Industrial Automation course -...
Transcript of Industrial Automation course -...
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 1
Industrial Automation course
Lesson 5
PLC - SFC
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 2
History
Before the 60s the SEQUENTIAL CONTROL was seen as EXTENSION OF THE CONTINUOUS CONTROL or in the best cases THE DIGITAL.
Since the '60s the theory on finite state automata is developed, whose formal models allow a deep MATHEMATICAL ANALYSIS, but hardly useful for DESIGN of the algorithms.
In the 70's the design of automation systems is quite elementary, is based on CIRCUITAL REPRESENTATIONS or TEXTUAL DESCRIPTIONS.
…
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 3
History
…
In the 1975, in France, a commission for the FORMALIZATION of a DESCRIPTIVE PROJECTUAL instrument oriented to the SEQUENTIAL CONTROL is instituted. GRAFCET is born.
In the 80s GRAFCET is defined as SEQUENTIAL FUNCTIONAL CHART in the IEC (International Electrotechnical Commission) 848.
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 4
Base elements
State (phase)
Definition
The STATE is a defined OPERATIVE CONDITION of a part of the complex system.
Graphical representation
NState number … or better a univocal Name
N
Initial state with the double border
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Base elements
State (phase)
Remarks
The STATE is an INVARIANT condition of the system under exam, it can change or being modified only as a result of an EVENT.
During the dynamic evolution of the complex system, each STATE of each part of the system can be only in two possible conditions: ACTIVE or INACTIVE.
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Base elements
State (phase)
Syntax
The PLC defines, for each state, two variables:
•MARKER, the Boolean variable that contain the condition of the state (it sais if it is active or not)
• TIMER, the internal variable that represents the elapsed time since the activation (if the state is active)
Usually
•MARKER variable: StateName.X
• TIMER variable: StateName.T
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Base elements
Action
Definition
All the OPERATIONS executed from the system under exam, when it is in a defined OPERATIVE CONDITION (STATE), are called ACTIONS.
Graphical representation
N Action 1
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Base elements
Action
Remarks
Each STATE of the system in made of a defined set of ACTIONS.
From the CONTROL SYSTEM field of view, considering that it is used for the execution of the LOGIC-SEQUENTIAL SOFTWARE, an ACTION corresponds to one or more PROCEDURES, that are a SET of INSTRUCTIONS, executed in series, until the STATE is ACTIVE (until an EVENT changes the STATE).
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Base elements
Action
Syntax
The PLC defines, for each action, three variables:
•𝐴𝑚, the univocal id of the action
•𝑄𝑚, that defines the action type (qualificator)
•𝑉𝑚, a Boolean variable that represents the state of the action
N.B.: In a lot of programming environments, the actions (and also the transitions) can be defined with one of the IEC 61131 programming languages
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Base elements
Action
Syntax
Qualificator types
•N: Normal non stored
If the action 𝐴𝑛 is of type N it will be executed each PLC cycle if the state connected to it is active
•P: Pulse
If the action 𝐴𝑛 is of type P it will be executed only the first PLC cycle in which the state connected to it is active
N.B.: Sometimes it is possible to find the qualificator I (Impulise)
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Base elements
Azione
Syntax
Qualificator types
• S: Set
If the action 𝐴𝑛 is of type S it will be executed each PLC cycleuntil the same 𝐴𝑛 action (in a subsequent state) will have the R qualificator
•R: Reset
If the action 𝐴𝑛 is of type R (as already described) stops the execution of a previous action enabled with a S qualificator
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Base elements
Azione
Syntax
Qualificator types
• L: Limited Time
If the action 𝐴𝑛 is of type L it will be each PLC cycle for t#Ts seconds, starting from the state activation
N.B.: if the state is disabled, the action is terminated
•D: Time Delayed
If the action 𝐴𝑛 is of type D it will be executed each PLC cycle after t#Ts seconds from the state activation
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Base elements
Azione
Syntax
Qualificator types
• SD: Stored / time Delayed
If the action 𝐴𝑛 is of type SD it is equal to a S action, with a delay of t#Ts seconds
• SL: Stored / time Limited
If the action 𝐴𝑛 is of type SL it is equal to a S action, disabled after t#Ts seconds (independently from the active state)
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Base elements
Transition
Definition(s)
The PASSAGE from a PREVIOUS STATE to a SUBSEQUENT one, because of an event, is called TRANSITION.
The LOGICAL check, that determines the happening of an EVENT is called CONDITION.
Remarks
Each CONDITION is associated to a TRANSITION.
Each CONDITION is defined as a LOGICAL FUNCTION, through a BOOLEAN EXPRESSION that can be TRUE or FALSE.
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Base elements
Transition
Graphical representation
N
N+1
Transition
Condition
Action 1
Action 2
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Evolution rules
Very important
Rule 1
A transition is defined as ENABLED if ALL the phases above of it are active.
A transition is defined as SUPERABLE if it is enabled and the condition related to it has the value «true».
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Evolution rules
Rule 2
When a transition is superable it is overcome: all the phases above the transition are disabled, while those below it are enabled
It is very easy: when the condition becomes true all the below phases are enabled and the above are disabled
N.B.: the correct syntax is always state - transition - state -transition - …
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Evolution rules
Example
2
3
False
11 2
True
3
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Base structures
Defined the previously defined rules, the base structures that can be created are the following:
If – then – else
1
2 3
4
A B
DC Convergence
Choice
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Base structures
Remarks
The A and B conditions must be
mutually exclusive.
If they were not it would risk having the
phase 4 activated «two times».
N.B.: As it will be showed in the last part of the course, this is a difference between the SFC and the Petri nets.
1
2 3
4
A B
DC
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Base structures
Parallel
1
2 3
4
A
BSynchronization
Parallelism
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Base structures
Remarks
The phases 2 and 3 are activated
simultaneously.
In the case of presence, in one of the
branches, of more than one phase, the
Check of the condition B is executed when the
Last phase of each branch is activated.
1
2 3
4
A
B
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SFC in a real case
In a lot of development environments (also in the case of the B&R Automation Studio) the SFC is integrated with the other IEC 61131 languages.
It means tat, in each phase (called «state»), it is possible to associate the execution of a portion of code written in one of the 1131 languages.
For this reason, not all the action types described before are used inside the tool.
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Example
Base example
What happens if A and B are both true?
The PLC continues to commutate cyclically between the state 1 and the 2
A
state2B
state1
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Example
Action and if – then – else example
Develop a software that allow, using the activation of an actuator for 1 second, to discard products with a weight lower to a defined threshold
Inputs
PC: Photocell
Weight: weight of the product
Outputs
Actuator: activation of the actuator
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Example
N.B.: With Automation Studio, if we want to use the IEC norm, in order to create actions, it is necessary to click on the button «Use IEC Steps» as depicted in the picture.
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Example
Action and if – then – else example
PC
Product Arrived
Weight<Threshold
Waiting
Discard
Discard.T>=t#1s
Weight>=Threshold
S Actuator
R Actuator
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Example
Temporally limited states example
Develop a software that activates the electro-valve U if the overfull sensor OFS remains active for more than 10 s and keeps it activated for 30 s
Inputs
OFS: Overfull sensor
Outputs
U: Electro-valve activation
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Example
Temporally limited states example
OFS
Sensor Active
SensorActive.t>=t#10s
Sensor Check
ValveActive
ValveActive.T>t#30s
NOT(OFS)
N U
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Example
Other languages actions example
Develop a software that advise for the passage of the tenth customer, using a photocell on the entrance of the shop
Inputs
PC: photocell
Outputs
Tenth: reach of the tenth customer
N.B.: usually, during the exercises, where it is required to implement a SFC software, if not specified, this type of solution is denied
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Example
Other languages actions example
PC
Customer Arrived
N>=10
Customer Wait
TenthReached
N<10
FALSE
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Example
Other languages actions example
N>=10
FALSE
EPC := FALSE;N := N + 1;
We can add actions on the Entrance (E), Cyclically (C) and on the eXit (X), in this case we use the structured text
PC
Customer Arrived
Customer Wait
TenthReached
N<10
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Example
Other languages actions example
N>=10
FALSE
ETenth := TRUE;
PC
Customer Arrived
Customer Wait
TenthReached
N<10
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From SFC to Ladder
It is possible to translate a software implemented in SFC, in a Ladder software, using simple rules, independent from the program itself.
In the course we will focus on the algorithm without stability search.
N.B.: These are standard rules, applicable to every SFC program!
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From SFC to Ladder
Consider this SFC evolution algorithm
Input reading
Phases initialization
Impulsive actions execution
Inputs reading
Definition of the superable transitions
Definition of the new condition
Outputs update
This algorithm is called withoutstability search.
In the case of non-stable phase (phase with, on the activation, the exiting transition set to true), this scheme «makes a mistake»:
• If the action is continuous must not be activated
• If it is impulsive, it must be
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From SFC to Ladder
Just for information, this is the algorithm with stability search
Inputs reading
Phases initialization
Impulsive actions execution
Inputs reading
Definition of the superable transitions
Definition of the new condition
Conditions isn’t changed
Impulsive actions execution
Outputs update N.B.: we will use the other one
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From SFC to Ladder
The Ladder must be defined with four sections:
• Initialization
Executed only during the first step of the software
•Actions execution
Enabling of the actions connected to the active phase
•Transitions evaluation
Enabling of all the active transitions
•Conditions update
Check of the passage between the active phases and the following
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 38
From SFC to Ladder
Initialization
JMP
𝐶𝑦𝑐𝑙𝑖𝑐
𝐶𝑦𝑐𝑙𝑖𝑐
LBL
.
.
.
𝐹𝑃
L
.
.
.
𝑇𝑟𝑢𝑒
U
𝑇𝑟𝑢𝑒
N.B.: in AS, this part can be directly implemented in the _INIT!
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 39
From SFC to Ladder
Actions execution
• For each continuous action a rung is used, connected in OR, with all the other that activate the same action
• For the stored activities it is possible to use SET – RESET coils
• For the impulsive actions it is possible to create the network for the generation of the impulse
𝐴𝑐𝑡𝑖𝑜𝑛𝐴𝑃ℎ𝑎𝑠𝑒𝐴
𝑃ℎ𝑎𝑠𝑒𝐵 𝐴𝑐𝑡𝑖𝑜𝑛𝐵
𝐴𝑐𝑡𝑖𝑜𝑛𝐶
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 40
From SFC to Ladder
Conditions evaluation
• For each phase, the outgoing conditions that enable the transition are verified
• In the case of parallel transitions it is necessary to use the AND structure with the Boolean signals of the phases
𝑇𝑟𝑎𝑛𝑠𝐴𝑃ℎ𝑎𝑠𝑒𝐴 𝐶𝑜𝑛𝑑𝐴
𝑇𝑟𝑎𝑛𝑠𝐵𝑃ℎ𝑎𝑠𝑒𝐶𝑃ℎ𝑎𝑠𝑒𝐵 𝐶𝑜𝑛𝑑𝐷
𝑇𝑟𝑎𝑛𝑠𝐵
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Politecnico di Milano – Universidad de Monterrey, July 2015, A. L. Cologni 41
From SFC to Ladder
Condition update
When a transition is active, all the above phases are disabled, while the below are enabled
𝑃ℎ𝑎𝑠𝑒𝐴𝑇𝑟𝑎𝑛𝑠𝐴
𝑇𝑟𝑎𝑛𝑠𝐵 𝑃ℎ𝑎𝑠𝑒𝐶
𝑃ℎ𝑎𝑠𝑒𝐷
𝑃ℎ𝑎𝑠𝑒𝐵
U
L
U
L