Measuring Distance with Laser Sensor Technology

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Manufacturing inspection applications that once required simple presence and absence detection of an object now ask sensors to solve demanding measurement and quality control tasks. Obtaining accurate and stable measurements is crucial to ensure consistent product quality and continuous production.

Laser sensor technology can solve these inspection applications with high-speed, high-precision performance. It can be used on multiple materials, reflective surfaces and colors, allowing manufacturers to collect continuous measurements in a range of industries, including applications with moving processes, stamped or machine parts, and soft or sticky parts.

Advanced laser sensors comprise a rugged, self-contained housing, a pinpoint laser emitter, a linear imager and user configurable outputs. Laser sensors require no external controller for adjustments. Operators can simply place the laser sensor in any location—including inaccessible areas of the machine or harsh environments—and make all necessary adjustments and configurations through various software tools.

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Adapts to Changing Targets
Automatic laser power and measurement rate control ensure reliable measurement under changing or challenging target conditions.

Linear imagers
The linear imager is one of the primary components of a modern-day laser distance sensor, defined as the eye of the sensor, and is made of hundreds or thousands of pixels arranged in a line. Some advanced laser sensors operate based on the principle of optical triangulation, which incorporates the linear imager. The linear image is used to detect precisely where the target is in front of the sensor—ultimately resulting in an accurate, stable measurement. A laser emitter transmits visible laser light through a lens, towards a target or object. The laser light is reflected diffusely from the surface of the target, where a receiver lens on the sensor then focuses that reflected light, creating a spot of light on the linear imager.

optical-triangulation-graphic

Theory of Operation: Optical Triangulation Graphic
Advanced laser sensors operate based on the principle of optical triangulation, which incorporates the linear imager, identifying where the target is in front of the sensor to achieve an accurate, stable measurement.

The target’s distance from the sensor determines the angle the light travels through the receiver lens; this angle then determines where the received light will hit the linear imager. If the target is far away (at the maximum specified range), then the light will fall toward the end of the imager closest to the laser emitter. Alternatively, if the target is at its closest position (at the minimum specified range), then the light will land at the opposite end of the imager farthest away from the laser emitter. The position of the light on the linear imager is calibrated in the factory for all valid target distances. The received light is processed through analog and digital electronics and analyzed by the digital signal processor (DSP), which determines the distance to the current target relative to the start of the measurement range very precisely by calculating the location of the received light on the linear imager and updating the sensor output to indicate the correct target distance.

Configurator software
A benefit of modern laser sensor technology is a configurator software tool, which allows straightforward, simple measurement applications and a resource to access tools for more complicated measurement applications. Configurator software makes it easy to set up the sensor and monitor its performance when adjusting sensor parameters. This software, which allows operators to control the sensor through a PC, provides data acquisition tools to graphically display measurement results. It also lets operators set measurement parameters, such as analog input scaling, averaging and sample size.

Configurator software can provide step-by-step actions to set-up unique measurement applications, such as using one sensor for displacement measurement, or pairing two sensors for a thickness measurement. For example, to obtain thickness measurements, two laser sensors self-synchronize together, requiring no external controller to adjust the sensors’ measurement rate or to calculate the thickness. The measurement data is communicated through a serial communication output, which is then displayed on the PC through the software. The thickness measurement can also be directly consumed as a 4-20 mA output for use in a PLC or data acquisition card. By providing graphical images during each step—including placement of wiring and connections in relation to the position of the sensor—the software enhances sensor usability. Advanced applications including a network of sensors for a mixture of displacement and thickness measurement is also possible.

Benefits of the technology
Laser sensor technology offers many benefits over traditional mechanical measurement devices, including non-contact measurement, small measurement area, high-speed data collection, solid-state design and flexible operation.

Non-contact measurement
With non-contact measurement, laser sensors do not experience any mechanical wear or contact with the target. Further, the target or object can be moving during the inspection, such as during tire run out measurement or measuring parts on a high volume assembly line. Non-contact measurement also increases sensor use on a wider variety of materials. Since the sensor does not impact the object, operators can easily obtain precise displacement or thickness measurements for soft or easily deformed materials, including rubber, plastic or wood—all without deforming the part or affecting the accuracy of the measurement.

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Non-Contact Measurement
Non-contact measurement allows operators to easily obtain precise displacement or thickness measurements for soft or easily deformed materials.

Small measurement area
Today’s advanced laser sensors feature a pinpoint laser emitter to measure even the smallest part features or targets. Plus, the laser emitter can easily align to a feature of interest on the target, quickly providing the measurements needed in the area of critical interest.

High-speed data
In comparison to mechanical measurement devices, such as a contacting probe or caliper, laser sensors have the capability to attain high-speed measurements—with some capturing more than 4000 measurements/sec. Mechanical measurement devices are often much slower, requiring the operator to carefully position the device to obtain an accurate reading. Laser sensors provide repeatable measurements without the risk of moving the object and impairing the final result. High-speed data capabilities also allow the sensor to be used to measure time varying distances. For example, a laser sensor can measure the vibration of a rotating shaft in real time to characterize its performance or indicate the need for maintenance.

Solid-state measurement
Laser sensors are built to last with a robust housing, allowing them to be mounted in heavy-duty industrial applications. Without the mechanical probes found in alternative measurement devices, there is minimal risk of damage to the sensor. Solid-state laser sensors do not generally require periodic calibration, reducing costs.

Flexible operation
Laser sensors offer increased performance with the ability to quickly adjust the laser intensity based on target color and surface finishes. For example, a laser sensor can measure the stack height of a rainbow of paper colors, ensuring the correct quantity is packed, on a high-speed production line. Or it can measure a stamped metal part without being affected by part color variation due to a surface heat treatment process. Automatic laser power and measurement rate control ensure reliable measurement under changing or challenging target conditions. Plus, to meet application requirements, the measurement rate can be fixed when a constant measurement output rate is necessary.

Downsides of the technology
While laser sensor technology provides an accurate measurement solution for a variety of industries, it does have one significant drawback from traditional mechanical measurement devices—cost. Laser sensors are more expensive than a caliper or mechanical micrometer, or a contacting LVDT displacement probe. In addition to the cost of the device itself, the added flexibility laser sensors provide beyond simple mechanical inspection of measuring one part and recording the data results in greater set-up complexity. As described earlier, however, some of today’s laser sensors include software that reduces start-up and run-time costs by providing step-by-step actions to assist with sensor set-up and the overall operation.

Application types
Laser sensor applications fall under three major categories: quality control, error-proofing and positioning. Quality control applications involve a process or machine producing a specific part, and measuring the part to make sure it meets quality requirements.

For example, in drywall manufacturing processes, it is important for operators to check for uniform thickness of drywall sheets to meet certain industry standards, including core and edge thickness, straightness of edges and length. To ensure these standards are met, a network of laser sensors is strategically placed to make continuous thickness measurements of the sheet. If any of the paired thickness sensors indicate a measurement that is out of spec, the sensor output can be used to immediately make online adjustments to the manufacturing line by adjusting a roller position with a servomotor. Online process control helps to eliminate scrapped material and improve the overall quality and consistency of the product.

thickness-measurement

Thickness Measurement
To obtain thickness measurements, two laser sensors self-synchronize together, requiring no external controller to adjust the sensors’ measurement rate or to calculate the thickness.

The second application category is error-proofing, such as inspecting metal thickness before it is inserted into the stamping press. To prevent damage to the stamping press and to avoid making bad parts, the operator has to load the correct sheet metal thickness into the press to create the exact part. A pair of laser sensors operating in synchronous mode can measure the thickness to ensure the correct material is loaded to preserve continuous and high quality operation. Another example of error-proofing is seen on assembly lines. A laser sensor is used to inspect for a missing part or to measure an individual component to ensure each element in the assembly has been installed correctly, resulting in a consistent stack height at a critical inspection point. This not only guarantees product quality, but also limits potential costs associated with the re-work or repair of missing or incorrect parts.

Laser sensors are also used for positioning applications. They measure the position of a machine part to ensure closed-loop process control during the manufacturing process. For example, a laser sensor can measure a specific surface on an air-powered cylinder to verify its position—confirming that the cylinder extends and retracts to the same end points each time. This ensures that that the manufacturing process is being performed correctly, accurately and reliably—minimizing production downtime and scrap.

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