Understanding Sensors: Types and Characteristics

Sensors

• A sensor measures a physical quantity and converts it into an electrical signal.
• Examples of sensors include temperature, displacement, position, motion, velocity, fluid, liquid flow, liquid level and light sensors.

Characteristics of Sensors

• Static and dynamic are the two types of sensor characteristics.
• Static characteristics define the steady-state relationship between sensor input and output.
• Dynamic characteristics define the relationship between sensor input and output when the measured quantity changes rapidly.

Static Characteristics

• Range: Defines the limits within which the input can vary; a thermocouple may have a range of 25-225 °C.
• Span: Difference between the maximum and minimum input values; the thermocouple with a range of 25-225°C has a span of 200°C.
• Error: The difference between the measured value and the true value; a sensor reads 29.8 mm when the actual displacement is 30 mm, resulting in a 0.2 mm error.
• Sensitivity: Ratio of change in output value per unit change in input value; a temperature sensor's sensitivity may be 10 mV/°C, resulting in 10mV with a 1°C temperature increase.
• Non-Linearity: Indicates the maximum deviation of a sensor's actual measured curve from the ideal curve.
• Hysteresis: The maximum difference in output at any measurement value within the sensor's specified range when approaching the point first with increasing and then with decreasing the input parameter.
• Stability: Ability of a sensor device to give the same output with a constant input over time.
• Dead band: The range of input values for which there is no output.
• Repeatability: Ability of a sensor to give the same output for repeated applications of the same input value under the same conditions.
• Accuracy: Closeness to the actual value.
• Precision: Ability of a sensor to reproduce a certain set of readings within a given accuracy; depends upon repeatability.
• Output Impedance: Impedance measured at the output of a sensor which needs to be known, allowing the electrical output of a sensor to interface with an electronic circuit.

Dynamic Characteristics

• Response time: Time elapsed for a sensor to give an output corresponding to a specified percentage (90-95%) of its steady value after a constant or step input is applied.
• Time constant: Measure of the inertia of the sensor, determining how it reacts to input changes, equal to 63.2% of the response time.
• Rise time: Time taken for the output to rise from 10% to 90% of its steady value.
• Settling time: Time it takes for the output to settle within a small percentage (2%) of its steady-state value.

Classification of Sensors

• Sensors are divided into active and passive types.
• Active sensors (self-generating) do not require external power for operation; an example is a thermocouple.
• Passive Sensors (external supply) require external power; an example is a photodiode.
• Sensors are divided into analog and digital types.
• Analog sensors produce a continuous output signal relative to the measured quantity; examples: LDR, strain gauge.
• Digital sensors work with discrete or digital data used for conversion and transmission; examples: IR, PIR.
• Sensors are divided into primary and secondary types.
• Primary sensors contain mechanical and electrical components, converting a physical quantity into a mechanical signal.
• Secondary sensors are deployed in cascade with a primary sensor, converting a mechanical signal into a more comprehensible electrical signal.
• Example: Bourdon tube (primary) and LVDT (secondary).

Selection of Sensors

• Operating principle, availability, cost, and performance figures must be considered when selecting a sensor for a particular application
• Availability consists of sources location, delivery schedule, payment options, continuation of supply.
• Cost involves the sensor and its delivery cost.
• Performance involves range, ease of use, power supply requirements, accuracy, hysteresis effect etc.

Displacement Sensors

• Displacement sensors measure the movement of an object by converting displacement into an electrical signal (resistance, capacitance, or inductance).
• Resistive displacement sensors include the potentiometer and strain gauge
• Capacitive displacement sensors include the capacitive element.
• Inductive displacement sensors include the LVDT.

Resistive Displacement Sensors

• Potentiometer: Consists of a resistance element with a sliding contact that moves along its length and can be wire wound or conductive plastic with a resolution of 0.5mm or 0.1µm, respectively
• Potentiometers relate the change in position (linear or rotary) to the change in resistance and convert it to a proportional voltage change through translational (linear) displacement x or rotary (angular) displacement , and the output voltage for an ideal potentiometer is given by equations involving supply voltage and a sensitivity constant.
• Strain gauge: A passive transducer that converts mechanical displacement into a change of resistance, it measures force, torque, pressure, acceleration and more
• Strain gauges operates on the principle of a thin metallic wire changing dimension and thus resistance, when strain is applied.
• Principles of working: Mechanical, Electrical and Piezoelectric
• Types based on mounting: Bonded or Unbonded.
• The electrical strain gauge is the most commonly used type, and a universal strain gauge consists of a thin, metallic, grid-shaped sensing element
• Strain gauge Operation involves a tightly bonded gauge to a a measuring object, so that the sensing element stretches or contracts according to the strain on the object, changing its length
• Change in length = Change in resistance and is determined by the equation R = pl/A, in which R is resistance, l is length and A is area.
• When strain or force is applied on a cantilever beam, the strain gauges mounted on the cantilever are strained, so that one gauge is under tension and one is under compression, giving measurable resistance change.
• The change in resistance measures the displacement of the beam and is used for measuring linear displacements of 1mm to 30mm with about ±1% nonlinearity error. is the resistance change, R is the original resistance of the strain gauge in ohms, G is the proportional constant, = strain in change of length.
• G is 2 for metal wire or metal foil strain gauge, G = -100 (or less for N-type semiconductor) and G = +100(or more for N-type semiconductor

Applications of Strain Gauges

• Measures displacement, force, residual stress, vibration, torque, bending, deflection, compression and tension.
• For displacement measurement using strain gauges, one strain gauge is attached to a flexible element in the form of cantilevers, rings, and U-shapes.

Inductive Displacement Sensors

• Linear Variable Differential Transducer/Transformer (LVDT) is the best example of inductive displacement sensors
• LVDT Principle: Operates on the principle of mutual induction, converting non-electrical energy from displacement into electrical energy.
• LVDT Construction: A cylindrical former contains one primary winding in the center and two secondary windings at the sides having the the same number of turns equal and opposite, resulting in net output voltages that is the difference in voltages between the 2 coils (S1&S2)
• Position: An iron core is in the center of the former moving to and fro
• AC excitation: Applied from 5-12V, having an operating frequency of 50 to 400 Hz
• LVDT Operation: Split into three cases based on core position - no external force, core at null. Move to left and move to right
• Case 1: With no external force, the core remains at the null position without movement so the voltages end up being equal
• Case 2: When an external force is applied, moving the steel iron core to the left, then the induced voltage in the secondary coil is greater than in coil 2, with a positive net output voltages output.
• Case 3: When an external force is applied, moving the steel iron core to the right, then the induced voltage in the secondary coil 2 is greater than in coil 1, with a negative net output voltage

LVDT Applications

• Measures displacement from fractions of a millimeter to centimeters and measures force, weight, and pressure (as a secondary transducer)

Force Sensor (Load Cell)

• Strain gauge load cells measures an applied force when a steel cylinder changes changes in dimension and has bonded strain gauges that stretch or compress, causing the gauges’ length and diameter to change dimension. The changes resistance or ouput voltage, measures the applied force.
• Construction: Consists of a steel cylinder (made up of four identical strain gauges) and have a vertical direction on which (R1 and R4) are mounted, and others with horizontal gauges is mounted circumferentially at right angles to gauges R1 and R4
• For operation these four gauges are connected in the form of bridge to convert the change in resistance to voltage.
• Case 1: With no load (force) on the steel cylinder, all four gauges registers the same, balancing the wheat stone bridge so the output voltage is zero
• Case 2: When compression load is applied on the steel cylinder, the vertical gauges R1 and R4 undergo compression and registers a decrease in resistance. While the horizontal gauges R2 and R3 undergo tension displaying an increase in resistance.
• Applications of Force Sensors: use in vehicle weigh bridges, force dynamo meters, and tension measurement of wires, to measure applied load

Temperature Sensors

• Detects changes in temperature and converts into electrical signals in the following types of sensors
• Bi-metallic strips (420°C)
• Thermocouples (-200°C to +2000°C)
• Resistance Temperature Detectors (RTD) (-200 to +600°C)
• Thermistors (-50 to 200°C)
• Thermodiodes and thermotransistors (-50 to 150°C)

Bi-Metallic Strips

• Consists by bonding two metals with different thermal expansion coefficients that are used for detecting and measuring temperature changes with a typical pair of brass and steel
• The principle is used when the most common for bi-metallic is in a thermal switch, where the brass is on the outside of the strip due to its greater expansion coefficient when hot.
• Advantages: No power source required and is low cost.
• Disadvantages: Low accuracy, used at 500 C, is limited to applications with manual reading required and unsuitable for low temps because the expansion of metals is similar.

Resistance Temperature Detectors (RTD)

• Operates on the principle of electrical resistance changing due to temprature, having metal resistance increase, that follows a linear relationship.

Thermistors

• A type of resistor used to measure changes in temperature, and a combination of the words thermal and resistor with a symbol in in figure 2.15 with a change in resistance as shown in figure 2.15.
• Types: Negative Temperature Coefficient (NTC) and Positive Temperature co-efficient (PTC)
• Negative temperature co-efficient (NTC) Thermistors consist of a metal oxide mixture chromium, cobalt, iron, manganese and nickel pressed forming a bead, disc or rod shape.
• Positive temperature co-efficient (PTC) Thermistors made-up of barium, lead and strontium titanite are used for the protection of motors and transformer windin
• Uses: air conditioning and refrigeration servicing, food processing, stoves and grills, textile or plastics processing, petrochemical, micro electronics, air, gas and liquid temperature measurement, exhaust gas measurement

Thermocouple

• Operates due to the dissimilar metal junction producing an electric potential when heated as a temperature function.
• The net open circuit voltage (Seebeck voltage) is a function of junction temp and metallic compound
• Seebeck coefficient a, provides relationship that is used to measure the change in temperature and is given
• Composition: Chromel (90% Nickel, 10% Chromium) and Alumel (95% Nickel, 2% Manganese, 2% Aluminium and 1% Silicon)

Thermodiode and Thermotransistors

• A semiconductor pn junction diode widely used as sensor, uses the temp of a doped semiconductor to effect rate carrier changes. with current through the function being determined by the change in temprature. LM3911 and LM35 circuits have such properties.
• LM35 is Calibrated in (Centigrade) has a Scale Factor of ± 10mV/°C, 0.5°C Ensured Accuracy (at 25°C), a Full -55°C to 150°C Range and is used in Remote Applications with operates at a range of 4v to 30v with non-linearity of ±¼°C

Light Sensors

• Converts light energy in electric energy and includes the following types of sensors.
• Light Dependent Resistor (LDR)
• Photo Diode
• Photo transistor

Light Dependant Resistor (LDR)

• It known as photoresistor, photoconductor, or photocells is a light device thats resistance is a light function and is made from semiconductor materials that includes Cadmium sulphide, lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb). Most of which has spec spectral curve.
• Operation: Based on when lights hits semiconducer material to increase conductivity in relationship to photon energy (hv). With valence and conducting band that determines conductivity. Dark resistance is measured when there is no light.
• Applications: automatic street light systems, bar code scanners, counting packages, light intensity meters, burger alarms

Photodiode

• Type of light detector with the ability to convert light energy to electrical is also called photo-detector, photo-sensor, or light detector except they are packaged with,fiber optics or a window. The material of photodiode is made up with semiconductor materials GaAs and InGaA
• Types of of photodiodes: PI, PIN and Avalanche
• Operation: PN junction: A design that operates in reverse bias where thermally generated minority charge carries cause small reverse saturation, when light comes over atom of atom to generate photocurrent.
• Applications: CD players, Smoke detectors, Space and Optical Communication

Phototransistor

• Either tri or bi-terminal semiconductor devices that have a light-sensitive base with larger base and collector regions, which structured in homojunction or heterojunction structures
• The circuit symbol contains two arrows to indicate light to base -Collector Equation: Ic = IB + (1 + )Ісво
• Operation: Open based transistor that increases ICBO when lighted on and amplifys light, that can that includes the following: Monitoring paper position and margin in printers, punch card readers and CD players.

Proximity Sensors

• Able to detect presence of objects with no physical contact and uses an electro-magnetic field/ radiation for the change of signal and comes in 4 types: Induction, Capacitive, Optical and Ultrasound
• Induction measures the position with metal objects. The induction coil oscillates with threshold circuit by the size of the sensor and material used with a 6 cm directionality.
• Capacitive for nonmetallic sensors such as plastic and metal can measured such as and are dependent on external load to make a connection.
• Optical can create retro-reflective, and beam and diffuse measurements. Using ON/OFF position or reflective with an indication being to the object.
• Ultrasound is an object emits and listens for a wave to an object that is in place of the electrical and sonic waves.

Ultrasonic Proximity Sensor

• The sensor uses a high-frequency sound wave instead of light, with a frequency above normal hearing (around 40KHz).
• Types of Sound: subsonic, sonic or ultrasound
• Process: Measure the transmit or short amount of wave to the measure time from echo.
• Application: Is used on measuring the trash and water.