Electronics Design LaboratoryLecture #3

ECEN 2270 Electronics Design Laboratory 1

ECEN 2270 Electronics Design Laboratory 2

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

ECEN 2270 Electronics Design Laboratory 3

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

Electronics Design Laboratory 3

ECEN 2270 Electronics Design Laboratory 4

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

ECEN 2270 Electronics Design Laboratory 5

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

ECEN 2270 Electronics Design Laboratory 6

NS

NS

Electronics Design Laboratory 6

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

ECEN 2270 Electronics Design Laboratory 7

NS

NS

Electronics Design Laboratory 7

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

ECEN 2270 Electronics Design Laboratory 8

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…

ECEN 2270 Electronics Design Laboratory 9

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

ECEN 2270 Electronics Design Laboratory 10

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 +=

+

_

ECEN 2270 Electronics Design Laboratory 11

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

ECEN 2270 Electronics Design Laboratory 12

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

ECEN 2270 Electronics Design Laboratory 13

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

ECEN 2270 Electronics Design Laboratory 14

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

ECEN 2270 Electronics Design Laboratory 15

Testing DC motor Spice model

Electronics Design Laboratory 15

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

ECEN 2270 Electronics Design Laboratory 16

Motor Simulation Results

Electronics Design Laboratory 16

+–

+

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 +=