Positioning Control Apparatus

1. Field of the Invention

The present invention relates to a positioning control apparatus, and more particularly, to an apparatus for performing positioning at high speed with high accuracy.

2. Description of Related Art

FIG. 1 is a block diagram illustrating a positioning control apparatus with a switching manner between speed control and positioning control which has generally been adopted as a head-positioning control apparatus for use in a magnetic disc apparatus. FIG. 1 is a typical example of the positioning control apparatus operating at high speed with high accuracy. The apparatus is described in such references as .circle.1 R. K. Oswald, Design of a Disk Head-Positioning Servo, IBM Journal of Research and Development, vol. 18, No. 6, November 1974, .circle.2 R. K. Oswald, Head Positioning Servo Design For The IBM 3344/3350 Disk Files, IEEE TRANSACTIONS ON MAGNETICS, vol. MAG-14, No. 4, July 1978, .circle.3 Takanami et al., Peripheral Circuits for 3.2 GByte Multi-Device Disk Storage, Transactions On Research and Utility By Nippon Telephone and Telegraph Company, vol. 31, No. 1, 1982.

In the description described hereinafter, a command will be assumed to be referred to as a command signal. In FIG. 1, reference numeral 1 designates an object to be controlled, concretely, a motor and an access head integrated with the motor. A speed signal 4 (a speed of the access head) of the object 1 is detected by a speed detector 3. The speed signal 4, then, is converted to a position signal 6 by an integrator 5, and a position error 7 between the position signal 6 and a position command 2 is obtained. A speed error 10, which is the difference between the speed signal 4 and a speed command 9 obtained with respect to the position error 7 from a speed reference curve 8 (a speed-position curve when the object 1 being accelerated at the maximum acceleration and decelerated at uniform deceleration), is inputted to a compensator 12 through a switch 11. In the compensator 12, frequency of the inputted speed error 10 is amplified and attenuated, and a control input 13 is outputted by the compensator 12 to the object 1. A position signal 15 (a position of the access head) of the object 1 is detected by a position detector 14, and a position error 16 between the position signal 15 and the position command 2 is inputted to the compensator 12 through the switch 11. In the compensator 12, frequency of the inputted position error 16 is amplified and attenuated, and the control input 13 is outputted by the compensator 12 to the object 1.

A positioning control apparatus of the prior art comprises both a speed control system for obtaining output response at high speed and a position control system for performing positioning with high accuracy. Those control systems arranged in parallel to each other can be switched over between the speed control system and the position control system by the switch 11, whereby positioning control is carried out at high speed with high accuracy.

Operations of the both control systems will now be described below. In the speed control system, the speed signal 4 is controlled so as to be in accord with the speed command 9 by such a process as that feedback compensation is applied through the compensator 12 to the speed error 10 between the speed command 9 and the speed signal 4 detected from the object 1 by means of the speed detector 3, and that the control input 13 is generated therein.

In order that a high-speed control is performed, however, there exists a problem of saturation of the control input 13 to the object 1. In order to obtain high-speed response by a control performing only the feedback compensation, it is necessary to momentarily provide an excessive control input. Then, in order to carry out high-speed control within the limits of control input, the apparatus of the prior art has employed a quadratic curve called a speed reference curve as shown in FIG. 2 so as to obtain the speed command 9. When the object is accelerated, electric current to be limited is brought into saturation by such as a driving amplifier, and when the object is decelerated, approximately two thirds of the electric current in comparison to that in acceleration is consumed so as to uniformly decelerate the object. This speed reference curve is established from the relation between a speed and a moving distance when the uniform accelerated motion is performed as will be described below. According to the relation between the speed v and the time t when the object is accelerated at uniform acceleration a, there is established such an equation as:

In addition, according to the relation between the moving distance s and the time t when the object is accelerated at uniform acceleration a, there is established such an equation as follows:

Since the moving distance s to be required is an error between a target position x.sub.T and a present position x, if the time t is eliminated by such equations (1) and (2) as described above, the speed reference curve is established to be such an equation as follows:

In the position control system, there is performed such control as to make the position signal 15 accord with the position command 2 by such a process as that feedback compensation is applied through the compensator 12 to the position error 16 between the position command 2 and the position signal 15 detected from the object 1 by means of the position detector 14, and that the control input 13 is thereby generated.

In the positioning control apparatus of the prior art, in order to realize high-speed positioning with high accuracy with as little control input as possible, a control for permitting high-speed movement and a control for carrying out high-accurate positioning have been performed by different systems. Both control systems can be switched over by the switch 11, however, it is necessary to decide both the switching conditions and the characteristics of both control systems so that the transient response simulated in accordance with the initial position and the initial speed at the time for a switch over can be allowed. In FIG. 2, x.sub.e designates a switching position.

As may be seen from the above description, in the positioning control apparatus of the prior art, in order to perform positioning control at high speed with high accuracy, control for high-speed movement has been realized in the speed control system, and so has been done control for high-accurate positioning in the position control system, respectively, whereby control is performed by switching over both control systems. This apparatus, however, has three problems as follows:

.circle.1 Since basically two control systems are switched over, transient response will be brought about unless the speed when those systems are switched over is made completely to be null, as a result, settling time will be longer.

.circle.2 Since the switching point is decided by simulating the transient response with trial and error, the control systems can not readily be designed.

.circle.3 The manner of switching over the systems makes those control systems complicated as well as a lot of circuits required therein result in more cost.

The control system of the apparatus of the present invention has both a feedback compensatory system and a feed-forward compensatory system. The feedback compensatory system comprises a first compensator and a position detector for detecting a position of an object to be controlled, and the feedforward compensatory system comprises a second compensator, a filtering means having a dynamic characteristic reverse to that of a system which consists of the second compensator and the feedback compensatory system, and a filtering means with a dynamic characteristic equal to that of the object. An input command signal to the control system is assumed to be a desired control signal with respect to the object.

The control system may also be adapted so that one portion of the feedforward compensatory system is eliminated, a signal after the input command signal is passed through the eliminated portion is stored in a memory, and when control is performed, the stored data in the memory is inputted to the other non-eliminated portion of the feedforward compensatory system.

The input command signal is selected so that an input signal to the second compensator and an input signal to the feedback compensatory system can be a continuous function of time.

It is an object of the invention to provide a positioning control apparatus capable of simultaneously performing both a high-speed response and a high-accurate positioning through an input is small.

It is another object of the invention to provide a positioning control apparatus wherein the high-speed response and the high-accurate positioning can more readily be realized by providing such a memory as described above.

It is a further object of the invention to provide a positioning control apparatus wherein positioning without transient response can be performed with a simple control system by selecting the input command signal as described above.

The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.

FIG. 1 is a block diagram of the positioning control apparatus of the prior art,

FIG. 2 is a view showing a speed reference curve employed in the positioning control apparatus of the prior art,

FIGS. 3 and 4 are block diagrams showing one embodiment of the positioning control apparatus of the present invention,

FIGS. 5a and 5b are charts illustrating a wave form of the input command signal and a differential wave form thereof,

FIG. 6 is a chart illustrating a wave form of the input command signal and a differential wave form thereof both in one embodiment of the present invention, and

FIG. 7 is a block diagram showing another embodiment of the positioning control apparatus of the present invention.

FIG. 3 illustrates one embodiment of the present invention, wherein reference numeral 1 designates an object to be controlled, which is an actuator (a motor and an access head integrated with the motor) employed in such as a magnetic disc driving apparatus. The object 1 is connected to a position detector 14 for detecting a position of the object 1, from the position detector 14 a position signal 15 being outputted to the object 1. The object 1 is also connected to a compensator 17, from which a control input 13 is sent to the object 1. A feedback compensatory system A.sub.1 comprises the object 1, the position detector 14, and the compensator 17. The following means are being connected in series to the feedback compensatory system A.sub.1 from a compensator 18 for smoothing response of the control system, a filter 20 having a dynamic characteristic reverse to that of a system where the feedback compensatory system A.sub.1 and the compensator 18 are connected, and to a filter 19 with a dynamic characteristic equal to that of the object 1. An input command 21 is inputted to the filter 19, from which a positioning command 22 is outputted to the filter 20, from which a command signal 23 of the control system is outputted to the compensator 18. A differential signal between the position signal 15 and a control signal 24 outputted from the compensator 18 is inputted to the compensator 17. A feedforward compensatory system A.sub.2 comprises the filter 19, the filter 20, and the compensator 18.

In FIG. 3, reference characters in each block designate dynamic characteristics of each block, and character s designates a Laplacean and so does character K a gain of the position detector 14.

Now a closed-loop characteristic W.sub.2 and a characteristic W.sub.1 of the control system both being shown in FIG. 3 will be defined, respectively as such equations (4) and (5) as follows: ##EQU1##

The control system of the apparatus of the present invention has such feedforward compensatory system A.sub.2 and feedback compensatory system A.sub.1 both being described above in order to perform, positioning at high speed with high accuracy. In this control system, high speed is maintained in the feedforward compensatory system A.sub.2, and so is high accuracy in the feedback compensatory system A.sub.1. That is to say, high-speed response of the control system is realized with both feedforward compensatory system A.sub.2 and the input command 21, and high accuracy of the control system is realized with the feedback compensatory system A.sub.1, respectively. Since the feedforward compensatory system A.sub.2 and the feedback compensatory system A.sub.1 are connected in series to each other, the control system has no need of being switched, by which operation the transient response is affected, then positioning at high speed with high accuracy can be realized by one control system alone. Furthermore, the control system of the apparatus of the invention can have each individual design for the high speed response and for the high-accurate positioning.

The input command 21 is selected so that the control input 13 can be small, that the position signal 15 can have a high speed output response and that the command signal 23 of the control system can be a continuous function.

Because the filter 20 has the dynamic characteristic reverse to the characteristic W.sub.1 (s) of the control system, the transfer characteristic from the position command 22 to the position signal 15 becomes 1, and the position signal 15 is in accord with the position command 22. If the object 1 is in accord with the filter 19, the value of the input command 21 becomes the same with that of the control input 13. As a result, in order to satisfy the limitation of the control input 13, there may be produced a wave form with both the upper and the lower limits as the input command 21.

A practical example of the design will now be described below.

Assuming that a moment of inertia of the object 1 which is an actuator is normalized to be 1, the dynamic characteristic P(s) of the object 1 is designated by such an equation (6) as will be described below. In addition, gain K of the position detector 14 is also assumed to be 1.

The following equation, for example, will be selected for C.sub.2 (s).

According to the above equation (4) and such equations (6) and (7) as described above, the closed-loop characteristic W.sub.2 (s) will be designated by the following equation.

Now, with consideration of only making the closed-loop characteristic stable, .alpha., .beta. and .gamma. are decided.

Assuming that C.sub.1 (s)=1 is made, such an equation as W.sub.1 (s)=W.sub.2 (s) will be established by the above equation (5). In other words, the reverse dynamic characteristic: 1/W.sub.1 (s) of the control system will be shown by the following equation:

The command signal 23 of the control system is calculated by adding the equation: 1/W.sub.1 (s) to the position command 22. The signal is easily differentiated, but it is difficult to perform such integration as 1/(s+.beta.). Then, in the embodiment of the present invention, C.sub.1 (s) is decided to be such an equation (11) as will be described below so that the reverse dynamic characteristic: 1/W.sub.1 (s) can be shown by an equation (10).

Such concrete transfer functions of each block as described above will all together be illustrated in FIG. 4.

The input command 21 has a waveform with the upper and the lower limits as described before, therefore, high-speed response can be shown in the position command 22 with both the maximum acceleration and deceleration of the object 1. Such a waveform as shown in FIG. 5(a) may be proposed for the input command 21. There being considered that the transfer function in the filter 19 is a double-integral, the command signal 23 can be obtained by adding all of a double-integral, single-integral, null-differential and single-differential of the input command 21 together. Assuming that the input command 21 is such a waveform as shown in FIG. 5(a), the single-differential waveform thereof is the one as shown in FIG. 5(b) having a non-continuity point. The non-continuity point causes transient action on the response waveform of the position signal 15, as a result, the settling time thereof becomes longer.

Then, in the embodiment of the present invention, there is employed a waveform (FIG. 6(a)) as the input command 21, so that the single-differential waveform (FIG. 6(b)) can be continuous in time.

When the input command 21 is produced, it is necessary to give thought to the following points. One is that the input command 21 must be produced so that the upper and the lower limits of the waveform thereof can be reduced after consideration of smoothness (continuity of n-time differential) thereof. The other is that the waveform of the input command 21 is produced so that the steady state value of the position command 22 can be the moving distance necessary at present.

As may be seen from the above equation (10), an exact differential is necessary in order to realize the characteristic of the filter 20, however, there exists a disadvantage that an imperfect differential is essentially necessary in order to realize the characteristic by an operational-amplifier circuit, and there is another disadvantage that noise with a high frequency is amplified. FIG. 7 is a block diagram of another embodiment of the present invention, wherein those elements numbered identically with the embodiment of FIGS. 1-2 perform the same or similar functions. In the embodiment of the present invention, a system with a dynamic characteristic reverse to that of the control system is realized as not hardware but software. It may be adapted that the command signal 23 of the control system is calculated in advance by means of the following equation (12) and, then, the obtained numeric data is stored in the memory 25. Then, when the control is carried out, the numeric data may be adapted to be read out from the memory 25 as the command signal 23 of the control system.

In the above equation (12), Yr(s) designates the command signal 23 of the control system, and ur(s) designates the input command 21.

In the embodiment of FIG. 7 of the present invention, there is arranged the control system wherein the two blocks of both filters 19 and 20 are removed from the feedforward compensatory system A.sub.2, however, there may also be arranged the control system wherein one portion in the block is removed.

Furthermore, in the embodiment of the present invention, an actuator is employed for the controlled object 1 and the degree of the compensator is to be a limited value, however, it goes without saying that each arrangement is not limited to as described above.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within the metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.