Motor speed control apparatus

Abstract: A motor speed control apparatus is provided which comprises period detecting means (3) which detects a period of the frequency signal (F s ) generated by a frequency generator (2) equipped into a motor (1) to output a mean speed signal (N) of the motor (1), drive means (4) which supplies the motor (1) with a driving current (i a ) in accordance with the control signal (C), and speed estimating means (7) which estimates a rotating speed (n') of the motor (1) from the control signal (C) and the mean speed signal (N) to obtain an estimated speed signal (n') and outputs the estimated speed signal (n') to speed control means as a speed feedback signal, the speed estimating means (7) comparing the estimated speed signal (n') and the mean speed signal (N) to generate a speed error (ΔN) and generating a predicted value (Δn) of the instantaneous speed error from the mean speed error (ΔN) thereby to correct the estimated speed signal (n'). The frequency band of the control system can be improved by using the estimated speed signal as the speed feedback signal. A motor speed control apparatus superior in disturbance control characteristic can be realized through the feed forward compensation of an estimated load torque (τ d ').

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a motor speed control apparatus having a frequency generator (FG). More particularly, it relates to a motor speed control apparatus capable of providing a stabilized rotation control system and high accuracy rotation of a motor even when a frequency which the frequency generator generates is low, that is, the motor is rotation-controlled at a low-speed, or the frequency generator for generating high frequencies cannot be employed from a viewpoint of structural and economical aspects thereof. 2. Description of the Prior Art A trend of down-sizing and light-weight video cassette recorders (hereinafter abbreviated as VCR) or camera incorporated type VCR (hereinafter called VCR Camcoder) is outstanding recently, necessarily leading to down-sizing motors to be used for them. Particularly, referring to capstan motors for running a tape at a constant speed, an outstanding reduction in rotor inertia of a rotor resulting from a progressive down-sizing trend contributes to a extreme degradation in rotation accuracy. Also, in order to realize a long-time recording by decreasing a tape running speed of the VCR, the capstan motor is required to be rotated at a low speed. However, when rotating at the low speed, a period of a frequency signal for detecting a rotating speed of the motor becomes large, resulting in an increase in detection delay of the rotating speed of the motor. As a result, when rotating the motor at the low speed, a control gain cannot be set so high that the speed control system of the motor can be stabilized. Accordingly, there arises a problem that a controllability of these motors may be degraded. The motor speed control apparatus in the prior art will be described below while referring to drawings. Fig. 13 is a block diagram showing a most basic structure of a conventional motor speed control apparatus. In Fig. 13, reference numeral 1 is a motor, 2 is a frequency generator for generating a frequency signal fs proportional to a rotating speed of the motor 1, 3 is a period detector for detecting a period of the frequency signal fs, 5 is a comparator for comparing a speed signal N outputted from the period detector 3 with a speed command signal Nr (constant value) to output a deviation signal ΔN, 6 is a controller for applying control compensations such as proportional and and integral compensations to the deviation signal ΔN to output a control signal C and 4 is a drive circuit for supplying the motor with a driving current in accordance with the control signal C. Next, the operation of the conventional motor speed control apparatus shown in Fig. 13 will be explained below. When the rotating speed of the motor is being controlled almost uniformly, the period of the frequency signal fs in a vicinity of an operation point is inversely proportional to the rotating speed of the motor. As a result, an output of the period detector 3 is made as the speed signal N. Namely, the rotating speed of the motor 1 is sampled by the frequency signal fs of the frequency generator 2, and a sampling result is held during the period thereof. As a result, the speed signal N of the rotating speed through the period detector 3 is not the detected value of an instantaneous rotating speed when sampling, but a mean rotating speed per period obtained by integrating the frequency signal fs over one period with respect to time. Fig. 14 is a waveform diagram showing a relation of the rotating speed of the motor and the rotating speed N outputted from the period detector 3, in which a reference numeral 61 denotes an instantaneous rotating speed n supposed to be changed as shown in the waveform diagram, and 62 denotes the rotating speed N outputted from the period detector 3, which is sampled by the frequency signal fs of the frequency generator 2 to show the mean rotating speed during a period T. The rotating speed N is, as shown in Fig. 14, changed in steps at time points t(i-1), t(i), t(i+1), ..... with the period of the frequency signal fs and holds a constant value for the time period of each step. The reference numeral 63 shown by a dotted line denotes an estimated rotating speed of the motor estimated from the rotating speed N shown at 62. As clear from Fig. 14, the estimated rotating speed is delayed in detection time by about T compared with the rotating speed n of the motor. As a result, when controlling the rotating speed using the conventional motor speed control apparatus as shown above, due to the time delay of detection as shown above, a control frequency band cannot be made higher excessively in order to stabilize a control system (about 1/12 of the sampling frequency in general), so that there arises a problem that the controllability of the rotation control system cannot be improved. In order to solve the above problem, some motor speed control apparatuses capable of improving the controllability have been proposed previously. For example, according to Japanese Laid-Open Patent Application NO. 61-30984, in case of a position control apparatus to be used in robots, NC machines tools and the like, a model expressing drive characteristics of the motor is provided in a control apparatus and a speed estimating unit is provided which is capable of estimating the instantaneous speed n, even during the time period where an output signal of a position detector is not changed, from a general torque τm of the motor and the detected speed value N obtained by operation at every time when the output signal of the position detector is obtained, whereby a motor speed control apparatus to be used in robots and NC machine tools is provided which is capable of reducing a detection time delay of the rotating speed of the motor by using an output of the speed estimating unit as a speed feedback signal. Explanations will be made below on the speed estimating unit as a member of a conventional motor speed control apparatus to be used in robots and NC machine tools as shown above. Fig. 1-5 is a block diagram of a conventional speed estimating unit. In Fig. 15, a reference numeral 21 is an integrator and a coefficient 1/(Jn•S) is a transfer function expressing driving characteristics of the motor, where Jn is a nominal value showing the rotor inertia of the motor and S is a Laplacian, 22 is a proportional integral compensator, where K1 is a proportional gain and K2 is an integral gain, and 23 is an integrator for obtaining a mean value N' during the period T of an output signal n' of the integrator 21 and expressed as follows ; N'= ∫n'dt T A reference numeral 24 is a comparator for comparing an output signal N' of the integrator 23 and the rotating speed N generated by the period detector 3 of the motor and outputting its deviation to a sample & holder 26. The sample & holder 26 holds a deviation output of the comparator 26 only during the period T and then, outputs it to the proportional integral compensator 22 as the deviation output ΔN, 28 is a multiplier for multiplying the control signal C by a coefficient (Ktn•gmn), where Ktn and gmn are nominal values of a torque constant Kt of the motor and a transform coefficient gm of the drive circuit 6, respectively. A reference numeral 25 is a subtracter which compares an estimated values τm' of generated torque of the motor 1 obtained by multiplying the control signal C by the coefficient (Ktm•gmn) by the multiplier 28 and an output τd' of the proportional integral compensator 22 to output a deviation therebetween to the integrator 21. Next, an operation of the conventional speed estimating unit will be explained in detail by referring to Fig. 15. In Fig. 15, the integrator 21 may be considered as a model for expressing the driving characteristics of the motor and related as follows; Jn · dn' dt = τm' - τd' Therefore, by applying a Laplace transform to an equation (2), n' = τm' - τd' Jn · s From an equation (3), the instantaneous rotating speed n of the motor can be estimated, and the estimated rotating speed n' can be obtained by integrating a difference between the estimated value τm' of a generated torque of the motor and the estimated value τd' load torque of the motor divided by the rotor inertia Jn of the motor with respect to time. As a result, in order to obtain the estimated rotating speed n', the load torque τd applied to the motor is required to be estimated, however, with the conventional speed estimating unit as shown in Fig. 15, the estimated τd' of the value is obtained by applying simple proportional integration to the deviation ΔN between the detected mean speed N by the period detector 3 and the estimated speed N' obtained from the equation (1). With the conventional motor speed control apparatus as shown above, the instantaneous rotating speed n' is estimated from the estimated value τd' of the load torque and the estimated value τm' of the generated torque of the motor, and the speed control of the motor 1 is achieved by the rotating speed n' instead of the speed feedback signal N shown in Fig. 13. Namely, by using the instantaneous rotating speed n' estimated by the speed estimating unit, the motor can be speed-controlled by using the speed signal having reduced detection delay as the feedback signal, so that a control gain of a speed control system can be improved compared with an apparatus achieving the speed control using the speed signal N having the detection delay of the period detector 3 as the feedback signal. With the structure as shown above, however, proportional integral compensation gains are required to be made large in order to estimate the load torque speedily and accurately. However, when using a method of estimating the load torque from the deviation between the estimated speed and the detected speed detected through operation at each time when the output signal of the position detector is obtained, it is required to form a model almost equivalent to a actual driving model as a speed estimating model. As a result, when taking the time delay due to rotational speed detection or the time delay due to sample holding into consideration, the proportional integral compensation gains for estimating the load torque cannot be set as sufficiently high as desired. Accordingly, in case that frequencies of load torque disturbance actually applied to the motor are distributed in the low frequency band only, here arises no problem practically, however, in case that the frequencies of load torque disturbance are distributed near or exceeding the control frequency band determined by the proportional integral compensation gains, there arises such a problem that if the speed control is carried out using the estimated rotating speed n', the speed change due to load torque disturbance cannot be satisfactorily reduced. EP-A-514 847 discloses a system and method for controlling a speed of a motor in an extremely low speed range having a rotary pulse encoder which outputs a pulse whenever a rotation axle of the motor has rotated through a predetermined angle, the extremely low speed range being defined such that a pulse interval of the output pulses is longer than a speed control period of the system are disclosed in which a least order disturbance, load torque estimated value observer is provided, the least order disturbance observer including: a first calculating block which calculates an estimated value of the motor speed of a motor model on the basis of a torque command and a load torque estimated value; a second calculating block which calculates an average value of the motor speed at each pulse interval on the basis of the speed control period and the pulse internal of the pulse encoder, a first deviating block which deviates between the average value of the motor speed derived from the second calculating block and an average value of the motor speed derived on the basis of the pulses of the pulse encoder, a second deviating block which deviates between the output value of the first deviating block and the deviated output of the first deviating block to derive the speed estimated value, a proportional gain observer which outputs the estimated load torque value from the deviated output of the first deviating block, and a third deviating block which deviates between the load torque command and the load estimated value from the proportional observer. SUMMARY OF THE INVENTION In consideration of the above-mentioned problems, an object of this invention is to provide a motor speed control apparatus, which makes possible to sufficiently enhance the control frequency band of the control system even if the output frequency of a frequency generator equipped into a motor and to be rotated at a high accuracy even if disturbance of the high frequency component of a load torque is applied to the motor. In order to attain the above object, a motor speed control apparatus of this invention comprises: frequency generating means for generating a frequency signal proportional to a rotating speed of a motor; period detecting means for detecting a period of the frequency signal to output a mean speed signal to proportional to a mean rotating speed of the motor; control means for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; drive means for supplying the motor with a driving current in accordance with the control signal; and speed estimating means for receiving the control signal and the mean speed signal and estimating a rotating speed of the motor from the control signal to obtain an estimated speed signal to be outputted to the control means as the speed feedback signal, wherein the speed estimating means comprises predicting means for comparing the mean speed signal and an estimated mean speed signal obtained by integrating the estimated speed signal with respect to time to generated a mean speed error and generating a predicted value of an instantaneous speed error form the mean speed error, and a feedback loop for correcting the estimated speed signal by the predicted value generated through the predicting means. In addition, a motor speed control apparatus of this invention comprises: frequency generating means for generating a frequency signal proportional to a rotating speed of a motor; period detecting means for detecting a period of the frequency signal to output a mean speed signal proportional to a mean rotating speed of the motor; control means for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; drive means for supplying the motor with a driving current in accordance with the control signal; and speed estimating means for receiving the control signal and the mean speed signal and estimating a rotating speed of the motor from the control signal to obtain an estimated speed signal of the motor to be outputted to the control means as the speed feedback signal, wherein the speed estimating means comprises predicting means for generating a predicted value of an instantaneous speed from the mean speed signal, and a feedback loop for comparing the estimated speed signal and the predicted value generated by the predicting means to generate a speed error and correcting the estimated speed signal by the speed error. Further in addition, a motor speed control apparatus of this invention comprises: frequency generating means a frequency signal proportional to a rotating speed of a motor; period detecting means for detecting a period of the frequency signal to output a mean speed signal proportional to a mean rotating speed of the motor ; control means for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means for correcting the control signal to output a corrected signal; drive means for supplying the motor with a driving current in accordance with the corrected signal; and speed & load torque estimating means for receiving the corrected signal and the mean speed signal, estimating a rotating speed of the motor and a load torque applied to the motor from the corrected signal to obtain an estimated speed signal and an estimated load torque signal, and outputting the estimated speed signal to the control means as the speed feedback signal and the estimated load torque signal to the correcting means, respectively, wherein the speed & load torque estimating means comprises predicting means for comparing the mean speed signal and an estimated mean speed signal obtained by integrating the estimated speed signal with respect to time to generate a mean speed error and generating a predicted value of an instantaneous speed error from the mean speed error, and a feedback loop for correcting the estimated speed signal by the predicted value generated by the predicting means. Still further in addition, a motor speed control apparatus of this invention comprises: frequency generating means for generating a frequency signal proportional to a rotating speed of a motor, period detecting means for detecting a period of the frequency signal to output a mean speed signal proportional to a mean rotating speed of the motor; control means for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means for correcting the control signal to output a corrected signal; drive means for supplying the motor with a driving current in accordance with the corrected signal; and speed & load torque estimating means for receiving the corrected signal and the mean speed signal, estimating a rotating speed of the motor and a load torque applied to the motor from the corrected signal thereby to obtain an estimated speed signal and an estimated load torque signal, and outputting the estimated speed signal to the control means as the speed feedback signal and the estimated load torque signal to the correcting means, respectively, wherein the speed & load torque estimating means comprises predicting means for generating a predicted value of an instantaneous speed from the mean speed signal, and a feedback loop for comparing the estimated speed signal and the predicted value generated by the predicting means to generate a speed error and correcting the estimated speed signal by the speed error. A motor speed control apparatus of the invention comprises: frequency generating means for generating a frequency signal proportional to a rotating speed of a motor; period detecting means for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means for correcting said control signal to output a corrected signal; drive means for supplying said motor with a driving current in accordance with said corrected signal; and speed & load torque estimating means for receiving said control signal and said mean speed signal, estimating a rotating speed of said motor and a load torque applied to said motor thereby to obtain an estimated speed signal and an estimated load torque signal, and outputting said estimated speed signal to said control means as said speed feedback signal and the estimated load torque signal to said correcting means. respectively, characterized in that said speed & load torque estimating means compnses: predicting means for comparing an estimated mean speed signal obtained by integrating said estimated speed signal with respect to time and said mean speed signal to generate a mean speed error and generating a predicted value of an instantaneous speed error from said mean speed error; and means for generating said estimated load torque signal from said predicted value generated by said predicting means. Further in addition, a motor speed control apparatus of this invention comprises: frequency generating means for generating a frequency signal proportional to a rotating speed of a motor; period detecting means for detecting a period of the frequency signal to generate a mean speed signal proportional to a mean rotating speed of the motor; control means for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means for correcting the control signal to output a corrected signal; drive means for supplying the motor with a driving current in accordance with the corrected current; and speed & load torque estimating means for estimating a rotating speed of the motor form the control signal to obtain an estimated speed signal to be outputted to the control means as the speed feedback signal, generating a predicted value of an instantaneous speed of the motor from the mean speed signal, comparing the predicted value and the estimated speed signal to generate a speed error, and generating an estimated load torque signal from the speed error to be outputted to the correcting means. With constitutions as shown above, the motor speed control apparatus of this invention can provide a speed estimating unit having wide frequency band, so that the disturbance of load torque applied to the motor and the rotating speed can be estimated accurately up to higher frequency levels. As a result, the motor speed control apparatus of this invention makes it possible to reduce the speed change due to the disturbance of load torque stably up to higher frequency band levels through the speed control of the estimated rotating speed using the feedback signal. Besides, the speed change due to the disturbance of load torque can be further reduced through the feed forward compensation using the estimated load torque by the speed estimating means. Consequently, according to this invention, such a motor speed control apparatus can be provided that the controllable frequency band of the control system can be sufficiently enhanced with a comparatively simple structure even if the frequency generating means equipped into the motor is low in output frequency and the rotational accuracy can be highly improved even if the disturbance of high frequency components of load torque is applied to the motor. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a speed control apparatus for motors according a first embodiment. Fig. 2 is a block diagram showing an example of a speed estimating unit as a member of the apparatus shown in Fig. 1. Fig. 3 is a waveform diagram for explaining the operation of a predicting circuit concerning to this invention. Fig. 4 is a block diagram of a predicting circuit concerning to this invention. Fig. 5 is a Bode diagram. Fig. 6 is a block diagram showing another example of a speed estimating unit as a member of the apparatus shown in Fig. 1. Fig. 7 is a block diagram of a speed control apparatus for motors according to a second embodiment. Fig. 8 is a entire block diagram for explaining the operation of an example of the apparatus shown in Fig. 7. Fig. 9 is an entire block diagram for explaining the operation of another example of the apparatus shown in Fig. 7. Fig. 10 is a block diagram of a speed control apparatus for motors according to a third embodiment. Fig. 11 is an entire block diagram for explaining the operation of an example of the apparatus shown in Fig. 10. Fig. 12 is an entire block diagram for explaining the operation of another example of the apparatus shown in Fig. 10. Fig. 13 is a block diagram of a speed control apparatus for motors in the prior art. Fig. 14 is a waveform diagram showing a relation between an instantaneous rotating speed n and a mean rotating speed N of a motor. Fig. 15 is a block diagram of a speed estimating unit in the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS A speed control apparatus for motors according to embodiments of this invention will be described below while referring to the drawings. Fig. 1 is a block diagram of a speed control apparatus for motors according to a first embodiment. In Fig. 1, a motor 1 is driven by a driving current ia and generates a driving torque τm to be rotated and the rotation axis is applied with disturbance torque τd, a frequency generator 2 generates a frequency signal proportional to a rotating speed of the motor 1, a period detector 3 detects a period of the frequency signal fs to output a rotating speed signal N, a drive circuit 4 supplies the motor 1 with a driving current ia in accordance with a control signal C, a speed estimating unit 7 obtains a generated torque τm' of the motor 1 in response to the control signal C supplied to the drive circuit 4 and detects an instantaneous rotating speed signal n' from the generated torque τm' and the rotating speed signal N to be outputted to a comparator 5 which compares the instantaneous rotating speed signal Δn' and a rotating speed command signal nr (constant value) to output a deviation signal Δn, and a controller 6 applies the deviation n with control compensations such as proportional and integral compensations and outputs the control signal C. With the structure as shown above, the operation of the speed control apparatus for motors will be explained in detail. In Fig. 1, in case that the rotating speed of the motor 1 is being controlled approximately invariably, since the period of the frequency signal fs is inversely proportional to the rotating speed of the motor 1 in the vicinity of its operation point, the output of the period detector 3 is made the detected rotating speed thereof. Namely, the rotating speed of the motor 1 is sampled by the period of the frequency signal fs of the period generator 2 and the detection result is held only for the period thereof. As a result, the detection result of rotating speed of the motor 1 does not show an instantaneous value of the rotating speed at the time point when sampling, but shows a mean value of the rotating speed for the period of the frequency signal fs, thus having a detection time delay. The speed estimating unit 7 estimates the instantaneous rotating speed during the term where the frequency signal fs outputted from the frequency generator 2 is not varied (where the zero-crossing point of signal is not detected) using the generated torque τm (= kt•ia = kt•gm•c; where kt is a torque constant of the motor 1 and gm is a transform constant of the drive circuit 4), which is generated in accordance with the control signal C outputted from the controller 6, and the detected mean speed value N outputted by the period detector 3. Here, the estimated value of rotating speed can be obtained, as shown in the equation (3), by integrating the deviation between the generated torque τm' of the motor 1 generated in accordance with the control signal C and the estimated load torque τd' applied to the motor 1 divided by the rotor inertia Jn of the motor with respect to time. As a result, the load torque τm is required to be estimated, however, a conventional speed control apparatus estimates the load torque τd by simply proportionally integrating the deviation between the mean speed detection value N of the period detector 3 and the mean value N' of estimated speed n'. Thus, the speed detection delay is reduced by estimating the instantaneous speed n' from thus estimated load torque τd' and the generated torque τm' of the motor 1 in accordance with the control signal C. Furthermore, the comparator 5 and the controller 6 control the rotating speed of the motor 1 with the instantaneous rotating speed n' estimated by the speed estimating unit 7 in response to the speed command signal nr as the feedback signal. Next, the speed estimating unit 7 as a member of the speed control apparatus for motors of this invention will be explained in detail below. Fig. 2 is a block diagram of the speed estimating unit 7 shown in Fig. 1. In addition, the members having the same functions as those of the conventional speed estimating unit shown in Fig. 15 are denoted at the same reference numerals and the explanations thereof will be omitted for avoiding duplication. In Fig. 2, the speed estimating unit 7 shown in Fig. 2 and the conventional speed estimating units shown in Fig. 15 are different in that with the conventional unit shown in Fig. 15, the output of a sample & holder 26 is directly sent to a proportional integral compensator 22, but with the speed estimating unit 7 shown in Fig. 2, the output of the sample & holder 26 is through a predictor 27 to the proportional integrated compensator 22. With the above-mentioned structure, the operation of the speed control apparatus of this invention will, be explained in detail by referring to the drawings. Fig. 3 is an explanatory diagram for the operation of the predictor 27 as a member of the speed estimating unit 7 shown in Fig. 2, in which the relation between the speed deviation ΔN (=N'-N) between the estimated mean rotating speed N' obtained through the speed estimating unit 7 and the mean rotating speed N generated by the period detector 3, and the speed deviation Δn' (=n'-n) between the estimated rotating speed n' (instantaneous value) and the actual rotating speed n (instantaneous value) of the motor is shown on a waveform basis. In Fig. 3, the reference numeral 31 denotes the speed deviation Δn (instantaneous value), which is supposed to be changed as shown in the diagram, 32 is the mean rotating speed deviation ΔN, which is sampled by the frequency signal fs of the frequency generator 2 and obtained during the period T. The mean rotating speed deviation ΔN is, as seen in Fig. 3, changed in steps at time points t(i-1), t(i), t(i+1) ..... with the period T of the frequency signal fs, thus holding it constant during the period T, 33 shown by the dotted line shows a rotating speed of the motor 1 estimated from the mean rotating speed deviation ΔN shown at 32, which has a detection time delay of about T compared with the rotating speed deviation ΔN of the motor as clearly seen in Fig. 3. Accordingly, in Fig. 15 showing the conventional speed estimating unit, if structured such that the mean rotating speed deviation ΔN is sent through the sample & holder 22 to the proportional integral compensator 22 for processing, the control frequency band cannot be so much increased in order to stabilized the control system of a loop A shown in Fig. 15 due to the above-mentioned detection time delay. On the other hand, the speed estimating unit 7 shown in Fig. 2 operationally estimates the instantaneous rotating speed deviation Δn of the motor from the mean rotating speed deviation ΔN and sends the estimated value to the proportional integral compensator 22 for processing, so that the control frequency band can be highly set independently of the detection time delay. In Fig. 3 the following equation (4) may be obtained between the instantaneous rotating speed deviation n and the mean rotating speed deviation ΔN as ; Δn(i)+Δn(i-1) 2 = Δ N(i) Therefore, by changing the above equation (4), the following equation (5) may be found as ; Δn(i)= 2·ΔN(i)-Δn(i-l) Fig. 4 shows the equation (5) by means of the block diagram, showing a block diagram of the predictor 27 as a member of the speed estimating unit 7. At (a) of Fig. 4, 41 is a multiplier whose constant is 2, 42 is a delay circuit to delay only one sampling period T (in this case, the period of the frequency signal fs of the frequency generator 2), and 43 is a subtracter for subtracting the rotating speed deviation Δn(i-1) obtained by the preceding operation of one sampling period T from the mean rotating speed deviation 2•ΔN(i) doubled through the multiplier 41. The operation results of the subtracter 43 shows the rotating speed deviation Δn(i) obtained by the equation (5). Namely, the predictor 27 makes possible to obtain the instantaneous rotating speed deviation Δn of the motor when sampling at the present time point from the mean rotating speed deviation ΔN when sampling at the present time point. Fig. 5 is a Bode diagram showing the transmission characteristics from the mean rotating speed deviation ΔN to the instantaneous rotating speed deviation Δn of the block diagram of the predictor 27 at (a) of Fig. 4. At (a) of Fig. 5, the reference numeral 51 denotes the amplitude characteristic of the black diagram shown at (a) of Fig. 4, and 52 at (b) of Fig. 5 is the phase characteristic of the block diagram shown at (a) of Fig. 4. Besides, the abscissas of Fig. 5 are normalized based on the sampling frequency fs (equal to the frequency of a frequency signal outputted from the frequency generator 2) . As seen from 52 at (b) of Fig. 5, the process of the predictor 27 shown in the block diagram of (a) of Fig. 4 characterizes the phase leading, but as seen from 51 at (a) of Fig. 5, the control system is unstable (the amplitude characteristic becomes infinite at frequency fs/2) and becomes unpractical. As a result, in order to stabilize the control system practically, the Bode diagram shown at (b) of Fig. 4 is used instead of that shown at (b) Fig. 4. In this case, however, the coefficient α at (b) of Fig. 4 is selected so as to be 0 < α < 1. The amplitude characteristic when the coefficient is α made 0.7 in the block diagram shown at (b) of Fig. 4 is shown at 53 at (a) of Fig. 5 and the phase characteristic thereof is shown at 54 at (b) of Fig. 5. By selecting α so as to be smaller than one, the control system of the predictor 27 can be stabilized and the phase leading characteristic can be obtained in the frequency range below fs/2. By including the predictor 27 having such a phase leading characteristic that shown above into the loop A of the speed estimating unit 7, one kind of differential compensation can be applied to the phase delay that the mean rotating speed deviation ΔN itself has as explained by referring to Fig. 3. As a result, it becomes possible to set larger the constants K1 and K2 of the proportional integral compensator 22, thus being capable of expanding the control frequency band of the loop A. As described above, according to this embodiment, the frequency band of the loop A of speed estimating unit can be made larger than that in the prior art and the load torque disturbance applied to the motor can be estimated up to higher frequency levels, so that the speed estimating unit 7 makes it possible to estimate the estimated rotating speed n' accurately up to higher frequency band levels. Consequently, the speed control apparatus for motors of this invention performs the speed control with the estimated rotating speed n' as the feedback signal, so that the speed change due to disturbance of load torque can be stably reduced up to higher frequency band levels. Fig. 6 is a black diagram showing another example of the speed estimating unit 7. In Fig. 6, the members having the same functions as those of the speed estimating unit 7 according to this invention shown in Fig. 2 are denoted at the same reference numerals and their explanations will be omitted or avoiding duplication. This example is different in structure from that shown in Fig. 2 in that in the case of the unit shown in Fig. 2, the integrator 23 is inserted into the loop A, but in the case of the unit shown in Fig. 7, as shown in Fig. 6, there does not use the integrator 23 and the estimated rotating speed n' as an output of the integrator 21 is directely to the comparator 24. On the other hand, the mean rotating speed N generated by the frequency generator 3 is sent through the predictor 27 to the comparator 24 to be applied with the subtraction therethrough and then, sent successively to the sample & holder 26 and the proportional integral compensator 22 for processing. The integrator 21, comparator 24, proportional integral compensator 22, and subtracter 25 from a loop B. With another example structuraled as shown above, the operation of the speed control apparatus of this invention will be explained in detail by referring to the drawings. The operation of the predictor 27 will be explained by referring to Fig. 14 already mentioned above. Fig. 14 is a waveform diagram showing the relation of the mean rotating speed N generated by the frequency generator 3 and the actual rotating speed n (instantaneous value) of the motor, and between them, there may be found the following equation (6) as ; n(i)+n(i-1) 2 = N(i) Therefore, by changing the equation (6), n (i)= 2 · N (i)- n (i-1) Namely, the instantaneous rotating speed n can be predicted from the mean rotating speed N by operation using the equation (7). The predicted rotating speed n" thus obtained and the estimated rotating speed n' are compared by the comparator 24. Besides, the operation of the equation (7) can be performed in the same way as that of the predictor 27 shown in Fig. 2 as well as is realizable by the operation expressed in the block diagram of Fig. 4. As a result, their explanations will be omitted for avoiding duplication. In the case of another example of the speed estimating unit 7 shown in Fig. 6, the integrator 23 generating phase delay is not included into the loop B, and the constants K1 and K2 of the proportional integral compensator 22 can be set large compared with the speed estimating unit 7 shown in Fig. 2. As a result, the loop B shown in Fig. 6 can be further expanded in control frequency band compared with the loop A shown in Fig. 2. According to the example shown in Fig. 6, as explained above, the frequency band of the loop B of the speed estimating unit 7 can be expanded and the load torque disturbance applied to the motor can be estimated up to higher frequency levels, so that the speed estimating unit 7 can estimate the estimated rotating speed n' accurately up to higher frequency levels. In addition, the example shown in Fig. 6 does not need such a integrator 23 that is required in Fig. 2, so that the speed estimating unit 7 can be made simple in structure. Fig. 7 is a block diagram of a speed control apparatus for motors according to a second embodiment. In this case, the members having the same functions as those in Fig. 1 will be denoted at the same reference numerals and their explanations will be omitted for avoiding duplication. The second embodiment shown in Fig. 7 is different in structure from the first embodiment shown in Fig. 1 in that in the case of the first embodiment, the output of the speed estimating unit 7 is only the estimated rotating speed n' and supplies it to the comparator 5, however, in the case of the second embodiment shown in Fig. 7, the estimated load torque τd' obtained by operation through a speed estimating unit 8 is used for performing feed forward compensation with the control signal C outputted from the controller 6. The reference numeral 9 denotes a corrector which receives the control signal C from the controller 6 and the estimated load torque τd' from the speed estimating unit 8, operates the compensation and outputs a corrected signal d to the drive circuit 4. With the structure as shown above, the operation of the speed control apparatus for motors according to the second embodiment will be explained in detail by referring to the drawings. Fig. 8 is an entire block diagram for explaining the operation of the speed control apparatus for motors shown in Fig. 7. In Fig. 8, the reference numeral 1 is a block diagram of the motor and 8 is a block diagram of the speed estimating unit according to the second embodiment, which is different from the speed estimating unit 7 according to the first embodiment in that it outputs both the estimated rotating speed n' and the estimated load torque τd' obtained by internal operation, and the estimated rotating speed n' is sent to the comparator 5 to be used as the feedback signal for speed control. On the other hand, the estimated load torque τd' is sent to the corrector 9 which comprises a multiplier 91 for multiplying a constant 1/(ktn•gmn) to be outputted and an adder 92 for adding the operation result of the multiplier 91 to the control signal C form the controller 6 to be outputted to the drive circuit 4 (transform constant is gm) as the corrected signal. Next, the operation of the speed control apparatus according to the second embodiment will be explained in detail. Also, referring to the speed estimating unit 8, the members having the same functions as those of the speed estimating unit 7 shown in Fig. 2 are denoted at the same reference numerals and their explanations will be omitted for avoiding duplication. In Fig. 8, the corrected signal d can be expressed as follows ; d = c + τd' Ktn · gmn As a result, the generated torque τm of the motor 1 can be expressed by multiplying the transform constant gm of the drive circuit 4 by the torque constant kt of the motor 1, or τm = Kt · gm · d = Kt · gm · c + τd' where, for the sake of simplification, ktn = kt and gmn = gm are supposed. Then, the estimated load torque τd' outputted from the speed estimating unit 8 is equal to the load torque τd applied to the motor 1 in the frequency range not exceeding the control frequency band of the loop A, so that the actual load torque τd and the estimated load torque τd' are canceled each other through the adder 12 as a component of the block diagram 1 of the motor. Namely, by applying the feed forward compensation to the estimated load torque τd' through the corrector 9, the motor 1 behaves as if the disturbance of the load torque τd is not applied to a transfer function 13 (its coefficient is 1/Js). Accordingly, the motor 1 can be driven at a high rotation accuracy without receiving any influence of the load torque disturbance on a practical basis. According to the second embodiment shown in Fig. 7, the frequency band of the loop A of the speed estimating unit 8 can be expanded by providing the predictor 27, and the load torque disturbance τd applied to the motor 1 can be estimated up to higher frequency levels. As a result, the speed estimating unit 8 can estimate the estimated rotating speed n' accurately up to higher frequency levels. Consequently, the speed control apparatus according to the second embodiment of this invention performs speed control using the estimated rotating speed n' obtained by the speed estimating unit 8, so that the speed change due to the load torque disturbance can be stably reduced up to higher frequency band levels. In addition, by applying the feed forward compensation to the estimated load torque τd' through the corrector 9, the load torque τd applied to the motor 1 can be canceled, so that the speed change due to load torque disturbance of the motor can be further reduced. Besides, the speed estimating unit 8 is structured based on that shown in Fig. 2 as clearly from the block diagram shown in Fig. 8. Fig. 9 is a block diagram showing another example of the speed estimating unit 8 of the second embodiment based on the structure shown in Fig. 6. In Fig. 9, the members having the same functions as those shown in Figs. 6 and 8 are denoted at the same reference numerals and their explanations are omitted for avoiding duplication. In the case of Fig. 9, the control frequency band of the loop B of the speed estimating unit 8 can be expanded, so that the same effects as with the example shown in Fig. 8 can be obtained. Besides, in the case of Fig. 9, there does not need the integrator 23 such as to be required in the case of the example shown in Fig. 8, so that the speed estimating unit can be made simpler in structure. As clear from the above descriptions, the speed control apparatus for motors according to the second embodiment of this invention makes it possible to stably reduce the speed change due to the disturbance of load torque up to higher frequency band levels by performing the speed control using the estimated rotating speed n' obtained by the speed estimating unit 8 and to cancel the load torque τd applied to the motor 1 by feedforward-compensating the estimated load torque τd' by the corrector 9, so that the speed change of the motor due to the load torque disturbance can be reduced. As shown above, according to the second embodiment of this invention, such a speed control apparatus for motors that is extremely small in speed change due to the disturbance of load torque can be provided. Fig. 10 is a block diagram of a speed control apparatus for motors according to a third embodiment. In Fig. 10, the members having the same functions as those shown in Figs. 1 and 7 are denoted at the same reference numerals and their explanations are omitted for avoiding duplication. The unit shown in Fig. 10 is different from that shown in Fig. 7 in that in the case of Fig. 7, the speed estimating unit 8 receives the corrected signal d outputted from the corrector 9 as its input, but in the case of Fig. 10, the speed estimating unit 10 receives the control signal c outputted from the speed controller 6 as its input. The estimated load torque τd' estimated by the speed estimating unit 10 is sent to the corrector 9. After correcting through the corrector 9, the corrected signal d thus obtained is sent to the drive circuit 4. The operating of the speed control apparatus for motors according to the third embodiment of this invention structured as shown above will we explained in detail while referring to the drawings. Fig. 11 is an entire block diagram for explaining the speed control apparatus for motors shown in Fig. 10. In Fig. 11, the members having the same functions as those shown in Fig. 8 are denoted at the same reference numerals and their explanations are omitted for avoiding duplication. In Fig. 11, 10 is a block diagram of the speed estimating unit according to the third embodiment, in which the subtracter 25 is not needed. On the other hand, the speed estimating unit 8 according to the second embodiment shown in Fig. 8 needs the subtracter 25 for subtracting the estimated load torque τd' from the estimated generated torque τm' obtained by multiplying the corrected signal d by the constant (Ktn•gmn) of the multiplier 28 thereby to obtain the signal τ' which is sent to the integrator 21. With the structure as shown above, the operation of the speed control apparatus for motors according to the third embodiment will be explained in detail. First, the estimated acceleration torque τ' as the input of the input of the integrator 21 of the speed estimating unit 8 shown in Fig. 8 may be expressed as follows; τ' = τm' - τd' = Ktn · gmn · d - τd' However, the corrected signal d of the corrector 9 may be expressed as follows; d = c + τd' Ktn · gmn Therefore, from the equations (10) and (11), the estimated acceleration torque τ' may be obtained as follows; τ' = Ktn · gmn · c By modifying the structure of the speed estimating unit 8 shown in Fig. 8 based on the equation (12), the structure of the speed estimating unit 10 shown in Fig. 11 maybe obtained. That is, the control signal c of the controller 6 is sent to the multiplier 28 to be multiplied by the constant (Ktn•gmn) thereby to obtain the estimated acceleration torque τ'. The estimated acceleration torque τ' is sent to the integrator 21 to be multiplied by the constant 1/(Jn s) thereby to obtain -the estimated rotating speed n'. On the other hand, the estimated load torque τd' is sent to the corrector 9 and similar to the case of Fig. 8, the estimated load torque τd' outputted from the speed estimating unit 10 is equal to the load torque τd applied to the motor 1, so that the actual load torque τd and the estimated load torque τd' can be canceled each other through the adder 12. As a result, similar to the case of the second embodiment shown in Fig. 8, the motor 1 can be driven at a high rotation accuracy. As explained above, according to this embodiment, the estimated load torque τd' applied to the motor and the estimated rotating speed n' can be estimated up to higher frequency band levels with simpler structure than that shown in Fig. 8. As a result, the speed control apparatus for motors according to the third embodiment of this invention makes possible to stably reduce the speed change due to the disturbance of load torque up to higher frequency band levels by speed-controlling using the estimated rotating speed n' obtained by the speed estimating unit 10 and to cancel the load torque τd applied to the motor 1 by feedforward-compensating the estimated load torque τd' by the corrector 9, so that the speed change due to the load torque disturbance of the motor can be further reduced. In addition, in Fig. 11, by feedforward-compensating the estimated load torque τd' by the corrector 9, the load torque τd applied to the motor 1 can be canceled, so that even if the predictor 27 is not particularly included into the speed estimating unit 10, the speed change due to the load torque disturbance of the motor can be satisfactorily reduced, thus being unproblematic on a practical basis. As clear from the block diagram shown in Fig. 11, the structure of the speed estimating unit 10 is based on that shown in Fig. 2. Fig. 12 is a block diagram showing another example of the speed estimating unit 10 according to the third embodiment, the structure of which is based on the structure of the unit shown in Fig. 6. In Fig. 12, the members having the same functions as those shown in Figs. 6 and 11 are denoted at the same reference numerals and their explanations are omitted for avoiding duplication. With the structure as shown in Fig. 12, the same effects as with the structure as shown in Fig. 11 can be obtained. In addition, in the case of the example shown in Fig. 12, there does not need to use such an integrator 23 that is required for the example shown in Fig. 11, thus the speed estimating unit 10 being able to be simpler in structure. According the third embodiment of this invention as explained above, such a speed control apparatus for motors that the speed change due to the disturbance of load torque is extremely small can be provided. Furthermore, in the first, second and third embodiments of this invention, the speed estimating unit receives the control signal from the controller as its input signal, however, it is needless to say that even if the driving current outputted from the drive circuit is used instead of the control signal, the same effects can be obtained. In addition, the integrator 21 and the proportional integral compensator 22 were explained above so as to be made of analog filters, but not limited thereto, they may be made of digital filters. Further in addition, each of the members forming the control apparatus may be realized by softwares by micro-computers. Also, the controlled output signal of each member in the above embodiments was explained as a continuous signal, but not limited thereto, it may be outputted as a discrete signal, and if the output sampling period is made sufficiently shorter than the period of an output frequency signal of the frequency generator, the same effects as those obtained in the continuous signal case can be obtained.

Claims: A motor speed control apparatus comprising: frequency generating means (2) for generating a frequency signal proportional to a rotating speed of a motor (1); period detecting means (3) for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means (6) for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; drive means (4) for supplying said motor with a driving current in accordance with said control signal; and speed estimating means (7) for receiving said control signal and said mean speed signal and estimating a rotating speed of said motor from said control signal to obtain an estimated speed signal of said motor to be outputted to said control means as said speed feedback signal, characterized in that said speed estimating means (7) comprises: predicting means (27) for comparing an estimated mean speed signal obtained by integrating said estimated speed signal with respect to time and said mean speed signal to generate a mean speed error and generating a predicted value of an instantaneous speed error from said mean speed error; and a feedback loop (A) for correcting said estimated speed signal by said predicted value generated through said predicting means. A motor speed control apparatus comprising: frequency generating means (2) for generating a frequency signal proportional to a rotating speed of a motor (1); period detecting means (3) for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means (6) for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; drive means (4) for supplying said motor with a driving current in accordance with said control signal; and speed estimating means (7) for receiving said control signal and said mean speed signal and estimating a rotating speed of said motor from said control signal to obtain an estimated speed signal of said motor to be outputted to said control means as said speed feedback signal, characterized in that said speed estimating means (7) comprises: predicting means (27) for generating a predicted value of an instantaneous speed from said mean speed signal; and a feedback loop (B) for comparing said predicted value generated by said predicting means and said estimated speed signal to generate a speed error and correcting said estimated speed signal by the speed error. The motor speed control apparatus according to claim 1 or 2, wherein said predicting means (27) comprises: a multiplier (44) for multiplying an input signal by a factor (1+α) (α is a real number, 0<α<1); a delay circuit (45) for multiplying an output of a subtracter (43) by a factor to delay by a time equal to a period of said frequency signal; and said subtracter (43) for subtracting an output of said delay circuit from an output of said multiplier thereby to generate an output signal. The motor speed control apparatus according to any of claims 1 to 3, wherein a period with which said speed estimating means (7) outputs said estimated speed signal is shorter than a period with which said period detecting means (3) outputs said mean speed signal. The motor speed control apparatus according to any of claims 1 to 4, wherein said speed estimating means (7) has the feedback loop (A, B) including a proportional term proportional to the speed error and an integral term having integrated said mean speed error with respect to time. The motor speed control apparatus according to any of claims 1 to 5, wherein a control band of the feedback loop (A, B) of said speed estimating means (7) is larger than a control band of said control means (6). A motor speed control apparatus comprising: frequency generating means (2) for generating a frequency signal proportional to a rotating speed of a motor (1); period detecting means (3) for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means (6) for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means (9) for correcting said control signal to output a corrected signal; drive means (4) for supplying said motor with a driving current in accordance with said corrected signal; and speed & load torque estimating means (8) for receiving said corrected signal and said mean speed signal, estimating a rotating speed of said motor and a load torque applied to said motor thereby to obtain an estimated speed signal and an estimated load torque signal, and outputting said estimated speed signal to said control means as said speed feedback signal and the estimated load torque signal to said collecting means, respectively, characterized in that said speed & load torque estimating means (8) comprises: predicting means (27) for comparing an estimated mean speed signal obtained by integrating said estimated speed signal with respect to time and said mean speed signal to generate a mean speed error and generating a predicted value of an instantaneous speed error from said mean speed error; and a feedback loop (A) for correcting said estimated speed signal by said predicted value generated by said predicting means. A motor speed control apparatus comprising: frequency generating means (2) for generating a frequency signal proportional to a rotating speed of a motor (1); period detecting means (3) for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means (6) for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means (9) for correcting said control signal to output a corrected signal; drive means (4) for supplying said motor with a driving current in accordance with said corrected signal; and speed & load torque estimating means (8) for receiving said corrected signal and said mean speed signal, estimating a rotating speed of said motor and a load torque applied to said motor thereby to obtain an estimated speed signal and an estimated load torque signal, and outputting said estimated speed signal to said control means as said speed feedback signal and the estimated load torque signal to said correcting means, respectively, characterized in that said speed & load torque estimating means (8) comprises: predicting means (27) for generating a predicted value of an instantaneous speed from said mean speed signal; and a feedback loop (B) for comparing said predicted value generated by said predicting means and said estimated speed signal to generate a speed error and correcting said estimated speed signal by said speed error. The motor speed control apparatus according to claim 7 or 8, wherein said speed & load torque estimating means (8) has the feedback loop (A, B ) including a proportional term proportional to the speed error and an integral term having integrated said mean speed error with respect to time. The motor speed control apparatus according to any of claims 7 to 9, wherein a control band of the feedback loop (A, B) of said speed & load torque estimating means (8) is larger than a control band of said control means (6). A motor speed control apparatus comprising: frequency generating means (2) for generating a frequency signal proportional to a rotating speed of a motor (1); period detecting means (3) for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means (6) for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means (9) for correcting said control signal to output a corrected signal; drive means (4) for supplying said motor with a driving current in accordance with said corrected signal; and speed & load torque estimating means (10) for receiving said control signal and said mean speed signal, estimating a rotating speed of said motor and a load torque applied to said motor thereby to obtain an estimated speed signal and an estimated load torque signal, and outputting said estimated speed signal to said control means as said speed feedback signal and the estimated load torque signal to said correcting means, respectively, characterized in that said speed & load torque estimating means (10) comprises: predicting means (27) for comparing an estimated mean speed signal obtained by integrating said estimated speed signal with respect to time and said mean speed signal to generate a mean speed error and generating a predicted value of an instantaneous speed error from said mean speed error; and means (22) for generating said estimated load torque signal from said predicted value generated by said predicting means. A motor speed control apparatus comprising: frequency generating means (2) for generating a frequency signal proportional to a rotating speed of a motor (1); period detecting means (3) for detecting a period of said frequency signal to output a mean speed signal proportional to a mean rotating speed of said motor; control means (6) for receiving an externally given speed command signal and a speed feedback signal and carrying out a control operation thereby to output a control signal; correcting means (9) for correcting said control signal to output a corrected signal; drive means (4) for supplying said motor with a driving current in accordance with said corrected signal; and speed & load torque estimating means ( 10) for receiving said control signal and said mean speed signal, estimating a rotating speed of said motor and a load torque applied to said motor thereby to obtain an estimated speed signal and an estimated load torque signal, and outputting said estimated speed signal to said control means as said speed feedback signal and the estimated load torque signal to said correcting means, respectively, characterized in that said speed & load torque estimating means (10) comprises: predicting means (27) for generating a predicted value of an instantaneous speed from said mean speed signal; means (24) for comparing said predicted value generated by said predicting means and said estimated speed signal to generate a speed error; and means (22) for generating said estimated load torque signal from said speed error. The motor speed control apparatus according to any of claims 7 to 12, wherein said predicting means comprises: a multiplier (44) for multiplying an input signal by a factor (1+α) (α is a real number, 0<α<1); a delay circuit (45) for multiplying an output of a subtracter (43) by a factor to delay by a time equal to a period of said frequency signal; and said subtracter (43) for subtracting an output of said delay circuit from an output of said multiplier thereby to generate an output signal. The motor speed control apparatus according to any of claims 7 to 13, wherein a period with which said speed & load torque estimating means (8, 10) outputs said estimated speed signal is shorter than a period with which said period detecting means (3) outputs said mean speed signal.

Inventors: INAJI TOSHIO, MATSUO KEISUKE, UEDA EIJI, INAJI, TOSHIO, MATSUO, KEISUKE, UEDA, EIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H02P5
IPC codes: G05B17, G11B15, H02P29, H02P23