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Announcements

Announcements. Bring two motors to lab this week. Motors can be purchased at the Scientific Supply Store - 211 Sciences Center Laboratory (2 nd Floor). Prepare the leads as shown on the class website. Can do this in lab if desired. Password Sync.

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Announcements

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  1. Announcements • Bring two motors to lab this week. • Motors can be purchased at the Scientific Supply Store - 211 Sciences Center Laboratory (2nd Floor). • Prepare the leads as shown on the class website. Can do this in lab if desired.

  2. Password Sync • …to login to Engineering labs, sync your password at www.auburn.edu/password. • Still having trouble? Visit Engineering Network Services in 270 Ross Hall during normal business hours for assistance.

  3. How a Motor Works • As current goes through armature it creates a magnetic field. • Opposites attract and likes repel causing rotational motion.

  4. Motor Parts • The brushes make electrical contact to the moving armature. Wires would get twisted.

  5. Motor Parts Continued • Current through the armature causes a magnetic field. The more current and more coils, the faster it turns.

  6. Motor Parts Continued • PTC (Positive Temperature Coefficient) Resistor - • As temperature increases, so does the resistance-prevents burning up the motor.

  7. Characteristics of Lego Motor • Constant voltage: Higher torque requires more current → lower speed. (Lego Car) • Constant torque: Voltage, speed, and current increase together. (Lab)

  8. Motors/Generators • Motor • Appling a voltage, turns the motor • Generator • Turn the rotor (gas engine, water power, etc.) generate a voltage

  9. Motor Equivalent Circuit • The faster the motor turns, the more the internal voltage cancels the driving voltage. • k is a constant that depends on the motor. • f is the rotational speed in revolutions/min (RPM).

  10. Motor Control Signals • Pulse Width Modulation (PWM)‏ • Pulse Position Modulation (PPM)‏

  11. Pulse Width Modulation (PWM)‏ • Waveform in which the width of a pulse is varied • Can be used as a communication scheme • Can be used to vary the speed of a motor

  12. Important Terms • Period (T) – the time to complete one cycle of the waveform (Units: seconds)‏ • Frequency (f=1/T) – the number of cycles per second (Units: hertz, Hz=1 cycle/s)‏ • Pulse width – amount of time the wave is high during one period • Duty Cycle – Pulse width / Period

  13. Controlling Motors with PWM • Think of the pulses as someone pulsing the switch below • The longer they hold it, the larger the pulse width

  14. PWM used to vary speed • As the duty cycle decreases, so does the average voltage delivered to the motors. • Hence your speed will decrease • Max speed = 100% duty cycle

  15. PWM for SERVOS • Servos are motors that you can tell to turn to a position. • Based on the duty cycle you send them, they move to a corresponding position.

  16. Pulse Position Modulation • Communication scheme in which the position of pulse varies in order to communicate a message.

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