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Sensors

Sensors (in medicine) are special technical devices that convert one physical phenomenon to another in equivalent terms.

In medicine and biology, sensors are often used that convert mechanical displacement, light intensity, temperature, and other physical quantities into electrical signals. Sensors, usually of small size and low weight, make it possible to conduct telemetric observations and recording, and to measure small and rapidly changing values. All sensors on the design and principle of converting the input signal into an electrical divided into dynamic and static.

Among dynamic sensors, piezoelectric and electrodynamic are widely used. Piezoelectric sensors are used to register various processes associated with the movement and vibration of body parts (pneumography, pulsography, mechanocardiography, sphygmography ). The thermocouple used to measure the temperature of organs and tissues also applies to dynamic sensors (Fig. 1).

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Fig. 1. Thermocouple (1) and thermopile (2)


Fig. 2. Scheme of the electrodynamic sensor: 1 - thrust; 2 - movable magnet; 3 - induction coil; 4- control spring.
Fig. 3. Hansen capacitive sensor circuit: 1 - output cable; 2 — isolation; 3 - fixed plate capacitor; 4 - movable plate of the capacitor; 5 - the place of application of pressure


Fig. 4. Scheme of the mechano-electronic sensor (mehanotron): 1 - flexible metal diaphragm; 2 - rod; 3 - movable anode; 4 - mesh; 5 - cathode; 6 - metal shell, connected to the anode. Fig. 5. Diagram of the photoelectric sensor: 1 - photocathode; 2 - anode
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Electrodynamic sensors (Fig. 2) are used to register various kinds of movements of body parts (ballistocardiography, dynamocardiography).

Static sensors, unlike dynamic, have an external power source. Being a controllable element of an electrical circuit, they convert a negligible effect from an object into a large electrical output signal. Controlled circuit elements can be resistance, inductance and capacitance. In accordance with this, resistor (ohmic), inductive and capacitive sensors are distinguished.

The ohmic sensors include a carbon microphone; strain gauge for recording voltages; thermistor for recording temperature. In the device of the latter, the principle of change in resistance of oxides of some heavy metals with a change in temperature is used, which allows determining it with a high degree of accuracy.

Most often they use capacitive sensors, which are highly sensitive to various kinds of displacements and work on changing the capacitance of a capacitor when the distance between its plates changes. They are used to register various movements that occur in organs and tissues, such as movement of limbs, movement of blood through vessels, and blood pressure (Fig. 3).

Special types of sensors include a mechano-electronic - mechanotron (Fig. 4) and an optoelectronic - photoelectric (Fig. 5), based on a change in the electron flux density between the electrodes of an electron tube or transistor. In mechanotrons, this is achieved by varying the distances between the movable anode and the cathode. They are very sensitive and are used to register mechanical oscillations of a very small amplitude (pulsation of cells, vibrations of the walls of capillaries). As a photoelectric sensor, a phototransistor or photocell is used, recording changes in the density of the light flux emanating from the object or passing through it (photometry of various tissues during their histological examination, oximetry - determination of blood oxygen saturation).