Product category:
Quality control systems/SPC software
News Release from: Panasonic Electric Works UK | Subject: Quality Checking With Lasers
Edited by the Manufacturingtalk Editorial
Team on 17 August 2004
Quality Checking With Lasers
In-line quality checking with lasers is now helping manufacturers to improve quality. This article describes the trend in laser measurement systems technology.
Ever growing demands for higher quality are leading to the increasing use of integrated measurement technology in industrial manufacturing with laser measurement technology becoming more and more important in this area This article describes the trend in laser measurement systems technology and uses a high-precision laser measurement system from Matsushita to illustrate the multiplicity of uses for the system in quality checking
This article was originally published on Manufacturingtalk on 4 Oct 2005 at 8.00am (UK)
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To optimise process and product, in-line quality checking - typically measurement of geometric values - is vital during various stages of production.
Developments in laser measurement technology have changed the evolution of industrial measurement systems.
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In addition to its non-contact, fast and friction-free optoelectronic measurement process the primary benefit of laser measurement is its high accuracy, previously only obtainable with tactile measuring systems.
Laser distance measuring sensors stand out by virtue of their greater measurement range than capacitive or inductive versions and higher resolution than ultrasound and radar sensors.
In addition they are not affected by electromagnetic fields.
Triangulation is one of the oldest procedures for optical non-contact distance measurement and remains the most widely used.
The development of semiconductor lasers has resulted in laser analogue sensors capable of being used in industrial applications and operating according to the triangulation principle with a visible red light laser and a transmit power of less than 1mW, which is in accordance with laser protection classes 1 or 2, for which no specific protective measures are required.
Triangulation using PSD-, CCD- and MOS- array sensors To measure distance sensors such as Position Sensitive Devices (PSD) can be used as detectors that have a strip-type photosensitive surface with one anode contact at each of its narrow sides.
The PSD used as a position detector delivers two output currents of differing sizes, depending on the position of the received maximum reflection and distance, information is derived from the ratio of the two currents to each other.
The evaluation is based on determining the focus of the distribution of the quantity of light on the entire active surface.
The advantages of PSD receivers lie in their fast response time, their low power consumption and their keen price.
The surface characteristics of the object being measured has an effect on the diffusion behaviour of the reflecting light and, in the final analysis, on how it is evaluated.
Reflective or shiny metallic surfaces for example create a directed reflection or directed diffuse light.
This distortion in the focus of light distribution is bound to lead to incorrect measurement of the distance whilst dark or low-reflective surfaces the currents created in the PSD element are so small that they can hardly be evaluated without great efforts on the electronics side.
In practice however highly reflective surfaces like semiconductor wafers and strongly absorbent materials like vehicle tyres are often encountered.
To overcome this a CCD array consisting of a large number of pixels rather than a PSD element is used in the sensor receiver.
A major advantage of a CCD array compared to a PSD element lies in the sequential readout of each individual pixel.
It is not just the absolute quantity of light that is important for evaluation but far more the relative strength of illumination or the distribution of the light over the entire CCD row.
The high computing power and complex evaluation electronics required are provided by fast microprocessors (or controllers) and special ASICs.
Very recently MOS arrays have also been used in laser distance measurement sensors.
As well as high photo sensitivity and greater brightness dynamics compared to laser sensors with CCD arrays, their major advantage is the far higher measurement frequency.
In contrast, with the laser sensors with a PSD array the resolution remains practically independent of the measurement frequency.
High precision with safety laser The particularly outstanding features of the HL-C1 Series laser analogue sensors from Matsushita are their high accuracy with a resolution of up to 1æm and a measurement frequency of 10kHz.
Using MOS technology not only achieves the optical characteristics of laser analogue sensors with a CCD array but also the higher dynamic characteristics so that even with difficult to measure objects such as light absorbent materials or particularly reflective surfaces the HL-C1 Series gives fast and highly precise measurements.
Two sensor heads are available.
One for standard applications such as rubber and materials with diffuse reflective surface properties.
The other is above all suitable for reflective surfaces and transparent objects.
Both versions are offered for measurement ranges 50mm +/-5mm and 80mm +/-20mm and depending on type, a resolution of maximum 1æm at a sampling rate of 10kHz is achieved.
Further features: * low level of temperature dependence.
* visible red light semiconductor laser in laser class 2, as well as..
* robust die-cast Zn housing with Protection Class IP67 for the laser sensor heads.
With the option of connecting two sensor heads and direct processing of the measured values, the control and checking unit of the device is typically suitable for thickness and distance measurement and can be configured for the material characteristics of the objects to be measured: * diffuse for standard applications.
* shiny for metals, for example,or..
* transparent for glass, for example.
The evaluation of the measured values or the analysis of the measurement curves using the installed software is adapted automatically.
In addition an illumination time of between 100 and 1000æs can be selected.
This enables the quantity of light on the object to be varied in order to obtain a best possible measurement result.
If the surface of tyres is to be measured for example a longer illumination should be chosen than when checking a CD-ROM.
Examples of applications for the HL-C1 Series include dynamic dimension checking or recording profiles and contours.
Specific applications are also conceivable, for example measurement systems used in tyre (black rubber) manufacturing or painted reflective surfaces of different colours in the automotive industry.
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