By Dr. Peter Adolphs, Managing Director
With new sensor elements and the increasing performance of microcontrollers, more efficient sensors can now be produced at a reasonable cost. This means that sensing techniques previously used only in measuring systems are now becoming useful in standard applications. However, the user has to address the question of how to make efficient use of the immense functionality of these sensors.
Blurring the dividing line between conventional photoelectric sensors and distance measurement solutions
In the field of optical sensors, there has been a clear trend towards measuring sensors that explore intrinsic light properties like phase and speed of light. It is fairly common to talk about these as time-of-flight (TOF) sensors. Low-cost devices, with integrated ASIC to determine object distances and based on the phase correlation principle, have been available for some time now. However, these devices have certain disadvantages in terms of measuring accuracy, sensitivity to extraneous light, their ability to suppress erroneous reflections from targets that are in the background, or their ability to process distance data from multiple simultaneous targets. These issues can be overcome reliably only with a more direct, true time-of-flight measurement of a light pulse.
With Pulse Ranging Technology (PRT), high-speed electronics are used to directly measure the time between emission and reception of a short, high-energy light pulse. Due to the high energy density of the pulse, this approach offers substantially increased immunity to extraneous light. Moreover, the ability to receive multiple reflected pulses makes it possible to discriminate accurately between several targets in the sensor’s field of view.
Modern microelectronics have driven down the cost of producing such devices, thus blurring the classic dividing line between light detecting sensors with a variable sensing distance and analog sensors. In fact, it is possible to use a PRT sensor either as a light-detecting sensor or as an analog sensor. It depends on the application as to whether the switching outputs should operate on reaching an arbitrarily configurable sensing distance, or whether the distance value itself should be sent directly to the PLC. In addition, the sensor offers many configuration options that allow it to be adapted to the application in question. Thanks to powerful microcontrollers, this range of functions can now be realized with minimal cost added compared to conventional long-range photoelectric sensors. With the ability to provide either a set point driven on/off output or distance analog data, users are faced with the new challenge of overcoming the need for a simple yet powerful interface with the PLC.
This interface must be able to transmit on/off switching signals as well as analog values quickly, allowing the transmission of configuration parameters and diagnostic data while keeping additional costs at a minimum. Only when these requirements have been addressed can this range of functions be performed with one device that’s suitable for all cases. For many years, automation professionals felt that a fieldbus like PROFIBUS or DeviceNet, and more recently, EtherNet would address this need. While these solutions are, in theory, capable of addressing all such needs, they are far too expensive to be included at the level of an ultrasonic or photoelectric sensor. In addition to the cost problem, the question of the physical interface, the additional special connectors required by these fieldbus technologies, made them unsuitable for the task.
Only when IO-Link appeared on the market did an interface become available that was suitable in terms of signal transmission technology, required processing power on the sensor and physical connectivity, resulting in minimal additional cost compared to sensors that offered just a conventional switching output. The serial bidirectional transmission system uses the signal level of a conventional electronic switching output, allowing it to transmit digitized analog values and parameters at 38.4 kBaud. In those cases where IO-Link-enabled sensors are connected to conventional PLC inputs, the sensor automatically reverts back to a conventional digital I/O operating mode.
IO-Link addresses the cost and space problems of fieldbus communication systems, since it takes advantage of the conventional M12 connectors together with conventional unshielded cable and addresses the compatibility issue when used with existing, non-IO-Link hardware. Since the vast majority of I/O cards and field-mounted I/O blocks are not yet supporting IO-Link, many of its interfacing benefits cannot be used once the sensor has been installed. The new SmartBridge technology attempts to address this shortcoming by intercepting the diagnostics data and transmitting it to a tablet (or smart phone) for visualization. Using the tablet, users will also have the ability to configure sensors using graphical elements.
The two-dimensional sensor element
Similar developments are taking place in sensor technology based on image processing techniques. Just as PRT sensors, these so-called vision sensors benefited from numerous engineering advances, along with massive price drops in powerful microelectronic components. As a result, high-end technology is now affordable and available in compact “sensor-like” form factors.
In the folding, binding and collation of print products, quality requirements are rising all the time. It is vital to ensure that the sheets are assembled in the correct sequence and that faulty sheets are removed.
Inside the sensor, complex image processing takes place at high speed. From the outside, the sensor behaves like a simple on/off switch that tells the machine whether the sheet is correct. This begs the question: Has the smart plug-and-play sensor finally been achieved? In theory, yes, but to work efficiently with any of the post-press systems, the sensor must be tightly integrated into the controls of the folding, collating and binding machines.
During set-up, it is desirable to give the user the opportunity to see the camera image, allowing lighting, focus or simple mechanical adjustment problems to be detected quickly. To optimize the print process, it is also helpful if the control system can access statistical information collected by the sensor during normal production runs. Lastly, it should also be possible to reconfigure the sensor remotely from the control system.
These three examples alone show that, for optimum integration of the sheet inspection sensor, a video interface and a fast serial interface are also needed. While a sensor that supports these features may be called “smart,” the interfacing question remains. How can the necessary data be transmitted effectively? The solution becomes smart only with the addition of a suitable interface technology, and Ethernet has become the de facto standard for applications in this performance class.
Ethernet is reasonably affordable, already widely used in industrial environments and offers sufficiently high data rates, so that even live-image transmission poses no problems. Combined with more capable wireless technologies and the increasing popularity of powerful tablet devices, it is expected that even high-performance hardware will soon take full advantage of graphical configuration and troubleshooting methods.
The smart interface for smart sensors
These examples make it clear that smart sensors will only be as good as the interfaces connecting them the control system, PLC and diagnostics device. With IO-Link enabling simple devices like ultrasonic and photoelectric sensors to transmit vastly improved process and diagnostics data at the low-end of the performance spectrum and Ethernet transmitting complex information with high-end requirements, two device interfaces are now available that cover essentially all application areas. It would be in the best interests of users and manufacturers if these interfaces were to become global standards.
Pepperl & Fuchs
www.pepperl-fuchs.us