The speed of sound depends on ambient temperature. As temperature rises, the speed of sound increases. As it gets colder, the speed of sound decreases. For ultrasonic sensors, this variance can affect accuracy, since a faster or slower echo return will make a stationary target appear closer or farther away.
Most of our ultrasonic sensors use an integral temperature compensation network to account for changing environmental conditions. Constant temperature changes throughout the day are not something that we spend much time thinking about. It would be difficult for some people to notice the difference between a 75 °F room and a 70 °F room. But, for ultrasonic sensor applications that require millimeter accuracy, it can make a world of difference.
We recently worked with a customer who was using a sensor from our M30 Plug series, and he was not getting the millimeter display from the Ultra 3000 software that he was expecting. There was consistently a 10 mm to 15 mm offset between the software reading and the distance he was measuring with a tape measure. Despite adjusting the sound cone size, sensitivity, and other variable parameters within the software, the software reading was still hovering around 10 mm away from the actual distance.
Our team simulated the customer’s application and noticed the same offset. We then identified one major issue—this series of sensors comes with a programming plug that provides temperature compensation. This plug has to be removed when using the programming cable (UC-30GM-R2). So any changes in temperature were not being accounted for in the sensor’s distance calculation.
To enable the sensor’s temperature compensation feature, the programming plug needs to be inserted into the sensor in the “T” position (molded on the plug). This indicates that the temperature compensation is active.
When you connect the programming plug to the sensor, communication with the computer stops. You get the voltage output information directly from the sensor. To accomplish this, we connected a voltmeter across the ground wire and the voltage output wire.
The voltage value has to be converted to a distance, using a conversion factor based on the current sensing window of the sensor. Since we used the UC2000-30GM-IUR2-V15 ultrasonic sensor with the factory default output window, the range was 200 mm … 2000 mm. The voltage output range over that span is 0 V … 10 V, so this was the necessary scaling factor to convert volts to mm:
------------- = 180 mm/V
We then multiplied the scaling factor by the voltmeter reading to give a distance between the target and sensor. Because this distance does not take into account the sensor’s dead band, the dead band (in this case, 200 mm) had to be added.
After inserting the programming plug and letting the sensor acclimate to the surroundings for about 10 minutes, we were able to use the scaling factor above to reduce the offset to under 2 mm. This is a drastic improvement for the customer’s application and is much more in line with what is expected of an ultrasonic sensor.
This application is a great reminder of the importance of temperature compensation on ultrasonic sensors. Ultra 3000 software is a useful tool for parameterizing your sensor, but in order to achieve maximum accuracy with a M30 Plug series sensor, you must disconnect the software and insert the programming plug.