You’re now familiar with contact methods for displacement measurement. In this article, George covers contactless options, including ultrasonic, radio and laser ranging.
Last month in Part 1 we reviewed major techniques of displacement measurement by contact methods. This time we’ll discuss contactless methods. The three best known principles used in robotics, control loops, process control, diagnostics, ranging, and other applications are: ultrasonic, radio, and laser ranging.
ULTRASONIC, RADIO & LASER
Ultrasonic distance measurement works by emitting pulses of ultrasonic energy while counting the time for their echoes to arrive. Then, from the known speed of sound, the distance the pulse has traveled to the target and back can be calculated. The standard speed of sound in air is 343.2 m (1,125.9′) per second. However, the speed varies with the air pressure, temperature, humidity, and the type of the media carrying it. In water, for example, the sound travels about 4.3× faster than in air. In iron, it travels approximately 15× faster than in air.
A well-known example of ultrasonic ranging is the original Polaroid One Step camera. Today, ultrasonic displacement measurement is utilized in systems where mechanical coupling of the object and the transducer would be impractical. Typical applications are the Navy sonar, fish finders, and gauging the level of materials (e.g., liquid, grain, and sand) contained in vessels and so forth. Ultrasonic frequencies are used extensively in medical applications such as sonography.
Unfortunately, ultrasonic measurement in open air can be corrupted by many factors, such as air movement, so a system designer must keep this weakness in mind. To reduce dependency on environmental factors, a “chirp” (i.e., a transmission of a modulated burst as opposed to a single frequency burst) is frequently employed. Ultrasonic frequencies used for ranging run from a few kilohertz in some sonars to hundreds of kilohertz, depending on the application. In all contactless displacement measurement techniques, the higher the frequency, the better resolution can be achieved, but at a penalty of increased signal attenuation leading to decreased range.
When the absolute distance is not important but its relative change is, Doppler effect (e.g., in ultrasonic or microwave intrusion detectors) is often employed. The effect is a frequency shift of the reflected signal, when the object moves relative to its source, be it sonic, radio, or light. The frequency-shifted echo is usually combined with the transmitted frequency, generating a beat frequency, which is further processed.
Light and radio waves travel at the speed of 299,792,450 mps, commonly rounded up to 300,000,000 mps or 186,411.4 miles per second. Radar is the most widespread application of radio frequency ranging. Due to the high speed of light and radio waves, sub-nanosecond, sophisticated electronics are needed for short distance ranging. Yet, the high speed of electromagnetic waves notwithstanding, resolution and precision down to millimeters is available with some equipment.
While studying laser ranging you are likely to encounter an acronym LIDAR. Analogous to RADAR, it stands for “Light Detection and Ranging,” and even its operation is similar to radar. Using a rotating mirror, LIDAR system scans a given area with a laser beam, evaluating its reflections. This technology is used for making high-resolution maps and other applications, many of them military. The older, so-called incoherent LIDAR method evaluated the amplitude of the reflected beam, transmitted by a very powerful laser. The new, coherent method determines the reflected beam’s phase shift, needs less power, and the laser is generally considered eye-safe.
TIME OF FLIGHT
The ranging method where a radio or a laser beam pulse is transmitted and the time for its reflection to arrive counted is often called “Time of Flight” method. The distance between the rangefinder and the measured object D can be calculated:
c is the speed of light. t is the time for the round trip from the rangefinder to the object and back. In terrestrial applications, due to the high speed of radio and light waves, the delay from the transmission to the reception is commonly measured as phase shift:
φ is the phase delay caused by the beam traveling to the object and back. Ω is the angular frequency of the wave. The angular frequency is related to the ordinary frequency, f, in Hertz as is also shown in the equation.
MULTIPLE FREQUENCY PHASE SHIFT
Besides the time of flight, “Multiple Frequency Phase-shift” method is sometimes used. For better accuracy, the phase shifts of multiple laser frequencies, reflected by the ranged object are measured and averaged. For tracking changes in distance, but not suitable for absolute distance measurement, interferometry is the most precise method. Interferometry is useful for investigating any type of waves. In a nutshell, the waves (i.e., transmitted and then reflected by an object) are superimposed on each other and the differences are studied. The process is performed with the help of sophisticated electronics and optics.
This technology is used in a wide variety of applications from golf ball distance measurement to military systems. SICK AG, for example, is a German company innovating in the field of laser distance and displacement measurement. Figure 1 shows the principle of their precision short displacement measurement instrument, which does not rely on the time of flight, but rather on trigonometry with the help of optics. These particular laser sensors are available within ranges from about 30 mm (1.18″) up to 500 mm (19.68″) and, depending on the design, can achieve resolution as high as 1 µm (about 0.039 mils).
A laser beam is projected onto the measured target while its image is “observed” through a lens and projected onto a sensor. The position of the measured object is derived by triangulation. Several different types of sensors are used: Position Sensitive Detectors (PSD), Complementary Metal Oxide Semiconductors (CMOS), and Charge Coupled (Devices CCD).
A PSD is a photodiode with a light-sensitive striped area. It generates two output currents whose ratio indicates where the laser beam image is striking its surface. The PSD sensor is very small and cost effective. CMOS sensors comprise light sensitive pixels and the position of the laser beam is evaluated by its brightness. CMOS sensors provide high resolution and accuracy for different object materials. They are very fast and exhibit great reliability. CCDs also offer high accuracy. Their sensitive elements are arranged in lines and thus indicate the beam position. The CCD sensors are fast, but not as fast as CMOS. They are also the sensors of choice in many digital cameras for autofocus.
Numerous rangefinders use optics to measure distance by triangulation. Typically, they have two lenses offset like eyes. Such devices may be active, where one lens projects a beam onto the object, while the other focuses on its reflection. Passive rangefinders focus both lenses on the object. Knowing the angles of the lenses, the distance can be calculated with the help of trigonometry.
Most digital cameras feature autofocusing, which is a closed-loop feedback system with a rangefinder and a motor to move the camera lens into focus. I already mentioned the Polaroid camera equipped with an ultrasonic rangefinder. This was just a dead-reckoning system. The lens was adjusted for the measured and calculated distance without a feedback to ensure the image was in focus. Thus, a picture taken through a window pane could be out of focus. Later models using infrared light and triangulation were not subject to this particular problem, but still did not ensure the image focus.
Currently, the two commonly used autofocus systems employ phase measurement or contrast analysis inside a closed-loop system. The phase measurement method uses two small lenses in opposite sides of the image-forming lens. When the image is out of focus, the beams from the two small lenses hitting electronic sensors are out of phase. The phase polarity indicates which direction the lens must be moved to attain focus. The principle is shown in Figure 2.
This electro-optical system comprising beam splitters, micro lenses, and apertures delivers two beams from the opposite sides of the main lens to two sensor arrays, one for each beam. The sensors’ signals are analyzed according to their intensity and phase. As can be gleaned from Figure 2, the resulting signal indicates the direction the lens must be moved to focus, as well as the magnitude of the focusing error.
In contrast, the other autofocusing method works by comparing the intensity of the adjacent pixels of the image which increases with improved focus. This method does not indicate which direction to move the lens; therefore, the lens must be moved through its entire range to find focus. It is used in many simpler digital cameras and is the cause of the annoying delay between squeezing the trigger and actually shooting. As this technique does not involve distance measurement, it is outside the scope of this article.
This wraps up our review of major contactless displacement/distance measurement methods. Next month, to conclude the series, we’ll review major position detection methods.
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • NOVEMBER 2016 #316 – Get a PDF of the issue