Laser triangulation outperforms vision systems

The latest laser triangulation sensors have inbuilt real time surface compensation algorithms that enable very high accuracy measurements at very high speeds of 10kHz or more.

For more than 10 years sensor manufacturers have developed laser triangulation sensors for measuring linear distance of a target from the sensor.

This linear distance can be referred to as movement, position, displacement, distance etc.

This technique has developed significantly in the last 3-5 years as the integration of digital electronics and high powered digital signal processors DSPs enable the laser to be less sensitive to target colour or texture and the surrounding environment, ambient light, and temperature changes.

The most recent laser triangulation sensors from Micro Epsilon have inbuilt real time surface compensation algorithms that enable very high accuracy measurements (typically less than 1um) at very high speeds of 10kHz or more.

Over a similar amount of time period, the market has seen the emergence of vision cameras for inspection of label orientation, to detection of surface defects and more recently to measure specific features of a component, for example a hole diameter, or a gap measurement of a groove or channel in a machined/formed component.

Many vision camera manufacturers and system integrators have tried to extend the vision camera principle of detection into the field of metrology.

However trying measure other features, such as height of a step, shape of an extruded profile, volume of a product as it is manufactured, positioning and guidance of robots is still today, very difficult, or not at all possible with vision camera measurement.

This is not due to the technical ability of the vision camera principle, as vision cameras have now developed to very high megapixel resolutions, but more because this principle was never really intended to be used for this type of metrology task.

Laser triangulation is far better suited to step height measurements, determining the profiles of extruded products and robot positioning and control, but this measurement technique is also limited due to the spot size of a laser sensor being very small and so multiple sensors are required, which makes this technique potentially as complex and expensive as vision cameras.

Then came the birth of a laser triangulation sensor producing a line instead of a spot, combined with a controller that could output the distance values across this line so instead of 1D measurements from a spot, 2D data were obtained from a line.

The limitation of early systems, and still some in the marketplace today, is they are quite large, have moving components inside the sensor head which makes them susceptible to damage from shock and vibration and are a relatively high price.

The new ScanControl 2800 from Micro Epsilon goes a very far way to addressing the above limitations.

The system has a solid state laser line projection with no moving components.

The system has been tested to ISO standards for shock and vibrations, capable of withstanding up to 15 and 2g, respectively.

A high quality CMOS imaging element with global shutter principle, combined with a high powered DSP, enables very high speed measurements without distortion of the image.

As the sensor is based on the laser triangulation principle, the system can really be used for metrology tasks as it is an absolute measurement principle.

Each sensor is calibrated at Micro Epsilon and is shipped with a traceable calibration certificate.

Data are output from the sensor along a FireWire interface which is based on the vision camera FireWire protocol, so users of vision cameras will find it simple to interface with.

However, as this technique is not based on vision camera principle, the traditional problems of vision camera integration do not apply: The ScanControl 2800 works just like a Micro-Epsilon laser triangulation sensor, which is able to regulate the light intensity every time a measurement is taken without the need for averaging or filtering.

Therefore no external lighting or consistent lighting is required.

The sensor can work outdoors on vehicles or in a factory completely unshielded from ambient light.

The depth of field of ScanControl 2800 is up to 250mm (z-axis), with a line length of 150mm (x-axis).

Providing the target is within this measurement window, the target will be measured accurately.

A vision camera requires the target to be positioned repeatably in front of the viewing lens.

Even the use of telecentric lenses (which are quite expensive and usually large) still only allow a small tolerance in z- and x-axis positioning of the target Conventional camera systems and also other line triangulation systems typically only operate at a maximum of 50frame/s.

The ScanControl 2800 has a very high speed CMOS matrix element and very high powered DSP, which enables the frame rate to be increased to as high as 4000profile/s.

Of course not all applications will require this high rate and so the sensor can be customised to the customer's exact requirements.

The high speed measurement rate now enables applications such as high speed robot positioning and control, real time measurement of extruded profiles, glue beads, weld profiles and also volumetric calculations with a high degree of accuracy not possible until now.

The ScanControl 2800 is an absolute measurement principle, and so the accuracy stated for the device is in microns not pixels or sub pixel resolutions.

Resolution can be as low as 10um for most target surfaces, with a three-sigma linearity error of better than 50um in the z-axis.

The resolution in the x-axis can be selected between 64, 128, 256, 512 and 1024 points.

By combining this with a customised line length, specific customer resolutions can be achieved.

The potential applications for the ScanControl 2800 are very large and to date have included: robot guidance and positing in the automotive industry; glue Bead profile monitoring - integrity checking, including height, width and volume; automated welding systems - integrity checking, including height, width and volume of weld; rail profile and gauge measurement in the railway industry; 100% parts inspection in a production line - including height, width, and volume; surface flatness and surface defect measurement; and integration into a CMM for surface mapping in reverse engineering.

So as can bee seen, the ScanControl 2800 system is not a vision camera, it has been designed to solve applications where visions cameras cannot.

ScanControl 2810 has the ability to perform certain processing functions inside the controller and then output simple analogue values and limit switches.

For example, if measuring an extruded product and the user only wants to log the width and height of the product and set a tolerance on these parameters, this can all be solved inside the ScanControl 2810 controller without the need for additional signal processing by a PLC or PC.

The height and width can be output with an analogue voltage to a simple display or logging device, and the built in high low limit functions can be output to an alarm indicator or safety cut off switch to if the product becomes out of tolerance via a simple open collector output.

This enables the cost of integration for the user to be reduced significantly and the installation process to be very simple.

The device can be configured via Micro Epsilon software and the "user specific" program is then stored into the controller's memory - even if the controller is switched off or loses power, it will restart in the configured "user specific" program when the power supply is returned to the controller.

Micro Epsilon products have led the marketplace for the last 30 years in terms of measurement performance and design.

The product range has always been developed with performance features that offer the customer real world advantages.

From initial conception, a strong emphasis has always been to design products suitable for both OEM and end user integration.

This has enabled Micro Epsilon to grow every year since the company was established.

The addition of ScanControl to the company's product range will no doubt enable the company to continue to lead from the front with a genuine world leading product.

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