Types, Characteristics, and Control of Motors in Intelligent Machines Design Lab - Prof. A, Lab Reports of Electrical and Electronics Engineering

Download Types, Characteristics, and Control of Motors in Intelligent Machines Design Lab - Prof. A and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! Sep-8-09—10:37 AM 1 EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 1 Today’s Menu • DC Motors • Servos • Stepper Motors • Pulse-Width Modulation • Solenoids EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 2 Electric Motors • An electric motor converts electrical into mechanical energy. > Motors come in all manner of shapes and sizes. – There are electromagnetic direct current (DC) motors and electromagnetic alternating current (AC) motors and multiple variations of each. > AC motors are typically used for large machinery (such as washers, dryers, machine tools, A/C units and the like) and are powered from an AC power source. – AC motors have such titles as single-phase, split-phase, capacitor start, and three-phase motors, to name a few. – AC motors are seldom used in our robots because a mobile robot’s power supply is typically a DC battery or battery pack. > DC motors appear in a large variety of shapes and sizes: – Permanent magnet iron core, permanent magnet ironless rotor, permanent magnet brushless, permanent magnet stepper, and hybrid stepper EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 3 DC Motors • For robotic purposes, a DC motor usually runs at too high a speed and too low a torque. > In order to swap these characteristics, they must be geared down. > Connecting the shaft of a motor to a gear-train causes the output of the shaft from the gear-train to rotate much more slowly and to deliver significantly more torque than the input shaft. • DC motors can be purchased with the gear-train already prepackaged inside the motor housing. They are called DC gear-head motors and are normally based on permanent magnet ironless rotor motors in order to be as lightweight as possible. EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 4 DC Motors • DC Motor Rotation > Most DC motors have two electrical terminals. Applying a voltage across these two terminals will cause the motor to spin in one direction, while a reverse polarity voltage will cause it to spin in the other direction. The amplitude of the voltage determines the motor speed. Polarity direction Amplitude speed > Stepper motors have six to eight terminals. Signals applied to these wires energize different coils inside the motor sequentially. The rotor is subsequently attracted to each portion and “stepped around” in a continuous fashion. The timing of these signals determines the motor speed, the phase between signals determines direction, and the number of wires determines the motor position. Sep-8-09—10:37 AM 2 EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 5 Servo Motors • Servos > Servo motors, or servos for short, are three-wire DC motors used extensively in the toy and model airplane industries, and in the steering on a radio-controlled car. This type of assembly incorporates a DC motor, a gear-train, limit stops beyond which the shaft cannot turn, a potentiometer for position feedback, and an integrated circuit for position control. > Servos use error-sensing feedback to correct the positioning of the device EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 6 Servo Motors • Servos > The three wires in a servo correspond to power, ground and a control signal input. – A pulse-width signal controls to what position the motor should move. – An electrical circuit directs the motor to rotate to the commanded position and keeps it there. > Servo motors can be extremely compact and easy to control. – They are mass produced for the toy industry and are therefore cheaper than any other DC gearhead motors. – They often find their way into mobile robot grippers, arms and legs. > Servos can be “hacked” by stripping the controller chips, potentiometers and removing the limit stops resulting in low- cost continuously revolvable DC gearhead motors suitable for small mobile robotic applications. EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 7 How a DC Motor Works • DC Motor > Electromagnetic forces in DC motors result when current- carrying conductors are placed in magnetic fields. – According to Lorentz’s law, the resulting force is perpendicular to both the direction of the current I, and the direction of the flux field, B according to the right hand rule (where the fingers curl from the direction of the current to the direction of the flux field and the thumb points to the direction of the created force). > Rotary motion requires a loop of wire. – Because forces are created in a direction perpendicular to both I and B’s direction, current going into the loop along one side generates a force in one direction while current going into the other side of the loop generates an opposite force. – The force disparity acting at a distance from the center of rotation, causes a torque. The loop will then rotate until the a force disparity no longer exists. EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 8 DC Motor Speed-Torque Curves • DC Motor > The speed and torque characteristics for a DC motor depend on a variety of parameters that have to do with the geometry of the motor, the materials involved, the number of windings, and the voltage at which the motor is driven. > These pertinent characteristics are often illustrated in a speed-torque graph for a given applied voltage. Efficiency, current, and power output are often plotted along with speed on the vertical axis against torque on the horizontal axis. > The speed-torque curve is linear with a negative slope and has a y-intercept dependent on the applied voltage. > The current increases linearly with torque and is independent of applied voltage. Legend = efficiency P = power I = current N = speed Sep-8-09—10:37 AM 5 EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 17 • TTL Transistor-Transistor Logic • A transistor looks like a current controlled switch. • When there is a voltage across the base-emitter (vbe>0), there is a current i b . • When i b > 0, the transistor conducts, and the collector to emitter junction looks like a wire (short circuit). • When i b 0, the transistor does not conduct, and the collector to emitter junction looks like an open circuit. Transistor-Transistor Logic From EEL-3701 Base Collector Emitter i b + – vbe EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 18 Controlling DC Motors • H-Bridges > By determining which pair of transistors is enabled, current can be made to flow in either of the two directions through the motor. Because permanent-magnet motors reverse their direction of turn when the current flow is reversed, this circuit allows bidirectional control of the motor. ==> EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 19 Controlling DC Motors • Controlling Motor Speed > To control the speed of the motor, the switches are opened and closed at different rates in order to apply a different “average voltage” across the motor. This is possible because the motor windings are inductive in nature and mechanical systems have a certain latency. The current across an inductor is given by the relation i =(1/L) v dt, where L is the inductance, i is the current and v is the voltage. The area (integral) under the voltage piecewise linear curve (by inspection) is proportional to the ratio of the amount of time the signal is high divided by the total, or the duty-cycle of the waveform. The dashed line in the figure shows the resulting average voltage applied to the motor. This technique is called pulse width modulation (PWM). The frequency is typically set to 20KHz, which is beyond the human hearing range. If switches S1 and S4 are used for pulse width modulation while switches S2 and S3 are left open, the voltage across the motor will be equal to and of the same polarity as Vcc when S1 and S4 are closed and 0 v when they are open. Speed duty-cycle = ton/ tperiod EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 20 Controlling DC Motors • Pulse Code Modulation vs Pulse Width Modulation > Pulse code modulation for servo motors is different than pulse width modulation for controlling the speed of DC motors. – In PCM some “intelligence” was added so that the pulse with was a code signifying to what position the servo motor should move. – In PWM we use varying pulse widths to create different average voltages across to the motor to change its speed. • Make your own or buy? > It is possible to design your own solid-state H-Bridge controller, but there are also a number of low cost single chip solutions on the market. – For example, one can obtain several engineering samples of 1A motor driver ICs from Texas Instruments (TI) free of charge! Sep-8-09—10:37 AM 6 EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 21 Solenoid • For our purposes, a Solenoid converts energy into linear motion (e.g., a linear solenoid). (Rotary solenoids also exist.) • Solenoid: An assembly used as a switch, consisting of a coil and a metal core free to slide along the coil axis under the influence of the magnetic field. • Solenoids generally have two stable positions, fully > Springs are often used to moved the solenoid to one of the stable positions and a current through the coil moves it to the other position. EEL-5666: Intelligent Machines Design Lab University of Florida, EEL 5666 © Drs. A. A. Arroyo & E. M. Schwartz 22 The End!