Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2000-03-07
2001-04-17
Ip, Paul (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S565000, C318S568220
Reexamination Certificate
active
06218801
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method for supervision of the movement control of a manipulator. The manipulator comprises a movable part which may be transferred relative to a stationary part or another movable part, at least one movement axis for positioning of the movable part, at least one motor which drives the axis and hence the movable part, a servo for control of the axis in accordance with reference values supplied, and a position sensor adapted to produce an output signal which defines the current position of the axis.
BACKGROUND ART
By manipulator is meant an industrial robot or external axes connected to the industrial robot, for example for orientation and movement of a work object or for movement of the robot itself. A manipulator comprises one or more arms which are movable in relation to one another, and a handling member which is provided with an attachment and which is movable relative to the arm which supports it. The handling member may, for example, be a single platform or a robot hand which is movable in one or more degrees of freedom. The manipulator is provided with a control system which controls the position and orientation of the handling member. For each one of the movement axes of the manipulator, servo equipment with a drive motor is provided. The servo system of each axis is supplied with a reference value for the angle of rotation of the axis and the drive motor of the axis brings the manipulator to move in the axis in question until the axis position corresponds to the reference value supplied to the servo system. By an axis are meant axis transmissions which may give rise to both rotation and translation of the movable arms and the handling member of the manipulator.
When using industrial robots with associated external axes, it is of great important to supervise the movement control such that no unwanted movements are imparted to any axes in the system or that no axes are subjected to abnormal disturbance torques. A plurality of different faults may arise and, independently of what fault has arisen, the fault must be detected rapidly and safely and the movement control must give the robot and external axes such movements that the consequences of the fault are minimized. The faults may arise in robots, external axes or in the process equipment which is used in the robot installation. Examples of components which may give sudden fault situations are cables, drive devices, motors, sensors, measurement electronics and control electronics.
Faults may also arise in the process which is coordinated with the movement of the robot, for example a spot welding gun or a gripper may get stuck in the wrong position. In addition, faults may arise because the operator makes a mistake, for example jogs the robot against an obstacle, starts an external axis too late, or forgets to set an i/o output which is to control a door through which the robot is to pass. Unless these fault situations are detected and acted on quickly, there is a considerable risk of injury to persons and damage to equipment. The robot and/or external axes may cause heavy collisions or equipment which has become stuck may be torn up. These fault situations may give rise to incorrect position, incorrect speed, incorrect acceleration or incorrect torque of one or more of the axes included in the system.
For supervision of the movement of the robot, there are currently normally used existing signals of the servo, and a fault situation is defined by any of these signals having passed an alarm limit. To avoid a false alarm, filtering of the signals is sometimes required, and in addition to the fact that the signals pass an alarm limit, it may also be required that the signal is above/below the alarm limit for a predetermined period of time. There are also solutions whereby the alarm limit depends on some signal of the servo, for example the measured axis speed. As filters there are normally used low-pass filters to avoid a false alarm because of brief “harmless” signal transients. A special type of filters which also occurs are so-called observer filters. U.S. Pat. No. 5,304,906, for example, describes an observer filter which is used for obtaining a signal which is to be used for supervising disturbance torques on the axis coupled to the servo. Disturbance torques are torques which normally do not influence the axes of the robot and may, for example, be generated by too high a tool load, tools which get stuck, or by collisions. These disturbance torques are normally nor measurable but must be reconstructed to be able to be detected. An observer is actually a feedback filter, which is constructed in the time plane with the aid of a dynamic model of the system in which the filter is to be used. This type of filter is especially interesting for dynamically reconstructing non-measurable signals. In the case of supervision, a reconstructed signal may be used as the signal to be supervised.
One disadvantage of observer filters, however, is that they do not become optimal for a general supervision purpose and that they only fulfil a function when a signal, which may be reconstructed, is to be supervised. Thus, an observer may be designed for supervision of unjustified disturbance torques on the axis, but it cannot be directly designed to detect, for example, an incorrect position, incorrect speed, and incorrect acceleration. It cannot, for example, with sufficient selectivity detect whether an unexpectedly high or low velocity is imparted to an axis, as is the case with a runaway situation. In addition, an observer is relatively slow and is difficult to get stable since mechanical resonances disturb the control, which is the case with elastic robots. By an elastic robot is meant a robot with a low natural frequency. An additional problem is to compensate for the dynamic influence of the mechanics on interference signals and control signals.
SUMMARY OF THE INVENTION
The object of the invention is to achieve a method for supervision of the movement control of a manipulator by means of which incorrect position, incorrect speed, incorrect acceleration, incorrect load, incorrect axis torque and incorrect disturbance torques may be detected.
What characterizes a method according to the invention will become clear from the appended claims.
The method has the following advantages:
a very high sensitivity,
a very fast detection,
little probability of false alarms,
detects all fault situations which may arise,
may also be used in elastic axes, that is, axes which are resilient.
The above-mentioned advantages are achieved by forming an alarm condition which comprises an alarm value and an alarm limit. Based on the model, the alarm value takes into consideration both stiff-body properties and elasticity properties of the manipulator. The influence of the elasticities on how normally situations and faults dynamically influence the controlled manipulator axes is modelled in the alarm condition with signal filters, which have essentially the same transfer functions as critical transfer functions between various quantities, for example torque and axis movement, in the manipulator. To obtain an optimum alarm condition, this is derived in accordance with known stiff-body models and elasticity models for the manipulator, starting from a signal combination which gives an output signal directly dependent on the type of fault to be supervised.
The alarm condition has the property of being very sensitive to incorrect disturbance torques, loads, accelerations, etc., while at the same time it is not released for normal operating cases. This is due to the fact that the alarm condition is based on signal combinations which directly describe the faults which are to be detected and that the alarm condition is model-based both with respect to a dynamic stiff-body model and a dynamic elasticity model of the manipulator.
In one embodiment of the invention, the alarm limit may consist of a constant. In a preferred embodiment of the invention, a model-based alarm limit is used instead, which, with knowledge of the modell
Brogardh Torgny
Elfving Staffan
Jonsson Ingvar
Moberg Stig
Skantze Fredrik
ABB AB
Connolly Bove & Lodge & Hutz LLP
Ip Paul
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