The demand for measurement tasks in which tactile and optical sensors are jointly used is set to rise more and more in the future. Here’s a technology that saves time and operating costs without compromising on reliable, precise measurement results. Article by ZEISS.
When it comes to maximum precision, coordinate measuring machines (CMMs) are an indispensable tool in industrial applications. To date, they have mainly been used for tactile measurement. In recent years, the need for and use of optical sensors is becoming increasingly significant. There are many reasons for this: the technical advancements being experienced in many sectors require increasingly complex parts; digitalisation and Industry 4.0 are changing manufacturing processes and thus also quality assurance; and customers have higher quality and efficiency demands, in general, nowadays. Many companies are therefore expressing the need for an all-round solution, that is, tactile and optical measurement on a CMM.
One example is the ZEISS CONTURA. Already in its fifth generation, ZEISS CONTURA is equipped with mass technology (multi-application sensor system) as standard, enabling tactile and optical measurement on a single machine. The multisensor platform means it is compatible with a variety of sensors from the ZEISS portfolio: sensors on the continuous articulating unit, star styluses or long styluses, optical or tactile, and scanning or with single point measurement. Thanks to the mass technology from ZEISS, the user acquires maximum flexibility.
Simple Sensor Switch
With ZEISS mass technology, when the sensors are operated on the continuous articulating unit, they are switched automatically. This applies to all optical sensors as well as the ZEISS VAST XXT and XDT tactile sensors.
During the sensor switch, the continuous articulating unit aligns itself in a 90 deg position, with the sensor pointing downwards. It then moves to a free place in the sensor magazine, which is usually attached to the reverse end of the measuring stage, pushes the safety flap back, moves downwards into a groove, and releases the magnetic locking mechanism in order to unlock the sensor. The new sensor is picked up in a similar way: the continuous articulating unit moves backwards and opens the safety flap, moves downwards and picks up the sensor magnetically. On the plate holding the sensor, there are three cylinder-shaped rollers which ensure that the counterpart is precisely positioned on the sensor.
Therefore, even after frequent switches, the sensor is reproducibly situated at the correct point. The measurement uncertainty is not increased by any significant extent due to the sensor bracket. Users do not need to worry that the accuracy may get out of hand if the sensor is switched repeatedly. Due to the high repetition accuracy during the sensor switch, it is not necessary to recalibrate the sensor after the switch has been carried out. Since the automatic exchange itself takes only a few seconds, ZEISS mass technology means an enormous boost in productivity – and thus time and cost savings.
The continuous articulating unit itself, as well as tactile probes from the ZEISS VAST XT gold series, are attached to the ZEISS CONTURA by means of a dovetail mechanism. This is a groove which the counterpart on the sensor or on the continuous articulating unit is pushed into and which, due to its shape and precise processing, does not allow any leeway whatsoever. Handling is easy too: the measuring technician loosens a screw mechanism and pulls the sensor or the continuous articulating unit out of the groove and inserts the new sensor. The sensor switch is completed within seconds. However, a repeated calibration is crucial during a sensor switch and is especially useful when using an active tactile sensor such as ZEISS VAST XT gold, which offers high measuring accuracy, short measurement times and long stylus lengths. All other sensors—passive, tactile as well as optical—are ideally operated on the continuous articulating unit—with all the advantages of the automatic sensor switch of ZEISS mass technology.
Optical Measuring Procedures
Optical measuring procedures are particularly interesting in parts with complex shapes if the user is required to record the surface quickly. This is useful in production in order to safeguard the quality of process steps, such as casting metal blanks or after grinding, in order to obtain a quick comparison between the current and target values of the CAD file. Optical sensors are also ideal for reverse engineering, that is, in order to generate CAD data from a prototype. Optical measurement procedures are often faster than tactile procedures and nonetheless sufficiently accurate. For sensitive parts which may not be touched, there is no alternative to optical sensors.
Various optical sensors can be more suitable depending on the application:
- Chromatic-confocal white light sensor: This type of sensor is used in the area of application of workpieces with sensitive, soft, reflective or low-contrast surfaces. It records the surface of sensitive parts which may not be touched—where tactile styluses are obviously excluded. This sensor even detects transparent painted surfaces above underlying metallic layers and is suitable for transparent layers with various refractive indices. For this purpose, the sensor uses white light, which includes all wavelengths of the visible spectrum. Even strongly reflective surfaces such as glossy metal parts either in automotive and engineering or knee implants do not need to be sprayed with a contrast medium, which other optical measurement methods usually require.
ZEISS offers such a pioneering chromatic confocal white light sensor: DotScan. The sensor can be rotated and swiveled in 2.5 deg steps so that it is always optimally aligned towards the surface. In conjunction with the optional rotary stage, it is suited, for example, to the quality control of parts with complex shapes as well as glass surfaces.
- Triangulation laser: suitable for the fast recording and inspection of freeform surfaces such as those required by casting tools or castings, bent sheets or plastic covers also require non-tactile measurement. The sensor moves above the part at a distance of a few centimetres and projects a line with laser light, which is thrown back from the surface into a sensor chip. Based on the angle, the sensor determines the distance from the part and therefore its surface shape. Each time the light is projected, the sensor determines hundreds of points in a line.
The maximum possible number of points with ZEISS LineScan is 700,000 measurement points per second—the number of rough points which are then calculated to provide actual measurement points in the software. Thus, point clouds which fully record the complex surfaces of even larger parts can be created in just a few minutes. Based on the point cloud, the ZEISS CALYPSO software calculates a chromatic representation using the CAD target data record as a comparison.
- 2D camera sensor: for very small or two-dimensional parts such as circuit boards or flat parts made of sheet metal that cannot be measured using contact means because it may result in deformation of their surfaces, the ZEISS ViScan 2D rotatable camera sensor is the ideal solution. It is capable of recording height-related information, thanks to the Autofocus function, as well as features various objective lenses, enabling increased flexibility in the working distance, area being recorded and accuracy.
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