A Test Bed Model of an Advanced Handheld Bone Drilling System

Tomislav Staroveski, Zlatko Catlak, Miho Klaic and Toma Udiljak

Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lucica 5, Zagreb, Croatia

Keywords: Bone Drilling Machine, Autonomous Drilling, Process Monitoring, Adaptive Control.

Abstract: Modern medical drilling systems utilized in bone and joint surgery are characterized with relatively low

level of automation, i.e., with no process monitoring and/or adaptive control characteristics, which could

potentially prevent mechanical and thermal bone damages. The quality of the drilling process depends

solely on the operator skills and tool characteristics. Therefore, a group of research activities have been

focused to the development of an advanced next generation hand-held drilling machine. It should provide

mechanical and thermal monitoring capabilities of the tool and bone, automated tool feed movement with

potential implementation of high-speed drilling regimes, as well as the application of an advanced adaptive

control algorithms for cutting forces and drilling temperature limitation. The system would reduce human

influence in drill guidance by allowing operator to define drilling location and desired tool direction/angle,

while all other activities would be performed autonomously by the machine monitoring and control system.

The test bed platform of such system which will be used in the final prototype shaping is presented in this

paper.

1 INTRODUCTION

Bone drilling interventions have nowadays become

usual and everyday practice in bone and joint

surgery as well as dental surgery. Bone is a complex

biological tissue with organic and mineral elements

whose interactions result in unique mechanical and

thermal properties. In order to avoid additional

mechanical and thermal bone damages, surgeon has

to take a special care concerning drill stability and

bone temperature during drilling process.

Quality of the drilling procedure depends on

several factors such as (Augustin et al., 2011): drill

design (type, number and flutes inclination, cutting

edge and drill point geometry, drill diameter),

machining parameters, drilling depth (cortical

thickness), cooling, drill wear rate, and drilling path

(drill position in relation to the bone).

Those factors can result in high drilling

temperatures and potential thermal osteonecrosis.

Most of them can also cause inadequate hole

geometry and high cutting forces. Higher forces can

cause drill point or cutting edge breakage, or even

complete drill body breakage. This results in

mechanical bone damages and longer postoperative

rehabilitation process.

Clinical practice in bone and joint surgery today

is based on drilling systems characterised with

relatively low level of automation, i.e., with no

process monitoring and/or adaptive control

characteristics. Drill guidance and handling is

completely controlled by the surgeon, and negative

friction or thermal influences are reduced by

applying cooling fluid externally on the bone surface

and drill shaft during the machining process. This

approach has very limited effect on the temperature

reduction because bone chips prevent contact of

cooling fluid with the cutting edges, and bone itself

has very low thermal conductivity (Davidson and

James, 2000.). Review of currently available

scientific papers and patents on handheld drilling

machines reveals the appearance of first solutions in

the form of prototype systems capable of controlling

thrust force and feed drive (Allotta, Giacalone,

Rinaldi, 1997), or advanced medical drills with

integrated sensors (von Freyberg et al., 2013, Hseih,

2012). The rest of the systems proposed in scientific

publications and patent documentation are robotised

concepts (Boiadjiev et al., 2013, Hsu, Lee and Lin,

2001) or systems and algorithms tested only on

laboratory machine tools (for drilling or milling).

Some studies have been performed on existing

commercial drilling machines.

All those systems combine force/torque sensors

190

Staroveski T., Catlak Z., Klaic M. and Udiljak T.

A Test Bed Model of an Advanced Handheld Bone Drilling System.

DOI: 10.5220/0006228601900193

In Proceedings of the 10th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2017), pages 190-193

ISBN: 978-989-758-216-5

and/or motor currents, vibration and acceleration

sensors (can be found in newer systems from 2012

and 2013) to identify drill transition between

different type of tissues, drilling depth (i.e., drilling

time) and feed.

Despite the proposed solutions and based on

numerous research studies in the field of medical

drilling, Pandey and Panda, 2013, stated in their

review paper that the development of more precise

automated bone drilling system to minimize human

error is needed.

In that sense, there are several new potential

enhancements of existing drilling systems applied in

bone and joint surgery worth to be studied, which

are related to:

 Real-time direct monitoring of drill path, as

well as drill wear rate and drilling temperature

using indirect monitoring techniques,

 Drilling process adaptive control based on the

criteria of maximum allowable mechanical

and thermal effects on drill and bone,

 Implementation of internally cooled surgical

drills and adequate clamping mechanism,

 Application of high-speed drilling regimes to

reduce bone drilling temperature (Shakouri et

al., 2014).

Drilling test bed system suitable for experimental

research of the abovementioned features is proposed

hereinafter. In the following section, a brief

decsription of its mechanical, electrical, control, data

acquisition and signal processing (DSP) elements, as

well as CAD model are presented.

2 TEST BED SYSTEM DESIGN

The mobile drilling test bed system will be

composed of three parts:

 Mechanical components and actuators,

 Process monitoring sensors,

 Control and DSP unit.

2.1 Mechanical Design and Actuators

The mechanical part of the proposed drilling system

is designed to provide:

 Compact body with integrated feed drive and

drill guide,

 Drilling experiments with or without engaging

automatic feed drive,

 Internal cooling option with easily

exchangeable drills,

 Installation of multiple process monitoring

sensors placed at the position nearest to the

signal source.

In order to achieve such characteristics, motors

of both drives, i.e., main spindle (3) and feed (7) are

fixed within the machine housing and placed

vertically one below other (Figure 1). Permanent

magnet synchronous servomotor (PMSM) with

integrated incremental encoder type Mecapion APM

SA01ACN-8 will be used for both drives. Their

characteristics are presented in Table 1. PMSM

motors were selected due to the constant torque vs.

RPM ratio over entire working range.

Figure 1: CAD model of the test bed drilling system.

A Test Bed Model of an Advanced Handheld Bone Drilling System

191

Table 1: Servomotor characteristics.

Servomotor characteristics

Size (H x W x D) in mm 40 x 40 x 125

Output, kW 0.1

Rated RPM/Max RPM 3000/5000

I, Arms 2.38

Rated Torque, Nm 0.318

Max Torque, Nm 0.955

Incr. Encoder, pulses/rev 8196

Moment of inertia, kg m

0.045 x 10

-4

Linear motion is performed by a pair of fixed

guideways (6) and a ball screw assembly (4) driven

by a feed motor and timing belt drive (5). Ball screw

nut is connected to the movable main spindle

mounting plate, which is then moved in forward or

backward direction.

Another pair of movable guideways (2), also

connected to the main spindle mounting plate, serves

as drill guide system. The drilling process is to be

operated by pushing the system against the bone and

maintaining the drill guide (11) in contact with the

bone during a drilling cycle.

Tool clamping unit (9) is mounted directly to the

main spindle motor and also serves as coolant

supply and a suitable base for placement of acoustic

emission (10) and vibration sensors (8). The tool is

clamped using collet chuck coupled to the motor

shaft within a sealed cylindrical compartment. The

coolant is fed to the drill through the compartment,

which is fixed in relation to the main spindle motion.

Suitable surgical drills with central 0,4mm coolant

channel have already been manufactured in-house

using Electrical Discharge Machining (EDM)

process.

Acoustic emission (AE) and vibration sensors

are placed in radial directions on the clamping unit,

which is the closest position to the motor spindle

front bearing in order to obtain the highest possible

signal quality. Inertial measurement unit - IMU (12)

was also installed on the device body close to the

centre of its mass.

2.2 Process Monitoring Sensors

Several types of signals will be acquired from the

system/process: drill bit position/path and drilling

machine orientation (IMU), AE, vibrations, cutting

forces, and servomotor currents.

The purpose of IMU, produced by Tinkerforge

type IMU Brick 2.0 (Figure 2), is to monitor drill

displacement caused by operator during automatic or

manual drilling, and also to establish maximal axial

forces which operator achieves during drilling with

respect to the device orientation. The IMU Brick 2.0

is equipped with a triaxial accelerometer,

magnetometer (compass) and gyroscope. It also

computes quaternions, linear acceleration, gravity

vector and heading, roll and pitch angles.

Figure 2: Inertial measurement unit (IMU Brick 2.0).

For the purpose of AE signals measuring, Kistler

industrial sensors type 8152B1 (measuring range 50

– 400 kHz) and 8152B2 (measuring range 100 – 900

kHz) coupled with 5125B interface modules will be

used. Vibration signals will be acquired by Kistler

triaxial accelerometer type 8688A50 coupled with

5134B amplifier unit (measuring range 0.5 – 5000

Hz) and cutting forces by triaxial Kistler

piezoelectric dynamometer 9257B coupled with

5017B charge amplifier. Force sensor will be

mounted on the table under the bone clamping

mechanism.

Figure 3: Acoustic emission, vibration and force

piezoelectric sensors.

The purpose of force measuring is only to

compare the force signals with the corresponding

servomotor current signals in order to analyse the

potential of current signals in drill wear and operator

trust force estimation, as well as to detect drill bit

exit from the bone.

2.3 Control and DSP Unit

Control system will be built from the following

modules:

 Dual axis PMSM servo drive module with the

power supply,

 Modular Control/DSP unit based on National

Instruments CompactRIO (cRIO) platform,

equipped with a suitable signal acquisition

modules,

 Vibration and AE signal conditioners,

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192

 Coolant pressure/ flow control unit,

 Compact industrial monitoring / data logging

PC with user interface.

Modules will be installed on three separate

vertical levels of the mobile rack cabinet. Dual axis

PMSM servo drive module with the power supply

will be installed on the first level. Selected type of

digital servo drives (AMC DZEANTU-020B200)

can be configured to operate in torque, velocity, or

position mode using a variety of external command

signals. In this application, main spindle drive will

operate in closed loop velocity mode, while feed

drive will be driven in closed loop position mode.

Drives have rated continuous current of 10A

RMS

and

can be powered with DC bus supply voltage of up to

175 VDC. This DC voltage is realized within

module using a set of serially connected switching

mode power supplies.

Both drives will use EtherCAT slave interface to

communicate with the cRIO Control/DSP unit

located in the second level. It will be equipped with

modules for acquisition of AE, vibration, force, and

temperature signals. Forces and current signals will

be sampled with the sampling rate of 1000 S/s, AE

signals with 10MS/s, vibration signals using 50 kS/s

and Euler angles at 100S/s. Other main spindle and

feed drive related parameters such as currents,

velocity and position will be acquired from the

EtherCAT bus. The same rack level will also contain

signal conditioners for AE and vibration signals.

Finally, third level will contain industrial PC,

which will mainly serve as a user interface for

experiment setup, data storage and offline data

analysis.

Coolant pressure / flow control unit will be

realized as independent module, providing

possibilities for controlling the coolant supply under

either constant pressure or constant flow rate.

Pressure/ flow set point reference will be provided to

the unit from the PC, using Ethernet interface and

MQTT protocol.

3 CONCLUSIONS

A summary of design details of a new handheld

medical drilling test bed platform is presented in the

paper. Beside existing features covered by several

already proposed solutions or prototypes, the new

system would have to ensure additional important

characteristics in the sense of drill path, drill wear

rate and bone temperature monitoring/estimation,

potential implementation of internally cooled

surgical drills and high-speed drilling regimes. It

should also provide implementation of adaptive

control algorithms, which will adjust drilling

regimes based on the criteria of maximum allowable

mechanical and thermal effects on bone and drill.

Realisation and implementation of those features

would be a substantial step toward semi- or

completely automated next generation drilling

machines, which would enable faster and more

reliable surgical procedure.

ACKNOWLEDGEMENTS

This work has been fully supported by the Croatian

Science Foundation under the project number IP-09-

2014-9870.

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