Transcript Fundamentals of Linear Electronics Integrated & Discrete

CHAPTER 21
Transducers
and
Actuators
Objectives
Describe and Analyze:
• Temperature Transducers
• Displacement Transducers
• Other Transducers
• Signal Conditioning
• Solenoids & Relays
• AC & DC Motors
Introduction
• Transducers convert one form of energy into
another. Examples: a microphone converts
sound to AC voltage; a speaker converts AC
voltage into sound.
• Actuators are electromechanical devices that
move, rotate, push, pull, and in general
“make something happen” when electrical
signals are sent to them.
A Generic Control System
Sensors and actuators are used to “close the loop”.
Temperature
• Temperature measures average kinetic
energy at the atomic level.
• Something is hot when its molecules are
banging into each other quickly.
• Something is cold when its molecules bump
each other slightly.
• At absolute zero temperature (0° Kelvin),
atoms stop moving.
Temperature Scales
• Two commonly used temperature scales are
Fahrenheit (F) and Celsius (C)
• Fahrenheit is used in the United States. The
rest of the world mostly uses Celsius.
• Water freezes at 0° C and boils at 100° C.
0° C = 32° F, and 100° C = 212° F
• A change of 1° K (Kelvin) = a change of 1° C
0° C = 273° K
Converting Temperature Scales
If you know the temperature on one scale, you
can convert it to the other scale:
°F
= 1.8  °C + 32
°C
= (°F – 32) / 1.8
A test: is it true that –40° F = –40° C ?
Thermocouples
• A thermocouple is a device made of two different
kinds of metal “welded” together to form a
junction. Electrons transfer from one metal to the
other with an energy proportional to temperature.
• The electron transfer produces a voltage
(Seebeck voltage) that is proportional to
temperature.
• The voltage is very low, and requires conditioning.
• Thermocouples can be used up to 2000° C, and
are reasonably linear.
Thermocouples
Platinum and rhodium are expensive.
RTDs
• Resistance Temperature Detectors (RTDs) rely
on the positive temperature coefficient of
resistance shown by metals. Platinum is
commonly used.
• RTDs have a very linear response to
temperature.
• RTDs make stable and accurate transducers.
• RTDs are fragile and expensive.
• RTDs require special meters to read them.
Thermistors
Thermal Resistors:
• Negative Temperature Coefficient (NTC)
• TEMPCO  –5% / °C
• Very sensitive
• Very nonlinear
• Inexpensive
• Come in various shapes and sizes
• Made from semiconducting ceramic
IC Temperature Transducers
Very linear response.
Displacement Transducers
Measure distance moved.
Units:
• Meters
• Centimeters
• Millimeters
• Micrometers
• And smaller
LVDTs
Can measure a 2-micrometer move.
Strain Gages
Commonly used, inexpensive, linear, small signal.
Pressure
<insert figure 21-18 here>
Pounds / in2 or Newtons / m2 (Pascals).
Pressure
Types of Pressure Measurements:
• Absolute Pressure: compared to a
vacuum.
• Gage Pressure: compared to ambient
pressure (atmospheric pressure).
• Differential Pressure: pressure change
across a boundary.
Flow Transducers
Measure how fast material moves.
Some examples:
• Gallons per minute (for fluids)
• Cubic feet per minute (for gases)
• Pounds per minute (for solids)
• Feet per second (actually, a velocity)
Flow
<insert figure 21-20 here>
Bernoulli’s principle: velocity up  pressure down.
Flow
• Bernoulli’s principle:
based on conservation of energy in a
moving fluid.
• Positive Flow Transducers:
based on rotation of a turbine caused
by direct contact with a moving fluid.
The “fluid” could be a slurry: a mixture
of solids and liquids.
Acceleration
Newton’s Law: Force = Mass  Acceleration
Hall-Effect Sensors
Voltage from current through a magnetic field.
Hall-Effect Sensors
Output proportional to magnetic field strength.
Solenoid Actuators
Moves when coil is energized.
Relays
Originally invented to relay telegraph signals.
DC Motors
Torque from interacting static magnetic fields.
Synchronous Motors
Rotor matches rotating magnetic field of stator.
AC Induction Motors
Stator induces current in rotor .
Induction Motors
• The “workhorse” of electric motors.
• Torque from interacting magnetic fields of
stator and rotor.
• 3-phase version has high torque, easy
starting, and can be reversed by swapping
two of the connections.
• Single-phase versions have less torque (for
their size) and need special starting circuits.
Stepper Motors
Accurate movement without feedback (servo) loop.
Stepper Motors
• Usually controlled by a microprocessor.
• Speed is in steps-per-second.
• Various step sizes: 360 steps/rev means 1
degree / step.
• Holding torque keeps motor from turning when
not being “stepped”.
• Large ones used in industrial robots and other
machines.
• Small ones used in printers and other
equipment.