High resolution vertical movement system for transducer and target separation in primary ultrasonic power measurement setup

• Methods for transducer and target separation measurement are described.
• Developed high resolution micro-stepping motor based vertical movement system.
• Technique includes correction of backlash error.
• Automatic null (zero) distance detection.

Radiation force balance approach is universally adopted and most widely used primary calibration method for total acoustic power radiated by an ultrasonic transducer. In this setup, the separation between the transducer and target (absorbing/reflecting) plays vital role in the overall measurement uncertainty of ultrasonic power due to attenuation of ultrasonic waves within the travel path. It becomes relatively more important at high frequencies as the ultrasonic attenuation is proportional to the square of frequency. Hence, the separation between the transducer and the target need to be measured with the best possible resolution and least uncertainty. In this article, two methods developed for transducer and target separation measurement have been described and compared. The first method uses displacement sensor in the feedback and controls stepper motor in micro stepping and adjust the separation with control program. It also uses the microbalance feedback approach to automatically detect the null (zero) distance. In the second approach, which does not use distance sensor in feedback and a triangle method is used to detect change in height. High torque stepper motor is used in micro-stepping mode to achieve best step resolution of one μm. Second method is free from error that cause due to electrical noise in sensor and has provision for backlash error correction. The article contains developmental details and functionalities of these methods.