SubscribeDelay time measurement with ultrasonic sensors Timing the interval between transmission and echo-return - delay time with ultrasonic sensors
Somewhere lightening hits and the glistening light reaches our eye without delay. The sound of thunder however reaches our ear several seconds later. As children it was fun for us to count the seconds between these two happenings. Then we multiplied the results with the sound’s propagation velocity in the air – as we had been taught in physics – the rough distance to the location of impact was determined this way and measurement of the delay time was born….
The UNDK 20 with electronic Teach-in and 20200 mm measuring range is currently the smallest ultrasonic sensor with analog output.
Many industrial ultrasonic sensors are based on the delay time measurement. As has been mentioned in the introduction, the “measurement representation” thereby is the propagation velocity of the sound in the air, which is 330 m/s under normal conditions.
The sensors utilized for these functions are however somewhat more precise then the ones described in our physics experiment mentioned in the introduction. Only an exact time base allows precise distance measurements, constant resolutions in the range of tenth of millimeters, as well as exact repeatable switchpoints
The Transducer
All the same the transducer is designed as transmitter (ultrasonic generator) as well as receiver (microphone with high sensitivity) and is optimized for frequencies in the range of 120 kHz up to 400 kHz, far beyond the human hearing capacity.
Activated by an oscillator the membrane of the transducer decouples ultrasonic packets – so called Bursts – within 50 to 100 microseconds into the air. These absorb the sonic waves and transmits them. A receiving period of approximately four to 50 ms follows each transmission sequence.
During operation the transducer alternates continuously between transmitting and receiving. If the transmitted switch packets encounter an object the sound is reflected and a small part of the energy is reflected back to the sensor, respectively the transducer. The output of the sensor reacts after registering a certain number of successive signals within the time frame selected by the user.
If the sensor is however already back to transmitting, the reflected signal arrives too late and can therefore no longer be received and evaluated. The object remains unrecognized. It is located too far away, outside the set detection range; in this particular instance the delay time “sensor – object – sensor” was too long.
As a result ultrasonic sensors are also excellent background fade-out units; in addition – if compared to other optical units with comparable characteristics – they are not easily influenced by shining and reflective objects located behind them.
Distance measuring Sensors
In some applications the mere digital information "Object present" or "Object not present" is not sufficient. Therefore there is increasing demand for absolute distance measuring sensors. Measurement distances from 20 mm up to 2,500 mm are possible depending upon size of the equipment and the type of transducer used.
Analog measuring sensors use the delay time measuring system as well: A D/A converter converts the sound delay-time, determined by the integrated microprocessor, into an analog signal. Voltage signals from 0 – 10 VDC or current signals from 4 – 20 mA are customary. Measuring ultrasonic sensors with analog output signals and electronic Teach-in are multi-functionally applicable devices.
The required measurement range can be set simply by pushing a button or can be set electronically with the external control input, the desired effective signal direction can be adapted increasing or decreasing to the application.
The resolution thereby is <0.3 mm over the complete measurement distance. A few of the sensors even offer an additional digital output with two open, programmable switching thresholds, which, for example, can be utilized as alarm output or for additional limitations to the measurement range. All settings can easily be reverted to the factory’s default settings.
Sensors in synchro-timing
If a larger number of ultrasonic sensors is utilized within a tight space, it can happen that these receive an echo, which is not produced by their own transmitted signal; one or several neighbors are transmitting as well. To prevent that in such instances the sensors are in each others way, Baumer Electric models, for example, are – with few exceptions - serially equipped with a synchronization input.
Communication between the units is established by connecting these “control” inputs of all sensors to be synchronized with each other. External control devices are not required. If control lines are connected to one another, the sensor with the largest detection range takes over by blocking the transmitters of all units it controls, until it is ready to transmit again.
Thereafter the whole group transmits again (maximum seven) simultaneously and with chronological synchronism. In so doing mutual interference can effectively be eliminated. In persistent instances multiplex types should be used.
Ultrasonic sensors in the application process
During the application process
Different from photoelectric sensors, which optics can be blinded by strong natural or artificial light sources or sensitivity can be affected by dirt deposits, ultrasonic sensors feel at home even in an unclean environment. Dust, smoke, fog, lint, etc. do not affect their performance.
Objects, such as transparent plastic foils, glass- and PETpackaging, reflective CD-surfaces, window glass or windshields, as well as highly polished metal parts can in most instances be detected safely and reliably.
As far as shiny, not diffuse reflecting objects are concerned, their superiority compared to optical sensors is clearly evident. Ultrasonic sensors however are also utilized to monitor rolling and unrolling processes, sag control, level measurement of granulates and liquids, as well as for positioning functions.
The application shows two ultrasonic proximity switches in PNP-format and with multiplex input, which detect pallets passing by below on a conveyor belt. The risk for pollution and soil for the transducers is quite large. The pallets are not standardized, they can consist of wood or plastic with a multitude of surface compositions and coloring.
Metal carriages loaded with a variety of goods and materials can safely be detected as well. In this application the first sensor detects the front edge of the fast approaching pallet and signals for it to slow down. The second sensor stops and positions the pallet.
In this application ultrasonic sensors were used for the following reasons: the color- and surface neutral detection performance of the sensors, the tolerance toward dust and other contamination, the availability of a multiplex version, as well as their simple installation and start-up. (Due to the fact that both sensors had to be installed close to each other, there was a danger that they would influence one another, this was eliminated with multiplexoperation).
Advantages of Teach-in
The following parameter can be determined simply by depressing a button and can be optimally adapted to the respective application: The measurement range initial value Sdc The measurement range final value Sde The effective direction of the analog output Signal (increasing or decreasing) Both switch points of the additional output At any time the sensor can be returned to the default factory setting The Teach-in key and the control line lock automatically five minutes after activation of the operating voltage. An unintentional change of the stored parameter is therefore impossible.