Data processing: measuring – calibrating – or testing – Calibration or correction system – Position measurement
Reexamination Certificate
2000-01-27
2002-06-18
Bui, Bryan (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Position measurement
C702S163000, C700S254000
Reexamination Certificate
active
06408252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a robot calibration system with a linear displacement measuring device.
2. Description of the Related Art
There are many known calibration systems for improving the positional accuracy of an industrial robot which are based upon a kinematic model of the robot. The movement of a single robot is controlled by algorithms executed within the processor for the robot (the robot's “controller”). These algorithms are based upon a mathematical model of the robot's geometry based on ideal, nominal parameters (ie. length of each link, twist angles between links, etc). However, the actual parameters (also known as “as built” parameters) of an individual industrial robot differ from the nominal ones due mainly to tolerances applied to each component in both the machining of components, sub-assembly of components, and final assembly of an industrial robot. Consequently, each individual robot of the same production model generally possesses a set of actual/“as built” parameters.
Therefore, a loss of “absolute robot positional accuracy” results, for example, when programming an individual industrial robot “off-line” (ie. programming by indicating Cartesian coordinates for a desired robot position rather tan driving the robot to that desired position), due to use of the nominal parameters of the robot (instead of the “as built” parameters) by the robot controller: the robot does not actually achieve the commanded Cartesian coordinates of position in space desired by the robot operator/programmer.
The process of identifying the set of actual/“as built” parameters associated with an individual industrial robot is often referred to as “robot calibration”. There are a number of different known methods which then use these actual/“as built” parameters, versus the nominal ones, to modify the positions in a prouction robot program to improve the robot's positional accuracy.
More specifically, due to the improved positional accuracy, robot calibration techniques permit the following operations to be performed without modification of robot positions by the robot operator/programmer (a process referred to as “touch up”): (1) programming of the robot “off-line” using a PC or workstation-based simulation software product; (2) restoring production robot programs following a collision between a robot and another entity (after regarding-calibration of the robot); and (3) transferring robot programs from one robot to another (ie. compensating for “as built” parameters of each robot which have been identified in the process of calibrating each robot).
The prior art systems for the calibration of a robot generally accomplish their functions by means of executing calibration robot programs on the robot controller which instruct the robot to move through a series of positions in its operational space while being monitored by a measurement device which is capable of determining the three dimensional location (ie. x, y, z location in a particular Cartesian coordinate system) of a point (often referred to as the Tool Center Point or TCP) of the end effector of the subject robot. In some cases, the measurement system provides more or less degrees-of-freedom but typically such measurement systems report measurement data in some type of “Cartesian” (or linear) format (e.g. 2-dimensional, 3-dimensional, or 6-dimensional).
Among all prior art systems, the purpose of the calibration procedure is to collect information concerning deviation between the actual (as identified by the measurement system) robot position achieved at each position in the calibration robot program and the corresponding commanded robot positions and then use that information to “deduce” or calculate the actual/“as built” parameters (ie. the differences between the “actual” robot and the “nominal” robot parameters). Typically the prior art systems used 3-dimensional or 6-dimensional measurement systems that include, for example, theodolites, laser interferometers, and camera/photogranmmetry systems.
In one known prior art system (the RoboTrack System distributed by Robot Simulations Ltd.) three measurement cables are secured to the end of the robot arm. The other end of each of the measurement cables is connected to a linear displacement measurement device which measures the extension and retraction of the cable due to the movement of the end of the robot arm. The linear displacement measurement devices are positioned at various known locations around the robot's operational envelope. Once the measurement cables have been connected to the robot, the displacement devices at each robot position measure the distances between the position achieved by the robot arm and the displacement devices. Using triangulation and other mathematical algorithms, the 3-dimensional position (x, y, z in a single Cartesian coordinate position) of the end of the robot arm and the end effector can be determined based upon the linear displacement data which is gathered from each of the measurement devices. This prior art system has numerous problems and in fact is generally only found in non-commercial facilities. Furthermore, this prior art system depends upon the accuracy of the 3-dimensional positional information, which means by nature of the triangulation process that the positional information “degrades” in several portions of the robot's operational envelope (particularly at the boundaries of such operational envelope). Therefore, in some instances, in fact, the absolute positional accuracy of the robot was not improved but rather worse than before the calibration procedure was performed with this prior art system.
Moreover, in addition to the restrictions upon overall accuracy of this prior art system attributable to triangulation and use of this “derived” 3-dimensional data, the linear displacement measurement devices themselves restricted measurement accuracy due to inherent design flaws. For example, each measurement cable of this prior art system exits the housing of the linear displacement measurement device at various angles/attitudes through a hole. By definition, as the cable can simply not bend at a “sharp” angle, the “rounding” of the cable when making contact with the edge of the exit hole contributed to error in the measurement data. Furthermore, this prior art system does not contain a design element to defeat overlap of the measurement cable as it retracts into the housing. This design issue concerning overlap contributes significantly to overall system error as the length of the cable extended is calculated based upon the assumed known and constant radius of the drum upon which the measurement cable retracts.
Other prior art systems have tried to overcome the overlap issue by employing a groove on the drum to force the cable to wind sequentially on the drum. However, as this groove method requires “spacing” on the drum surface, the groove method naturally restricts the amount of measurement cable which can be held by each linear displacement device, and consequently restricts the amount of the robot's operational envelope in which measurements can be recorded. Finally, the groove method does not prevent cases in which the cable “jumps” out of one groove and rests on top of another portion of the drum at unpredictable intervals.
One prior art system avoids any cable issue entirely and the cable itself by employing a radial-distance linear transducer referred to in the art as (an LVDT or “ball-bar”) instead, that is often referred to as the telescopic ball-bar system. The ball-bar mechanism of this prior art system has a magnetic chuck permanently mounted at one end, and a removable high precision steel ball mounted at the opposite end. Extension bars permit the nominal length of the ball-bar to be increased in order to reach more of the robot's operational envelope, but these extension bars add significant weight (and corresponding force) at the measurement point and therefore degrade the accuracy of the measurement data recorded wit
Bui Bryan
Dynalog, Inc.
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
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