Having been involved in the machine vision industry for over 40 years, it has been fascinating to follow the many iterations of laser-based displacement sensing. These changes have included an evolution as it has evolved from analog to digital operation, and from basic capabilities to today’s simple-to-use all-in-one measurement systems. Laser displacement sensors, which measure the distance from a sensor to a surface, superseded traditional displacement sensors, primarily dial indicator gages and LVDTs (linear variable displacement transducers). These traditional sensors were based on technologies that used a mechanical contact on the surface being measured. Contact measurement techniques are very effective and inexpensive for many common applications, but for in-line and high-speed applications, contact sensors have serious limitations. These include limited frequency response, potential damage to finished surfaces, errors due to deformation of soft materials, and errors from tip wear and dirt contamination.
Displacement sensors, both analog and digital, rely on laser beams and triangulation to determine distance.
Analog Laser Displacement Sensors
To overcome the limitations of contact sensors, the first laser-based displacement sensors were commercially introduced in the early 1970s, implementing laser triangulation principles. Early displacement sensing devices employed analog solid-state position sensing detectors (PSDs) as imagers. In today’s almost totally digital world, most people would consider analog sensors antiques, suitable only for museum display. On the contrary, PSD-based displacement sensors continue to be the sensor of choice for many highly demanding applications because of their high resolution, high measurement data rates (70 kHz or more), and capability of adjusting laser power in real-time to maintain accuracy for varying surface reflectivity. It is amazing that the basic design of these analog sensors has remained essentially unchanged for over nearly 40 years – remarkable longevity in the rapidly changing technology world.
Laser-based displacement sensors overcome the limitations of contact sensors by high speed non-contact operation. Another advantage of laser-based sensing is that the laser beam can be focused to a very small “probe size”, as low as 0.025mm. This tiny probe size allows accurate measurement of extremely small features, such as surface roughness or the critical edge radius of a turbine blade.
Analog triangulation sensors do have some limitations, however. They are often bulky and only function when a single laser spot is formed on the imager. Multiple spots, from reflections or external light, for example, produce invalid results when using a PSD-based sensor.
Digital Laser Displacement Sensors
To overcome PSD limitations, the next step was development of digital imaging based displacement sensors, most commonly using a linear array imager with a single row of pixels. These digital sensors are now widely available from a large number of providers, with a broad range of specifications and prices. Most of today’s digital displacement sensors (as well as analog sensors) are data acquisition devices only, acquiring data which is communicated to a host computer or PLC, where it is analyzed with application-specific software developed by the system integrator. An external PC or other processing device is often required to control sensor operation, providing triggering, exposure control, and data processing.
Analog displacement sensors provide only an analog output of the centroid position of all light in the image, with no ability to process or display the image. A user cannot determine if the image is properly focussed, or if external light is affecting the measurements. With digital-based sensors, a video output of the full image is available, which can be processed and filtered to remove secondary reflections or other unwanted parts of the image, or to isolate specific regions of interest in the imaged spot. The latest generation of digital displacement sensors have the image processing and filtering capabilities located inside the sensor package. Thanks to video output capabilities, the image can be visualized on a display screen, which is extremely useful for setup and diagnostic purposes.
Continuing developments in displacement sensors leverage the latest high speed digital imaging technologies, featuring easy-to-implement all-in-one operation. These smart sensors offer a built-in web server that connects to common web browsers for ease of setup and implementation, supporting multiple languages. Sensors can be configured and integrated from any computer and operating system. Another feature of all-in-one operation is inclusion of built-in measurement tools to compute range data, calculate differences, apply sensor offsets and travel calibration, make pass/fail decisions, and communicate information in digital or analog formats--all from inside the sensor. No added software and no machine vision expertise are required.
Control of exposure time is very important for accurate data, particularly when properties of the surface being measured have varying reflectivity or roughness. Digital all-in-one sensors have built-in dynamic exposure control to automatically adjust the exposure after each frame, providing maximum dynamic range with precise measurements.
Where multiple sensors are required for an application, today’s all-in-one displacement sensors include the capability to easily synchronize multiple sensors and combine data from multiple sensors into a measured value, such as a differential thickness or height difference. Master hubs can provide power distribution, laser safety interlock, I/O handling, and microsecond-level data synchronization.
Turning Displacement Sensors Into Profilers
Today’s latest generation of displacement sensors achieve data rates of up to 32 kHz, with resolutions to the micron level. By leveraging high speed with on-board smart processing, displacement sensors can perform 2D profiling on rapidly moving products along the direction of travel that far exceed the speed of today’s high-end laser line profile sensors. This new capability of turning 32kHz displacement sensors into 32kHz line profilers offers a unique data acquisition solution to scanning applications and breathes new life into a mature technology.
Posted by Dr. Walt Pastorius