Electronics Design Laboratory Lecture #3

Electronics Design LaboratoryLecture #3

ECEN 2270 Electronics Design Laboratory 1


Lessons from Lab 1• All relevant dates are in the course calendar, which is

available on both D2L and on the course website.

• All lab materials are on the course website

http://ecee.colorado.edu/~ecen2270/

• If you have questions, ask the instructors

Electronics Design Laboratory 2


Experiment 2 – Robot DC Motor• Part A: working with a load, modeling, finding model

parameters based on experiments– 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

• Part B: speed sensor circuitry: hardware implementation, verification, and testing



Two DC motors, each driving a wheel• Each DC motor has an optical

encoder for sensing rotational frequency and direction

• A gear box connects the motor to the wheel

IDC

wheel

12 pulses per motor shaft rotation

-10 < VDC < +10 V

motorshaft

wheelshaft

+VCC

Robot Platform

DC Motors

64:1gear

Optical Encoder

_

+_

ENCAENCB

DC Motor


Current produces magnetic field (which is why conductors have inductance)

Basics: Current, Magnetic Field, Force

Electronics Design Laboratory 5ECEN 2830, Spring 2011

i

𝐵𝐵

i

𝐵𝐵

𝐵𝐵

𝐵𝐵𝑒𝑒

�⃗�𝐹

�⃗�𝐹

Magnetic fields tend to align with each other. As a result, mechanical force is exerted on a coil of wire carrying current i when the coil is placed in external magnetic field


NS

NS


Simple DC Motor

Torque [Nm] DCkIT =k = motor constant [Nm/A]

+_

VDC

iDC Use permanent magnets to create a fixed magnetic field

• If a shaft is connected to the rotating coil, we have a motor!• The torque of this motor is directly related to the DC current

in our wire loops

DC voltage creates DC current. ‘Split rings’ reverse

polarity every half turn

�⃗�𝐹 𝐵𝐵

𝐵𝐵𝑒𝑒


NS

NS


Back EMF

+_

VDC

IDC

We now have a time varying magnetic field through the coil… Faradays law tells us this

should be generating an electromotive force, i.e. an induced voltage!

dtdVEMFΦ

−=Induced EMF [V]

Rate of change of magnetic flux Φ through the coils (“armature

winding”)

ωkVEMF =

k = motor constant [Nm/A], [V/(rad/s)]

Induced EMF [V] Speed [rad/s]

�⃗�𝐹 𝐵𝐵

𝐵𝐵𝑒𝑒


Basic DC Motor Relationships

Electronics Design Laboratory 8ECEN 2830, Spring 2011

ωkVEMF =Induced EMF [V]

Speed [rad/s]

NS

NS

+_

IDC

Torque [Nm] DCkIT =�⃗�𝐹 𝐵𝐵

𝐵𝐵𝑒𝑒 VDC

For analysis, it would be nice to have an equivalent circuit of the motor…


DC motor equations

ωkVEMF =

Electrical model (armature circuit)

EMFDC

MDCMDC Vdt

dILIRV ++=

Mechanical model

loadTBdtdJT ++= ωω

DCkIT =

J = moment of inertiaB = friction coefficient

extintload TTT += Load torque is a combination of internal gearbox load and external load


DC motor equivalent circuit model

+–

+

VDC

_

IDCLM RM

VEMF = kω T = kIDC

ω

Tload1/BJ

EMFDC

MDCMDC Vdt

dILIRV ++=loadTB

dtdJT ++= ωω

ωkVEMF = DCkIT =

• Consider how to measure all circuit parameters from the model• Requires measurement of

• input terminals, VDC and IDC

• frequency ω in rad/s use optical encoder

extintload TTT +=

+

_


Optical encoder

Encoder output pulses, frequency fenc [Hz] is proportional to speed

counterclockwise

clockwise

Encoder pulse output AEncoder pulse output B

Encoder pulse output AEncoder pulse output B

In Lab 2, only one encoder pulse output is needed. Optional extra credit uses both pulses to determine direction


Encoder circuit

+VCC = +5 VGND

Pulse out APulse out B

Photo-transistorsshort 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

LEDs shine through a

spinning wheel with

notchesSpinning disk goes here


Speed conversions

n = wheel speed, rotations per second [rps]

ω = wheel rotational speed [rad/s]

fenc = frequency of encoder pulses [Hz]

Example: wheel speed is 1 rotation per second: 1 rps

( )

( )( )

( )( )secradk8.41264

Hz7681264secrad2

rotationradians2

secrotation1

≈×=

≈×=

=

=

=

ωω

ππω

enc

enc nf

n

n


DC motor Spice sub-circuit model

Model parameters to be determined by experiments:

RM, k, J, B, Tint

Encoder model: correct speed to fenc frequency conversion has already been done, no need to change anything in this part of the model

Input and output ports defined

• Download the model from the Experiment 2 website• Only edit the model designated parameters


Testing DC motor Spice model


External load torque Text attached here

External load must sink to

ground

• Simulation set up to1. Start motor: bring up VDC, over first 1ms2. Pulse load torque: 0A (no load) for first 50ms, 1A for next 50ms3. Stop motor: bring down VDC from 100ms to 101ms, 10V to 0V


Motor Simulation Results


+–

+

VDC

_

IDCLM RM

VEMF = kω T = kIDC

ω

Tload1/BJ

EMFDC

MDCMDC Vdt

dILIRV ++= loadTBdtdJT ++= ωω

ωkVEMF = DCkIT =

• Consider waveforms and model in each mode: motor start, load change, motor stop

extintload TTT +=

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Electronics Design Laboratory Lecture #3