DC Motors, Stepper Motors, H-bridges
• DC Motors
• Stepper Motors
• Motor Control Circuits
– Relays
– H-bridges
AC versus DC Motors
• AC motors are not very flexible w/o a transmission
– They can only turn in one direction
– The speed is controlled by the design of the motor and
the frequency of the AC current source (60Hz)
• DC Motors can be operated more flexibly
– They can turn in either direction based on the polarity
of the applied voltage
– The speed is controlled by the magnitude of the voltage
DC Motors
• A DC motor has coils on a rotating electromagnetic
armature in a fixed or variable magnetic field
• A commutator connects the DC power source in
sequence to the coils in the armature as it turns
• Commutator end view:
Fixed /Variable Field Magnet
+ Brushes
Electromagnetic
Armature Coil
(1 of Several)
Commutator
Fixed/Variable Field Magnet
- +
DC Motors
• A continuous voltage across the brushes will
keep the motor turning in either one direction
or the other depending on the polarity
– A higher voltage across the brushes will make the
motor turn faster
– A lower voltage across the brushes will make the
motor turn slower
• The commutator sparks as it turns creating EMI
or possible explosion hazard
Stepper Motors
• A stepper motor is a DC motor that has fixed
magnets on the armature
• It does not use a commutator to automatically
energize/de-energize different magnetic coils
• The ends of the coil windings in the field are
alternately energized and de-energized by an
external control circuit in a desired sequence
• The order and dwell time of the voltage to each
coil controls the direction and the speed
Stepper Motors
• A stepper motor can be held in a fixed position
by pausing the sequence and keeping one coil
energized for the duration of the hold time
• Hence, a stepper motor can be used in similar
applications as a servomotor (studied in CS341)
– It can be moved to and held in a desired position
– It can be rotated continuously at a controlled speed
• The control is all externally implemented
Types of Stepper Motors
• Many possible geometric arrangements of the:
– Fixed magnets on the armature
– Electromagnetic coils in the field around the armature
• References:
http://homepage.cs.uiowa.edu/~jones/step/types.html
• Our Mercury Motor is a Bipolar Stepper Motor
Stepper Motors - Heat
• Stepper motors are designed to operate at a
high temperature and can get hot (~ 80 deg C)
• If this causes a problem, try the following:
– Lower the voltage (But I found the Mercury motor
will not step/hold reliably at 9 VDC versus 12 VDC)
– Turn off the power to the coils when not in use
(Expect that the motor will slip out of position)
– Mount the motor on a heat sink to dissipate heat
– Use a fan to create air flow over motor / heat sink
Motor Control Circuits
• A motor control circuit:
– Provides power to the motor coils in either polarity
– Allows the external logic to control direction/speed
• There are two basic types of motor controllers
– Electromechanical relay based controllers
– Semiconductor based controllers (H-bridges)
• Each type has its advantages and disadvantages
• We’ll use our elevator motors as an example
Relay Motor Control Circuit
• Two relays can be used to provide on/off and
up/down control for our elevator DC motor
To Arduino
Output Pins
& Ground
NO
To Motor
Power
Supply
+ 9 VDC
NC
Ground
Red
Up Down
+
Black
NC
NO
+
Yellow
Black
Safety switch: Stops “up”
when the car hits switch
Relay Motor Control Circuit
• Advantages
– Uses simple, robust electromechanical devices
– Can handle high voltages and large currents
• Disadvantages
– Requires periodic maintenance (cleaning contacts)
– Relatively slow due to inertia in mechanical parts
– Contact make/break/bounce sparks creating EMI
or possible explosion hazard
• Example: Used under the hood in cars
Relay Motor Control Circuit
• A DC motor such as the drive motor for our
elevator system requires one circuit per motor
– The direction is determined by one relay or the other
being turned on - closing normal open (NO) contact
• A bipolar stepper motor such as the Mercury
Motor requires two circuits per motor
– Each coil is turned on and off in a selected direction
in sequence to change the position continuously
– One coil can be left on in a selected direction to hold
in a desired position
Bipolar Motor - Relay Shield Wiring
Arduino
Uno
Don’t
Connect
Positive
Sides NO
NC
+ 12 VDC
Stepper
Motor
Coil A
Black
Stepper
Motor
Coil B
Brown
NO
NO
NC
NC
NO
Orange
Yellow
NC
Connect
Grounds
Relay
Coil 0
Clockwise
+
+
+
+
Counter CW
Pin 7
Pin 6
Pin 5
Pin 4
D0
D1
D2
D3
Relay
Coil 1
Relay
Coil 2
Relay
Coil 3
H-bridge Motor Control Circuit
• An electronic version of previous relay circuit
• Can be implemented with individual components
http://www.youtube.com/watch?v=A_JNjAFo1f4
• Can use an L298 H-bridge component
– Two independent H-bridge circuits per component
– Needs external “snubber” or “flyback” diodes
H-bridge Motor Control Circuit
• Half of an L298 H-bridge can be used to provide
on/off and up/down control for a DC motor
+9 or 12 VDC
Vs
+5 VDC Vss
Pin 4
Pin 9
Pin 2
Input 1
Arduino
Uno
L298
Pin 5
H-bridge
Enable A
Pin 6 (Side A)
Input 2
Pin 7
Pin 8 Pin 1 Pin 3
Snubber
Diodes
Connect
Grounds
Red
Safety
Switch
Yellow
Black
Snubber
Black
Diodes
Up Down
-
+
+
-
H-bridge Motor Control Circuit
• Advantages
– No periodic maintenance is required (no contacts)
– Fast switching speed available (limited by motor)
– No EMI or possible explosion hazard due to sparks
• Disadvantages:
– Limited voltage and current handling capacity
– Vulnerable to EMI/EMP or damage due to electrical
transients (may need metal shielded container and
“snubber” or “flyback” diodes)
H-bridge Motor Control Circuit
• A DC motor such as our elevator system drive
motor requires one H-bridge per motor
– The direction is determined by the inputs (1 or 2)
– The motor can be turned on and off by the Enable A
• A bipolar stepper motor such as the Mercury
Motor requires two H-Bridge circuits per motor
– The direction is determined by the ordering of the
logic signals to the inputs (1, 2, 3, or 4)
– The speed is controlled by the speed of sequencing
H-bridge Motor Control Circuit
• The Motor Shield provides uses both halves of
an L298 component – one to control each coil
of the stepper motor
• It is an electronic version of the Relay Shield
diagram shown earlier
• It includes the “snubber” diodes on the board