Position control system comprising a digital algebraic adder circuit

A system for controllably driving a motor to place a movable element at a position indicated by a command signal from an external source produces, in response to a phase staggered incremental position signal pair, one and the other of two position pulse sequences at a time depending on an instantaneous angular velocity of the motor. Responsive to the command signal and the position pulses, a mode signal is produced to indicate a position control mode while a position error is within a predetermined range and otherwise a velocity control mode. Likewise, a signal is produced that represents at least one reference angular velocity and the position error during the respective modes. Responsive to the position pulses alone, gate pulses are produced either with two different pulse widths depending on the modes or with a single pulse width. During presence of each gate pulse and dependent on the sense of the instantaneous angular velocity indicated by the position pulses, a digital algebraic adder circuit calculates an algebraic sum of the reference angular velocity or the position error and a reference feedback value, which may either be memorized in the system or supplied from the source and may take two digital values depending on the modes. A digital signal representative of the sums and either the reference angular velocity or the position error during presence and absence, respectively, of the gate pulses is converted to an analog signal, which is smoothed and amplified to provide a motor drive signal.

BACKGROUND OF THE INVENTION 
This invention relates to a position control or positioning control system 
responsive in general to a sequence of position commands supplied from an 
external source for intermittently controllably positioning a movable 
element at successively commanded positions. More specifically, this 
invention ralates to a position control system for either a carriage or a 
type or print wheel, drum, or cylinder of a serial printer of the type 
known as an impact type in the art. 
An impact type serial printer for use in combination with an electronic 
digital computer comprises a carriage, a carriage motor for reciprocating 
the carriage along a predetermined path, and a position control system for 
controllably driving the motor so as to successively intermittently place 
the carriage at desired linear positions commanded by the computer. A 
rotary type wheel carrying a plurality of type elements is mounted on the 
the carriage together with a type wheel motor for rotating the type wheel. 
A similar position control system contrallably drives the type wheel motor 
to intermittently place the type wheel at desired rotational positions 
commanded also by the computer. Successively selected type elements are 
thereby placed at a printing position determined relative to the carriage. 
A position control system for controllably positioning a movable machine 
element, such as the carriage or the type wheel, to a commanded position 
comprises an increment encoder or position transducer mechanically or 
otherwise coupled to the motor. It is already known to make the encoder 
produce a pair of phase staggered or displaced incremental position 
signals, which is in effect representative of an angle of rotation of the 
motor and a current or instantaneous angular velocity thereof and hence a 
current position and an instantaneous speed of the movable element. The 
position signals are subtracted from the command signal so as to provide 
an error signal representative of a position error between the current and 
the commanded positions. The motor is controlled to render the position 
error zero. It is to be noted here that the commanded positions are 
selected from a plurality of predetermined positions with repetition 
allowed. 
In U.S. Pat. No. 3,954,163 issued to Andrew Gabor on May 4, 1976, a 
position control system for an impact serial printer is disclosed wherein 
each of a motor for a carriage and another motor for a rotary print wheel 
is controllably driven by a first motor drive signal at first until the 
movable element reaches a point spaced a predetermined distance from each 
commanded position and then by a second motor drive signal until the 
element eventually rests at the commanded position. By the use of a 
velocity logic unit depicted in FIG. 7 of Gabor at 67 and in his FIG. 12 
in two parts at 157 and 158, an analog velocity signal is produced from 
the position signal to indicate a current speed of the element, namely, a 
current angular velocity of the motor. An analog velocity reference signal 
is produced in response to an external control signal and to the position 
signal. The first motor drive signal is given by algebraically summing the 
velocity reference signal and the analog velocity signal. An analog 
position error signal is produced from an external command signal and the 
position signal to indicate a position error of the element. The second 
motor drive signal is given by algebraically summing the position error 
signal and the analog velocity signal. Use of analog signals, however, 
makes it difficult to achieve expected results in productivity and 
maintenance of the position control systems. The above-mentioned velocity 
logic unit is complicated in structure. Furthermore, it is rendered 
difficult to provide a compact system. This means that the position 
control system becomes bulky and accordingly highly expensive. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a position 
control system operable on a digital basis. 
It is another object of this invention to provide a position control system 
of the type described, which is simple in structure and is therefore 
compact and not expensive. 
It is still another object of this invention to provide a position control 
system of the type described, which is capable of optimally controlling a 
variable angular velocity of a motor for a movable element and 
consequently positioning the element at each commanded position at high 
speed and with a high precision. 
It is a further object of this invention to provide a position control 
system of the type described, wherein use is unnecessary of a complicated 
velocity logic unit for producing an analog signal indicative of an 
instantaneous angular velocity of the motor in response to incremental 
position signals. 
It is possible to manufacture essential parts of a position control system 
according to this invention as an integrated circuit. 
A position control system to which this invention is applicable is for use 
in combination with a movable element, a motor rotatable with a variable 
angular velocity to controllably vary a current position of the element, 
and a command signal generator for producing a command signal indicative 
of a commanded position of the element. The angular velocity has either of 
two senses of rotation at a time. The system comprises an increment 
encoder to be coupled to the motor to produce a pair of polarity variable 
incremental position signals and servo control means responsive to the 
command and the position signals for supplying a motor drive signal to the 
motor to make the motor move the element eventually to the commanded 
position. The position signals vary their respective polarities with a 
phase difference therebetween in response to every predetermined 
incremental angle of rotation of the motor. Each of the position signals 
thereby varies its polarity at a rate dependent on a current angular 
velocity of the motor. The phase difference is representative of the sense 
of the current angular velocity. In accordance with this invention, the 
servo control means comprises position pulse producing means responsive to 
the incremental position signals for producing position pulses 
representative of an angular position of the motor and the current angular 
velocity, mode signal producing means responsive to the command signal and 
the position pulses for producing a mode signal during an interval of time 
during which the current position is within a predetermined distance from 
the commanded position, and control signal producing means responsive to 
the command signal, the position pulses, and the mode signal for producing 
a control signal representative of a digital control datum. The control 
datum gives a position error between the current and the commmanded 
positions during presence of the mode signal and otherwise gives at least 
one reference angular velocity. The servo control means further comprises 
gate pulse producing means responsive to the position pulses for producing 
gate pulses of a predetermined pulse width at a time at a rate dependent 
on the current angular velocity and digital adder means responsive to a 
reference signal representative of a digital reference feedback value at a 
time, the gate pulses, the position pulses, and the control signal for 
calculating during presence of each of the gate pulses an algebraic sum of 
the reference feedback value and the control datum to produce a digital 
signal. The algebraic sum is one of an arithmetic sum and a difference 
that is determined by that sense of the current angular velocity to which 
the above-mentioned each gate pulse is related. The digital signal is 
representative of the algebaric sums and the control data during presence 
and absunce, respectively, of the gate pulses. The servo control means 
still further comprises a driving signal producing circuit responsive to 
the digital signal for producing the motor drive signal. 
The reference signal may be produced by memory means comprised by the 
digital adder means. Alternatively, the reference signal is supplied to 
the digital adder means from the command signal generator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It is to be noted at the outset that signals and lines for transmitting the 
respective signals are often designated by same reference characters. 
Referring now to FIG. 1, a position control system to which the present 
invention is applicable is for use in combination with a movable element 
15, such as a carriage of a serial printer of the impact type, a motor 16 
for controllably rotating an axle 17 thereof with a variable angular 
velocity. Merely for convenience of illustration, the axle 17 is depicted 
to pass through the motor 16. The axle 17 is coupled to the element 15 by 
means of an endless belt 18 to controllably vary a current or 
instantaneous position of the element 15. The position control system is 
for use in combination also with a command signal generator or external 
source 19, such as an electronic digital computer, for producing a digital 
command signal indicative in general of successively commanded positions 
for the element 15. It is possible to assume without loss of generality 
that the axle 17 is kept in a predetermined direction. The "angular 
velocity" as called herein is therefore specified by its magnitude and 
sense. Furthermore, the expression is used herein such that the "motor" 16 
is rotatable in one or the other sense at a time. 
Further referring to FIG. 1, the position control system comprises an 
increment encoder or position transducer 21 and a servo control circuit 
22. The increment encoder 21 is mechanically or otherwise coupled to the 
motor 16 to produce a pair of polarity variable incremental position 
signals 24 and 25 and supplies the same to the servo control circuit 22. 
As will later be described in detail, the incremental position signals 24 
and 25 vary their respective polarities in response to every predetermined 
angle of rotation of the motor 16 and have a phase difference therebetween 
that is representative of the sense of a current or instantaneous angular 
velocity of the motor 16. A rate at which changes occur in the polarity of 
the incremental position signals 24 and 25 depends on the current angular 
velocity. The command signal for each commanded position or end portion is 
produced in response to a new data request signal 26 supplied from the 
servo control circuit 22 in the manner described later and comprises a 
distance signal 27 and a sense signal 28, which are supplied to the servo 
control circuit 22. The distance signal 27 is usually representative of a 
distance of the end point from a next previous commanded position or start 
point and consists of a predetermined number of bits, such as eleven bits. 
The sense signal 28 indicates a plus or a minus sense, such as 
counterclockwise and clockwise sense, of rotation of the motor 16 at a 
time and may be an additional bit of the distance signal 27, such as a 
twelfth bit. Responsive to the incremental position signals 24 and 25 and 
the command signal 27-28, the servo control circuit 22 supplies a motor 
drive signal 29 to the motor 16 to make the motor 16 move the movable 
element 15 eventually to the end point as will become clear as the 
description proceeds. 
Referring to FIG. 2, a position control system according to a first 
embodiment of this invention is shown together with the motor 16 and the 
command signal generator 19. The position control system comprises the 
increment encoder 21 and the servo control circuit 22. As described 
hereinabove, the increment encoder 21 produces a pair of incremental 
position signals 24 and 25. The servo control circuit 22 supplies a new 
data request signal 26 to the command signal generator 19 and receives 
thereform a distance signal 27 and a sense signal 28 as a command signal. 
Responsive to the position signals 24 and 25 and the command signal 27-28, 
the servo control circuit 22 supplies a motor drive signal 29 to the motor 
16. The servo control circuit 22 comprises a position pulse or pulse 
sequence producing circuit 31 for sensing the changes in the polarity of 
the incremental position signals 24 and 25 and also the phase difference 
therebetween to produce position pulses representative in effect of a 
current angular position of the motor 16 and the current angular velocity. 
In the example being illustrated, the position pulses are produced as 
pulses of a first or a second position pulse sequence 32 or 33 depending 
on the phase difference. Position pulses of the first and the second pulse 
sequences 32 and 33 are produced when the current angular velocity has the 
plus and the minus sense, respectively, with an interval T.sub.i 
representative of the magnitude of the current angular velocity. 
Further referring to FIG. 2 let it be presumed that the sense signal 28 
indicates the plus sense of rotation of the motor 16. When produced, the 
distance and the sense signals 27 and 28 are stored in a control circuit 
35 of the servo control circuit 22. At first, the control circuit 35 
produces a mode signal 36 indicative. of a velocity control mode as named 
herein and a digital control signal 37 representative of a position 
reference angular velocity predetermined for the motor 16. As will 
presently be understood, the control signal 37 passes through a digital 
algebraic adder circuit 38 to become a digital output signal 39. 
Responsive to the output signal 39, a driving signal producing circuit 41 
produces the motor drive signal 29 to put the motor 16 into rotation in 
the pulse sense. The position pulse producing circuit 31 begins to produce 
the first or plus position pulse sequence 32. Responsive to the position 
pulses 32, a gate pulse producing circuit 43 produces a sequence of gate 
pulses 44 of a predetermined pulse width T.sub.p with the interval T.sub.i 
of the position pulses 32. Supplied with the mode signal 36, a memory or 
register circuit 45 of the servo control circuit 22 produces a reference 
signal 47 representative of a first digital reference feedback value 
A.sub.1. Responsive to the plus position pulse sequence 32, control signal 
37, gate pulses 44, and reference signal 47, the algebraic adder circuit 
38 calculates a difference between the reference angular velocity and the 
reference feedback value A.sub.1 during the presence of each of the gate 
pulses 44 by subtracting the latter from the former to make the output 
signal 39 represent the difference and the reference angular velocity 
during presence and absence, respectively, of the gate pulses 44. 
Responsive to the position pulses 32, the control circuit 35 calculates a 
position error between the current and the commanded positions of the 
movable element 15 (FIG. 1). preferably, the reference angular velocity is 
stepwise reduced as the position error decreases. For convenience of 
description, a combination of the algebraic adder circuit 38, gate pulse 
producing circuit 43, and memory circuit 45 are referred to as a feedback 
circuit 49. 
The driving signal producing circuit 41 shown in FIG. 2 comprises a 
digital-to-analog converter (not depicted in FIG. 2) for converting the 
digital output signal 39 to an analog signal, which is a sequence of 
velocity feedback pulses as called herein. When the reference signal 47 
represents the first digital reference feedback value A.sub.1, the 
velocity feedback pulses have a pulse height proportional to the feedback 
value A.sub.1, a common pulse width equal to the gate pulse width T.sub.p, 
and a pulse interval equal to the position and gate pulse interval T.sub.i 
and consequently have an average voltage proportional to A.sub.1 
.multidot.T.sub.p /T.sub.i. The position pulse interval T.sub.i is 
inversely proportional to the magnitude of the current angular velocity of 
the motor 16. The velocity feedback pulses therefore indicates, when 
smoothed, the motor speed. It is thus possible to give the motor 16 with 
velocity feedback of a level proportional to A.sub.1 .multidot.T.sub.p and 
to control the motor 16 to make the magnitude of the current angular 
velocity follow the reference angular velocity or velocities until the 
position error decreases to a predetermined value, when the control 
circuit 35 makes the mode and the control signals 36 and 37 indicate a 
position control mode as named herein and represent the position error, 
respectively. Responsive to the mode signal 37 indicative of the position 
control mode, the reference signal 47 is made to represent a second 
digital reference feedback value A.sub.2. The digital output signal 39 now 
represents the position error and a difference between the position error 
and the second feedback value A.sub.2 during absence and presence, 
respectively, of the gate pulses 44. In the position control mode, the 
velocity feedback is proportional to A.sub.2 .multidot.T.sub.p. When 
smoothed, the velocity feedback pulses serve as a damping signal for the 
motor speed to make the motor 16 eventually place the movable element 15 
at rest at the end point. The control circuit 35 produces the new data 
request signal 26 and receives the command signal 27-28 for a new end 
point. By selection of the feedback values A.sub.1 and A.sub.2, it is 
possible to separately provide optimum velocity feedback in the velocity 
and the position control modes. 
Still further referring to FIG. 2, let it now be surmised that the sense 
signal 28 indicates the minus sense of rotation of the motor 16. The 
digital control signal 37 represents a minus reference angular velocity 
for the motor 16. The position pulses are of the second or minus position 
pulse sequence 33. In the velocity control mode, the digital algebraic 
adder circuit 38 calculates a sum of the minus reference angular velocity 
and the first digital reference feedback value A.sub.1 during presence of 
the gate pulses 44 to make the digital output signal 39 represent the sum 
and the minus reference angular velocity during presence and absence, 
respectively, of the gate pulses 44. In the position control mode, the 
digital output signal 39 represents the position error and a sum of the 
position error and the second digital reference feedback value A.sub.2 
during absence and presence, respectively, of the gate pulses 44. 
Turning to FIG. 3, a control circuit 35 for use in a servo control circuit 
22 used in a position control system according to this invention comprises 
a down counter 51 in which the distance signal 27 is set as an initial 
count. The control circuit 35 further comprises a register 52 for holding 
the sense signal 28 and an up-down counter 53 for counting down the plus 
position pulses 32 and counting up the minus position pulses 33 to supply 
carry down and up signals to the down counter 51 through an OR gate 54. 
Responsive to the carry down or up signals, the down counter 51 counts 
down the initial count to produce a first position error signal 55. 
Supplied with the sense signal 28 held in the register 52 and the first 
position error signal 55 as an address signal, a memory circuit 56 
delivers the reference signal to a line 57. Responsive to the first 
position error signal 55, a first zero detector 58 makes the mode signal 
36 indicate the velocity and the position control mode when the first 
position error signal 55 does not represent and represents zero, 
respectively. Supplied with the mode signal 36 indicative of the velocity 
control mode, a selector 59 supplies the reference signal to the line 37. 
For the distance signal 27 of eleven bits, the up-down counter 53 is 
preferably a three-bit counter that produces a second position error 
signal 61 representative of the position error in the position control 
mode. Responsive to the mode signal 36 indicative of the position control 
mode, the selector 59 supplies the second position error signal 61 to the 
line 37. A second zero detector 62 produces the new data request signal 26 
when the second position error signal 61 becomes zero during the position 
control mode. It is possible to make the first zero detector 58 produce 
the new data request signal 26 a prescribed interval of time after the 
mode signal 36 is switched to indicate the position control mode. 
Alternatively, the command signal genarator 19 may be programmed to switch 
a predetermined duration of time after production of the command signal 
27-28 for an end point to the command signal 27-28 for a new end point. 
Referring to FIG. 4, the memory circuit 45 of the feedback circuit 49 used 
in the position control system illustrated with reference to FIG. 2 
comprises first and second registers 63 and 64 preset with signals for the 
first and the second digital reference feedback values A.sub.1 and 
A.sub.2, respectively, and a selector 65 responsive to the mode signal 36 
indicative of the velocity and the position control modes for selectively 
making the reference signal 47 represent the first and the second feedback 
values A.sub.1 and A.sub.2, respectively. The gate pulse producing circuit 
43 comprises an OR gate 66 for supplying either of the plus and the minus 
position pulses 32 and 33 to a monostable or one-shot multivibrator 67 to 
make the latter produce the gate pulses 44. The digital algebraic adder 
circuit 38 comprises a flip-flop circuit 68 set and reset by the plus and 
the minus position pulses 32 and 33, respectively, and a digital algebraic 
adder 69 supplied with the gate pulses 44 for calculating the algebraic 
sum, namely, the above-mentioned difference and sum while the flip-flop 
circuit 68 is set and reset, respectively. The flip-flop circuit 68 may be 
set and reset by the minus and the plus position pulses 32 and 33, 
respectively. 
Referring temporarily to FIG. 5, a feedback circuit 49 for use in a 
position control system according to a second embodiment of this invention 
comprises similar parts designated by like reference numerals as in FIG. 
4. The memory circuit 45 comprises a single register 71 preset with a 
signal representative of a single reference feedback value A.sub.0 for 
both the velocity and the position control modes. Instead, the gate pulse 
producing circuit 43 comprises first and second monostable multivibrators 
72 and 73 responsive to either of the plus and the minus position pulse 
sequences 32 and 33 for producing first and second gate pulses having 
first and second predetermined pulse widths T.sub.p1 and T.sub.p2, 
respectively, and a selector 74 responsive to the mode signal 36 
indicative of the velocity and the position control modes for selecting 
the first and the second gate pulses, respectively. With the position 
control system according to the second embodiment of this invention, the 
optimum velocity feedback is proportional to levels A.sub.0 
.multidot.T.sub.p1 and A.sub.0 .multidot.T.sub.p2 in the velocity and the 
position control modes, respectively. 
Referring to FIG. 6, a feedback circuit 49 for use in a position control 
system according to a third embodiment of this invention comprises similar 
parts designated by like reference numerals as in FIGS. 4 and 5. It is to 
be noted that the mode signal 36 is used only in the control circuit 35 
(FIG. 2) to make the digital control signal 37 represent the reference 
angular velocity or velocities and the position error in the velocity and 
the position control modes, respectively. If used, the second zero 
detector 62 is also controlled by the mode signal 36 to produce the new 
data request signal 26. The velocity feedback is proportional to a level 
A.sub.0 .multidot.T.sub.p both in the velocity and the position control 
modes. As is obvious from FIG. 6, the position control system is 
simplified in structure according to the third embodiment. 
As a modification of the gate pulse producing circuit 43 for use in the 
position control system according to the second embodiment of this 
invention, use is possible of a single monostable multivibrator of which 
time constant is switched in response to the mode signal 36 to produce the 
first and the second gate pulses. The feedback circuit 49 may be dispensed 
with the memory circuit 45 for the reference signal 47, with the command 
signal generator 19 programmed to supply the reference signal 47 to the 
digital algebraic adder circuit 38 or the digital algebraic adder 69. 
Referring now to FIG. 7, the driving signal producing circuit 41 comprises 
a digital-to-analog converter 75 mentioned above for converting the 
digital output signal 39 to an analog signal 76 consisting of the 
above-mentioned position error signal or of the reference angular velocity 
signal and velocity feedback pulses, a low-pass filter 77 for smoothing 
the analog signal 76 to produce a smoothed analog signal 78, and a power 
amplifier 79 for amplifying the smoothed analog signal 78 into the motor 
drive signal 29. As shown, the low-pass filter 77 may comprise an 
operational amplifier. The cutoff frequency of the low-pass filter 77 
should be such as to smooth the velocity feedback pulses and to allow 
passage of the control signal 37 representative of the reference angular 
velocity signal or the position error signal. 
Referring to FIGS. 8 and 9, the increment encoder 21 may comprise a 
rotatable opaque disk 81 mounted on the motor shaft 17 and having 
azimuthally equally spaced radial slits or position information patterns 
82, a light source 83 for illuminating the radial slits 82, and a fixed 
opaque plate 84 provided with a pair of slits 90 arranged for alignment 
with the radial slits 82. The encoder 21 further comprises a pair of 
optical sensors 85 and 86 aligned with the slit pair and energized by an 
electric power symbolized at +V to produce electric signals when 
illuminated, a source 87 of a reference voltage, and comparators 88 and 89 
for comparing the respective electric signals with the reference voltage. 
The azimuthally equal spacing of the radial slits 82 is decided in 
consideration of the predetermined incremental angle of rotation of the 
motor 16. The number of the radial slits 82 may be, for example, six 
hundred. By making the spacing between the slit pair 90 differ from the 
azimuthally equal spacing by a quarter, it is possible to make the 
comparator 88 and 89 produce the incremental position signals 24 and 25 
exemplified In FIGS. 9 (a) and (b) and described more in detail in the 
following. 
Referring to FIGS. 9 (a) and (b), the incremental position signals 24 and 
25 have phases designated by A and B as depicted at 24p, 25p, 24m, and 25m 
and are now called A-phase and B-phase incremental position signals 24 and 
25, respectively. The A-phase and the B-phase signals 24p and 25p depicted 
in FIG. 9 (a) are produced when the angular velocity of the motor 16 has 
the plus sense. The A-phase and the B-phase signals 24m and 25m shown in 
FIG. 9 (b) are produced for the minus sense of rotation of the motor 16. 
With the azimuthally equal spacing of the radial slits 82 (FIG. 8) defined 
to be 360.degree. in space degree, the phase difference between the A and 
the B phases is equal to 90.degree. when the motor 16 is in rotation. 
Referring now to FIG. 10, the position pulse producing circuit 31 for the 
incremental position signals 24 and 25 of the type illustrated with 
reference to FIGS. 9 (a) and (b ) may comprise first and second monostable 
multivibrators 91 and 92 supplied with the A-phase incremental position 
signal 24 both directly and through a first inverter 93, respectively. 
Responsive to build up of the respective pulses of the A-phase signal 24, 
the first monostable multivibrator 91 produces first pulses of a short 
duration, such as one hundred nanoseconds. Responsive to build down of the 
respective A-phase pulses 24, the second monostable multivibrator 92 
produces second pulses of the short duration. When the angular velocity of 
the motor 16 has the plus sense, a first AND gate 94 enabled by the pulses 
of the B-phase signal 25 allows passage therethrough of the first pulses. 
A second AND gare 95 is enabled by the B-phase pulses 25 supplied thereto 
through a second inverter 96 and allows the second pulses to pass 
therethrough. When the motor 16 rotates in the minus sense, a third AND 
gate 97 is enabled by the inverted B-phase pulses for the second pulses. A 
fourth AND gate 98 is enabled by the B-phase pulses 25 also for the second 
pulses. Supplied with the first and the second pulses from the first and 
the second AND gates 94 and 95 and from the third and the fourth AND gates 
97 and 98, first and second OR gates 99p and 99m produces the plus and the 
minus position pulse sequences 32 and 33, respectively. 
Referring to FIG. 11, the position pulse producing circuit 31 for the 
incremental position signals 24 and 25 of the type described in 
conjunction with FIGS. 9 (a) and (b) may alternatively comprise a first 
AND gate 101 having two inputs supplied with the A-phase pulses 24 
directly and through a delay circuit 102 for a short delay, such as one 
hundred nanoseconds, followed by a first inverter 103, respectively. A 
second AND gate 104 also has two inputs, which are supplied with the 
delayed A-phase pulses and the A-phase pulses 24 through a second inverter 
105, respectively. The first and the second AND gates 101 and 104 produce 
first and second short pulses of the type described with reference to FIG. 
10. The illustrated position pulse producing circuit 31 further comprises 
a third inverter 106, third through sixth AND gates 107, 108, 109, and 
110, and first and second OR gates 111 and 112 that correspond to the 
circuit elements 96, 94, 95, 97, 98, 99p, and 99m (FIG. 10), respectively. 
Turning to FIGS. 12 (a) and (b), the control circuit 35 (FIG. 2) produces 
the new data request signal 26 and receives the distance and the sense 
signals 27 and 28 as the command signal to produce the control signal 37 
when the movable element 15 (FIG. 1) is brought at rest to a start point. 
It is assumed that the sense signal 27 indicates the plus sense of 
rotation of the motor 16. The control signal 37 indicates a first positive 
reference angular velocity 116 for the motor 16. As described, the control 
signal 37 passes through the digital adder circuit 38 at first to put the 
motor 16 into rotation in the plus sense. The digital adder circuit 38 
subtracts the first digital reference feedback value A.sub.1 from the 
first reference angular velocity 116 only during presence of the gate 
pulses 44 of the common width T.sub.p. The analog signal 76 vary between 
top and bottom levels that correspond to the first positive reference 
angular velocity 116 and the angular velocity 116 less than the first 
reference feedback value A.sub.1, respectively. The motor drive signal 29 
has a positive level 117 during an accelerating period 118 of the motor 
speed. The motor speed increases as exemplified by a broken line to the 
first reference angular velocity 116 until the motor drive signal 29 is 
given a zero level 120 in average to keep the current angular velocity at 
the first positive reference angular velocity 116. When the position error 
of the movable element 15 decreases to a certain prescribed value, the 
memory circuit 56 switches the control signal 37 to make the same indicate 
a second positive reference angular velocity 121 in preparation for 
operation in the position control mode. The analog signal 76 vary between 
two levels corresponding to the second positive reference angular velocity 
121 and that velocity 121 less the first reference feedback value A.sub.1, 
respectively. The motor drive signal 29 is given a negative level 122 to 
drive the motor 16 in a decelerating period 123 of the motor speed. 
Referring to FIGS. 13 (a) and (b), the control circuit 35 (FIG. 2) makes 
the control signal 37 represent a decreasing position error 126 when the 
position error of the movable element 15 (FIG. 1) is reduced to the 
predetermined distance as the motor 16 is driven in the manner described 
with reference to FIGS. 12 (a) and (b). The analog signal 76 now vary 
between two levels that correspond to the decreasing position error 126 
and that position error 126 minus the second reference feedback value 
A.sub.2, respectively. Inasmuch as the pulse interval common to the 
position and the gate pulses 32 and 44 increases, the motor drive signal 
29 is given a level 127 that is substantially equal to zero at the 
beginning of operation in the position control mode, takes a varying 
negative value to decelerate the motor speed, and eventually becomes zero 
as the position error of the movable element 15 converges to zero. 
As regards the operation illustrated with reference to FIGS. 12 and 13, it 
is now readily understood that the motor speed is controlled to 
successively follow the reference angular velocities, such as 116 and 121, 
in the velocity control mode and to decrease to zero in accordance with 
the position error of the movable element 15 in the position control mode. 
According to the embodiment described in conjunction with FIG. 5, the 
common gate pulse width is T.sub.p1 and T.sub.p2 in the velocity and the 
position control modes, respectively, with the reference feedback value 
kept at A.sub.0 throughout both modes. With the embodiment partly shown in 
FIG. 6, both the gate pulse width and the reference feedback value are 
maintained at T.sub.p and A.sub.0, respectively, throughout the velocity 
and the position control modes. 
Finally referring to FIG. 14, a position control system according to a 
fourth embodiment of this invention is illustrated together with the motor 
16 and the command signal generator 19 of the type described with 
reference to FIG. 1. The position control system comprises the increment 
encoder 21 of the type described and a servo control circuit 22 to be 
presently described. The increment encoder 21 supplies the above-mentioned 
incremental position signal pair 24 and 25 to the servo control circuit 
22. Responsive to the new data request signal 26, the command signal 
generator 19 supplies the servo control circuit 22 with the distance and 
the sense signals 27 and 28 as the command signal. The servo control 
circuit 22 supplies the motor drive signal 29 to the motor 16. Merely for 
convenience of description, the driving signal producing circuit 41 is 
depicted outside that block 22' of the servo control circuit 22 which 
deals with the digital signals. 
Further referring to FIG. 14, the block 22' comprises a programmable logic 
array 131 that may be an integrated circuit comprising two stages of AND 
and OR gates, such as that provided by the use of two bipolar 
field-programmable logic arrays 82S 100 (sixteen inputs and eight outputs 
with forty-eight product terms) described in "Data Manual" published 1976 
by Signetics Corporation, page 60 (`Memories`). By the use of a monostable 
multivibrator 67 and a delay circuit 102 as in FIGS. 4 and 6 and in FIG. 
11, it is possible to make the programmable logic array 131 function as 
the position and the gate pulse producing circuits 31 and 43 shown in FIG. 
2. The block 22' further comprises a counter-decoder 132 comprising a 
cascade connection of a counter and a decoder (not separately shown) to 
serve as the down counter 51 and the register 52 (FIG. 3). In the example 
being illustrated, the distance signal 27 is an eleven-bit signal. 
Responsive to a down count signal 133, the counter-decoder 132 produces 
the first position error signal 55 of three bits and, as the sense signal 
reproduced by the register 52, either of one-bit forward and reverse 
signals 134 and 135. The block 22' still further comprises a counter 136 
that receives the plus or the minus position pulses 32 or 33 from the 
programmable logic array 131 and serves as the up-down counter 53 (FIG. 3) 
for supplying the programmable logic array 131 with the second position 
error signal 62 of three bits in the illustrated example and either of 
carry up and down signals 137 and 138. The programmable logic array 131 
thus serves as that part of the embodiment described with reference to 
FIG. 2 and the example illustrated in FIG. 3 which comprises the digital 
adder circuit 38, OR gate 54, memory circuit 56, first zero detector 58, 
selector 59, and second zero detector 62. The programmable logic array 131 
supplies the digital output signal to the driving signal producing circuit 
41. 
While a few preferred embodiments of this invention and modifications 
thereof have thus far been described, it is now readily possible for those 
skilled in the art to put this invention into practice in various other 
ways. For example, it is possible to use a mode signal, such as 36, 
indicative only of the position control mode with the selector 59 made to 
select a single reference angular velocity in the absence of this mode 
signal. Use of such a mode signal does not preclude use of another mode 
signal for selecting one of a predetermined number of digital reference 
angular velocities at a time and is equivalent to use of a mode signal 
indicative of the velocity control mode alone. It is possible to use a 
position control system according to this invention for two motors, such 
as 16, for a carriage and a print wheel of a serial printer of the impact 
type with the new data request signal 26 used also as a switching signal 
for making the system alternatingly serve for the respective motors. This 
system is equally well applicable to positioning of any other movable 
element particularly when intermittent movement of the element is 
mandatory. For this purpose, the down counter 51 or the counter-decoder 
132 alone may be used to produce a single position error signal with the 
first zero detector 58 or the correponding element of the programmable 
logic array 131 used to switch the mode signal 36 between indication of 
the velocity and the position control modes and further to indication of 
the new data request and others and with such a single position error 
signal supplied to the memory circuit 56 or the equivalent in the 
progammable logic array 131 as the address signal. For the system 
illustrated with reference to FIG. 14, it is readily possible to make a 
programmable logic array comprise the counter-decoder 132 and/or the 
counter 136. It should clearly be understood that the control circuit 35, 
feedback circuit 49, driving signal producing circuit 41, increment 
encoder 21, and position signal producing circuit 31 are described with 
reference to FIGS. 3 through 11 only by way of example. For example, the 
algebraic adder 69 is equivalent to an arithmetic adder or a subtractor. 
Although the system has not been invented primirily for a numerical 
control system for a machine tool or similar apparatus, it is possible 
with at least one system according to this invention to carry out three 
dimensional control of a set of motors for a movable element by making the 
command signal comprise a direction signal, such as that for the 
"curvature data" described in U.S. Pat. No. 4,061,907 issued to Kiyokazu 
Okamato et al, assignors to the present assignee, by which a set of 
reference angular velocities for the motor set is selected at least for a 
certain duration of time.