Circuit arrangement for motor speed control
This is a continuation of application Ser. No. 08/445,982, filed May 22, 1995 now abandoned, which is a continuation of application Ser. No. 08/323,190, filed Oct. 14, 1994 now abandoned, which is a continuation of application Ser. No. 07/963,649, filed Oct. 20, 1992 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a circuit arrangement for controlling the speed of rotation of a motor specifically a capstan motor for a magnetic tape recorder. 
     BACKGROUND OF THE INVENTION 
     In a magnetic tape recorder a capstan servo control controls the transport speed of a magnetic tape. The magnetic tape then engages with the shaft of a capstan motor. The speed of the shaft is measured by counting clock pulses produced between two successive speed pulses. A speed error is obtained by the subtraction of a set value from the actually counted number of revolutions. Such a servo arrangement is known from DE-PS-28 33 981. 
     The speed pulses are customarily produced by optically scanning a tacho disc which is rigidly connected to the shaft of the capstan motor. For achieving sufficient control accuracy, a line graduation applied to the tacho disc should have a very fine structure for obtaining a maximum number of speed pulses per revolution of the tacho disc. However, the number of equidistant lines may not be arbitrarily increased for reasons of typography, because errors in the line graduation also detrimentally affect the accuracy of control. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the; present invention to improve the accuracy of control in a circuit arrangement of the type mentioned in the opening paragraph. 
     This object is achieved by producing a series of data words whose arithmetical mean constitutes a nominal value. The series of data words is compared with an actual value that can be tapped from the output of a counter. This comparison produces a setting signal which controls the speed of rotation of the motor. Because the data words are incremented or decremented by finite whole values, it leaves intervals between the finite whole values which cannot be used for motor control. Applicants use the series of data words to create an arithmetical mean which can fall within these intervals, thereby providing more accurate control of motor speed. 
     The circuit arrangement according to the invention is of advantage in that adjacent-valued digital values which are separated by a finite value interval (&#34;0&#34;, &#34;1&#34;) as a result of quantization, present a fine graduation when used in connection with a servo circuit, so that the accuracy with respect to time is enhanced without changing the existing line graduation of a tacho disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the invention is represented in the drawing and further explained in the following description, wherein: 
     FIG. 1 shows a block diagram in accordance with the invention, 
     FIG. 2 shows the block diagram of a further embodiment for determining actual values in accordance with the invention, and 
     FIG. 3 gives a representation of the series of data words which drive a nominal value. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference character 1 in FIG. 1 denotes a magnetic tape which is pressed against a capstan shaft 3 by a pressure roller. When the capstan shaft 3 revolves in the direction of an arrow 4 the magnetic tape 1 is moved in the direction of an arrow 5. The capstan shaft 3 is driven by a capstan motor 6. For determining the speed of rotation of the capstan motor 6, the capstan shaft 3 comprises, rigidly attached thereto, a tacho disc 7 which in the present exemplary embodiment has a specific line graduation along its circumference. 
     It is assumed that n lines are applied equidistantly along the circumference of the tacho disc 7, so that when the tacho disc 7 is optically scanned by a scanning element 9 there will be n speed pulses during each revolution of the tacho disc 7. A speed pulse signal tapped from the scanning element 9 is applied to the reset input RST of a counter 10 as well as the read/write input R/W of a memory 11. A high-frequency clock signal is present at a clock input CLK (terminal 12) of the counter 10. The pulse repetition frequency of this clock signal is much higher than the pulse repetition frequency of the speed pulse signal, provided that there is a normal recording operation. When a clock signal is used having a pulse repetition frequency of 9 MHz and the speed pulse signal has a pulse repetition frequency of 3 kHz, the counter 10 will produce the value of 3000 (digital actual value). If, due to a change of the speed of rotation of the tacho disc 7, the pulse repetition frequency of the speed pulse signal changes, the result can be tapped from the output of the counter 10, and thus the digital actual value, will change in accordance with the ratio of the pulse repetition frequency of the clock signal to that of the speed pulse signal. 
     The obtained digital actual value is buffered in the memory 11; at the same time the counter 10 is reset and the counting operation is recommenced. The digital actual value available at the output of the memory 11 is applied to an ACTUAL input of a NOMINAL/ACTUAL value comparator 13. In response to a nominal/actual value comparison of available digital values, the NOMINAL/ACTUAL value comparator 13 produces a digital setting quantity which is subsequently subjected to digital-to-analog conversion for deriving an analog setting signal therefrom. 
     At a NOMINAL input of the digital NOMINAL/ACTUAL value comparator 13 a sequential series of data words (for a digital nominal value) is available, whose arithmetical mean also contains intermediate values of adjacent data words which, as a result of their quantization, cannot be available, in principle, as separate data words. In order to clarify these circumstances, reference is made to the representation of FIG. 3. Line a of FIG. 3 represents the pulse variation of a speed pulse signal tapped from the scanning element 9. T denotes therein the duration of one speed pulse period and 4T the duration of 4 speed pulse periods. It is assumed that the sequential series of data words representing a digital nominal value comprises four speed pulse periods and that in accordance with line b of FIG. 3 the following digital nominal values: 3000, 3000, 3000 and 3000 are applied to the NOMINAL input of the digital phase comparator 13 in a word-alternating fashion. The arithmetical mean of this sequential series of data words is the digital nominal value 3000.00. If one of the nominal values in the sequential series of dam words of the duration 4T adopts the value 3001, according to line c of FIG. 3, the digital nominal value will adopt the value 3000.25. Accordingly, in accordance with line d of FIG. 3, if two data words of the value 3001 and two data words of the value 3000 are available, the digital nominal value will be 3000.50 and, in accordance with line e of FIG. 3, the digital nominal value will be 3000.75 if three data words of the value 3001 and one data word of the value 3000 are available. 
     Each series of data words is produced in a digital combining circuit 14 which is driven by a control unit 15. The control unit 15 in combination with the reference signal 16 provides the arithmetic means to be produced by the digital combining circuit using a series of data words. The digital combining circuit 14 comprises, for example, an arrangement of counters for determining the duration of the series of data words and the value of the data words which are to be produced each time. A digital nominal value is produced by the control unit 15, sent to the digital combining circuit which creates a series of data words. Control unit 15 is synchronized by a reference signal available at a terminal 16 which is connected to the digital combining circuit 14 by way of an m-bit wide data bus, where m is greater than the number of bits of the NOMINAL input of the NOMINAL/ACTUAL value comparator 13. In the control unit 15 the 9 MHz clock signal is produced, and the reference signal is available at terminal 16. 
     The analog setting signal which can be tapped from the output of the NOMINAL/ACTUAL value comparator 13 is applied, via a low-pass filter 17, to a motor output stage 18 and amplified there. A setting signal which can be tapped from the output of the motor output stage 18 controls the speed of rotation of capstan motor 6. 
     FIG. 2 shows the block diagram of a further embodiment for determining the digital actual value to be used by the digital combining circuit 15. In FIG. 2 the components that correspond to those shown in FIG. 1 are denoted by like reference characters. This further embodiment comprises a second counter 19, arranged in parallel with the counter 10, which is also supplied with the clock signal available at the terminal 12 and the speed pulse signal produced by the scanning element 9. The speed pulse signal is also applied to an inverting input of the counter so that, under the control of the speed pulse signal, the two counters alternately count clocks of the clock signal available at terminal 12 and, alternately apply the detected digital actual values to the digital phase comparator 13. 
     In the present exemplary embodiment it was assumed that a sequential series of data words which improves comprised four data words: the accuracy with which the capstan servo control can be set by a factor of four. For better setting accuracy, the number of data words may be increased. However, in this case it should be considered that the control time constant can be adjusted accordingly. 
     In a capstan servo control the fielding speed of the magnetic tape 1 depends, in essence, on the diameter of the capstan shaft 3 as well as the speed of rotation of the capstan shaft 3. Since the oblique tracks recorded on magnetic tape nowadays are not more than a few micrometers wide (about 20 μm), strict requirements are to be made on the dimensional stability of the capstan shaft 3. Tolerances in point of product engineering of a capstan shaft and the entailing track deviations may be corrected by applying a specific sequential series of data words to a NOMINAL input of the NOMINAL/ACTUAL value comparator 13. The sequential series of data words to be used for this correction may be determined, for example, during an initialization phase by measuring the feeding speed of the magnetic tape 1, while the control pulses recorded on a control track of the magnetic tape 1 can be utilized. 
     The invention has been explained, for example, with the aid of a capstan servo control. Needless to observe that this principle may also be applied with the same degree of success to other digital control circuits.