Feature Machine Vision Systems
All-in-one displacement sensors T
As Dr Walt Pastorius, LMI Technologies, explains, laser displacement sensors measure the displacement (distance) from a sensor to a single point on an object using the triangulation principle. They are used in a broad variety of factory automation applications, including gauging, error-proofing and closed loop control
he first practical laser displace- ment sensors for industrial appli- cations became available in about 1971. These early devices were analog-based, using a solid state posi- tion sensing detector (PSD) as the imager. While in today’s digital world analog sensors are considered antiques, displacement sensors based on PSD imaging continue to be the sensor of choice for a number of appli- cations. These analog sensors offer very high measurement data rates capable of adjusting laser power in real time and achieving a high dynamic range of operation. PSD sens- ing, however, only functions when a single laser spot is formed on the imager. Multiple spots from reflec- tions, for example, produce invalid results on a PSD. To improve upon the PSD design, the next step was development of dig- ital-based displacement sensors, most commonly using a linear array imager with a single row of pixels. These dig- ital sensors are widely available from a large number of providers. At the low end, inexpensive sensors provide mid-level performance, suitable for basic applications.
At the upper end are sensors that provide measurement speeds in the range of 16 to 80kHz, with resolutions down to fractions of a micron. Examples of high performance dis- placement sensors are LMI’s SLS and Optocator families, which continue to be popular today for many demanding applications. It should be highlighted that the basic design of these sensors is over 30 years old - remarkable longevity in a dynamic market. All of today’s digital displacement sensors are data acquisition devices only, acquiring data which is commu- nicated to a host computer or PLC where it is analysed with application specific software developed by a system integrator. The external PC may also be needed to control sensor operation, providing triggering, expo- sure control and data processing.
Keeping things simple To dramatically simplify integration of displacement sensors, LMI is offering a new series of sensor based on the Gocator product line, leveraging the latest high speed digital imaging tech-
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nologies. All Gocator sensors offer a built-in web server that connects to common web browsers for ease of set-up and implementation, sup- porting multiple languages. Sensors can be configured and integrated from any computer or even mobile devices, regardless of operating system. No added soft- ware and no machine vision expertise is required. Once set- up is complete, the PC can be removed. Most importantly, Gocators run a real-time oper- ating system and can process measurements up to 100kHz with low latency. Most PC envi- ronments are not designed to handle acquisition interruptions beyond 1kHz and therefore require special I/O hardware.
G1 sensors eliminate the need for external hardware and related software development with built-in measure- ment tools. Measurement tools compute range data, calculate differences, apply sensor offsets and encoder calibration, and can make pass/fail decisions, all inside the sensor. Where applications require multiple sensors, they can be synchronised into a single network with an LMI Master.
Setting the correct sensor exposure is a critical step in sensor operation. The sensors are provided with two modes for setting exposure - single and dynamic. When measuring sur- faces of uniform reflectivity, the single or fixed exposure mode is used. For measurement of surfaces
which have varying reflectivity, the dynamic exposure mode will auto- matically adjust the exposure after each measurement frame, adapting to the changing surface to ensure accu- rate measurements. For ease of set- up, both modes are provided with an auto-set function, as well as manual setting capability.
Further features
Sensors are precision calibrated over the measurement range at the factory. The all-in-one design has all optical components rigidly mounted on a single mechanical structure to guaran- tee measurement stability in harsh factory settings.
Packaging is designed for industrial factory environ- ments, with IP67 rating. Two available compact form factors make it easy to mount sensors in congested areas. Weight, depending on model, is 1 to 2kg. Compared to current
Above and below: the G1 sensors are small and
lightweight and have a narrow housing
equivalent SLS and DLS sensors, G1 sensors are both smaller and lighter. For applications where displacement sensors are mounted on robots or moving vehicles, light weight and rigid sensor construction are impor- tant factors.
Communications with a host com- puter is provided by Gigabit Ethernet using standard PLC protocols (EtherIP or Modbus) or simple TCP/IP command and data exchange. Measurement values can also be com- municated by digital, analog and serial outputs, including Selcom serial (an LMI proprietary synchro- nous serial output). Scan triggering can be internal time-based, exter- nally triggered or driven from an encoder input. An open source SDK is available for users who wish to create custom solutions.
The G1 series represent the latest generation of displacement sensors. All-in-one sensing makes integration simple, and the built-in web browser makes integration easy - vision experi- ence is not required.
LMI Technologies
www.lmi3d.com T: +31 45 850 7000
Enter 200 MARCH 2013 Automation
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