Course Code: MDP 454, Course Name:, Second Semester 2014 Actuators & Mechanisms
Lectures Joints (Fasteners, Connectors) Power/Energy Conversion (Electrical Motors) Transmission Support (Bearings) Power/Energy Transmission (Gears, Belt Drives, Power Screws) Structural Support (Frames Shafts Axles Spindles) Tools Stress Analysis, Failure Theories Dynamics, Statics, Etc.
Electric Motors Principles and Applications
Power/Energy Converters Rotary Electrical Input -> Mechanical Rotary Motion/Torque =DC Motor =AC Motor =Stepper Motor Combustion -> Mechanical Rotary Motion /Torque =Gasoline Engine =Gas Turbine
Power/Energy Converters Linear Electrical Input -> Mechanical Linear Motion/Torque =Lead screw linear actuators =Solenoids Pressure -> Mechanical Linear Motion/Torque =Hydraulic Pumps =Hydraulic Actuators =Pneumatic Actuators =Compressors
Motor Actuators Motor Actuators: Types Theories Applications
DC Motors Just as the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding. In the animation the commutator contacts are brown and the brushes are dark grey. A yellow spark shows when the brushes switch to the next winding.
DC Motor Applications Automobiles: Windshield Wipers Door locks Window lifts Antenna retractor Seat adjust Mirror adjust Anti-lock Braking System -Cordless hand drill -Electric lawnmower -Fans -Toys -Electric toothbrush -Servo Motor
Brushless DC Motors A brushless dc motor has a rotor with permanent magnets and a stator with windings. It is essentially a dc motor turned inside out. The control electronics replace the function of the commutator and energize the proper winding.
Brushless DC Motor Applications Medical: centrifuges, orthoscopic surgical tools, respirators, dental surgical tools, and organ transport pump systems Model airplanes, cars, boats, helicopters Microscopes Tape drives and winders Artificial heart
Full Stepper Motor This animation demonstrates the principle for a stepper motor using full step commutation. The rotor of a permanent magnet stepper motor consists of permanent magnets and the stator has two pairs of windings. Just as the rotor aligns with one of the stator poles, the second phase is energized. The two phases alternate on and off and also reverse polarity. There are four steps. One phase lags the other phase by one step. This is equivalent to one forth of an electrical cycle or 90.
Half Stepper Motor This animation shows the stepping pattern for a half-step stepper motor. The commutation sequence for a half-step stepper motor has eight steps instead of four. The main difference is that the second phase is turned on before the first phase is turned off. Thus, sometimes both phases are energized at the same time. During the half-steps the rotor is held in between the two full-step positions. A half-step motor has twice the resolution of a full step motor. It is very popular for this reason.
Stepper Motors This stepper motor is very simplified. The rotor of a real stepper motor usually has many poles. The animation has only ten poles, however a real stepper motor might have a hundred. These are formed using a single magnet mounted inline with the rotor axis and two pole pieces with many teeth. The teeth are staggered to produce many poles. The stator poles of a real stepper motor also has many teeth. The teeth are arranged so that the two phases are still 90 out of phase. This stepper motor uses permanent magnets. Some stepper motors do not have magnets and instead use the basic principles of a switched reluctance motor. The stator is similar but the rotor is composed of a iron laminates.
More on Stepper Motors Note how the phases are driven so that the rotor takes half steps
More on Stepper Motors Animation shows how coils are energized for full steps
More on Stepper Motors Full step sequence showing how binary numbers can control the motor Half step sequence of binary control numbers
Stepper Motors Applications Film Drive Optical Scanner Printers ATM Machines Pump Blood Analyzer FAX Machines Thermostats
Switched Reluctance Motor A switched reluctance or variable reluctance motor does not contain any permanent magnets. The stator is similar to a brushless dc motor. However, the rotor consists only of iron laminates. The iron rotor is attracted to the energized stator pole. The polarity of the stator pole does not matter. Torque is produced as a result of the attraction between the electromagnet and the iron rotor in the same way a magnet is attracted to a refrigerator door. An electrically quiet motor since it has no brushes.
Switched Reluctance Motor Motor scooters and other electric and hybrid vehicles Industrial fans, blowers, pumps, mixers, centrifuges, machine tools Domestic appliances
Brushless AC Motor A brushless ac motor is driven with ac sine wave voltages. The permanent magnet rotor rotates synchronous to the rotating magnetic field. The rotating magnetic field is illustrated using a red and green gradient. An actual simulation of the magnetic field would show a far more complex magnetic field.
AC Induction Motor The stator windings of an ac induction motor are distributed around the stator to produce a roughly sinusoidal distribution. When three phase ac voltages are applied to the stator windings, a rotating magnetic field is produced. The rotor of an induction motor also consists of windings or more often a copper squirrel cage imbedded within iron laminates. Only the iron laminates are shown. An electric current is induced in the rotor bars which also produce a magnetic field.
Huge List of applications Aircraft Window Polarizing Drives Antenna Positioning and Tuning Devices Audio/Video Recording Instruments Automated Inspection Equipment Automated Photo Developing Equipment Automated Photo Slide Trimming & Mounting Equipment Automatic Carton Marking & Dating Machines Automatic Dying and Textile Coloring Equipment Automatic Food Processing Equipment Automatic I.V. Dispensing Equipment Automatic Radio Station Identification Equipment Automotive Automotive Engine Pollution Analyzers
Huge List of applications Baseball Pitching Machine Blood Agitators Blood Cell Analyzer Warning Light Flashers Railroad Signal Equipment Remote Focusing Microscopes Resonator Drives for Vibraphones Silicone Wafer Production Equipment Solar Collector Devices Sonar Range Recorders and Simulators Steel Mill Process Scanners Tape Cleaning Equipment Tape Input for Automatic Typewriters
Huge List of applications Telescope Drives Ultrasonic Commercial Fish Detectors Ultrasonic Medical Diagnostic Equipment Voltage Regulators Water and Sewage Treatment Controls Weather Data Collection Machines Welding Machines X-Ray Equipment XY Plotters
Linear Stepping Motor
Solenoid The duty cycle factor, labeled f on the curves is defined as: where Ton is the on-time and Toff is the off-time.
Analysis of Electric Motors Electric Motors convert electrical power to mechanical power. I, V Electrical Power I : Current V: Voltage Electric Motor F, v or T, w Mechanical Power F: Force v: Velocity T: Torque w: Angular Velocity Electrical Power = I*V Mechanical Power = F*v for linear motor = T*w for rotary motor
Electric Motors Features of basic motor types DC Motors: speed and rotational direction control via voltage = Easy to control torque via current = low voltage = linear torque-speed relations = Quick response AC Motors: smaller, reliable, and cheaper = speed fixed by AC frequency = low torque at low speed = difficult to start
Principle and Analysis of DC Motor V in R i i x e ind x i x F, v x B l i = V in /R (Ohm s Law) B is constant When the switch is on, the wire will experience a sudden increase of force from 0 to F. v will increase. e ind will be induced in the direction opposite to i. i will decrease. F will decrease. v will decrease. Eventually v reaches to a constant velocity.
DC Motor under external load At steady-state, apply F load to slow down the wire. The net force will decrease. F - F load v will decrease. e ind will decrease. i will increase. F will increase. v will increase. V in R i i x F load x i x B F, v x l Again, v eventually reaches a constant velocity. v new < v
Velocity Vs. Force Relation DC Motor Basic Trend: F load v ; F load v v F load Note: v is the steady-state velocity.
Motor Output Torque: T A A T T A K T i T i K K T i T 0 0 0 for 0 for 0 T: motor output torque K T : torque constant (motor specific, based on the construction) i A : armature current [Amp] f: magnetic flux [webers] = [T m 2 ] T 0 : torque loss (e.g. frictional) Basic Equations: PM DC Motor: Theory of operation
PM DC Motor: Theory of operation Current Line: i A i K 1 T T 0 T KT T 0 /K T f Slope = 1/K T f T
PM DC Motor: Theory of operation Electromotive Force (EMF): e ind K E n e ind : induced voltage or EMF K E : EMF constant (motor specific, based on construction) n: rotational speed of the motor [rpm]
PM DC Motor: Theory of operation Armature Voltage (Counter EMF): V A V in e ind e ind works in opposite direction to input voltage V in. e ind : induced voltage (voltage generated within motor) V in : input voltage V A : actual voltage available in the armature
PM DC Motor: Theory of operation Armature Current i A V in e R A ind R A : armature resistance
Torque-Speed Curve (Different Motors)
Types of DC Motors -speed/torque curve is very steep, soft performance, high no-load speed and starting torque -good speed regulation to two times full-load torque -high starting torque and soft speed characteristic, limited no-load speed -linear speed/torque curve, current draw varies linearly with torque
Load Lines Working load Vs. Speed Constant Torque (or force) is independent of speed =Example - when weight has to be lifted or load moved against dry Coulomb friction. Linear Torque is proportional to speed. =Example - stirring pancake batter with egg-beater Often a consequence of laminar flows Quadratic Torque is proportional to square of speed =Examples - fans, air drag on aircraft automobiles. Often a consequence of turbulent flows
Load Lines Operation occurs at intersection of motor s operation line and load line, i.e. where load curve and torque-speed curve intersect Constant
Operating Regions Two main operations regions: I. Start Up Torque Torque required to change speed in a motor. =Normally used for starting a motor from a dead stop. =Inertia and required response time is important. II. Operating Torque Torque required to operate motor at constant speed. =Since motor is operating at a constant speed, inertia is not as important as the driven load.
Operating Regions
Operating Torque Example The figure shows the torque speed curve of a DC motor. Find the speed and motor current for the following: No-load and stall conditions Lifting a 10-oz load with a 2 in radius pulley. A motor driving a robot arm with a weight.
Operating Torque Example Solution (a) If voltage is applied to motor with no load attached to shaft, motor would turn at its no-load speed of 1000 rpm On the other hand, if the shaft was clamped so it could not turn, motor would exert the stall torque of 100 in-oz on clamp and draw 260 ma of current
Operating Torque Example Solution (b) Torque equals force times distance Thus motor torque to lift the weight is T = 2 in. * 10 oz. = 20 in-oz From graph, at torque of 20 in-oz, Speed has declined to 800 rpm Current is up to 125 ma
Operating Torque Example Solution (c) Motor attached to a 12-in. robot arm (weighing 10 oz) Apple (weighing 8 oz) rests on end of arm Torque due to arm and apple: T = (6 in.*10 oz.) + (12 in.*8 oz.) = 156 in-oz
Operating Torque Example Solution (c) From graph, torque of 156 in-oz exceeds stall torque of 100 in-oz, motor will not be able to lift load Inserting a gear train between motor and load might solve the problem Consider gear ratio of 5:1 Torque required of motor is then only 1/5 of original torque: Tnew = 156/5 = 31.2 in-oz.
Operating Torque Example Solution (c) Motor will now rotate at 690 rpm and a require a current of 150 ma However, because of gear train, load will only rotate at 690/5 = 138 rpm
Thank You For Your Attention! Questions?