 14 there is an abrupt change of current from decoder 18.

If the character selected is e.g., at position 2 on the printing element, counterclockwise of position 2 the error is positive, at position 2 it would be zero and clockwise of position 2 the error voltage would be negative.

If the character selected is at position 1 on the printing element as defined by the discontinuous point on the code disc, clockwise and counterclockwise motion from that position results in a positive error signal. Thus, a counterclockwise direction is commanded regardless of the actual direction of error.

As a result of this, serious positioning error could occur when selection of the character at position 1 or 22 is desired. For example, if the code disc 14 and printing element 11 are at position 10 and the character in position 1 is selected at the keyboard, momentum may carry the printing element 11 and therefore the code disc past the code discontinuity into position 22. As previously explained this results in the wrong error voltage polarity. If the servo system were ideal, it would not overshoot at all but certain system variables e.g., friction, lack of lubrication in the motor bearings, or slight misalignment of the components of the motor shaft cannot be completely controlled etc., cause it. Therefore, a possibility of overshoot always exists and incorrect error voltage will be generated when overshoot occurs at positions 1 and 22.

Therefore, the logic circuitry of FIG. 4 has the additional function of providing a voltage step or level at this critical position of overshoot so that the error signal as seen by the servo amplifier 25 is bidirectional and independent of the position selected.

NAND circuit L4 has one input from terminal B and the other from OR circuit Lx, and inverter circuit L7. The output terminal of NAND circuit is connected to the base of transistor Q3.

A NAND circuit L6 has its output connected to an inverter circuit L7. NAND circuit L5 has one input connected to terminal C and the other input connected to the output of the inverter circuit L7, and OR circuit Lx. The output of the NAND circuit L5 is connected to the base of the transistor Q4.

When the input terminals B and C to OR circuit Lx have unlike signals e.g., 1, 0 or 0, 1, its output will be high, if the input to inverter L7 is low. The signals will be unalike whenever the selected character is upper case and the encoded position is lower case or vice versa. At this time NAND circuits L4 and L5 each have a high on one of their input terminals.

Now if there is a high on terminal B, the output of the NAND circuit L4 will go low. Transistor Q3 will cease conducting causing a positive input signal to be supplied the servo amplifier 25.

Transistor Q4 and Q5 cease conducting, to apply a negative signal to the servo amplifier 25 if terminal C (instead of terminal B) has a high. This is so because the NAND circuit L5 will then have two highs and provide a low on the base of the transistor Q4.

Thus, without added logic the servo amplifier would receive bidirectional signals whenever the selected position and the encoded position are in different cases using the "homing" bit in an identical way as described with respect to the fine positioning mode.

Naturally, overshoot protection is needed only when the selected positions are 1 and 22 and because it is only at this position that danger of overshooting past the discontinuity of the code is present.

Therefore, the NAND circuits L4 and L5 inputs from the OR circuit Lx are inhibited for all except positions 1 and 22.

This is accomplished by the NAND circuit L6 having input terminals D and E. The output of the NAND circuit L6 is connected as the input to the inverter circuit L7 whose output terminal is connected to the output terminal of the OR circuit Lx.

When the inputs on the terminals D and E are both highs, it can be seen that, the NAND circuits L4 and L5 are enabled, (if B and C are dissimilar) and transistor Q3 or transistors Q4 and Q5 may be turned off. The inputs on the terminals are high only when overshoot may occur i.e., selection to positions 1 and 22.

Thus, even if overshoot occurs, the motor 12 is driven in the correct direction.

When this overshoot protection is put into operation, it is necessary to eliminate the still present error signal from being supplied to the servo amplifier 25 since it is the signal which at that position contains the incorrect directional information.

Since the output of the OR circuit Lx is high (as previously explained) when an overshoot is present, the input of inverter circuit L3 goes high causing the output of inverter circuit L2 to go high (the output of the inverter L1 having no effect). When the output of inverter circuit L2 goes high, the transistors Q1 and 23 turn off disconnecting the summing amplifier 21 from the servo amplifier 25. The signals on D and E are obtained from the Gray to binary code converter 17 (the fourth bit indicative of the discontinuity) and from optical pickup 16 respectively.

Both signals on D and E are the "unconverted" Gray coded signals. The signal on E is available directly at the optical pickup amplifier output, but the signal on terminal D can not be used in its condition as it is at the optical pickup amplifier output, but must be inverted. The Gray to binary converter 17 includes inverters in its logic, and the signal on terminal D is taken away from the Gray to binary converter 17 after it is inverted, but not yet converted.

Claims

I claim:

1. A single element printer control for positioning a printing element in response to a coded command,

position encoding means providing two discrete binary codes for each position, each of said two codes differing from each other in the least significant bit.

selection means providing a discrete binary code for each character selected for printing,

means converting the coded output from each of said position encoding means and said selection means into analog voltages,

first summing means connected to said converting means combining said analog voltages and providing an error voltage representative of the difference in magnitude and direction of the commanded and actual positions of the printing element,

servo motor means connected to said summing means coarsely positioning the printing element to the commanded position in response to said error voltage,

fine positioning means connected to said position encoding means and responsive to changes in the least significant bits of the binary code of printing element position to finely position the printing element to the commanded position,

said fine positioning control means including detector means connected to said summing means responsive to zero error voltage to actuate said fine positioning control means,

said fine positioning also comprising,

transistor switch means normally connecting said first summing means to said servo motor means,

first logic means connected between said transistor switch means and said detector means for opening said transistor switch means when said error signal becomes zero,

second logic means connected between said detector means and said servo motor means and responsive to zero error voltage for providing directional driving voltage to said servo motor means in response to the least significant bits from said position encoding means,

said second logic means further including a positive and negative source of voltage,

first transistor means connecting said positive source of voltage to said servo motor means in response to zero error voltage and a high least significant bit,

second transistor means connecting said negative source of voltage to said servo motor means in response to zero error voltage and a low least significant bit.

2. A single element printer control system for positioning a printing element in response to a coded command,

position encoding means providing two discrete binary codes for each position, each of said two codes differing from each other in the least significant bit.

selection means providing a discrete binary code for each character selected for printing,

means converting the coded output from each of said position encoding means and said selection means into analog voltages,

first summing means connected to said converting means combining said analog voltages and providing an error voltage representative of the difference in magnitude and direction of the commanded and actual positions of the printing element,

servo motor means connected to said summing means coarsely positioning the printing element to the commanded position in response to said error voltage,

fine positioning means connected to said position encoding means and responsive to changes in the least significant bits of the binary code of printing element position to finely position the printing element to the commanded position,

said fine positioning control means including detector means connected to said summing means responsive to zero error voltage to actuate said fine positioning control means,

said servo motor means comprising current limiting means for limiting driving current to said servo motor means regardless of the additive effects of back EMF when polarity of the driving current is abruptly reversed.

3. A single element printer control system for positioning a printing element in response to a coded command,

position encoding means providing two discrete binary codes for each position, each of said two codes differing from each other in the least significant bit,

selection means providing a discrete binary code for each character selected for printing,

means converting the coded output from each of said position encoding means and said selection means into analog voltages,

first summing means connected to said converting means combining said analog voltages and providing an error voltage representative of the difference in magnitude and direction of the commanded and actual positions of the printing element,

servo motor means connected to said summing means coarsely positioning the printing element to the commanded position in response to said error voltage,

fine positioning means connected to said position encoding means and responsive to changes in the least significant bits of the binary code of printing element position to finely position the printing element to the commanded position,

said fine positioning control means including detector means connected to said summing means responsive to zero error voltage to actuate said fine positioning control means,

rate damping means including sensor means connected to said servo motor means providing a signal representative of the instantaneous speed of said servo motor means,

second summing means connected to said first summing means and said sensor means for combining said error voltage and said speed signal,

amplifier means connected between said second summing means and said servo motor means applying a driving current to said servo motor means having a polarity dependent on the ratio of error voltages to speed signal,

said servo motor means comprising current limiting means for limiting driving current to said servo motor means regardless of the additive effects of back EMG when polarity of the driving current is abruptly reversed.

4. A single element printer control system in accordance with claim 3 wherein said fine positioning means comprises,

transistor switch means normally connecting said first summing means to said amplifier,

first logic means connected between said transistor switch means and said detector means for opening said transistor switch means when said error signal becomes zero,

second logic means connected between said detector means and said amplifier and responsive to zero error voltage for providing directional driving voltage to said amplifier in response to the least significant bits from said position encoding means.

5. A single element printer control system in accordance with claim 4 wherein said second logic means includes a positive and negative source of voltage,

first transistor means connecting said positive source of voltage to said amplifier in response to zero error voltage and a high least significant bit,

second transistor means connecting said negative source of voltage to said amplifier in response to zero error voltage and a low least significant bit.

6. A single element printer control system in accordance with claim 5 wherein said second logic means includes means for causing said first and second transistor means to connect said positive or negative voltage source to said amplifier in response to high and low least significant bits, respectively when predetermined characters are selected regardless of the presence of error voltage.

7. A single element printer control system according to claim 2 wherein said current limiting means comprises a symetrical load current limiting power amplifier having,

first and second multistage amplifier sections,

said first section comprising a PNP transistor in the output stage having its emitter connected to a positive power source through a load resistance,

said second section comprising a NPN transistor in the output stage having its emitter connected to a negative power source through a load resistance,

an output terminal connected in common to the collectors of said transistors,

each of said first and second amplifier stages having a transistor in the input stage having bases connected in common to an input terminal,

load current limiting means for applying a current to the emitter of the input transistor of the first section when current through the load resistor of the output transistor reaches a predetermined amount and for applying a current to the emitter of the input transistor of the second section when current through the load resistor of the output transistor of the second sections reaches a predetermined amount,

whereby the currents applied to the input transistors prevent the magnitude of the load currents from exceeding said predetermined amount.

8. A single element printer control system according to claim 2 wherein said current limiting means comprises a symetrical load current limiting power amplifier having,

first and second differential amplifiers,

said first differential amplifier comprising first and second PNP transistors having their emitters commonly connected to a positive power source,

said second differential amplifier comprising first and second NPN transistors having their emitters commonly connected to a negative power source,

first amplifier means for amplifying all positive going signals comprising at least an input transistor having an emitter connected to the collector of said first PNP transistor and an output transistor having an emitter connected to the base of said first PNP transistor,

second amplifier means for amplifying all negative going signals comprising at least an input transistor having an emitter connected to the collector of said first NPN transistor and an output transistor having an emitter connected to the base of said first NPN transistor,

the bases of each of said input transistors connected in common to an input terminal,

the collectors of each of said output transistors being connected in common to an output terminal,

a load resistor in the emitter circuit of each of said output transistors,

a first load resistor connected between a positive source of voltage and the emitter of one of said output transistors,

a second load resistor connected between a negative source of voltage and the emitter of the other of said output transistors,

means biasing said first and second differential amplifier to permit current flow only in each of said second transistors thereof,

each of said differential amplifiers responsive to the current in either of the load resistors attaining a predetermined value to cause the respective first transistors thereof to become conductive,

whereby current is fed back to reduce conduction of the appropriate input transistor.

9. A single element printer control system according to claim 1 wherein said servo motor means comprises,

a DC torque motor having its output shaft connected to the printing element,

said amplifier means being connected between said second summing means and said DC torque motor,

said amplifier means including current control means maintaining said driving current at near peak value without exceeding a predetermined value even when the polarity of the driving current is abruptly changed.

10. A single element printer control system according to claim 9 wherein said current control means comprises,

first and second multistage amplifier sections,

said first section comprising a PNP transistor in the output stage having its emitter connected to a positive power source through a load resistance,

said second section comprising a NPN transistor in the output stage having its emitter connected to a negative power source through a load resistance,

an output terminal connected in common to the collectors of said transistors and to the energizing coil of said DC torque motor,

each of said first and second amplifier stages having a transistor in the input stage having bases connected in common to an input terminal,

said input terminal connected to said second summing means,

load current limiting means for applying a current to the emitter of the input transistor of the first section when current through the load resistor of the output transistor reaches a predetermined amount and for applying a current to the emitter of the input transistor of the second section when current through the load resistor of the output transistor of the second sections reaches a predetermined amount,

whereby the currents applied to the input transistors prevent the magnitude of the load currents from exceeding said predetermined amount.

11. A single element printer control system according to claim 9 wherein said current control means comprises,

first and second differential amplifiers,

said first differential amplifier comprising first and second PNP transistors having their emitters commonly connected to a positive power source,

said second differential amplifier comprising first and second NPN transistors having their emitters commonly connected to a negative power source,

first amplifier means for amplifying all positive going signals comprising at least an input transistor having an emitter connected to the collector of said first PNP transistor and an output transistor having an emitter connected to the base of said first PNP transistor,

second amplifier means for amplifying all negative going signals comprising at least an input transistor having an emitter connected to the collector of said first NPN transistor and an output transistor having an emitter connected to the base of said first NPN transistor,

the bases of each of said input transistors connected in common to an input terminal,

said input terminal connected to said second summing means,

the collectors of each of said output transistors being connected in common to an output terminal,

said output terminal being connected to the energizing coil of said DC torque motor,

a load resistor connected between a positive source of voltage and the emitter of one of said output transistors,

a second load resistor connected between a negative source of voltage and the emitter of the other of said output transistors,

means biasing said first and second differential amplifier to permit current flow only in each of said second transistors thereof,

each of said differential amplifiers responsive to the current in either of the load resistors attaining a predetermined value to cause the respective first transistors thereof to become conductive,

whereby current is fed back to reduce conduction of the appropriate input transistor.