Distance measurement using contact-type measuring tools has been used for decades in applications such as plumbing, land surveying, and construction work. However, the demand for quick and accurate distance measurements has spurred the growth of contactless-type distance measuring instruments, spanning ultrasonic, IR, and laser measurement techniques. In this article, we will discuss laser distance measuring technology, including its benefits, operation and types.
Contact vs Contactless Measurement
Distance measurement can be performed with either contact-type or contactless measurement devices. Contact-type devices such as Architect's scale, Caliper, Gauge blocks, Screw gauge, Vernier calipers, Tape measures, Thread pitch gauges, and Yardsticks are direct measurement devices. The drawbacks of contact-type measurement is that these devices have to be used in fixed locations which sometimes have poor visibility, or the presence of blind spots, dust, or debris that can impede the measurement. Known as Electronic Distance Measurement, contactless measurement methods use modulated waves or pulse signals of sound waves or light waves to calculate the distance. EDM devices have a wave transmitter, a receiver, and a controller. They calculate the distance of an object using the time difference between transmitted and received waves. Benefits of contactless measurement include, the ability to detect and measure moving objects, the detection of small objects over long distances, and the measurement of the distance to a wide range of objects regardless of their shape, color, or surface texture.
Types of Contactless Measurement
There are four main types of contactless distance measurement technologies. They include ultrasonic, radar, optical and laser. Ultrasonic distance measurement devices or sensors measure distance by taking advantage of sound waves. Ultrasonic sensors send out a sound wave at a specific frequency and listen for the sound wave to bounce back. By recording the elapsed time, once can calculate the distance between objects. This technology has limitations related to variables such as object size, material, and changes in temperature and humidity, affecting the accuracy of the measurement. In radar distance measuring devices, a short radio pulse with very high pulse power is transmitted. A small portion gets reflected from the radar when the pulse hits the radar. The radar antenna receives this energy and evaluates the time of travel to determine the distance. This technology is costly and is typically used for precise long-range distance measurements suitable for industrial, aircraft landing, submarines, and defense applications. Optical measuring systems use microwave, infrared (IR), and visible light waves to determine exact distances. IR distance measuring instruments can measure up to the 5 km with an accuracy of + or – 10mm per km. The optical waves may attenuate due to atmospheric conditions such as water vapors, carbon dioxide, temperature, barometric pressure, gaseous mixture, rain, snow, dust, aerosols, and bacteria. Modulation techniques are used to reduce the error and achieve accurate distance measurement.
Laser Distance Measurement
Laser measuring devices emit laser light through a process called optical amplification based on the stimulated emission of photons. This technology produces coherent light with high intensity. The laser beam is very narrow and concentrated on a small area. The laser beam color will vary with the wavelength of the light (i.e., red, 660 & 635 nm; green, 532 & 520nm; and blue-violet, 445 & 405nm).
Laser radiation can be created using gas, liquid, or semiconductor materials. Usually, semiconductor diodes are used in distance measuring instruments. A p-n junction of a semiconductor diode forms the active medium or laser medium. Based on the potential of causing injury to human eyes and skin, lasers are classified into various classes; class 1 and 2 are used for laser distance measuring instruments. Every laser distance measuring equipment manufacturer follows the ISO 16331-1 standard that specifies procedures for checking compliance with performance specifications of handheld laser distance measuring instruments.
Laser distance measuring tools have very smooth operation. These devices offer an adjustable sensing range and memory to store measured values in memory. These instruments can be used as sensors because measured values can be sent through IO-Link to host systems like PLC. Figure 1 illustrates the operation of a laser distance measuring instrument.
Figure 1: Block diagram of a laser distance measuring instrument
A laser emitter generates a pulse of light than through a lens. A receiver element (i.e., photodiode) detects the reflected wave and sends it to a signal processing unit. The signal processor converts the reflected wave to digital form and then sends it to a microprocessor. The microprocessor calculates the distance to the target and displays the measured values.
Laser Distance Measurement Operation
The distance calculation is done through precision optics and laser physics using the triangulation, time of flight, or phase-shift method. Triangulation method is used for short distance measurement and time of flight or phase-shift methods are used for long distance measurement.
Triangulation: Triangulation refers to a procedure in which a distance or positions are determined by measuring angles to it from known points (laser sender and receiver position). A laser transmitter projects a laser spot on an object, and the reflected light falls on the receiving element at a certain angle. The receiver lens creates a triangle shape on the light receiving element using reflected waves. Using angle and distance between transmitter and receiver, distance to the object can be calculated.
Figure 2: Triangulation of a laser wave
An adjustable field sensing technique can be used to adjust the range of measurable distance. Figure 2 shows two differently aimed detectors (R1 and R2). When the object is closer than the cut-off distance (Object A), the near detector (R1) detects stronger signals than the far detector (R2), and the sensor responds to the object. If the object is beyond the cut-off distance (Object B), the far detector (R2) light signal is stronger than the near detector (R1) light signal, and the sensor ignores the object. The actual cutoff distance for lower reflectance targets is slightly shorter than for higher reflectance targets at any given cutoff setting; this phenomenon is known as color sensitivity. Objects with a reflectivity less than 90% reflect less light back to the sensor and require proportionally more excess gain to be sensed reliably.
Time of Flight: In this method, using the reference speed of light and the difference between transmit time and received time, a controller (range finder) calculates the distance of an object using the formula.
Distance = ½ (speed of light x time)
Phase correction measurement process: In this method, distance can be calculated by comparing the phase of the emitted and the reflected wave using the following formula:
[Where d is distance, TIF is the period of intermediate frequency signal, TX is the time corresponding to received wave, and λ0 is wavelength.]
There are numerous advantages to laser distance measurement. They include the following:
• One person can take measurements independently, which reduces effort, time, and energy.
• The sensing function is not affected by environmental factors such as the wind, rain, snow, fog, light, humidity, and air temperature.
• The laser beam is always dead straight, so there is no bending or sagging, resulting in high linearity and accuracy of the measured value.
• IO link (Bluetooth or USB) allows transferring saved or measured values to a PC or PLC.
• The distance measurement is independent of the target surface.
• They provide accuracy even in short distance (millimeter distance).
• Laser measuring instruments are available in different sizes.
• They consume very low energy.
Examples of Laser Distance Measurement Devices
|Laser Sensor, Q4X Series||Laser Sensor, LM Series||Laser Sensor, WORLD-BEAM QS18LD Series||Laser Emitter, MULTI-BEAM® Series|