The system hardware
Basic Block of System Hardware
The basic block here describes the whole system in which EMG electrodes attached to human hand is acquired and amplified using Instrumentation Amplifier. Then filtering is done to remove 50Hz interference noise and selective frequencies are allowed using wide Band Pass filter. Then filtered signal is fed to a comparator or Analog to Digital Converter which serves as a input for the microcontroller to actuate the six servo motors of a Robotic Arm.
EMG SENSOR/EMG ELECTRODES:
Different kinds of medical instruments require different types of electrodes. For instance, the ECG requires surface electrodes; the EMG uses either surface electrodes or needle electrodes. Electrodes are actually a type of transducer, which extracts the signals. We can obtain the EMG signal simply by placing a surface electrode on the skin enveloping the muscle or by applying an inserted electrode in the muscle.
The two electrode configurations are
- Monopolar configuration
- Bipolar configuration
In the monopolar configuration, one electrode is placed on the particular muscle site. Other one is a reference electrode, which is placed on a site with minimal electric association with the inserted site. The drawback of this monopolar configuration is that it detects not only the signal from the muscle of interest but also unwanted signals from around the muscle of interest.
In the bipolar configuration, two electrodes with a small distance between each other are placed in the muscle to pick up the local signals within the muscle of interest. A differential amplifier amplifies the signals picked up from the two electrodes with respect to the signal picked up by a reference electrode. Because the interference signals from a distant source are essentially equal in magnitude and phase as detected by the two electrodes, the common mode rejection capability of the differential amplifier eliminates the unwanted signals.
TYPES OF ELECTRODES
Electrodes used in EMG can be classified into two types:
- Surface electrode (or Skin) and
- Inserted (Wire or Needle) electrode.
Surface electrodes may be constructed as either passive of active units. In passive units electrodes consist of detection surface that senses the current on the skin through its skin-electrode interface. In the active configuration, the input impedance of the electrode is greatly increased by electronic means, rendering it less sensitive to the impedance of the electrode-skin interface. The electrode impedance can be reduced by applying conducting gel in the skin-electrode interface and can be further reduced by removing the dead surface layer on the skin along with protective oils; this is best done by light abrasion of the skin. Active electrodes are one which can be either resistively or capacitively coupled to the skin. Although the capacitively coupled electrodes have the advantage of not requiring a conductive medium, they have a higher inherent noise level. Also these electrodes do not have long-term reliability. For these reasons they have not yet found a place in electromyography. The disadvantages of surface electrodes are that they cannot effectively detect signal from muscles deep beneath the skin and that because of poor selectivity, they cannot eliminate cross-talk from adjacent muscles
Surface Electrodes are applied in Time-Force relationship of EMG signal, Kinesiological studies of surface muscles, Neurophysiologic studies of surface muscles, Psycho physiological studies, interfacing an individual with external electromechanical devices.
Applications of Needle Electrodes are in manipulating MUAP Characteristics and also in the manipulation of Control properties of motor units (firing rate, recruitment) and find its field in exploratory clinical electromyography.
Wire Electrodes are applied in Kinesiological study of deep muscles, neurophysiologic studies of deep muscles, limited studies of motor unit properties and in Comfortable recording procedure for deep muscles.
Needle and wire electrodes are common in EMG detection. They have diameter in the order of 150 to 25 micrometers. Because of the extremely small surface they are used to pick up action potentials from single motoneurons and are also painless. They are highly non-oxidizing, stiff wire with insulation. Alloys of Platinum, Silver, Nickel and Chromium are preferable. The alloy of 905 Platinum with 10% Iridium offers appropriate combination of chemical inertness.
SELECTION of Electrode Material
Generally, Noble metals like Gold, Silver, and Platinum are chosen because of their Corrosion resistance. Silver has better mechanical strength. Platinum is used for pacemaker casing. Stainless steel and their alloys are possible alternatives for their low cost and good strength.
Many types of electrodes require additional conductive gel to enhance the electrical connection. However, older types of conductive gel contained some ingredients that increased the chance of bacterial growth, such as, polysaccharides and some kind of thickener, which are food for bacteria. Most conductive gel consists of sodium chloride, which provides good electrical contact. However, chloride ion can cause skin irritation.
Antibacterial and antifungal ingredients are added into the conductive gel to prevent bacterial growth. The ingredients, such as, methyl P-Hydroxyl benzoate, zephiran chloride and xylenol have been added to prevent and retard bacterial growth.
Important properties that conductive gel should possess are:
- Has low moisture vapor transmission rate so that the gel can stay moist during use and for the shelf life.
- The gel should have some antibacterial/fungi or disinfected with gamma ray to prevent microorganism and mold growth.
- Have ionic salts or surfactants so that it provides low electrolyte-skin impedance. Causes less skin irritation.
Placement of Electrodes:
Electromyogram Sensor is made of Ag/AgCl electrodes and it consists of a 3-pin jack to connect to Instrumentation Amplifier.
Here we have one electrode as a common ground placed on the wrist end of the hand and other two electrodes separated from each other by few inches placed on
- Extensor Carpi Radialis
- Flexor Carpi Radialis-2 Pairs
- Biceps Brachi