Outline from Webster, Biomedical Instrumentation (see course home page for details on how to purchase the book).

Transducer - a device that converts energy from one form to another.

Sensor - converts a physical parameter to an electrical output.

Actuator - converts an electric signal to a physical output.

Displacement measurements - these can measure size, shape, and position of the organs and tissues of the body. Example of direct measurements is to determine the change in diameter of vessels and changes in volume and shape of cardiac chambers. Example of indirect measurements is to quantify movement of liquids using a microphone.

Consist of potentiometers and strain gages.

Potentiometer - is a resistor in which there is a moveable conductive arm or slide. With a change in position of the slide along the length of the resistor, the resistance from the slide to a base point changes. Resistance increases as the length of the resistor in the circuit increases. Potentiometers usually have constant resistivity (resistance per length) so that there is a linear relationship between position of the slide and resistance. The resistance can be transformed into a voltage when current is applied to the circuit.

Strain gages - are usually fine wires which are bendable or stretchable. When the wire is strained within its elastic limit, the resistance changes according to a known function. If the wire is strained past its elastic limit, it would damage the strain gage material and it will no longer function properly. Resistance is given by the equation:

we arrive at the equation:

where the first term on the right is the dimensional effect and the second term is the piezoresistive effect. The gage factor is:

Elements such as nickel and silicon are used as strain gage materials and these have differing gage factors. The choice of gage factor will depend on the length that the material will be strained during normal measurement. When very little strain will be put on the device so that the length changes only slightly, the gage factor should be large so that the change in resistance will be detectable. When large strains are put on the material which produce a large change in length, the circuit designer may wish to use a material with a small gage factor so there is not a too wide swing in resistance which might affect the circuit. Unbonded strain gages are connected to a housing only at their ends, whereas bonded strain gages are made to adhere to a bendable or stretchable substrate, usually by means of an epoxy.

Bridge circuits are commonly used in the electronics which measure small changes in resistance. The bridge may consist of up to four variable resistors. The variable resistors in the circuit are either the strain gages, or the potentiometers.

Inductive sensors are often used to measure displacements. The equation for inductance is:

A capacitor consists of two parallel conductive plates, attached to wires at the ends, with an insulating material sandwiched in between. The equation for capacitance is:

Where k is the sensitivity, which increases as the plate separation decreases. By combining the last two equations we obtain:

which is an expression which shows the percent change in C about any neutral point is equal to the per-unit change in x for small displacements. When a capacitative sensor is connected to a voltage source and an amplifier, it can be shown that the transfer function is:

One should recognize the transfer function as being a high pass filter. As the frequency increases, the output is a constant E/x₀. As the frequency approaches zero, so too does the output.

Piezoelectric materials generate an electrical potential when mechanically strained, and conversely an electric potential can cause physical deformation of the material. The induced charge in response to a mechanical strain is

q = k F

where k is the piezoelectric constant and F is the applied force. Assuming that the system acts as a parallel plat capacitor where the voltage V across the capacitor is charge q divided by capacitance C, then:

Hence, the voltage across the piezoelectric material is directly proportional to the applied force. It will be larger when the surface area A of the material is smaller and when the plate separation x is increased.

Temperature measurements provide information concerning the state of the patient and whether there is an infection or inflammation, which raises the temperature locally or throughout the entire body. There are several types of temperature measurement devices.

Thermocouples are based on the principle that a voltage difference exists across the junction of two dissimilar metals. An empirical formula is usually used to model the relationship of temperature to voltage difference at the junction of the form:

E = a T + 0.5 b T² + ...

Where a and b are constants, T is the temperature in degrees Celsius, and E is the electromotive force (voltage drop).

Thermistors are semiconductors made of ceramic material that are thermal resistors with a high negative temperature coefficient. This means that the resistance of thermistors decreases as the temperature increases. These devices are small in size and they can be made << 0.5mm. An empirical relationship is also used to describe these devices, of the form:

Where T is the absolute temperature in degrees Kelvin.

Radiation thermometry is based on the known relationship between the surface temperature of an object and its radiant power. This principle makes it possible to measure the temperature of a material without physical contact with it. It is based on the expression of the radiant flux per unit area and wavelength:

Optical measurements are used, for example, to determine the content of a solution. Often, filters and lenses are used to gather and focus the light from an intense lamp. Tungsten wire filament lamps are most often used for illumination.