Three-dimensional sensing versus machine vision systems

Three-dimensional (3D) sensor systems are rapidly increasing in importance in the automation sector. In particular, they are moving from having previously been high-priced to a point now where they are affordable for general automation.

Millions of optical sensors are used in measuring or switching systems, determining the distance or the presence of an object by emitting a single beam of light. Various forms of light are used, dependent upon the application, with red light the most common, being economical and easy to use, because of it being easily visible. Ultraviolet light is used in sensing UV colouring - for example, when looking for the presence of glue or labels on glass. Contrast sensors, which are also called contrast scanners, use red, green or blue light - or indeed all of these in the form of white light - to look for specific colours such as print registration marks. Laser light, being highly energetic, is suitable for sensing over a greater distance. It also provides a finer resolution and hence accuracy for sensing edges - for example, delicate cogs, or holes in a PCB.

More recently camera-based vision systems which use surface illumination have become more common. These vision systems analyse the light/dark distribution of elements (pixels) within a two-dimensional (2D) surface, usually at a high resolution. If this 2D data is combined with distance information from an energetic single-beam system, this additional depth information provides a third dimension.

2D camera plus z-axis sensor

An alternative technology to combining a camera image and a sensor is to project a laser line across a section, which provides not only the location of each pixel in an x/y co-ordinate system and its brightness value, but also the distance in the z axis.

For example, Leuze electronic's recently launched line profile sensor, the LPS36, projects a line up to 600mm long, which is equivalent to using a chain of 376 laser distance measurement sensors working together. The LPS sensor benefits from being perfectly calibrated the moment it is switched on and is used to take cross-sectional profiles or slices across a tray, for example. A vision software package constructs these profiles into a 3D image of the tray and its contents for a bin-picking system like Faude's Flexvision system [see 'Bin-picking robot is guided by laser profile sensor - Ed].

The line profile sensor offers cost and time savings compared with the complexity of combining distance measurement sensors, let alone 376 of them. Vision software like MVTec's HALCONsoftware has 3D vision libraries available, saving users the time-consuming task of carrying out software development and the need to program their own algorithms.

Cost comparisons

The LPS, and other profiling sensors as they become available, offer the measurement of dimensions or the position of objects at a price level well below vision systems - for example Leuze's LPS sensor is three-quarters the price of the company's smart camera, which, in turn, is half the price of many competitors' units.

Their ability to provide distance measurement, namely the third dimension, makes them considerably less sensitive to differences in brightness or colour of an object. The energetic laser provides illumination of a point on the surface; in lighting terms, this is a factor of 1000 times that of a vision system and is therefore robustly independent of changes in ambient light. Despite this startlingly high factor, the safety of the operator's eyes is preserved.

By now you might be asking "So why have 3D sensors not been chosen thus far?" Previously 3D sensors were expensive and large. The extremely complex processing of the 3D point clouds was difficult and expensive to manage, hampering their widespread acceptance. Now, however, sensors have become extremely compact, integrated data pre-processing has become available and the price/performance ratio has reached an affordable level for general machine builders and their applications. Therefore 3D sensors are now at a point where they are about to become a much more common sight.

Applications, for example, like determining the 3D co-ordinates for the pick position of a robot's grabber, or determining the volume of an object, which require height information in addition to length and width measurements, are best done and now affordable with a line profile sensor.

Leuze electronic displayed one of the systems working on a bin-picking robot at its stand at the SPS exhibition in November 2010 and a video of this is on YouTube. Follow the link for more information about the LRS 36 and LPS 36 light-section sensors.