"The Five Senses of Sensors" is a five-part series discussing the availability and advances in sensors that mirror and mimic human senses. This first segment provides an overview of sensors used in applications requiring sound.
Sensors that detect sound or "hear" are simply microphones. While we are familiar with the dynamic version as it represents the common "microphone" in the music industry, electrostatic and piezoelectric sensors are also used in measurement and as detectors in such applications as industrial, medical, robotics, and identification and tracking.
Electrostatic microphones are frequently used for measurement since they are easily downsized, have flat frequency responses over a wide frequency range, and provide markedly high stability. In comparison, the piezoelectric microphone is extensively used primarily as a microphone for low-frequency sound-level meters. Piezoelectric crystals generate a voltage when force is applied, and the same crystal can be used as an ultrasonic detector. Some systems use separate transmitter and receiver components while others combine both into one piezoelectric transceiver.
Ultrasound is often used in range finding – commonly referred to as SONAR (sound navigation and ranging). Similar to RADAR (radio detection and ranging), an ultrasonic pulse is generated in a specific direction. If an object is in the path of the pulse, part or all of the pulse is reflected back to the transmitter as an echo. Measuring the difference between pulse transmission and echo indicates the distance of the object. Measurement is affected by salinity and temperature when measuring SONAR pulses in water and is used when distances are short and the accuracy of the distance measurement is desired to be greater.
Ultrasonic sensors, specifically transceivers, if they send and receive, work like RADAR or SONAR by generating high frequency sound waves and evaluating the echo that is received back by the sensor. Calculating the time interval between sending the signal and receiving an echo determines the distance to an object. However, shapes of surfaces, material consistency, and density, all have the potential to distort readings.
The following outlines the various applications that use sound sensors.
Ultrasonic sensors are used to detect the existence of objects (digital) and distance (analog) in factory automation and processing. Sound is better for this application than light because sound is not affected by reflectivity, translucence, or color.
The Honeywell 94X Series Ultrasonic Distance Sensors, for example, offer enhanced sensing range capability. The non-contact distance sensing is used for non-invasive measurement. Its reduced sensitivity to light intensity, reflectivity, and opacity of a target offers flexibility and greater accuracy. With a sensing distance of up to 19.65 feet, it detects over a longer distance than other method so there is no need for close proximity to a target. Applications include level measurement, presence/absence detection, and measurement of distance.
Figure 1: One M30 option provides a sensing distance of between 300 mm to 3500 mm, with a 1.0 Hz switching frequency, and is housed in a M30 x 1.5 mm plastic PBTB.
In addition to finding an object or measuring distance within the industrial environment, ultrasonic sensors have several other important applications. For example:
- Ultrasonic transducers are used in ultrasonic cleaning. Connected to a solvent-filled pan, when a square wave is applied, vibration energy is imparted resulting in a cleaning action. At frequencies from 20 to 40 kHz, vibration collapses microscopic cavitations near the surface, resulting in cleaning of lenses, optical, and industrial parts.
- Boom microphones offer noise rejection and high-frequency crossover of near and far field responses. Not only are they incorporated in standard headsets and audio systems, industrial, commercial, and governmental designers are using them in new helmet designs for military, aircraft, and high-noise manufacturing environments. (See Knowles' Design Guide)
- Ultrasonic testing to find flaws in, or measure thickness of, metals or plastics can be performed in a non-destructive manner. Low-frequency ultrasound (50-500 kHz) is ideal for inspecting less dense materials like wood, concrete, and cement.
- The ultrasonic welding of plastics at high frequency and low amplitude vibration creates heat from friction between the materials.
Ultrasound can also be applied to chemical reactions and processes. Called sonochemistry, it uses the 20 kHz to 1 MHz range of the sonic spectrum whereas frequencies well in excess of 1 MHz are used as medical and diagnostic ultrasound.
Our most familiar experience of medical ultrasound sensors is during pregnancy. In addition to the uses in obstetrics, ultrasound is effective for scanning soft tissue throughout the body. Non-invasive, relatively inexpensive, and portable, its non-contact nature also eliminates potential contamination in medical applications.
The medical transducer is typically passed over the surface of the body. A transmit signal, or short burst of ultrasonic energy, goes out and looks for a return signal within a very short period of time. This return timeframe equals the time it takes for the energy transmitted to pass through the vessel. It is these signals that will receive additional signal processing.
Ultrasound-based diagnostic medical imaging techniques are used to capture the size, structure, and lesions of muscles, tendons, and internal organs with real-time tomography images. Ultrasound is also useful in:
- Breaking kidney stones and gallstones
- Tumor ablation (destruction without removal) guided by MRI
- Delivering chemotherapy to brain cancer cells using high-frequency ultrasound
- Cleaning of teeth
- Cataract treatment
- Bone growth stimulation
- Therapeutic tooth and bone regeneration
Surprisingly, there are downsides to its use. Ultrasound enhances inflammatory response, and can heat soft tissue. The mechanical pressure wave produced by ultrasound can actually cause microscopic bubbles in living tissue and distortion of cell membranes as it causes molecular friction heating the tissue.
According to the American Institute of Ultrasound in a Medicine Consensus Report on Potential Bioeffects of Diagnostic Ultrasound, long-term effects of ultrasound exposure in a diagnostics are still unknown.
In addition to obstetrics, sensors are used to treat hearing impairment. There are two requirements for the hearing impaired: face-to-face conversations and the use electronic equipment. Hearing aids therefore must switch input modes. A nanosensor based on giant magneto resistance (GMR) is used and embedded in an IC. GMR effect requires conduction layers thinner than the mean free path of conduction electrons, so the critical conduction layers in hearing aids are thin. This GMR technology is also used in pacemakers and implantable defibrillators for high-speed communication.
Identification and tracking
Ultrasound ID (USID) is a real-time locating system technology used to automatically track and identify the location of objects in real-time using simple, inexpensive nodes attached to – or embedded into – objects and devices that transmit an ultrasound signal to communicate location to microphone sensors. Applications include positioning systems, patient tracking, asset tracking, workflow, and locating personnel in mines.
The unique natural attributes of ultrasound make USID technology ideal for real-time systems requiring reliable location data with high levels of accuracy.
When compared with RFID solutions, ultrasound has several advantages:
- It does not require line of sight.
- The ultrasound waves are immune to interference and they do not interfere with sensitive equipment in the environment.
- Security is high so that eavesdropping is nearly impossible.
- Location is easy to achieve within a room, as ultrasound does not penetrate solid walls.
Ping Sensors from Parallax are popular for robotics. The numerous software drivers make the sensors easy to implement in a wide range of designs, in addition to its low cost and substantial list of features.
Using ultrasound rather than infrared solves the challenges of changes in ambient light, the short range, and the need for calibration inherent in infrared solutions.
Ultrasound is reliable in any lighting condition. It can be used indoors or out. It is fast enough to take care of collision avoidance for a robot, but not fast enough to track a flying object like a ball. It can handle being moved or shaken, as long as the motion is not very fast. It is so flexible that it can be reliably positioned on a rolling or walking robot, or placed on a moving articulated sensor pod.
Ultrasonic rangefinders are commonly used as collision detectors so they are placed low, but not low enough to reflect off the ground with a wide beam. It can also be angled down and placed higher to avoid the reflection dilemma.
Parallax's PING)))™ ultrasonic sensor provides a very low-cost and easy method of distance measurement. This sensor is ideal for any number of applications that require measurements be performed between moving or stationary objects. Naturally, robotics applications are very popular but this product also is useful in security systems or as an infrared replacement. The sensor includes an activity status LED and uses just one I/O pin.
Figure 2: The PING))) sensor works by transmitting an ultrasonic (well above human hearing range) burst and providing an output pulse that corresponds to the time required for the burst echo to return to the sensor.
The Parallax PING))) ultrasonic distance sensor provides precise, non-contact distance measurements from approximately 2 cm (0.8 inches) to 3 meters (3.3 yards). It is very easy to connect to microcontrollers such as the BASIC Stamp®, SX or Propeller chip, requiring only one I/O pin. It works by transmitting an ultrasonic (well above human hearing range) burst and providing an output pulse that corresponds to the time required for the burst echo to return to the sensor. By measuring the echo pulse width, the distance to target can easily be calculated. Features:
- Provides precise, non-contact distance measurements within a 2 cm to 3 m range.
- Simple pulse in/pulse out communication.
- Burst indicator LED shows measurement in progress.
- 20 mA power consumption.
- Narrow acceptance angle.
- 3-pin header makes it easy to connect using a servo extension cable, no soldering required.
- Power requirements: +5 VDC.
- Communication - Positive TTL pulse.
- Dimensions: 0.81 x 1.8 x 0.6 in (22 x 46 x 16 mm).
- Operating temp range: +32 to +158 °F (0 to +70 °C).