Coordinate-measuring machine

A coordinate measuring machine based on the principle of coordinate measuring technology and includes a suitable for measuring spatial coordinate measuring system. It consists of a measuring head system (switching or measuring sensor ) whose range is extended by a traversing or positioning system with incremental position or angle sensors. In addition, other software and hardware components for the evaluation of the detected coordinate values for computer correction of systematic bias and to control the axes are required.

3.1 Mechanical ( tactile ) probing
3.2 Non-contact probing

4.1 multi-sensor coordinate measuring machines - Measuring with multiple sensors
4.2 X-ray computed
4.3 Laser Tracker
4.4 Indoor GPS ( Global Positioning System)
4.5 Machine-integrated measuring technology

5.1 Constructive measures against temperature-induced measurement errors
5.2 Constructive measures against vibration
5.3 Numerical Measures

Basic principle and applications

Basic designs

A coordinate measurement system has a CNC - controlled positioning system, or a hand-operated movement system that allows the measuring head system (sensor) and the measurement object to be moved in their spatial position relative to one another so as to detect the respective measuring points. Each traversing the positioning system is assigned at least one length measurement system that measures the relative position with fine resolution. Individual sensor data points may be transformed into a common coordinate system and linked to each other due to the known positions of the positioning unit thus.

Usually used as length measuring systems for the relative positioning of the axes incremental measuring systems with electronic data acquisition and measurement scales on the material (eg glass scale ) or optical (eg laser interferometer ) basis.

The measuring range and the device coordinate system defined by the axes and their guides, drives and incremental measuring systems. Coordinate measuring machines of conventional design having a Cartesian coordinate system devices. Coordinate measuring machines, whose tours encompass a cylindrical or spherical coordinate system, but also widespread and are working with a combination of incremental path and angle sensors.

Conventional construction - Cartesian coordinate system units

The most commonly used "classic" devices are Cartesian rectangular coordinate measuring systems. The orthogonal guides span a Cartesian coordinate system. Significant structural and functional groups of a Cartesian coordinate measuring machine are:

Measurement table: mostly hard rock
Measurement scales for each axis: eg photoelectric incremental length measuring system with Zerodur scales ( low thermal expansion coefficient).
Superposition of the individual axes: eg aerostatic bearings ( air bearings ) to realize a low friction between the moving parts. Tracks from hard rock (granite).
Drive: for example, the movement of the axes realized in CNC CMMs by electric drives (including gear and vibration-damping elements) in a closed loop. The task of the driving system is only the movement of the axes, not the provision of the information on the position of the axes. This task is performed by the length measuring system, which transmits the current position to the control ( for positioning ) and evaluating (for calculation of the probed coordinates values).
Measuring and probing systems: see section sensors
Control and evaluation: To control the measurement process and the movements, for controlling the position or trajectory ( eg, varying the travel speed ), to manipulate the values measured in terms of a mathematical correction of systematic errors (using previously determined calibration values) and in terms of transformation of the coordinate values and the evaluation of the measurement points according to the associated geometry element.

According to DIN EN ISO 10360-1:2003-07 following basic types, a distinction for the realization of three mutually perpendicular movable guides:

Cantilever design: a cantilever arm on which the measuring head system being mounted to be movable in the vertical direction. Two more axes perpendicular to each other are movable in the horizontal direction. The horizontal movement can be achieved either by a movable table or a fixed table by means of movable assembly elements of the boom. Most of this construction is used for measuring devices with small measuring range, but very good accuracy.
Bridge construction: The moving axes and the clamping region are separated. The movable in the horizontal direction of the bridge carrying the sleeve with the measuring head system which can be moved in the horizontal direction along the bridge in vertical direction. Coordinate measuring machines in bridge design allow a wide measuring range and thus the detection of very large workpieces up to entire vehicles or components of the aircraft.
Gantry design: On the portal transverse beam, the bearing sleeve, the measuring head system ( vertical movement ) is arranged, which is movable in the horizontal direction along the portal transverse beam. The portal is supported by two legs at the edges of the device table and movable in the horizontal direction along the instrument table. Also are constructions with fixed portal and mobile devices table spread. Coordinate measuring machines in portal form offer small variations in measurements with good accessibility and a sufficiently large measurement range. Therefore, they represent the most common construction dar. usual is a measuring range of approximately 1 m.
Stand construction: In a coordinate measuring system in post and beam construction with horizontal arm, the sleeve is movable with the measuring head system in the horizontal direction, which can be moved in the vertical direction (also called column ) along a stand. The second horizontal movement can be realized by moving the stand along the measuring table or a movable table. This design is often used for measurement of bodywork and large sheet metal parts, as three sides of the measuring range are freely accessible.

Basic types of coordinate measuring systems according to DIN EN ISO 10360-1:2003-07

Bridge construction

Portal design

Post and beam construction

Unconventional design - cylindrical or spherical coordinate system

Coordinate measuring systems of unconventional design, measure in cylindrical or spherical coordinates. These include laser tracker, Gelenkarmmessgeräte and X-ray computed tomography.

To reduce the measurement errors - caused by random and systematic deviations rotational leadership - unconventional arrangements of the length measurement and drive systems used in micro and nano coordinate measuring machines. By realization of the Abbe comparator principle in several measuring axes, the use of laser interferometric length measuring systems and a parallel metrology, which in all three axes measures all position values directly to the moving sensor or the movable in all three axes platform with the measuring object, the measurement error and measurement uncertainties can significantly reduce.

Extensions

Through the use of an additional turntable or rotary swivel device also unfavorably situated measuring elements can be touched. With such enhancements, the workpiece can be rotated in one or more axes. The changed position of the workpiece is taken into account in the calculation and transformation of the measured coordinate values in the workpiece coordinate system. Alternatively or additionally, articulating means for the sensors.

Sensors

Coordinate measuring systems can be equipped with switching and measuring sensors. Switching sensors provide a measurement point when recording only one trigger signal which initiates the reading of the length measurement systems. However, Measuring sensors have their own internal measurement range of a few millimeters. The internally measured sensor value is superimposed with the length determined by the position of the sensor measuring systems.

Sensors for coordinate measuring systems can also be divided in terms of their physical principle. Until the 90s, tactile sensors were the most used Antastsensoren in coordinate measuring machines. With improved sensor technology, powerful processing technology and increased demands, optical, opto - tactile and X-ray sensors are used nowadays propagated. An overview of sensors in coordinate measurement systems is also given in.

To enhance the universality of coordinate measurement systems, including several different sensing principles may be combined in a co-ordinate measuring system. This coordinate measuring systems are called multi-sensor coordinate measuring systems.

Since it is not possible with a single sensor or push button to release all measuring tasks, it is possible in most coordinate measuring systems, this exchange. With probe changing facilities to change to the automatic measuring process can be integrated.

Mechanical ( tactile ) probing

Probing Systems with tactile sensors ( eg, based on the electro- mechanical principle) and measuring systems (eg with inductive or capacitive measuring sensors ) into switching systems.

The probing of the workpiece surface is performed by the probe. Depending on the measuring task this can have different geometric shapes of the sensing element (usually balls ) and materials (often industry Rubin, carbide, silicon nitride ) are used.

Since the probing a measuring force of the order of 0.01 N acts to 0.2 N, the stylus, which must be considered in measuring bends. The deflection of the stylus is taken into account when calibrating the probe ( Taststiftkalibrierung, button qualification) and automatically corrected with the following measurements. In addition, in the calibration of the probe, the diameter is designed and manufactured for multiple sensing elements of relation to each other (eg star probe the distances between the centers of the probe spheres to each other). The calibration of the probe is conducted on a very accurate ball Normal ( form deviation <0.2 microns ), which is touched with any used button at least five or more points according to a defined by the device manufacturer Einmessstrategie.

Non-contact probing

For non-contact probing optical, electrical and x-ray tomographic sensors can be used. In principle any electrical or optical sensor may be used in co-ordinate measuring systems to enhance the measuring range by means of the positioning system.

Optical sensors

Triangulation sensors (eg laser triangulation sensor line, photogrammetry, fringe projection )
Image processing sensors (eg, edge detection, binary processing, gray level processing)
Contrast methods (eg autofocus )
Konfokalsensoren (eg chromatic Confocal, Foucault sensor or laser focus method )
Interferometer ( eg laser interferometer, white light interferometer )

Electric Sensors

Tunnel current sensors (such as tunnel current or switching measurement)

X-ray tomography sensors

Consisting of an X-ray tube and a detector ( the measured by a measuring object, which is located between the X-ray tube and detector and is irradiated, attenuated X-rays )

More advanced equipment technology

Multi-sensor coordinate measuring machines - Measuring with multiple sensors

A very high universality is achieved by the combination of a plurality of different sensors in a coordinate measuring machine. For each characteristic to be measured, the optimum sensor can be selected. The measurement results of the different sensors are in a common coordinate system. For this purpose, the position of the sensors is pre- calibrated to each other. This makes it possible to combine the results from different sensors to measure features that are only poorly or not measurable with a sensor.

The various sensors are attached to either an AC port on the sleeve of the coordinate measuring machine and the measurement procedure successively loaded automatically (sensor changer ), or are permanently mounted on the vertically positionable spindle side by side. Through devices with multiple sleeves which permit a separate vertical positioning of the individual sensors, the risk of collision is reduced. Also, sensors are attached by means of retraction axes to quill and extended only when needed, thereby avoiding collisions.

X-ray computed

Next tactile and optical sensors, which, apart from the thickness measurement always detect the outside geometry, the method of X-ray computer tomography can be employed in order to in addition to the outer geometry measure the internal geometry of a workpiece. When measuring with these sensors operate numerous influences ( ring artifacts wobble artifacts Kegelstrahl-/Feldkamp-Artefakte, beam hardening, partial or Partialvolumenartefakte ), which affect the accuracy of the measurement result. Through the use of additional tactile sensors, systematic errors of tomography can be partially corrected. As a component must always be completely irradiated during imaging, and the use of massive components or for components with very different absorption coefficients is limited ( due to non irradiated and irradiated areas).

While coordinate measuring systems on the X-ray computed tomography for industrial applications are based commercially available, industrial coordinate measurement systems are based on ultrasound computer tomography ( USCT ) and neutron computed tomography ( NCT ) is still in the research stage.

Laser Tracker

See Tracking interferometer.

Indoor GPS ( Global Positioning System)

See indoor positioning.

Machine-integrated metrology

Tool measuring instruments are specialized instruments for testing, adjustment, alignment and adjustment of cutting tools. There are several variations, the interpretation meet the requirements of specialization.

Only two axes (tool height and width ) and an axis of rotation to test the tool length, the tool diameter and concentricity are required for the manufacturing process milling, so for rotating tools such as twist drills or cutter heads. Instead of a measuring sensor for measuring a contact to a surface of the tool measuring devices usually occurs in a two-axis movable overhead projector, which reflects the shadow or profile of the tool on a large projection screen with crosshairs. When measuring the projection unit is the the edge or corner of the tool in the crosshairs process is. From the path results in the tool length or diameter. By focusing on the crosshairs and then turn past the runout can be checked and corrected until the rotation shows no deviation of the shadow edge on the reticle.

Tool gauges for cutting tools for the manufacturing process turning come formally from two axes, but usually have three to the height of the cutting edge (for the so-called " turning over or under center ") in respect of the tool holder level check to the rotation axis on the lathe can.

Causes and measures to reduce measurement errors

For each measurement, there are some divergences between the displayed by the meter measured value and the actual value of the geometric size (reference value). These measurement errors can be divided into random and systematic errors. For coordinate measuring systems, many structural and computational measures are applied in order to keep measurement errors low. While you can correct systematic errors mathematically, make random errors the measurement result uncertain.

Important causes of measurement errors in coordinate measuring machines are:

Environmental conditions: temperature (temperature fluctuations, gradients, radiation ), vibration, moisture, dirt
Workpiece / DUT: Form errors, micro shape ( roughness), the material (Young's modulus in tactile probing ), reflectance ( at optical probing ) Dimensions / weight, temperature, flexibility (eg, filigree structures ), cleanliness
Instrument: Technical design, stage errors, probe, probing force and direction, measurement and evaluation
Operator: care, stretching, push-button configuration, probe calibration, monitoring of the CMM

Constructive measures against temperature-induced measurement errors

Precision machined guide body with excellent thermal properties.
Sleeve and the portal traverse of materials with high heat conductivity (for example aluminum). The high thermal conductivity reduces the adjustment time for the restoration of accuracy by temperature fluctuations.
Adherence to internationally agreed reference temperature of 20 ° C by temperature-controlled environment, tempering the measurement objects and avoidance of temperature change on the meter (thermal insulation of the device structure; avoid direct hand heat by thermo gloves, avoidance of radiation by light and sun, etc.)
Standards of materials with minimal thermal expansion coefficient
Temperature monitoring of the measuring chamber, the measuring range of the measured object and measuring system elements

Design measures against vibration

To reduce the vibration from the environment to the measuring equipment, can be used as foundation on which are all metrological relevant institutions to serve a massive concrete slab, which in turn, through a gravel trap and an insulating buffer layer (special polymer) is separated from the ground. A circumferential parting line decouples the base plate from the rest of the building. Through these measures, the maximum permissible vibration amplitudes according to VDI / VDE 2627 can be met.
An integrated measurement system in the coordinate system for the passive (or active ) pneumatic vibration damping minimizes the transmission of ground vibrations and leveled the device table for unbalanced load caused by the workpiece weight.

Calculated measures

Calculated correction of static and dynamic influences of all 21 leadership deviations ( regular monitoring and, if necessary, recalibration to avoid drift)
Available flat deformations of the measurement table - caused by temperature gradients - are compensated for by measuring the temperature difference between the plate top and bottom.
Calculated correction of probe bending due to contact forces (with tactile sensors)
Computational correction of variations in temperature of the workpiece and the individual components ( for example, the scales) of the coordinate measurement system

Terms

The term " measuring machine " (or " CMM " ) is always to be avoided. It is correctly called " meter " (or " coordinate measuring machine " ), as in art, a "device" as a signalumsetzendes or information-processing system to increase the sensory or mental capacity of a human being is defined as a "machine" an energy - or stoffumsetzendes system to increase physical performance of a person describes.

Since devices for coordinate measurements are now complex systems, that is, represent a composite of several devices, one has to internationally agreed in the context of the ISO / TC 213 WG10 in 2013 on the future only the term " coordinate measuring system " in standards to be used ( English: " Coordinate Measuring system").

Standards and Guidelines

DIN EN ISO 10360 series: Geometrical product specifications ( GPS) - Acceptance and reverification tests for coordinate measuring machines (CMM ). An overview is given on the website of ISO/TC13.

VDI / VDE 2617 series: Accuracy of coordinate measuring machines - Characteristics and their testing. An overview is given on the website of GMA FA " 3:31 coordinate measuring machines ".

Numerical control Transformation (function) Rotary table Inductive sensor Structured-light 3D scanner List of types of interferometers

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