Biomechatronic Systems

Transcription

1 Biomechatronic Systems Unit 4: Control Mehdi Delrobaei Spring 2018

2 Open-Loop, Closed-Loop, Feed-Forward Control Open-Loop - Walking with closed eyes - Changing sitting position Feed-Forward - Visual balance control Closed-Loop (negative feedback) - Body temp. control - Blood sugar control - Vestibular balance control Closed-Loop (positive feedback) - Internal bleeding (hemorrhage)

3 Sample of Closed-Loop Control: Blood Glucose control

4 Sample of Closed-Loop Control: Body Temperature Source: schoolbag.info

5 Neuro-Musculo-Skeletal System (NMS) Human motor control is a combination of three systems: (1) Neural: the brain, the spinal cord and the peripheral nerves, (2) Skeletal: body bones and joints, including the spine, and (3) Muscular: mainly the muscles and tendons. Proprioception: conscious or unconscious awareness of joint position, posture, and equilibrium condition. Sensory-Motor Integration: the process of perceptionaction coupling. Sensory-Motor Plasticity: changes in the cortex that are linked to changes in movement or behavior Source: eglaufwellnesscenter.com

6 Proprioception

7 Sample Biomechatronic Control Source: M.Sc. Thesis by Roozbeh Borjian

8 Sample Biomechatronic Control Deep Brain Stimulation (Open-Loop Control) Implantable Cardioverter Defibrillator (Closed-Loop Control) Source: youtube.com

9 Sample Biomechatronic Control: Deep Brain Stimulation * DBS is an effective treatment for movement disorders, including Parkinson s disease and essential tremor. However, the mechanisms of action through which DBS works are unclear. Device tuning is essentially a trial and error process with associated difficulties of time, expense, and patient discomfort. * Kuncel, Alexis M., and Warren M. Grill. "Selection of stimulus parameters for deep brain stimulation." Clinical neurophysiology (2004): :34 PM 9

10 Anatomical Targeting Successful treatment with DBS depends on accurately placed electrodes. Anatomical targeting involves: Determining where to put the electrode, where to direct the electric current. Common targets for DBS: The subthalamic nucleus (STN) for Parkinson s disease, The GPi for dystonia and Parkinson s disease, The VIM of the thalamus for essential tremor. 6:34 PM 10

11 Variation in Anatomical Targeting It is unknown whether the excitation of the neural elements in the STN and/or in the surrounding fiber tracts is responsible for the clinical improvements. Determining exactly where the DBS lead is located is difficult because anatomical images acquired from magnetic resonance imaging have distortions due to the presence of the DBS electrode. 6:34 PM 11

12 Electrical targeting Electrical targeting is used to control neural activation by controlling the spread of the electric field and by selectively activating neural elements. The spread of electric field depends on: the location of the active contact, the electrode geometry, the electrical properties of the surrounding tissue. 6:34 PM 12

13 Electrical targeting: Effect of contact location on distribution of the electric field (a) Model of the DBS lead with four contacts centered in the STN, surrounded by white fiber tracts and grey matter. Distribution of electric potential, magnitude of the electric field, and magnitude of the activating function when monopolar stimulation (V=1v) applied in (b) white matter (conductivity of 0.2 S/m) through Contact 0 and (c) in grey matter (conductivities of 1 S/m in the direction parallel to the fibers and 0.1 S/m in the direction perpendicular to the fibers) through Contact 1. The magnitude of the activating function represents the strength of the polarization on the surrounding neural elements. 6:34 PM 13

14 Electrical targeting: Effect of electrode geometry on distribution of electric field 6:34 PM 14

15 Stimulus parameters Medtronic s Soletra Model 7426: voltage ranging from V in V increments, pulse widths from ms in 30 ms increments, frequencies from Hz in 5 Hz increments. (a) (b) 25,480 available combinations of pulse width, frequency, and voltage. - The manufacturer has recommended a charge density limit of 30 μc cm 2 ; Charge density = (Amplitude * Duty-cycle)/30. - Even with the charge density restriction, there are still 12,964 combinations of voltage, pulse width, and frequency available. - Typical DBS parameter settings to treat PD: For STN DBS: 3 V, 82 ms, 152 Hz For GPi DBS: 3.2 V, 125 ms, 162 Hz 6:34 PM 15

16 Stimulus parameters In post-surgical management of DBS, the goal is to find an optimal combination of pulse width, frequency, and voltage. The optimal combination would best reduce symptoms, minimize side effects, and minimize power consumption. Low power consumption would increase battery life and decrease the risk of tissue damage. Finding the optimal setting is complicated because the optimal settings for reducing symptoms, minimizing side effects, and minimizing power consumption may be different. 6:34 PM 16

17 Stimulus parameters: Strength duration relationship The threshold stimulus amplitude required to excite neural elements, Ith; decreases as pulse width increases. - Irh: the minimum current needed to excite neural elements with a pulse of infinite duration, - PW: pulse width, - Tch: the chronaxie, defined as the threshold pulse width when the stimulus amplitude is equal to twice the Irh. 6:34 PM 17

18 Stimulus parameters: Charge duration relationship The threshold charge to excite neural elements increases as the pulse width increases. - Irh: the minimum current needed to excite neural elements with a pulse of infinite duration, - PW: pulse width, - Tch: the chronaxie, defined as the threshold pulse width when the stimulus amplitude is equal to twice the Irh. Increasing pulse width has opposite effects on the threshold current and threshold charge. Short pulse widths require high current but low charge, and are thus more efficient at exciting neural elements. 6:34 PM 18

19 Stimulus parameters: Charge density limit for non-damaging stimulation Charge and charge density are important stimulation parameters to consider when determining the threshold and severity of tissue damage. Charge density is defined as the charge divided by the geometric surface area of the electrode, which is 0.06 cm 2 for the DBS contact. Medtronic provides a level of charge density above which tissue damage may occur, thus restricting the stimulus parameter combinations. The charge density limit, 30 μc/cm 2, was extracted from previous studies on charge, charge density, and tissue damage 6:34 PM 19

20 Setting the stimulus parameters: Pulse Width Short pulse widths have been found to increase the dynamic range between clinical benefit and adverse side effects (therapeutic window). The results suggest that DBS devices should be programmed with the shortest possible pulse duration, and that future generation stimulators should include lower ranges of pulse widths. The disappearance of wrist rigidity with the patient resting as the clinical constant effect to obtain required clinical effect, RCE. 6:34 PM 20

21 Setting the stimulus parameters: Frequency Tremor, rigidity and akinesia are only reduced for frequencies above 50 Hz. The effectiveness increases linearly with increasing frequency, the maximum occurs around 130 Hz. There is a further small, nonlinear increase in efficacy above 130 Hz, up to the maximum frequency of the device. High frequency stimulation requires higher stimulus amplitudes, so decreases battery life. Frequency is often set to 130 Hz as a compromise between power consumption and clinical efficacy. 6:34 PM 21

22 Setting The Stimulus Parameters: Amplitude The stimulus amplitude required to activate neural elements depends on the spatial relationship between the electrode and the nerve fiber. DBS studies have shown that the clinical benefits saturate above a certain voltage. Tremor, bradykinesia, and rigidity progressively improved between two and three volts and did not continue to improve beyond 3 V. A linear increase in voltage does not correlate to a linear increase in the volume of neural elements excited, but it increases the power consumption. Voltages above 3.6 are generally avoided clinically. 6:34 PM 22

23 Conclusions and recommendations Currently, there are several challenges to rational selection of optimal stimulation parameters for DBS treatment. These challenges include: the large number of degrees of freedom in stimulus parameters and electrode geometries, the variability and uncertainty in electrode positioning, the unknown effects of stimulation, the complexity and diversity of responses to DBS. Fundamental understanding of electrical stimulation of the nervous system obtained through theoretical and empirical studies can guide selection of appropriate stimulus parameters. Less time-intensive tuning methods based on an understanding of the influence of changes in parameter settings on the effects of DBS must be developed. 6:34 PM 23