Driving circuit of a grating

A driving circuit for rotating the grating of a monochromator thereby changing the wavelength of light which is emitted from the monochromator. This driving circuit includes an operating panel for supplying wavelength data, a data converting circuit which obtains a trigonometric function value corresponding to the wavelength data when the data converting circuit receives the wavelength data, and which converts the wavelength data to angle data on the basis of the trigonometric function value, and a motor driver for rotating the grating into the angular position corresponding to the angle data.

BACKGROUND OF THE INVENTION 
This invention relates to a driving circuit of a grating for rotating the 
grating of a monochromator and, more particularly, to a driving circuit of 
a grating in which, when wavelength data is input, angle data is 
calculated from the wavelength data, and the grating is caused to rotate 
on the basis of this calculated result. 
Conventionally, in one type of monochromator optical equipment, in the case 
of detecting the wavelength distribution or the like of light as shown in 
FIG. 1, a incident light 3 is split into spectra by a grating (diffraction 
grating) 1 rotatably provided at a predetermined portion of the main body 
of the monochromater (not shown). A light component having a desired 
wavelength is thus taken from the incident light 3 by a slit apparatus 
(not shown) and focusing it in a light receiver apparatus (not shown). 
With such a grating system, however, when the incident light 3 is provided 
thereto with only an angle .alpha. of inclination from a normal line 2, 
which is perpendicular to a lattice plane 1.sub.a of the grating 1, a 
primary light 4 having a wavelength .lambda., represented by the equation 
EQU .lambda.=d(sin .alpha.+sin .beta.) . . . (1) 
is emitted in a direction with an angle .beta. of inclination from the 
normal line 2, wherein d indicates a lattice constant of the grating 1. On 
the other hand, in this case, a difference angle .gamma. between the 
incident light 3 and the primary light 4 shown in FIG. 1 is determined 
dependent upon the location of a mirror (not shown) to supply the incident 
light 3 to the grating 1 and the location of a mirror (not shown) which 
receives the primary light 4. Therefore, assuming that 
.beta.-.alpha.=.gamma. (constant) and by substituting this for equation 
(1) and further by modifying the equation thus obtained, 
##EQU1## 
can be derived. In this equation (2), since 2d.times. cos 
##EQU2## 
is a constant, this is substituted by a constant A. Also since the angle 
##EQU3## 
is the angle between a normal line 2 and an axis 5 which divides the angle 
.gamma. into two, this is substituted by a variable .theta.; thus, 
equation (2) will be 
EQU .lambda.=A.multidot.sin .theta. . . . (3). 
As will be understood from equation (3), to emit the primary light 4 of the 
wavelength .lambda. from this grating 1 when the incident light 3 is 
supplied to the grating 1, it is necessary to rotate the grating 1 by only 
the angle .theta. 
##EQU4## 
corresponding to the wavelength .lambda.. 
FIG. 2 is a diagram showing one example of a sine bar as one of the devices 
to perform such wavelength/angle conversion. As illustrated in this 
diagram, the sine bar 12 is constituted in a manner such that a rod member 
9 which abuts a movable member 8 is rotated around an axis 10 in 
association with the movement of the movable member 8 in the X direction 
by means of a motor or the like (not shown), and at the same time, a 
grating mounting plate 11 fixed to this axis 10 is also rotated. 
Therefore, when the amount of travel S of the movable member 8 is small, 
the relation 
EQU S=R.multidot.sin .theta. . . . (4) 
is satisfied between the travel amount S and the rotational angle 
(.theta..sub.1 -.theta..sub.2) of the grating mounting plate 11. In this 
case, R indicates a distance from the axis 10 to the contact point of the 
rod member 9 and movable member 8, and .theta. represents the difference 
angle between the angle .theta..sub.1 before deformation and the angle 
.theta..sub.2 after deformation. As can be seen from a comparison of 
equations (3) and (4), if the movable member 8 can be moved by only the 
amount S 
##EQU5## 
corresponding to the wavelength .lambda., the primary light 4 of the 
wavelength .lambda. can be emitted from the grating 1. 
However, since the amount of change of the arm length R of the rod member 9 
increases with an increase in the travel amount S, such a sine bar 12 
cannot cover a wide wavelength range. Also, even when the travel amount S 
lies within a narrow range, the arm length R slightly changes and the rod 
member 9 and the movable member 8 are mutually worn out due to contact 
with each other, so that a high degree of accuracy cannot be obtained. 
Further, such a sine bar 12 cannot be miniaturized due to the structure 
thereof, so that there is an inconvenience such as an increase in mounting 
space for the grating driving apparatus. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a new and 
improved driving circuit of a grating in which a wide wavelength range can 
be covered and a high degree of conversion accuracy can be obtained, and 
further, the whole grating driving apparatus can be miniaturized. 
According to the present invention, a driving circuit of a grating, which 
rotates the grating of a monochromator and changes the wavelength of light 
emitted from the monochromator, comprises: a means for supplying 
wavelength data; a data converting means which obtains a trigonometric 
function value corresponding to the wavelength data when it receives the 
wavelength data, thereby converting the wavelength data to the angle data 
on the basis of the trigonometric function value; and a driving means for 
rotating the grating into the angular location corresponding to the angle 
data. 
With such a constitution, it is possible to provide a driving circuit of a 
grating in which a wide wavelength range can be covered and a high degree 
of conversion accuracy can be obtained, and further, the whole grating 
driving apparatus can be miniaturized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will be described below with respect to embodiments shown in 
the drawings. FIG. 3 is a block diagram showing the first embodiment of 
the driving circuit of a grating according to the present invention. FIG. 
3 shows an operating panel 14 for optical equipment equipped with the 
present driving circuit. The wavelength data .lambda. input from this 
operating panel 14 is supplied to a wavelength/angle converter (hereafter, 
abbreviated as a .lambda./.theta. converter) 15. The .lambda./.theta. 
converter 15 comprises a data converter 30, readout means 15B, an angle 
converter 15C, and a memory 15A. When the wavelength data .lambda. is 
supplied from the operating panel 14, this wavelength data .lambda. is 
divided by the constant A in the data converter 30 to obtain the data 
.lambda./A. A readout signal D corresponding to this data .lambda./A is 
supplied through the readout means 15B to the memory 15A. Data 
corresponding to respective wavelength intervals 1 to n, as shown in the 
table below, is stored in the memory 15A as follows: the sine value data 
(.lambda./A).sub.1 corresponding to the angle .theta..sub.1 [where, 
(.lambda./A).sub.1 =sin .theta..sub.1 ]; the sine value data 
(.lambda./A).sub.2 corresponding to the angle .theta..sub.2 [where, 
(.lambda./A).sub.2 =sin .theta..sub.2 ]; the sine value data 
(.lambda./A).sub.n corresponding to the angle .theta..sub.n [where, 
(.lambda./A).sub.n =sin .theta..sub.n ]; and the gradients .alpha..sub.1, 
.alpha..sub.2, . . . , .alpha..sub.n between these respective angles 
.theta..sub.1 -.theta..sub.2, . . . , .theta..sub.n-1 -.theta..sub.n. When 
a readout signal D is supplied from the .lambda./.theta. converter 15, the 
data in the wavelength interval indicated by this readout signal D is read 
out and is supplied to the readout means 15B. Namely, when the readout 
means 15B outputs the readout signal D indicative of the wavelength 
interval 2, the value 0.0628 of the data (.lambda./A).sub.2 corresponding 
to the wavelength interval 2 is selected from the memory 15A as shown in 
the table below and is supplied to the readout means 15B. In addition, in 
the case where the readout means 15B selects any one of the wavelength 
intervals 3 to n, one value of the sine value data (.lambda./A).sub.3 to 
(.lambda./A).sub.n corresponding to the selected wavelength interval is 
also similarly supplied to the readout means 15B. 
______________________________________ 
Wave- 
length Sine value 
interval data Angle data 
Gradient data 
n (.lambda./A).sub.n 
.theta..sub.n 
.alpha..sub.n 
______________________________________ 
1 0 0.degree.00' 
57.3284 
2 0.0628 3.degree.36' 
57.5079 
3 0.1254 7.degree.12' 
57.9710 
4 0.1875 10.degree.48' 
. . . . 
. . . . 
. . . . 
n - 2 0.9921 82.degree.48' 
n - 1 0.9981 86.degree.24' 
600.0000 
n 1.0000 90.degree.00' 
1894.7368 
______________________________________ 
After the readout means 15B fetches the sine value data from the memory 15A 
in response to the readout signal, it compares the value of the 
above-mentioned data .lambda./A, which is the former arithmetic result, 
with the value of the sine value data (.lambda./A).sub.m (m is an integer 
indicative of any one of 1 to n) read out from the memory 15A. When 
(.lambda./A).sub.m &lt;.lambda./A, the readout means 15B reads out the sine 
value data (.lambda./A).sub.m+1 corresponding to (m+1), which is larger 
than the wavelength interval m at this time. On the other hand, when 
(.lambda./A).sub.m &gt;.lambda./A, it reads out m, the sine value data 
(.lambda./A).sub.m-1 corresponding to (m-1) which is smaller than the 
wavelength interval m at this time, and repeats this comparison operation, 
thereby obtaining the value of m which satisfies the relation 
[(.lambda./A).sub.m &lt;.lambda./A&lt;(.lambda./A).sub.m+1 ], namely, the 
wavelength interval m in which the above-mentioned data .lambda./A is 
included. Next, the angle converter 15C for converting to the angle data 
subtracts the sine value data (.lambda./A).sub.m corresponding to the 
wavelength interval m from the data .lambda./A and temporarily stores this 
subtraction result [.lambda./A-(.lambda./A).sub.m ]. Further, the angle 
converter 15C reads out the angle data .theta..sub.m and gradient data 
.alpha..sub.m corresponding to the wavelength interval m from the memory 
15A and performs the arithmetic operation shown in the following equation: 
EQU .theta.=.alpha..sub.m [.lambda./A-(.lambda./A).sub.m ]+.theta..sub.m . . . 
(5). 
In this case, the angle data .theta..sub.m indicates the sine angle 
corresponding to the data (.lambda./A).sub.m, and the gradient data 
.alpha..sub.m represents the gradient angle from this angle data 
.theta..sub.m to the next angle data .theta..sub.m+1. Thus, the angle data 
.theta. for the data .lambda./A is obtained from equation (5). On the 
other hand, in the case where the equation .lambda./A=(.lambda./A).sub.m 
is satisfied in the foregoing comparison operation, the readout means 15B 
reads out the angle data .theta..sub.m of the waveform interval m 
corresponding to this (.lambda./A).sub.m from the memory 15A and outputs 
this as the angle data .theta. from the angle converter 15C. 
Therefore, for example, the wavelength .lambda. is input from the operating 
panel 14 and in correspondence with this, when the value of the data 
.lambda./A lies between the sine value data (.lambda./A).sub.2 and 
(.lambda./A).sub.3, as shown in FIG. 4, the .lambda./.theta. converter 15 
reads out the angle data .theta..sub.2 and gradient data .alpha..sub.2 
corresponding to the sine value data (.lambda./A).sub.2 from the memory 
15A. Also, the converter 15 performs the arithmetic operation shown in 
equation (5) to obtain the angle data .theta. and supplies it to a motor 
driver 17. The motor driver 17 compares the angle data .theta. with the 
value of an angle detection signal .theta..sub.p output from a rotation 
detector 18 to detect the rotational angle of the grating 1, and it 
controls a motor 19 so as to make them coincident. When the motor 19 is 
driven by this motor driver 17, a grating mounting plate 20, on which the 
grating 1 is mounted, is rotated in correspondence to this. The rotational 
deformation amount at this time is detected by the rotation detector 18 
and is fed back to the motor driver 17. 
Next, the operation of this first embodiment constituted as described above 
will be described with reference to the flow chart shown in FIG. 5. 
First, when the circuit is activated, the .lambda./.theta. converter 15 
executes step S1 and maintains the standby mode until the wavelength data 
.lambda. is input. When the operating panel 14 is operated and the 
wavelength data .lambda. is input, the .lambda./.theta. converter 15 
executes the next step S2 and compares the data .lambda./A obtained in 
correspondence with the wavelength data .lambda. with the sine value data 
(.lambda./A).sub.m previously stored in the memory 15A, thereby 
discriminating to see if there exists the wavelength interval m which 
satisfies .lambda./A=(.lambda./A).sub.m. When this condition is not 
satisfied, namely, when (.lambda./A).sub.m 
&lt;.lambda./A&lt;(.lambda./A).sub.m+1, the converter 15 executes step S3 and 
performs a linear approximating operation as shown in equation (5), 
thereby obtaining the angle data .theta. corresponding to the data 
.lambda./A. Next, in step S4, the .lambda./.theta. converter 15 supplies 
the angle data .theta., obtained by the above-mentioned arithmetic 
processing, to the motor driver 17 and driving the motor 19 to rotate the 
grating 1 by only the angle corresponding to the angle data .theta.. 
Thereafter, the processing is returned through step S5 to the initial step 
S1. 
On the other hand, in the above-described operation, when it is determined 
that there is no need to interpolate in step S2, namely, when the value of 
.lambda./A input coincides with the sine value data stored in the memory 
15A, step S3 is omitted and steps S4 and S5 are directly executed. 
In addition, although this embodiment has been explained with respect to 
the example in the case where the gradient data .alpha..sub.1 
-.alpha..sub.n are stored in the memory 15A, these gradient data 
.alpha..sub.1 -.alpha..sub.n may be obtained by substituting 
.lambda.=.lambda..sub.n for the gradient 
##EQU6## 
which is derived by differentiating both sides of .lambda.=A sin .theta.. 
Further, the circuit may be constructed so that the angle data .theta. is 
obtained by the program operation based on the wavelength data .lambda., 
supplied from the operating panel 14, by use of a microprocessor (not 
shown) having an ROM (read only memory) in which programs are stored and 
an RAM (random access memory) or the like serving as the working area, and 
the angle data .theta. obtained in this way is output. In this case, when 
the wavelength data .lambda. is input from the operating panel 14, the 
microprocessor instead of the converter 15 obtains the data x 
(x=.lambda./A) on the basis of this wavelength data .lambda. and the 
constant data A stored in the ROM. At the same time, it obtains the angle 
data .theta.(.theta.=sin.sup.-1 x) on the basis of this data x and the 
programs stored in the ROM; namely, on the basis of a first program to be 
sequentially operated a limited number of times for each term on the right 
side in the Maclaurin expanded expression, indicated by 
##EQU7## 
and a second program to sequentially add the respective values obtained in 
accordance with the first program. The angle data .theta., which is the 
result of this arithmetic operation, is supplied through an interface (not 
shown) to the motor driver. In this way, even if the wavelength data 
.lambda. is converted to the angle data .theta. by the program operation, 
the grating 1 can be rotated by only the angle corresponding to the 
wavelength data .lambda. in a manner similar to the foregoing, first 
embodiment. 
In addition, the operation shown in equation (6) may be performed in such a 
manner that the wavelength data .lambda. is converted to the angle data 
.theta. by use of a logic circuit (not shown) for performing such an 
operation; namely, by use of a .lambda./.theta. conversion logic circuit 
which has: a first logic circuit to output the input data x as it is; a 
second logic circuit to operate 
##EQU8## 
from the data x; and an adder to add the values obtained in these logic 
circuits. 
As described above, even if the arithmetic logic circuit is constituted by 
hardware, a similar effect as that mentioned above can be obtained. 
Further, in this case, the operating speed can be made fast. 
Also, although the wavelength data .lambda. is converted to the angle data 
.theta. on the basis of the Maclaurin expanded expression of the arcsine, 
sin .sup.-1 x may be derived by integrating 
##EQU9## 
or by use of other operational expressions. 
FIGS. 6A and 6B are block diagrams showing the second embodiment of this 
invention. FIG. 7 is a waveform diagram to explain this second embodiment. 
In FIG. 6(A), an analog circuit 26 is provided for converting 
.lambda./.theta. having an arcsine generator (voltage generator) 27 to 
generate an arcsine waveform S, as shown in FIG. 6(B), a sampling circuit 
28 to sample and hold an output of this arcsine generator 27, a 
controlling circuit 29 to control the sampling circuit 28 and arcsine 
generator 27, and the data converter 30. When the wavelength data .lambda. 
(in this case, this wavelength data .lambda. is the voltage) is supplied 
from the operating panel 14, the analogue circuit 26 obtains the value of 
.lambda./A and, thereafter, it makes the arcsine generator 27 operative in 
response to an output of the controlling circuit 29, thereby allowing the 
generation of the arcsine waveform S to be started. A sampling signal 
S.sub.p is output from the controlling circuit 29 after only the delay 
time corresponding to the value of .lambda./A from this starting time 
(e.g., time t.sub.0 shown in FIG. 7). The sampling circuit 28 is made 
operative in response to this sampling signal, and this sampling result 
.theta. is supplied as the angle data .theta. to a motor driver 17.sub.a. 
Therefore, in this embodiment, the wavelength data .lambda. is converted 
to the angle data .theta., thereby enabling the angle of the grating 1 to 
be controlled. 
On the other hand, although the output of the arcsine generator 27 is 
delayed and is sampled in this embodiment, in place of this, it is also 
possible to provide a sine generator for generating a sine waveform and to 
obtain the angle data .theta. by measuring the time period from the time 
when such sine generator starts generating the sine waveform to the time 
when an output of the sine generator coincides with the value .lambda./A. 
As described above, in the driving circuit of a grating according to this 
invention, when the wavelength data is supplied, the .lambda./.theta. 
converter converts the wavelength data to the angle data, and the motor 
driver rotates the grating in accordance with this angle data. Therefore, 
the wavelength/angle conversion can be performed over a wide range, and 
the wavelength data can be converted to the angle data with a higher 
degree of accuracy than in the mechanical wavelength/angle converting 
apparatus, such as a sine bar or the like. Further, the whole grating 
driving apparatus can be miniaturized since the wavelength/angle 
conversion is electrically performed. 
In the monochromator, the grating is disposed on the axial line of the 
optical system so that the angle thereof can be varied from this axial 
line. The light supplied through an optical fiber or the like is split 
into spectra by the grating or the like. By selecting the spectrum through 
a slit in the slit apparatus, only the light of a desired wavelength from 
the light which is split into spectra is obtained, and, e.g., the strength 
of the light of each wavelength are analyzed. In a spectrum analyzer in 
which the wavelength of light is displayed on an abscissa axis and the 
strength of the light of each wavelength is displayed on an ordinate axis, 
the angle of the grating can be accurately set from the position 
information of the abscissa, namely, from the wavelength data according to 
this invention.