Transcript EET 2261 PowerPoint Slides - Sinclair Community College

EET 2261 Unit 13
Controlling Stepper Motors and Servos

Read Almy, Chapter 21.

Lab #13 due next week.

Final Exam next week.
Stepper Motors and Servos
•There are many kinds of electric motors. (See
Wikipedia article.)
•In many motors, the rotor spins continuously,
with no way of precisely controlling the motor’s
rotational position.
•Stepper motors and servos are two widely
used kinds of motors whose position can be
precisely controlled.
Coverage of Motors in the
Textbook
•Our textbook does not discuss stepper motors
or servos (except in Question 7 on page 200).
•But Chapter 21 discusses pulse width
modulation (PWM), which is the technique
used to control servos.
Stepper Motors
•A stepper motor is a digitally controlled motor
that allows precise control over the position of
the motor’s rotor.
•Changes in the digital input rotate the motor’s
rotor by a precise amount, which is called a
step or step angle.
•Depending on the motor, this step angle may
be as small as 1 (or less) or as large as 45.
•On our motors, the step angle is 3.6.
Our Stepper Motor
•Our motor is made by
Howard Ind., part
number 1-19-4202.
•As shown on its
specifications sheet,
it’s a unipolar motor
with a 3.6 step angle.
Interfacing the Microcontroller to
the Stepper Motor
•Several widely used motor driver ICs exist for
interfacing motors to microcontrollers.
•On our Dragon12 board, the TB6612FNG driver
serves this purpose (U12 near the board’s lower left
corner).
Toshiba TB6612FNG
•This chip contains a popular driver design
called an H-bridge. It is often used to control
DC motors and stepper motors.
•TB6612FNG datasheet.
Connections to TB6612FNG
•On the Dragon12 board, the TB6612FNG is
connected to Ports B and P of the HCS12.
•Figure from Dragon12 Schematic Diagram 5.
Code for Turning a Stepper Motor
•Once you’ve made the proper connections and
configured the HCS12’s ports, stepper motors
are easy to program. The following code will
turn our motor counterclockwise (CCW):
Servos
•A servo motor (or servo) is a package
containing a DC motor connected via gears to
a shaft.
•It also contains a feedback circuit that
precisely controls the shaft’s angle of rotation.
•Unlike a motor whose rotor spins continuously,
a servo is usually used to set the shaft to a
specific angle of rotation and then hold it there
for a while.
Servos in RC Vehicles
•Servos are commonly used in small robotics
and in radio-controlled (RC) airplanes, cars,
and boats.
•In an RC airplane, for example, servos may
control the plane’s throttle, rudder, elevators,
ailerons, landing gear, etc.
•Next slide shows inner workings of an RC
airplane.
Servos and Stepper Motors
Wiring a Servo
•Servos have three wires:
•Power (red)
•Ground (black)
•Control signal (yellow or white)
Connecting Servos to the
Dragon12
•The Dragon12 board has four connectors
(near the center of the top edge) for servos,
labeled PP4, PP5, PP6, PP7.
•Jumper J35 lets you
choose whether to
power the servo from
the board or from an
external supply. We’ll
power it from the board,
but to do so we must have the power adapter
plugged in.
Dragon12 Connections
•On the Dragon12 board, the servo outputs are
connected to Port P of the HCS12.
•Figure from Dragon12 Schematic Diagram 5.
Our Servo
•Our servo is made by
Hitec, part number
HS-311.
•See ServoCity’s
webpage for detailed
specifications.
Controlling a Servo
•The servo’s control signal is a 50 Hz pulse
train. Therefore, what is this signal’s period?
•As shown on the next slide, the width of the
pulse is crucial in controlling the servo.
Period = ?
Controlling a Servo
•The control signal’s pulse width determines
the shaft’s angle of rotation.
•Typically this pulse width ranges from about
0.5 ms to about 2.5 ms, interpreted as follows:
•1.5 ms pulse width: 0 rotation.
•Pulse width less than 1.5 ms: rotate
counterclockwise (up to 90, for our servo).
•Pulse width greater than 1.5 ms: rotate
clockwise (up to 90).
Pulse Width Modulation
•The term pulse width modulation (PWM)
refers to the technique of varying a signal’s
pulse width to control a device such as a servo.
(PWM is used to control other kinds of devices,
too, including DC motors.)
PWM Using the HCS12
•One way to perform PWM using the HCS12
would be to write a loop that sets an output pin
HIGH and LOW at the right times to generate
pulses of the desired width and frequency.
•This approach would tie up a lot of the CPU’s
time.
•A more efficient way is to use the HCS12’s
built-in PWM block. Using this approach, once
we have configured the PWM block correctly, it
will generate pulses of the desired width and
frequency without tying up the CPU.
Pulse Width Modulation (PWM)
Block
•The Pulse Width
Modulation (PWM) block
shares pins with generalpurpose I/O Port P.
•Figure from p. 6 of
textbook or page 23 of
Device User Guide).