Note: Descriptions are shown in the official language in which they were submitted.
1~7537~ 1
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BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to devices and circuitry
for operating a stepping motor, and in particular for operating a
s~epping motor to position a type carrier in a printing device in
combination with a position transmitter connected to the motor
shat.
Descrip~ion of the Prior Art
It is known to those skilled in the art of teleprinting
and typewriter design to position a type carrier such as, for
example, a typing cylinder or a typing disk, by the use of a
stepping motor ollowed by a printing operation undertaken with a
printing hammer according to the selected type character.
A device of this type, in which a typewri~er employs a
typing cylinder which is positioned by means of a stepping motor,
is known from German OS 2,156,093, corresponding to United States
Patent 3,823l265. The typing cylinder is connected to ~ timing
disk which serves as a position transmitter, the timing disk having
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marks thereon corresponding to the number of type columns carried
on the typing cylinder. The marks are scanned by a scanning device
during pre-adjustment of the type carrier. The driving stepping
motor is connected by suitable gearing with the typing cylinder
and the associated timin~ disk.
In conventional devices wherein a type carrier is pre-
set by the use of a stepping motor, it is customary to bring the
motor to a stop in a na~ural stop position. The number of such
stop positions can be multiplied by th~ use of appropriate gear
mechanisms so as to coincide with the number of type columns or,
if a typing disk is employed as the type carrier, to coincide with
the number of typing spokes on the typing disk~ This conventional
technique of obtaining the desired num~er of stop positions by
a gear mechanism has the disadvantages of reducing the control
precîsion and being susceptible to wear due to abrasion necessitating
conscientious preventive maintenance such as lubrication.
The use of a commutator-controlled direct current motor
in place of a stepping motor similarly does not overcome the dis-
advantages because of the limited service life and reliability of
the commutator, which experiences significant wear as a result of
abrasion by the brushes.
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SUMNARY ~F THE INVENTION
It is an object of the present invention to provide an
apparatus for operating a stepping motor in a printing device for
positioning the type carrier by which the stepping motor is directly
connected to the type carrier, such as a typing disk. It is a
further object of the present invention to provide such an apparatus
by which the type carrier can be adjusted as precisely as possible
~n a relatively short adjustment time~
The above objects are inventively achieved by an apparatus
for operating a stepping motor which operates the motor in a manner
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so as to simulate a commutator-controlled direct current motor. Circuitry
including a plurality of switches respectively associated with th~ windings
of the stepping motor is provided with an additional switching arrangement
which alternately activates the winding switches. The circuitry generates
a stator field which is controlled in dependence upon a position trans-
mitter which is displaced with respect to the permanent rotor field of the
stepping motor by a pre-set phase difference. The stepping motor is main~
tained in a holding position by logic circuitry which inverts the stator
field in dependence upon an analog control signal received from the posit-
ion transmitter.
Thus, in accordance with a broad aspect of the invention, thereis provided, in an apparatus for operating a stepping motor for positioning
a type carrier in a printing device, said stepping motor having a rotor
corotatably connected to said type carrier and to a timing disk for an
optical position transmitter in said printing device, the improvement of:
a means for operating said stepping motor to simulate a commutator-control-
led direct current motor including a plurality of electronic switches
connected to individual windings of said stepping motor, a rotary field
; switch for alternatingly actuating individual ones of said switches, said
means generating a stator field in accordance with signals received from
said optical position transmitter, said stator field being displaced by
a predetermined phase difference with respect to a permanent rotary field
of said stepping motor.
The circuit disclosed and claimed herein results in an operation
of the stepping motor which is analogous to a direct current motor and in
effect operates as an "electronic commutator." Driving the stepping motor
in this manner achieves the advantages of a direct current motor without
the disadvantages of a genuine commutator, so that the service life and
reliability of the motor is significantly improved for the reason that the
wear normally associated with the action of the brushes on the commutator
in a direct current motor is eliminated. The stepping motor can be operated
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so as to stop precisely at positions corresponding to the number of spokes
on a typîng disk by suitable inversion of the stator field.
For pre-adjustment of the typing disk~ a digital coarse control
loop is included in the control circuitry~ A target point to which the
typing disk is to be set is entered by a target address which is supplied
to a device for generating a rotar~ field for operating the stepping
motor so that the stepping motor is accelerated to a maximum speed which
is stored in a memory and which is dependent upon the distance to the
selected target position. The acceleration phase of the stepping motor
may be followed, if necessary, by a coasting phase of constant rotational
velocity.
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which in turn is followed by a brake phase which is determined by
a brake curve also stored in a memory. The stepping motor is
braked until the typing disk is posi~ioned in the region of the
target point at which time an analog fine control loop takes over
and, in dependence upon the amplitude of the si~nal received from
the position transmitter, undertakes precise positioning to the
target point.
By the use of a control arrangement of the type described
above, the control and positioning of the typing disk is undertaken
digitally at first until the typing disk is positioned in the region
of the target point, at which time positioning of the typing disk
is undertaken by an analog process so that the disk can be adjusted
with the required precision. This apparatus thus unites the advan-
tages of digital control of the stepping motor with the advantages
of precision adjustment heretofore obtainable only by ~he use of a
servo direct current motor.
Moreover, a particularly rapid adjustment of the type
carrier is possible with the above apparatus, because both the
optimum acceleration characteristics and braking characteristics
~asso~ciated with a particular motor are stored for use, and can be
called for each acceIeration and braking process.
DESCRIPTION OF THE DRAWINGS
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Fig. 1 is a block dia~ram of an apparatus for positioning
a typing disk in a printing device constructed in accordance with
the principles of the present invention~
Fig. 2 is a graphic representation of ~he output signals
of the position transmitter employed in the apparatus of Fig. 1.
Fig. 3 is a graphic represent~tion of the manner by which
the rotary field is controlled by the apparatus shown in Fig. 1 as
a function of the position of the rotor of the stepping motor.
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Fig. 4 is a schematic illustration of a three-phase
stepping motor of the type employed in the present invention during
operation with open winding.
Fig. 5 is a schematic representation of a three-phase
stepping motor of the type employed in the present invention with
short-circuited winding.
Fig. 6 is a graphic representation of the motor torque
for compensation of the natural stop position.
Fig, 7 is a graphic representation of a signal for
inverting the rotary field.
Fig. 8 is a block circuit diagram of the apparatus shown
in Fig. 1 showing the coarse control loop in grea~er detail.
Fig. 9 is a graphic representation of a posi~ioning process
undertaken by the apparatus of Figs. 1 and 8.
Fig. 10 is a graphic representation of the output of the
~ ~ poosition~transmitter during the positioning process of Fig. 9.
;~ ~ Fig. 11 is a block circuit diagram of the analog control
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loop employed in the apparatus of Figs. 1 and 8.
Fig. 12 is a circuit diagram of the value former employed
in the fine control loop of Fig. 11.
Fig. 13 is a circuit diagram of the two-point current
regulator and three-phase power bridge employed in ~he circuits of
igs. l and ~. ~
DESC~IPTION OF THE PREFER~ED EMBODIMENTS
An apparatus is schematically shown in Fig. 1 for posi-
tioning a typing disk T in a printing device such as, for example,
~a teletypewriter. The apparatus includes a three-phase stepping
motor SM ha~ing a drive shaft which is corotationally coupled with
the typing disk T and which is also corotati~nally coupled to a
timing disk TS which is part of an incremental optical position
transmitter PG. The timing disk TS has a number of apertures or
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slots corresponding to the number of spokes on the typing disk T,
which apertures are scanned by an optical arrangement known to those
skilled in the art, not shown in greater detail in Fig. 1.
The further elements of Fig. 1 together with the stepping
motor form a closed control loop which includes a digital cOarse
control loop DG which is connectable as required to the position
transmitter PG by a switching device SE, as well as an analog fine
control loop AF for precise positioning of the typing disk T, which
is also connectable to the position transmitter PG via the switching
device SE. The control loops ~F and DG are each connected to a
rotary field switch DS which controls the s~epping motor SM, as
well as to a two-point current regulator SR and a three-~hase power
bridge LB~ By means of a conventional input device ~not shown) a
signal SP such as a ta.rget signal or target address for the desired
position of the typing disk T is supplied to the control loop.
After a completed positioning of the typing disk T, a printing
hammer release signal is generated on line HF which brings about
the~clearing and release of th~ printing hammer corresponding to
the selected typing disk position.
The outputs of the position transmitter PG are graphically
shown in:Fig. 2. Those output signals include a triangular control
signal~:RS which:is shown in Fig.. 2 in dependence upon the angle of
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rotation DW of the stepping motor SM.
In addition to the control signal RS, the position
transmitter PG also releases a rectangular index signal ID and a
signal TTL, from which the direc~ion of movement of the typing disk
T is deriv~d. The index signal ID and the direction signal TTL
are displaced with respect to the control signal RS by 90~.
The ~riangular control signal RS is required for ~he
analog regulating process for precise positioning of the typing
di~k T. As can be seen in Fig~ 2, certain portions of the ~riangular
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signal RS are in registr~ with portions of the signal TTL which
are at O potential. Those portions of the signal RS correspond
to the precision regulating region. The deviation of the potential
of the signal RS in the vicinity of a printing point A is converted
in the precision regulating region into a proportional motor current
for controlling the'stepping motor SM~ At the zero passage A,
depending upon the'sin, the motor direction, and corresponding
torque, is invPrted ~ia the rotary field switch DS, whereby the
position of the typing disk T is maintained. A signal AP has
shaded regions shbwn beneath the control signal RS which define
the clearing region in the vicinity of ~he printing position A
during which time the corresponding printing hammer is activated.
A brushless four-pole three-phase'stepping mo~or SM
serves as the drive for the typing disk T. The stepping motor SM
has the advantage over a commutator-controlled direct curren~ motor
of operating with less abrasion and additionally has smaller external
dimensions. As described above, the stepping motor SM is driven to
simulate such a direct current motor by a suitable controlled elec-
tronic rotary field.
During a revolution of the rotor of the stepping motor
SM, the individual phases Pl, P2 and P3 are sequentially connected
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to a current generator via the power bridge LB, as graphically
shown in Fig. 3 and schematically represented in Figs. 4 and 5.
The result of such operation is the voltage pro~ression U shown in
Fig. 3. The current course I during a motor phase Pl is also shown
in Fig. 3. Beginning from the synchronized position, the angle of
rotation DW is represented on the abscissa of the graph showing
individual stop positions O to 11 which occur in the case of stepping
motor operation with a short-circuited winding.
In order that the torque can be'released ~o the motor
shaft, the stator fiel.d of the ~tepping motor SM must be phase
displace'd with'respect' to the permanent rotor field by a pre-set
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phase difference angle, which for the purposes of this discussion
is referenced PD, but is not shown in the drawings. The torque is
theoretically proportional to the value of the sin PD and the motor
current, which builds up the stator field. In the case of a
homogenous field curve, the maximum is attained at a phase difference
angle PD of 90 (mid-way between the stator and rotor field) al~hough
the phase difference may be in the range of 60 to 120~ The
stepping motor SM is controlled as shown in Fig. 4 such that of
the three legs, in each case only two legs have current passing
through them. In Figs. 4 and 5, the current vector is designated
by i and the phases are designated by Pl, P2 and P3. The field
switch over of the rotary field occurs in the region of + 30.
This results in the twelve switch over points shown in Fig. 3.
In the case of a conventional typing disk having 108 occupiable
positions, the controlling arrangemen~ disclosed herein relays
the stator field after nine mark or aperture divisions of the timing
disk TS associated with the position transmitter PG are passed.
For satisfactory operation of the motor SM, it is neces-
sary to relate the state of the stator field, as was done with the
typing disk, to the synchronization position. In the case of
static current flow, the initial position of the stator field is
determined and the timing disk TS of the position transmitter PG
is oriented accordingly. me required phase difference of 90 is
produced by the control arrangement, so that by means of this
l'electronic commutator" the stepping motor SM behaves as a commutator-
controlled direct current motor with twelve brushes.
- A typical operation for stepping motors with a 30
control system, of the type shown in Figs. 4 and S, whereby alter-
natingly one winding conne~tion remains free, that is, two windings
are operated in parallel, has disadvantages in the context of
printing devices. ~ecause of the complete current degradation in
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each case in the sh~rt-circuited winding, increased eddy curren~
losses result. The varying power consumption during switch over
from the operating mode of Fig. 4 ~o an operating mode of Fig. 5
brings about torque fluctuations.
In order to maintain the stepping motor SM in a printing
position A, by means of the control arrangement shown in Fig. 6 the
rotary field is inverted by the rotary field switch DS, which cor-
responds to a phase rotation of 180~. In Fig. 6, the vector BR
designa~es the rotor fieId direction and magnitude, the vector Ml
represents the stator field with an associated inverse stator field
represented by the vector M2, M designates the resultant torque
vector and LW represents the load or stress angle of the stepping
motor SM. The switch over of the rotary field proceeds with a
frequency of approximately 5 kHz, whereby the position transmitter
PG supplies the control signal RS for this process by means of
the operational sin of the analog control signal RS. The current
curve for the stepping motor I is shown in Fig. 7 plotted wi~h
respect to time t, wherein TL and TR respectively represent ~he
operating duration for left and right (or countercloclcwise and
clockwise) rotation of the rotor. The'resulting moment M shown
in Fig. 6 for the printing position A is determined by the equa~ion
M = (Ml TL ~ sin LW) - (M2 TR sin LW).
A schematic block dia~ram is shown in Fig. 8 of the
apparatus of Fig. 1 showing the coarse control loop DG, outlined by
the dot and dash line, in greater detail. The digital course control
loop DG obtains signals or the determination of the position in
digital form which is indicated by RSD which is a digitalized version
of the position signal RS. The loop DG also receives the rotation
direction signal TT~ from the position transmitter PG. Proceeding
from the'synchronized position, the positioning address SP is
compared by a comparator VG with'the'actual position of the typin~
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disk T which is determined by a known e~aluated circuit AUS, A logic
circuit LA provided with the outpu~. of the comparator VG and an
optimum brake curve supplied by a brake curve memory BKS selects the
direction of rotation which provides the shortest connection or move-
ment to the new posi~ion and the typing disk approaches the new posi-
tion with maximum momen~, that is, with constant current. The digital
control loop DG thus compares the velocity in each case given the
time between the zero passages of the signal RS (and the correspondin&
digital signal RSD~ of the position transmitter with the desired value
and determines whether to acceIerate, brake, or switch over to a
coasting phase. The complete braking occurs by means of current
reversal and takes place with maxi~um moment~
If the desired position is attained and the type which
is to be printed is in the vicinity of the printing position A,
switch over occurs to the analog position control loop AF, which
takes over the precision control of the motor to move the typing
disk T to the prin~ing position. The digital con~rol loop DG becomes
active again only with a new address or as a result of a mechanical
movement of the typing disk.
Positioning of the typing disk T to the next position
proceeds via the shortest distance with the use of a velocity eurve
o~ the type shown in Fig. 9. The rotary field is generated such
that ~he motor attains its maximum r4tational velocity of appro-
ximately 1400 revolutions per minute a~ter approximately 20 mark
di~isions or apertures of the timing disk TS are scanned by the
position transmitter PG, corresponding to approximately 70~ rotation.
As shown in Fig. 9, wherein the rotational ~elocity of the motor
is presented on the ordinate and is plot~ed in dependence upon the
distance between two positioni~g points A and B on the abscissa,
the positioning pro~eeds f~m an initial position AN to the end
position B as follows. The stepping motor SM is at firs~ accelerated
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up to position C. The point C is the'position where the velocity
is attained which is pre-set by the braking curve. This acceleration
phase is followed by a phase of constant velocity or coasting from
point C to point Cl. From the point Cl to the tar~et or desired
point B, the motor is braked corresponding to the'brake curve which
is shown in Fig. 9 and which is stored in the brake curve memory BKS~
The brake curve contains the'rela~ionship between the velocity and
distance up to the desired position B for an optimum braking process.
The curve is determined based on ~he most unfavorable conditions
with respect to the magnetic tolerances of the motor, so that the
motor comes to a stop in the target position B without significant
overshoot. Different motors of course display different braking
characteristics, however, these different characteristics are equalized
by the braking curve associated with each motor which undertakes a
~continuous switch over between braking and coasting resulting in the
step-shaped brake curve of the type shown in Fig. 9~ Each mo~or has
an individual brake curve associated therewith, calculated and plotted
in dependen e upon the'unique characteristîcs of the particular motor.
The brake curve is stored in the brake curve memory in
the form of intervals3 so that the'angular velocity of the steppin~,
motor SM is decreased to such an extent that ~omplete brakin~ can be
undertaken in the vicinity o the target position B by the analog
precision control loop AF. The actual braking proceeds such that
at the beginning of each period of the received position transmitter
signal RS, the measured actual velocity is compared with the pre-set
value. In dependence upon the result of this comparison, during
this signal period, a switch over occurs to braking or coas~ing as
needed. When th~ distance to the desired position is smaller than
5 mark divisions, thls comparison is undertaken at each half mark
division. In the next to last half mark division before the target
position, depending upon thP ~easured vel'ocity, a selection is made
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as to whether to permit the motor to coast, to emit a short brake
pulse, or to emit a long brake pulse. In the last half mark division
before the target position, the motor is actuated with a brake pulse
having a duration dependent upon the measured velocity. Thus, the
motor enters into the target position with a~propriate velocity. In
the re~ion of the target position B, as stated above, the analog
control loop AF brings the motor to the precise position. It is,
of course, possible that the starting point AN and the target point
B will be disposed closer to one another than in the example dis-
cussed above, so ~hat no coasting period is necessary in between.
An accelerating and braking curve progression of this sort is also
shown in Fig. 9, between the points Al, C2 and B.
A precise depiction of the positioning process from a
point AN to a point B is shown in Fi~ 10, in which the control
signal RS is plotted in dependence upon time t. The progression in
Fig. 10 corresponds to the first example discussed in connection
with Fig.~9, nameIy from the point AN to the point B. An accelera-
tion region exists from the point AN to the point C, followed by a
region of constant speed or coasting, which is not shown in Fig. 10,
and finally followed by the actual braking which occurs in the region
between the points Cl and the target point B. A deceleration region
is referenced with V, which is divided into a number of different
regions including region Vl, wherein the comparison takes place
between the actual and desired value at the beginning of each period
of the control signal RS, whereby the duration of a period of the
control signal RS corresponds to one mark division.
Other comparisons are undertaken in the spacing of five
mark divisions from the desired posi~ion B between the desired and
actual values of the velocity in each half period of the signal RS,
designated by the regions V2, Y3 and V4~ In the last half mark
division V4 before the attainment of the target position B, the
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velocity of the stepping motor is modulated to a constant target
arrival velocity. The urther positioning is then taken over by the
analog precision control loop AF. The entire process is divided into
a digital control region DRB and an analog control region ARB.
For the precise regulation of the typing disk T to the
printing position an analog control loop of the type shown in Fig. 11
in detail is employed. During the analog control phase ARB, a
constant direction rotary field is supplied to an inverter IN based
on information received from an analog evaluator AUS' and an analog
control unit DRS~. The inverter IN supplies a signal to the rotary
field switch DS to invert the rotary field after attainment of the
printing position in order to hold the motor in the requested target
position.
The analog precision control loop AF employs the amplitude
of the analog transmitter signal RS of the position transmi~ter PG
as a regulated value, whereby the motor moment is regulated in
dependence upon this value in rate and amplitude in such a manner
that the tor comes to rest with an attenuated movement in the zero
passage of the control signal. For monitoring, an additional ampli-
fier (not shown) is provided, to which is supplied the transmitter
signal RS, and which supplies an output signal which indicates whether
the typing disk T is positioned sufficiently precisely in the printing
position. If the printing position is attained, by means of the
analog evalua~or AUS', after a pre-set time of approximately 2 ms,
during which the position may no longer change, the printing is
cleared.
After a printing operation, the amplification of the
analo~ precision control loop is lowered to such an extent that the
motor SM still is maintained in the desired position, h~wever, a
disturbance noise which is only weakly audible as a result of the
constant rotary field inversion via the inverter IN is produced.
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The analog precision control loop AF contains a propor-
tional-differential regulator PD, which is connect d to an int gral
regulator IR. A value former BB is post-connected to the propor-
tional-differential regulator PD, which is connected throu~h a two-
point current regulator SR with the rotary field switch DS.
The integral regulator IR performs the function of sub-
stantially increasing the amplification for signals which are almost
static at a low an~ular velocity or standstill, so that the influence
of ~he permanent motor stop moment is suppressed and a better posi-
tioning precision is attained. For fast movements, however, the
integral regulator IR is substantially without effect.
The proportional-differential regulator PD performs the
function of compensat;ng the lagging mechanical and electrical phase
displacement of the motor SM. The amplification progression is
thereby accommodated to the motor movement such that the attenuation
of the rotational movement proceeds in the zero passage of the output
signal of the position transmitter RS. By this operation, the
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oscillation tendency of the entire analog control loop is significantly
attenuated.
The output signal of the proportional-differential
regulator PD is supplied to one side of the inverter IN and is also
supplied to the value former BB, which converts the deviation from
the zero point into a proportional direct voltage.
This direct voltage is converted in the two-point current
regulator SR into a proportional current and thus into a proportional
motor moment.
The value former BB shown in Fig. 12, which is employed
in the fine control loop AF, is a precision rectifier with a low
zeroing error~ The value formar BB consists of two operational
amplifier~ OPl and OP2, which have a common output and which are
connected such that one amplifier has an amplification factor of
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~1 and the other amplifier has an amplification factor of -1. For
this purpose, the inverting input of the first operational amplifier
OPl is directly coupled to the output thereof, while the invertin~
input of the second operational amplifier OP2 is connected through
a resistor Rl which may, for example, have a value of 10 kQ, to the
common output UA. An input signal is supplied to the inverting in-
put of the amplifier OP2 through a resistor R2, which may, for
example, have a value of 10 kQ, and is supplied directly to the non-
inverting input of the amplifier OPl. The'operation of the ~alue
former BB on an input signal OV, supplied to the input W , is shown
by the resulting output signal OV' appearing at the output UA, which
is negatively rectified.
As described a~ove, the stepping motor SM is operated
with an impressed current which in the'target or printing position
can be regulated to zero. This occurs as shown in Fig. 13 by means
of the operation of the rotary field switch DS in combination with
a power bridge LB having six transistors T connected in Darlington
pairs for actuating the stepping motor SM with a corresponding current.
For the coasting phase, the power bridge LB is in a blocking
state and the motor is essentially currentless. The two-point
regulator operates with a set frequency of approximately 40 kHz,
which is supplied to an operational amplifier OP3. In the amplifier
OP3, the preampliied current signal, which is measured by a
resistor R3, is compared in the analog control loop AF to the output
signal of the value former BB, or, in the'digital coarse control
loop DG, is compared to a constan~ voltage. The result of either
comparison controls a switching transistor ST. Corresponding to
the control signal received at the base of the transistor ST, the
transistor activates the'stepping motor SM through a feeder reactor
L with an operating voltage of approximatPly 40 volts. Upon dis-
connection of the switching tranaistor ST, in order to avoid im-
precisions in controlling the motor, it is nece'ssary to quickly
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dissipate energy which is stored in the entire circuit arrangement.
This is undertaken by means of a feedback transformer RT, the
primary winding of which is connected through a coasting diode D3
with the output of the switching transistor ST, and the secondary
winding of which is connected through a feedback diode D2 with a
direct voltage source S~ The windings of the feedback transformer
R~ are connected electrically oppositely 7 SO that the ratio amounts
to 1:2. The feedback transformer RT dissipates the energy stored
in the circuit in the disconnected state of the switching transistor
ST by diverting the'energy back to the'voltage'supply S.
In the disconnected state of the switching transistor ST,
the output of the transistor ST is at a potential of -20 volts as a
result of the presence of the feeder reactor L. This voltage leads
to a voltage of 40 volts at the feedback transformer RT on the
secondary side,' whereby through thP feedback diode D2 a feedback of
the stored energy results to the direct voltage source S. In addi-
tion to the diodes D2 and D3, a further coasting diode Dl is dis-
posed between the'output of the feeder reactor L and the voltage
source S. The coasting diode, which.is associated w~th the stepping
motor SM, perorms the function of guiding the'energy stored in the
feeder reactor L back to the'direct voltage supply S during blocking
of the transistors T through the rotary field control switch DS.
Al~hough modifications and ehanges may be suggested by
those skilled in the art it is the intention of the inventors to
'embody within the patent warranted hereon all changes and modifica-
tions as reasonably and properly come within the scope of their
contribution to th~'art.
