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.

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.

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

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

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

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

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

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

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).

Motor Control Signals Pulse Width Modulation (PWM)‏ Pulse Position Modulation (PPM)‏

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

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

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

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

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.

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