Position control system

A position control system equipped with a driving unit including a mover and a stator giving a relative moving force to the mover comprises a position detector for outputting polyphase alternating signals different in phase from each other, a carrier signal generator for generating carrier signals to be modulated by the signals thus outputted, a digital sample-holder for demodulating a position-angle information signal of the modulated carrier signals and a digital calculator for calculating a driving signal to the driving unit. Thus, the position control system makes it possible to provide with a high resolution positioning, low power consumption, high stiffness, prevention of out of stepping, and high speed and vibrationless settling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a position control system for moving a certain 
movable member to a selected position for positioning and more 
particularly, to an apparatus for moving a head member such as, for 
example, the data-transducer of a disk drive or the printing head of a 
printer for positioning and their methods. 
2. Description of the Prior Art 
Recently, enhancement of performances of information equipment is 
surprising and accompanied with which, a driving means used for the 
data-transducer of a disk drive or the printing head of a printer has been 
required to provide with a position control system operable at a higher 
speed and accuracy as well as a finer resolution. 
Conventional driving means for use in such information equipment have 
frequently used an open-loop position control system utilizing a stepping 
motor. 
Hybrid-Permanent-Magnet type of stepping motor has been widely used for 
this purpose. This type of motor has a rotor consisting of a permanent 
magnet magnetized in the direction of rotating axis and a couple of 
magnetic materials mounted on both sides of the permanent magnet. These 
magnetic materials are respectively provided with arrays of magnetic 
pole-teeth engraved at constant pitches in the circumferential 
peripheries. On the other hand, the motor of this type has a stator 
consisting of a magnetic material core having groups of magnetic 
pole-teeth on its surface in confronting relation to the rotor and a 
plurality of coils provided in the slots of the magnetic material core. 
Current-feeding circuit for the stepping motor of hybrid-permanent-magnet 
type includes bridge-circuits respectively structured of a plurality of 
semiconductor switches and these coils are connected between the middle 
points of these bridge-circuits. So connected that if these switches are 
successively switched, currents can be alternatively flowed in these coils 
in order. As a result, the rotor changes the position to rotate due to the 
series of magnetization of these coils. With the most typical 2-phase 
feeding stepping motor, the rotor rotates by 1/4-teeth pitch (90.degree. 
in terms of electric angle) by synchronizing with an inputted stepping 
pulse signal. 
The usual open-loop position control system utilizing the stepping motor 
shown above is well-known in U.S. Pat. No. 4,568,988; "Position Control 
System of a Disk System". Such an open-loop position control system offers 
the advantage of using electronic control circuit simple in structure. 
Input signals to the electronic circuit are the stepping pulse signal and 
a command signal for the moving direction. The outputs from the electronic 
control circuit are the feeding-currents. The electronic control circuit 
consists mainly of the above-mentioned bridge-circuits and a sequence 
circuit for switching the semiconductor switches in response to the 
above-mentioned input signals. The usual open-loop position control system 
supplies currents to the coils of the stepping motor through the 
bridge-circuits and the sequence circuit in response to these input 
signals to rotate the rotor. As a result, a head member such as, for 
example, the data-transducer of a disk drive or the printing head of a 
printer can be moved for positioning. The term "open-loop" means that the 
moving distance of head member can be determined by the number of stepping 
pulses of an input signal only. 
Such a conventional position control system as utilizes a stepping motor as 
a driving means makes possible that by the number of stepping pulse 
signals and the command signal for moving direction, for example, the head 
member can be moved for positioning, so that the control can be easily 
achieved by means of a micro-processor. In addition, the system offers the 
higher effectiveness of power as compared to a system utilizing a driving 
means such as, for example, the voice coil motor or servo-motor. This is 
because a power device such as the stepping motor having tooth-shaped 
magnetic pole-teeth in the magnetic circuit is superior in the magnetic 
effectiveness and an extremely large torque can be generated even by a 
small current. As a result, this system was characterized by being less in 
power consumption. 
However, some problems have been pointed out on the speed and accuracy of 
this system. A case in point is that when the stepping motor as a driving 
means is to be stopped for positioning, it will become vibrative around a 
standstill position. This means that it may require a considerable 
settling time for positioning. In order to decrease the vibration thereby 
to reduce the settling time, the rotor or movable members can be provided 
with a mechanical viscous damping. However, this may result in a complex 
structure or may cause the high speed operation to be prevented due to 
this viscous damping, which have been pointed out for this system as 
disadvantages such as not to be pointed out for that utilizing a voice 
coil motor or servo-motor. 
Also, the stepping motor can obtain a speed synchronized with the stepping 
pulse signal. As a result, in order to move a moving member at a higher 
speed, the frequency of a stepping pulse signal can be increased on a 
theoretical basis. When drived at a higher speed, however, differing from 
the case when drived at a lower speed with a large torque, because of the 
effect of a time constant or magnetic hysterisis loss that a coil 
possesses, the rising of a current will be delayed, and the torque can not 
be developed effectively, resulting in the generation of a small torque. 
In this case, to attempt to unreasonably enhance the frequency of the 
stepping pulse signal will easily cause an out of stepping to take place. 
A position control system having the timing of a stepping pulse signal 
controlled by providing sensor elements on the moving member of the 
stepping motor in order to prevent the out of stepping has been disclosed 
in U.S. Pat. No. 4,044,881 as an example showing the application for a 
serial printer. This system senses the presence of the magnetic pole-teeth 
to produce a pulse signal and then, controls the timing of the generation 
of a stepping pulse signal in response to this pulse signal thereby to 
prevent the out of stepping. However, this method can do the timing 
control only, so that when, for example, a vibration or impact is applied 
from the outside, it is difficult to prevent such a disturbance. 
In addition, if the magnetic pole-teeth pitch of each of the rotor and 
stator is made small to increase the number of teeth, one step angle can 
be decreased, so that the resolution of positioning of a moving member is 
ought to be increased. However, in practice, not only there exists a 
limitation upon the mechanical accuracy, but also the switching frequency 
of the current of each coil is increased, so that the effectiveness can be 
rapidly decreased. This is explained in detail in "Parameters Governing 
the Dynamic Performance of Permanent-Magnet Stepping Motors" (A, Hughes 
Proc. Sixth Annual Symposium of Incremental Motion Control System and 
Devices, 1977, pp. 39-47). In addition, in order to improve the resolution 
of positioning, if the magnetic pole-teeth pitches of the rotor and stator 
are made small to increase the number of them thereby to decrease one step 
angle, there exists such a trend that the stiffness and position holding 
ability against a vibration or impact from the outside become small. 
SUMMARY OF THE INVENTION 
An object of the present invention is to eliminate occurrence of an out of 
stepping which may take place in a conventional open-loop position control 
system utilizing a stepping motor as a driving means thereby to provide a 
position control system operable at a higher speed. 
Another object of the present invention is to provide a position control 
system capable of eliminating disadvantageous vibrative positioning and 
longer settling time for positioning which have been pointed out in the 
conventional position control system. 
Still another object of the present invention is to provide a position 
control system capable of obtaining a higher resolution of positioning and 
a higher value of stiffness. 
A further object of the present invention is to provide a position control 
system superior in power effectiveness and low in power consumption. 
In order to attain the above-mentioned objects, a position control system 
of the present invention has a driving means including a mover and a 
stator which can move the mover relative thereto by giving a relative 
moving force to the mover. The position control system comprises: 
a position detecting means for outputting polyphase alternating position 
signals which are different in phase from each other in response to a 
relative movement of the mover to the stator; 
a carrier signal generating means for generating a carrier signal modulated 
by the polyphase alternating position signals from the position detecting 
means; 
a digital position signal generating means including a digital sample-hold 
means for demodulating phase-angle information of the carrier signal thus 
modulated to digitally produce relative position information indicative of 
a relative position between the mover and the stator; 
a digital calculating means for calculating a driving signal in response to 
an error between the relative position information and an externally given 
position command signal; and 
a current feeding means for feeding currents to the driving means in 
response to the driving signal. 
Further preferably, the position control system of the present invention 
comprises; 
the mover and the stator at least one of which equips with magnetic 
material members having arrays of magnetic pole-teeth engraved at constant 
pitches; 
a memory means which is provided in the digital calculating means and has a 
tabulated data according to a specific function; and a function generating 
means for reading a data from the tabulated data according to the specific 
function of the memory means in response to the error signal between a 
relative position information signal between the mover and stator and a 
position command signal from the outside thereby outputting the data thus 
read as a driving signal to the feeding means. 
So structured as above that the position control system of the present 
invention effectively operates as follows; 
First, the system makes it possible to generate the torque of the driving 
means always effectively, contributing to a reduction in power 
consumption. This is achieved in such a way that the position detecting 
means detects a relative position between the mover and stator of the 
driving means, a relative position information signal between the rotor 
and the stator is digitally produced a higher resolution from the position 
detecting means, an error signal is calculated between the relative 
position information signal and a position command signal from the outside 
and the digital calculating means effectively controls the driving signal 
to the driving means in response to the error signal thus obtained. 
In addition, the movable member can be moved at a higher speed without 
taking place the out of stepping. This is because the position detecting 
means detects a relative position between the mover and stator of the 
driving means to digitally produce a relative position information signal 
therebetween and the movable member is moved by a closed-loop position 
control in response to thus digitally produced relative position 
information signal. 
Also, at the same time, the movable member to be positioned can be quickly 
settled to a target position under depressed vibrative condition. This is 
achieved by providing the control system with a suitable damping by the 
digital calculating means. 
In addition, the digital position signal generating means makes the 
position control with a higher resolution possible. 
Further in addition, a higher stiffness than would be obtained 
conventionally can be easily obtained by the digital calculating means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Position control system according to one embodiment of the present 
invention will be described below while referring to the drawings. 
FIG. 1 is a block diagram showing a position control system according to 
the embodiment of the present invention. In FIG. 1, a rotary-type motor 
such as, for example, the stepping motor is used as a driving unit. The 
reference numeral 2 indicates a rotor (mover) which includes a permanent 
magnet 2a and arrays of tooth-shaped magnetic pole-teeth 2b made of 
magnetic material. 3a and 3b are respectively coils wound in the slots of 
a magnetic material core, which constitute a stator 3 together with the 
magnetic material core in confronting relation to the rotor 2 via a 
predetermined space. A driving unit 1 consists mainly of the rotor 2 and 
the stator 3. Movable member to be positioned is indicated at 4. At 5a is 
indicated a position detector which detects a position of the rotor 2 or a 
position of the movable member to be positioned and outputs 2-phase 
sine-wave alternating position signals sa and sb which are different in 
phase by 90.degree. from each other. 5b is a digital position signal 
generator which receives the 2-phase sine-wave alternating position 
signals sa and sb outputted from the position detector 5a and outputs the 
digital position signal b of the rotor 2 with a high resolution on a 
digital basis. The digital position signal b is a signal for indicating 
the absolute position of the movable member to the positioned in the 
movable range at a high resolution level with a certain reference point as 
the origin. A comparator (COMP) is indicated at 7, which receives the 
digital position signal b of the rotor 2 and a position command signal a 
which is supplied to an input terminal 7a from the outside and outputs a 
position error signal c (=a-b). 8 is an electric angle calculator which 
receives the position error signal c to execute a calculation for 
servo-control and outputs an electric angle signal e. 9 is a function 
generator (FUNC. GENERATOR) which outputs 2-phase sine-wave signals sc and 
ss equivalent to driving signals for the driving unit 1 in response to the 
electric angle signal e. 6 is a digital calculator including the 
comparator 7, electric angle calculator 8 and function generator 9. 10 is 
a current feeding circuit which amplifies the 2-phase sine-wave signals sc 
and ss equivalent to the driving signals outputted from the function 
generator 9 to feed currents to the coils 3a and 3b of the driving unit 1, 
respectively. 
FIGS. 2 (a) and (b) each is a block diagram showing an example of the 
position signal generator 5b in FIG. 1. In FIG. 2 (a), 11a and 11b are 
amplifiers (AMP) respectively for amplifying the 2-phase sine-wave 
alternating position signals sa and sb different in phase by 90 from each 
other outputted from the position signal detector 5a. 12a and 12b are 
modulators (MODU) which are a kind of multiplier for modulating the high 
frequency carrier signals 16a and 16b respectively by the 2-phase 
sine-wave output signals from the amplifiers 11a and 11b. A carrier signal 
generator 14a divides a reference clock generated by an oscillator (OSC) 
14b into 1/n thereby to produce the carrier signals 16a and 16b. The 
carrier signals 16a and 16b differ in phase by 90 from each other and are 
sent through the carrier signal generator 14a to the modulators 12a and 
12b, respectively. Summing circuit (SUM) 13a sums up the signals modulated 
through the modulator 12a and 12b. Law-pass filter (LPF) 13b removes 
harmonic components from the modulated output signal summed-up by the 
summing circuit 13a to extract a basic frequency component only. 
Sample-hold pulse signal generator (S-H PULSE GENERATOR) 13c converts the 
basic frequency component into a square wave and then, generates a 
sample-hold pulse signal u. 15a and 15b indicate a digital counter for 
demodulating phase-angle information contained into the square wave of the 
basic frequency component and a temporary memory (MEMO) for sample 
holding, respectively. The digital counter 15a counts the reference clock 
to produce a digital sawtooth wave. 
The temporary memory 15b is a temporary storing circuit such as, for 
example, the memory, register or latch circuit and sample-holds a count 
value of the digital counter 15a by means of the sample-hold pulse signal 
u. 17 indicates a digital sample-holder which comprises the sample-hold 
pulse signal generator 13c, digital counters 15a and 15b and the 
oscillator 14b and serves to demodulate the phase-angle information 
contained into the square wave of the basic frequency component. The 
digital counter 15a counts the reference clock, but if the maximum count 
value is expressed by n, by demodulating the phase-angle information 
contained into the square wave of the basic frequency component, the 
digital sample holder 17 can digitally execute n-division (interpolation) 
of the range of one period pitch of the 2-phase sine-wave alternating 
position signals sa and sb which are different in phase by 90.degree. from 
each other outputted from the position signal detector 5a. However, such 
execution has only an ability to discriminate phase-angles within the 
range of one period pitch of the alternating position signals sa and sb. 
Therefore, to make possible the discrimination of phase-angles in wider 
range than above, a reversible counter 18a is provided for counting the 
number of the alternating position signals that exceeds one period pitch. 
Reversible count pulse generator (COUNT PULSE GENERATOR) 18b produces an 
up-count pulse or a down-count pulse respectively every time when the 
content of the temporary memory 15a is rapidly changed from zero (or a 
value near zero) to n (or a value near n) and from n (or a value near n) 
to zero (or a value near zero). The reversible counter 18a receives the 
up-count pulse or down-count pulse to up-count or down-count, 
respectively. 18c indicates an origin.initial reset signal input terminal 
for the reversible counter 18a. 19 indicates a position register for 
summing the content of the reversible counter 18a and the content of the 
temporary memory 15b respectively as an upper bit portion and a lower bit 
portion. 20 is an output terminal of the position register 19 to 
constitute an output of the digital position signal generator 5b (a 
digital position signal b for the rotor 2). 
Here, the principle of the digital position signal generator 5b will be 
explained. 
Suppose that when the 2-phase sine-wave alternating position signals sa and 
sb from the position signal amplifiers 11a and 11b are expressed in terms 
of E.sub.A (.theta.) and E.sub.B (.theta.), respectively, the following 
can be established; 
EQU E.sub.A (.theta.)=E COS (2.pi..theta./.theta..sub.p) (1a) 
EQU E.sub.B (.theta.)=E SIN (2.pi..theta./.theta..sub.p) (1b) 
where, .theta. is a rotational angle (position) of the rotor 2, 
.theta..sub.p is one period pitch of the sine-wave alternating position 
signals outputted from the position signal detector 5a, and E is a crest 
value of the sine-wave position signals. On the other hand, if the carrier 
signals 16a and 16b are expressed in terms of C.sub.A (t) and C.sub.B (t), 
respectively, the following can be obtained; 
EQU C.sub.A (.theta.)=COS (2.pi.f.sub.C t) (2a) 
EQU C.sub.B (.theta.)=SIN (2.pi.f.sub.C t) (2b) 
where, f.sub.C is a carrier frequency. If the result obtained through the 
summing circuit 13a after modulated by the modulators 12a and 12b is 
expressed in terms of P(t, .theta.), from Eqs.(1a), (1b), (2a) and (2b), 
the following may be found as; 
##EQU1## 
This means that phase-angle member with a phase-angle information of 
2.pi..theta./.theta..sub.p is contained into a carrier signal having the 
frequency of f.sub.C. That is, P(t, .theta.) has the position information 
converted into the form of phase-angle information. As a result, the 
modulation of the phase-angle information of P(t, .theta.) makes it 
possible to recognize a position of the rotor 2. In addition, in order to 
detect a position of the rotor 2 accurately from P(t, .theta.), the 
position signals E.sub.A (.theta.) and E.sub.B (.theta.) are required to 
have a waveform of sine-wave which has less distortion with respect to the 
relational position .theta.. Large distortion results in a bad linearity. 
The low-pass filter 13b serves to eliminate the harmonic elements 
contained in P(t, .theta.). Generally, this is because the carrier signals 
16a and 16b are substantially a square wave containing a lot of harmonic 
waves. Next, the signal P(t, .theta.) summed-up through the circuit 13a 
after modulation is required to be demodulated to separate the phase-angle 
information 2.pi..theta./.theta..sub.p only to be fetched. In this 
embodiment, a phase difference of the signal P(t, .theta.) to the carrier 
signals is demodulated by the digital sample holder 17, which is carried 
out in such a simple method that the reference clock is counted by the 
digital counter 15a first to form a digital sawtooth wave, then a 
sample-hold pulse signal u is formed from the signal P(t, .theta.), and 
the digital sawtooth wave of the digital counter 15a ia sample-held by 
means of this pulse u. Concretely, the content counted by the digital 
counter 15a is sent with the sample-hold pulse signal u to the temporary 
memory 15b. The sample-hold pulse signal u does the waveform shaping of 
the signal P(t,.theta.) to make it square, which is produced from the edge 
of its rise (or its fall). So structured as above that the phase-angle 
information (2.pi..theta./.theta..sub.p contained in the signal P(t, 8 ) 
is digitally held in the temporary memory 15b. If the carrier signal 
frequency is expressed by f.sub.C and a reference clock n.multidot.f.sub.C 
having a frequency n-times of f.sub.C is counted by a digital counter 
whose maximum count value is n, the phase-angle difference can be measured 
at a 1/n resolution per period. This means that it has a resolution to 
divide a phase angle of 2.pi. into equals parts of n. The phase-angle 
2.pi. corresponds to one period pitch .theta..sub.p of sine-wave 
alternating position signals outputted by the position signal detector 5a. 
As a result, the digital position signal generator 5b can interpolate the 
one period pitch .theta..sub.p of the alternating position signals from 
the position signal detector 5a at an equal space of 2.pi./n to a 
rotational position of the rotor 2. However, the content to be counted by 
the digital counter 15a is a digital sawtooth wave whose maximum count 
number is n and this sawtooth is repeatable. Therefore, the digital 
counter 15a has only an ability to discriminate a rotational position of 
the rotor 2 within the range of one period pitch of the alternating 
position signals. As a result, to make the discrimination possible to do 
in a wider range, the reversible counter 18a is provided to count the 
number of the alternating position signals that exceed one period pitch 
.theta..sub.p. The reversible count pulse generator 18b produces an 
up-count pulse or a down-count pulse respectively every time when the 
content of the temporary memory 15b is rapidly changed from zero (or a 
value near zero) to n (or a value near n) or vice versa thereby to send it 
to the reversible counter 18a for up-counting or down-counting. As a 
result, the reversible counter 18a indicates upper bits of the position 
information and on the other hand, the temporary memory 15a indicates 
lower bits of the position information. The content of the reversible 
counter 18a and that of the temporary memory 15b respectively are summed 
up in the position register 19 as the upper bit portion and the lower bit 
portion and stored thereinto. 
FIG. 2 (b) is, similar to FIG. 2 (a), a block diagram showing an example of 
the digital position signal generator 5b shown in FIG. 1. This resembles 
in structure to that shown in FIG. 2 (a). What is different between the 
both is the structure of the digital sample holder 17. Explanation will be 
made on this difference as follows; Also, in FIG. 2 (b), any blocks having 
equal or equivalent functions to those in FIG. 2 (a) have same names and 
numbers. The digital sample holder 17 in FIG. 2 (b) also is to obtain a 
phase difference of the signal P(t, .theta.) to the carrier signals (that 
is, to phase-demodulate), which means that their operational purposes are 
the same. Concretely, first, the digital sample holder 17 in FIG. 2 (b) 
differs from that in FIG. 2 (a) in that the sample hold pulse generator 
13c forms the sample hold pulse signal u from the carrier signal 16a. 
Accompanied with which, a digital sawtooth wave formed by the digital 
counter 15a is periodically reset by a reset pulse signal r generated 
through a reset pulse signal generator 13d in response to the signal P (t, 
.theta.). This constitutes another different point from the preceding 
example. With the above-mentioned structure, a digital sawtooth wave is 
formed based on the signal P(t, .theta.) to sample-hold it by a carrier, 
thus the phase difference between the both being held in the temporary 
memory 15b, which is just opposite in method to the preceding example 
shown in FIG. 2 (a). In FIG. 2 (a ), though equivalently, the digital 
sawtooth wave is formed based on the carrier signals and sample-held by 
means of the signal P(t, .theta.). The system shown in FIG. 2 (b) is 
slightly complex in structure compared with that in FIG. 2 (a), but 
substantially equivalent functions can be obtained. 
FIG. 3 is a block diagram showing a concrete example of the digital 
calculator 6 in FIG. 1. In this embodiment, the comparator 7, electric 
angle calculator 8 and function generator 9 of the digital calculator 6 
are respectively structured of an A/D converter 21, a computer 22, a 
memory 23 and D/A converters 24a and 24b. The A/D converter 21 converts a 
position command signal a sent from the outside to input terminal 7a into 
a digital signal. The computer 22 is structured of an arithmetic logical 
unit for executing calculation and a sequencer for controlling the 
proceedings, and operates in accordance with a predetermined built-in 
program (described later on) stored in the ROM (read only memory) area of 
the memory 23. First, the computer 22 receives an output from the A/D 
converter 21 and a digital position signal b of the rotor 2 outputted from 
the digital position signal generator 5b, stores them into a register or 
RAM (random access memory) area of the memory 23, and then, carries out a 
predetermined digital calculation including adding-subtracting process for 
producing the electric angle signal e. Also, the computer 22 refers to the 
function tables of sine-wave and cosine-wave being stored in the ROM area 
of the memory 23 in response to this electric angle signal e to produce 
signals SC and SS and sends them respectively to the D/A converter 24a and 
the D/A converter 24b. The D/A converters 24a and 24b carry out the 
digital-analog conversion of the 2-phase signals SC and SS into 2-phase 
analog signals sc and ss and output them. In addition, in case that the 
position command signal a sent from the outside to the input terminal 7a 
is digital, the A/D converter 21 can be omitted. 
Next, built-in program being stored in the ROM area of the memory 23 will 
be outlined while referring to the flow chart shown in FIG. 4. 
FIG. 4 is a basic flow chart showing an example of the built-in program 
being stored in the ROM area of the memory 23 shown in FIG. 3. In this 
case will be started from 1. A first process 31 waits an interruption from 
a timer. The time generates an interrupt signal by predetermined time .pi. 
and if interrupted, it will be shifted to ". That is, the following 
processes will be performed by sampling period .pi.; Process 32 is for 
acquisition of data to receive the digital position signal b and stores it 
in a predetermined register or RAM area Qb. Process 33 is for control 
calculation and carries out a predetermined control calculation in 
response to the position error signal c between the position command 
signal a and the digital position signal b thereby to produce an electric 
angle signal g in order to position the rotational position of the rotor 2 
to a reference position shown by the position command signal a. Process 34 
is for limitation of signal and restricts the electric angle signal g 
within a predetermined limit for producing an electric angle signal h. 
Process 35 is for output of the result obtained by adding corrections to 
the calculation results shown above and refers to the function tables of 
sine-wave and cosine-wave being stored in the ROM area of the memory 23 in 
response to the digital position signal b of the rotor 2, the electric 
angle signal h obtained through the process 34 and the like thereby toe 
produce 2-phase signals SC and SS, and sends them to the D/A converters 
24a and 24b, respectively. After completion of the process 35, it will be 
returned to 1. 
Next, detailed explanations will be made on the processes 32 to 35 while 
referring to FIGS. 5 (a), (b), (c) and (d). 
FIG. 5 (a) is a flow chart showing a concrete example of the process 32 in 
FIG. 4. In FIG. 5 (a), processes 36 and 37 respectively store the position 
command signal a and the digital position signal b of the rotor in their 
predetermined registers or RAM areas Qa and Qb. Process 38 stores the 
content of a predetermined register or RAM area Qn holding the value at 
the preceding sampling time into another area Qn-1. Provided, the area Qn 
is a register or RAM area for storing the digital signal corresponding to 
the position error signal c between the position command signal a and the 
digital position signal b. Next, the process 39 newly calculates the 
digital signal corresponding to the position error signal c between the 
position command signal a and the digital position signal b (c=a-b or 
(Qn).rarw.(Qa)-(Qb)) and stores it in the area Qn. That is, at this time, 
the position error signal between the present rotational position of the 
rotator 2 and the position command signal is stored in the area Qn and the 
position error signal at the preceding one sampling time is to be stored 
in the area Qn-1. The processes show above correspond to the process by 
the comparator 7. Hereinafter, (Qn) will indicate the content of Qn. 
FIG. 5 (b) is a flow chart showing a concrete example of the process 33 in 
FIG. 4. In FIG. 5 (b), first, a process 41 multiplies the content of Qn 
holding the value corresponding to the position error signal c (=a-b) 
between the position command signal a and the digital position signal b at 
the present time by K1 and stores the result thus obtained into a 
predetermined register or RAM area P. The content of the P becomes a 
factor proportional to the content of Qn. Process 42 subtracts the content 
of Qn-1 holding the value corresponding to the position error signal c at 
the preceding one sampling time from the content of Qn holding the value 
corresponding to the position signal c at the present time and stores the 
result thus obtained into a predetermined register or RAM area .DELTA.Q. 
Process 43 multiplies the content of Qn by sampling time .tau., that is, 
calculates the value (Qn).multidot..tau., then adds the content of a 
predetermined register or RAM area Sn-1 at the preceding one sampling time 
to the result thus calculated and stores the added result into another 
area Sn. That is, it carries out the following equation (4) and stores the 
obtained result into a variable Sn; 
EQU (Sn)=(Sn-1)+(Qn).multidot..pi. (4) 
In Eq. (4), the content of Qn indicates the position error signal c between 
the position command signal a and the digital position signal b, and if 
there exists a static position error between the position command signal a 
and the digital position signal b, the term Qn.multidot..pi. will be added 
to the term (Sn-1) by every sampling. As a result, as the content of Sn 
increases with a time, the static position error between the signals a and 
b can be eliminated by making a closed-loop servo-system based on this. If 
the sampling period is substantially small, the content of Sn shown in Eq. 
(4) can express the result obtained by integrating the position error 
signal c by time. That is, the process 43 is to obtain an integral factor 
by accumulating the error signal by time. Next, a process 44 multiplies 
the content of Sn by K2 and stores the result thus obtained into a 
predetermined register or RAM area I. The content of the area I becomes an 
integral signal. Next, in a process 45, the value (.DELTA.Q)/.pi. obtained 
by dividing the content of the .DELTA.Q by the sampling period .pi. is 
multiplied by K3 and the result thus obtained is stored into a 
predetermined register or RAM area D. Namely, the processes 42 and 45 
respectively obtain a differential factor by differentiating the error 
signal by time. The content of D becomes a differential signal. Process 46 
sums up the all contents of the areas P, I and D respectively obtained 
through the processes 41, 44 and 45 to produce the electric angle signal g 
and stores the result thus obtained into a predetermined register or RAM 
area G. Then, it goes to $. The above-mentioned processes are known as the 
PID compensation which makes it possible to provide stability in the 
system as well as to improve the stiffness (or the position holding torque 
against a vibration or an impact from the outside) and the position 
traceability to the position command signal a. 
FIG. 5 (c) shows a flow chart showing a concrete example of the process 34 
in FIG. 4. Process 48 compares the absolute value of the electric angle 
signal g obtained through the process 33 with a preset constant g.sub.MAX. 
If .vertline.g.vertline.&gt;g.sub.MAX, it goes to a process 49. If not so, 
the signal g is stored in a predetermined register or RAM area H in a 
process 50 and directly goes to %. In the process 49, the sign of the 
signal g is retained and it is also stored in the area H with its value as 
g.sub.MAX, then going to %. The signal of this H is called an electric 
angle signal h. The process 34 is for limiting the electric angle signal 
to a predetermined range, which is called limitation of electric angle. 
The principle and functions thereof will be described in detail later. 
FIG. 5 (d) is a flow chart showing a concrete example of the process 35 in 
FIG. 4. In a process 56, addition of the electric angle signal h obtained 
through the process 34 and the digital position signal b is carried out to 
produce the electric angle signal e thereby storing it into a 
predetermined register or RAM area E. Processes up to the production of 
the electric angle signal e as described above constitute main processes 
of the electric angle calculator 8. Processes 57 refers to the cosine-wave 
function table being stored in the ROM area of the memory 23 in response 
to the electric angle signal e obtained through the process 56 (in general 
case, the upper bits of the signal e are neglected, being in response to 
the lower bits of the signal e) thereby to produce a cosine-wave signal 
SC=fc(e). Similarly, a process 58 refers to the sine-wave function table 
being stored in the ROM area of the memory 23 in response to the electric 
angle signal e thereby to produce a sine-wave signal SS=fs(c). Finally, 
the 2-phase signals SC and SS are respectively sent to the D/A converters 
24a and 24b. The above-mentioned processes constitute main processes of 
the function generator 9. (Explanations on successive operations by 
referring to FIGS. 1 and 3 are that the D/A converters 24a and 24b 
respectively convert the digital signals SC and SS into analog signals sc 
and ss and send them to the current feeding circuit 10. The 2-phase 
signals sc and ss are amplified through the circuit 10, then converted 
into 2-phase current signals (or voltage signals) proportional to the 
signals sc and ss, and sent to the 2-phase coils 3a and 3b provided on the 
magnetic material core of the stator 3, respectively.) 
Then, the flow of program is returned to the top process 1 for waiting the 
next timer interruption. 
Next, explanations will be made on the functions and operations of a 
position control system according to an embodiment of the present 
invention while referring to the drawings attached. First, the principles 
of torque generation and position control will be explained using FIG. 6. 
FIG. 6 schematically shows the torque generation mechanism of the driving 
unit 1 in FIG. 1. In FIG. 6, 3a and 3b are a coil. .PHI.m indicates the 
magnetic pole vector of the rotor 2. (provided, the rotor 2 includes a 
magnet having a couple of magnetic pole for the sake of simplification, 
but practically, magnet having a couple of multi-poles or having arrays of 
magnetic pole-teeth of multi-poles is generally used.) The position 
detector 5a serves to detect a relative rotational position of the 
magnetic pole vector .PHI.m to the coils 3a and 3b. This relative 
rotational position is expressed by b, but the b has an arbitrary relative 
reference position as its origin, so that its range can exceed 2.pi. (rad) 
(=360.degree.). Here, if currents to be flowed in these phases are 
respectively expressed by I.sub.A and I.sub.B, then torques to be 
developed thereinto can be expressed as follows; 
##EQU2## 
where, Kt=torque coefficient, 
b=position signal of the rotor 2 
The currents to be supplied to the 2-phase coils 3a and 3b of the rotor 2 
are changed into sine-wave in response to the 2-phase signals sc and ss 
outputted from the function generator 9. To the function generator 9 is 
inputted the electric angle signal e (=b+h) which is obtained by the 
electric angle signal h produced by the electric angle calculator 8 and 
the digital position signal b of the rotor 2 thereby to output the 2-phase 
sine-wave signals sc and ss in response to the signal e. As a result, the 
currents to be supplied to A-and B-phases of the coils 3a and 3b are 
respectively proportional to the 2-phase sine-wave signals sc and ss 
outputted from the function generator 9 and can be expressed as follows; 
EQU I.sub.A =I.sub.O .multidot.COS.multidot.(b+h) (6a) 
EQU I.sub.B =I.sub.O .multidot.SIN.multidot.(b+h) (6b) 
where, I.sub.O =crest value of current. 
This means that the current magnetization vector .PHI.i obtained by 
composing those of the coils 3a and 3b is formed at a position of (b+h) as 
shown at 61 in FIG. 6. In this case, however, the total generating torque 
can be found by Eqs. (5a), (5b), (6a) and (6b) as follows; 
##EQU3## 
This equation (7) explains that in the position control system of the 
present invention, the position control of the rotor becomes possible by 
freely controlling the digital position signal b and the phase of the 
composed current magnetization vector .PHI.i of a plurality of coils in 
response to the position error signal c between the signal b and the 
position command signal a. Namely, by controlling the variable h in 
accordance with Eq. (7), the total generating torque T can be changed to 
provide the system with a desired characteristic for positioning. Here, if 
a position of the rotor 2 is expressed by (b-.DELTA.b) and a value of the 
position command signal a is b, then the position error c will become b, 
the variable h can be calculated based on this, and a torque can be 
generated in accordance with Eq. (7), so that the rotor 2 is rotated until 
.DELTA.b becomes zero. Finally, the position of the rotor 2 becomes b, 
which means that .DELTA.b, h and torque all become zero. Also, in the 
electric angle calculator 8, the electric angle signal g is produced by 
composing a proportional element, differential element and integral 
element of the position error signal c outputted from the comparator 7. 
The electric angle signal e is based on this signal g, so that by the 
action of the differential element which is a time differential signal 
contained in the signal e, a damping can be given to the control system 
electrically. As a result, vibration of the rotor 2 to be generated in 
positioning can be prevented, resulting in the reduction of the settling 
time. In addition, by the action of the integral element contained in the 
electric angle signal e, the static position error to be generated between 
the position command signal a and the digital position signal b can be 
depressed. This is because when and error is generated between the 
position command signal a and the digital position signal b by a friction 
or load torque, the position error signal c (=a-b) does not become zero, 
so that the integral element can be increased with a time according to Eq. 
(4). This serves to act the digital position signal b so as to gradually 
approach to the position command signal a, and the static position error 
can be depressed. 
Next, functions and principle of the electric angle limiter in the process 
34 shown in FIG. 4 and FIG. 5 (c) will be explained using FIGS. 7 and 8. 
FIG. 7 shows a static torque characteristic of a driving unit showing an 
example of the driving unit shown in FIG. 1. In each of FIGS. 7 and 8, the 
ordinate indicates a generating torque T and the abscissa indicates an 
electric angle signal e. As shown in Eq. (7), the generating torque T 
becomes a sine-wave function of the electric angle signal h, so that the 
generating torque T increases with a gradual increase in h from zero, and 
when e =e1 in FIG. 7, or h=.pi./2 (rad), the torque T peaks to give a 
value of T1, and when h exceeds .pi./2 (rad), it decreases. Also, if h 
exceeds .pi. (rad) to a point of e=e2 in FIG. 7, the sign of the 
generating torque is inverted to generate a torque T2 opposite in 
direction to the command signal and the control system can become 
unstable, resulting in the out of stepping in the worst possible case. As 
a result, it is necessary to control the absolute value of the electric 
angle signal h so as not to exceed .pi./2.pi. (rad). 
FIG. 8 shows a characteristic diagram showing an example of the limitation 
characteristic of the process 34 shown in FIG. 4 and FIG. 5 (c). In FIG. 
8, the absolute value of the electric angle signal g is compared in 
advance with a predetermined constant g.sub.MAX (=.pi./2 [rad]) to limit 
its range and the result thus obtained is made as the electric angle 
signal h. Thus, the electric angle signal e can be limited within a 
certain range. With the structure as above, even wen the absolute value of 
an electric angle signal calculated through the electric angle calculator 
8 exceeds .pi./2 (rad), by setting the operational signal (or the electric 
angle signal e) to a value to make the generating torque T maximum, the 
driving unit can be effectively accelerated and decelerated. As a result, 
whenever a position to be positioned is given by the position command 
signal a, the rotor 2 rotates smoothly to the position without giving out 
of stepping. In addition, g.sub.MAX =.pi./2 (rad) is used as a limit in 
this embodiment, it is obvious that so far as the limit is below .pi. 
(rad), the driving unit can be provided with stable acceleration and 
deceleration. 
In addition, in the above-mentioned examples, a motor having arrays of 
magnetic pole-teeth such as so-called stepping motor can be introduced 
into the mechanical part of the driving unit. 
For example, as schematically shown in FIGS. 9 (a) and (b), a stepping 
motor of VR (variable reluctance) type can be applied, in which one of the 
rotor and the stator has magnetic material bodies 71 having arrays of 
magnetic pole-teeth 72 engraved at constant pitches and the other has a 
magnetic material core 74 having groups of magnetic pole-teeth 73 and a 
plurality of coils 75 wound on the magnetic material core 74. 
In addition, as schematically shown in FIGS. 10 (a) and (b), a stepping 
motor of PM (permanent magnet) type can be applied, in which one of the 
rotor and the stator has a multi-pole magnetized permanent magnet 81 and 
the other has a magnetic material core 83 having groups of magnetic 
pole-teeth 82 in confronting relation to the magnet 81 and a plurality of 
coils 85 wound on the magnetic material core 83. 
Further in addition, as schematically shown in FIGS. 11 (a) and (b), a 
stepping motor of hybrid-permanent-magnet type can be applied, in which 
one of the rotor and the stator has magnetic material bodies 92 having 
arrays of magnetic pole-teeth 91 engraved at constant pitches and the 
other has a magnetic material core 94 having groups of magnetic pole-teeth 
93, a plurality of coils 95 wound on the magnetic material core 94 and a 
permanent magnet 96 for generating a bias magnetic field. 
Still further in addition, as schematically shown in FIGS. 12 (a) and (b), 
a hybrid PM type stepping motor can be applied, in which one of the rotor 
and the stator has magnetic material bodies, 101 and 102 respectively 
having arrays of magnetic pole-teeth 105 and 106 engraved at constant 
pitches and a permanent magnet 100 for generating a bias magnetic field 
and the other has a magnetic material core 108 having groups of magnetic 
pole-teeth 107 and a plurality of coils 103a, 104a, and 104b wound on the 
magnetic material core 108. Such a motor that is provided with arrays of 
magnetic pole-teeth makes it possible to produce a large torque using a 
small current, so that its application largely contribute to power saving 
and compactization. 
FIGS. 13 (a), (b) and (c) are concrete examples of the position detector 5a 
in FIG. 1. 
FIG. 13 (a) exemplifies a position detector using a plurality of magnetic 
sensors for magnetically detecting magnetic pole-teeth engraved at a 
constant pitch. In which, 100 indicates a magnetic resistor element and 
102 is a detecting bias magnet. The magnetic resistor element 100 is under 
application of a magnetic field generated by the detecting bias magnet 102 
disposed on its back side. As the magnetic pole-teeth 101 engraved at a 
constant pitch moves in front of the magnetic resistor element 100, the 
magnetic field is modulated and polyphase sine-wave alternating position 
signals are outputted. 
FIG. 13 (b) exemplifies a position detector using a plurality of magnetic 
sensors for magnetically detecting a multi-pole magnetized permanent 
magnet, in which 104 indicates a Hall effect element for outputting 
polyphase sine-wave alternating position signals as the multi-pole 
magnetized permanent magnet 103 moves. 
FIG. 13 (c) exemplifies a position detector using an optical detecting 
element for optically detecting a relative move between the rotor and the 
stator. In which, 107 is a slit plate and 108 is an optical detecting 
element having a luminous element and light receiving element disposed in 
opposite relation to each other. The slit plate 107 is provided with 
either stripe pattern or stripe-shaped holes. As the slit plate 107 moves, 
polyphase sine-wave alternating position signals are outputted. In FIG. 13 
(c), magnetic pole-teeth 109 engraved at a constant pitch can be used 
instead of the slit plate 107 for picking-up the above-mentioned optical 
detecting element. 
In addition, the driving unit 1 shown in FIG. 1 used a 2-phase, 2-coil 
motor, but the same effects can be obtained even by using a motor having 
phases and coils more than three. For this, the function generator 9 and 
the current feeder 10 should be modified in the number of phases. The 
function generator 9 does not have a limitation upon its function having 
to be of sine or cosine, but it has to be of periodically repeatable 
waveform. Also, it is allowed to be a function generating a waveform so as 
to correct a distortion the generating torque waveform of the driving unit 
1. 
In addition, in the above explanations, a rotation-type motor was used as 
the driving unit 1, but it is obvious that a linear motor can be applied 
for the driving unit 1 in FIG. 1. 
Further in addition, the above explanations were made so that the position 
detector 5a for the rotor of the driving unit is to be connected directly 
to the rotating axis of a motor, but it can be connected to a movable 
member to be driven. Also, it is unnecessary to be of a rotary type and it 
is obvious that it can be of a linear type. Furthermore, its output is not 
necessarily to be of 2-phase or sine-wave, being capable of having phases 
more than three. 
Further in addition, in examples shown in FIGS. 3 and 4, the processes were 
explained on a software basis, but they can be made on a hardware basis.