Operation and design of ultrasonic sensors

A special sonic transducer is used for the ultrasonic proximity sensors, which allows for alternate transmission and reception of sound waves. The sonic waves emitted by the transducer are reflected by an object and received back in the transducer. After having emitted the sound waves, the ultrasonic sensor will switch to receive mode. The time elapsed between emitting and receiving is proportional to the distance of the object from the sensor.

Digital output

Sensing is only possible within the detection area. The required sensing range can be adjusted with the sensor's potentiometer or by electronic Teach-in (Teach-in button or remote Teach-in). If an object is detected within the set area, the output will change state which is visualized by the integrated LED.

Target detection

Sonic waves are best reflected from hard surfaces. Targets may be solids, liquids, granules or powders. In general, ultrasonic sensors are deployed for object detection where optical principles would lack reliability.

Standard target

The standard target is defined as a square flat object of following sizes:

  • 15 x 15 mm for Sde up to 250 mm
  • 30 x 30 mm for Sde up to 1000 mm
  • 100 x 100 mm for Sde > 1000 mm

The target should be mounted perpendicular to the axis of the sensor.

Size

To ensure a reliable object detection, the reflected signal must be large enough. The intensity of the signal depends on the size of the object. Using a standard object, the full scanning distance Sd is available.

Surfaces

Detection of sound absorbent materials will result in a reduction of the maximum sensing distance. The maximum sensing distance can be achieved as long as the maximum roughness of the object does not exceed 0,2 mm.
Typical sound absorbing materials are:

  • foam rubber
  • cotton / wool / cloth / felt
  • very porous materials

Sonic cone profiles

The sonic cone profile charts as found in the spec sheets of this catalog represent the active sensing areas for ultrasonic sensors. The charts demonstrate the short-range sonic side lobes, which widen the sensor's close-range aperture angle. Due to sound absorption and air diffusion, the lobes decrease at longer ranges. Size, shape, surface properties and the direction of target detection have very high influence on the lateral detecting region of an ultrasonic sensor. Sonic cone profiles apply to the whole product family, e.g. a 100 - 1000 mm profile is representative for all related sensors of the same sensing range - digital or analog outputs, etc.

Measuring method

Standard square targets made of steel are used to determine the shape of typical sonic cone profiles.

  • 15 x 15 mm for Sde up to 250 mm
  • 30 x 30 mm for Sde up to 1000 mm
  • 100 x 100 mm for Sde > 1000 mm

The targets are positioned perpendicularly to the sensor's reference axis, approached sideways at different distances. The sonic cone profile is then plotted by connecting the measured points with a line. The cone shape can vary if round or differently shaped objects are detected.