Analog distance measurement with photoelectric sensors

Balluff Pty Ltd
Friday, 26 November, 2010


This article discusses two powerful distance measurement photoelectric sensor technologies: triangulation and time-of-flight.

Now that high-performance sensors are more affordable than ever, many manufacturers today are looking for ways to improve their industrial process control without investing a lot of time and money. Measurement sensing technology has stabilised in price due to manufacturing improvements and increases in production volume. This means that continuous position feedback can be implemented at a reasonable cost, thus allowing process owners a new level of flexibility and control to create highly profitable solutions.

Modern photoelectric sensors offer more useful industrial solutions, handling a wider range of functions than ever before. The longer range characteristics of contemporary photoelectric sensors have enabled new designs to operate in environments that would have destroyed conventional photoelectric designs just a few years ago. Advancements in emitters, receivers, lenses and processing circuits have led to products with not only longer ranges but better resolution and higher switching frequencies too.

Measurement using triangulation

 
Figure 1: Distance sensing using triangulation.

Distance measurement photoelectric sensors utilising triangulation technology use the laws of geometry to assess how far a target is from the sensor. In Figure 1, the emitter is sending light towards the target through its outgoing lens. The target reflects light back to the receiver, and as its position changes, so does the angle at which the light comes back through the receiving lens. This returned light will strike the receiving element at a different position based on how far away the target is from the sensor. Processing electronics convert this position into a highly accurate analog output.

Sensors with this technology are capable of long sensing ranges, limited only by the size of the lenses and the separation between the emitter and receiver in the sensor housing. The emitters for these types of sensors are typically standard visible red or highly focused laser LEDs. Resolution is directly related to the receiving element’s ability to discriminate the angles of incoming light. The two most common types of receiver elements found in these sensors are the PSD (position sensitive device) and the CCD (charged-coupled device).

Position sensitive devices (PSDs)

In general terms, position sensitive devices are diodes with extended light-sensitive surfaces. When light strikes the diode, it converts this energy into an electrical current that is divided into two parts, based on the point on the diode where the light strikes. A processing circuit then converts these partial currents into the analog output of the sensor.

  

 
Figure 2: A typical PSD element - light-sensitive edges are oriented to maximise light capture.

Charged-coupled devices (CCDs)

Unlike PSDs, which are a single sensing element, charged-coupled devices are an array of detectors with capacitors connected to each element. The presence of light on the detector causes the capacitor to charge. Independent of this light detection, a processing circuit is continuously cycling through the detectors looking for a change in intensity at each element. This distribution of the light intensity on the array is then converted to the analog output signal.

The question of performance between PSDs and CCDs has been the source of debate for many manufacturers and customers alike. Both technologies have been continuously improved for many years and are very effective at providing an accurate measurement of the distance to a target. The key differences between the two technologies are:

  • PSDs are best for applications that require a faster response time.
  • CCDs are best for applications that require a better resolution.

There is also a trade-off in resolution versus sensing range, independent of the receiving element used. Generally speaking, the longer the range, the less resolution that is achievable.

  

 
Figure 3: A typical CCD element - sensing elements are laid side-by-side to form an array.

Time-of-flight measurement sensing

A newer technology available in the market today is called time-of-flight sensing. Working similarly to radar, these laser sensors emit a light waveform with specific characteristics that can be marked. This light travels from the sensor, reflects off the target and returns. The amount of time it takes for the light to make this trip is recorded from the marked waveform, and the processing electronics convert this to an analog output.

 
Figure 4: Distance sensing using time-of-flight.

Time-of-flight sensing offers the best balance of long-range sensing and high resolution. Laser emitters are used in time-of-flight sensing because of the required high switching frequency and the need for a tightly defined beam to maximise the distance that can be measured. The difficult task of receiving these light waves is left to a special type of detector called an avalanche photodiode (APD), which features a high degree of internal amplification. Leading sensor companies offer a number of photoelectric sensing families that utilise the latest versions of triangulation and time-of-flight sensors. A summary of typical performance characteristics is shown in Table 1.

The bottom line

Common applications for these technologies include part contouring, roll diameter measurement, positioning of assemblies, thickness detection and bin level measurement. Today’s industrial measurement photoelectric sensors are finding new applications in manufacturing environments across the world.

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