Chapter 5: Transducers

Input and Output Transducers
Active and Passive Transducers
Wheatstone Bridge or Full Bridge
Integrated Sensors – included embedded amplifier
Force Sensitive Resistors
Piezo Film and Devices
Nitinol – Muscle Wire
Solenoids, Motors
Pressure, Force and Strain
Pressure PadBlood Pressure Measurements
Oscillometric
Tomography
Finger Cuff
Optical Transducers and Sensors: Pulse Oximeter, Plethysmogram, CO2 Sensor, NIRS, fNIR
Pulse Oximeter
Photo Plethysmogram
CO2 Gas Sensor
NIRS and fNIR
Plethysmogram Transducers
Oxygen Paramagnetic Sensor
Thermocouples and Thermistors
Goniometers and Torsiometers
Angular Rate Sensor (Gyro)
Respiratory Effort
Heart Sounds
Vibromyography
Fiber Optic Temperature and Pressure
Accelerometer
Inclinometer
Humidity
Thermopile
Hall Effect
Laser Doppler Flow
Radar Doppler
Electrodermal Activity and Response
Bioimpedance and Cardiac Output
Pyroelectric Sensors
Josephson Junctions (SQUID Magnetometer)
Inductive Sensors
Capacitive Sensors
Resistive Sensors
Proximity Sensors
Magnetoresistive Sensors
Electronic Stethyscope or Stethysphone
fNIR Transducer
MEMS Devices

Input and Output Transducers

A transducer is a physical device which transforms one kind of energy into another. Most commonly, and in regards to the field of physiological monitoring, an input transducer is used to convert various kinds of physical energy (such as related to pressure, temperature, force, sound and acceleration) into electrical energy (such as related to electrical current or voltage). An input transducer is also called a sensor. An output transducer operates in reverse, namely it will convert electrical signal energy into energy in a different physical domain.

An example of an input transducer is a microphone. A microphone will convert received sound pressure waves (acoustical energy) into proportional electrical energy in the form of a changing potential. An example of an output transducer is a speaker. A speaker converts a varying electrical current into a proportionally changing sound pressure wave.

When using transducers to sense changes in the physical environment, it’s very helpful to convert the physical variable being measured into a corresponding electrical signal. This is because there is sophisticated technology available to amplify, filter and digitize electrical signals. Similarly, when attempting to create a stimulus in the physical environment, it’s optimal to process an electrical signal (typically in digital form, via software) in the desired fashion prior to converting the signal into energy of a different physical form.

For maximum flexibility and capability it’s typically best to sense the environment using the desired input transducer, then employ an amplifier/conditioner to direct the electrical signal from the transducer to an analog to digital converter. Once digitized the signal can be subject to whatever software-based processing. When attempting to influence the environment, via some type of physical stimulus, it’s optimal to employ software to establish the nature of the stimulus signal and then use a digital to analog converter to transform the digital -software produced- signal back into an electrical signal form. Finally, an output transducer can be used to transform the electrical signal into the desired physical signal.

Active and Passive Transducers

Input transducers (sensors) come in two basic types – active and passive. An active sensor simply converts the ambient physical variable monitored directly to a corresponding electrical signal. Examples here include solar cells, piezo-electric devices and thermocouples. Solar cells or photo-voltaic devices directly convert absorbed photons to electrons. Piezo-electric sensors convert physical strain directly to electrical charge. Thermocouples convert heat differentials directly to voltage differentials. A passive sensor does not directly convert physical energy into electrical energy. Instead, passive sensors convert the ambient physical variable monitored into a variable impedance, such as capacitance, inductance or resistance. In order to produce a corresponding electrical signal, these circuit elements require activation as part of a energized network. This signal voltage can be then be amplified and converted into a digital form.

Output transducers, common types including speakers, actuators, LEDs and Piezo devices, operate by converting electrical signals into corresponding physical changes. Many output transducers are considered to be passive, because the driving electrical signal can incorporate sufficient energy to produce the desired physical change. In the case of speakers, the electrical signal is an electrical current. The electrical current creates a proportional electromotive force, via a coil, that acts to push against the magnetic field established by a permanent magnet. The coil is attached to a flexible membrane, called a cone, and as the electrical current varies though the coil, the membrane moves to produce audible sound pressure waves. Many actuators behave similarly to the speaker. A supplied electrical current acts to create an electromotive force, via a coil, to push against an existing magnetic field to effect a physical motion. LEDs operate according to the principle of electroluminescence whereby electrons are recombined with electron holes and, in that process, photons are released. Piezo-electric output transducers respond to the application of external voltage by expanding or contracting along a specific crystal axis. As the applied voltage changes, the crystal will physically distort in correspondence.

Wheatstone bridge or full bridge

A common transducer circuit network configuration is called the Wheatstone Bridge or full bridge. This type of input transducer incorporates four circuit elements (impedances) configured as shown in Figure X. All four of the impedances vary in proportion to the sensed physical signal. As the physical signal increases, two of the impedances increase as the other two decrease. Four sensing elements act to quadruple the sensitivity of the transducer, compared to the situation where just one sensing element is used. Additionally, if all sensing elements have identical construction, then their less desirable characteristics, such as drift with time and temperature cancel each other out. Furthermore, because a full bridge is designed to connect to a differential amplifier, this results in improved CMRR. Accordingly, a full bridge is a very stable and sensitive transducer design.

Integrated Sensors

An integrated sensor incorporates sensing element(s), electrical network, amplification and conditioning. This type of sensor typically require an excitation voltage and the sensor output can connect directly to an analog to digital converter. Integrated sensors, for nearly any type of physical energy sensing, are becoming increasingly available. Recent developments with integrated sensors employ MEMS technology. Silicon micro-machines are presently being used to measure acceleration, pressure, air flow and rotational rate.

Force Sensitive Resistors

A Force Sensitive Resistor (FSR) converts force or pressure into a corresponding resistance value. A FSR is a laminated sensor which incorporates a conductive polymer and conductive electrodes. The conductive polymer changes resistance in a repeatable manner with applied force to its surface. The conductive polymer is applied as sensing film and consists of both electrically conducting and non-conducting particles suspended in a matrix. Applying a force to the surface of a the sensing film causes particles to touch the conducting electrodes, changing the resistance of the film. All resistive based sensors, including FSRs, only require a simple interface design and can operate satisfactorily in moderately hostile environments. However, FSRs can be damaged if pressure is applied for a long time periods. Compared to other force sensors, the advantages of FSRs are their size, low cost and good shock resistance. A disadvantage of FSRs are their low accuracy. FSRs are used as input transducers.

Piezo Film and Devices

Piezo devices (film, ceramics) convert strain variation present within the film or ceramic into corresponding charge manifesting across the sensor surface. Piezo transducers are bidirectional, meaning that they can be used as both input and output transducers. Piezo elements, called “benders” are useful as touch indicators and as buzzers. Ceramic piezo devices are very useful in difficult environments, such as where liquid egress is commonplace or where the ambient magnetic field is very strong, as in the fMRI.

Nitinol – Muscle Wire

Nitinol is a Nickel-Titanium alloy which undergoes a state change when the material reaches a specific transition temperature. The state change results in a physical contraction of the material when the transition temperature point is exceeded. When the temperature drops below the transition point, the material expands back to it’s original state. Typically, Nitinol can only be used for relatively long cycle time mechanical actuation, as thermal inertia is a limiting factor on actuation relaxation. Nitinol is used as an output transducer.

Solenoids, Motors, Speakers

Solenoids, motors and speakers are devices which convert electrical energy (current) into physical motion. These are electromechanical transducers. These electromechanical transducers usually operate as output transducers, but can be used as input transducers too.

Pressure, Force and Strain

Force and strain transducers are typically constructed as full bridges. Often, these transducers include amplified outputs. Typically, they are designed so one leg of the full bridge decreases in impedance, and another leg increases in impedance, with applied force. A simple conceptual example involved the flexing of a metal beam. When flexed under strain, one side of the beam stretches and the other side compresses. If strain measurement elements have been bonded to each side of the beam, the stretched strain element increases in resistance and the compressed strain element decreases in resistance. Pressure transducers are typically MEMS devices.

Pressure Pad

A pressure pad is a compliant air bladder which can be placed on different parts of the body to measure some physical displacement of the body. A commonly known type of pressure pad is the blood pressure cuff. A blood pressure cuff is wrapped around the upper arm and, as it begins to inflate via an air pump, becomes sensitive to volumetric changes in the cuff-surrounded arm. The air in the cuff is able to couple to the changes in pressure of the brachial artery.

Blood Pressure Measurement

Oscillometric Method

Based on the observation that beat-to-beat pressure fluctuations in a blood pressure cuff vary as a consequence of slowly changing pressure in that cuff. The fluctuations start at pressures higher than systolic and can be measured below diastolic. The fluctuations are maximized near mean arterial pressure. Systolic and diastolic pressures are estimated via an algorithm based on the measured fluctuation amplitude changes, during slow pressure decrease, from above systolic to below diastolic. The artery being measured becomes fully occluded, during the test, so the method is not suitable for beat-to-beat blood pressure measurements.

Tonometry Method

Based on the observation that when mechanically pressing an artery against a bone, the pressure fluctuations in the mechanical pressor are proportional to the arterial pressure. This method typically employs measurement at the wrist because the radial artery rides on top on the radius bone. The pressure sensing aspect (pressor sensor) of the mechanical pressor must be placed on the center of the artery for proper measurement. In practice, this requirement has been accomplished by employing an array of sensors situated on the artery. The radial artery is never fully occluded, so the method can provide beat-to-beat blood pressure measures.

Finger Cuff Method

Based on the observation of photoplethysmogram variations, in the finger, when subject to changing pressure provided by a partially occluding cuff. The photoplethysmograph signal is used as an input to a servo that controls the pressure in the cuff. The cuff pressure becomes reflective of the finger arterial pressure. The finger artery is never fully occluded, so the method can provide beat-to-beat blood pressure measures.

Optical Transducers and Sensors: Pulse Oximeter, Plethysmogram, CO2 sensor, NIRS, fNIR

Optically-based transducers are increasingly used in physiological monitoring. There are many types of physiological variables which can be sensed using optical methods. Blood oxygen level (SpO2), carbon dioxide percentage in expired air, and the blood volume pulse (BVP) can all be sensed using optical sources and detectors. Blood oxygen is measured in accordance to Beer-Lambert equation. In this case two wavelengths of light are introduced through blood invivo. The log of the ratios of the absorbances is proportional to the saturation of oxygen in the blood. Carbon dioxide concentrations in air are measured in a similar manner. The BVP can be measured with a single light source and detector by simply measuring the change in optical transmission from source to detector.

Plethysmogram Transducers

There are several types of Plethysmogram Transducers. These types of transducers are used for measuring volume variations in a limb, organ or other body part. There are four basic types:

Impedance Plethysmography: Employs the principle that impedance changes are related to body volume under measurement. Typically, an alternating current is directed through a tissue volume and the resulting alternating voltage, developed across the tissue volume, is simultaneously measured. In this manner, both the magnitude and phase of the tissue impedance can be measured for any specific alternating current frequency.

Photo Plethysmogram Transducer: Employs photonic-based transmission or reflection on body part to measure changes in tissue volume as related to variations in blood volume. Typically used to measure blood volume pulse (BVP).

Pneumo Plethysmogram Transducer: Employs a partially-inflated pressure cuff placed around the digit, limb or other body part. As volume of tissue surrounded by cuff changes, pressure changes in accordance. Pressure variations can be measured using a pressure transducer coupled to cuff volume.

Strain Plethysmogram Transducer: Employs a liquid metal (mercury or indium-gallium) strain gauge transducer placed circumferentially around the digit, limb or body part of interest. As the underlying tissue volume varies, the associated circumference changes are sensed by the liquid metal transducer as it stretches or contracts to follow the changes. The liquid metal transducer consists of flexible tubing (usually silicone-based) filled with liquid metal. As tubing is stretched, the average diameter of tubing narrows, thus increasing the end-to-end resistance of the transducer. These transducers have low resistance, on the order of a few ohms or less. To operate properly, the transducer should have a circumference of about 90% of the tissue volume circumference to be sensed.

Oxygen Paramagnetic Transducer

This transducer measures oxygen concentration in air via the principle of paramagnetism. Paramagnetism refers to a property of materials in which they are attracted to a magnetic field. Paramagnetism results from the presence of least one unpaired spin in the material’s atoms or molecules. Gaseous oxygen (O2) has two unpaired electrons so it has paramagnetic characteristics. Most other gases are not attracted by magnetic fields, including carbon dioxide (CO2) and nitrogen (N2).

The oxygen in the air sample, measured by the transducer, is attracted to a strong magnetic field in the transducer. As the oxygen is displaced inside the air sample, an element is moved which converts the oxygen movement to voltage. Any oxygen present in the air sample will be attracted to the strongest part of the magnetic field inside the transducer. One method, to detect this movement of oxygen, involves two nitrogen filled glass spheres that are mounted on a rotating element within the magnetic field. A mirror is mounted centrally on the suspension. Light is shone onto the mirror. The reflected light is directed towards two photocells. Oxygen attracted to the magnetic field will displace the nitrogen filled spheres, causing the element to shift its angle. The photocells detect the angular movement and generate a signal. The signal generated by the photocells is passed to a feedback system. The feedback system sends current through a wire mounted on the suspension. This current creates an electromotive force, which acts to keep the element in its original position. The current flowing through the wire is measured and is proportional to the concentration of oxygen within the gas mixture.

Thermocouple and Thermistors

Thermocouples and thermistors are designed to measure relative changes in temperature. A thermocouple consists of two conductors of different metals (or metal alloys) that produce a voltage around the point where the two conductors are in bonded contact. The voltage is related to the difference of temperature of the bonded contact junction to other parts of those conductors. As the bonded junction increases in temperature, the voltage increases along the junction line connection. The voltage does not generate at the point of junction, of the two thermocouple metals, but instead along the lengths of the two metal conductors that are exposed to temperature differences . Both lengths of metals are influenced by the same temperature difference because they are bonded at the probe end (thermocouple junction) and sensed at the open end (reference or “cold” junction). For absolute temperature measurement, the temperature at the reference junction is measured with a diode or thermistor to establish the cold junction compensation.

The types of metals used in the thermocouple have differing temperature to voltage coefficients. The voltage across the junction does not change linearly with respect to temperature, so linearization of the thermocouple output is required. There are also a variety of commercially available integrated circuits that linearize and condition outputs signals from a variety of thermocouple types.

Thermistors are semiconductor-based transducers constructed out of silicon that has been slightly doped with other elements. Thermistors are very sensitive to temperature changes. Typically, basic thermistors have an inverse and non-linear relationship to applied temperature. Like thermocouples, thermistors come in many shapes and sizes.

Goniometers and Torsiometers

Goniometers and Torsiometers are transducers which can be used to convert angular, bending or rotational motion to voltage. In the context of physiological monitoring, these transducers can be attached to a variety of body locations to measure changes in physical relationships from one part of the body to a proximal part. For example, a dual channel Goniometer can be situated to cross the joint of the wrist to measure both extension/flexion and side/side motion of the wrist.

A Torsiometer can be placed along the spine to measure twisting of the torso. Very small goniometers can be used to measure finger curling or grasping.

Angular Rate Sensor (Gyro)

An angular rate transducer is also called an electronic gyroscope. This type of transducer measures angular rate in the form of degrees per second.

Respiratory Effort

Respiratory effort transducers measure the physical displacement of the thorax and/or abdomen that result from breathing. There are many kinds of respiratory effort transducers. Technologies used employ variations in inductance, capacitance and resistance. There are also respiratory effort transducers based on piezo sensing technology.

Heart Sounds

These transducers are microphones. Some heart sound measurement transducers attach directly to the surface of the body above the heart. Heart sounds are largely in the frequency range of 20-300 Hz. Heart sound transducers can be used to detect different aspects of cardiac pumping action, including valve opening and closing.

Vibromyography

This transducer is used to measure the minute physical accelerations associated with muscle contraction. These accelerations have a relationship to the force manifested by the muscle.

Fiber Optic Temperature and Pressure

Fiber optic transducers are based on principles that associate certain physical changes at the tip of the fiber probe which affect the transmission of light through the fiber. Light intensity, polarization or phase may be affected by the physical environment sensed by the probe tip, subject to specific tip treatments or coatings.

Accelerometer

An accelerometer can measures the rate of change of velocity or the strength of a gravitational field. In the context of physiological recording, accelerometers are typically used to to measure limb and body movement. Another physiological measurement use for accelerometers is Vibromyography (VMG), the measurement of vibrations associated with the contraction of skeletal muscle tissue. VMG is more useful than EMG to measure absolute muscle force between subjects.

Accelerometers can be constructed from a variety of materials. If the accelerometer’s frequency response needs to extend to static values of acceleration, then the accelerometer is typically a Silicon Micromachine (MEMS) unit. If the accelerometer just need to sense changes in acceleration, then the device can be crafted from piezo film or ceramic.

Inclinometer

This sensor is designed to measure changes in orientation angle. An inclinometer is especially useful for determining angle, when the recorded values are near 0 degrees. Inclinometers are suitable for ergonomic investigations relating to head position and body posture.

Humidity

A humidity sensor measures the water vapor content in the air or in other gas mixtures. There are two types of humidity sensors; absolute and relative. An absolute humidity sensor measures the mass of water vapor in a known volume of gas. A relative humidity sensor measures the actual moisture level in air compared to the saturated moisture level at the same temperature and pressure. This sensor reports the ratio of these values as a number between 0 and 100%. If ambient temperature is measured, along with relative humidity, it’s possible to calculate absolute humidity and the associated actual water vapor pressure.

Thermopile

A thermopile is a device which converts thermal energy into electrical energy. A thermopile consists of a number of thermocouples, usually connected in series but sometimes in parallel. Thermopiles generate an output voltage proportional to a relative temperature difference. A thermopile become increasingly to thermal energy changes as its number of series-connected, component, thermocouples increases. Thermal cameras employ large arrays of densely packed thermopiles to record a complete thermal image or video.

Hall Effect

A Hall effect sensor measures the change in the surrounding magnetic field and converts that measurement into voltage. The Hall effect is the production of a voltage difference across an electrical conductor, at a right angle, to an electric current in the conductor and a magnetic field perpendicular to the current.

Laser Doppler Flow

This sensor employs a laser and detector to measure blood flow (perfusion) via Doppler methods. When illuminated via laser light, moving blood will Doppler-shift the light wavelength. As blood flows towards the laser source, the reflected laser light increases in frequency. As blood flows away from the laser source, the reflected laser light decreases in frequency. Increasing blood perfusion in tissue will result in increased blood flow both towards and away from the laser source (probe). This increase in blood perfusion is also referred to as an increase in blood cell motility. A laser Doppler flow (LDF) system measures blood cell flow as related to blood cell motility. Motility is calibrated in Blood Perfusion Units (BPU). A motility standard may consist of a solution of latex microspheres in water, undergoing Brownian motion, to provide a standard calibration value.

Radar Doppler

This sensor employs radar to measure body or internal organ activity via Doppler methods. When illuminated via radar, moving reflective surfaces will Doppler-shift the radar’s frequency wavelength. As movement is towards towards the radar source, the reflected energy increases in frequency. As movement is away from the laser source, the reflected energy decreases in frequency.

Electrodermal Activity and Response

This sensor measures conductance of the skin as influenced by the eccrine glands. Typically, this sensor is used to measure the conductance between two surface electrodes which are attached to body locations with a high density of eccrine glands, such as the fingertips, palms, soles of the feet and forehead.

Bioimpedance and Cardiac Output

This sensor employs a constant current to measure the time-varying impedance of the torso to provide an indication of cardiac output. Cardiac output is the measure of how much blood is pumped by the heart over a certain time period. As blood is pumped out of the heart, with each beat, a small drop in torso impedance is recorded. The derivative of this impedance has been shown to be proportional to stroke volume. Stroke volume multiplied by heart rate is equal to cardiac output.

Pyroelectric Sensors

Sensors of this class covert incident thermal radiation (heat) into a voltage difference. Many piezoelectric sensors are also pyroelectric sensors. Kynar piezo film is a good example of a sensor technology that behaves in this described manner.

Josephson Junctions (SQUID Magnetometer)

Superconductivity research has resulted in the ability to measure very small magnetic fields, via a Superconducting Quantum Interference Device (SQUID) magnetometer based on two parallel Josephson junctions. These sensors are designed to measure very small magnetic fields, such as those associated with Magnetoencephalography (MEG). These sensors must be held at close to zero degrees Kelvin to produce the required superconductivity effect. The magnetic field associated with neuronal activity of the brain is very small, on the order of 10-1000 femto Tesla, a few cm from the surface of the scalp. In comparison, the magnetic field of the earth is about 50 milli Tesla.

Inductive Sensors

These sensors are simply conductive coils. Coils may be wrapped around cores of differing permeability to enhance performance for specific applications. Generally considered, these sensors operate by principle of inductance variation as the sensor is brought close to materials of interest.

Capacitive Sensors

These sensors consist of parallel conductive plates. In a typical case, one plate of the sensor capacitor may be the material being sensed as this operates as a proximity or touch sensor.

Proximity Sensors

Proximity sensors can be crafted from a variety of technical approaches. Inductive, capacitive, ultrasonic, optical and radar methods can be used to develop a proximity sensor. Inductive and capacitive methods employ circuit topologies that are sensitive to the component variation as it changes in response to affecting elements in the environment. Ultrasonic, optical and radar methods are sensitive to the reflections of a pulsed or continuous wave transmitted into the environment.

Magnetoresistive Sensors

There are several types of magnetoresistive (MR) sensors. The basic principle of all MR sensors is that the resistance of the sensor is influenced by the application or change of an external magnetic field.

The Anisotropic Magnetoresistance (AMR) effect occurs in ferromagnetic materials. The materials’ resistance changes with the direction of the applied magnetic field.

The Tunnel Magnetoresistive (TMR) effect occurs in material sandwiches consisting of two (or more) ferromagnetic layers with a separating isolation layer. The resistance between the ferromagnetic layers depends on the vector angle of the applied magnetic field.

The Giant MagnetoResistive (GMR) effect occurs in material sandwiches consisting of two (or more) ferromagnetic layers with a separating, non-magnetic, metallic layer. The resistance between the ferromagnetic layers depends on the degree of parallel equivalence, in the layers, when subjected to an applied magnetic field.

Electronic Stethophone

This transducer convert the acoustical pressure signals sensed, in a specific area, to a voltage waveform capable of being amplified. This device is typically used to provide an indication of heart and/or lung sounds.

fNIR Transducer

The fNIR transducer introduces two or more wavelengths of light through a tissue volume for the purposes of evaluating specific chemical composition in the volume. Typically, these transducers are used to measure changes in the concentration of oxygenation and deoxygenation in the sensed tissue volume.

MEMS Devices

Micro-Electro-Mechanical Systems (MEMS) consist of miniaturized mechanical and electro-mechanical elements that are made using micro-fabrication methods. MEMS devices include flow sensors, pressure sensors, force sensors, accelerometers, gyroscopes, microphones, optical switches, micro-fluidic systems, ultrasonic transducers and energy harvesting systems.