Double Ball-Bar Measuring System Thermal Error Compensation
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Solution Overview
Problem
Double ball-bars in measurement apparatuses face geometric and thermal errors due to misalignment, deflection, and temperature changes, which affect the accuracy of measurements in hexapod machines, especially when used in uncontrolled ambient temperatures.
Innovation Solution
A double ball-bar measuring system with a calibration unit and at least two double ball-bars, including a measuring double ball-bar and a reference double ball-bar, where the calibration unit provides multiple reference center distances and supporting members to establish a center distance function, allowing for the compensation of geometric and thermal errors through displacement and thermal error measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a double ball-bar is used to measure center distance, then measurement capability is provided, but geometric errors (misalignment, deflection) and thermal errors cause measurement precision to deteriorate
Solution Approach 1:
The patent introduces a reference double ball-bar as an intermediary element that measures thermal errors independently. This reference ball-bar is positioned away from the measurement area but experiences the same thermal environment, allowing its thermal expansion/contraction to be measured and used to compensate for thermal errors in the actual measurement, thereby isolating the measurement system from thermal interference
Solution Approach 2:
The system implements feedback by continuously monitoring thermal errors through the reference double ball-bar and using this information to compensate for errors in real-time. The thermal error measurements from the reference ball-bar feed back into the measurement calculation process, allowing dynamic correction of thermal effects on the measured center distance
Solution Approach 3:
The patent changes the measurement parameter from directly measuring center distance to measuring the difference between the reference ball-bar length change and the actual ball-bar length change. By measuring relative changes rather than absolute values, and by establishing a relationship between reference ball-bar displacement and actual ball-bar center distance, the system compensates for thermal expansion effects
2Measurement precision
If the readhead of optical or magnetic linear encoder is used, then displacement measurement is enabled, but the readhead generates heat causing thermal error in the double ball-bar
Solution Approach 1:
The reference double ball-bar serves as an intermediary that captures the thermal effects generated by the readhead and encoder system. Since the reference ball-bar is in the same thermal environment but not subjected to measurement loads, it accurately reflects the thermal expansion caused by the readhead, allowing this thermal interference to be measured and compensated
Solution Approach 2:
The patent converts the harmful thermal effect generated by the readhead into a useful measurement signal. By using the reference double ball-bar to measure the thermal expansion caused by the readhead, the system transforms the harmful heat generation into beneficial thermal error data that can be used to compensate for errors in the actual measurement
3Adaptability or versatility
If double ball-bar measures in uncontrolled ambient temperature, then adaptability to workshop environment is improved, but thermal expansion or contraction causes thermal errors
Solution Approach 1:
The system uses feedback to continuously monitor and compensate for thermal errors in uncontrolled environments. The reference double ball-bar provides real-time thermal error data that feeds back into the measurement calculation, allowing the system to maintain accuracy despite ambient temperature variations without requiring controlled environmental conditions
Solution Approach 2:
The patent changes the measurement approach from absolute center distance measurement to differential measurement, where the measured value is the difference between the reference ball-bar displacement and the actual ball-bar displacement. This parameter transformation makes the measurement immune to uniform thermal expansion effects, enabling accurate measurements in uncontrolled temperature environments
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves highly accurate measurements by compensating for geometric and thermal errors, ensuring precise data processing and maintaining measurement accuracy even in varying ambient temperatures.
Implementation Method 1
The displacement sensor for a double ball-bar includes linear variable differential transformer (LVDT)
Implementation Method 2
The displacement sensor for a double ball-bar includes laser interferometer
Implementation Method 3
The displacement sensor for a double ball-bar includes linear magnetic encoder
Implementation Method 4
The displacement sensor for a double ball-bar includes linear optical encoder
Implementation Method 5
A change in ambient temperature will cause the double ball-bar to expand or contract between the two ball and socket joints, which results in thermal errors in the measured center distance
Implementation Method 6
the readhead of an optical linear encoder or a magnetic linear encoder is also a heat source and may cause thermal error in the double ball-bar. Although the power of the electrical circuit in the readhead is low, the generated heat may raise the temperature of the readhead and the neighboring elements notably as their mass is small
Data Source
AI summary
A double ball-bar measuring system includes a calibration unit, at least two double ball-bars and a measuring module. Among the double ball-bars, at least one is a measuring double ball-bar and at least one is a reference double ball-bar. The measuring double ball-bar is installed on a measurement apparatus to measure or calibrate a target machine. The reference double ball-bar is disposed on the calibration unit to measure thermal errors. When the target machine is driven for measurement or calibration, the geometric and thermal errors of the measuring double ball-bar are compensated so that the measurement apparatus can achieve highly accurate measurement.