Electricity: motive power systems – Positional servo systems – Adaptive or optimizing systems including 'bang-bang' servos
Reexamination Certificate
2000-09-06
2001-08-28
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Adaptive or optimizing systems including 'bang-bang' servos
C318S568100, C318S610000
Reexamination Certificate
active
06281650
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the general field of controlling machine tools. More particularly, the present invention relates to methods of tuning one or more axis velocity control loops. In one embodiment, this is achieved by direct measurement and analysis of frequency response functions obtained by means of random noise excitation of each of such axes of motion, and under various machine operating conditions, constructing a piecewise matching of frequency response functions so obtained into a composite frequency response function for tuning.
BACKGROUND OF THE INVENTION
The modem manufacturing facility is making increasing use of computers, microprocessors and other machine-based “intelligence” for the purpose of producing articles of uniform high quality. Herein, the machine processing an article will be referred to as the “processing machine” and the article undergoing such processing will be referred to as the “workpiece.” The portion of the processing machine physically interacting with the workpiece will be denoted as the “tool” understanding thereby that the tool may be a mechanical cutting, drilling or shaping implement, laser, waterjet or any other device, implement, or form of energy interacting with the workpiece and causing changes therein.
The computer control of the processing machine may take the form of a microprocessor or computer embedded within the particular processing machine and dedicated to the control of that single machine. Computer control may also take the form of a detached mini-, midi-, personal, or mainframe computer, physically separate from the processing machine but electronically connected (wired or wireless) thereto and controlling the machine's processing of the workpiece. A computer may thus control one or more processing machines and may be physically remote from the processing machine, perhaps by many miles utilizing intranet, internet or other modem data transmission technology.
For the purposes of illustration, the present invention will be described in terms of the common case in which the controlling computer is embedded within the processing machine and dedicated to the control of just that machine. However, the present invention is not so limited and numerous particular embodiments of computer control are included within its scope, as would be obvious to those having ordinary skills in the art.
Computer-Numerical-Control “CNC” is the designation for the common machining technique whereby the workpiece and the tool are positioned, oriented and moved through space according to trajectories determined by instructions stored in a computer or microprocessor. In full generality, three variables determine the position of the tool relative to the workpiece in space at any given instant of time while three more variables determine the instantaneous orientation of the tool relative to the workpiece. Thus, the fill control of a workpiece and tool would require the specification of 6 variables as functions of time, determining thereby the trajectories of workpiece and tool and the orientations thereof as each traverses its designated trajectory. Control variables in addition to location and orientation may be required to specify precisely the processing of the workpiece. For example, in laser processing additional commands may be issued by the controlling computer to determine beam power, beam on or off, continuous or pulsed. In practice, CNC machines would control several types of motion and orientation of the workpiece and tool but would typically lack the full flexibility to control every parameter in the processing of the workpiece. Although CNC or computer control of a processing machine, is referenced herein, it is not intended thereby to limit or to exclude any particular form of computer control of orientation, location, trajectory or other processing parameters.
The use herein of “tuning” of the control system or control loop refers to the adjustment of parameters in the control loop to achieve the desired performance of the processing machine. The systems for controlling the workpiece and/or tool during processing must meet several criteria to be effective and robust, producing high quality results in practical production environments. One important criterion for the control system is that it be stable in response to disturbances which may be introduced into the production process in an uncontrolled, random and unpredictable manner. Such disturbances may result from random fluctuations in the torque generated by motors driving the machine, torque ripple, variations in materials properties of the workpiece, random noise in the electronic or mechanical systems, vibrations external to the machine, and many other possible disturbances well known in the art. An unstable control system will not cause the machining process to return to its desired state when such a disturbance is encountered. An unstable system will run as far as the machine allows in a particular direction, oscillate with increasing amplitude or oscillate with a non-decreasing amplitude in response to such a disturbance. Thus, absence of instability, or stability, is one condition necessary for practical operation of a control system.
Improperly tuned machines may affect the quality of the workpiece in one or more undesirable ways. Unstable systems are clearly unacceptable in that any slight disturbance to the processing of the workpiece will typically lead to large processing errors. However, the control system must also be tuned to have adequate margins of stability in order to achieve good quality processing under all reasonable working conditions. That is, the control system must not only be stable but also remain stable when characteristics of the machine change over time during operation. Therefore, it is necessary that the control system remain stable as machine components wear, frictional forces may change over time and/or inertia of various machine components may change.
In addition to being stable with adequate margins of stability, the control system needs to reject disturbances sufficiently strongly such that any perturbations reaching the tool-workpiece interface will be too small, and endure for too short a time period, to affect the quality of the final workpiece. Thus, good stability, margin of stability and disturbance rejection are among the characteristics of a properly functioning machine control system. Achieving these criteria in a practical production environment is one purpose of the present invention.
Several problems may arise for improperly tuned machines even though the control system is stable. For example, variations in the torque output of drive motors, “ripples,” invariably present in practical motors will affect the surface finish of the workpiece if the control system too lightly damps such disturbances Additionally, commands from the typical CNC device for controlling the motion of tool and/or workpiece may overshoot in velocity, position or both, resulting in improperly processed parts. Such commands may result in oscillations and cause imperfections in the workpiece beyond the range of acceptability.
In some cases, improper tuning may even result in audible “squeals” being produced by the machine during processing, resulting in a perception of poor quality and perhaps rapid wearing of the machine components. These are a few of the important criteria to be achieved when a machine control loop is adequately tuned. Other criteria are discussed elsewhere herein.
There are many approaches to control loop tuning. In general, there are two broad classifications control loop tuning. In time domain tuning, the time response of the system to certain inputs is measured. Frequently, an abrupt “step” input is provided and the response of the machine with time then determined. Other inputs may be employed within the scope of time domain tuning.
An alternative but mathematically equivalent procedure for analyzing control systems is to make use of the frequency domain, related to the time domain by the Laplace transform. A lin
Leykin Rita
Nappi Robert E.
Siemens Energy & Automation Inc.
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