Various applications in different fields, such as structural health monitoring, civil engineering, and manufacturing industries primarily employ displacement measurement in their quality control and production processes. It helps manufacturers ensure that the dimensions of a product are accurate and it performs exactly as required.
Laser displacement measurement systems are known to provide non-contact, high-speed and high-precision measurement. They enable stable measurements to ensure consistent product quality and continuous production. For example, in some applications, test stands, components and environmental conditions can generate EM noise, which tampers the measurement accuracy. Also, in components with complex surfaces and geometries, it becomes very hard to detect distance with contacting sensors. In such cases, non-contact laser displacement measurement is employed to provide results without any signal integrity loss. The sensing transducer can be embedded in close proximity to electromagnetic noise-generating components in order to return distance to objects in its field of view.
In this article, we will discuss laser displacement measurement in detail, including the measuring principle, benefits offered, types of sensors and their selection guide and some industrial applications of these sensors.
A laser displacement measurement can be carried in different ways, depending on the application requirement. CMOS technology is often used for short-range operations and utilises hundreds or thousands of pixels arranged in a line.
The measurement is based on the Laser Triangulation Principle which includes a CMOS detector and a solid-state laser light source. Here distance is measured by angle calculation. A laser beam is projected on the target under measurement and a part of this beam is reflected via focusing optics onto the detector. As the target shifts, the laser beam moves on the detector. A lens focuses this reflected light into a small spot onto the CMOS detector. The received light is then processed through analog and digital electronics and analysed by the digital signal processor.
Triangulation devices, such as displacement sensors based on CMOS technology, may be built on any scale, but the accuracy falls off rapidly with increasing range. These sensors are ideal for machine integration, factory automation and component inspection applications.
The Time-of-flight Principle is often used in long-range displacement measurements. In this method, the distance is measured based on the time in which the emitted laser beam returns to the sensor after hitting the target. The time of flight sensors use a transmitter diode to generate a very short pulse of narrow-spectrum red or infrared light which reflects from the target object. This reflected light is received by a sensitive, laser energy detector, which is also known as receiver diode.
Another approach of measuring long-range distance is Phase Measurement. A laser beam with sinusoidally modulated optical power is sent to the target object. The strength of the laser is varied rapidly to produce a signal that changes over time. Also, the time for a single round trip is not measured directly. The signal from the laser is compared with the delayed signal returning from the target to indirectly measure the time delay. This phase-comparison approach is generally ideal to measure range up to 150m.
The increasing adaptation of laser displacement measurement technology has changed the whole test and measurement scenario. This is due to the advantageous features and benefits the technology has to offer.
Highly reflecting surfaces such as reflecting plastics, mirror glass or polished metals are generally difficult to measure, due to light getting reflected back into the sensor itself. The real-time surface compensation feature allows the degree of reflection from the measurement object to be compensated for during the on-going exposure and in real-time. The active surface compensation feature ensures stable control regardless of color & brightness. The sensors are capable of quickly adjusting the laser intensity based on target color and surface finishes. Automatic laser power and measurement speed control enable reliable measurement under changing or challenging target conditions. In fact, even with the change of surface from black to white the signal shows no sudden changes or erroneous data.
Measurement in Small Targets
During measurements, the sensors measure distance that is roughly equivalent to the typical surface location within the laser spot. In order to measure effectively in uneven surfaces, the laser spot must be 25% smaller than the feature being measured. Therefore, the laser beam projected by the laser displacement sensor generates a small light spot on the target surface, which is required for a high spatial resolution. Spatial resolution is the measure of the smallest object that can be resolved by the sensor. With a small light spot, better spatial resolution can be attained. The smaller the measuring range, the smaller the light spot.
The laser line sensors use an oval light spot and special software algorithms to filter out disturbances caused by surface roughness, defects, indentations or tiny holes on metallic surfaces. Additionally, the laser emitter allows objects to be detected in holes or recesses where the reflections from the side walls would make it impossible for other contact-based sensors.
Mechanical measuring devices can often be too slow to measure or may require the operator to carefully position the device to obtain a reading. As compared to other distance measurement systems, laser displacement measuring systems are able to attain high-speed measurements capturing more than 4000 measurements per second. The target can be moving during the inspection as in case of a high volume assembly line measurement application. Additionally, the sensors can provide repeatable measurements without the risk of moving the object and impairing the final result. This makes the sensors suitable to be used in measuring time-varying distances.
Multiple Measuring Ranges
Laser displacement sensors are available in various measuring ranges from 2–1000 mm that covers a large number of applications across many different industries. This means that some of these sensors can be used to measure from a large distance to the target using a very small light spot. These special sensors enable non-contact displacement measurements in difficult and dangerous surfaces such as hot metals.
A production process involves high-speed measurement of various physical parameters to ensure that a high-quality product is manufactured. A laser distance sensor enables smooth and non-contact measurements to comply with regulations, conditions, standards and parameters associated with the product. Therefore, different configurations of laser sensors are available for the measurement of various parameters such as thickness, gap, height, position and distance.
A profile sensor based on laser triangulation principle is suitable for measuring the thickness of even very fine strips and sheets made of metals, plastics, wood and many other materials.
The gap measurement is best performed with a point laser sensor or a laser scanner. Depending on the inspection situation, a single scanner applied on a robot arm can measure different gaps in a static or dynamic mode. The laser gap measurement is performed in various applications such as automobile interiors, bearings, and many glass materials.
For measuring the height of different platforms such as dams, elevators, humans or concrete blocks, long-range laser sensors are employed. The sensors can be installed remotely in a control cabin, away from direct sunlight or conditions that can lead to corrosion.
Position and distance measurement is performed where components need to be aligned, adjusted or fitted in a certain position. The blue laser sensors are highly suitable for position measurement due to their sharp projection of the light spot. The sensors transmit the distance information to the controller or perform the complete system regulation with its signals.
Laser Displacement Measurement can be performed using sensors with various configurations. The choice of the laser displacement sensor depends on the following factors.
Measuring Range and Stand-off Distance
There are laser sensors available for both short and long measuring ranges. Laser triangulation sensors measure from a large distance to the target using a very small light spot. Even in difficult surfaces such as hot metals, these sensors are able to perform reliably providing stable measurement results. Check out the selection guide for laser triangulation sensors.
Then, we have long-range laser sensors or laser range-finders to perform measurements from a safe distance. These are ideal for measuring ranges up to 3,000m.
Level of Precision Required
A laser sensor uses a red laser diode as the source. It penetrates deeper into the target surface, which becomes a problem in situations when accuracy cannot be compromised. In these situations, a blue laser sensor is preferred. The sensor generates a much more focused laser to the surface and does not penetrate as deep into the object. This is why blue laser sensors perform well when measuring objects with shiny, reflective or highly-polished surfaces. For matt or low reflective surfaces, a red laser sensor can be used.
Size of Laser Spot
To measure effectively on rough surfaces, tiny holes on metallic surfaces, defects or indentations, sensors with oval light spots are employed. The oval light spot and special software algorithms filter out disturbances caused by such surfaces. For smaller measuring ranges, laser sensors with smaller measuring spots are preferred.
Position Measurement in Automobile
Almost every industry is now employing laser displacement measurement systems in its production processes. The technology is used in the implementation of industrial automation for precise and reliable profile measurement of components. In the automobile industry, several positioning tasks are performed using laser displacement sensors. For instance, the position is measured in giant gantry cranes to make warehousing easier and convenient. Even while positioning car components, these sensors ensure that every component such as windows, car body, and engines is positioned and applied correctly.
Plasterboard Thickness Measurement
Products such as chipboard, plasterboard and paper require stringent thickness measurement to meet the required specifications and minimise the wastage of the amount of raw materials used. Non-contact laser displacement measurement systems are compact and can be easily mounted in the manufacturing site to attain the exact thickness of the plasterboard.
Positioning in PCB boards
In a PCB board, laser sensors measure whether any excessive or incorrect soldering is done that may cause the board to become bowed or not straight edgeways or if any components are missing on the board. Operations such as this can occur at high speed and are much more accurate and repeatable with a sensor than having a human operator perform the same tasks.
Edge Detection while Fabric Cutting
Fabric Manufacturers laser sensors for cutting the fabrics into the precise size to reduce material waste. The sensor is directly mounted on one of the cutting blades and determines the exact position of the materials before processing.
Gap Measurement of Car Bodywork Parts
In car interiors and bodywork parts, the design plays a major role along with technical aspects to ensure customer satisfaction. Hence, certain minor flaws such as different gap sizes on both sides of a bonnet and a door that doesn’t close flush, the width and height of a gap and the gap between the single cockpit elements cannot be overlooked. Laser sensors are employed in such applications to provide output of values such as gap width, flushness, angle, etc with a single fast measurement within a fraction of a second.
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