SERVO CONTROL CIRCUIT

Abstract: The servo control circuit suppresses an incompletely integrated term contained in the integrated gain in a speed feedback loop of a speed control block (5) at a moment when a deviation between a position signal from a machine system and a movement instruction has exceeded a predetermined reference value. This makes it possible to control the torque instruction to the current control block (6) such that a torque is minimised that develops overhsooting according to static frictional characteristics of the machine system. The operation equivalent to the analog servo system by non-linearly operating the coefficients of a CR time-constant circuit during, electric charge and discharge is realised in the digital servo control system aided by a computer.

Description: This invention relates to a servo-control circuit which may provide a digital servo system by using a microcomputer. In an ordinary digital servo system, the output of a proportional integrating circuit rises in the manner of a linear function when one pulse is inputted as a position command, and the mechanical system begins moving when torque exceeds the static frictional force of the mechanical system. When the integration gain of the proportional integrating circuit is low, overshoot develops in positioning the mechanical system and positioning precision declines as a result. Conversely, when the integration gain is high, overshoot does not occur but the rate of increase in torque takes on the form of a quadratic function and time is required for the mechanical system to actually begin moving once the position command has been applied, or one pulse may not be enough to produce movement. Thus, it is necessary to select how the integrating gain in the digital servo system should be set depending upon the mechanical system of the object under control. However, finding an optimum value by variably setting a coefficient in an appropriate manner, as in an analog servo system, is not easy. Accordingly, when it is attempted to set an optimum value for the mechanical system on each occasion to meet the mutually contradictory demands for positioning accuracy and servo response in a software servo system for controlling the conventional digital servo system, a problem that arises is increased cost of system design. The present invention has been devised to solve the foregoing problem and its object is to provide a servo-control circuit which, in controlling the position of a mechanical system, maintains the precision of one-pulse feed without diminishing response. JP-A-60-5306 discloses a servo-controlling device wherein amplifier gains are switched when the positional error is larger than a predetermined value. Such a servo-controlling device is in accordance with the precharacterising part of the attached claim 1. EP-A-0 207 166 discloses the conventional structure of a proportional integrator, but does not disclose that an incomplete integration term according to the attached claim 1 is suppressed when the positional error exceeds a set reference value. According to the present invention there is provided a servo-control circuit for driving a mechanical system and having position and velocity feedback loops, the circuit comprising arithmetic means for computing an error between position feedback from the mechanical system and a move command, and a velocity control block having a gain which is arranged to be altered in dependence upon said error, characterised in that said gain is expressed by the expression k₁/(S + k₃) + k₂, wherein k₁ represents the integration gain of the velocity feedback loop, k₂ represents the proportional gain of the velocity control block, and k₃ is an incomplete integration term which is suppressed when said error exceeds a set reference value, thereby to achieve the alteration in said velocity control block gain. The servo-control circuit of the invention is adapted to suppress an incomplete integration term, which is contained in the integration gain of the velocity feedback loop, at the moment the error exceeds a predetermined reference value. Therefore, overshoot in the mechanical system due to an excessively large torque command can be prevented by setting the incomplete integration term contained in the integration gain at an optimum value conforming to the mechanical system. Brief Description of the Drawings Fig. 1 is a block diagram illustrating an embodiment of the invention, Fig. 2(a) is a view illustrating the same embodiment in the form of a circuit arrangement corresponding to the case of an analog servo system, and Fig. 2(b) is an explanatory view illustrating the state of an optimum torque command in the same embodiment. An embodiment of the present invention will now be described in detail with reference to the drawings. Fig. 1 is a system block diagram for realizing control with a digital servo system. Numeral 1 denotes a mechanical system which is the object of control. Here an input/output relationship is stipulated by a transfer function which includes a coefficient typified by motor inertia J m . A move command inputted to this servo system is applied to an adder circuit 2 together with a backlash correction signal or a pitch error correction signal. The adder circuit computes an error E p between the command and a position signal fed back from a linear scale, an inductsyn or a pulse coder. An arithmetic block 3 including a position gain K p computes a velocity command from the error E p and inputs the velocity command to an adder circuit 4, which computes an error E v between the command and a velocity signal fed back from a pulse coder. Numeral 5 denotes a velocity control block constructed from a transfer function in which the incomplete integration term k₃ thereof is varied depending upon whether the error E p is zero or not. The output of this velocity control block 5 decides, via a current control block 6, a command current applied to a servomotor which drives the mechanical system 1. A characterizing feature of the present invention is that the incomplete integration term k₃ included in the integrating gain of the velocity feedback loop is controlled in the velocity control block 5 to prevent mechanical system overshoot caused by stick slip or the like. Control of the incomplete integration term k₃ by the velocity control block 5 will now be described with reference to the analog servo system and signal waveforms shown in Fig. 2. In Fig. 2(a), a velocity error signal applied to a post-amplifier 12 from a preamplifier 11 to which a velocity command is inputted is supplied via a resistor Rx proportional to the integration gain of a command Tcmd shown in (b) of Fig. 2. A series circuit composed of two resistors R₁, R₂ is connected in parallel with the amplifier 12. The resistor R₁ corresponds to the proportional gain k₂ of the velocity control block 5 of Fig. 1, and the resistor R₂ corresponds to the incomplete integration gain k₃. A capacitance C₁ connected in parallel with the resistor R₂ corresponds to the integration gain k₁ of the velocity control block 5. When one pulse of a move command is inputted at a time t₁, as shown in Fig. 2(b), the torque command Tcmd rises along a straight line proportional to time if the value of resistor R₂ is made infinite. When the actual movement of position is detected at a time t₂ at which the torque command attains a predetermined value in dependence upon the static frictional characteristic of the mechanical system, the incomplete integration gain k₃ corresponding to the resistor R2 is set to a predetermined value. More specifically, if the resistor R₂ is made small before the start of movement of the mechanical system, positional accuracy deteriorates. In this case, in other words, torque gradually increases along a curve indicated by the dashed line in Fig. 2(b), so that the static frictional torque is not overcome even when one pulse of the move command is applied. As a result, it appears as if backlash is present even when there is linear scale feedback. Conversely, if the resistor R₂ is left infinitely large even after the mechanical system starts moving (at time t₂), excessive overshoot develops in the mechanical system after the start of movement. Accordingly, if the moment at which the error between the position signal from the mechanical system and the movement command exceeds the preset reference value is grasped based on, e.g., a change in the error E p inputted to the arithmetic block 3 having a position compensation function, and if the incomplete integration term included in the integration gain of the velocity feedback loop is controlled, the torque command curve becomes as shown by the solid line in Fig. 2(b) and overshoot caused by excessive torque will not occur. In a case where the mechanical system is composed of a drive mechanism that does not include a gravity axis, it will be necessary to consider a fixed error value to the torque command value. Essentially, however, it will suffice to control the incomplete integration term in accordance with the position error E p such that the reference value of the position error is set to zero. The servo-control circuit of the present invention suppresses an incomplete integration term, which is contained in the integration gain of a velocity feedback loop, at the moment an error between a position signal from a mechanical system and a move command exceeds a set reference value. Therefore, overshoot in the mechanical system due to an excessively large torque command can be prevented by setting the incomplete integration term contained in the integration gain at an optimum value conforming to the mechanical system.

Claims: A servo-control circuit (2 to 6) for driving a mechanical system (1) and having position and velocity feedback loops, the circuit (2 to 6) comprising arithmetic means (2) for computing an error (E p ) between position feedback from the mechanical system (1) and a move command, and a velocity control block (5) having a gain which is arranged to be altered in dependence upon said error (E p ), characterised in that said gain is expressed by the expression k₁/(S + k₃) + k₂, wherein k₁ represents the integration gain of the velocity feedback loop, k₂ represents the proportional gain of the velocity control block (5), and k₃ is an incomplete integration term which is suppressed when said error (E p ) exceeds a set reference value, thereby to achieve the alteration in said velocity control block gain. A servo-control circuit according to claim 1, characterised in that the reference value of said error (Ep) is set to zero in a case where the mechanical system is composed of a drive system that does not include a gravity axis.

Inventors: MATSUBARA SHUNSUKE, SAKAMOTO KEIJI, MATSUBARA, SHUNSUKE, SAKAMOTO, KEIJI
Applicants: FANUC LTD, FANUC LTD.
Application year: 1988
Main classification: G05B19
IPC codes: G05B11, G05D3, G05B13, G05B19