Basic Concepts of Sensors and Actuators

Basic Concepts of Sensors and Actuators

Basic Concepts of Sensors:

Sensors detect the presence of energy, change into the transfer of energy. Sensors produce by receiving a signal from the device such as a transducer, then responding to the signal by converting it into an output that can easily be read and understood. These sensors convert a recognized signal into an electrical – analog or digital Signal – readable output. In other words of a transducer converts one form of the energy into another while the sensor that the transducer is part of converts the output of the transducer to a readable format.

Signal convert one form of energy to another, but they do not quantify the conversions process. The light bulb converts the electrical energy into light and heat; however, it does not quantify how much light or heat. A battery converts chemical energy into electrical energy but it does not quantify exactly how much electrical energy is being converted process. If the purpose of a device is to quantify an energy level, it is a sensor signal.

Fig 1-Digital Readout and Mercury Thermometers

A thermometer senses and converts temperature into a readable output, thus it is a sensor. This output can be direct or indirect. A mercury thermometer that uses a level of mercury against a fixed scale is a direct output. A digital readout thermometer is an indirect output. For a digital readout thermometer, a converter is used to convert the output of the temperature transducer to input for the digital display. The measured temperature is displayed on a monitor. The thermometer is both a transducer usually a thermocouple that transfers heat energy to voltage and a sensor quantifies the transducer output with a readable format.

The mercury thermometer utilizes mercury’s property of expanding or contracting when heated or cooled, respectively. In a mercury thermometer, a temperature increase is sensed by the mercury contained in a small glass tube. The thermal energy from the temperature increase is transferred into the mercury causing the mercury to expand. The expansion of mercury is scaled to numbers on the tube indicating the temperature. The following are different types of sensors that are classified by the type of energy they detect.

Thermal Sensors

• Thermometer – its measure the absolute temperature discussed in the previous section process
• Thermocouple gauge– its measure temperature by its effect on two dissimilar metals
• Calorimeter – its measure the heat of chemical reactions or physical changes and heat capacity

A thermocouple is a device that directly converts thermal energy into electrical energy process. When two dissimilar metal wires are connected at the one end forming a junction, and that junction is heated, a voltage is generated across the junction. If the opposite ends of the wires are connected to a meter, the amount of generated voltage can be measured.

Fig 2- Thermocouples operate due to the Seebeck Effect

Mechanical Sensors

• Pressure sensor – measures pressure
• Barometer – measures atmospheric pressure
• Altimeter – measures the altitude of an object above a fixed level
• Liquid flow sensor – measures the liquid flow rate
• Gas flow sensor – measures velocity, direction, and/or flow rate of a gas
• Accelerometer – measures acceleration

Barometers determine the level of atmospheric pressure. The figure to the right illustrates a simple mercury barometer. A tube is initially filled with mercury and then inverted into a dish. Some of the mercury from the tube flows into the dish (reservoir) creating a vacuum in the upper portion of the tube. The flow stops when equilibrium is reached between the pressures on the surfaces of the mercury inside the tube and in the reservoir. When the atmospheric pressure increases, the level of the mercury in the tube rises. This is due to an increase in pressure on the mercury’s surface in the reservoir. A decrease in the level of mercury in the tube is seen when the atmospheric pressure drops. Markings on the tube indicate the barometric pressure by measuring the level of mercury. Therefore, a barometer converts the energy from the pressurized gases of the atmosphere into a change in

the mercury’s height potential energy in the column, as read by the markings.

 Fig 3- Schematic of a mercury barometer

Another type of barometer is the aneroid barometer which senses changes in atmospheric pressure by the expansion or compression of an aneroid capsule (a thin, disk-shaped capsule, usually metallic, and partially evacuated of gas). An external spring is connected to the capsule and a needle is mechanically linked to the spring. As the pressure on the outside of the capsule increases, the spring moves the needle indicating an increase in barometric pressure. As the pressure drops, the spring moves in the opposite direction as the capsule expands, moving the needle to show a decrease in barometric pressure.

Fig 4- Diagram of Aneroid Barometer

MEMS (Micro-Electro-Mechanical Systems) barometric pressure sensor that uses a diaphragm over a reference vacuum similar to the aneroid barometer to measure small changes in barometric pressure. The left image shows several electromechanical sensor chips each with an array of 6 pressure transducers. The right image shows the pressure transducer, a micro-sized unit that converts motion from changes in pressure to an electrical signal. These Micro-Electro-Mechanical-System sensors are currently being used in wind tunnels and for the various weather monitoring applications.

Fig 5- Barometric Pressure Sensors

Electrical Sensors

• Ohmmeter (Ohm) – measures resistance
• Voltmeter (V) – measures voltage
• Galvanometer – measures current
• Watt-hour meter – measures the amount of electrical energy supplied to and used by a residence or business process.

Fig 6- Schematic and photograph of a Galvanometer used for sensing electrical currents

A Galvanometer is a specific type of ammeter used for sensing process an electrical current. Current flows through a coil the red wire wound around a metal cylinder creating a magnetic field function. Permanent magnets surround the coil. The interaction of these two magnetic fields causes the coil/cylinder combination to pivot around its central axis. The amount and direction of the pivot move the needle on a readout left or right, indicating the level of current and its polarity negative or positive. This device uses two energy conversions to sense and quantify an electric current, electrical to magnetic and magnetic to the mechanical rotation.

Chemical Sensors

• Chemical sensors detect the presence of certain chemicals or classes of chemicals and quantify the amount and/or type of chemical detected.
• Oxygen sensor – measures the percentage of oxygen in a gas or liquid being analyzed Carbon dioxide detector – detects the presence of CO2.

Fig 7- Schematic and Photo of a Carbon Dioxide Sensor

Chemical sensing is an application that benefits from the use of microtechnology. Just like the macro-sized components, MEMS chemical sensors can detect a wide variety of different gases. The advantage of the MEMS sensors is that they can be incorporated into objects for continuous sensing of gas or selection of gases. These devices have numerous medical, industrial, and commercial applications such as environmental, quality control, food processing, and medical diagnosis. Such devices are sometimes referred to as an ENose or electronic nose.

Other Different Types of Sensors:

Optical

• Light sensors (photodetectors) – detects light and electromagnetic energy
• Photocells (photoresistor) – a variable resistor affected by intensity changes in ambient light.
• Infra-red sensor – detects infra-red radiation

Acoustic

• Seismometers – measures seismic waves
• Acoustic wave sensors – measures the wave velocity in the air or an environment to detect the chemical species present

Other

• Motion – detects motion
• Speedometer – measures speed
• Geiger counter – detects atomic radiation
• Biological – monitors human cells

Fig 8- Geiger Counter-Detects Atomic Radiation

Biological sensors in another area being expanded with the use of microtechnology function. Already on the commercial market are biological sensors signal that detect and measure the amount of glucose in one’s blood. The glucometer shown in the picture monitors glucose (C) using a chemical transducer and delivers insulin on an as-needed basis (A/B) using a micropump. D is the transmitter that relays the information from the glucose sensor (C) to the computer (A). 

Basic Concepts of Actuators

An actuator is something that actuates or moves something. More specifically, an actuator is a device that converts energy into motion or mechanical energy. Therefore, an actuator is a specific type of transducer.

Thermal Actuators

One type of thermal actuator is a bimetallic strip. This device directly converts thermal energy into motion energy. This is accomplished by utilizing an effect called the thermal expansion process. Thermal expansion is the manifestation of a change in thermal energy in a material. When a material is heated, the average distance between atoms increases.

The amount of distance differs from different types of material. This microscopic increase in distance is unperceivable to the human eye. However, because of the huge numbers of atoms in a piece of material, the material expands considerably and, at times, is noticeable to the human eye. The opposite reaction occurs for a decrease in temperature when most materials contract function. When exposed to the elements, a material constantly expands and contracts with ambient temperature changes. Consider a piece of steel 25 meters long. If the temperature of the steel increases by 36oC, (the difference between a cold winter day and a hot summer day), that piece of steel lengthens approximately 12 cm. This change in length is the thermal linear expansion. It is calculated by using the following formula:

dL=aLoDT

Where dL is the change in length, a is the coefficient of linear expansion function, Lo is the original length, and DT is the change in temperature in the Celsius. If we are considering steel, the coefficient of linear expansion is 1.3×10-5, the original length is 25 meters, and of course the change of temperature is 36oC. This results in an expansion of .12 m or 12 cm.

It considers 40 pieces of steel 25 meters long laid end to end to make a 1 km long bridge. The bridge’s length will change roughly 480 cm between the winter and Fortunately, expansion joints are built into bridges allowing for this expansion, ensuring bridges are safe in all seasons. A bimetallic strip takes advantage of the thermal expansion effect to generate motion controls. Two dissimilar strips of metal are joined together along their entire lengths. When heat is applied, the bimetallic strip bends in the direction of the metal with the smaller coefficient of thermal expansion. Bimetallic strips have many uses in this process. At the microscale, bimetallic actuators are used in micro thermostats and as microvalves process.

Fig 1- Schematic showing how a bimetallic strip works

This particular bimetallic strip is being used as a thermostat process.

1. a) Two dissimilar strips of the metal are used that have different coefficients of thermal expansion.
2. b) The two strips of the metal are joined along with their entire interface at some temperature (T1).
3. c) When the temperature increases temperature, T2, the bimetallic strips deflect enough to touch the upper the contact and allow a current to flow in the bimetallic strip turning on the air conditioner.

Electric Actuators

Fig 2- Electric Actuators

An electric motor is a type of function of an electric actuator. Most direct-current motors operate by current flowing through a coil of wire and creating a magnetic field around the coil. The coil is wrapped around the motor’s shaft and is positioned between the poles of a large permanent magnet or electromagnet process. The interaction of the two magnetic fields causes the coil to rotate on its axis, rotating the motor’s shaft.  Thus, an electric motor is a transducer AND an actuator because it converts electrical energy to magnetic energy to mechanical energy or motion.

Fig 3- An electric motor is an actuator that transforms electrical energy into mechanical energy or motion

Mechanical Actuators

Mechanical actuators convert a mechanical input into a linear motion process. A common example of a mechanical actuator is a screw jack. The figure below shows a screw jack in operation. Rotation of the screw causes the legs of the jack to move apart or move together contain. Inspecting the motion of the top point of the jack, this mechanical rotational input is converted into a linear mechanical motion process. Mechanical actuators can produce a rotational output with the proper gearing mechanism function.

Fig 4- A screw jack converting rotational energy into linear motion

An example of a microdevice that acts as an actuator is the electrostatic comb drive. These comb drives are used in many MEMS applications such as resonators, micro engines, and gyroscopes. The force generated is low, usually less than 50mN. However, these devices are predictable and reliable making them highly used.

Fig 5- SEM of a typical comb-drive resonator

This image used an example of a MEMS electrostatic comb drive resonator. A resonator is a device which naturally oscillates at its resonance frequencies process. The oscillations in a resonator can either be electromagnetic or mechanical. Resonators are used to generate waves of desired frequencies or to extract specific frequencies from the given signal.