Sensors and Transducers

Sensors are transducers used to obtain information about the outside world

Terminology that might appear in specs of a sensor:

range = limits between which input can vary

span = max val - min val

error = measured val - true val

accuracy = a range for the error, often expressed as a % of full scale.

sensitivity = ratio of delta out / delta in (e.g., 0.5 ohm/degree C)

hysteresis error = error due to hysteresis
What is Hysteresis?

non-linearity error

repeatability/reproducibility - same input may give different outputs at different times.

repeatability = ((maxval - minval)/fullrange) x 100

stability = steadyness of output when input is held constant

drift = change in output over time (constant input)

zero drift = change in output when input = zero

dead band = range of input vals that produce no output

dead time = time lag between change in input and response in output

resolution = smallest change in input that produces an observable change in output

output impedance

Example: specs for a strain gauge pressure transducer

ranges: 70 to 1000 kPa, 2000 to 70000 kPa (kiloPascals, about 1/101th atm)
supply voltage: 10 V d.c. or a.c. r.m.s.
full range output: 40 mV
non-linearity and hysteresis: +- 0.5% full range output
temperature range -54 degree C to +120 degree C when operating
thermal zero shift: 0.030% full range output/ degree C
(when temp changes by 1 degree C, output for zero input changes by this amount)

static characteristics = values for steady-state conditions
dynamic characteristics = values characterizing behavior when conditions change

response time = time needed for transducer to respond to step input; time from step to when output reaches n % of steady-state output (n often = 95)

time constant = 63.2% response time

rise time = time taken for output to rise some specified %age of steady-state output. Often, time for rise from 10% to 90 or 95% of steady-state value.

settling time = time taken for output to settle to within some %age of steady-state value. E.g., with 2% of s.-s. value

Sensor types

Displacement, position, proximity sensors

Potentiometer - rotary and linear
Secret Life of Pots The Humble Potentiometer Linear potentiometers
Let Rp = resistance of pot
RL = resistance of load
x = fractional position of slider
Vs = source voltage
VL = output voltage

VL / Vs = x / ((Rp / RL) x (1 - x) + 1)

If RL = infinity, VL = x Vs. Output is linear.

Error = x Vs - VL = Vs (Rp / RL) (x^2 - x^3) (non-linearity error)

Strain gauge - can be made of thin wire, metal foil, semiconductor, conductive plastic foam
How they work the strain gage

Delta R / R = G e

where G = gauge factor, e(epsilon) = strain

Typically, G = 2 for wire and foil, +/- 100 for semiconductors

Capacitive element - plates can move away from each other, by each other, or dielectric can slide from between them

Let A = area of plate overlap
d = distance between plates
C = capacitance

C = er e0 A / d where er(epsilon r) = relative permittivity of dielectric, e0(epsilon 0) = permittivity of free space

If plates move away from each other, C is a non-linear function of displacement:

C = er e0 A / (d + x)

If three plates are used to form two capacitors (two outer plates fixed, the middle plate gets displaced) we have a push-pull displacement sensor.
How they work

The two capacitors can be put in different arms of an a.c. bridge. the out-of-balance voltage is proportional to displacement x.

linear variable differential transformer - a transformer with a sliding ferrous rod core (primary coil between two secondary coils)
Construction Backup

Constant a.c. fed to primary coil, varying a.c. comes out of difference of secondary coils, depending on position of rod. Fairly linear

There is also a rotary variable differential transformer that uses a heart- shaped core. Detects rotations of core of +/- 40 degrees.
Another design Backup

Eddy current proximity sensor - two coils, no core. Constant a.c. to reference coil. Sensor coil picks up induced a.c. Amount varies when a conducting object comes near because eddy currents are produced in the object.
Overview backup

Inductive proximity switch - single coil around a core. When metal object is near the coil, its inductance changes. Coil is part of a resonant circuit; frequency of circuit changes.
examples

Optical encoders - Position of disc determined by effect of holes on light beam.
digital encoders

Incremental encoder - Uses two rings of holes, offset from each other by 1/2 width of hole; allows direction of motion to be deduced. A single hole in a third ring indicates a "home" position.

Absolute encoder - one track is used for each bit of the binary number representing a position; the binary number for the disc position is read directly off the light sensors. Glitches can occur when more than one bit changes at the same time or nearly the same time (due to misalignment, for example).
absolute encoders

Gray code is often used instead of the usual sequence of binary numbers so that only one bit changes at a time-less chance for errors if sensor is out of alignment.

   Normal binary      Gray code
   -------------      ---------
      0000              0000
      0001              0001
      0010              0011
      0011              0010
      0100              0110
      0101              0111
      0110              0101
      0111              0100
      1000              1100
      1001              1101
      1010              1111

Pneumatic sensor - low pressure air escapes from end of sensor. If object nearby, pressure detector in sensor detects increase in air pressure.
non-contact pneumatic sensors

Proximity switch -

microswitch - electrical switch requiring little force to operate it. (This is also called a snap action switch.)
photo catalog page-we'll use SW143-ND switch
reed switch - electrical switch closed by bringing a magnet near it.
magnetic reed switch
photosensitive device - light or infra-red; object breaks beam or reflects light back to sensor (as in my robots)
introduction range-finding sensors

Hall effect sensor - When a current flows through a conductor, a voltage forms sideways across the conductor if it is in a magnetic field (the electrons are deflected to the side, just as they would in a vacuum). Hall effect sensors come as ICs that are either linear in output, or behave as threshold detectors. Can operate very fast (100kHz).
the Hall effect

Motion detectors

Incremental encoder - holes in disc break light beam; # of pulses per second gives speed
encoder module

Tachogenerator - ferromagnetic tooth wheel spins next to a pick-up coil with permanent magnet core. Magnitude of induced current is proportional to speed. Also, individual pulses can be counted.
DC tachogenerator

An alternative form is just an a.c. generator. shaft turns coil in a magnetic field. Amplitude and frequency of generated current are proportional to speed of shaft.

Pyroelectric sensor - polarized lithium tantalate generates a charge that is proportional to the temperature that it detects. Motion of heat source (person) is detected when it moves by two of these sensors, causing a difference in their outputs. A parabolic mirror or Fresnel lens is often used to focus infrared radiation onto the sensor.
the basic detector
kit version with Fresnel lens

Force detectors

Strain gauge load cell --- a cylindrical tube with strain gauges attached. Strain gauges are temperature sensitive too, so this has to be accounted for in signal conditioning. Errors are typically about +/- 0.03% of full range for non-linearity error, and +/- 0.02% of full range for hysteresis and repeatability errors.
wide variety

Fluid pressure sensors

Position of a diaphragm or bellows can be detected with strain gauges or linear variable differential transformers. Mechanical part could be a Bourdon tube (C-shaped or coiled tube that straightens out as pressure increases)
pressure measurement Diaphragm Backup
a Bourdon tube Backup

Piezoelectric sensors - voltage is proportional to applied pressure.
Piezoelectric sensors more technical detail big catalog

Tactile sensors - two layers of piezoelectric polyvinylidene fluoride (PVDF) film separated by a soft film. A.c. to one layer causes it to vibrate. Vibrations transmit to other layer, causing it to generate a current. Vibration is stronger when layers are pushed together.
many approaches to tactile sensing

Liquid flow

Orifice plate - constriction in pipe, with pressure sensors in front and behind. Flow is proportional to sqrt of pressure difference.

Orifice plate is often a disc with a hole in the middle. Pressure sensors are one pipe diameter upstream, 1/2 pipe diameter downstream. Other tap locations are common.
differential pressure flowmeters backup medical applications

Turbine meter - multibladed rotor inside the pipe. Rotation of turbine may be detected by magnetic pick-up coil. More accurate, but more expensive.
turbine flowmeters backup

Liquid level

Float - position of float detected by potentiometer, linear variable differential transformer, or strain-gauge.

Differential pressure - pressure between liquid at bottom of vessel and the atmosphere or the gas at the top of the tank is measured. (Every pressure sensor measures the difference between two pressures; sometimes one side of the sensor is vented to the atmosphere, sometimes it is vented to another point in the system being monitored, e.g., the top of a tank or other side of an orifice.)

Temperature

survey presentation Bimetallic strip - two metals attached to each other. Bends when temperature changes. This can close switch. magnet in switch can attract piece of soft iron on strip to cause hysteresis.
thermometer

Resistance temperature detector - resistance is often variable with temperature. Platinum is linear over common temperature ranges, copper is somewhat nonlinear (exponential) and Nickel is linear above 400 degrees C.

Thermistor - special resistor made of mixtures of metal oxides (semiconductors). Look like beads, discs or rods. Typically,

             B/t
  R(t) = K e

thermistors

Some thermistors have positive temperature coefficients (like the wire resistors do). (The temperature coefficient is the derivative of the resistance with respect to the temperature. Hence, the sign of the temperature coefficient is the opposite of the sign of the B constant in the formula.)

Thermodiodes and transistors - semiconductor devices. In a diode, the current I is

              eV/kT
     I = Io (e      - 1)

T is degrees Kelvin, e (in exponent) is the charge on an electron, k and Io are constants. Solving for V,

V = (kT/e)ln(I/Io + 1)

If I is held constant, V is proportional to T in degrees Kelvin.

A common chip based on this is LM3911, which has a sensativity of 10 mV/ degrees C.

Transistors behave similarly. A chip based on temperature sensitive transistors is the LM35. also 10 mV/ degree C. (The difference between base-emitter voltages of two transistors with different collector currents is proportional to temperature on the Kelvin scale.)
LM35 sensor

Thermocouples - The junction of two different metals has a voltage that is a function of temperature. Two thermocouples are used; one is kept at a reference temperature, the other is the sensor. The voltage difference between the two junctions is a quadratic function of temperature.
thermocouples

Light sensors -

Photodiodes
photodiodes
phototransistors -
IR phototransistors
Photo Darlington - emitter of phototrans. connected to base of power trans., both have common power to collectors.
phototransistors and photodarlingtons
photoresistors - cadmium sulphide or cadmium selinide
picture connecting photoresistors

Switches -
Switches

    Single Pole Single Throw (SPST)   _/ _

                                         _
    Single Pole Double Throw (SPDT)   _/ _

    Double Pole Double Throw (DPDT)   _ |/ _
                                        o
                                      _  / _
                                        o
                                        |

Problem: switch bounce.
Software solution: when closed switch is detected, wait a while (20 msec, for example), if switch is still closed, accept that it is closed.
solutions
Hardware solution: SR flip-flop can be used to debounce SPDT switch.


       +5 V--R---+                  (R = a resistor)
                 |  +-----+
               --+--|S   Q|--
              /     |     |
       +-----o      |     |
       |       --+--|R    |
     -----       |  +-----+
      ---  +5 V-R+
       -

Flip-flop will change state when switch reaches other pole, but is not affected by bounce at that pole, since a pulse is needed at the other input to SR flip-flop to cause change.

D flip-flop can be used to debounce a SPST switch.

        /     +------+
    ---o  o---|D    Q|-----
              |      |
           ---|clk   |
              +------+

D flip-flop only changes when clock signal changes.

Keypads -
contact key, membrane pad

These notes are based in part on W. Bolton, Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering, Second edition, Addison Wesley Longman, 1999.