Electronics Design Laboratory
Lecture #3
ECEN 2270
Electronics Design Laboratory
1
Lessons from Lab 1
• Use the course calendar, it has all relevant dates
• Use the course website, it has all lab materials
• If you have questions ask the instructors
ECEN 2270
Electronics Design Laboratory
2
Experiment 2 ‐ Robot DC Motor
• Broader objectives: working with a load
–
–
–
–
–
Understand the physical behavior of the load: DC motor
Developing an electrical model for the DC motor as a load
Experimentally finding model parameters
Performing design and simulation using models
Hardware implementation, verification, and testing
ECEN 2270
Electronics Design Laboratory
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Two DC motors, each driving a wheel
• Each DC motor has an optical encoder for sensing direction and rotational frequency
• A gear box connects the motor to the wheel
Robot Platform DC Motors
wheel
+
‐10 < VDC < +10 V _
IDC
wheel
shaft
Optical Encoder
VCC +_
ENCA
ENCB
DC Motor
64:1
gear
motor
shaft
12 pulses per motor shaft rotation
ECEN 2270
Electronics Design Laboratory
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DC Motor basics
• DC currents create magnetic fields

B
iDC
iDC

B
Ampere’s law relates current iDC to the magnetic field, B
iDC
• Magnetic fields want to alight with each other! The will exert force on each other…


B2
ECEN 2270

F

B1
F
iDC

B2

B1
 

F  iDC l  B2
Force [N]

F
Electronics Design Laboratory
Unit
Length
Vector
Current [A] Magnetic flux density vector [T]
5
Simple Motor
N
S

F

B1

B2
DC voltage creates DC current. ‘Split rings’ reverse polarity every half turn
+
_
iDC
VDC
Use permanent magnets to create a fixed magnetic field
N
S
T  kI
• If an arm is connected to the rotating coil, we have a motor!
• The torque of this motor is directly related to the DC current DC
Torque [Nm]
in our wire loops
k = motor constant [Nm/A]
ECEN 2270
Electronics Design Laboratory
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Back EMF
N
S

F
We now have a time varying magnetic field… Faradays law tells us this should be generating an electromotive force! 
B1

B2
+
_
iDC
N
S
VDC
Rate of change of magnetic flux  through the armature winding
VEMF
Induced EMF [V]
d

dt
VEMF  k
Induced EMF [V]
Speed [rad/s]
k = motor constant [Nm/A], [V/(rad/s)]
• For analysis an equivalent circuit would be nice… ECEN 2270
Electronics Design Laboratory
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Building an Equivalent Circuit.
N
S

F
VEMF  k

B1

B2
Induced EMF [V]
+
_
iDC
N
S
ECEN 2270
ECEN 2830, Spring 2011
Speed [rad/s]
T  kI DC
Torque [Nm]
• DC Voltage source powers motor
• Motor coils are simply one long piece of wire Electronics Design Laboratory
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DC motor equations
Electrical model (armature circuit)
VDC  RM I DC
VEMF  k
dI DC
 LM
 VEMF
dt
Mechanical model
d
TJ
 B  Tload
dt
T  kI DC
Tload  Tint  Text
ECEN 2270
J = moment of inertia
B = friction coefficient
Load torque is a combination of internal gearbox load and external load
Electronics Design Laboratory
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DC motor equivalent circuit model
IDC
LM
RM

+
+
VDC
VEMF = k
+
–
T = kIDC
1/B
J
Tload
_
_
VDC  RM I DC
dI DC
 LM
 VEMF
dt
VEMF  k
d
TJ
 B  Tload
dt
T  kI DC
Tload  Tint  Text
• Consider how to measure all circuit parameters from the model
• Requires measurement of
• input terminals, VDC and IDC
• frequency  in rad/s  use optical encoder
ECEN 2270
Electronics Design Laboratory
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Optical encoder
Encoder output pulses, frequency fenc [Hz] is proportional to speed
counterclockwise
Encoder pulse output A
Encoder pulse output B
clockwise
Encoder pulse output A
Encoder pulse output B
In Lab 2, only one encoder pulse output is needed. Optional extra credit uses both pulses to determine direction
ECEN 2270
Electronics Design Laboratory
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Encoder circuit
+VCC = +5 V
GND
Pulse out A
Pulse out B
LEDs shine through a spinning wheel with notches
Photo‐transistors
short a node to ground whenever light is shined on them
Logic inverters shape the sensed signals into square‐wave output pulses
Encoder connector takes VCC and ground and supplies ENCA and ENCB
Spinning disk goes here
ECEN 2270
Electronics Design Laboratory
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Speed conversions
Example: wheel speed is 1 rotation per second: 1 rps
rotation
sec
radians 
rad

  n  2
  2
sec
 rotation 
f enc  n 64  12  768Hz
n 1
enc   64 12   4.8k
rad
sec
n = wheel speed, rotations per second [rps]
 = wheel rotational speed [rad/s] fenc = frequency of encoder pulses [Hz]
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Electronics Design Laboratory
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Input and output ports defined
Model parameters to be determined by experiments:
RM, k, J, B, Tint
ECEN 2270
Spice model
Encoder model: correct speed to fenc frequency conversion has already been done, no need to change anything in this part of the model
• Download the model from the Experiment 2 website
• Only edit the model designated parameters
Electronics Design Laboratory
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Testing Spice model
External load torque Text attached here
External load must sink to ground
• Simulation set up to
1. Start motor: bring up VDC, over first 1ms
2. Pulse load torque: 0A (no load) for first 50ms, 1A for next 50ms
3. Stop motor: bring down VDC from 100ms to 101ms, 10V to 0V
ECEN 2270
Electronics Design Laboratory
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Motor Simulation Results
• Consider waveforms and model in each mode: motor start, load change, motor stop
IDC
LM
RM

+
VDC
VEMF = k
+
–
T = kIDC
J
1/B
Tload
_
VDC  RM I DC  LM
dI DC
 VEMF
dt
VEMF  k
ECEN 2270
d
 B  Tload
dt
Tload  Tint  Text
T  kI DC
TJ
Electronics Design Laboratory
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