Tremor Suppression Through Impedance Control
Stephen Pledgie1, Kenneth Barner2, Sunil Agrawal3
University of Delaware
Newark, Delaware 19716
duPont Hospital for Children
Wilmington, Delaware 19899
1 Ph.D. Candidate, Biomechanics and Movement Science Program
2 Department of Computer and Electrical Engineering
3 Department of Mechanical Engineering
4 Extended Manipulation Laboratory
This paper presents a method for designing tremor suppression systems that achieve a
specified reduction in pathological tremor power through controlling the impedance of the hu-
man-machine interface. Position, rate, and acceleration feedback are examined and two tech-
niques for the selection of feedback coefficients are discussed. Both techniques seek a desired
closed-loop human-machine frequency response and require the development of open-loop hu-
man-machine models through system identification.
The design techniques were used to develop a tremor suppression system that was subse-
quently evaluated using human subjects. It is concluded that non-adaptive tremor suppression
systems that utilize impedance control to achieve a specified reduction in tremor power can be
successfully designed when accurate open-loop human-machine models are available.
Tremor is an involuntary, rhythmic, oscillatory movement of the body . Tremor
movements are typically categorized as being either physiological or pathological in origin.
Physiological tremor pervades all human movements, both voluntary and involuntary, and is
generally considered to exist as a consequence of the structure, function, and physical properties
of the neuromuscular and skeletal systems . Its frequency varies with time and lies between 8
and 12 Hz. Pathological tremor arises in cases of injury and disease and is typically of greater
amplitude and lower frequency than physiological tremor. In its mildest form, pathological
tremor impedes the activities of daily living and hinders social function. In more severe cases,
tremor occurs with sufficient amplitude to obscure all underlying voluntary activity [3, 4].
A number of digital filtering algorithms have been developed for the purpose of remov-
ing unwanted noise from signals of interest and have thus found application in tremor sup-
pression. Riviere and Thakor have investigated the application of adaptive notch filtering for the
purpose of suppressing pathological tremor noise during computer pen input [5, 6]. When a ref-
erence of the noise signal is available, adaptive finite impulse response (FIR) filters can produce
a closed-loop frequency response very similar to that of an adaptive notch filter . Gonzalez et
al. developed a digital filtering algorithm that utilized an optimal equalizer to equilibrate a
tremor contaminated input signal and a target signal that the subject attempted to follow on a
computer screen . Inherent human tracking characteristics, such as a relatively constant tem-
poral delay and over and undershoots at target trajectory extrema, were incorporated in a
“pulled-optimization” process designed to minimize a measure of performance similar to the
squared error of the tracking signal.
To improve an individual’s ability to perform manual tasks in a physical environment, it
is necessary to suppress tremor-related movements. This can be accomplished by applying re-
sistive forces to the user’s limb to attenuate movements that occur at or near tremor frequencies.
The mechanical impedance of the human-machine interface is altered due to the activity of a set
of actuators driven by a displacement feedback controller.
Several projects have investigated the application of viscous (velocity dependent) resis-
tive forces to the hand and wrist of tremor subjects for the purpose of suppressing tremor move-
ments [4, 7, 9, 10, 11]. Experimentation with varying levels of velocity dependent force feed-
back showed, qualitatively, that tremor movements could be increasingly suppressed with
increasing levels of viscous force feedback, but that concurrent resistance of voluntary move-
ment may occur.
Closed loop functional electrical stimulation (FES) has been shown to be effective in
suppressing tremor movements in patients with essential tremor, parkinsonian tremor, and cere-
bellar tremor [12, 13]. In this approach, tremorogenic muscles are stimulated out-of-phase to
cancel the tremor forces generated by affected muscles. Investigators were successful in deter-
mining closed loop configurations that attenuate 2 - 5 Hz tremor movements with minimal at-
tenuation applied to voluntary movements in the 0 – 1 Hz range.
Previous investigations into non-adaptive feedback tremor suppression systems have not
utilized quantitative performance criteria during the design of the feedback control system. They
addressed the question of whether or not velocity dependent resistive forces (damping) could ef-
fectively suppress tremor movements, but were not concerned with achieving a specified statisti-
cal reduction in the tremor.
The objective of this research was the development of a methodology that incorporates
quantitative performance criteria as well as position, rate, and acceleration feedback into the de-
sign of a non-adaptive tremor suppression system. The remainder of this paper is divided into
six sections. Section 2 presents the results of an analysis of pathological tremor movements.
teristics within the bi-articular muscles of the limb as well as the inertial properties of the limb
segments. A constant-coefficient second order linear model is unable to represent such a de-
pendence. For this reason, localized inaccuracies of the human-machine models may exist and
lead to degraded performance. Because the subjects performed tasks that did not require large
joint excursions, the errors introduced by the configuration dependent impedance were tolerable.
Additionally, a model developed under the conditions of a relaxed and primarily passive limb
may not be applicable under the conditions of an actively controlled limb. There may exist a
more suitable method of system identification that can be employed to model the properties of an
It is suggested that future investigations utilize adaptive feedback that seeks an “optimal”
level of tremor reduction. The average power of the user’s movements could be taken as the cost
function for either an adaptive FIR filter or the Weighted-Frequency Fourier Linear Combiner
(WFLC) developed by Riviere and Thakor [11, 12]. The existence of a physical plant in the
system will, in all likelihood, require the use of a noise reference signal that has been pre-filtered
using a model of the plant
Improved tremor suppression may also be achieved with the use of higher order feed-
back. Such systems will provide for better control of the closed loop frequency response but
may suffer from significant noise amplification and instability problems.
In conclusion, it has been demonstrated that a non-adaptive tremor suppression system
can be designed such that movements at a designated frequency experience a specified level of
attenuation. When second order feedback is present, additional frequency domain constraints,
such as the preservation of lower frequency voluntary movements, can be addressed.
This research was funded by the National Institute on Disability and Rehabilitation Re-
search (NIDRR) of the U.S. Department of Education under grant #H133E30013.
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Stephen Pledgie: email@example.com
Kenneth Barner: firstname.lastname@example.org
Sunil Agrawal: email@example.com
Tariq Rahman: firstname.lastname@example.org