4 LABORATORY - SERIAL INTERFACING AND PROGRAMMING

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A stepper motor is unlike other motors. When a voltage is applied the motor does not turn

continuously, it only moves a small increment. There are normally a set of four or

more inputs. When these are turned on-off in a set pattern the motor shaft will

rotate forward or backwards. A typical stepper motor might have 200 steps per

revolution, or steps of 1.8 degrees. These motors often require somewhat sophisticated controllers. One type of controller is called an indexer. It can be given commands to move the motor, and then it takes care of pulsing the motor outputs to

drive the motion.

The stepper motor controllers to be used in this laboratory are integrated into the turntables in the material handling system. The controller is integrated into the turntable

stations so that it can rotate the turntable up to 360 degrees with a stepped motor,

eject a cart using two outputs to solenoid valves, and detect a cart present with a

diffuse photoelectric sensor. The controller has an RS-422 port that can be used to

communicate, and load programs. This will be connected to an RS-232C port

using a special interface cable that converts the current loop to voltage values. The

communication settings for the turntables are 9600 baud, 8 data bits, no parity, 1

stop bits, no flow control.

The programming commands for the controller are summarized below.

DCB-241 Commands

abort

@ soft stop

C reset

+ move in positive direction

- move in negative direction

[ read nonvolatile memory

] read hardware limits

\ write to nonvolatile memory

^ read moving status

A port read/write

B jog speed

C restore

D divide step rates

E enable auto power off

F find home



G go from address

I initial velocity

K ramp slope

L loop on port

M move at constant speed

O set origin

P program mode

Q query program

R index to target position

S store parameters

T set trip point

V slew velocity

W wait

X examine parameters

Z display position



Figure X.14 - Stepper Motor Control Board Commands (DCB-241)

When writing programs command lines can be up to 15 characters long, including spaces.

Spaces are used to separate commands and arguments. Characters used in programs can be either upper or lower case. A sample program is given below.



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Pre-Lab:

1. Go to the web site www.stepcontrol.com and look at the product documents for the

DCB-241 stepper driver.

In-Lab:

1. Use a terminal program to communicate with the stepper motor controller. You will

need a special communication cable, and the boxes can be opened with a flat

bladed screwdriver. Plug the communication cable into the lower connector. (Note:

if the unit already has power don’t touch the exposed 120Vac power on the power

supply.) Connect an air supply and power to the unit. (Note: don’t forget to turn on

the power on the front of the cabinet.)

2. Use the following commands (in sequence) to verify that the turntable is operating

properly, and to explore basic commands. (Note: comments are provided for

understanding, but should not be entered into the controller.)

C -- this should reset the unit

-- this should print out the line ’V2.03’, if not there are problems

-- this should print ’#’

Z -- read the current position

O -- set the current position as the origin

Z -- print the current position

R1000 -- this should rotate the turntable

Z -- should now be 1000

R-1000 -- this should rotate the turntable the other way

Z -- should be zero again

A8 - kicks the cart one way (notice the lights on the solenoids)

A16 - kicks the cart the other way

A0 - turns off all solenoids

] -- this will check the input ports, bits 7 and 8 are for the cart present detectors

3. Enter the following program so that the turntable operates automatically. The list below

also includes the commands to download and enter the program. Again comments

should not be entered, and line numbers are automatically generated. When the

program has been entered it can be run with the command ’G0’.

P0 -- put the controller in programming mode and start the program at location ’0’

0 O0 -- set the current position to the origin with a value of 0

4 R10000 -- more the controller 10000 steps in the positive direction

8 W0 -- wait until ’0’ ms after the motion is complete

11 R-10000 -- move 10000 steps in the opposite direction

15 W100 -- wait until ’100’ ms after the motion is complete

18 J 4 3 -- jump to address ’4’ four (3+1) times, a basic for loop (you may need to

change ’4’ if your line numbers don’t match)

22 A8 -- eject the cart

24 W1000 - wait for 1 second

27 A0 - shut off the solenoid valve

29 P0 -- the end of the program

4. Write a C++ program to communicate with the stepper motor controller over RS-232. It



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should allow the user to enter a motor position from the keyboard, and the controller should automatically move.

Submit (individually):

1. The source code listings for the motor control program.



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7. PROGRAMMABLE LOGIC CONTROLLERS (PLCs)



• CONTROL - Using artificial means to manipulate the world with a particular goal.



• System types,



• Continuous - The values to be controlled change smoothly.

e.g. the speed of a car as the gas pedal is pushed

• Logical - The values to be controlled are easily described as on-off.

e.g. The car motor is on-off (like basic pneumatics).

Note: All systems are continuous but they can be treated as logical for

simplicity.

• Logical control types,



• Conditional - A control decision is made by looking at current conditions

only.

e.g. A car engine may turn on only when the key is in the ignition and

the transmission is in park.

• Sequential - The controller must keep track of things that change and/or

know the time and/or how long since something happened.

e.g. A car with a diesel engine must wait 30 seconds after the glow

plug has been active before the engine may start.

Note: We can often turn a sequential problem into a conditional by

adding more sensors.



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CONTROL



CONTINUOUS



LINEAR



LOGICAL



NON_LINEAR



CONDITIONAL



e.g. MRAC

e.g. PID



BOOLEAN



SEQUENTIAL

EVENT BASED

TEMPORAL



e.g. COUNTERS

e.g. FUZZY LOGIC

EXPERT SYSTEMS e.g. TIMERS



Examples:



continuous:



logical:

conditional:



sequential:



mixed (continuous and logical) systems:



• A Programmable Logic Controller (PLC) is an input/output processing computer.



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• Advantages of PLCs are:



- cost effective for complex systems

- flexible (easy to add new timers/counters, etc)

- computational abilities

- trouble shooting aids

- reliable

- easy to add new components

• Ladder logic was originally introduced to mimic relay logic.



7.1 BASIC LADDER LOGIC



• The PLC can be programmed like other computers using specialized “languages.”



- Ladder Logic - a programming technique using a ladder-like structure. It

was originally adopted because of its similarity to relay logic diagrams

to ease its acceptance in manufacturing facilities. The ladder approach

is somewhat limited by the lack of loops, etc. (although this is changing).



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OUTPUTS



HOT



NEUTRAL

INPUTS



POWER NEEDS TO FLOW THROUGH

THE INPUTS TO THE OUTPUTS



- Mnemonic - instructions and opcodes, similar to assembly language. It is

more involved to program, but also more flexible than ladder logic.

This will be used with the hand held programmers.



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e.g. for an Omron PLC



00000

00001

00002

00003

00004

00005

00006



LDI

AND

LD

AND

ORB

OUT

END



A

B

C

D



the mnemonic code is equivalent to

the ladder logic below



E



A



B



C



E



D



END



• There are other methods that are not as common,



- sequential function charts/petri nets

- state space diagrams

- etc.



7.2 WHAT DOES LADDER LOGIC DO?



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7.2.1 Connecting A PLC To A Process



PROCESS



Connections to

Actuators



Feedback from

sensors/switches

PLC



• The PLC continuously scans the inputs and changes the outputs.



• The process can be anything - a large press, a car, a security door, a blast furnace, etc.



• As inputs change (e.g. a start button), the outputs will be changed. This will cause the process to change and new inputs to the PLC will be received.



PLC program changes outputs

by examining inputs



Set new outputs



THE

CONTROL

LOOP

read inputs



process changes and PLC pauses

while it checks its own operation



7.2.2 PLC Operation

• Remember: The PLC is a computer. Computers have basic components, as shown below:



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Keyboard

Input



SVGA Screen

Output

80586

CPU



Serial

Mouse

Input



133/200 MHz

Light output



2.1GB Disk

Storage



32MB Memory

Storage



• In fact the computer above looks more like the one below:



inputs



Keyboard



input memory



output memory



computer



Input Chip

CPU ‘586



Mouse



outputs



Screen memory

chips



monitor



Serial Input Chip

digital output chip



LED display



Flow of Information

Disk Controller



Memory Chips



Disk



storage



• Notice that in this computer, outputs aren’t connected to the CPU directly.



• A PLC will scan a copy of all inputs into memory. After this, the ladder logic program is run

once and it creates a temporary table of all outputs in memory. This table is then written to the



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outputs after the ladder logic program is done. This continues indefinitely while the PLC is running.



• PLC operation can be shown with a time-line -



Self input logic output

test scan solve scan



0



Self input logic output

test scan solve scan



Self input logic

test scan solve



ranges from 1 to 100 ms



time



PLC turns on



SELF TEST - Checks to see if all cards error free, resets watch-dog timer,

etc. (A watchdog timer will cause an error, and shut down the PLC if

not reset within a short period of time - this would indicate that the ladder logic is not being scanned normally).

INPUT SCAN - Reads input values from the chips in the input cards and copies their values to memory. This makes the PLC operation faster and

avoids cases where an input changes from the start to the end of the

program (e.g., an emergency stop). There are special PLC functions

that read the inputs directly and avoid the input tables.

LOGIC SOLVE/SCAN - Based on the input table in memory, the program is

executed one step at a time, and outputs are updated. This is the focus

of the later sections.

OUTPUT SCAN - The output table is copied from memory to the output

chips. These chips then drive the output devices.



7.3 LADDER LOGIC

• Ladder logic has been developed to mimic relay logic - to make the computer more acceptable to companies and employees.



• Original efforts resisted the use of computers because they required new skills and

approaches, but the use of ladder logic allowed a much smaller paradigm shift.