Heat generation is a well-known problem in bone drilling. Excessive heat generation during drilling operation on bone causes necrosis of the tissue. This paper presents measurements and analysis of temperatures in ultrasonically assisted drilling of bone. The main objective was to find critical drilling parameters (drill speed and feed rate) and ultrasonic parameters (frequency and amplitude) for producing higher temperatures in bone. The increase in drilling speed and feed rate were found to increase the temperatures in bone. The frequencies above 15 kHz were found to produce higher temperatures in bone. The feed rates above 30 mm/min and the amplitude of vibration up to 20 mm were found to have no significant effect on bone temperature. Ultrasonically assisted drilling may be used as an alternative to conventional drilling in orthopaedic surgical procedures to avoid death of bone cells.

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This article (1) proposes the use of a varying sensing resistor for digital charge estimators of piezoelectric actuators, (2) shows the usefulness of this proposal and (3) derives formulae to estimate the proposed varying sensing resistor. Digital charge estimators are used for charge-based precise position control of piezoelectric actuators. In existing digital charge estimators, an uncalculated resistor is used for a wide range of operating areas. Improper choice of the sensing resistor causes excessive drop in accuracy of charge estimation and/or in the driver voltage across the actuator. These issues are shown to nearly vanish with the use of proposed varying resistors.