Device for driving stepper motor type measuring instrument

A measure obtained by a counting circuit (2) is converted by a processing circuit (3) to an indication angle signal at a predetermined conversion cycle. A smoothing circuit (4) comprises a division circuit (11) for calculating the difference between the previous indication angle data .theta..sub.0 and the present indication angle data .theta..sub.1 which is outputted after the conversion cycle T and has changed, and sequentially accumulating the angle data corresponding to T/n (where n is an integer of 2 or more) shorter than the conversion cycle T at a division cycle T/n and a voltage memory (12) for converting and outputting a two-phase driving signal for driving a stepper motor type measuring instrument (6) on the basis of the indication angle data .theta..sub.i outputted from the division circuit (11) every division cycle T/n. A voltage signal is generated through an output circuit (5) for converting the signal to a voltage signal to be applied to a two-phase excitation coil of a stepper motor. This voltage signal drives the stepper type motor measuring instrument (6) and rotates a pointer (9) fixed to the end of a driving shaft at angle indication corresponding to a scale (8) of a dial (7).

TECHNICAL FIELD 
The present invention relates to a drive apparatus for a stepping motor 
type meter used as a measurement movement for facilitating digital control 
in place of an ammeter comprising a movable coil type or cross coil type 
rotating magnet or the like and more particularly, for a stepping motor 
type meter which indicates measurements based upon signal input of a 
frequency proportional to a measurement amount such as the travelling 
speed or engine revolutions of an automobile. 
BACKGROUND ART 
Generally, this type of meter rotates a pointer fixed to one end of a drive 
shaft of the movement of the meter corresponding to an input signal and 
indicates a measurement by way of contrast and legibility against a dial 
provided with numbers or a scale for showing a measurement amount, and is 
normally constructed so as to indicate a linear indication characteristic 
by a scale of substantially equal increments. 
In particular, in the case of the stepping motor the stepping operation of 
the magnet rotor is determined by the number of tooth of the toothed yoke 
and the pitch formed thereby so that in order to attain a smooth rotating 
operation, it is necessary to increase the number of teeth and reduce the 
pitch of the teeth, or alternatively, perform a so-called micro-step drive 
by means of a drive signal and the type of stepping motor is selected 
based on the allowable size of the stepping motor main unit meeting the 
condition of use and the cost of including a drive circuit. 
Further, such a stepping motor as this is desired to be compact regardless 
of its applications. Since a so-called PM type stepping motor has a simple 
structure, it is made easy to use due to its improved rotor magnet and 
toothed yoke. 
Further, in contrast to the digitization of the processing circuit 
(controlled by a microcomputer), this type of stepping motor in which 
pulse signal control is performed has attracted attention also as a 
movement for an indicating meter, which indicates a comparative readout on 
a dial scale by means of a pointer--for example, it can be used in a 
speedometer or engine tachometer of an automobile, or as a heat gauge or 
thermometer by way of A/D processing of a detection signal, with various 
proposals for practical applications forthcoming, such as those disclosed 
in Unexamined Japanese Patent Application S61-129575, Unexamined Japanese 
Patent Application H1-223312, etc. 
In addition, it is common for a plurality of indicating meters such as a 
speedometer or tachometer to be used simultaneously in this type of 
measuring apparatus, and in cases where a processing circuit is provided 
for each of these meters it is necessary to use the least expensive 
circuit components in view of the production cost, to which extent 
indication control having a slow computing speed becomes inevitable. 
Also, with the digitization of signal processing, time division processing 
by means of a digital processing circuit such as a microcomputer from a 
control system in which each indicating meter is processed is developing, 
wherein the computation processing cycle is allocated by the 
responsiveness of each meter corresponding to the subject being measured 
(speedometers and tachometers have fast cycles, heat gauges and 
thermometers have slow cycles), necessitating processing circuits with 
high computation processing speeds according to an increase in the number 
of driving indicating meters. 
However, where expensive processing circuits are not used in view of the 
production cost, or where many indicating meters are time-divisionally 
driven, there is a limitation to the compression of a computation 
processing cycle allocated to one indicating meter. For example, given 
that a drive update cycle necessary to smoothly rotate the pointer 
corresponding to a measurement amount which varies to a great degree, such 
as engine revolutions, is 10 milliseconds, and the allocation limit of the 
computation processing cycle of the processing circuit is only 20 
milliseconds, the pointer can only move intermittently with respect to 
changes in the measurement amount and a smooth indication characteristic 
cannot be attained. 
In particular, although a stepping motor attains smoothness by correcting 
the wave form of the drive signal on the basis of a step operation 
corresponding to the pitch of the teeth on the toothed yoke, where a 
measurement data updating cycle is large with respect to changes in a 
measurement amount, changes in the drive signal to the stepping motor 
itself are also large. Also, although when changes in the measurement 
amount are fast the intermittent movement of the pointer is relatively 
inconspicuous due to its fast movement, where the responsiveness of the 
stepping motor itself is excellent or, conversely, where changes in the 
measurement amount are gradual, it was confirmed that large changes in 
this data directly caused intermittent movement of the pointer and there 
was the problem that a smooth indicating characteristic as an indicating 
meter could not be achieved. 
The present invention has for its object to enable smooth drive to an 
extent which is imperceptible even where the conversion processing 
capability, i.e. conversion processing cycle, of a processing circuit for 
computation processing measurement data and converting and outputting an 
indication angle signal is not sufficiently short to attain smooth 
movement of the stepping motor with respect to changes in the measurement 
amount. 
DISCLOSURE OF INVENTION 
The present invention provides a measuring apparatus having a drive circuit 
for driving a stepping motor based on a digital signal corresponding to a 
measurement amount and which displays the measurement amount by indicating 
an increment on a scale on a dial corresponding to the measurement amount 
by means of a pointer fixed to an end of a drive shaft of the stepping 
motor, and is characterized by comprising a processing circuit for 
inputting a digital signal corresponding to the measurement amount and 
converting it to an indicating angle signal in a predetermined cycle, and 
a smoothing circuit for obtaining a difference between successive before 
and after indicating angle signals output in the conversion cycle of the 
processing circuit and dividing the difference into integral parts 
corresponding to a divisor thereof in a separating cycle which is shorter 
than the conversion cycle of the processing circuit, and modifying the 
indicating angle signal for every divided indicating angle part as the 
division result in every separation cycle during subsequent conversion 
cycles. 
Also, the present invention is characterized by comprising a processing 
circuit for inputting a digital signal D corresponding to the measurement 
amount and converting it to an indicating angle signal .theta. in a 
predetermined cycle, and a smoothing circuit for obtaining a difference 
.DELTA..theta. with respect to an indicating angle signal .theta.h prior 
to a newest indicating angle signal .theta.i output in the conversion 
cycle of the processing circuit, and modifying, by sequential addition, an 
indicating angle corresponding to .DELTA..theta./n in the previous 
indicating angle signal .theta.h, at every short cycle corresponding to 
1/n (n is a plural integer) of the cycle of the processing circuit. 
Further, the present invention is characterized by the processing circuit 
being constructed so that, as well as producing indicating signals for 
driving a plurality of indicating meters including other indicating 
meters, it also produces and outputs an indicating signal to each of the 
indicating meters by means of a time division process, and the conversion 
cycle is a period equal to or longer than a period in which processing is 
possible, allocated by the time division process. 
Still further, the present invention is characterized in that the 
above-mentioned plurality of indicating meters are all stepping motor type 
meters. 
In addition, the present invention is characterized in that an updating 
process of the indicating angles modified at every cycle shorter than the 
conversion cycle commences from a point in time either substantially 
synchronous with the conversion cycle or after one cycle of the shorter 
cycle. 
Finally, the present invention is characterized by comprising a processing 
circuit for inputting a digital signal corresponding to the measurement 
amount and converting it to an indicating angle signal .theta. in a 
predetermined cycle T, and a smoothing circuit for obtaining a difference 
.DELTA..theta. with respect to an indicating angle signal .theta.h prior 
to a newest indicating angle signal .theta.i output in the conversion 
cycle T of the processing circuit, and modifying, by sequential addition, 
an indicating angle corresponding to .DELTA..theta./n in the previous 
indicating angle signal .theta.h, at every short cycle T/n corresponding 
to 1/n (n is a plural integer) of the cycle of the processing circuit, 
wherein the short cycle T/n is scale 8 milliseconds or less.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows the basic structure of the present invention and, explaining 
this with the speedometer of an automobile as an example, once a frequency 
signal proportional to the travelling speed of the automobile which is the 
measurement amount is input from an input terminal 1, in a counter circuit 
2 the rises and falls in the input signal are detected and counted by 
predetermined gate times (gate time method) or by counting a different 
high frequency clock signal by means of the input signal (frequency 
measurement method) and computing the forward speed which changes moment 
by moment as digital data D. 
The measurement amount obtained by the counter circuit 2 is converted to an 
indicating angle signal .theta.i at a predetermined conversion cycle T in 
the processing circuit 3, is changed by a smoothing circuit 4 
corresponding to every angle 1/n of a successive before and after angle 
difference .DELTA..theta. of the converted indicating angle signal 
.theta.i at every separation cycle T/n (where n is a plural integer), 
which is shorter than the conversion cycle T, drives the stepping motor 
type meter 6 via an output circuit 5 (not necessary when the meter is 
capable of being driven by the output of the smoothing circuit 4) such as 
a voltage converter or the like, and rotates a pointer 9 fixed to the end 
of a shaft driven at angular increments corresponding to those on a scale 
8 of a dial 7. 
Each of the above circuits properly processes input signals and drives the 
stepping motor type meter 6 as a driving circuit, and the processing 
circuit 3 can be constructed by a microcomputer which includes the counter 
circuit 2 so that the measurement indicating characteristic can be 
arbitrarily set to indicate the forward speed, comprises a memory (ROM) 10 
in which indicating angle data (indicating angle signal) .theta.i 
corresponding to the counted digital data D has been stored, and takes in 
the digital data at a predetermined conversion cycle and reads out the 
indicating angle signal .theta.i of the memory address corresponding 
thereto. 
This processing circuit 3, in cases where an inexpensive IC whose 
processing speed is slow with respect to a single indicating meter drive 
such as the speedometer described here is used, or in cases where other 
indicating meters not shown in the drawings, such as an engine tachometer, 
a heat gauge, a thermometer, an oil pressure meter, and a voltage meter 
(these meters may all be stepping motor type meters, or other measuring 
movements such as a cross coil type meter, movable coil type meter, or the 
like may be jointly used) are simultaneously time division driven, the 
processing cycle allocated to the speedometer has insufficient processing 
capability to attain smooth responsiveness. 
Also, storing of the indicating angle data .theta.i in the memory 10 of the 
processing circuit 3 holds numbers of data for attaining a desired 
resolution corresponding to a prior indicating region from a minimum (MIN) 
and maximum (MAX) of the digital data D corresponding to the measurement 
amount, stores indicating angle data i in 0.5.degree. units with respect 
to an indicating angle from 0.degree. (MIN) to 360.degree. (MAX) for 
example, and reads out this stored indicating angle data .theta.i 
corresponding to the digital data D in a predetermined conversion cycle T. 
The smoothing circuit 4 obtains a difference of changed indicating angle 
data .theta.1, i.e. an angular difference .DELTA..theta. 
(.theta.1-.theta.0) of successive before and after indicating angle data, 
output after the conversion cycle T from the previous indicating angle 
data .theta.0 with respect to the indicating angle data .theta.i output at 
the conversion cycle T from the processing circuit 3, comprises a 
crossover network 11 for sequentially adding angle data corresponding to 
.DELTA..theta./n at every separation cycle T/n (where n is a plural 
integer) which is shorter than the conversion cycle T and a voltage memory 
12 for converting this and outputting a two-phase drive signal for driving 
the stepping motor type meter 6 based on the indicating angle data .theta. 
for every separation cycle T/n output by the crossover network 11, and 
produces a voltage signal as shown in FIG. 2 for example via the output 
circuit 5 for converting to a voltage signal to be applied to the 
two-phase exciting coil of the stepping motor. 
Although the signal wave form for driving the stepping motor can be set to 
an arbitrary two-phase signal by the number of teeth and pitch of the 
toothed yoke, and data of all driving wave forms corresponding to all 
indicating angles within 360.degree. can be stored in the voltage memory 
12, here 60.degree. portions of voltage data formed by dividing 360 into 
six are provided in the voltage memory 12 as stepping motor drive signals 
of the stepping motor type meter 6, and have a structure which reduces the 
memory capacity in a system which uses this data in every angle region. 
In other words, the voltage wave form of the drive signal applied to the 
two-phase exciting coils A and B with respect to the 360.degree. rotating 
angle of the pointer 9 (coupled magnet rotor) of the stepping motor type 
meter 6, as shown in FIG. 2, imparts a change of substantially SIN and COS 
wave forms within a 60.degree. angle, and expands to all of the 60.degree. 
angular regions a through f by way of this voltage wave form. This 
expanding process can be determined by indicating angle data .theta. 
corresponding to the indicating region of the digital data D, stores drive 
voltage data V in the voltage memory 12 at a resolution of 60.degree. 
divided into 512 (storing data at 60/512.degree. angular differences from 
.theta.0 with respect to 0.degree. to 60.degree. with respect to 
60.degree.), and with respect to the digital data D of indicating angle 
regions b to f as well as the reading out of the voltage data V 
corresponding to the digital data D, as well as determining each 
indicating region, it can read out the voltage data V within the 
determined region from the voltage memory 12 and obtain an indicating 
position on a meter by a combination with the determined region. 
With regard to this, where the teeth of the yoke of the stepping motor are 
arranged in at least one group each in the indicating regions of FIG. 2, 
because the polar position of the magnet rotor moves only to the 
excitation position within the region, rather than the drive signals 
differing for each indicating region, by determining the indicating region 
based on the digital data D the voltage data V read out from the voltage 
memory 12 can be read out; for example, if the digital data D corresponds 
to the indicating angle 150.degree., the region is determined as c and by 
reading out data corresponding to 30.degree. within the region from the 
voltage memory 12 the drive signal is as shown in FIG. 2. 
Here, the drive wave form shown in FIG. 2 is a substantially curved wave 
form in order to attain a smooth operation of the mechanical step 
operation determined by the tooth pitch of the yoke of the stepping motor 
by means of so-called micro-steps. In fact, SIN and COS wave forms are 
digital minute stepped wave forms and the pitch of the micro-steps is set 
the speed of change of the measurement amount indicated by the meter, i.e. 
the angular speed of the pointer, and the processing capability of the 
processing circuit. 
The applicants of the present invention, performed fabrication and 
characteristic tests on an actual indicating meter based on such a basic 
structure, and as the stepping motor main unit serving as the meter 
movement using the stepping motor type meter 6, wound an exciting coil for 
exciting the two-phase drive wave form around two stacked resin bobbins, 
stacked toothed yokes above and below each bobbin and used a PM die of 
well-known structure to rotatably support the magnet rotor in the 
intermediate portion of this bobbin stack. 
In each toothed yoke, six teeth are formed positioned opposite each other 
at each phase so that over the entire circumference 24 teeth are arrayed, 
and are formed having a mechanical step pitch of 15.degree. and so that 
four teeth correspond within each angular limit of 60.degree. of the 
indicating regions a through f shown in FIG. 2. 
The processing circuit 3 performs time division driving for simultaneously 
drive processing other meters or alarm displays by means of a 
microcomputer, and a conversion cycle T of 16 milliseconds is set 
allocated to the speedometer and the movement of the pointer 9 was 
observed when the digital data D was changed to various angular speeds. 
The setting of the division cycle T/n of the crossover network 11 in the 
smoothing circuit 4 at this time was performed stepwise by changing n, the 
angular speed was changed under the division cycle T/n thus set, and an 
allowable value at which the rotation of the pointer 9 was seen to rotate 
with sufficient smoothness even when there was a gradual change in the 
digital data D, i e. forward speed, was selected. 
Although the smoothness of the rotation of the pointer 9 differs according 
to the observational powers of the person observing it, the applicants of 
the present invention, as well as determining visual smoothness, actually 
rotated the pointer 9 at various angular speeds, and selected a division 
cycle having relatively uniform changes due to the data characteristic 
wave form with respect to a plurality of angular speed axes of a rotation 
indication characteristic observed at the rotation speed change rate of 
the pointer 9 (attained by fixing an optical rotary encoder to the pointer 
axis and measuring changes in the angular speed of the pointer, for 
example). 
In actuality, as well as further setting the conversion cycle T at 16 
milliseconds, the angular speed .omega. is changed to a low speed region 
of 10.degree./sec units and a high speed region of 100.degree./sec units 
within a range of 10.degree./sec to 1080.degree./sec, and under these 
conditions the cycle is set from 16 milliseconds to units of 1 millisecond 
as a shorter cycle, and the rotational smoothness of the pointer 9 
confirmed. 
As a result, smoothness is-visually favorable within the entire range of 
angular speeds, and a cycle which is relatively uniform even by way of 
characteristic data of the rotational speed change rate is 8 milliseconds 
or less, while a cycle which is very uniform as indicated by the 
characteristic data and imparts stable smoothness without visual 
irregularities as viewed by an observer is 4 milliseconds or less. 
Accordingly, the division cycle T/n applied to the embodiment of the 
present invention, where the conversion cycle T has been set at 16 
milliseconds, an allowable indicating characteristic can be attained in 
the 8 milliseconds corresponding to half this cycle as a visual or 
comparative meter with rotation speed change rate, and it was judged by 
repeating characteristic verification in this manner that a division cycle 
in which the rotation smoothness of the pointer 9 is favorable is 8 
milliseconds or less, and optimally 4 milliseconds or less. Also, where 
the conversion cycle T is 20 milliseconds for example, the division cycle 
T/n is set at 5 milliseconds (n=4) with 10 milliseconds (n=2) which is 
close to 8 milliseconds as the limit, and where the conversion cycle T is 
15 milliseconds, the division cycle T/n is set to 5 milliseconds (n=3), 
whereupon it was determined that smooth rotational indication of the 
pointer 9 can be attained. 
FIG. 3 (proceeding circuit) shows the change characteristic of the 
indication angle data .theta. where n=4 and the division cycle T/n (4 
milliseconds) is set with respect to the conversion cycle T (16 
milliseconds), and with respect to a change of the 16 millisecond 
conversion cycle allocatable to the speedometer in the processing circuit 
3 which converts, by means of a data table in the memory 10, the digital 
data D corresponding to the forward speed, data is updated in a four 
millisecond division cycle, but if a change to the indication angle data 
in this 16 millisecond conversion cycle, i.e. the difference 
.DELTA..theta. of successive before and after indication angle data, is 
taken as 2.degree., the data added and updated in every four millisecond 
division cycle becomes 2.degree./4=0.5.degree., and as shown in the 
drawing, with respect to the 2.degree. difference from indicating angle 
data .theta.0 at the point T0 to the indicating angle data .theta.1 at the 
point T1 after the conversion cycle, the change at the point T1 is 
obtained, updated upward stepwise by 0.5.degree. each time from the 
initial division cycle lapse time, four milliseconds after the conversion 
cycle T1, and subsequent changes from T2 onwards are performed by the same 
process. 
Note that, other than the timing of FIG. 3 where the 0.5.degree. 
computation update is at the initial division cycle time which is four 
milliseconds from the conversion cycle T1, if processing from the 
conversion cycle T1 can be made momentary it is possible to start from a 
time substantially simultaneous with the conversion cycle T1. 
The 2.degree. change every 16 milliseconds at this time is the rotation 
angle of the pointer 9, and where the speedometer is such that 240.degree. 
on the scale 8 of the dial 7 thereof is set to 180 Km/h, the change of the 
indication angle data 2.degree./16 milliseconds corresponds to 1.5 Km/h 
(180/240*2) changing in 16 milliseconds, this corresponds to an 
acceleration increase of 94 Km/h in one second, and although this is a 
change that cannot exist in actual forward motion, the division result 
shown in FIG. 3 is set by the division cycle T/n as a numerical value for 
ease of understanding. 
In actuality, although changes in speed during normal forward motion do not 
occur as angular speed to an equivalent extent during sudden acceleration 
or sudden deceleration, with regard to an engine tachometer, changes occur 
extremely rapidly, therefore there are times when it reaches 2.degree. per 
16 milliseconds, i.e. an indication angle of 125.degree. or more on the 
dial of the meter is one second, and rotational indication in even smaller 
angular steps can be attained in 4 millisecond cycles at every 0.5.degree. 
during such changes. 
In such a case, although rotation in a 0.5.degree. division cycle is in 
numerically large step angles, because the angular speed is a quick 
movement in the 4 millisecond cycle, it has been confirmed that this is 
visually a smooth movement, and where the angular speed is slow, for 
example, an acceleration of 10 Km/h in one second in a normal forward 
acceleration, movement in extremely small steps of approximately 13/sec, 
i.e. 0.05.degree. in the 4 millisecond cycle at 240.degree.*10/180 can be 
attained. 
With the stored data resolution of the present embodiment, 60/512.degree., 
i.e. 0.117.degree. unit data is possible, thus in fact the 0.05.degree. 
step possible in the 4 millisecond cycle cannot be achieved, and although 
0.117.degree. step operation is possible in an enforced 8 millisecond 
cycle, a step operation in at least the units of basic data resolution can 
be attained and visually there are no problems whatsoever. Further, if 
data having more detailed basic resolution, e.g. resolution of 
60/1200.degree. or more, is stored, the step operation of 0.05.degree. in 
4 milliseconds can be achieved, and an extremely smooth indication 
characteristic can be achieved by a step operation which largely cannot be 
visually observed. Consequently, also with regard to the smoothness in 
this angular speed, a characteristic which is visually allowable was 
confirmed with approximately 8 milliseconds or less as the data update 
cycle, and this was determined to be a division cycle which could 
guarantee favorable smoothness within the range of the angular speed used 
as the indicating meter. 
In a processing circuit-constructed within such a driving apparatus, as 
well as a microcomputer being used and various types of meter driving 
controls being performed by a computation program, the smoothing circuit 
can be constructed by combining gate circuits by means of a semiconductor, 
and in particular the microcomputer is effectively utilizable when there 
are many load controls. 
As described above, according to the drive apparatus for stepping motor 
type meter of the present invention, the processing circuit for inputting 
measurement amount data obtains a difference of changed indicating angle 
data 1, i.e. an angle difference .DELTA..theta. (.theta.1-.theta.0) of 
successive before and after indicating angle data, output after the 
conversion cycle T from the previous indicating angle data .theta.0 with 
respect to the indicating angle data .theta.i output at the conversion 
cycle T and produces a divided voltage signal having a minutely stepped 
wave form by means of a smoothing circuit for sequentially adding angle 
data corresponding to .DELTA..theta./n at every separation cycle T/n 
(where n is a plural integer) which is shorter than the conversion cycle 
T. 
Thereby, although a smooth movement could not be attained in the conversion 
cycle T of the processing circuit, a smooth indication characteristic can 
be attained in the division cycle T/n, and in particular where the 
division cycle is 8 milliseconds or less, or optimally 4 milliseconds or 
less, a stepping motor type meter having a smooth indication 
characteristic which is not visibly unnatural can be attained. 
Consequently, in relation to production costs, where expensive processing 
circuits are not used, or where there is a limit to compression of the 
computation processing cycle assigned to one meter when many meters are 
time division operated, and where, for example, allocation of the 
computation processing cycle of the processing circuit cannot follow the 
drive update cycle necessary to smoothly rotate the pointer corresponding 
to a measurement amount in which changes are severe, such as engine 
revolutions, and the pointer can only move intermittently with respect to 
changes in the measurement amount, a smooth indication characteristic can 
be attained as the indicating meter by movement during the division cycle 
by means of the smoothing circuit. 
Industrial Applicability 
As described above, the present invention can be applied to use in an 
electrical movement of an indicating meter when working towards 
miniaturization, for inputting a physical measurement amount as an 
electrical signal and performing corresponding indication on a scale of a 
dial by means of a pointer.