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Sensors Address System-Level Design Issues
System engineers benefit from simplified design
Randy Frank, Contributing Editor -- Design News, April 28, 2008
The ability to sense common or unusual parameters is a great start for any control system. However, those sensors that take into account system-related aspects distinguish themselves from the competition. While usually more expensive than units that just provide the sensing function, the cost is more than made up at the system level, in reduced time-to-market and other advantages. Sensors with design considerations for and specific focus on system-level details for interfacing, communication, packaging and ease-of-use make life easier for system designers and end users.
Interfacing Sensors
While many sensors include some or all of the identified key system factors, one aspect frequently stands out as a primary design consideration. In most cases, system-level integration, like many of the other design considerations, is not revolutionary but more of an incremental improvement to the company's previous generation of sensors.
System-level integration typifies a growing interface trend from several sensor manufacturers. In addition to its low-profile and surface-mount design that is compatible with standard printed circuit board (PCB) assembly methods, Kavlico's P6050 allows system designers the ability to select a standard analog voltage output or a digital serial peripheral interface (SPI) output.
Using a 5V dc supply, the amplified analog output is 0.5 to 4.5V dc, linearly proportional to pressure. With its integrated 10-bit analog to digital converter and digital output, the digital output version allows system designers to eliminate additional components. This option saves space, component cost and reduces overall design complexity.
Communicating Data
Communicating sensed information to the system through either a wired or wireless network is another system aspect sensor manufacturers have addressed with another level of integration. In some instances, the communication function is separate from the sensor but targets specific sensor types. For example, MicroStrain's G-Link® Micro Datalogging Transceiver system, combined with a MEMS accelerometer, provides a high-speed, wireless, triaxial accelerometer node, designed to operate as part of an integrated wireless sensor network system.
Triggering the sampling and logging of data from 70m is simplified with the bi-directional RF communications link. Additionally, the link can request the transmission of real-time data to a host PC for acquisition and analysis. The system can handle simultaneous real-time streaming from up to 16 nodes in the 2.4 GHz range.
Application-Specific Packaging
Packaging that addresses unique requirements of specific applications has been one of the first system-level capabilities implemented by sensor suppliers. In many cases, packaging determines whether the sensor will work in an application.
For example, Banner Engineering's WORLD-BEAM Q20 photoelectric sensors, in a IP67-sealed housing with industry-standard 25.4 mm mounting, delivers a variety of sensing modes. With a water-tight, small rectangular housing, the Q20 is rated up to 1,200 psi for wash-down environments and provides reliable sensing in a space-saving package.
Other application-specific packaging addresses system requirements with industry standard form factors and standard mechanical interfaces. Designed specifically for use in heavy-duty vehicle engine exhaust gas recirculation (EGR) systems, Honeywell's one-piece R300 Series temperature sensor has a M14 x 1.5 mounting thread and an integral connector. With a continuous working temperature range of -40 to 275C (-40 to 527F) and the ability to handle excursions up to 300C (572F), the unit also qualifies for sensing temperatures in other heavy-duty vehicle engine applications including oil, coolant, fuel and air inlet.
Similarly, Kavlico's P528 family of ceramic capacitive refrigeration pressure sensors also have a threaded body and integral connector but these units sense pressure in refrigeration compressors, rooftop chillers and refrigerant recovery systems. Handling pressures in the 0-15 to 0-500 psia ranges, the sensor has brass housing with a ¾-inch external hex and pressure ports that include 1/8-27 NPT, ¼-18 NPT and ¼ SAE female threads with Shrader deflators. The integral electrical connector is a Packard Metri-Pak 150.
Ease of Use
Perhaps the ultimate system level design feature is any aspect that makes the sensor easier to use. In spite of increased functionality that could have resulted in a vision sensor that was more complex and difficult to use, simplicity was a primary design aspect of Pepperl+Fuchs' VOS300 Series vision sensors.
Although the VOS300 combines a camera, illumination, digital outputs, process data and five evaluation methods into a single housing, the sensor is easily configured without programming knowledge and operated without the need for detailed image processing experience. Targeting inspection applications beyond the capability of traditional photoelectric sensors, the vision sensor provides a single discrete output indicating general “pass/fail” status. Using an Ethernet connection, additional data can be read from the sensor to determine the pass or fail condition of each feature check.
Addressing specific system requirements gets easier with improvements on existing products. This is the case with Cognex Checker 232. Compared to the previous Checker 202, the 232 has the same features, plus functions to inspect smaller features and a wider field of view. With a much longer working distance that allows mounting further from the inspection area, the sensor simplifies detection and inspection measurements without requiring a complete vision system.
With its PK pressure sensors, Turck demonstrates easily programmed sensors for pneumatic and robotic applications. Detecting the switch point in air and inert gas, in pressure ranges from -29.5 to 0 inches of Mercury (-1 to 0 bar) and 0 to 145 psi, the sensor uses simple push-button programming. The menu can be inverted for additional programming flexibility.
As sensor designers add more attributes to sensors that address system-related issues, system designers can focus on even more advanced systems.
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