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Why choose a laser based sensor?

11 October 2015

Launching the remarkable Banner Engineering Q3X and Q4X laser sensor ranges to the UK market, Turck Banner's Ian Manning takes a closer look at the differences between LED and laser based object and distance sensing technologies.

Modern Photoelectric sensors typically employ one of two light technologies, either LED or laser and the choice of light source can be as important for a reliable application as the choice of sensing mode - opposed, retro-reflective or diffuse.

For practical purposes the main differences between laser light and LED light is how it reacts with the outside world. The key property is 'coherence'. Both laser and LED use a diode to generate the light, the laser then bounces this light back and forth within its housing to generate more of the same wavelength of light (monochromatic, single colour), and more critically all the waves are in phase with each other (coherent). LEDs also generate near monochromatic light, they have a broader spectrum of wavelengths than lasers but the critical feature is that the light is incoherent, the wavelengths are randomly produced.

All natural light and most man-made light is incoherent and has many wavelengths which is why it spreads out from its point source as it travels. Laser light, on the other hand, remains in a tightly focused beam over great distances.

It is this coherence of laser light that gives laser sensors their unique properties of very narrow beams that can travel great distances without spreading out, making them especially suited for certain types of application and a very poor choice for others. Let’s looks at the pros and cons:

In order for an object to be detected the light from the emitter must reach the receiver or the light from the emitter must be blocked from reaching the receiver. This is true for all optical sensing modes, the object can act to either block the beam or reflect the light to the receiver. 

Initial set-ups are often fairly simple but maintaining reliability can be difficult, especially if the environment is dirty or vibration is present. The emitted light pattern from an LED based sensor spreads out very much like a simple torch light, the receiver has a similar field of view. As long as the beam pattern of the emitter falls on the lens of the receiver and the field of view of the receiver covers the lens of the emitter a light beam will be established between the two. 

With a wide beam pattern and field of view the emitter and receiver can move and rotate a considerable amount without the beam being broken, as often occurs if the units are knocked or move in relation to one another due to vibration. A laser emitter has a very narrow beam pattern and although this beam can travel a very long way, small amounts of vibration will move the beam off the receiver and break the beam.

Laser based sensors are restricted in the amount of light power they can produce as the risk of blinding someone would make them too dangerous to use in a normal working environment. Whilst conditions are good this low power does not cause any problems as it remains concentrated in a small area; however, dirt, oil, snow and so on can disperse, block or divert the beam. This can also be an advantage when detecting clear objects and semi opaque objects: where an LED beam may pass straight through, the laser beam will be effected.

The small spot size of the laser sensor makes it ideal for detecting small objects or small features on an object. It also allows objects to be detected in holes or recesses where the reflections from the side walls would make it impossible to deploy LED sensors. The single wavelength of the laser light produces a precise colour that allows the detection of small colour differences that an LED sensor would struggle with.

Where laser sensors really excel is in distance sensing. The narrow beam is ideal for triangulation, where the position of the beam reflected from an object will change with the distance that the object is away from the sensor, the intensity of the beam will remain relatively unchanged with distance.

The opposite is true for an LED sensor. Intensity changes with distance but the reflected beam is too wide to gain any accurate position information. As the intensity also changes with object colour and reflectivity, attempting to use a standard LED sensor for distance measurement is normally a poor choice. The laser sensor can detect colour or reflectivity with a degree of immunity from distance changes.

The ability to detect the object distance allows the sensor to overcome a number of common sensing problems. A background suppression or fixed field sensor will ignore a shiny object that is beyond its cut-off point and still be able to detect a dull object just a few millimetres in front, whilst a foreground suppression sensor will ignore objects closer than its cut off point.

A combination of the two can create a window of detection between its near and far cut-off points. This window of detection is commonly employed together with an analogue output (0-10V or 0/4-20mA) to monitor the position of an object within the window.

After more than four decades designing and manufacturing photoelectric sensors, Banner Engineering has now turned its attention to laser sensor technology with the launch of the Q3X and Q4X ranges.

Q3X laser contrast sensor 

Featuring high-speed part detection as fast as 250µs, the Q3X laser contrast sensor can capture up to 2,000 events per second and detect small changes in contrast up to 300mm away. There are also background suppression models available in 50 and 100mm ranges with 60 and 100mm background suppression respectively.

These features make the Q3X ideal for solving applications with small contrast changes where a background needs to be ignored. Out of the box, the Q3X LD50 operates like a fixed-field laser sensor and detects targets within its 50mm sensing range. However, the sensor can be programmed for low contrast sensing within this range, while ignoring objects at least 60mm distant. This allows the sensor to accurately detect its target without background objects affecting its performance. 

This fixed background suppression is key in overcoming the main problem of standard diffuse-mode sensors where objects in the background affect the application. A typical application is label detection on a bottle where contrast differences between the label and bottle provide reliable detection but the sensor must ignore the shiny metal rail in the background. The shiny metal background could cause a false trigger for standard diffuse-mode sensors

The Q3X is designed with a novel angled, three-digit display of signal intensity, which provides high visibility of operating conditions from multiple angles. Two tactile buttons located below the display facilitate quick and easy set-up.

A robust nickel-plated zinc die-cast housing ensures reliable performance even in environments with exposure to cutting fluids and oils. Sensors are also rated to IP67, IP68 and IP69K for enhanced protection against water immersion and high-pressure wash-down.

Q4X laser distance sensor

Featuring superior, versatile sensing performance, the Q4X reliably detects distance changes as small as 1mm and covers a 25 to 300mm range across multiple target colours, materials and surfaces. With the ability to detect the presence/absence of a target or an object’s orientation, the Q4X solves a variety of common sensing problems.

The Q4X is ideal for difficult distance-based sensing applications as it easily detects objects regardless of target surface reflectivity, including black foam on black plastic, black rubber in front of metal, multi-colour packaging and targets of all colours.

Additionally, the sensor provides superior resistance to ambient light interference. Analogue versions with 4-20Ma and 0-10V output options are also available. Applications range from fill-level indication, part positioning and roll diameter to loop control and thickness/height verification.

The Q4X provides a clear readout from its angled four-digit display, easily viewed from multiple positions. The series is further enhanced with the inclusion of a dual teach mode option. Featuring new firmware, the dual teach mode version combines window thresholds on both target distance and target reflected intensity, allowing it to reliably solve difficult applications with ease.

The Q4X can detect clear objects without requiring a retro-reflector. Dual mode can be used to error-proof applications by ensuring the correct colour part is located at the correct position.

The sensors are contained within robust housings rated to IP67, IP68 or IP69K, allowing use in wet and high-pressure wash-down environments, while durable FDA-grade stainless steel resists mechanical impact, over tightening, extreme vibration and aggressive cleaning procedures.

The Q4X provides reliable detection of: black foam in front of black plastic in automotive interior and exterior assembly; parts of varying colours in consumer packaged goods and automotive industries; black rubber in front of metal in robotic assembly; multi-coloured packaging and secondary packaging in food, beverage and cosmetics; part-in-place for error proofing of assembly steps; small parts for automated assembly processes, such as caps, seals, o-rings, washers, vials and jars; a wide array of parts in vibratory feeder bowl equipment, including part level detection in the bowl and part present in the feeder track; and any other challenging part-detection application that you might care to think of.

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