Measuring arm
A measuring arm is a portable, manually operated measuring device with which the geometry of a component can be measured. In contrast to a coordinate measuring machine, a measuring arm does not have any linear measuring standards, but the position of the probe on the head of the measuring arm is recorded with incremental measuring systems in its joints. Measuring arms can be used directly in the production area and do not need an air-conditioned measuring room. Compared to coordinate measuring machines, however, the achievable accuracy is roughly an order of magnitude lower.
history
Measuring arms were developed in the late 1970s. They were looking for a measuring device with which the geometry of large workpieces could be measured without having to bring the workpiece to the measuring device. The original design was described by inventor Homer Eaton in 1974 in US Patent No. 3,944,798. At the time of the invention, Eaton's measuring arm was intended solely for measuring angles on curved pipelines. In the 1980s, Eaton merged with Romain Graiger, founded the Romer company and developed measuring arms for general industrial applications in production metrology .
Layout and function
Similar to a human arm (with upper arm, forearm and hand), a measuring arm consists of two longer arm parts and the sensor head. Each arm part and the sensor head are connected to one another via two joints . The arms are designed for high rigidity so that the arms do not bend, which would lead to an increased measurement uncertainty . As a rule, they consist of a fiber-reinforced plastic tube . Thanks to the six joints, the sensor head can be rotated to any point in three-dimensional space and the sensor can be rotated in three spatial angles (a total of six degrees of freedom ).
Either a tactile button or an optical sensor such as a laser light section sensor is used as the sensor head .
A measuring arm records many individual points (when using a tactile probe) or large point clouds (when using a light section sensor). From this, geometric elements are first calculated (e.g. circle, plane, cylinder) from which the features to be measured (dimensional tolerances such as the diameter or shape and position tolerances such as flatness or cylindricity) are derived and based Compliance with the tolerance must be checked. For this processing of the measuring points, the same software is generally used that is also used for coordinate measuring machines.
accuracy
Due to the six joints connected in series, the uncertainties in the rotation angle measurement of each encoder are propagated to its nearest neighbor. The situation is different with classic coordinate measuring machines, since the linear incremental encoders are aligned in three orthogonal spatial directions and errors in each linear position measuring system are not transferred to the other axes. For this reason, measuring arms are less accurate than coordinate measuring machines with a similarly large measuring volume. The uncertainty in the probing of a point is usually several tens of micrometers with measuring arms, whereas coordinate measuring machines can achieve an uncertainty of a few micrometers.
operation area
Measuring arms are "distributed over various fields". The automotive industry "represents the largest group of customers with its high quality requirements". According to the "golden rule of precision measurement technology" , the measurement uncertainty should be only a tenth to a fifth of the tolerance. Therefore, measuring arms are only suitable for measuring features with tolerances in the range from a few tenths to several millimeters. Examples for the use of measuring arms can be found in "measurements in the vehicle interior", which "are only possible to a very limited extent with a stationary measuring system". "For example, the entire dashboard, the pedals or mounting brackets in the center of the vehicle and at the rear of the vehicle can be measured." Other examples are
- the measurement of cast parts, rough fits and gap dimensions
- the reverse engineering
Individual evidence
- ↑ Patent US3944798 : Method and apparatus for measuring direction. Applied on April 18, 1974 , published March 16, 1976 , applicant: Eaton-Leonard Corporation, inventor: Homer L. Eaton.
- ↑ a b Hans-Jürgen Ossenbühl: Optical and tactile: the measuring arm in use . In: Peter Ulrich (Hrsg.): Vehicle test: methods and procedures . expert Verlag, Renningen 2006, ISBN 3-8169-2579-0 , p. 56-65 .