Strategy of Controlling a Free Hanging Humerus in an Experimental Test Setup
by using an Inertial Measurement Unit IMU
1
Daniel Tomas, 2Patrick Stäheli, 2 Walter Siegl, 1Daniel Baumgartner
IMES Institute for Mechanical Systems, ZHAW Winterthur, Switzerland
2
IEFE Institute for Energy Systems and Fluid Engineering, ZHAW Winterthur, Switzerland
1
INTRODUCTION
METHODS
The human shoulder was often biomechanically
analysed by using experimental test setups. Active
motion of the glenohumeral joint was performed with
a so called dynamic shoulder model. Necessary
shoulder muscle forces were measured to abduct the
upper arm to a specific arm angle.
Detailed functional models were built by Wuelker et
al [1] and Sharkey et al [2] in the early 90’s to
analyse the force distribution at the rotator cuff.
These models are able to measure a stepwise increase
of the muscle forces up to a maximum value. If a
specific arm angle has to be achieved, the muscle
forces need to be incrementally adapted.
Inertial Measurement Unit IMU
In order to acquire the mentioned humeral movement,
an Inertial Measurement Unit (Sparkfun, USA) was
used and placed distally at the humerus dummy.
The IMU is able to measure rotation around every
axis of its orthogonal coordinate system, as well as
the acceleration in every direction of the coordinate
system.
By attaching the IMU to the humerus and processing
the provided signals, it is possible to measure the
abduction angle (Alpha) and the rotation angle (Beta)
of the humerus. The angles Alpha and Beta are used
as Set Point (SP) for the two control loops.
Figure 1: Dynamic shoulder models of Wuelker et al.
(left) and Sharkey et al., including a humerus and scapula
of the shoulder.
Mentioned existing models are therefore not position
controlled with respect to predefined arm angles. In
order to be able to achieve reproducible arm angles,
the system, and in particular the humerus, needs to be
controlled by a system which acquires the current
arm position.
As a consequence, a novel shoulder model was built
by considering a closed loop control system using an
Inertial Measurement Unit (IMU) as a 3D position
sensor. The IMU signal is used to steer the individual
muscle forces in real time to achieve predefined
humerus angles.
If the system is able to reproducibly apply specific
arm angles, primary implant stability tests for
multiple cycles will be possible to perform.
Figure 2: Schema of the novel shoulder simulator
including actuators and measurement devices for muscle
and joint loads.
KMD
IMU
EPOS
MMA
SMA
MMR
SMR
SM
Kraftmessdose/Load cell
Inertial Measurement Unit
Maxon EPOS 2
Master Motor Abduction
Slave Motor Abduction
Master Motor Rotation
Slave Motor Rotation
Scapula Motor
(Deltoideus Muscle)
(Supraspinatus Muscle)
(Infraspinatus/Teres Minor)
(2 Segments of Subscapularis)
Controller
The controller includes two control circuits, one for
the abduction (CA) and one for the rotation (CR) of
the humerus. The CA is configured as a master and
the CR as a Slave.
Each of these control loops includes two linear
actuators, again based on a Master/Slave
configuration, each with its own controller (Fig. 3
and 4).
Abduction control circuit
The CA system receives its initial SP from the user as
a target angle for Alpha. Based upon the difference
between the SP and the actual Alpha value the CA
calculates a target position for the position controller
of the Master Motor Abduction (MMA) and sends it
over the CAN-Bus. Based on the angle Alpha, the
Scapula Motor (SM) contributes always with one
third of the total humerus abduction.
The load cell, which is mounted in line with the
MMA, measures the applied force needed to perform
the abduction of the humerus, one third of this actual
force value will be the new target value for the Slave
Motor Abduction (SMA), which is sent over the
CAN-Bus to the corresponding position controller.
Figure 3: Block diagram to control the abduction angle of
the humerus. Scapular rotation provides half of the
thoraco-humeral abduction.
MCA
SCA
MCR
SCR
Master Controller Abduction
Slave Controller Abduction
Master Controller Rotation
Slave Controller Rotation
Rotational control circuit
The second controller CR uses a SP from the user as
a target angle for Beta and sends the calculated
values to the Master Motor Rotation (MMR) which
follows the same behaviour as the MMA. The Slave
Motor Rotation (SMR) obtains a user input as SP for
the target force. Both SPs for the MMR and SMR
should be kept over the complete abduction cycle.
Figure 4: Block diagram to control the rotational angles of
the humerus around the longitudinal axis.
RESULTS
By now a first attempt of the CA system has been
programmed, implemented and tested.
After first measurements, the realised system is able
to perform the required boundary conditions. Master
motor (representing Deltoideus) and Slave motor
(representing Supraspinatus) are in the predefined
ratio of 3:1.
After first measurements, one abduction cycle 0°85°-0° needs about 20 seconds. This does not fulfil
the expected goal of around 5 seconds. Especially for
long term testing, the cycle time has to be reduced
significantly, so that a ten thousand cycle test can be
finished in less than fourteen hours.
Afterwards, the CR system, which has been tested
individually, can be implemented and tested; this can
only be done with a correctly working CA system.
DISCUSSION AND CONCLUSIONS
So as to achieve the mentioned limit, the CA system
has to be made faster without losing the already
gained accuracy. Finally, a combination of Abduction
and Rotation will be in the focus for further
investigations.
REFERENCES
1. Wuelker N, et al. Acta Orthop. Scand., 65(4):
442-446, 1994
2. Sharkey NA, et al., Journal of Orthopaedic
Research, 12 (5): 699-708, 1992
ACKNOWLEDGEMENTS
The scientific work was financially supported by the
Zurich University of Applied Sciences.