CHAPTER 21: Transducers and Actuators potx

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. [...]... (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,... 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... 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... 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 Bernoulli’s principle: velocity up  pressure down Flow • Bernoulli’s principle: based on conservation of energy in a moving fluid • Positive Flow Transducers: ... 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 . CHAPTER 21 Transducers and Actuators Objectives Describe and Analyze: • Temperature Transducers • Displacement Transducers • Other Transducers • Signal Conditioning. 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. 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 °