Gantry table cuts the cost of 3D laser scanner

An economical and accurate gantry table was key to the development of a new line of laser scanners that provides precision reverse engineering at a price point 50 per cent lower than in the past. A leading manufacturer of 3D laser digitising systems for rapid inspection applications - including verification, manufacturing, rapid prototyping, reverse engineer and process control - had been looking for the opportunity to enter the lower-cost end of the market but found it difficult to provide the accurate motion control required for such a system at a cost that would allow an economical product to be developed.

Isel's gantry base provided high-precision table movements over a wide range of motion at a very economical cost. As a result, the company was able to introduce a series of 3D laser digitisers at half the price of what used to be its lowest cost product. The new product filled an important gap in its capability by enabling it to offer a very accurate reverse engineering tool at a price that can be afforded by small and medium-sized firms.

Its software engineers had developed line-sensor technology that dramatically reduces scanning time by collecting data substantially faster than conventional measuring technologies. The technology uses laser triangulation, an active stereoscopic technique where the distance of the object is computed by means of a directional light source and a video camera. The scanner emits a Class II laser light that scans an object, and the scanning systems use line range profiling that spreads the laser beam into a line and collects data up to 100 times faster than point range sensors - at a rate of up to 15,000 co-ordinates per second. Using triangulation-based technologies, 3D co-ordinate locations on the surface of the object are captured by a sensor on a charge coupled device (CCD) in the probe, according to scan density and pattern parameters set by the user. This means details of the object's shape and finish are captured with high accuracy.

Triangulation

High-density, true surface information permits exact duplication of complex shapes and the digitising eliminates the inaccuracies of mechanical probe offsets deflection, vector analysis, and probe size or shape. Laser scanning non-contact technology also enables difficult materials such as clay, fabric, wood, foam, plaster, glass, metal and plastic to be scanned without damage to the surface of the object. A laser beam is projected from the sensor head onto a scanning object. The object scatters the light, which is then collected by a video camera located at a known triangulation distance from the laser. Using trigonometry, the 3D spatial (XYZ) co-ordinates of a surface point are calculated. The CCD camera's 2D array captures the surface profile's image and digitises all data points along the laser. With specialised inspection software from a specialist partner supplier, the laser scan data can be easily compared to a CAD file so deviations from nominal can be displayed graphically. Other software programs enable NURBS surfaces to be applied to the scan data to create a CAD model of the scanned object. STL and CNC tool paths can be derived from the scan data for duplication and manufacturing.

The 3D laser digitising systems are used for rapid inspection applications including computer-aided verification and quality control, reverse engineering, rapid prototyping and manufacturing. Typical parts scanned include plastic and rubber components, EDM electrodes, extrusions, moulds, dies and castings. The system's scanning speed and accuracy are suitable for quality inspection. By scanning an existing product or tool, editing, and outputting the scan data to a CAD system, improved product designs can be completed very quickly, thereby reducing the design cycle time. Models that have been produced by a sculptor or rapid prototyping system can also be scanned for input into a CAD package.

Although very successful, the need for a lower-cost scanning tool for reverse engineering was identified. All the machines previously used servo controls while other machines were available at a lower price but they had manual motion to control the scanning head. The problem with this approach is that manual motion introduces jerkiness and instability that reduces the accuracy of the resulting 3D model. What was required was a motion control system that would provide the accuracy of servo control at an economical cost. The Isel gantry was the answer, offering travel ranging from 200 x 250 x 130mm up to 1400 x 2400 x 500mm and a mechanical system delivering durable, play-free motion that is rigid and stable. Yet the cost was low enough, enabling the company to introduce a new line of machines that each cost about 50 per cent less than existing machines of comparable size. The benefit of using Isel was that not only does Isel supply machines for CNC applications, it also makes them available for other OEM applications such as scanning and inspection.

Rigid, accurate and smooth

Each Isel gantry base incorporates a linear motion system with high load-carrying capacity, rigidity and positioning accuracy. Heavy cast aluminium side plates support the X axis, providing increased stiffness. All axes include a home reference switch and a far end limit switch with +/-0.01mm repeatability. The profile rail and bearing block systems produce very smooth, play-free motion, thereby facilitating high-accuracy scans or cuts. Another appealing feature of the machine was its use of anti-backlash ball screws. These have excellent power transmission capabilities due to the rolling ball contract between the nut and screw, which also feature extremely small backlash. Rolling ball contact between the nut and screw provides excellent rigidity, ensuring long life because wear is considerably lower than ACME screws and nuts with sliding friction contact. The ball screws also contribute to the bases' exceptionally high positioning accuracy per axis of 0.10mm/300mm, resolution of 0.01mm and repeatability per axis of 0.01mm.

It took just one week to complete the design of the new laser scanner, the process primarily consisting of integrating the existing scanner head and software with the Isel gantry base. The result is a new series of 3D laser digitisers that scan parts from all orientations and then transfer the data back into a common co-ordinate system. The machine control software controls scanning motion, sensor settings and also contains advanced data-editing automation features.

These laser scanners have already established a large base of new customers in a wide range of different industries and customers report that they can produce models in 1/20 to 1/10 of the time they took when they were creating CAD models by hand.

Micromech is the UK systems specialist and distributor for Isel.