Function Generator and Application Thereof

1. Field of the Invention

The present invention relates to a function generator which is supplied with an angular signal and produces function sin .phi. and cos .phi. or a function closely approximating a circular function. (In the case that a pair of functions X = f.sub.1 (.phi.) and Y = f.sub.2 (.phi.) are plotted along the X and Y axes of a rectangular coordinate system, if the locus of any pair of coordinates (X, Y) falls on a circle when the angle .phi. is linearly varied, the functions X = f.sub.1 (.phi.) and Y = f.sub.2 (.phi.) are referred to as circular functions in the present application).

2. Description of the Prior Art

In the hitherto known digital resolver converter or the like, a circuit which is supplied with an angular signal in a digital form and produces as an output a resolver signal in an analog form has been employed. For better understanding of the invention, a brief description will first be made of general arrangements and features of the heretofore known circuits employed for the digital resolver converter by referring to FIGS. 1 to 4.

FIG. 1 shows schematically a typical function generator employed for the digital resolver converter or the like. It is assumed that the function generator l shown in FIG. 1 is supplied with an angular signal .phi. of a digital quantity, whereby there appears at an output terminal 3 a function signal expressed by

and at the other output the function is given by the following expression:

The angular signal .phi. applied to the input terminal 2 in a digital form comprising bits a.sub.0, a.sub.1, a.sub.2 , . . . , a.sub.n. The range in which the angular quantity applied to the input is varied is assumed to be 0.degree. to 90.degree., that is, within the first quadrant for the convenience' sake of the explanation. Further, it is also assumed that the weighting of the individual bits is made in accordance with the binary notation in a manner defined by the following expression:

The range of .phi. given by the above expression (3) is of 0.degree. to 90.degree. .times. (1 - 2.sup.116 n.sup.-1).

By selecting n to be a sufficiently great number, .phi. can cover substantially the whole range from 0.degree. to 90.degree..

Reference numeral 5 indicates an input terminal for a reference signal Es required to generate the analog function. Application of the reference signal Es at the input terminal results in the generation of products of desired functions and Es at the outputs. In the case of the digital resolver converter or the like, an alternating current signal required for the excitation of the resolver is used as the reference signal Es. In other applications, a direct current signal may be used for the reference signal.

If the functions f.sub.1 (.phi.) and f.sub.2 (.phi.) having the following relation: ##EQU1## can be produced by the function generator 1, with a practically tolerable accuracy from the engineering viewpoint, the output signals X and Y may be satisfactorily utilized as the resolver signals. In this connection, it is noted that, even if the conditions that f.sub.1 (.phi.) .varies. cos .phi. and f.sub.2 (.phi.) .varies. sin .phi. are not always satisfied, the receiver servo system supplied with the above resolver signals X and Y can be operated normally, so far as the relation (4) is met, and the angular signal .phi. can be accepted with a practically tolerable accuracy.

As an example of such a function, the following functions ##EQU2## and ##EQU3## have been already reported in the periodical "ELECTRONIC DESIGN", Vol. 18, No. 7, Apr. 1, 1970, p. 56.

FIG. 2 shows a vector defined by the functions produced by the function generator shown in FIG. 1. The function f.sub.1 (.phi.) defined by the expression (5) is taken along the abscissa, while f.sub.2 (.phi.) satisfying the expression (6 ) is taken along the ordinate. In this case, the length of vector R and the angle .theta. can be, respectively, given by the following formulas:

If the constant K appearing in the formulas (5) and (6) is selected so that

and K' is so selected that R becomes equal to 1 when .phi. = 0, the vector length R will be varied in a manner shown in FIG. 3 as the signal .phi. varies from 0.degree. to 90.degree..

Further, if the term ##EQU5## of the expression (4) becomes ideally equal to tan .phi., the term .phi. of the expression (8) becomes equal to .phi.. However, in reality, the above condition can not be realized, and there arises an inevitable error between .theta. and .phi., which error will be varied in dependence on the values of the angle .phi. as is illustrated in FIG. 4.

When the functions given by the expressions (5) and (6) are utilized as X-axis signal and Y-axis signals, the angular error of the vector of these function signals is in the order of 0.032.degree. at maximum as can be seen from FIG. 4. Such errors lie in a tolerable range from the engineering viewpoint and hence the above functions can be satisfactorily utilized for the digital resolver converter and the digital synchro converter in practice.

The length of vector should ideally be constant independently from the angular signal .phi.. However, the vector length is decreased about 14 % at maximum at 45.degree. as is illustrated in FIG. 3. Although such variation may be tolerated in the case of the digital resolver converter or the like under certain circumstances, it can be allowed in the other applications such as display devices or the like.

As will be appreciated from the foregoing discussion, the function generator which can produce the functions defined by the expressions (5) and (6) has a drawback that the vector length of the functions is subjected to variations in dependence upon the angular values and therefore can not be called an ideal circular function generator. In other words, the functions expressed by the formulas (5) and (6) will certainly satisfy the condition given by the expression (4) in respect of the angular value with a high accuracy. These functions, however, are not proportional to cos .phi. and sin .phi. and for this reason incurs a result that the vector length will not remain constant.

FIG. 1 shows schematically an example of a function generator employed hitherto for conventional digital resolver converter or the like.

FIG. 2 shows a vector diagram of the functions produced by the function generator shown in FIG. 1.

FIG. 3 shows graphically a variation of the vector length of the functions produced by the conventional generator such as shown in FIG. 1.

FIG. 4 illustrates angular errors of the same.

FIGS. 5 and 6 show graphically examples of correcting function for correcting or compensating the variation in the vector length according to the invention.

FIG. 7 is a block diagram showing schematically a general arrangement of a function generator according to an embodiment of the invention.

FIG. 8 shows exemplarily errors in the vector length after having been corrected according to the invention.

FIGS. 9 and 10 show more concretely a circuit arrangement of a function generator which is capable of correcting the error in the vector length according to the invention.

FIG. 11 is a block diagram showing in detail a function generator shown in FIG. 1.

FIG. 12 is a circuit diagram of another embodiment of a vector length correcting function generator according to the invention.

FIG. 13 is a block diagram of a display apparatus to which the function generator of the invention is applied.

FIGS. 14a and 14b respectively show PPI and spiral images which can be produced by the apparatus shown in FIG. 13.

FIG. 15 is a block diagram of a triangular function computing apparatus employing the principle of the invention.

The present invention is intended to eliminate the above described disadvantages and contemplates as a first object the provision of a function generator apparatus comprising a firt function generator which is supplied with an angular signal .phi. to generate at the outputs thereof functions ##EQU6## or modifications thereof in which K and K' are given constants and f.sub.2 (.phi.)/f.sub.1 (.phi.) is substantially equal to tan .phi. with a tolerable accuracy, wherein a second function generator for generating a function closely approximating the function 1 /.sqroot.f.sub.1.sup.2 (.phi.) + f.sub.2.sup.2 (.phi.) is connected in series with the first function generator, to thereby obtain at the outpus of the apparatus the functions approximating to cos .phi. and sin .phi. or circular functions substantially insusceptible to the variation in the vector length thereof.

A second object of the invention is to obtain functions approximating to cos .phi. and sin .phi. or circular functions having a vector length subjected to little variation by employing as the vector length correcting function the function ##EQU7## or ##EQU8## wherein A and B are given constants or a modification thereof.

A third object of the invention is to realize a sine or cosine function or a circular function with a high accuracy by a circuit of a relatively simplified construction.

Another object of the invention is to provide a digital resolver or digital synchroconverter, the outputs of which is substantially made immune to the variation in the vector length by employing the above described function generator.

A further object of the invention is to provide a display apparatus which can easily produce a circle with the aid of the function generator according to the invention.

To accomplish the above-mentioned objects of the invention, there is proposed according to the invention a further generator circuit for correcting the variation in the vector length by utilizing the variation in the angular signal as a variable, which circuit is connected in series with the function generator of the type hereinbefore described, to thereby maintain the vector length as constant as possible.

FIGS. 5 and 6 graphically show examples of the correction function for correcting or compensating the variation in the vector length. At this point, it should be recalled that the vector length of the functions f.sub.1 (.phi.) and f.sub.2 (.phi.) expressed by the formulas (5) and (6) can be given by the formula (7) and undergoes the variation such as shown in FIG. 3 in dependence on the variation of .phi. when K = 0.00617.

Starting from the above recognition, according to the invention, a function f.sub.5 (.phi.) which together with the function given by the formula (7) can produce a constant product is employed. This function f.sub.5 (.phi.) can be expressed as follows:

or ##EQU9## When the above function f.sub.5 (.phi.) is graphically represented with K equal to 0.00617, it takes a form such as shown in FIG. 5. In this connection, the constant K' is so selected that f.sub.5 (.phi.) becomes equal to 1 when .phi. is zero.

A function generator 6 which can produce a function approximating the function shown in FIG. 5 is connected in series with the hereinbefore described function generator 1 at the reference signal input terminal thereof as is shown in FIG. 7.

The function generator 6 is externally applied with the angular signal .phi. in a digital form at the input terminal 8 and additionally supplied with a reference signal Es at another input terminal 7.

The function generator 6 thus produces at the output thereof a product of a function approximating the formula (11) and the reference signal Es. When the output product is supplied to the function generator 1, the latter will produce at the function outputs 3 and 4 the signals which approximate the following functions (12) and (13) multiplied by Es. Namely, ##EQU10##

If the above functions are represented in a vector diagram with f.sub.3 (.phi.) taken along the X-axis and f.sub.4 (.phi.) along the Y-axis, the length of the vector is determined in the following manner: ##EQU11## This means that the vector length is constant.

On the other hand, the angle .theta. can be determined as follows: ##EQU12## It will thus be understood that the result is the same as the hereinbefore mentioned formula (8) and errors can be represented in the same form as the one shown in FIG. 4.

In this way, the vector length can be made constant by connecting the vector length correcting function generator 6 to the reference input of the function generator 1.

Now, a circuit arrangement of the function generator 6 will be described. The function to be generated by the generator 6 must be the one given by the formula (11), which takes a form shown in FIG. 5 when graphically represented after numerical computation with K = 0.00617. K' is so selected that f.sub.5 (.phi.) is equal to 1 when .phi. = 0. As can be seen from FIG. 5, the function f.sub.5 (.phi.) is symmetrical along the abscissa about the point corresponding to 45.degree., which means that the circuit capable of generating the function in the range of 0.degree. to 45.degree. can also easily generate the function in the range beyond the point corresponding to 45.degree.. For the production of such a function, one generally resorts to a principle of linear segment approximation. However, in accordance with the present invention, a method is provided in which, when the angle .phi. applied in digital form is varied linearly, the function generator 6 produces an output, in analog form, having a continuous, curved characteristic.

The function shown in FIG. 5 takes values of 1 at 0.degree. and 90.degree. and the values greater than 1 between 0.degree. and 90.degree., exclusive. Accordingly, the function f.sub.5 (.phi.) may be expressed as follows:

The function f.sub.6 (.phi.) will then take a profile shown in FIG. 6 which corresponds to the form of f.sub.5 (.phi.) subtracted by 1 therefrom.

In the range from 0.degree. to 45.degree., the function shown in FIG. 6 can be approximated by the following function: ##EQU13##

On the other hand, in the range from 45.degree. to 90.degree., the function f.sub.6 (.phi.) can be approximated by the function ##EQU14## which is symmetrical to the function f.sub.7 (.phi.) about the point of 45.degree.. In the formulas (17) and (18), A and B represent constants.

Accordingly, in the range from 0.degree. to 45.degree., ##EQU15## while in the range of 45.degree. to 90.degree., ##EQU16##

The formulas (19) and (20) do not perfectly coincide with the formula (11). However, if a certain degree of error is tolerated, the former approximates the latter with a reasonable accuracy.

After the correction by the function f.sub.5, the vector length of vectors defined by the functions f.sub.1 (.phi.) and f.sub.2 (.phi.) may be given by the following expressions: ##EQU17## when 0.degree..THETA..phi. < 45.degree., and ##EQU18## when 45.degree. .ltoreq. .phi. < 90.degree. .

The vector length Rh thus takes constantly the value substantially equal to 1 even if the angular signal .phi. is varied.

When errors of the functions expressed by the formulas (21) and (22) relative to 1 are calculated on the assumption that A = 0.0314, B = 0.00897, K = 0.00617 and K' = 0.01728, they may be graphically represented in a form such as shown in FIG. 8, from which it can be seen that the error is always smaller than 0.005.

Stated alternatively, it is possible to suppress the variation in the vector length less than 0.5 %, when the correction is made by the functions of the formulas (19) and (20) generated through the function generator 6. It will be noted that the error of 0.5 % is reasonably tolerable, although it depends on the practical applications.

Referring to FIG. 7, when the functions given by the formulas (5) and (6) are generated from the function generator 1, while the functions of the formulas (19) and (20 ) are produced by the function generator 6, there will appear at the function output terminals 3 and 4 the signals which can be expressed as follows: