By |November 12, 2018|Categories: Industrial Sensors, Knowledge Base|

Ultrasonic Sensor FAQ: Detection Range and Accuracy

To choose the right ultrasonic sensor for a particular application, it is necessary to determine which factors affect the detection range and accuracy of ultrasonic sensors and how incorrect measurements can be corrected.

1. What Factors Affect the Detection Range of Ultrasonic Sensors?

The surface structure and angle of an object have an impact on the detection range of ultrasonic sensors. The maximum detection range is achieved with a flat surface (standard reflector) positioned at an exact right angle to the sensor axis. Very small objects or objects that partially deflect the sound reduce the detection range accordingly. An angle of 90 ° must be adhered to as closely as possible for objects with smooth surfaces. If the object has a rough surface texture, the angle can deviate further. Dust and high humidity also dampen sound waves in the air. However, since ultrasonic sensors from Pepperl+Fuchs feature large operating reserves, this fact has little effect on the detection range.

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The maximum detection range is achieved with a flat surface (standard reflector) that is exactly at right angles to the sensor axis.

2. What Factors Influence the Sound Cone of an Ultrasonic Sensor?

The detection behavior and response curve of a sound cone are shown in curves. These curves show the area from which a particular object can be reliably detected. The reflection properties of an object are primarily responsible for the response curve. Objects with a large surface area and optimal orientation (large sound cone) are more easily detected than small, round objects or objects with poor reflective properties (small sound cone).

3. Can the Sound Cone Be Modified Using Software?

In ultrasonic sensors with an adjustable sound cone, sensor sensitivity and the sound cone can be fine-tuned. The sensing range for optimal object detection can be adjusted by programming or a teach-in function. Sensors from Pepperl+Fuchs are programmed using the software tools ULTRA 3000, SONPROG, PACTware (IO-Link), or ULTRA-PROG-IR. Programming can reduce the overall sensitivity of ultrasonic sensors. This means that the sound cone is reduced in length (axial sound cone modification) and width (lateral sound cone modification). The lateral and axial sound cone widths can be adjusted independently of each other via appropriate parameters. The axial sound cone modification primarily decreases the length of the sound cone, while the lateral sound cone modification decreases the width. Reducing the size of the sound cone allows disruptive objects or attachments on machines or in containers to be suppressed.

4. How Does Ultrasound Behave in Overpressure Conditions?

The speed of sound decreases by less than 1 % between sea level and 3000 m altitude. Atmospheric fluctuations at a specific location are also negligibly low and the effects on the speed of sound are scarcely measurable. If an application involves overpressure of more than 1 bar compared to standard pressure, the speed of sound increases while air damping decreases. This means that ultrasonic sensors have an increased detection range in the case of overpressure compared to standard pressure conditions. From overpressure of around 3 bar above standard pressure, parameter settings in the sensors must be changed so that the increased detection range and lower air damping do not cause interference and measurement errors.

Safety Notice:
In installation situations with overpressure, it is important to note that most standard ultrasonic sensors are not designed to be installed with just their front in the pressure range when the rest of the sensor is outside the pressure range. Special models are available for these installation situations. However, installing standard models completely in the overpressure area is not a problem. Only the information above regarding possible parameter adjustments at higher pressures must be considered.

5. What Does Accuracy Mean for Ultrasonic Sensors?

Accuracy or absolute accuracy is the difference between the measured value output by the ultrasonic sensor and the actual measurement distance. When using ultrasonic proximity switches in industrial work areas from -25 °C to +70 °C, absolute accuracy of 1 % – 3 % is realistic. If the ambient conditions are constant, higher accuracies can be achieved. In this case, it is advisable to switch off the temperature compensation function in the programming tool. Another option is to use an ultrasonic reference sensor. This involves mounting a second sensor of the same type parallel to the measuring sensor and aligning it to a fixed object. If ambient conditions in the measuring range change, the distance to the object will also change due to the altered speed of sound. To avoid this error, the measured value of the measuring sensor must be corrected, e.g., in the controller.

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From a practical point of view, absolute accuracies of 1-3% are realistic for the industrial use of ultrasonic proximity switches in the operating range from -25°C to +70°C.

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