Mechanical Sensors

Mechanical Sensors Class of sensors to measure mechanical phenomena For example Displacement, location, position sensors Strain sensors Motion sensors Pressure sensors Flow sensors

Displacement, location, position sensors In industrial process, it is sometimes required to measure position location of object on conveyor system orientation of steel plates in a rolling mill liquid or solid level

Potentiometer sensors Potentiometer consists of wire wound around a rod with fixed resistor R Movement of wiper change the resistance of the potentiometer

Example A potentiometric displacement sensor is to be used to measure work-piece motion from 0 to 10 cm. The resistance changes linearly over this range from 0 to 1 k. Develop signal conditioning to provide a linear, 0- to 10- V output

Capacitive and Inductive Sensors The displacement measurement due to the change in capacitive or inductive Capacitive

There are 3 ways to change capacity variation of plate separation variation of plate area variation of dielectric constant

Inductive a motion of permeable core changes the inductance AC bridge can be used to detect the change in inductance

Variable Reluctance Sensor A motion of coil varies the magnetic flux coupling between the two coils Used in measure translation and rotational motion called linear variable differential transformer (LVDT)

LVDT LVDT consists of 3 coils The primary is connected to AC source, thus inducing ac voltage in coils 2 and 3 coil 2 and 3 are connected in series with opposition direction when core is in middle, zero voltage output when core is move to one side, the net voltage will be increase

The circuit for LVDT is shown below

Level Sensors Level sensors measure the level of solid and liquid Example Mechanical Electrical Ultrasonic

Example The level of ethyl alcohol is to be measured from 0 to 5 m using a capacitive system. The following specifications define the system: for ethyl alcohol: K = 26 (for air, K = 1) cylinder separation: d = 0.5 cm plate area: A = 2RL where R = 5.75 cm = average radius L = distance along cylinder axis Find the range of capacity variation as the alcohol level varies from 0 to 5 m.

Strain Sensors The sensor to measure the deformation of solid object due to stress Stress can be categorized in 3 types Tensile stress Compressional stress Shear stress

Strain Strain is defined as the fractional change in length of the sample due to stress

Stress-Strain Curve In linear region, the slope is constant which is called modulus of elasticity, Young’s Modulus

Modulus of Elasticity Material Modulus (N/m2) Aluminum 6.89x1010 Copper 11.73x1010 Steel 20.70x1010 Polyethylene 3.45x108

Example Find the strain that results from a tensile force of 1000 N applied to a 10 m aluminum beam having a 4x10-4 m2 cross-sectional area

Strain Gauge Principles The resistance of sample material is After the application of stress The change in resistance due to stress is

Example Find the approximate change in a metal wire of resistance 120  that results from a strain of 1000 m/m

Measurement Principles The strain gauge (SG) is glued to the element whose strain is to be measured When stress is applied, the element and the SG is deformed at the same manner

Metal Strain Gauges The device used to measured strain Gauge factor is defined as Indicates how easy the strain can be measured Large GF means large change in resistance for a given strain

Construction Wire or foil Design to make it long in order to give large resistance change Sufficiently fine, not to resist the deformation Unidirectional Nominal value is 60, 120, 240, 350, 500 and 1000 

Signal Conditioning The bridge circuit is used to detect the change in resistance of the SG

The resistor value is set equal to nominal resistance of unstrained SG The resistance of active SG is The offset voltage is then

Finally, or

Example A strain gauge with GF = 2.03 and R = 350  is used in the bridge. The bridge resistors are R1 = R2 = 350  and the dummy gauge has R = 350 . If a tensile strain of 1450 m/m is applied, find the bridge offset voltage if Vs = 10.0 V. Find the relation between bridge off-null voltage and strain. How much voltage results from a strain of 1 micro

Load Cells The direct application of SG is the measurement of force or weight The load cell measure the deformation produces from the fore or weight A beam or yoke assembly is used that has several SGs mounted

Example The load cell consists of an aluminum post of 2.500 cm radius with a detector and compensation strain gauges. The 120  strain gauges are used in the bridge with V = 2 V, R1 = R2 = RD = 120.0 , and GF = 2.13. Find the variation of bridge offset voltage for a load of 0 to 5000 lb.

Motion Sensors Motion sensors are designed to measure the rate of change of position, location, or displacement The primary form of motion sensor is the accelerometer The speed, and position can be found from integration

Principle of Accelerometer In industrial application, the design of accelerometer is based on Newton’s Law and Hooke’s Law of Spring

Natural Frequency A spring and attached mass always exhibits oscillations at some characteristic natural frequency. This natural frequency is given by

Damping The friction that eventually brings the mass to rest is defined by a damping coefficient , which has the units of s-1

If the spring-mass system is exposed to a vibration, then the resultant acceleration of the base is given by

Example An accelerometer has a seismic mass of 0.05 kg and a spring constant of 3.0x103 N/m. Maximum mass displacement is ±0.02 m. Calculate (a) the maximum measurable acceleration in g, and (b) the natural frequency

Type of Accelerometers Potentiometer LVDT Variable Reluctance Piezoelectric

Potentiometer Attaching the spring mass to the wiper arm of a potentiometer The displacement is converted to resistance

LVDT In these instruments, the LVDT core itself is the seismic mass. Displacements of the core are converted directly into a linearly proportional ac voltage.

Variable Reluctance The test mass is usually a permanent magnet. The measurement is made from the voltage induced in a surrounding coil as the magnetic mass moves under the influence of an acceleration.

Piezoelectric The piezoelectric accelerometer is based on a property exhibited by certain crystals where a voltage is generated across the crystal when stressed.

Example An accelerometer outputs 14 mV per g. Design a signal-conditioning system that provides a velocity signal at 0.25 V for every m/s, and determine the gain of the system and the feedback resistance ratio

Pressure Sensor Pressure sensor is use to measure the pressure of fluid and gas in industrial application

Pressure Principles Pressure is force per area that fluid exerts on its surroundings Static Pressure The fluid is not moving Dynamic Pressure The fluid is moving

Units for Pressure Gauge pressure SI unit: N/m2 (Pa) English unit: lb/in2 (psi) Atmosphere unit (1 atm = 14.7 psi = 101.325 kPa ) Gauge pressure Describe a pressure in a relative sense, compared to atmospheric pressure

Head Pressure Used to describe pressure for liquid in tank or pipe The pressure that are produced by the weight of liquid above that measured point

Example A tank holds water with a depth of 7.0 ft. What is the pressure at the tank bottom in psi and Pa (density = 103 kg/m3)

Pressure Sensors (p > 1 atm) A device measures pressure more than 1 atm Normally, convert pressure into physical displacement which is then converted to electrical signal

Diaphragm A thin, flexible piece of metal as shown in the picture If a pressure p1 exists on one side, p2 on the other side, the net force is then

Bellows Convert pressure into physical displacement The displacement is converted to voltage using LVDT

Bourdon Tube A hard metal tube is flattened and one end is closed The other end is attached to header by which pressure is introduced

Electronic conversions Convert mechanical displacement to electrical signal by mechanical linkage Connect to potentiometer Connect to strain gauge Connect to LVDT

Diaphragm with Feedback

Solid State Pressure Sensor Integrated circuit sensor Range from 0 to 100 kPa (0 to 14.7 psi) The silicon wafer acts like a diaphragm that deflects in response to a pressure difference

Pressure Sensors (p < 1 atm) Measurement of pressure less than 1 atm are done using purely electronic method The device is based on the rate at which heat is conducted and radiated away from the heated filament Heat loss is proportional to number of gas molecule per unit volume (Pressure)

Pirani Gauge Determine the filament temperature thought a measurement of filament resistance Pirani gauge is used with bridge High pressure causes gas molecules to collide with filament and gas molecule absorb energy from the filament, resulting in cooling of the filament

Use to measure a very low pressures about 10-3 to 10-13 atm Ionization Gauge Use to measure a very low pressures about 10-3 to 10-13 atm A regulated electron current (typically 10 mA) is emitted from a heated filament. The electrons are attracted to the helical grid by a dc potential of about +150 volts Some electrons collide with gas molecules in the gauge envelope causing a fraction of them to be ionized The gas ions are attracted to the central ion collector wire by the negative voltage on the collector (typically a minus 30 volts). Ion currents are on the order of 1 mA/Pa. This current is amplified and displayed by a high gain differential amplifier +150 V -30 V

Flow Sensors Manufacturing processes involve the movement of raw materials, product, and waste throughout the process Automobile parts through assembly line Methyl chloride through pipe Flow can be categorized into solid, liquid, and gas

Solid-Flow Measurement Such as crushed material or powder carried on conveyer The flow rate is measured in mass or weight per unit of time Kg/min lb/min

Flow rate is measured by weighting the amount of material on a platform of length L

The flow sensor to measure weight passing through the platform Load cell LVDT

Liquid Flow The unit of flow are Volume flow rate (gals/min, m3/h, ft3/h) Flow velocity: express as the distance the liquid travels in the carrier per unit of time (m/min, ft/min)

Mass or weight flow rate: expressed as mass or weight flowing per unit of time

Example Water is pumped through a 1.5 in diameter pipe with a flow velocity of 2.5 ft/s. Find the volume flow rate (ft3/min) and weight flow rate (lb/min). The weight density is 62.4 lb/ft3

Restriction Flow Sensor Introduce a restriction in the pipe. Measure the pressure drop across the pipe With restriction, velocity of fluid increases but pressure decreases

Example Flow is to be controlled from 20 to 150 gal/min. The flow is measured using an orifice plate system. The orifice plate is described by K =119.5 (gal/min)/psi0.5 . A bellow measures the pressure with an LVDT so that the output is 1.8 V/psi. Find the range of voltages that results from the given flow range.

Obstruction Flow Sensor Operates by the effect of obstruction placed in the flow stream Rotameter flow meter Moving vane flow meter Turbine flow meter

Magnetic Flow Meter Charged particle move across the magnetic field, potential is established across the flow