process control with twido plc lab guide

8/10/2019 Process Control With Twido Plc Lab Guide

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Process Control using TWIDO PLCLaboratory Guide

Embedded and Ambient Systems Laboratory

BMEVIIIA350

Author:KOVCSGbor

[email protected]

Budapest University of Technology and Economics

Department of Control Engineering and Information Technology

2014
mailto:[email protected]:[email protected]:[email protected]


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Process Control using TWIDO PLC Contents

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Contents

1 TWIDO controllers ........................................................................................................................... 3

2 Basics of ladder diagram programming........................................................................................... 4

2.1 Basic logic elements ................................................................................................................ 4

2.2 Operandslanguage objects .................................................................................................. 5

2.3 Basic function blocks ............................................................................................................... 6

2.4 State machines ........................................................................................................................ 9

3 Measurement tasks ....................................................................................................................... 12

3.1 Manufacturing line control .................................................................................................... 12

3.2 Control of a 3D manipulator .................................................................................................. 13

3.3 Control of a chemical process model .................................................................................... 15

3.4 Control of a signalled pedestrian crossing ............................................................................ 16

4 Appendix: Laboratory report skeleton .......................................................................................... 18


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Process Control using TWIDO PLC TWIDO controllers

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1 TWIDO controllersTWIDO is the PLC family of Schneider Electric designed for low complexity applications, containing

both modular and compact devices. During the measurement, different models of compact

controllers will be used.

Being compact devices, PLCs used during the measurement enclose the power supply unit, the CPU

and the IOs in the same housing. Parts of a compact TWIDO PLC are shown byFigure 1.

1 Covers of input and output connectors

2 Hinge door

3 RS485 connector (programming

interface)

4 Place of operator display

5 Input connectors

6 Connector for extension modules

(located at the right side)

7 Input, output and status LEDs

8 Output connectors

9 Setpoint potentiometers

10 Place of second serial port

11 Power connection

12 RTC / EEPROM expansion slot

Figure 1Parts of a compact TWIDO PLC

Depending on the model, PLCs might have 24V DC or 230V AC power inputs, from which the power

supply unit generates lower voltage supply for the CPU and other modules.

The CPU is responsible for operating the PLC, running the operating system and the user program. It

is equipped with 32kBytes of RAM, of which the program memory has the capacity of 3000

instructions. The RAM includes 256 bit registers and (depending on the use of function blocks) at

most 3000 word registers. The CPU allows simultaneous use of 128 counters and 128 timers. The

RAM is battery-powered to store its contents during power outage, but an external 32kByte EEPROM

cartridge can also be used. The controller can also be equipped with a real-time clock (RTC) module.

Depending on the model, compact TWIDO PLCs might have up to 24 digital IOs with 0-24V voltage

levels, decoupled from the CPU. Analogue setpoint potentiometers are also available in the base

unit. PLCs in the lab have relay outputs, but models with solid-state digital outputs are also available

both in sink and source configurations. States of inputs and outputs are displayed on the front panel

LEDs. Further inputs and outputs (both digital and alalog ones) can be connected to the base unit in

forms of expansion modules.

Every TWIDO PLC is equipped with an RS485 communication interface, serving both as programming

interface and a MODBUS communication port for connection to other controllers. Othercommunication interfaces (ASI, CAN or Ethernet) are available as expansion modules.


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Process Control using TWIDO PLC Basics of ladder diagram programming

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2 Basics of ladder diagram programmingAmong the PLC programming languages defined by the standard IEC-1131 (and later IEC-61131), the

ladder diagram is the most popular one used in simple applications. In the followings, the basic

concepts of ladder programming will be presented using the syntax of its TWIDO implementation.

2.1

Basic logic elements

As initially designed to mimic relay logics, ladder diagram consist of rungs, each implementing a

logical function. In each scan cycle, the PLC reads the inputs, evaluates the logic functions from the

top to the bottom, and updates the outputs. Basic logic elements of ladder diagrams are shown by

Table 1.

Contacts Coils

Normally open (NO) contact Normally opern (NO) coil

Normally closed (NC) contact Normally closed (NC) coil

Rising edge sensitive contact Set (latch) coil

Falling edge sensitive contact Reset (unlatch) coil

Table 1Basic logic elements

Inputs of a logical function correspond to the contacts, while its outputs correspond to the coils. The

current flow starts from the left side of the rung, and directed towards the right through the contacts

and then the coils. The normally open (NO) contact can be interpreted as a simple pushbutton: it

conducts if the value of the associated variable (written over the contact in the diagram) is true (1).

The normally closed (NC) contact works the opposite way: it conducts if the corresponding variable is

false (value 0), so it can be considered as a normally closed pushbutton. Serial interconnection of

normally open contacts (resp. normally closed contacts) realize an AND (resp. NAND) operation,

while their parallel interconnection implements an OR (resp. NOR) operation.

Normally open (NO) coils set the corresponding variable to 1 if energized, and set it to 0 otherwise.

Similarly, normally close (NC, negated) coils set the corresponding variable to 1 if not energized.

Figure 1Implementation of a logical function

The ladder diagram above implements the logical function Y=NOT(A OR (B AND NOT C)). The top

branch on the left conducts if Ais true, while the bottom one conducts if Bis true and Cis false (B

AND NOT C). The coil on the right is energized if either of the branches on the left conduct, i.e. if the

condition A OR (B AND NOT C)is true. However, since the coil is a normally closed one, the variableCis set to the value NOT(A OR (B AND NOT C)).


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It is important to emphasise that the operation of the rung implements a logical function, which sets

the output no matter its condition is satisfied or not. Beginners often make a mistake by associating

the above ladder rung with the instruction IF (A OR (B AND NOT C)) THEN (Y=0)of high-

level programming languages. However, this is not correct: the instruction above does not change

the value of Y if the condition is not satisfied. On the other hand, the logical function implemented by

the ladder rung sets the output to 1 in that case. Therefore, the logical function should be associated

with the instruction IF (A OR (B AND NOT C)) THEN (Y=0) ELSE (Y=1).

Variables associated to the coils are set to the value corresponding to the part of the rung left from

them in each scan cycle. If we would like to add memory-like behaviour to a variable, Set and Reset

coils should be used. The Set coil sets the corresponding variable to 1 if energized, and does not

change its value if not energized. Clearing the variable can be achieved by a Reset coil in a similar

way.

Due to the cyclic behaviour of PLCs, the user program can not directly set the output bits. Instead, it

sets the bits of the output map stored in the memory, which are copied to the physical outputs after

the end of the program scan. Therefore, if an output bit is assigned by coils in multiple rungs, the last

assignment will overwrite the results of the formers in the output map, so therefore those

assignments do not appear at the physical output. It is strongly recommended to write each output

only in one single rung of the diagram.

2.2

Operands language objects

Operands used by different operations are referred to as language objects by the TwidoSuite

development environment. These objects can be bit, word, double word or (in case of some PLC

models) floating point language objects. During the measurement only bit and word objects will be

used. Language objects, except to immediate values, are referenced by their identifier. The identifier

starts with the %character, followed by the object type (1 or 2 characters) and the number of the

object. In case of function blocks, the identifier is extended by the identifier of the function block

variable the objects represents.

Bit object include digital inputs and outputs, memory bits and function block outputs. Inputs, outputs

and memory bit objects are identified by the characters I, Q and M, respectively. Addresses of inputs

and outputs are composed of two numbers delimited by a dot (.). The first number corresponds to

the module the given port is located at (the base unit, used during the measurement, has the

number 0), while the second is the number of the given input or output inside the module. In case of

memory bits, the address is one single number from 0 to 255.

Word objects include analog inputs and outputs (IW and QW), memory words (MW) and constants

(KW), and some function block variables. Referencing input, output and memory word objects is the

same as referencing their bit object counterparts.

Variables of function blocks (see next section) can be referenced by adding a dot (.) and the variable

identifier to the identifier of the function block object (like when referencing a property of an object).

Language objects used during the measurement are shown byTable 2.


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Language

objectDescription Example

%Ix.y Digital input y of module x %I0.4Digital input 4 of the base unit

%Qx.y Digital output y of module x %Q0.0Digital output 0 of the base unit

%Mi Memory bit number i %M4Memory bit number 4

%MWi Memory word number i %MW3Memory word number 3

%KWi Word constant number i %KW3Word constant number 3

%Ci Counter number i %C2.VValue of the counter number 2

%TMi Timer number i %TM7.QOutput of the counter number 7

Table 2Language objects

2.3

Basic function blocks

Beside simple logic elements, more sophisticated blocks, the so-called function blocks can also be

used in ladder diagram. The most important timer, counter, compare and operation function blocks

of TWIDO will be presented in the sequel.

Timers

TWIDO compact PLCs allow the simultaneous use of 128 timer blocks (%TM0%TM127) which might

be of different types (TON, TON, TP). Output Qof the timer block is a delayed copy of the input IN

and its actual behaviour depends on the type of the counter. The time delay can be set by the time

base (base) and preset parameters. The time base can be 1ms, 10ms, 100ms or 1 min (pay attention

that 1 minute is the default setting), and the delay can be expressed as the multiple of the time base

defined by the preset value (e.g. using 10ms time base and a preset value of 30 will result in a delay

of 300ms): .

Figure 2The Timer function block

The output of the timer (%TMi.Q, read-only), the preset value (%TMi.P) and the current value of

the counter (%TMi.V, read-only) can be addressed from the user program.


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Figure 6The Counter function block

The value 1 of the R(Reset) input sets the current value of the counter to 0, while the value 1 of the

S(Set) input sets it to the predefined preset value. Rising edges on the CUand CDinputs increment

and decrement the counter value, respectively.

The current value of the counter can be in the range of 0 to 9999. Overflow (9999 0)is signalled by

the value 1 of the output F(Full), while underflow (0 9999)is signalled by the output E(Empty).

The value of the output D (Done) is set to 1 if the current value equals the preset value. These

outputs can be referenced as language objects, e.g. as %Ci.D.

However the current value and preset value of the counter do not appear as outputs (as being not

logical values), they can be accessed through compare and operation function blocks using the

language objects %Ci.V and %Ci.P, respectively. Note that the current value of the counter is

read-only.

The Compare function block

Non-logical operators (e.g. memory words or counter values) can be compared to each other using a

compare function block.

Figure 7The Compare function block

The block is evaluated only if energized, i.e. the condition to its left evaluates to true. The

comparison operation can be = or while the operators can be word or double

word objects, including immediate values (seeFigure 7).

The Operate function block

Non-logical assignments and arithmetic operations can be carried out using an Operate function

block. The block is evaluated only if energized, i.e. the condition to its left evaluates to true: in that

case, the operation defined above the block is executed.

Assignment is represented by the operator :=. The left side of the operator can be any non-read-

only language object, while any language object or immediate value can be present at the right side.


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Right side might also include an arithmetic formula using addition (+), subtraction (-), multiplication

(*), division (/), remainder (REM), square root (SQRT), increment (INC), decrement (DEC) or

absolute value (ABS) operations, depending on the data types of the operands.

Figure 8The Operate function block

2.4 State machines

Operation of controlled processes can often be described by finite state machines, which are then

implemented by a ladder diagram. Generally Moore-model automata are used, i.e. the outputs

depend only on the current state.

As an illustrative example, consider the control of a simple heating device: the equipment can be

turned on by pressing a pushbutton. In that case, only one of the two heaters is turned on (LOW

mode). Upon a second press of the button, the other heater is also turned on (HIGH mode). The third

press of the button turns off the equipment. The state transition diagram of the process is illustrated

byFigure 9.

Figure 9State transition diagram of the heating device

Implementation of state machines can be carried out in three simple steps:

1.

definition of the transitions

2. definition of the output mapping

3. initialization of the state machine

At first, a memory bit (the state register) should be assigned to each state. The value of a state

register is 1 if and only if the corresponding state of the state machine is active, otherwise it is 0. In

the sequel the registers assigned to the three states of the heating device will be referred to as

S_OFF, S_LOWand S_HIGH.

Implementation of a state machine means the definition of the mapping which gives the next state

as the function of the current state and the conditions of the transitions. Therefore, in order to

execute a transition, it has to be evaluated whether the initial state of the transition is the current

state and whether the condition of the transition is met. If both evaluate to true, the current state

has to be replaced to the destination state of the given transition, i.e. the state register bit of the


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current state has to be set to 0 and the state register bit of the destination state has to be set to 1.

The rung for implementing a generalized transition is shown byFigure 10.

Figure 10General template for implementing state transitions

Here and are the state registers associated to the origin

and destination state of the transition, respectively. The condition of the transition is modelled here

by a simple contact, but in general, any logical function can be plugged in there.

The ladder diagram describing the state transitions of the heating device is given byFigure 11.

Figure 11Ladder diagram implementing the state transitions of the heating device

When implementing the output mapping, the rule of writing an output only in one single rung has to

be respected. In case of state machines with multiple states, it is useful to make a table containing

the state of each output in the different states. Let us denote the outputs of the heating device by

the symbolic names HEATER1 and HEATER2 (corresponding the on/off state of the two heaters).

Then the table of the output mapping is given byTable 3Output mapping of the .


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State S_OFF S_LOW S_HIGH

HEATER1 0 1 1

HEATER2 0 0 1Table 3Output mapping of the heating device

Based on the table, the output mapping can easily be implemented as illustrated byFigure 12.

Figure 12Ladder diagram implementing the output mapping of the heating device

However the implementation of the transitions can assure that no more than one state can be active

simultaneously, each state register bit is set to its default value 0 upon starting the PLC, i.e. there will

be no active state. In order to start the state machine from its initial state, the corresponding state

register bit has to be set to 1.

One solution, which is platform-independent, i.e. can be used in any environment, is to set the state

register of the initial state to 1 if all state registers are of value 0 (seeFigure 13).

Figure 13General solution to initialize the state machine

A more sophisticated (and, especially in case of a high number of states, more convenient) solution is

to use the current state of the PLC as condition for setting the initial state of the state machine. PLCs

usually use a system bit or system word to notify the user program that the current scan cycle is the

first one after startup, so this information can be used to initialize the state machine. TWIDO PLCs set

the system bit %S13 to 1 during the first scan, so the initialization of the state machine can be

implemented as shown byFigure 14.

Figure 14Initialization of the state machine on a TWIDO PLC


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Process Control using TWIDO PLC Measurement tasks

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3 Measurement tasksOne of the following tasks will be assigned for each group. The measurement tasks model such

control problems, which are solved using PLCs in the industrial practice.

3.1 Manufacturing line control

The manufacturing line model consists of four conveyors, two pushers connecting the perpendicular

conveyors, a milling machine and a drilling machine (seeFigure 15).

Figure 15Layout of the manufacturing cell

Photo sensors are installed along the conveyors. Parts arriving in front of the sensor break the light

beam and therefore their presence is signalled by the logical value 0 of the corresponding photo

sensor. Conveyor motors and machines can be operated by setting the corresponding PLC output to1. In case of pushers, the direction of motors can be set by an other PLC output (1: forward, 0:

backward). Limit switches are placed at the forward and backward extremities of the pushers, which

signal if pressed by the device.

The control of the manufacturing line should achieve the following operation. When a part is put

onto Conveyor 1 (in front of Photo sensor 1), the conveyor should be started in order to forward the

part to Pusher 1. Passing the part to the pusher can be achieved by operating the controller for 2

seconds after the part has passed Photo sensor 2. The pusher should push the part to Conveyor 2 (for

smooth operation, Conveyor 2 should be operating during the pushing operation), and then retract

to its back position.

As the part arrives to the milling machine, Conveyor 2 should be stopped, and the machine should be

operated for 3 seconds. Afterwards, the conveyor should forward the part towards the drilling

machine: it should be operated for approximately 4 seconds after the part left the milling machine,

and Conveyor 3 should be turned on in the meantime.

As the part arrives to the drilling machine, Conveyor 3 should be stopped and the machine should be

operated for 5 seconds. Afterwards, the conveyor should forward the part to Pusher 2 (again, 4

seconds of conveyor operation is needed after the part leaves the machine).


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Pusher 2 should forward the part to Conveyor 4 (which should be started during the extension of the

pusher for smooth operation) and then retract to its back position. Conveyor 4 should be operated

for additional 2 seconds after the part has passed by Photo sensor 5, and then stopped.

Measurement task

Implement a PLC program which achieves the operation described above. You can assume that onlyone part is present in the whole line.

Additional task

Modify the program to deal with multiple parts present in the line simultaneously.

Hints

Implement distinct state machines for the elements of the line (input conveyor, output

conveyor, machines, pushers) and operate them simultaneously. Take the time to design the

state machines on a piece of paper before implementing them.

Take care not to move pushers to incorrect directions upon the activation of the limitswitches.

If you experience any problem (e.g. pusher off the rail) stop the line using the emergency

shutdown button and ask for assistance.

3.2

Control of a 3D manipulator

The task is to relocate parts from an output conveyor to an input conveyor using a 3D manipulator.

The manipulator is a 3-degrees-of-freedom arm, equipped with a mechanical gripper, as shown by

Figure 16.

Figure 16The 3D manipulator

Joint 1 is a rotational one, which rotates the whole arm around its vertical axis (the positive direction

is the clockwise one). Joint 2 is a translational one, which allows lowering (positive direction) or lifting

(negative direction) the horizontal segment of the arm. Joint 3 allows extending (positive direction)

or retracting (negative direction) the horizontal segment. The gripper can be opened by positive

motion of its motor and closed by negative motion. There are two PLC outputs corresponding to each


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joint and the gripper. The first turns the motor on or off, while the second sets the direction of

movement (0: negative direction, 1: positive direction).

Reference positions of the joints and the gripper (a given angular position for Joint 1, top position for

Joint 2, fully retracted position for Joint 3, closed position for the gripper) are sensed by limit

switches, providing absolute position information. Joints are also equipped with pulse switches,which output a square signal when the given joint is moved. By summing these pulses with respect to

the sign of the direction of movement (i.e. incrementing the counter in case of positive direction and

decrementing it in case of negative direction), the relative position of the joints from their reference

position can be obtained. Positions of the manipulator are given using these values (the so-called

joint coordinates), e.g. the coordinate (30,0,20) means that Joint 1 has turned 30 pulses from its

reference position to the positive direction, Joint 2 is in its reference (top) position and Joint 3 has

been extended by 20 pulses.

The manipulator should be controlled as follows. After the start of the controller, the position of the

arm is unknown, so it has to be calibrated, i.e. all joints have to be moved to their reference positionsindicated by the limit switches. At first the gripper has to be moved to negative direction until the

limit switch signals, then Joint 2 (lifting), Joint 3 (retract) and finally Joint 1 (turning to counter-

clockwise direction) has to be moved to negative direction.

After calibration, the manipulator should be moved to Position 1 over the output conveyor and the

gripper should be opened. If a part arrives at the output conveyor (the value of the photo sensor

input becomes 0), the arm should be lowered to Position 2, and the part should be picked up by

closing the gripper. Then the arm should be fully lifted (Position 1 again), and moved to Position 3

over the input conveyor. Then it should be lowered to Position 4 and the gripper should be opened.

After lifting the arm back to Position 3, it should be recalibrated by moving it to its reference

position. After the calibration, it should be moved to Position 1 and the controller should wait for the

next part to pick up.

Measurement task

Implement a PLC program which operates the manipulator as described above.

Hints

Implement a state machine, which controls the successive movements defined above. Take

the time to design the state machine on a piece of paper before implementing it.

Use counters to count the pulses of each joint and the gripper (take care of the direction ofthe movement). Use the appropriate limit switches to reset these counters.

When the goal of a movement is the reference (0) position of a joint, use the limit switch for

sensing the goal position instead of the less reliable counters.

Joints 1 and 3 should be moved only if the limit switch of Joint 2 is active (the vertical

segment is fully lifted).

The manipulator might harm itself and parts of the manufacturing cell. If you experience a

dangerous situation, stop the movement immediately by the emergency shutdown button

and ask for assistance.


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3.3 Control of a chemical process model

The model consists of two tanks, two pumps, a mixer, a level sensor, two pushbuttons and a status

light (green lights signalling the operation of the pumps and the mixer are in serial connections with

the corresponding motors). Pump 1 pumps the liquid from Tank 1 to Tank 2 (storing, betrol in

Hungarian), while Pump 2 operates in the reverse direction (discharging, kitrol in Hungarian).

Level of Tank 2 is measured by a floating level sensor, which triggers a digital switch if the level

reaches the high level, and another if the level reaches the low level. The layout of the model is

shown byFigure 17.

Figure 17Layout of the chemical process model

In discontinuous operating mode, the operation is started by pressing the START button. Pump 1

should fill Tank 2 (the pump should be operated until the high level sensor becomes active), and then

the liquid should be mixed for 5 seconds. Then the liquid should be pumped back to Tank 1 (the

pump should be operated until the low level sensor becomes active), and the process should be

stopped. During the whole procedure, the lamp should be lit.

In continuous operating mode, the process should not be stopped after discharging, but a new cycle

should be started (i.e. the liquid should be pumped again to Tank 2 etc.). In this operating mode, the

process can be stopped by pressing the STOP button. Upon pressing the button, the operation should

not terminate immediately, but after discharging (when Tank 2 becomes empty). In order to notify

the operator that the request for stop has been processed, the lamp should blink during the

termination (otherwise it should be lit continuously).

Measurement tasks

1. Implement a PLC program which implements the discontinuous operating mode.

2.

Modify the program such that it realizes the continuous operating mode.


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Additional task

Modify the program such that it allows emergency shutdown. If the STOP button is pressed in the

termination phase (i.e. it is pressed at a second time), switch off all motors regardless the current

operation. Take care to pump all liquid from Tank 2 to Tank 1 before starting the normal operation

after an emergency shutdown.

Hints

It is advised to solve the problem using a state machine. Take the time to design the state

machine on a piece of paper before implementing it.

Use system bit %S6to implement blinking of the light. The system bit outputs a 0-1 sequence

with 1Hz frequency.

3.4

Control of a signalled pedestrian crossing

On roads with heavy vehicle traffic but rare needs of pedestrians to cross the road, pedestrian

request-based signalling is commonly used. Pedestrians arriving the side of the road can request a

green signal by pressing a button, which is granted for them after a while. You can find such a

pedestrian crossing at the tram station of Goldmann Gyrgy tr.

Figure 18Layout of the signalled pedestrian crossing

The controller should operate the traffic lights the following way. Initially, the lights show green for

the vehicles and red for the pedestrians. If a crossing request is signalled by pushing the button, the

WAIT (VRJON) light should be lit to notify the pedestrian about processing the request. After 2seconds, the light for the vehicles should be turned to yellow, then, after another 2 seconds, to red.

After 1 second, the pedestrian light should be turned to green for 5 seconds and the WAIT light

should be turned off. Then, the green light for the pedestrians should start blinking for 5 seconds,

and then turned to red. The same time the light for the vehicles should show a red-yellow signal, and

after 2 seconds, turned to green.

Measurement task

Implement a PLC program which can control the traffic lights accordingly to the description above.


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Additional task

Implement the feature of night mode.By switching the mode switch to night mode, the yellow

lights for the vehicles should start blinking while the lights for the passengers should be turned off.

Take care of returning to a safe state (red for all directions) when switching back to daylight mode.

Hints Lights and buttons of the two sides are connected to common IOs.

It is advised to solve the problem using a state machine. Take the time to design the state

machine on a piece of paper before implementing it.

Use system bit %S6to implement blinking of lights. The system bit outputs a 0-1 sequence

with 1Hz frequency.


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Process Control using TWIDO PLC Appendix: Laboratory report skeleton

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4 Appendix: Laboratory report skeleton

Laboratory Report

Embedded and Ambient Systems Laboratory (BMEVIMIA350)

Process Control with TWIDO PLC

Date: ..

Group:

Students: ()

()

The Laboratory Report is to be sent by email with the project documentation attached to the address

[email protected] within 7 days from the measurement session. Reports sent late are not

accepted.
mailto:[email protected]:[email protected]:[email protected]


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Process Control using TWIDO PLC Appendix: Laboratory report skeleton

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Measurement task

Outline of the solution

Project documentation

Project documentation generated by TwidoSuite is attached in file .

< Attention! Only documentation generated from a well-commented project (i.e. descriptivesymbolic names, comments attached to each rung) is acceptable! >

process control with twido plc lab guide

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