EET 2261 Unit 13
Controlling Stepper Motors and Servos
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Read Almy, Chapter 21.
Homework #13 and 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.
Electromagnets
•Recall that an electromagnet behaves like a
magnet only when current is passing through it.
It’s a magnet that you can “turn on” or “turn off.”
•Electromagnets are
constructed by wrapping wire
around an iron-alloy bar.
(Illustration from Wikipedia.)
Magnets in a Stepper Motor
•A stepper motor has a permanent magnet
connected to its rotor and electromagnets
connected to its stator.
Simplified image from
John Iovine’s PIC
Microcontroller
Project Book, 2nd ed.
•The digital signals controlling the motor turn
the electromagnets on and off in sequence,
which results in rotation of the rotor.
Rotating a Stepper Motor
(Simplified Picture)
In this example,
how big is each
step?
From John Iovine’s PIC
Microcontroller
Project Book, 2nd ed.
Half-Stepping
•In the previous illustration, only one of the
electromagnets was on at a time.
•We can get finer resolution by sometimes
turning on two electromagnets at a time. This
technique is called half-stepping.
Types of Stepper Motors
•Different designs result in several types of
stepper motors:
•Unipolar
•Bipolar
•Universal (which combine characteristics of
unipolar and bipolar motors)
•Our motor is a unipolar stepper motor.
Wiring a Stepper Motor
•The number of wires on a stepper motor
varies, depending on the motor’s type.
•Bipolar Stepper Motor: 4 wires
•Unipolar Stepper Motor: 5 or 6 wires
•Universal Stepper Motor: 8 wires
Online Tutorial
•The website at
http://homepage.cs.uiowa.edu/~jones/step/
contains an excellent online tutorial on stepper
motors, including this animation showing the
operation of a unipolar motor.
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 Problems
•Two interfacing problems arise when driving a
motor from a digital system:
1. Motors introduce a great deal of electrical
“noise” into a system. This noise can
disrupt the operation of sensitive digital
circuits such as the HCS12.
2. Motors consume more current than most
digital outputs can supply. Therefore we
can’t connect the motor directly to the
HCS12’s output pins.
Solution to Interfacing Problem #1
(Noise)
•Use separate power supplies, one for the
motor and one for the HCS12.
•We’ll power our motor from
the 9 V pin in the Dragon12
board’s lower left corner.
For this to work, we must
plug in the Dragon12’s
power adapter, instead of
just powering the board
from its USB port.
•If needed, we could also use an optoisolator, such as the
ILQ74, to provide further isolation between the HCS12 and
the motor.
Solution to Interfacing Problem #2
(Supplying Adequate Current)
•Use one of the following circuits between the
HCS12 and the motor:
•Power transistors, such as the TIP120.
•Motor driver IC, such as the ULN2003 or the
TB6612FNG H-bridge. The Dragon12 board has a
TB6612FNG (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
Servo: Exploded View
Servo: Block Diagram
•Diagram above demonstrates feedback, in which a
system’s output is fed back in as an input to provide
more accurate control over the output.
•Feedback is a fundamental concept in most control
systems.
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).
Block Diagram of Pulse Width
Modulation (PWM) Block
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The PWM block’s
eight channels let
us generate up to
eight different
PWM signals at
the same time.
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See page 14 of the
PWM_8B8C Block
User Guide.
Special-Function Registers
Associated with the PWM Block
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The 39 special-function registers located at
addresses $00A0 to $00C7 let us control the
operation of the PWM block.
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See page 35 of Device User Guide.
Special-Function Registers That
We’ll Use
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The following registers are the most
important ones for using the PWM block:
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PWM Enable Register (PWME)
PWM Polarity Register (PWMPOL)
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PWM Prescale Clock Select Register (PWMPRCLK)
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PWM Scale A Register (PWMSCLA)
PWM Clock Select Register (PWMCLK)
PWM Channel Period Registers (PWMPERx):
one register for each of the eight channels
PWM Channel Duty Registers (PWMDTYx): one
register for each of the eight channels
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PWM Enable Register (PWME)
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The bits in this register let us enable or
disable each of the eight PWM channels. If
we enable a particular channel, then its I/O
pin cannot also be used for general-purpose
I/O as part of Port P.
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Figure from p. 19 of PWM_8B8C Block User Guide.
PWM Polarity Register (PWMPOL)
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For each PWM channel, we can choose
whether the PWM signal starts HIGH and
then goes LOW in each cycle, or vice versa.
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Figure from p. 20 of PWM_8B8C Block User Guide.
PWM Clocks
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The PWM block has four clocks that are
derived from the system’s bus clock:
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Clock A
Clock B
Clock SA (Scaled A)
Clock SB (Scaled B)
PWM Channels 0, 1, 4, 5 can use either
Clock A or Clock SA
PWM Channels 2, 3, 6, 7 can use either
Clock B or Clock SB.
The next three slides discuss the registers
that control these clocks.
PWM Prescale Clock Select
Register (PWMPRCLK)
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The bits in this register set the frequencies
of Clock A and Clock B, as follows:
Figures from p. 23 of
PWM_8B8C Block User
Guide.
PWM Scale A Register
(PWMSCLA)
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The 8-bit value in this register sets Clock
SA’s frequency, according to the formula:
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Figures from p. 28 of PWM_8B8C Block User Guide.
PWM Clock Select Register
(PWMCLK)
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Each PWM channel has one bit in this
register to select which clock it uses:
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For PWM Channels 0, 1, 4, 5, we can choose
between Clock A and Clock SA.
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For PWMM Channels 2, 3, 6, 7, we can choose
between Clock B and Clock SB.
Figure from p. 22 of PWM_8B8C Block User Guide.
PWM Channel Period Registers
(PWMPERx)
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Each PWM channel has one of these
registers.
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The value in this register determines the
period (cycle time) of the signal generated
by the PWM channel:
Signal period = PWMPERx × Period of selected clock source
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Figure from p. 31 of PWM_8B8C Block User Guide.
PWM Channel Duty Registers
(PWMDTYx)
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Each PWM channel has one of these registers.
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Assuming PWMPOL has been set so that the signal
starts HIGH, the value in this register determines
the pulse width (time high) of the signal generated
by the PWM channel:
Pulse width = PWMDTYx × Period of selected clock source
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Figure from p. 32 of PWM_8B8C Block User Guide.
Sample Code for Configuring
PWM Channel 4
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Note that code is incomplete: you must
decide what values to load into PWMPER4
and PWMDTY4.