20-Bio-B8 Applied Optics_Photonics · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2013 — 04-Bio-B8 Rehabilitation Engineering. Three hours, open book, non-communicating calculator permitted. Seven questions of equal value (20 marks each); five constitute a complete paper and only the first five appearing in the answer book are marked. All seven are solved here as a complete study resource. Every question is an essay/design question (block-diagram assistive-technology system design, or descriptive explanation).
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Approach. A pair of surface EMG electrodes on the trapezius supplies a volitional command signal, which is conditioned and converted to a proportional stimulation-current command that a second, current-controlled stimulator delivers to surface (or percutaneous) electrodes over the forearm finger-flexor mass, closing the hand with a grip strength that tracks the strength of the shoulder contraction.
Block functions and specifications.
| Block | Function | Specification |
|---|---|---|
| Surface EMG electrodes (trapezius) | Bipolar Ag/AgCl pair placed along the upper trapezius muscle belly (~2 cm inter-electrode spacing), with a third reference electrode over an electrically neutral bony landmark (acromion); picks up the volitional myoelectric command. | Input impedance > 10 MΩ at the amplifier; skin prep to < 5 kΩ contact impedance. |
| Instrumentation amp + band-pass filter | Differential amplification (high CMRR rejects mains/motion artifact common to both electrodes) and band-pass filtering to the raw sEMG spectrum. | Gain 1000–5000×; CMRR > 100 dB; pass-band 10–500 Hz. |
| Rectifier + envelope low-pass filter | Full-wave rectification followed by low-pass filtering extracts the slowly-varying EMG envelope, which tracks overall contraction intensity (motor-unit recruitment and firing rate). | Low-pass cutoff 0–10 Hz (linear envelope detector). |
| Proportional controller | Compares the envelope to a resting/noise threshold (rejects baseline EMG noise so the hand does not close involuntarily), then maps supra-threshold envelope amplitude linearly onto a desired stimulation-current command. | Adjustable threshold (calibrated per-subject to ~10–15% of the subject's maximum voluntary trapezius contraction); linear gain to 0–100% of full-scale stimulation current. |
| Current-controlled stimulator | Generates charge-balanced biphasic current pulses whose amplitude (or pulse width) is set by the controller, driving the forearm flexor electrodes; current control (not voltage control) keeps delivered charge, and hence force, insensitive to electrode-skin impedance drift. | Biphasic pulses, 20–50 pulses/s, 0–60 mA (or 0–300 µs pulse width at fixed amplitude), charge-balanced to avoid electrochemical tissue injury. |
| Stimulating electrodes (forearm flexors) | Surface (or percutaneous, for more selective activation) electrodes placed over the flexor digitorum superficialis/profundus motor points on the volar forearm, close the fingers around the object when driven above motor threshold. | Self-adhesive Ag/AgCl surface pads, 2–4 cm², positioned by motor-point mapping (stimulate and observe finger flexion) before each session. |
The stimulation pulse rate (20–50 pulses/s) is chosen above the muscle's tetanic-fusion frequency so the forearm produces a smooth, sustained grip rather than a series of discrete twitches, while staying low enough to delay the onset of electrically-stimulated muscle fatigue (which occurs faster than in voluntary contraction because FES recruits motor units in a fixed, non-physiological order).
