When someone is looking for a device to detect the presence of a target object, or to determine target distance (for example, in level measurement applications), I often suggest an ultrasonic sensor as one possible solution. Most of these sensors operate as a single unit that emits sound pulses and evaluates the echo of each pulse as it is reflected back from the target object.
One of the advantages of using an ultrasonic sensor is the fact that the detection field – also called a “sound cone” – can cover a relatively large area compared to other sensing technologies. However, in order to make the best use of this feature (and to avoid any potential pitfalls), it’s important to understand what the sound cone looks like and how the sensor responds when target objects enter it.
General sound cone shape
Ultrasonic sound pulses originate from the circular transducer of the sensor. As each pulse is generated, it travels through the air in front of the transducer, spreading outward to form a three-dimensional cone of sound waves as it moves away from the sensor. Any object in this cone-shaped area (illustrated in the figure below) can be detected by the sensor if it has a surface capable of reflecting sound back to the sensor’s transducer. Note, however, that the fact an object is detected by the sensor does not automatically mean that the sensor’s output will be affected by the object’s presence – more on that later.
One of the first things you should know about the sound cone is that it contains an area directly in front of the transducer known as the “unusable area,” deadband, or blind zone. Take care that nothing is allowed into this area, since objects entering the unusable area cannot be accurately detected by the sensor. The size of the unusable area varies from model to model and is listed on each sensor’s data sheet.
Determining the size of the detection area
An ultrasonic sensor’s sensing range is listed on its data sheet, and represents the length of the sound cone along its central axis. But what about the sound cone width? To help you determine how wide the sound cone gets for a particular ultrasonic sensor, most data sheets include a response curve diagram, which depicts the size and shape of the area in front of the sensor where targets with certain characteristics are able to be detected by the sensor.
The example below shows response curves for a sensor with 6 m range. In this case, the targets used to generate the response curves are a flat plate 100 mm square, and a round bar with 25 mm diameter. Because the flat plate has a relatively wide surface that can reflect most of the ultrasonic signal that hits it directly back to the sensor, there is a much larger area where this target can be detected. On the other hand, because the round bar target is smaller, with much of its surface area curved so that the ultrasonic signal that hits it is deflected away rather than returned to the sensor, there is a much smaller area where this target can be detected. Your target can be compared to one of these to give you an idea of what the sensor’s response to it might look like.
Relationship between object detection and sensor output
Most ultrasonic sensors are teachable, so it’s important to understand that an object may be present in a sensor’s detection area, but may not affect the sensor’s output. For example, a sensor’s data sheet may state a 30 mm...500 mm sensing range, meaning the sensor is able to detect an object anywhere within that range and evaluate the distance to that object. But if that sensor has been programmed as normally open with a switching point at 150 mm from the sensor, then the output will be active (i.e., switch to the closed state) only when the target is within the taught-in range – in this case, between 30 mm...150 mm from the sensor. If a target is not in the active area, even if it’s within the sensor’s range and detected by it, the output won’t be active.
Can the sound cone be changed?
Some ultrasonic sensors allow the user to change the sound cone width, either by pushbutton configuration, or advanced programming through a software interface. This is especially useful when there are objects near the edge of the sound cone that you don’t want to detect, such as a container wall.