WEARABLE TREMOR REDUCTION DEVICE (TRD) FOR HUMAN HANDS AND ARMS

Transcription

1 Proceedings of the 2018 Design of Medical Devices Conference DMD2018 April 9-12, 2018, Minneapolis, MN, USA DMD WEARABLE TREMOR REDUCTION DEVICE (TRD) FOR HUMAN HANDS AND ARMS Sreekanth Rudraraju California State University Fresno Fresno, California, USA The Nguyen California State University Fresno Fresno, California, USA BACKGROUND Parkinson s disease (PD) and essential tremor (ET) are two common but unrelated diseases that cause movement disorders often involving with severe tremor. The two diseases affects tens of millions of people worldwide, but there is no known cure for them. The tremor not only pose difficulty in completing daily tasks but also impair patients social confidence. The objective of this project is to develop a wearable tremor reduction device for the upper limb. The device is obviously different from any previous devices because it is compact, lightweight, comfortable to wear and effective. It is expected to help patients manage the tremor symptoms and regain their normal life. Many researchers and inventors have come up with potential solutions to control the tremor experienced by movement disorder patients. One approach to tremor suppression is to create a noninvasive wearable tremor suppression exoskeleton called WOTAS (Wearable Orthosis for Tremor Assessment and Suppression) (Rocon et al., 2007). It was under the DRIFTS (Dynamically Responsive Intervention for Tremor Suppression) framework to devise a wearable orthosis which uses a DC motor to reduce tremors at each joint (Manto, et al., 2003). It can be seen that WOTAS and similar devices are too bulky to be worn on human body even though they are effective in suppressing the tremor. Several mechanical devices have been patented for the suppression of tremor in the lower arm, dating back as early as 1979 where the device aims to reduce tremors caused by cerebral palsy by using a forearm and hand brace connected to vertical post by a series of mechanical linkages (Terry and Hoyt, 1979). Tremor suppression can be as simple as strapping the arm down (Thomas, 1988) or more sophisticated methods (Asatourian et al., 2002, Handforth, 2002 and Kalvert, 2004). It is clear that none of these and similar patents have made to market successfully due to their relative ineffectiveness or sizes. Another approach for arm tremor suppression is to use tuned vibration absorbers (TVAs) or tuned mass dampers (TMDs). (Li, 2000) and (Hashemi et al., 2004) describe in good details the application of the TVA on a model arm. The model arm was created from linkages representing the upper and lower arm. These links were connected via springs representing muscles. The tremor was reproduced by a DC motor mounted on the arm. The study showed that the TVA was effective to reduce tremor at the arm. Their successes with the model arm reinforce the potential of using a vibration absorber for tremor suppression. However, their studies stopped at the method validation step. The presented tremor reduction device (TRD) in this paper also uses the principle of TVAs, but the innovative design and arrangement of components allow it to be highly potential for commercialization. METHODS Due to Parkinson s disease, muscles in the body will make involuntary motions particularly at the hands and arms. This device is developed for reducing the unwanted motion in the lower arm and hand. Fig.1 shows the schematic of the human arm represented by a single Degree of Freedom (DOF) massspring-damper (MSD) system in which muscles are treated as springs with damping. The hand is considered as the mass m 1. When excitation happens due to muscles, it will be transferred to the hand, and therefore the hand starts shaking without control. Fig. 2 shows the schematic of the hand with the tremor reduction device (TRD) represented by the two DOF systems. The TRD is the secondary mass m 2 attached to the main mass m 1, i.e. the hand, via a pair of spring and damper. As it can be seen, the operational principle of the TRD (m 2 ) is the same as a tuned vibration absorber (TVA). When mass m 1 (hand) is excited by the base excitation via the spring and damper (muscle), mass m 2 (TRD) will oscillate vigorously to take away kinetic energy from the mass m 1. In the design, very low 1 Copyright 2018 ASME

2 friction bearings and low damping springs are used so that mass m2 moves freely even for small disturbances at the hand. EOM. Springs are represented with their stiffness constants K s and damping coefficients C s. (a) Fig.1 Hand alone (1DOF) Fig.2 Hand with TRD (2DOF) As stated earlier, TRD bases on mass tuned damping principle, in which mass m2 and springs in TRD device are tuned to absorb the energy developed in the mass m1 (hand). Motion of m1 is transferred to m2 through the springs. Motion directions of m1 and m2 are out of phase in the 2DOF MSD system. Due to the opposite phase, the m2 s motion creates counter force opposite to the moving direction of m1. As a result, the motion of m1 is reduced. For effective absorption, the resonant frequency of the TRD system (determined by mass m2 and spring k2) should match with the frequency of the excitation, i.e. the tremor frequency in this case. (b) (c) Fig.4 FBD of (a) Mass m1 (b) Mass m2 and (c) Mass m3 Equations of motion are derived based on the Newton s second law of motion. Equations (1-3) represent corresponding EOMs of m1, m2 and m3. (1) (2) (3) Table.1 List of parameters and values for study Parameter Value Units Hand mass (m1) 1.69 kg Muscle stiffness (K1) 400 N/m Muscle damping coefficient (C1) 5 N-s/m TRD masses (m2 and m3) 0.2 kg TRD springs stiffness(k2,k3,k4,k5) 80 N/m TRD damping coefficients (C2,C3,C4,C5) 0.25 N-s/m Fig.3 Schematic of hand with TRD Fig.3 represents the typical arrangement of TRD attached to hand in an actual operational condition. In this arrangement, the TRD is divided into two identical units. Each unit included a mass connected directly to the hand via a spring with low damping. Thus, the masses are denoted m2 and m3 and arranged on the opposite sides of the hand. This orientation is to address the tremor in one translational direction, i.e. vertical in the shown configuration, and the rotational tremor as well. This orientation also provides balance in the hand. During the design process, it is desired to obtain the analytical calculations to identify the required mass, spring stiffness and damping coefficient. Therefore, equations of motion (EOM) are derived for masses m1, m2 and m3 to analyze the motion of entire system. Free body diagrams (FBDs) of mass m1, m2 and m3 are represented in Fig. 4 for deriving the Fig.5 Simulink Model of Hand with TRD 2 Copyright 2018 ASME

3 For experimental and simulation purpose, the masses and springs are selected with the values listed in Table.1. The numbers are applicable for a typical adult with 150 pounds of weight and tremor frequency at 4.5 Hz. For Parkinson s disease patients, the tremor frequency often ranges from 3 to 7Hz. The 4.5Hz is used in this study because it fits with one of the patients collaborating in the research. If the TRD is proven to be effective with a particular frequency, it will be effective for any frequency by adjusting its mass and spring characteristics. Using the above equations and values from the table, a Simulink model (Fig. 5) is developed to predict the motion of m 1, m 2 and m 3. Input of the model is a sinusoidal excitation with a preset frequency and 50mm amplitude. These parameters are selected based on the frequency and amplitude of a patient s tremor. To verify the theoretical results obtained from the Simulink model, a prototype of the tremor reduction device (TRD) is designed and manufactured. The prototype is tested on the experimental setup as well on a Parkinson s disease patient. The designed TRD is a metallic flat rectangular box with dimensions 109mm*72mm*9mm. Mass and springs are arranged in this box following the principle of a tuned vibration absorber to suppress the motion in the hand. The actual TRD prototype includes two identical units. Each TRD unit weighs 215 grams including moving mass, springs, metal box and case. Figures 6(a-c) represents TRD with mounting arrangement on the mannequin hand using case with straps. (a) (b) Fig.7 (a) Experimental setup, (b) Close-up view of TRD From the observation of the Parkinson s disease patients, it is understood that hand usually moves mostly in one direction in a certain activity. The direction may change when the patient does another activity. Therefore, the device is designed to address the tremor in one direction. To achieve tremor suppression in all orientations of the hand, the TRD units can be adjusted to an appropriate position along the circumference of the patient s wrist. Thus, when changing activities, the patient only needs to loosen the strap, rotate the TRD units to a new position that is suitable to the intended activity, and tighten the strap for effective tremor reduction. In the test setup shown in Fig.7, the TRD units are positioned to address the tremor in horizontal or vertical direction. Fig.8 TRD with Brace (a) (b) (c) Fig.6 (a) TRD, (b) TRD and Iphone-5c, (c) TRD with case Effectiveness of the TRD is tested on the experimental setup as shown in the Fig.7. A mannequin hand is connected to a base excitation via a pair of springs with known stiffness and damping to represent muscles. In this setup, horizontal and vertical displacement excitations are created by electric motors whose frequencies are controlled to represent the tremor frequencies. Two TRD units (one on top, and on the bottom) are wrapped around the hand using leather casings as shown in Figures 7(a-b). Laser distance sensors are positioned to capture the displacement of the hand in horizontal and vertical directions. While testing the TRD on the mannequin hand and actual human hand, it is observed that the wrist of the mannequin hand is rigid, while that of the human hand is flexible. The flexibility of the human wrist makes the tremor control of the hand difficult. To address this problem, regular hand brace is worn at the hand and wrist to make them relatively rigid but still allow the fingers and palm to stretch comfortably. To this end, the behavior of the human hand is similar to that of the mannequin hand, and the TRD is effective. The brace also serves as the base to avoid direct contact between the device and skin for comfort. Fig.8 showcases the brace between the wrist and the TRD. It can be seen that the two TRD units sit a the top and bottom of the hand and are effective in horizontal direction in this configuration. 3 Copyright 2018 ASME

4 helps reduce a total of 80% of tremor amplitude. This amounts of tremor reduction correlate well with the predicted results from Simulink model. Fig.9 Simulink results with and without TRD RESULTS Simulated results from the Simulink model are shown in Fig.9. Results show that the displacement of the hand (m1) is 4.5 mm with TRD attached, and 30 mm without TRD. It is equivalent to 85% reduction in tremor amplitude. It is worth noting that the simulation is in ideal condition where the damping is only viscous with very low value. Excitation Frequency (Hz) Fig.10 Acceleration of the mechanical arm with and without TRD Another method for assessing the tremor level is using accelerometers. Fig.10 shows the acceleration (in gravitational acceleration g s) of the mechanical arm illustrated in Fig.7 with and without the Tremor Reduction Device. The dotted line represents the acceleration of the arm without the device, while the bold solid line represents that of the arm with the device worn at wrist. It can be seen that the tremor frequency is about 5.5Hz. Without the device, the peak acceleration is averaged at 1.3g. With the device, the peak acceleration is averaged at 0.4g. Those numbers indicate that the device helps reduce 70% of the tremor acceleration amplitude. This number is significant and promising. Table.2 Experimental results with setup Hand Displacement (mm) Only Hand with dead Hand with two hand mass of 530gr TRD units 530gr to To ensure the effectiveness of the TRD, it was tested on the mannequin hand and actual human hand. Table.2 represents the results obtained from the experimental setup illustrated in Fig.7 with the mannequin hand. The experiments were conducted in three settings: without the TRD, with a dead mass equivalent to the TRD s weight and with the TRD to verify the effectiveness of the device. These results are obtained only for the horizontal excitation with 50mm amplitude and multiple frequencies around 4.5 Hz representing the common tremor at patients. Displacements were measured at end of the hand using laser distance sensors. At each frequency, three trials were run, and each trial was about 3 minutes. The numbers were read when the mannequin hand reached steady state in each trial. As mentioned previously, the TRD is designed to address specific frequency that a patient exhibits. It is shown in Table.2 that the designed resonant frequency of the TRD is matching with the excitation frequency at 4.5 Hz where the TRD is the most effective. At other excitation frequency, the reduction is significantly less. Focusing at the excitation frequency around 4.5 Hz, it is observed that attaching the dead mass onto the hand only reduces about 30% of the tremor. However, the TRD Fig.11 Tracings of pre-drawn lines by a Parkinson s disease patient with and without TRD (Green and Red markings with TRD; Black markings - without TRD) 4 Copyright 2018 ASME

5 For checking the performance of the device on patient, the TRD is worn at a patient s wrist with the brace. Patient s job was to trace the pre-drawn line on the white board with markers with different colors. The physical traces were recorded and shown in Fig.11 for evaluation purpose. The free-hand (nothing attached) tracing is in black color and with the highest width amplitude. The green colored tracing represents the case of the patient hand with the TRD. Using the width the tracing curves, the tremor reduction is estimated to be about 75%. This number is very significant in allowing the patient to resume several daily life activities. INTERPRETATION Results from the analytical model, tests on experimental setup and on the patients are matching well. Thus far, two levels of frequencies were tested extensively, 4.5Hz and 5.5Hz, because there were two Parkinson s disease patients being involved with the research. The stated values are the frequencies of their tremor. The parameters in the simulated models, designs of the TRD and experiments were to verify with the two said patients. The overall tremor reduction is about 70% to 80% with the TRD worn at the patient s wrist. The preliminary analytical and experimental results were evaluated with only horizontal orientation. However, the TRD device is believed to work with other directions as well. The immediate work of this study is to verify the effectiveness of the device in various directions of tremor. This device is compact and lightweight at about one pound. It is easily worn and adjustable on the patient s wrist. With the enclosing case design, the TRD unit can be quickly and easily slip in and out. The sleek design makes the patient feel confident wearing the device in the public to manage the tremor symptoms. If patients do not wish to reveal the TRD, its size is compact enough to be easily hidden under long sleeves. Moreover, the device is purely mechanical and therefore fully functional all the time without involving with any power source or harmful electromagnetic waves. Since this device is adjustable around the wrist, patients, while wearing the TRD, can perform any activity they desire. Future works involve with improving the size, weight and appearance of the device to make it ready for market. Also, the device will be verified on various patients with different wrist sizes, tremor frequencies and amplitudes to understand the limitation of the device. It is very important to understand the tremor behavior (frequency and amplitude) of each patient to design the TRD suitable for specific person. The device is expected to be ultimately in the market by early ACKNOWLEDGMENTS The New California Venture LLC is acknowledged for providing seed fund to start this project. REFERENCES Rocon, E, Belda-Lois, J. M., Ruiz, A. F., Manto, M., Moreno, J. C. and Pons, J. L., "Design and Validation of a Rehabilitation Robotic Exoskeleton for Tremor Assessment and Suppression," IEEE Transactions on Neural Systems and Rehabilitation Engineering, vol. 15, no. 3, pp , Sept. 2007, doi: /TNSRE Manto, M., Topping, M., Soede, M., Sanchez-Lacuesta, J., Harwin, W., Pons, J., Williams, J., Skaarup, S. and Normie, L., "Dynamically responsive intervention for tremor suppression," IEEE Engineering in Medicine and Biology Magazine, vol. 22, no. 3, pp , May-June 2003, doi: /MEMB Terry, T. E. and Hoyt, L. J., Sr., Cerebral palsy arm and hand brace, Patent US A, 17 Apr Thomas, R. A., Arm constraint, Patent US A, 15 Nov Asatourian, A., Hyun, B. S. and Lipaz, G., Method for Treating Tremors, Patent US B1, 26 Mar Handforth, C., Firm-contact apparel prosthesis for tremor suppression and method of use thereof, Patent US A1, 22 Jan Kalvert, M. A., Adjustable and tunable hand tremor stabilizer, Patent US B2, 4 May Li, L., Modelling and tremor and suppression of human arm with Parkinson s disease, Master s thesis, Department of Mechanical Engineering, University of Waterloo, Ontario, Canada, Hashemi, S. M., Golnaraghi, M. F., and Patla, A. E., Tuned vibration absorber for suppression of rest tremor in Parkinson s disease, Medical, Biological Engineering and Computing, vol. 42, pp , 2004, doi: /BF Copyright 2018 ASME