Sensors

Instructor: Shuvra DasMechanical Engineering Dept.

University of Detroit Mercy

Flowchart of Mechatronic Systems

Sensors

• Transducers convert one form of energy into another. Two types: Sensors and Actuators

• Devices that permit the measurement of physical quantities such as Force, Stress, Temperature, velocity, pressure, flow rate etc. (converts physical quantity to electrical signal)

Sensor TypesSensor Applications Examples

Mechanical Position, disp., velocity, Acc.,vib.,stress/strain, pressure

Electrical Voltage, current, resist., cap.,inductance, magnetic, radiation

AcousticAnd Flow

Sound intensity, visc., flow rate,ultrasound NDE, frequency

Chemical andBiological

Enzymes, pH, concentration, gases,hum.,

Optical Intensity, wavelength, phase, vision,interference, polarization, scattering,etc.

Thermal Temperature, infrared radiation, etc.

Some Sensors: Potentiometers

• Measures Linear or angular position• Measures voltage between the wiper and one end of

pot• Voltage linearly related to distance• Simple, common• Disadvantage: Subject to wear and tear• Disadvantage: difficulty with displacement that

varies with time quickly (limitation for dynamic measurements)

Potentiometers

• For linear measurement it consists of a wire coil, a moving contact or wiper and a DC source with constant voltage V

• V0 = (V/ymax)y

Potentiometers

• Rotary potentiometer is used for angular measurement.

• V0 = (Vi/span in degrees)y

• Sensitivity r = ymax/N; N= number of turns/coils

Example: Throttle sensor

• Refer to doc file…pot_example.doc

Some Sensors: Optical Encoders

• Converts motion into a series of digital pulses. • By counting a single bit or a set of bits the pulses

can be converted into relative or absolute position measurements.

• Rotational encoders are manufactured in two basic forms: the absolute encoder and the incremental encoder (allows measurement of relative position of the shaft).

Optical Encoders: Absolute

• Digital signal linear or angular position• Useful in motion control• A light source shines light on pad• Reflected/transmitted light picked up by

photodiodes• The code increment is converted to displacement• A n bit code corresponds to 2n combinations

representing range

Optical Encoders:Absolute

• Several concentric tracks divided into sectors

• for n=4 tracks divided into 2n = 24=16 sectors

• Dark = 0 , light = 1 in each sector

• resolution = 360/ 24=22.5 (with 4 bits)

• resolution = 360/ 25 =11.25 (with 5 bits)

• Can have upto 22 tracks

– resolution = 360/222=0.000086

Optical Encoders:Absolute

• The code used is not binary but modified binary or Gray code.

• Advantage: only one window changes at a time.

Binary to Gray

Decimal code Binary Gray

0 0000 00001 0001 00012 0010 00113 0011 00104 0100 01105 0101 01116 0110 01017 0111 01008 1000 11009 1001 110110 1010 111111 1011 111012 1100 101013 1101 101114 1110 100115 1111 1000

Incremental encoder

• Simpler in design

• With relative motion a succession of rows read

• A and B are about 1/4 cycle out of phase so with disc rotation the actual phase difference determines direction

• Speed is from freq. Of pulses.

• An index row counts number of total revs.

Light detecting Transducers

• Light sensitive detectors, or photocells can be categorized as either thermal detectors or photon detectors.

• Thermal detectors include a temperature sensitive element, which is heated by incident light.

• The photon detectors respond directly to absorbed photons.

Light Detecting Transducers

• Some of the photon devices are:

• photoemmissive\ or photomultiplier

• photoresistive

• photovoltaic (solar cell)

• photodiode

• phototransistor

Photoresistors

• Photoresistors are variable resistors that function in ways similar to potentiometers

• The change in resistance is caused by a change in light level.

• The active elements are made of chemicals such as Cadmium Sulphide

• The electrical resistance is inversely proportional to light intensity.

Physics of Light

• Light can be considered as energy carrying particles, photons

• Also, it can be considered as an electromagnetic wave.

• Energy of photon:

• E hf hc/ – h – Planck’s constant (6.6326 X 10-34 Ws2)– f - Frequency– Wavelength (m)– c - Speed of light (300,000 Km/s)

Photoconductive Sensors

• Photoresistor: A piece of semiconductor material placed between conducting end plates forming a sandwich

• materials used:– Cadmium sulfide (CdS)– Cadmium selenide

(CdSe)– Germaium (Ge)– Silicon (Si)

Photoconductive Sensors

• Principle: Resistance changes as luminous intensity changes

• Kphoto - sensitivity of photoresistor I - change in

luminous intensity R - change in

resistance

• Kphoto = I/ R

RCTime

• BS2 command that can be used to compute the value of variable resistance.

• Syntax: RCTIME <Pin>, <State>, <Variable>

• Measure time while Pin remains in State and put the result on Variable; usually to measure the charge/ discharge time of resistor/ capacitor (RC) circuit.

• We will use that on our following light program, by comparing the RCTIME of the left and the right photo sensors (resistor on our case) and use the comparison result to decide whether we we should turn right or left.

RCtime

RCTime

• The RC circuit can be represented as a first order system:

• Rdq/dt+q/C = V; time constant - RC• Through the above command in BS2 time required

to discharge (or charge) the capacitor in an RC circuit is measured and is related to the time constant and thus RC. From that data the variable value of R may be found.

• See stamp manual for more details.

programming

• ' Measure RC time for left photoresistor.– high left_pin ' Set detector to output-high.

– pause 3 ' Pause for 3 ms.– rctime left_pin,1,left_photo ' Measure RC time on left.

• ' Measure RC time for right photoresistor.– high right_pin ' Set detector to output-high.– pause 3 ' Pause for 3 ms.– rctime right_pin,1,right_photo ' Measure RC time on right.

programming

• ' Take the difference between right_photo and left_photo, then decide what to do.– if abs(left_photo-right_photo) > 2 then check_dir

– ' Check if difference between RC times is within the deadband, 2 in this case.

– ' If yes, then forward. If no then skip to check_dir subroutine.

The Hall Effect

• An electrical conductor is subjected to a magnetic field.

• The magnetic field is at right angles to a current flow inside the conductor.

• The magnetic field pushes the moving electron towards the bottom edge of the conductor (right hand rule).

The Hall Effect

• The bottom edge becomes negatively charged while the upper edge becomes positively charged.

• An EMF will develop across the width of the slab that is in proportion to the amount of flux, current, and D.

Applications of the Hall Effect:Measuring Blood Flow

• Tiny voltage probes are attached on either side of the blood vessel.

• Poles of an electromagnet are placed perpendicular to these probes.

• The amount of voltage generated due to the Hall effect created by the ions within the blood supply is a direct indication of a persons blood pressure.

Applications of the Hall Effect:Tachometer

• As the metal tooth passes the Hall detector, the redirected magnetic field passes through the Hall device

• The generated emf indicates one rotation.

• The same system can be adapted for a flow monitoring system for fluids by replacing the gear with a rotating turbine and its blades.