Photoelectric sensors for direct distance measurement

“Sensing by Ranging”

The future of photoelectric sensors lies in direct distance measurement

Pepperl+Fuchs: Photoelectric sensors have always had the capability of measuring the intensity of incidental light with light-sensitive components (photodiodes or phototransistors). This principle is as simple as it is effective and is ideal for the design of photoelectric sensors. However, it is always difficult if the reflection characteristics of the object to be detected change the light incidence at the diode. In principle, a thru-beam sensor detects any object, but a reflector that moves unintentionally into the beam of a retro-reflective sensor remains undetected as an object. This physical characteristic is particularly disruptive for optical sensors that measure directly to the object. Different colored objects reflect different amounts of light back to the receiver and all optical sensors have something called a black/white difference, i.e. different switching points for different objects. This applies to energetic sensors in particular, but background suppressors are always affected as well.

The indirect measuring method described is the reason behind any unsatisfactory results. Direct measurement is the only way to reliably measure the distance to an object where the reflection characteristics are constantly changing. While this realization is nothing new, it has rarely been implemented in the industrial optical sensor sector due to high costs.

Distance measurement using the time-of-flight principle

The VDM28 from Pepperl+Fuchs was designed for maximum measuring distances of 15 m and operates according to the Pulse Ranging Technology (PRT) principle. Although the device falls in the price category of a high-quality photoelectric sensor, a genuine time-of-flight principle has been implemented. The extremely short time (the light only requires approx. 3 nsec. for 1 m) that a very steep laser light pulse takes to travel from the transmitter to the receiver must be measured here. Fig. 1 illustrates the principle in more detail. The main advantage of this principle over the more commonly used phase correlation measurement is its unique nature. If several objects are positioned within the light beam from the transmitter, the pulse reflected from the next object arrives at the receiver before any other pulses that are reflected by objects. An intelligently designed electronic system can therefore distinguish between the reflection pulses from each individual object located in the light beam and measure the distances separately. In such cases, the phase correlation method used frequently in low-cost sensors always results in a mixed phase scenario that provides no clear indication of the distance. Only the PRT principle is 100 % capable of detecting multiple targets and can therefore be used in safety-critical applications.

Figure 2 shows the VDM28 used on a stock feeder for specialized fine positioning in double-depth racks. In this application, locating the relevant marking through precision distance measurement and distinguishing clearly from echoes reflected by the many other objects in the beam path is essential.

However it should not go unmentioned that PRT technology can also be used successfully in the optical high-end sensor applications. Figure 3 shows the VDM100 used to control positioning of a stock feeder. The sensor uses reflectors to reach sensing ranges of up to 500 m. A measurement accuracy of a few millimeters is achieved, which remains constant across the entire measuring range and is not affected by an increase in distance, unlike other measuring principles. In the past, this was only possible by measuring the light propagation time directly. The PRT principle is so efficient that a completely harmless class 1 infrared measuring laser can be incorporated into this high-precision measurement device.

The two examples impressively demonstrate the scalability of the PRT concept. From measuring photoelectric sensors to high-end measuring technology, a wide spectrum of requirements can be fulfilled using the same measuring principle. Even diverging requirements for device manufacturing costs can be fulfilled by using components customized for the relevant application.

Distance measurement using triangulation

The main strengths of the PRT principle are without doubt the long measurement range and extended sensing range. There is also a trend of sensing by ranging with smaller detection ranges. Figure 4 shows the principle of triangulation. The received beam is diverted towards various light-sensitive photoelectric receivers in a multipixel array (MPA) depending on the distance to the object. For distances up to approx. 800 mm, the method is both simple and extremely accurate. Either a preset threshold can be retrieved or an analog measured value issued in the sensor. Multiple target capability is not usually required due to the small sensing range and the well-focused transmitter beam.

Triangulation and PRT offer a measuring principle that directly measures the distance to the object and is largely independent of the reflection characteristics of the object. The user is free to configure both sensors in various ways using an IO-Link interface. Put simply, a measuring sensor is a sensor with defined switching properties. The extremely cost-effective IO-Link provides the perfect interface between analog sensors and the controller environment. The IO-Link is downward compatible to the classic switching output yet still facilitates the transfer of measured data and parameters in real time.

Outlook

PRT technology has succeeded in providing a time-of-flight measuring principle that is both suitable for compact sensors and acceptable in price. The degree of accuracy is adequate for many applications and the costs are almost as low as those of a high-quality photoelectric sensor. As the popularity of these sensors increases over time, the costs will decrease even further and the common background suppression diffuse mode sensor will ultimately be replaced completely.

Moreover, transferring the principle of triangulation to a large-surface receiving array and a light spot projected in the shape of a line results in the laser light method, which in industrial vision is an extremely popular method of measuring heights in 2D. The height profile of several points on the object is measured along the projected laser line at the same time. The possibility of using this principle in optical sensors has only just begun to be explored. Further developments are eagerly anticipated.