Positioning Control System

1. Field of the Invention

This invention relates to a positioning control system for accurate positioning of numerical-controlled machines.

2. Description of the Prior Art

A positioning control system heretofore employed for numercial-controlled machines (for example, as shown in FIG. 1) which employs a closed loop type servomechanism. With this conventional system, as shown in FIG. 1 velocity information detected by a tachogenerator or a like velocity detector T1, and positional information detected by a resolver or a like position detector R1, both detectors being mounted on a motor M, are fed back to the input side, wherein they are applied to adders AD1 and AD2, respectively. In the adder AD1, a comparison is made between commanded positional information from a control unit CONT and the detected positional information from the position detector R1, and the resulting difference signal is provided as a velocity command to the adder AD2 via an amplifier AMP1. In the adder AD2, the velocity command and the detected velocity information from the velocity detector T1 are compared, and the motor M is driven via a velocity amplifier AMP2 in such a manner as to reduce the difference between these quantities to zero, by which a feed screw V is driven to position a movable machine part MC.

In the conventional system described above, in which the velocity feedback is derived from the motor M, it is impossible to correct vibration of the mechanical system which is produced by its various elements at the end of the positioning operation; therefore, accurate positioning may in some cases be impossible. With the use of an arrangement in which the velocity feedback is derived directly from the mechanical system, the vibration can be suppressed. In general, however, when the mechanical system is included in the velocity feedback loop, various characteristics of the mechanical system exert an adverse influence on the velocity control loop; namely, even in a steady state in which no velocity difference exists between the motor M and the movable machine part MC, that is, when no vibration occurs, the characteristics of the mechanical system are fed back to make the velocity control loop unstable.

In the past, an attempt has been made to prevent vibration of the mechanical system by employment of a positioning control system such as depicted in FIG. 2. This system includes another feedback loop, for the positional information of the mechanical system, in the arrangement of FIG. 1. With this system, the position of the machine is detected by a position detector R2 mounted on the movable machine part MC, the commanded positional information is applied to a deviation set circuit SM to detect the expected rotational angle of the output shaft of the motor M, and then the two items of information are compared by adder AD3 to thereby detect an error of the whole mechanical system. The error signal thus detected is applied to a pulse oscillator OCL to yield correction pulses corresponding to the error. The correction pulses are provided to an adder AD4, wherein they are added to the commanded positional information from the control unit CONT, thereby correcting the commanded rotational angle of the motor M to eliminate the error. Since the error signal derived from the adder AD3 also includes an error resulting from vibration of the mechanical system, it is possible theoretically to suppress the vibration of the mechanical system; nevertheless, this system is intended, in general, to correct position errors, such as backlash error, pitch error and so forth. The gain of this system having the position detector R2 is limited by the natural frequency of the mechanical system, so that in the case of the natural frequency being low, the gain for a very small displacement is reduced; thus the response to a very small displacement (such as mechanical vibration or the like) is essentially retarded, resulting in a failure to sufficiently suppress the vibration. Moreover, the use of the deviation set circuit is disadvantageous economically.

It is an object of this invention to provide a positioning control system for numerical-controlled machines which prevents vibration of a mechanical system, to provide for enhanced accuracy in positioning.

Another object of this invention is to provide a positioning control system for numerical-controlled machines in which vibration of a mechanical system at the moment of completion of the positioning operation is detected and a motor is controlled in a manner to suppress the vibration, thereby to provide enhanced accuracy in positioning. Briefly stated, the present invention comprises a servo system for feeding back detected positional information from a position detector and detected velocity information from a velocity detector for detecting the velocity of a motor, and means for detecting the velocity of a movable machine part. The difference between the velocity information of the movable machine part detected by the detecting means and the velocity information of the motor is fed back to control the motor.

FIGS. 1 and 2 are block diagrams showing conventional positioning systems; and

FIGS. 3, 4 and 5 are block diagrams respectively illustrating embodiments of this invention.

In FIG. 3, there is illustrated in block form an embodiment of this invention as applied to an ordinary positioning control system of the type where the position and the velocity feedback are obtained from the motor M. Reference character T2 indicates a velocity detector; AD5 and AD6 designate adders; and AMP3 identifies an amplifier. The parts corresponding to those in FIG. 1 are marked with the same reference characters.

Commanded positional information from the control unit CONT is compared by the adder AD1 with detected positional information from the position detector R1, and the resulting amplified difference signal (interpreted, as a velocity command) and the detected velocity information from the velocity detector T1 are compared, by adder AD2. If is simply driven via the velocity amplifier AMP2 so that the difference detected by AD2 is reduced to zero, such an operation would be carried out in exactly the same manner as in the prior art.

The velocity detector T2 is composed of, for example, a combination of a tachogenerator and a rack and pinion, a combination of an Inductosyn or a like linear position detector and a frequency-voltage converter, or the like, and is mounted directly on the side of a numerical-controlled machine. The velocity detector T2 detects the velocity of the movable machine part MC, and the detected velocity information is applied to the adder AD5, where it is subtracted from the detected velocity information from velocity detector T1. The difference signal is amplified by the amplifier AMP3 and provided to the adder AD6 for subtraction from the output signal from the adder AD2. The velocity amplifier AMP2 drives the motor M to position the movable machine part MC.

At the moment of completion of the positioning operation, the detected positional information from the position detector R1 and the commanded positional information from the control unit CONT become equal to each other and the output signal from the adder AD1 is reduced to zero, thus tending to stop the motor M from rotating. Accordingly, the detected velocity information from the velocity detector T1 coupled with the motor M should become zero. At this time, if a vibration occurs in the movable machine part MC, then detected velocity information corresponding to the vibration is outputted from the velocity detector T2, and in the adder AD5, the detected velocity information from the velocity detectors T1 and T2 are compared, and the resulting difference signal is provided to the adder AD6. Consequently, by the output from the velocity amplifier AMP2, the motor M is driven, and in this case, the motor M is controlled so that the difference signal from the adder AD5 is reduced to zero, thus suppressing the vibration of the movable machine part MC. By suppressing the vibration of the mechanical system in this way, the positioning operation is rapidly completed, ensuring accurate positioning. This embodiment involves addition of the velocity detector T2, the amplifier AMP3 and the adders AD5 and AD6, and hence is not complicated in construction.

As will be appreciated from the arrangement shown in FIG. 3, in the present system, only when there is a difference in velocity between the movable machine part MC and the motor M is the difference fed back to produce the effect of suppressing the velocity difference. At a steady velocity, when no velocity difference is present between the movable machine part MC and the motor M, only ordinary velocity feedback is (through adder AD2) is effective so that there is no possibility of various characteristics of the mechanical system exerting adverse influence on the control loop. Moreover, in the case where the natural frequency of the machine is low, the response of the velocity feedback loop can be raised above the limits defined by the natural frequency of the machine; therefore, the gain of the position feedback loop can be selected to be large, and the response to a very small displacement is improved.

FIG. 4 illustrates in block form another embodiment of this invention as applied to a positioning control system in which ordinary position feedback is derived from the position detector R2, such as an Inductosyn or the like, disposed directly on the side of the machine. In FIG. 4, parts corresponding to those in FIG. 3 are identified by the same reference characters. As will be obvious from FIG. 4, the embodiment of FIG. 4 is nearly identical in construction and in operation with the embodiment of FIG. 3, except that the position detector R2 is mounted on the movable machine part MC; therefore, detailed description need not be repeated. In view of the fact that positioning accuracy is generally enhanced by detecting the positional information from the machine side, the present embodiment may be higher in the positioning accuracy than the system of FIG. 3.

FIG. 5 illustrates in block form a further embodiment of this invention, in which the velocity detector T2 used in FIG. 4 is left out, and the positional information from the position detector R2 mounted on the machine side is processed to obtain the velocity of the movable machine part MC.

A position-velocity converter FV receives the detected positional information from the position detector R2 and calculates and outputs the moving velocity of the movable machine part MC at that time. The position-velocity converter FV is composed of a shaping circuit for wave form shaping a detected pulse signal from the position detector R2 into a constant-pulse-width signal, and a low-pass filter for receiving the output signal from the shaping circuit. The velocity information derived from the position-velocity converter FV is applied to the adder AD5, and then the same operations as those in the embodiments of FIGS. 3 and 4 are performed, by which vibration of the mechanical system is suppressed to ensure accurate positioning. Since the position detector R2 is used for detecting both the position and the velocity information, the arrangement of the machine side can be simplified.

As has been described in the foregoing, according to the present invention, the difference between the velocity of the movable machine part and the motor velocity is fed back to the servo system in the positioning control system for a numerical-controlled machine, in addition to direct velocity feedback from the motor and position feedback from the motor or the machine side, and vibration of the mechanical system, which may be produced by various elements of the mechanical system, is minimized; thus, accurate positioning can be performed. Further, in the case of the steady machine velocity, when no velocity difference exists between the movable machine part and the motor, only the ordinary velocity feedback from the motor is effective, avoiding the likelihood of the control loop being adversely affected by the various characteristics of the mechanical system. Accordingly, by applying the system of this invention to the positioning of a numerical-controlled machine, accurate positioning can be achieved with an economical construction.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.