Temperature control in robotic bone drilling process
Authors: Boiadjiev George , Boiadjiev Tony , Delchev Kamen , Chavdarov Ivan , Kastelov Rumen 
Published: April 2022

The orthopedic manipulation “bone drilling” is the most commonly used in the surgical treatment of bone fractures. The bone drilling process is characterized with a set of input and output parameters. The first ones define the conditions of the process execution and the second ones determine the outcome. The input parameters as feed rate [mm/s] and drill speed [rpm] have the most importance for the outcome, namely thermal and mechanical damages of the bone tissue.Many scientific researches related to the input parameters influence to the bone drilling process are published. The purpose is to find such an input parameters combination which is able to guarantee optimal result concerning the outcome parameters of the bone drilling process.The difference between the experimental results of many studies appears and even becomes bigger for the sake of wide variety of test conditions used by researchers. However, the following common dependencies stand out:-to avoid thermal osteonecrosis the drilling process must be executed with possibly maximal feed rate value together with possibly minimal drill speed value. -to avoid traumatic osteonecrosis the drilling process must be executed with possibly minimal feed rate value and possibly maximal drill speed value.So that the requirements concerning the drill speed and the feed rate values are contradictory and in the same time such values must be maintained during the bone drilling process aiming to obtain the optimal results. In manual drilling these parameters are controlled by the surgeon on the base of his practical experience. But the control of these parameters and the achievement of their optimal values can be successfully realized only under its robotized execution.Feed rate control during an automatic bone drilling process using an orthopedic drilling robot ODRO has already been introduced in our previous publications. The drill speed control is the next step in the development of ODRO.During bone drilling a significant amount of already done mechanical work directly converts into thermal heat elevated at the tool – bone interface. Since the critical temperature for thermal osteonecrosis is the most rigorously defined to be 47°C, the purpose of drill speed control during bone drilling process is to maintain the temperature below this critical threshold.Currently drill bit rotation in our case is realized on the base of MAXON BLDC controller/driver. With this type of controller, the motor speed is controlled entirely by hardware. It not allows software implementation of an algorithm for controlling the speed of the motor. The created and developed algorithm as well as the obtained and presented experimental results have a purpose to demonstrate the abilities of the controller TMCM 1630 for drill speed control during bone drilling process in dependence on the data characterizing the temperature deviation.This work presents basic characteristics of ODRO as well as a new drill speed control algorithm. It is one more additional step for improving the functionality of ODRO together with feed rate control during drilling and various specialized working regimes.