ADVANCES in technology have given today's ultrasonic sensors robust, accurate sensing capabilities that were not available a few years ago. These new technologies have made ultrasonic sensors easier to use, more flexible and affordable.
These enhanced features open up a new range of applications far beyond the abilities of older ultrasonic sensor designs. Today's ultrasonic sensors allow machine designers to find new, creative solutions in the industrial marketplace.
Years ago, ultrasonic sensors were often a secondary choice in sensor technology. Designers chose ultrasonics only when other sensing technology would not work. Acceptable applications usually involved transparent objects, long sensing distances, or changes in target colour.
New technology gives today's ultrasonic sensors robust capabilities for withstanding harsh environments:
* The introduction of IP67 and IP69K rated housings allow ultrasonic sensors to be used in wet sensing environments, such as bottle washing machines.
* Built-in temperature-compensation circuitry compensates for significant changes in temperature that may occur during a normal day or shift operation.
* A special coating on the face of the sensor shields the transducer from damage from harsh chemicals.
* Advanced filtering circuits allow ultrasonic sensors to ignore ambient factory noise.
* New transducer designs offer much greater protection from physical damage and dirty environments.
Improved ease of use
Features on the new generation of ultrasonic sensors that make them easier to use include push-button setup, DIP-switch programming, and multiple programming options.
* Push-buttons integrated into the sensor housing make setting the near and far limits of the sensor range easy. Simply place the target in front of the sensor and press the push-button. The sensor automatically learns the window size and target distance. Easy setup means the same sensor can be used in many different applications.
* DIP-switch programming means a single sensor can be customised for specific applications. Some features that can be programmed are response time, type of output, analog or discrete, and special settings for fill level applications.
* Ultrasonic sensors usually have multiple outputs included in a single sensor. A sensor can have two separate discrete outputs to allow sensing at two different distances with the same sensor. Other sensors have both discrete and analog outputs in the same sensor, so a single sensor can both measure and provide alarm output.
These features make ultrasonic sensors very flexible and a viable alternative to other sensor technology. Because of their ease of use and application flexibility, ultrasonic sensors are contributing to significant productivity gains and quality improvements in industrial automation.
Fundamentals of using ultrasonic sensors
An ultrasonic sensor houses a transducer that emits high-frequency, inaudible acoustic waves in one direction when the transducer element vibrates. If the waves strike and bounce off an object, the transducer receives the echoed signal. The sensor then determines its distance from the object based on the length of time between the initial sound burst and the echo's return. Typically, a sensor has a near and a far limit that are set with push-button programming. The sensor determines whether an object is present within those limits.
For example, when an ultrasonic sensor mounted above a tank of liquid or bin of pellets emits waves into the container, the length of time it takes for the echo to return indicates whether the container is full, empty, or partially filled.
Some ultrasonic sensors use a separate emitter and receiver transducer. These opposed-mode ultrasonic sensors work well in applications that require edge detection, faster response time or that have wet environments.
Ultrasonic sensors should be a first choice for detecting clear objects, liquids, dense materials of any surface type (rough, smooth, shiny), and irregular shaped objects. Ultrasonic sensors are not suitable outdoors, in extremely hot environments, or in pressure tanks; nor can they detect foam objects.
The following are key considerations when choosing an ultrasonic sensor for an application:
* Range and size. The size of the object being detected affects the maximum range of ultrasonic sensors. The sensor must detect a certain level of sound to activate its output. A large part reflects most of the sound to the ultrasonic sensor, so the sensor can detect the part at its maximum sensing distance. A small part reflects a much smaller portion of the sound resulting in a significant reduction in sensing range.
* Object to detect. The ideal object for an ultrasonic application is a large, flat, dense object perpendicular to the face of the transducer. The most difficult objects to detect are small, or made of a sound-absorbing material such as foam rubber, or are at an angle to the transducer. Some difficult objects can be detected by teaching the sensor to detect a background surface and then respond to an object that comes between the sensor and the background.
Detecting liquids is easy if the liquid has flat surface perpendicular to the face of the sensor. To compensate for a turbulent surface, the sensor can be programmed for a longer response time, which averages the turbulence changes for a more consistent reading. There is no good solution to help an ultrasonic sensor accurately sense a liquid with a foamy surface, because the foam deflects sound in different directions.
Sensing irregular shaped objects is possible using an ultrasonic sensor in retrosonic mode. In retrosonic mode, the ultrasonic sensor detects a flat background, such as a wall. Any object passing between the sensor and the wall blocks the sound waves. The sensor detects the interruption and recognises that an object is present.
* Vibration. Vibration sometimes can affect the accuracy of a distance measurement, whether the sensor itself or a nearby machine vibrates. Using a rubber anti-vibration device as part of the sensor mounting reduces vibration. Guide rails also are sometimes used to eliminate or minimise part vibration.
* Attenuation. Temperature compensation designed into a sensor can adjust for slow changes to ambient temperature. It does not adjust for temperature gradients or fast changes in ambient temperature.
* False results. Sound waves can reflect from nearby objects, such as a guide rail or fixture pole. To reliably detect the target object, it is necessary to reduce or eliminate the effects of nearby sound- reflective surfaces. To avoid false detection of nearby objects, many ultrasonic sensors have an indicator LED to guide the operator during installation, ensuring that the sensor is mounted correctly and reducing the risk of false results.
Lower costs and ease of use have led machine designers to incorporate ultrasonic sensors into applications that were once considered too difficult or too costly. Industrial applications for ultrasonic sensors include detecting fill level; detecting clear objects and materials; controlling loop tension; and measuring distance. Industries that use ultrasonic sensors include packaging, bottling, material handling, automotive, and more.
Here are just a few application examples:
* In a bottling plant, to detect bottles.
* In a food processing plant to detect and control the fill level of liquid in a tank.
* On a packaging line to control web speed of a material by monitoring the loop tension between two rollers.
* To detect bolt heads on an automotive engine block.
Today, industrial use of ultrasonic sensors is rapidly expanding. A technology that was once expensive and unreliable is now simple to use and affordable. Ultrasonic sensors are routinely used to improve product quality in process monitoring, to detect defects, to determine presence or absence and for other applications. The sensors also improve productivity by reducing scrap and machine downtime due to bad parts. Future product development in this technology will continue this trend. The challenge now is to make industry aware of the potential of ultrasonic sensors in all areas of manufacturing, including quality control, process control and inspection.
*Lee Kielblock is a senior application engineer with Banner Engineering Corporation. Banner is represented in Australia by Micromax .