Laser sensor delivers results

Jon Lawson

Chris Jones explains how a non-contact laser sensor provides 10 times higher measurement repeatability in wax distortion tests 

A UK-based manufacturer of investment casting wax is using a laser displacement sensor to measure the distortion and contraction of wax materials used in investment casting. The non-contact, high-performance sensor is providing 10 times higher measurement repeatability than the previous LVDTs and touch-probe gauges.

Blayson Olefines, a key supplier to the worldwide investment casting industry, manufactures a wide range of high quality wax materials. Its customers come from a variety of industries including aerospace, gas turbines, medical, automotive, electronics, marine and construction.

The wax products are used solely for investment casting, a process that uses metal moulds to produce expendable wax-based patterns of the part to be cast. These parts can be extremely complex and may incorporate cavities. The patterns are mounted and coated with a ceramic material by building up layers of sand and liquid binder until a suitable shell is formed. The wax is then removed from the shell using high-pressure steam. The shell is fired in a furnace to 1,000°C resulting in a precision mould into which the molten metal can be poured. After cooling, the mould is broken open and the casting is removed and then finished.

Phil Hancock, technical manager at Blayson, based in Waterbeach, Cambridge, comments: “We place a strong emphasis on R&D. Our customers demand the highest in terms of wax performance and our products therefore have to meet tight manufacturing tolerances. Our latest developments include wax products specifically designed for turbine blade manufacture, as well as reduced memory pattern wax.”

In 2013, Micro-Epsilon UK approached Blayson to introduce a suitable non-contact laser displacement sensor for measuring displacement of wax materials. As part of its Six Sigma programme, Blayson uses the Gauge Repeatability and Reproducibility (R&R) analysis technique for measuring the amount of variation in a measurement system arising from the measurement device itself. Gauge R&R is a statistical tool that uses an analysis of variance (ANOVA) random effects model to assess a measurement system’s performance. This technique is not limited to gauges but to all types of measuring instruments and test methods. Typically, the technique is used to examine the P/T ratio, which is the ratio of the precision of a measurement system to the total tolerance of the manufacturing process of which it is a part.

The optoNCDT 2300 is a high-speed, high accuracy non-contact laser sensor. As with all Micro-Epsilon laser sensors, it is a self-contained sensor that requires no separate controller, yet still provides an extremely high measuring speed of up to 50kHz and resolution of 0.0015% FSO (Full Scale Output). The sensor is therefore ideal for high-speed dynamic applications such as vibration measurement and profile scanning of uneven, rapidly changing surfaces.

Two years later…

In two years of using the laser sensor, Blayson reports that it has had no reliability issues. The sensor paid for itself within months and has performed reliably on all colours of wax (yellow, green and blue) with no signal noise when performing measurements on wax materials. This ensures reliable measurement data every time.

Although competing sensors claim to offer similar measuring rates and resolution, its makers say that the optoNCDT 2300 is the only sensor that offers a 50kHz measuring speed and an integral controller. Unlike some other sensors, the optoNCDT 2300 achieves high resolution and high measuring speeds without any averaging.

The sensor uses Micro-Epsilon’s new A-RTSC (Advanced Real Time Surface Compensation) technology, which enables it to automatically compensate in real time for difficult-to-measure surfaces. A-RTSC is a further development of the company’s RTSC feature, which, when combined with high-speed software algorithms, dramatically reduces signal noise at high measurement speeds. When users need to measure against a rapidly changing surface, they ideally require a sensor that is able to automatically adjust the laser pulse duration (or laser on time) of the sensor to give them the optimum exposure time on the CCD for that particular surface. This, in turn, provides a higher accuracy measurement due to lower noise level on the output signal. This is the basis of RTSC and helps to achieve resolutions down to 0.3µm and linearity down to ± 0.6µm for the 2mm measurement range. Data output is via Ethernet, RS422 or EtherCAT. Analogue outputs can also be offered by using the CSP2008 Din Rail mounted controller from Micro-Epsilon.

User adjustable feature

The optoNCDT 2300 has a user adjustable measurement rate. This feature enables precise and stable measurements on low reflectivity surfaces, for example, matte black or shiny metals. Rather than having a fast exposure time resulting in low light levels on the CCD line, the adjustable rate enables the user to slow down the measurement rate, which then enables a longer time interval for CCD exposure, resulting in a more stable, higher resolution measurement.

The sensor can now be configured remotely by using a web browser interface. This direct connection means the user can store parameters for a particular application, which can then be uploaded to one or multiple sensors, reducing set up time considerably. The sensor also provides various options for signal processing and data output, including peak selection (peak-to-peak), masking of the video signal, data reduction, averaging, exposure time, error status, time stamp, filtering and statistics (min and max).

The optoNCDT 2300 is available in seven models with measuring ranges from 2mm up to 200mm. The sensor is also extremely compact, measuring just 80mm by 75mm by 30mm.

Chris Jones is MD at Micro-Epsilon UK.