Laser measurement for quality control

30 August 2011

Dr Jie Lin describes the developing trends in laser measurement technology and looks at a range of diverse applications for quality control in manufacturing processes which have benefitted from the use of this technology.

Laser measurement technology has been a beneficiary of the rising demand for quality assurance, owing to its non-contact, maintenance-free and rapid measurement capabilities. To optimise manufacturing processes and improve product quality, in-line quality control during the various manufacturing stages has become indispensable, for example measuring geometric parameters such as distance, thickness, diameter and surface contours.

Laser measurement technology has made huge advances in the past 20 years. Besides the fast, contactless and maintenance-free method for measuring using opto-electrical systems, the main advantage of laser measurement technology using opto-electrical systems lies in its precision, which previously could only be obtained by tactile measurement systems. In comparison to other contact-free methods of measurement, laser displacement sensors have the advantage over capacitive or inductive distance sensors by being able to measure a larger area. In comparison to ultrasound and radar sensors, laser displacement sensors resolution is more precise. Also, laser sensors are not affected by electromagnetic fields.

The triangulation principle is the oldest and still the most common method used for optical, contact-free distance measurement in industrial sensor technology. The advancement of the semi-conductor laser led to the development of laser analogue sensors suitable for use in industry, which operate according to the triangulation principle using visible red lasers and emitting power of less than 1 mW within the scope of laser protection class 1 or 2. These protection classes require no special protective measures. This paved the way for the safe and widespread use of laser sensors across industry. Given the continuous increase in performance, combined with decreasing prices in microelectronics and microprocessors, more ASICs and faster microprocessors (or microcontrollers) could be integrated into laser sensors. The complex signal processing of modern laser measurement systems can meet the precision and speed required by automation technology. In addition, premium, special optical/micro-optical lens (systems) guarantee the realization of focused laser beams on the one hand, and, on the other hand, minimize optical representation errors on the receiving element.

Triangulation measurement
Position sensitive devices (PSDs) were mostly used as detectors for cost reasons when laser analogue sensors were first developed. However, laser analogue sensors and laser measurement systems introduced during the past 10 years rely instead almost exclusively on CCD or CMOS linear sensors. One advantage CCD/CMOS linear sensors have over PSD elements is their ability to read out each individual pixel sequentially and directly. Other than with PSD, not only is the absolute amount of light processed important (i.e. the determination of the most intense point within the distribution of light), but rather the relative illumination or distribution of light over the entire CCD or CMOS line. Fast microprocessors/microcontrollers and special ASICs have solved the problem of performing the demanding computations and providing the complex analytical electronic components required.

The use of CCD and CMOS sensors smoothed the way for implementing laser analogue sensors throughout industry, to measure standard objects as well as for more demanding applications with complex surface and/or material properties - highly reflective surfaces such a chrome-plated piece parts or metal surfaces painted with light colours and highly absorptive materials such as rubber or tires. CMOS sensors have become more popular in laser analogue sensors. They are well suited to use with measurement systems because of their greater dynamic range. Besides greater sensitivity to light and luminosity dynamics in comparison to laser analog sensors with CCD sensors, laser analog sensors with CMOS sensors exhibit other technical advantages too – an attainable sampling rate of 100 kHz, direct access to each individual pixel as well as ‘on-chip integration of the electronic components needed to analyse the data as well as lower power consumption. CMOS sensors are also generally more cost-efficient than CCD sensors.

The latest generation
One example of the latest generation of CMOS sensors is the HL-G1 series from Panasonic, which sets new standards for measuring distance optically. Features include resolutions down to 0.5mm, high sampling rates of 10 kHz and a minimal linearity error (£ ±0.1% FS). The all-in-one concept of the HL-G1 series integrates the processing and control unit into the sensor head. The sensor can be configured using function keys on the device.

The CMOS linear image sensor can achieve the same sensing results for optical characteristics such as being independent of colour and surface as laser displacement sensors with CCD receiving elements. In addition, the shutter speed for the sensor receiver can be adjusted automatically or manually allowing the amount of light reflected onto the sensor to be varied to obtain the best measurement result.

The sensor is typically used in applications such as dynamic measurement control of material thickness and diameter, or to determine profiles or contours, as well as in special applications such as for the measurement during manufacturing processes in applications such as tyre production (black rubber) or metal surfaces of various colours as can be found in the automotive industry.

*About the author
Dr Jie Lin is the european general manager responsible for sales and product management of sensors, automation components and UV curing systems at Panasonic.

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