GB2163915A - Web drive apparatus - Google Patents

Abstract

An impact printer ribbon drive apparatus in which an elongate ink ribbon extending between and wound around opposite reels is driven at substantially constant ribbon speed and constant ribbon tension independent of varying ribbon pack radius on the two reels. Each one of the reels is driven by a different one of a pair of identical DC permanent magnet motors (M1, M2). Each of the motors may be utilized as either the supply motor or as the take-up motor. The motors are electrically connected in series with a common node (25) between the two motors coupling the motors to a current sink. A first switching circuit 20/22, 24/28 couples the take-up motor to an unregulated voltage source 21 and the supply motor to ground. The duty cycle of the first switching circuit is varied as to maintain the voltage drop across the pair of motors substantially constant. A second switching circuit 26 couples the current sink to the common node between the two motors. The duty cycle of the second switching circuit is varied so as to maintain a substantially constant current level Ic drawn from the common node. <IMAGE>

Description

SPECIFICATION Web drive apparatus The present invention relates generally to apparatus for driving an elongated web member or "towel" ribbon between a pair of rotatable reels, and more particularly to apparatus for driving an ink ribbon between a discharge reel and a take-up reel in an impact printer.

It is well known in the art to provide an impact printer in which the individual hammers of a hammer bank or the raised characters on a font impact an inked ribbon against print paper supported by a platen to effect printing. The printing may be formatted utilizing different methods such as a dot matrix where the desired characters or other printed indicia are formed by a series of dots made by the action of a hardened stylus impacting the ink ribbon against the print paper. Typically, print hammers are mounted in a hammer bank, and the hammer bank is reciprocated relative to the print paper to optimize usage of the hammers. The ink ribbon may be routed around fixed guides and mounted on a pair of rotating reels, one reel serving as a supply reel and the other reel serving as a take-up reel.Typically the ink ribbon may be driven in opposite directions between the opposite ends thereof to equalize usage and wear of the ribbon.

The purpose of the ribbon drive system is to drive an ink ribbon from a supply reel to a take-up reel at a fairly constant ribbon tension and at a fairly constant ribbon speed. Generally, a motor, or a pair of motors, is utilized to drive the ribbon. One method utilizes two AC motors in which one motor drives the ribbon take-up reel while the remaining motor opposes the motion of the take-up motor thereby tensioning the ribbon.

Another method utilizes one motor to drive the ribbon take-up reel and keeps tension on the ribbon by resisting the ribbon motion at the supply reel with mechanical or electrical drag.

The ribbon drive system described in U.S. Patent No. 3,941,051, utilizes a pair of AC motors to drive the opposite ribbon reels. One motor serves as the leading or take-up motor while the trailing or supply motor operates as a torquer to exert constant torque thereby tensioning the ribbon. The leading motor is driven at constant speed which results in a varying ribbon speed because the diameter of the ribbon pack varies as the ribbon is driven from one reel to the other. Generally, the ribbon speed will double as the ribbon pack diameter is doubled for constant rotational speed of the take-up motor. Similarly, ribbon tension will vary inversely with ribbon pack diameter on the reel connected to the torque motor.

Another ribbon drive system is described in U.S. Patent No. 4,177,731 which system employs two DC permanent-magnet motors energized by a circuit having a pair of servo circuits which couple the motors between sources of constant voltage and a common current path and which are alternately actuated depending upon the desired direction of ribbon drive. The voltage drop across the leading or take-up motor is maintained substantially constant while the current through the trailing motor or supply motor is varied so as to maintain the sum of the two motor currents constant.

The present invention provides apparatus for driving an elongated web member, extending between and having the opposite ends thereof wound on opposite ones of a pair of rotatable elements, at substantially constant speed and under substantially constant tension, said apparatus comprising means for mounting the pair of rotatable elements for rotation with said elongated web member extending therebetween; a pair of motors, each motor being arranged to be coupled to a respective one of the pair of rotatable elements, said motors being electrically connected in series and having a common node therebetween coupled to each motor of said pair; first switching means coupled to the inputs of both of the motors, the first switching means being responsive to a set of control signals and to a first electrical signal to couple a variable voltage source to the input of one of the motors so as to maintain a substantially constant voltage drop across said pair of motors, said one of the motors operating as a web member take-up motor and the other one of the motors operating as a web member supply motor; second switching means coupled to said common node and to a current sink to provide a common current path coupled to each of the motors via said common node, the second switching means being responsive to a second electrical signal so as to maintain the current drawn from the common node by the current sink substantially constant; first generating means responsive to the voltage level of the variable voltage source to generate said first electrical signal to vary the duty cycle df the first switching means so as to maintain the voltage drop across both motors substantially constant; and second generating means, including differential amplifier means for comparing the current drawn from the common node to a reference value, the second generating means being responsive to the level of current drawn from the common node to generate said second electrical signal to vary the duty cycle of the second switching means so as to maintain the current drawn from the common node at a predetermined substantially constant value.

In apparatus as set forth in the last preceding paragraph, it is preferred that the first switching means comprises a plurality of electronic switches, first switches thereof being provided for coupling the variable voltage source to the input of both of the pair of motors, second switches thereof being provided for coupling the input of both of the pair of motors to ground, the first switches being responsive to the set of control signals to couple the variable voltage supply to one of the pair of motors operated as a take-up motor and the second switch being responsive to the first electrical signal alternately to connect and disconnect the other one of the motors to and from ground so as to maintain a substantially constant voltage drop across both motors, said other one of the motors operating as a supply motor.

In apparatus as set forth in either one of the last two immediately preceding paragraphs, it is preferred that the second switching means comprises an electronic switch responsive to said electrical signal, the electronic switch coupling said common node to said current sink, and a current sensing resistor to provide a voltage signal indicative of the level of current drawn from said common node.

In apparatus as set forth in the last but one immediately preceding paragraph, or the last preceding paragraph as appended thereto, it is preferred that said first switches and said second switches comprise power transistors.

In apparatus as set forth in the last but one immediately preceding paragraph, or the last preceding paragraph as appended thereto, it is preferred that said electronic switch comprises a power transistors The present invention also provides an impact printer comprising a platen; a print hammer means disposed opposite the platen, the print hammer means being capable of being reciprocated relative to the platen; a pair of motors, each motor being provided for rotating a respective one of a pair of rotatable reels, each of which receives a respective end of an elongate inked print ribbon wound thereon, which ribbon can be fed from one reel to the other between the print hammer means and a printable medium located against the platen, and the motors being electrically connected in series and having a common node therebetween electrically connected to each motor of said pair; first switching means coupled to the inputs of both motors, the first switching means being responsive to a set of control signals and to a first electrical signal to couple a variable voltage source to the input of one of the motors and to couple the input of the other one of the motors to ground so as to maintain a substantially constant voltage drop across said pair of motors thereby maintaining a substantially constant ribbon speed independent of varying the radius of ribbon wound on the rotatable reels, said one of the motors operating as the print ribbon take-up motor and said other one of the motors operating as the print ribbon supply motor; second switching means coupled to said common node and to a current sinkto provide a common current path coupled to each of the motors via said common node, the second switching means being responsive to a second electrical signal so as to maintain the current drawn from the common node by the current source substantially constant thereby maintaining a substantially constant print ribbon tension; first generating means responsive to the voltage level of the variable voltage source to generate said first electrical signal to vary the duty cycle of the first switching means so as to maintain the voltage drop across both motors substantially constant; and second generating means, including differential amplifier means for comparing the current drawn from the common node to a reference value, the second generating means being responsive to the level of current drawn from the common node to generate said second electrical signal to vary the duty cycle of the second switching means so as to maintain the current drawn from the common node at a predetermined substantially constant value.

A ribbon drive apparatus in accordance with a preferred embodiment of the present invention drives the print ribbon at substantially constant ribbon speed and ribbon tension independently of the varying ribbon packs as the print ribbon is transferred from a supply reel to a take-up reel. The ribbon drive apparatus comprises an elongated web or towel ribbon wound on a supply reel and connected to a take-up reel. The supply reel and the take-up reel are driven to two essentially identical motors; either motor can serve as the supply motor or take-up motor. The two motors are connected in series and a current sink draws current from the common node between the two motors. The current drawn from the common node is proportional to ribbon tension and is maintained substantially constant so asto maintain a constant ribbon tension.To maintain a constant ribbon speed, the sum of the voltage drop across both motors is kept substantially constant; this allows the voltage drop across the individual motors to vary and thereby adjust for the varying radius of the ribbon pack on their respective ribbon reels.

The ribbon drive motors are powered by a unipolarvariable voltage supply. The duty cycle of the motors is increased or decreased as the voltage level of the variable supply decreases or increases so as to keep a constant voltage across the motors over the range of the variable supply. Similarly, the duty cycle of the current sink drawing current from the common node between the two motors is varied as a function of the current drawn from the common node so as to maintain a substantially constant current from the common node. Both motors drive in the same direction when moving the ribbon from one reel to the other, with the leading or take-up motor supplying the greater torque. When tensioning the ribbon, the motors each pull their end of the ribbon onto their respective reels, thus pulling aginst each other tensioning the ribbon.The sum of the motor current is limited thus limiting motor torque. Since the motors are essentially identical, the current divides equally between the two motors and each motor exerts approximately the same torque thereby tensioning the ribbon with limited ribbon movement.

There now follows a detailed description, which is to be read with reference to the accompanying drawings, of apparatus according to the present invention; it is to be clearly understood that this apparatus, embodied as an impact printer, has been selected for description to illustrate the invention by way of example and not by way of limitation.

In the accompanying drawings: Figure 1A is a perspective view of a web ribbon utilizing a ribbon drive apparatus in accordance with the invention; Figure 1B is a plan view of an impact printer utilizing a ribbon drive apparatus in accordance with the invention; Figures 2A, 28 and 2C are block diagrams illustrating the three operating modes of a ribbon drive apparatus in accordance with the invention; Figure 3A is a block diagram of the ribbon logic system and the current control loop of the ribbon drive apparatus illustrated in Figure 2; Figure 3B is a schematic diagram of the drive motor power system of the ribbon drive apparatus illustrated in Figure 2; and Figure 4 is a timing diagram which illustrates the relationship between the various control signals and the operation of the ribbon drive apparatus illustrated in Figure 2.

For the purposes of illustration and explanation the invention is described in conjunction with an impact printer as shown in Figure 1 B. The purpose of the ribbon drive apparatus is to drive an elongate web type ink ribbon from a supply reel to a take-up reel past an array of print hammers at a substantially constant ribbon speed and constant ribbon tension. It will be understood by those skilled in the artthatthe ribbon drive can be used in environments where an elongated web member other than an ink ribbon is required to be driven at constant speed and with constant tension without departing from the scope and spirit of the present invention.

Referring now to Figures 1A and 1 B, an ink ribbon comprises an elongated sheet of suitable material, typically either nylon or mylar, and is wound on and connects two cylindrical ribbon reels 10 and 11. The ribbon 1 is driven from one reel to the other reel around guide rods 4 and between a print hammer bank 4 and the print paper 2. The paper or other imprintable media 2 comprises one or a number of webs of conventional edge perforated, continuous or fan-folded sheet fed past a horizontal printing station. The print hammer bank 4 comprises an array of print hammers (not shown) which can be individually actuated.By actuating a hammer at the appropriate time, the hammer is propelled forward under spring tension causing an included printing tip to impact the ink ribbon against the print paper 2 and a print plate or platen 3 thereby printing a character or a dot on the print paper 2, following which the print hammer returns to its initial position.

As the hammer bank reciprocates back and forth, transverse to the direction of ink ribbon 1 movement, the ink ribbon 1 is driven past the print paper 2, first in a direction from the reel 11 to the reel 10. At both ends of the ribbon 1 near where the ribbon 1 attaches to the reel, there are conductive strips of a suitable material, such as aluminium foil, attached to the ribbon 1. Each time the ribbon 1 is unwound to its end on one of the reels 10, 11, the condition is sensed and the direction of drive is reversed. In this manner usage and wear of the ribbon 1 are distributed throughout the entire length of the ribbon 1, while at the same time fresh portions of the ribbon 1 are constantly available for the various print hammers (not shown) as they impact the ribbon 1 against the paper 2.

Each of the ribbon reels 10 and 11 is coupled to substantially identical motors Ml and M2 (Figure 1) by shafts 16 and 18 respectively. Although both motors M1 and M2 are substantially identical and either motor can serve as both supply or take-up motor, motor M2 is designated as the supply motor and motor M1 is designated as the take-up motor. As previously noted, the ribbon 1 is driven from the supply reel 11 to the take-up reel 10 under the control of the motors M2 and M1, respectively, and the drive motor control circuit (as shown in Figure 3A). When the ribbon 1 is completely wound upon the take-up reel 11, the condition-is sensed at the supply reel 10 to provide a signal to the motor control circuit which causes the motors M1 and M2 to reverse direction.The reel 10 then becomes the take-up reel and the reel 11 becomes the supply reel with the ribbon 1 moving in the opposite direction. Both motors M1 and M2 drive in the same direction when moving the ribbon 1 in a given direction with the pulling or take-up motor applying the majority of the torque to keep the ribbon 1 under tension (DC permanent-magnet gear motors designated as GM9413 manufactured by Pittman may be utilized for this purpose). As the ribbon 1 is unwound or wound on a reel 10 or 11 the radius of the ribbon pack wound on the reel varies from an inner, minimum radius 14 of 1.8cm, to an outer, maximum radius 12 of approximately 4.2cm.

Referring now to Figures 2A, 2B and 2C,the ribbon is driven by two series connected motors M1 and M2, each motor driving one of the ribbon reels 10 and 11 (as shown in Figure 1A). A current sink takes a constant current I, from the common connection 25 between the two motors M1 and M2. It can be shown that T= KllC - K2 Equation A where T is the ribbon tension; K1 is a constant which describes both motor constants; and K2 is a constant which includes system friction.

The derivation of Equation A is given in Appendix A hereto.

K1 is a function of ribbon pack radius and, since the ribbon pack radius varies with the amount of ribbon wound on each reel, K1 and K2 are not true constants. Utilizing matched motors, the maximum variation in Ka will be about 8.5 percent. Therefore, a fairly constant ribbon tension may be maintained by maintaining lc constant and K1I, much greaterthan K2.

A further purpose of the ribbon drive system is to drive the ribbon at a fairly constant speed. The voltage drop across a DC motor is given by: V = KbW+ Rip + LAVA where V is the voltage drop across the motor; R is the armature resistance; w is the angularvelocity of the DC motor; Kb is the back EMF constant for the DC motor; 1A is the armature current; and LA is the armature inductance.

Applied to the matched DC motors of this embodiment of the present invention, the voltage drop across both motors, Bboth, is given by: Vboth = Kiwi + R1IA + LA1 iA1 + Kb2w2 + R21A2 + LA2A2 In steady state operation, iA1, 1A2 = and the terms RilAl and R2lA2 are small and relatively constant.Thus, we can say, Kb1 Kb2 Vboth = r1Ww + r2w2 r1 r2 For a constant ribbon speed, the term r1w1 will be constant (similarly the term r2w2 will be constant and equal to r1w1) and Both, the sum of the voltage drops across the motors M1 and M2, will be substantially constant. The voltage drop across each individual motor M1 and M2 is allowed to vary as the individual motors adjust two the radius of the ribbon pack on their respective ribbon reels while at the same time keeping the voltage across both motors M1 and M2 constant to keep the ribbon speed substantially constant. Note that constant ribbon speed is achieved independently of the ribbon tension control.

Referring now to Figure 2A, electronic switch 20 is closed and electronic switches 22 and 24 are open while electronic switches 26 and 28 are cycled at different rates to enable the motors M1 and M2 to drive the ribbon 1 (as shown in Figure 1 B) in a first direction. A single variable voltage supply +V is applied on a line 21. By cycling the electronic switch 28 at a predetermined rate the average voltage applied across the motors M1 and M2 is maintained at a fairly constant value. A signal designated as LONG PULSE cycles the electronic switch at the proper rate to maintain the desired average voltage across the motors M1 and M2. LONG PULSE is a signal at approximately 20 KHz in which the pulse width is a function of voltage +V on the line 21.

Similarly the electronic switch 26 is cycled at a predetermined rate by a signal designated as SHORT PULSE.

SHORT PULSE is a signal at approximately 20 KHz in which the pulse width is a function of the current lc flowing from the common node 25 through the switch 26.

Referring now to Figure 2B, the electronic switch 22 is closed and electronic switches 20 and 28 are open while electronic switches 24 and 26 are cycled in the same manner as described above to drive the ribbon 1 (as shown in Figure 1 B) in a second direction, the second direction being opposite to the above first direction.

Referring now to Figure 2C, electronic switches 20 and 22 are closed while electronic switches 24 and 28 are open to set up a tensioning mode which reduces any slack in the ribbon 1 (as shown in Figure 1) without appreciably moving the ribbon. Electronic switch 26 is cycled by SHORT PULSE to limit the sum of the motor currents thereby limiting motor torque. Since the motors M1 and M2 are substantially identical, the current divides substantially equally between the two motors, causing the two motors to produce approximately the same torque resulting in the ribbon being tensioned with little or no movement.

The ribbon drive is illustrated in Figures 3A and 3B, and can be divided into three subsystems: the ribbon I logic system, the current control loop and the drive motor power system.

Referring now to Figure 3A, the ribbon logic system is represented by block 40. The function of the ribbon logic system 40 is to convert commands from the printer command system into on/off signals which drive the power transistors in the drive motor power system which energizes the ribbon drive motors M1 and M2 (as shown in Figure 3B). The inputs to the ribbon logic system 40 are DIRECTION 1 (DIR 1) on line 38, DIRECTION 2 (DIR 2) on line 39, SHORT PULSE on line 71 and LONG PULSE on line 75. The outputs ofthe ribbon logic system 40 are the driving signals for the five power transistors in the drive motor power system called TL, TR, BR, BC and BL on lines 41,47,43, 51 and 53 respectively.

The printer command system sets the data on input lines 33 and 35 and then clocks the direction latch 30 by setting a ribbon stobe command, RIBBON STROBE, on line 37 low and then high to latch the data on lines ) 33 and 35 onto lines 38 and 39. The DIR 1 on line 38 and DIR 2 on line 39 have the following effect on the ribbon- drive: DIRECTION 1 DIRECTION 2 EFFECT 0 0 Ribbon Drive Off 1 0 Drive Ribbon From Supply 11 to Take-up Reel 10 0 1 Drive Ribbon From Take-up to Supply Reel 11 1 1 Tension Ribbon The outputs of the ribbon logic system as a function of the inputs DIRECTION 1 (DIR 1) and DIRECTION 2 (DIR 2) are as follows:: DIR 1 DIR 2 TR TL BR BC BL 0 0 OFF OFF OFF OFF OFF 1 0 OFF ON LONG SHORT OFF PULSE PULSE 0 1 ON OFF OFF SHORT LONG PULSE PULSE 1 1 ON ON OFF SHORT OFF PULSE The current control loop is illustrated in Figure 3A. The function of the current control loop are (1) to generate a pulse designated LONG PULSE on line 75 which regulates the ribbon drive motor voltage supply (+40 to +65 volts unregulated) by shortening the duty cycle for higher voltages and lenghthening the duty cycle for lower voltages in order to maintain a substantially constant voltage drop across the two drive motors M1 and M2 (as shown in Figure 3B), and (2) to generate a pulse designated SHORT PULSE on line 71 to maintain the current taken from the common node 75 between the drive motors M1 and M2 (as shown in Figure 3B) substantially constant. The frequency of LONG PULSE is approximately 20 KHz and its duty cycle varies from 35 percent to 60 percent as a function of the value of the ribbon drive motor unregulated voltage supply; the frequency of SHORT PULSE is the same as LONG PULSE. The inputs to the current control loop are the unregulated voltage supply on line 73 and a signal from a current sensing resistor 62 (as shown in Figure 3B) on lines 61 and 63.

The LONG PULSE generating block 70 receives an input voltage on line 73 and generates an output pulse on line 75 which is shorter the higher the input voltage level to the block is and longer the lower the input voltage level. In this manner, the switching period times the unregulated input voltage is maintained at a constant level. The output is low-pass filtered and compared to a 5 percent internal reference level. LONG PULSE is coupled to the ribbon logic system 40 via the line 75. SHORT PULSE is the signal which controls the current level drawn from the common node 25 of the two ribbon drive motors M1 and M2 (as shown in Figure 3B). The rising edge of LONG PULSE enables SHORT PULSE by turning on flip-flop 72.An operational amplifier 76 measures the current drawn from the common node 25 as represented by the signal on lines 61 and 63 which is compared to a reference voltage level by a comparator 77. When the current level exceeds a set value, the output signal from the comparator 77 clears the SHORT PULSE flip-flop 72 turning off SHORT PULSE. When LONG PULSE goes low, SHORT PULSE is also turned off. To prevent SHORT PULSE from switching at a lower rate than LONG PULSE, SHORT PULSE is ANDed with LONG PULSE at an AND gate 74 and coupled to the ribbon logic system 40 via the line 71. The current drawn from the common node 25 (as shown in Figure 3B) is set at approximately 300 milliamps. The arnount of tension in the ribbon is directly related to the value of the reference voltage set at the comparator 77.

Referring now to Figure 3B, the purpose of the drive motor power system is to supply power to the two ribbon drive motors M1 and M2 in accordance with commands from the printer command system. The inputs to the drive motor power system from the ribbon logic system 40 (as shown in Figure 3A) are the power transistor control signals TR, TL, BR, BC and BL on lines 41,47,43,51 and 53 respectively. The power transistors 20,22,24,26 and 28 are protected from transients in the +5 volt (not shown) and the +12 volt supply wherever the +5 volt supply (not shown) falls below a predetermined reference level.

PNP power transistors 20 and 22 and NPN power transistors 24, 26 and 28 serve as electronic switches to supply the proper currents to ribbon drive motors M1 and M2 (as shown in Figures 2A, 29 and 2C). The diodes 50, 52, 54, 56 and 58 in series with the power transistors prevent the transistors from turning on in the reverse direction and the diodes 55, 57, 59, 67 and 65 across the series diode-transistor pairs protect the power transistors from inductive kicks. The power transistors 24, 26 and 28 are further protected by R/C snubbing networks 68, 66 and 69 respectively. The snubbing network 66 of the power transistor 26 take the form it has to prevent the snubber capacitor 63 charging current from prematurely shortening SHORT PULSE on line 51.Current sensing resistors 60 and 64 sense the current of the non-pulling ribbon drive motor and generate a signal used in a ribbon fault warning system (not shown). Current sensing resistor 62 senses the current being drawn from the common node 25 and generates a signal on lines 61 and 63 to be used in the current control loop to generate SHORT PULSE (as shown in Figure 3A).

Referring now to Figures 3A, 3B and 4, the operation of the ribbon drive will be explained utilizing the timing diagram illustrated in Figure 4. Once a command is clocked into the direction latch 30, the DIR1 and DIR2 lines, line 38 and line 39 respectively, hold the state until the next command is clocked in. Typically, the ribbon drive will commence operation from the off state. The printer command system first instructs the ribbon drive to tension the ribbon. Tensioning the ribbon is accomplished by driving the ribbon reels 10 and 11 (as shown in Figure 1 B) against one another and controlling the ribbon tension by controlling the drive motor M1 and M2 currents. RIBBON STROBE 92 on line 37 clocks the direction latch 30 setting DIR1 90 and DIR2 91 on lines 38 and 39 high representing ones to the ribbon logic system 40.LONG PULSE 93 also serves as an internal clock to the ribbon logic system 40. On the next rising edge of LONG PULSE 93, TR 94 on line 47 and TL 95 on line 41 go high which turns on power transistors 22 and 20 respectively applying power to the ribbon drive motors M1 and M2. Both BR 96 and BL 97 on lines 43 and 53 respectively remain low holding power transistors 28 and 24 off. BC 98 on line 51 goes high which applies SHORT PULSE to turn power transistor 26 on and off at the SHORT PULSE rate thereby controlling the current drawn from common node 25.

To move the ribbon in direction 1 (i.e., to move the ribbon from the supply reel 11 to the take-up reel 10 as shown in Figure 1A), the printer command system sets DIR1 on line 38 high and DIR2 on line 39 low. To prevent simultaneous conduction of the power transistors in the drive motor power system, the ribbon logic system holds all output lines lowfor a set time period. At the expiration of the time delay, the next rising edge of LONG PULSE 93 sets TL on line 41 high and keeps TR on line 47 low turning on the power transistor 20 and keeping the power transistor 22 off. BR 96 goes high which applies LONG PULSE on line 43 to the power transistor 28, cycling the power transistor 28 on and off at the LONG PULSE rate which maintains the voltage drop across both motors M1 and M2 substantially constant.BC 98 on line 51 also goes high which applies SHORT PULSE to the power transistor 26, cycling the power transistor 26 on and off at the SHORT PULSE rate to control the current drawn from the common node 25 and maintain it substantially constant. BL 97 on line 53 remains low holding power transistor 24 in the off state.

To drive the ribbon in the opposite direction (from the take-up reel 10 to the supply reel 11, as shown in Figure 1A), the printer command system sets DIR1 90 on line 38 low and DIR2 91 on line 39 high. The falling edge of DIR1 90 sets all of the ribbon logic system 40 output lines low and triggers the time delay. At the expiration of the time delay, the next rising edge of LONG PULSE 93 sets TR 940n line 47 high which turns on the power transistor 22 and keeps TL on line 41 low which maintains the power transistor 20 off. BL 97 goes high applying LONG PULSE 93 on line 53 to the power transistor 24 cycling power transistor at the LONG PULSE rate to maintain a substantially constant voltage across the ribbon drive motors M1 and M2.

BR 96 remains low keeping the power transistor 28 turned off. BC 98 goes high applying SHORT PULSE on line 51 to the power transistor 26 cycling the power transistor 26 at the SHORT PULSE rate thereby maintaining the current drawn from the common node 25 at a substantially constant value. The ribbon drive will remain in this state until a state change is directed by the printer command system.

AppendixA For a DC motor: KTI = Jw + Dw + f0 [sgn(w)j + Text, where KT is the motor torque constant; lis the armature current; J is the motor inertia; D is the motor damping factor; F0 is the motor starting friction; withe motor angular velocity; and sgn(w) is a constant which: sgn(w)=1 if woo sgn(w) = 0 if w = 0 sgn(w)= -1 ifw < 0;and Text is any external torque applied to the motor.Applied to a system of two coupled DC motors as described herein and illustrated in Figure 1: J1 D1w1 f01 Tr1 l1 = w1 + + [sgn(w1)] + KT1 KT1 KT1 kT1G J2 D2W2 f02 Tr2 l2 w2 + + [sgn(w2)] KT2 KT2 KT2 KT1G and lc = l1 - 12 (as shown in Figure 2) where l1 is armature current for motor M1; 12 is armature current for motor M2; r1 is the radius of the ribbon pack on reel 10 coupled to motor M1; r2 is the radius of the ribbon pack on reel 11 coupled to motor M2; T is the ribbon tension; and G is the motor gear ratio (identical for both motors M1 and M2); During steady state operation, both wi and w2 are small, therefore rearranging terms, we have: G D2w2 D1w1 f02 f01 lc + - + [sng(w2)] - [sgn(w1)] r1 + r2 KT2 KT1 KT2 KT1 KT1 KT2 Assuming both W1 and w2 are positive, the term F02 F01 KT2 KT1 is small and may be neglected.

Typically, D < < KT and for rmax # 2.4 rmin, the term D2 w@ D1 KT2 - w1 is KT1 For matched motors as described herein, the term G KT1KT2 r1KT2 + r2KT1 is fairly constant; and T = K1lc - K2 where G D2w2 D1w1 k2 = r1 + r2 KT2 KT1 KTl KT2 For the matched motors utilized in the apparatus described herein, the term K2js small and does not vary by more than about 20 percent.

Claims (7)

1. Apparatus for driving an elongated web member, extending between and having the opposite ends thereof wound on opposite ones of a pair of rotatable elements, at substantially constant speed and under substantially constant tension, said apparatus comprising: means for mounting the pair of rotatable elements for rotation with said elongated web member extending therebetween; a pair of motors, each motor being arranged to be coupled to a respective one of the pair of rotatable elements, said motors being electrically connected in series and having a common node therebetween coupled to each motor of said pair;; first switching means coupled to the inputs of both of the motors, the first switching means being responsive to a set of control signals and to a first electrical signal to couple a variable voltage source to the input of one of the motors so as to maintain a substantially constant voltage drop across said pair of motors, said one of the motors operating as a web member take-up motor and the other one of the motors operating as a web member supply motor; second switching means coupled to said common node and to a current sink to provide a common current path coupled to each of the motors via said common node, the second switching means being responsive to a second electrical signal so asto maintain the current drawn from the common node by the current sink substantially constant;; first generating means responsive to the voltage level of the variable voltage source to generate said first electrical signal to vary the duty cycle of the first switching means so as to maintain the voltage drop across both motors substantially constant; and second generating means, including differential amplifier means for comparing the current drawn from the common node to a reference value, the second generating means being responsive to the level of current drawn from the common node to generate said second electrical signal to vary the duty cycle of the second switching means so as to maintain the current drawn from the common node at a predetermined substantially constant value.

2. Apparatus according to claim 1 wherein the first switching means comprises a plurality of electronic switches, first switches thereof being provided for coupling the variable voltage source to the input of both of the pair of motors, second switches thereof being provided for coupling the input of both of the pair of motors to ground, the first switches being responsive to the set of control signals to couple the variable voltage supply to one of the pair of pair of motors operated as a take-up motor and the second switches being responsive to the first electrical signal alternately to connect and disconnect the other one of the motors to and from ground so as to maintain a substantially constant voltage drop across both motors, said other one of the motors operating as a supply motor.

3. Apparatus according to either one of claims 1 and 2 wherein the second switching means comprises an electronic switch responsive to said electrical signal, the electronic switch coupling said common node to said current sink, and a current sensing resistor to provide a voltage signal indicative of the level of current drawn from said common node.

4. Apparatus according to claim 2 or claim 3 as appended to claim 2 wherein said first switched and said second switches comprise power transistors.

5. Apparatus according to claim 3 or claim 4 as appended to claim 3 wherein said electronic switch comprises a power resistor.

6. Apparatus for driving an elongate web member, substantially as hereinbefore described with reference to the accompanying drawings.

7. An impact printer comprising: a platen; a print hammer means disposed opposite the platen, the print hammer means being capable of being reciprocated relative to the platen; a pair of motors, each motor being provided for rotating a respective one of a pair of rotatable reels, each of which receives a respective end of an elongate inked print ribbon wound thereon, which ribbon can be fed from one reel to the other between the print hammer means and a printable medium located against the platen, and the motors being electrically connected in series and having a common node therebetween electrically connected to each motor of said pair;; first switching means coupled to the inputs of both motors, the first switching means being responsive to a set of control signals and a first electrical signal to couple a variable voltage source to the input of one of the motors and to couple the input of the other one of the motors to ground so as to maintain a substantially constant voltage drop across said pair of motors thereby maintaining a substantially constant ribbon speed independent of varying the radius of ribbon wound on the rotatable reels, said one of the motors operating as the print ribbon take-up motor and said other one of the motors operating as the print ribbon supply motor;; second switching means coupled to said common node and to a current sink to provide a common current path coupled to each of the motors via said common node, the second switching means being responsive to a second electrical signal so as to maintain the current drawn from the common node by the current source substantially constant thereby maintaining a substantially constant print ribbon tension; first generating means responsive to the voltage level of the variable voltage source to generate said first electrical signal to vary the duty cycle of the first switching means so as to maintain the voltage drop across both motors substantially constant; and second generating means, including differential amplifier means for comparing the current drawn from the common node to a reference value, the second generating means being responsive to the level of current drawn from the common node to generate said second electrical signal to vary the duty cycle of the second switching means so as to maintain the current drawn from the common node at a predetermined substantially constant value.