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Oct 18, 2013 (4 years and 5 months ago)

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DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
1

DC
Motors

Electric Circuits

/

System Design

Overview
:

Rube Goldberg
(1883
-
1970)
was a Pulitzer Prize
-
winning
artist, famous for his cartoons
portraying

fictional inventions.
Each

invention

consisted of a
long, complex sequence of steps to carryout

a

si
mple
While

his inventions
regularly
violat
ed
some
engineering principles, there was

a
lways

a
great deal of
real engineering to be found in his
humorous

designs
.
Although h
e made his career as an artist,
Rube Goldberg
was educated as an enginee
r. H
is
legacy continues today through
the

annual
National Rube
Goldberg Machine Contest held at Purdue University
. The
event is organized by the local chapter

of Phi Chapter of Theta Tau, the National Student Engineering Organization
.

In this activity, you will l
earn the basics of operating a small DC electric motor to achieve one
step in a
, and you will work with other students to assemble
a Rube Goldberg
machine.

CAUTION:

Th
is

equipment
described here
is only appropriate for controlling low
-
vol
tage
, low
-
current

DC
devices, such as
small
battery
-
powered motors.
NEVER CONNECT THE CIRCUIT
TO AC DEVICES OR TO ANY VOLTAGE SOURCE THAT EXCEEDS 12 VOLTS OR
3
00 mA.

PART 1

Some

Basics

of
DC

Motor
s

and Relays

DC motors come in a large variety of sizes an
d shapes, but they all depend upon the interaction
between electric currents and magnetic fields to produce a
rotation.

Most (but not all) can rotate
either forward or backward depending upon the direction of the electric current.

Electromechanical relays
are similar to motors in their dependence on electric currents and
magnetic fields, but they are designed simply to control the back and force movement of a
mechanical switch.
Like motors, relays are available in a wide variety of shapes and sizes,
includi
ng the 2 types shown in the photo at the top of this page.
A relatively small current is

needed to activate a relay, enabling

the relay
to

control a much larger current. In many
applications
,

relays have been replaced by transistors or other purely electro
nic switches

which
can be less expensive and which are not subject to the possible failure of a relay’s moving parts.
On the other hand,

relays remain important in man
y applications

because
they can be much more
efficient in conducting relatively large cur
rents with virtually no drop in voltage.

Relays are often used to control DC motors, both to turn them on and off and to control their
direction of motion. The schematic circuit diagram on
the next page shows an “H
-
Bridge,” which
you will use to control a
DC motor. There are also photos that show the inner workings of
two
types of
relay
s
.

Although the relays are
quite
different electrically and physically
, both use a
current flowing through an electromagnetic to move a mechanical switch.

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
2

Circuit Schematic

Diagram

SPST Relay

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
3

DPD
T Relay

Question 1 on the Report Form s
hows black
-
and
-
white versions of the motor control circuit, with
the relays shown in all of their possible states
.
The top

relay is a single
-
pole, single
-
throw (SPSP)
relay, which comp
letes an electrical pathway when the relay’s magnet is energized. The other
relay is a double
-
pole, double
-
throw (DPDT) relay, which two pathways (2 poles) and which
directs the electric current in either of two different directions (2 throws). In your cir
cuit, the
SPSP relay will be magnetized when your calculator sends the signal “1.” The DPDT relay will
be magnetized when your calcualtor sends the signal, “2”. Both relays will be energized by the
signal “3,” since 3 = 1 + 2.

Directly on the diagrams on t
he Report Form, draw lines and arrows to show how current will
flow from the battery positive for each of the 4 possible signals. Also indicated how the
motor
will respond by checking either “forward,” “reverse”

or
“off.”

Use a different color of pen, penc
il or marker on the diagrams above to show the path of
electric current which will activate the electromagnet.for each
magnetized

relay.

Answer the other question in Part 1 of the Report Form.

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
4

DC Circuits

REPORT FORM (Part 1)

NAME(S) ____________________
________________________________________________

1)

Directly on the diagrams on the Report Form, draw lines and arrows to show how current will
flow from the battery positive for each of the 4 possible signals. Also indicated how the
motor will respond by che
cking either “forward,” “reverse” or “off.”

Signal =

0

SPST relay:
not magnetized

DPDT relay:
not magnetized

Motor: Forward ___ Reverse ___ Off ____

Signal =
1

SPST relay:
magnetized

DPDT relay: not magnetized

Motor: Forward ___ Reverse ___ Off

____

Signal =
2

SPST relay: not magnetized

DPDT relay:
magnetized

Motor: Forward ___ Reverse ___ Off ____

Signal =

3

SPST relay:
magnetized

DPDT relay:
magnetized

Motor: Forward ___ Reverse ___ Off ____

2)

Use a different color of pen, pencil o
r marker on the diagrams above to show the path of
electric current which will activate the electromagnet.for each
magnetize
d

relay.

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
5

3)

The SPST relay shown has a coil resistance of 250
Ω and is rated for a control voltage of 6 V.
Use Ohm’s law to find the control current which will flow through the electromagnet’s
coil

when the relay is activated.

4)

The SPST relay is rated for a “maximum switched current” of 500 mA. Why is this value not
t
he same as the current calculated in question 3?

5)

The 2 relays used here are SPST and DPDT. Describe with words or a diagram how a
SPDT

relay would operate.

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
6

PART
2

Building and Testing the Control Circuit

CAUTION: The motor here must use a DC voltage of
12 V or less and it must use a current
of 500 mA or less.

Be extremely alert for short circuits, which can cause fires and can cause
the batteries to over heat and spill harmful chemicals. DO NOT INSERT BATTERIES
INTO THE BATTERY PACK UNTIL AFTER THE CIRCU
IT HAS BEEN CHECKED

The pictures on the next page show a recommended board layout for using a SPST relayt and a
DPDT relay to control a DC motor. The green lines represent metal clips inside a solderless

#276
-
175. Solid 22AWG wire should fit easily into any of the
rd to complete other connections. If your motor or battery pack uses
stranded wire, it may be necessary either to make the attachment to the board using a shor
t
jumper wire with alligator clips.

Assembl
e the components and connection on the breadboard as shown.

Note that the coil
connections on the DPDT relay are furthest from the other pins and that these go to the
left
in the suggested layout. This means that
the lettering on the DPDT relay will face
towards the back.

After the breadboard has been approved by your instructor, use the calculator, CBL2 and
Binary Basic Trainer to test the operation of your motor, verifying that it turns both
directions and goes o
ff with the appropriate calculator instructions. If the system does not
function as it should, use the troubleshooting guide which follows the pictures to find and
correct the problem.

Describe the results of your tests on
Part 2 of
the Report Form.

DC Motors

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, 200
6

page
7

Bo
ard Layout

DC Motors

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, 200
6

page
8

DC Circuits

REPORT FORM (Part 2)

NAME(S) ____________________________________________________________________

1)

Describe below the steps you took to determine whether your control circuit is functioning as
as it should.

2)

Describe any proble
ms you encountered with the system and explain how you attempted to
identify and correct the problems. Also desribe the final result of your troubleshooting.

3)

In your system, what is the source of energy which powers the motor?

4)

In your system, what is the s
ource of energy which powers the relays?

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
9

PART 3

A Rube Goldberg Megasystem

CAUTION: The motor
s

here must use a DC voltage of 12 V or less and must use a current
of 500 mA or less.

Be extremely alert for short circuits, which can cause fires
or
cause the

batteries to over heat and spill harmful chemicals.

In the spirit of Rube Goldberg, the goal of this section is to move a ping pong ball from one end
of the room to the other in the most complex and entertaining way possible. Work with others in
the class

to design a megasystem which satisfies these requirements:

There must be at least 5 distinct and different steps to the ball’s motion. (The more steps, the
better.)

The entire motion must be planned in advance, with control of each step assigned specific
ally
to one team. Control may
only
be exercised by entering appropriate signal codes into a
graphing calculator.

Once the trial begins, no one except the instructor may touch the ping pong ball directly.

Complete Part 3 of the Report Form to describe the

Megasystem.

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
10

DC Circuits

REPORT FORM (Part 3)

NAME(S) ____________________________________________________________________

1.

In the table below, describe each step in moving the ping pong ball. Add more pages as
needed.

Step #

People Responsible

Descriptio
n

1.

2.

3.

4.

DC Motors

Participant Handout

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11

5.

6.

7.

8.

9.

DC Motors

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12

Trouble Shooting

Sooner or later, you will encounter problems with the control circuit. Although it can certainly be
frustrating to find that an experiment or demonstration is not working as expected, most
problems

(like those in most other electrical systems) will be bad connections that can be quickly
corrected. Real
-
world systems don't always work perfectly, and it is extremely valuable to know
how to deal with the inevitable problems.

The key strategy for diagn
osing and repairing
problems with the control circuit depends upon
understanding that the circuit merges a flow of
information from the calculator and the CBL
with one or more flows of energy from the
batteries. If you don't see the desired action, it
is
possible that a faulty or damaged component
is causing the problem, but it is far more likely
that the flow of either information or energy is
blocked at some point. You need to find the
blockage and eliminate it.

Check the information path:

Measurement

Expected result

Problem solution

To check the calculator
-
CBL2 connection
, exit
the SIGNAL program if
it is running and restart
the program. The
program checks to
verify that the CBL2
(
or
LabPro
)

is responding
to the calculator and
gives an error message
i
f the two devices are
not communicating.

If there is no error, the
SIGNAL program goes
quickly to the welcome
screen. When you actually
send a signal, the yellow
and green LEDs on the
CBL2 or LabPro should
flash.

The most common problems involve
cable between the calculator
and CBL2 or LabPro. Be sure it is
inserted fully into both devices. You
may also need to try a difference
cable, particularly if the tip is bent.

Be sure the CBL2 or LabPro has good
batteries or that it is connected to the
pr

SIGNAL

program to verify
that it has not been changed by
accident.

Check to see if the
Binary Basic Trainer is
responding to the CBL2

or LabPro
by
using the
SIGNAL program’s
“Count 0 to 15” option.

If there is no error, all 4
L
EDs on the Binary Basic
Trainer should light and in
turn and go out again
according to the binary
sequence.

Verify that the cable from the CBL2
to the Binary Basic Trainer is snapped
completely into place at both ends.

Verify that the Binary Basic Trainer
’s
batteries are good and in place.

If using the LabPro, verify that the
cable
is attached to
DIG/SONIC
1
, not
DIG/SONIC
2
.

DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
13

Check to see if
information is reaching
the

SPST power

relay
,
by using the SIGNAL
program’s “Key Press”

a汴e牮r瑥汹
=
“0” and “1.”
=
By⁬=獴敮楮s⁣汯獥汹⁯爠

-

you switch from “1” to
“0.” (
f琠t猠

=

=

=

“1
” is sent.
)
=

Trace the wires which lead from one
of the Binary Basic Trainer’s “+”
terminals to the relay’s input and

=

Verify that the SPST relay’s 4 pins
a牥⁡汬⁩渠=桥⁣潲oec琠桯te猠潦⁴桥=

=

If necessary, use a voltmeter to fi
nd
the break in the connection. Send a
continuous “1” from the calculator
and place the voltmeter’s negative

Basic Trainer’s “1” attaches to the

a牴楮r⁷桥牥⁴桥⁷楲i⁦=潭⁴桥=
Binary Basic Trainer’s “+” terminal
a瑴ac桥猠瑯⁴桥⁢牥a摢潡牤⸠r桥=

=
=

ec欠瑯⁳ee⁩映

DPDT direction

relay
, by using the
SIGNAL program’s
“Key Press” option to
press alternately “0”
and “
2
.”
=
By⁬=獴敮楮s⁣汯獥汹
=

=

relay coming on when “
O

“0” is sent.
=

Trace the wires which lead from one
of the Binary Basic Trainer’s “+”

䑐䑔a
relay’s input
a湤⁲n獥a琠慳⁮te摥搮
=

Verify that the
DPDT
relay’s
U
=

a牥⁡汬⁩渠=桥⁣

=
f映湥ce獳慲yⰠ畳I⁡⁶潬=浥瑥爠瑯⁦楮搠

=
pe湤⁡=
continuous “2” from the calculator
and place the voltmeter’s negative

Basic Trainer’s “2” attaches to the

ard. Move the voltmeter’s

Binary Basic Trainer’s “+” terminal
a瑴ac桥猠瑯⁴桥⁢牥a摢潡牤⸠r桥=

=
DC Motors

Participant Handout

Jan. 2
2
, 200
6

page
14

Check the energy path:

Measurement

Expected result

Problem solution

V
erify that en
ergy is

by using

a voltmeter to
measure the voltage
where the 2 wires from
the batter
y pack attach

The voltage at
should be close
to the nominal
value expected
from the
1.5 V time the
number of
batteries).

Remove the batteries and reinstall them,
making sure none are backwards

Verify the b
atteries are not dead. Replace them
if needed.

Check for a short circuit

. If
the battery pack leads show 6 volts when they
are not connected to the breadboard but the
voltage drops when the are connected to the
board, there may be an un
desired connection
from positive to ground on the board.

Verify that the main
batteries can operate the
motor

by connecting
at the point where the

The motor
should one
continuously in
one di
rection
regardless of any
signal from the
calculator.

Verify that the voltage and current
requirements of the motor match the capability
of the batteries.

Use an ohmmeter to check for a short circuit,
and open circuit in the motor and replace the
motor if
needed.

Verify that the SPST
power relay can switch
the motor on and off
,

by

moving the motor’s
with the opposite side
of the SPST relay. Use
the SIGNAL program’s
“Key Press” option to
send alternate signals of
“0” and “1.”

The motor

should run in one
direction when
you send “1” and
go off when you
send “0.”

Check that all 4 pins of the SPST relay are
seated correctly and that the connecting wire
are in the correct location

Replace the SPST relay, if needed.

Verify that the
DPDT
dire
ction relay can
switch the motor’s
direction
,

by
reconnecting the
the correct output of the
DPDT relay. Use the
SIGNAL program’s
“Key Press” option to
send alternate signals of
“1” and “3.”

The motor
should run in one
direction wh
en
you send “1” and
in the opposite
direction when
you send “3.”

Check that all 8 pins of the DPDT relay are
seated correctly and that the connecting wires
are in the correct location

Replace the DPDT relay, if needed.