University of Michigantilbury/logiccontrol/talks/... · 2006-10-25 · Acknowledgments • Joint work with Euisu Park and Pramod Khargonekar of University of Michigan • Funding

Modular Logic Controllers forMachining Systems:

Formal Representation andAnalysis using Petri Nets

Dawn TilburyMechanical Engineering and Applied Mechanics

University of Michigan

Acknowledgments

• Joint work with Euisu Park and PramodKhargonekar of University of Michigan

• Funding provided by NSF/ERC forReconfigurable Machining Systems at UM

• Industrial partners from Lamb Techniconfor motivation, examples, and feedback

Outline

• Motivation: Logic control problem forhigh-volume machining systems

• Background• Automatic cycle logic control• Multi-mode logic control• Implementation and future work

High-volume machining systems

• Cycle time determinedby projected demand– 500,000 parts/year

one part every 20 sec.– One operation per station

• Dedicated transfer machine:– Fixed material handling, part flow– Highly accurate and repeatable operations– 10-12 stations, buffer at beginning/end– 5,000-10,000 I/O points (sensors/actuators)– 10-20 transfer machines per transfer line

Control of machining systems

• Continuous variable control (servo, CNC)– Positioning, feed for metal removal, fixturing,

and transfer operations: mainly SISO loops

• Discrete-variable (Logic) control (PLC)– Sequencing of operations– Coordination and synchronization of stations– User interface (pushbuttons & display)– Fault diagnosis and fault handling– Safety: gates and interlocks– Machine services: coolant, lubrication

Logic control problems

• Control software is 50% of machiningsystem development time and cost– Often written in low-level language (ladder)– Specification for automatic cycle: timing bar

chart, but only requires 10% of control code– No specification for alternate control modes,

error handling, diagnostics

• No formal verification beforeimplementation

• Long testing and debugging cycles result

Industry trends

• Faster time to market for new products– Must reduce system development time

• Shorter product lifecycles– Need to change manufacturing systems to

produce new products

• Increased quality– Integrate new technologies into existing

manufacturing systems

Reconfigurable machining systems

• Modular: easy assembly, potential reuse• Upgradable: new technology integration• Convertible: quick changeover to new part

Logic control challenges in RMS

• Modular structure– Same granularity as machine modules– Ease of configuration/reconfiguration

• Mathematical basis– Enable formal verification of correctness– High-level abstraction for ease of

understanding control system functionality

• Industrial implementation– Realistic complexity (thousands of events)– Target system: PLC or PC code generation

Outline


• Background– Transfer lines and Petri nets

• Automatic cycle logic control• Multi-mode logic control• Implementation and future work

Transfer line example

• Modular, standardized machining systems• Each station self-contained with controls• Coordination through transfer bar

Control specification

• Developed bymachine designers

• Operation sequence• Causal

dependencies• Home operation

– Mechanical stability

• Hydraulics• Cycle time

Timing bar chart specifies auto mode

Advance Clamp

Return Clamp

Rapid Advance Positioning Slide

Reset Main Slide

Feed Main Slide

Decel

Rapid Return Positioning Slide

CradleModule

Transfer BarModule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Total; Cycle Time = 22.2 second

Read Part Seated Air ChecksModule

0.7

0.3

0.30.3

2.5

0.5

0.30.3

0.5

2.5

19.6

15.4

22.2

63.1

50.0

Servo

29.6

20.5

36.5Servo

Return Cradle (Transfer Position)

2.9

3.8

1.0

1.0

4.9

6.5

1.5

0.5

1.5

2.1

Servo

1.3

Servo

0.6

0.9

9.7

0.6

9.0

Raise Transfer (1st Lift)

Raise Transfer (2nd Lift)

Raise Transfer (3rd Lift)

Raise Transfer (4th Lift)

Advance Transfer

Lower Transfer (1st Lower)

Lower Transfer (2nd Lower)

Lower Transfer (3rd Lower)

Lower Transfer (4th Lower)

Return Transfer

OPERATION GPM SEC 22

Advance Cradle (Machining Position)

Lower Rotate

Advance Grippers

Raise Rotate

Return Rails

Advance Rotate 270 Deg.

Return Grippers

Return Rotate 270 Deg.

Advance Rails

6.3

2.2

4.7

8.3

1.7

3.9

1.0

2.3

2.8

1.0

2.85

0.5

2.0

1.0

2.0

2.0

ModuleMill 1

RotatingTableModule

Clamp 1

Logic control block diagram

• Inputs: sensors and operator commands• Outputs: servo or hydraulic devices

• Petri net: Bipartite graph– Places (states) model operations: active or idle

• Token (dot) marks active state

– Transitions model events: from sensors oroperator commands

Formal representation of control

Advantages of Petri net models

• Large base of existing theory• Verification of key properties of control• Hierarchical structures for complexity

management• Implementation in PLC

– SFC = IEC standard programming language– One-one translation from Petri net to SFC

Petri net model capabilities

• Synchronization– p10 cannot begin until p7,p8,p9

have finished

• Conflict– Only one of p3, p4 can occur– Transition conditions t1, t2

must be mutually exclusive

• Concurrency– p5, p6 active simultaneously

• Causal dependency– p1 occurs before p2

t1

t2

p7

p8

p9

p1 p2

p3

p4

p6

p5

p10

Petri net properties for logic control

• Live: every transition can eventually occur– No deadlocks– All operations and events can happen

• Safe: No more than one token per place– Ongoing operations are not requested– Boolean state representation (active, inactive)

• Reversible: Initial state always reachable– Guarantees cyclic behavior of system

Supervisory control problem

• Petri net control models desired closed-loop behavior of system

• Control actions generated based ondesired state

Machining System

ControlGenerator

Operator Commands

Sensors

DesiredStates

Control Actions

Petri NetControl

Outline



Auto mode

• Normal operation cycle– Unclamp parts– Raise transfer– Advance transfer– Lower transfer– Clamp parts– Cycle machining stations– Return transfer

• Overlapping to minimize cycle time• Fault diagnosis and fault stop

Advance Clamp

Return Clamp


Reset Main Slide

Feed Main Slide

Decel


CradleModule

Transfer BarModule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21



0.7

0.3

0.30.3

2.5

0.5

0.30.3

0.5

2.5

19.6

15.4

22.2

63.1

50.0

Servo

29.6

20.5

36.5Servo


2.9

3.8

1.0

1.0

4.9

6.5

1.5

0.5

1.5

2.1

Servo

1.3

Servo

0.6

0.9

9.7

0.6

9.0





Advance Transfer





Return Transfer



Lower Rotate

Advance Grippers

Raise Rotate

Return Rails


Return Grippers


Advance Rails

6.3

2.2

4.7

8.3

1.7

3.9

1.0

2.3

2.8

1.0

2.85

0.5

2.0

1.0

2.0

2.0

ModuleMill 1

RotatingTableModule

Clamp 1

Timing bar chart -> Petri net

• Add waitstates beforesynchronizedtransitionsto modeloperationtermination

Mill Head

20mm RapidAdvance

10mm Decel. 553mm Feed

30mm RapidReturn

Reset Main Slide

Engine Block

SynchronizedTransition

W Wait to Start

20mm

10mm

553mm

30mm

553mm

Rapid Advance

Decel.

Feed

Rapid Return

Reset Main Slide


Reset Main Slide

Feed Main Slide

Decel


2.1

Servo

1.3

Servo

0.6

0.9

9.7

0.6

9.0

ModuleMill 1

• Each mechanical module gives one directedcircuit

Hierarchical representation

• Reduce complexity of Petri net bycombining sets of places/transitions

• Preserves liveness, safeness, reversibility

Combining control modules

• Merge synchronized transitions• Synchronized transitions retain

physical meaning

t3

t4

p1

p2

p3

t1

t2

t1 & t3

t2 & t4

W W

W W

p4

p6

p5

p8p7

p1

p2

p3

p4

p5

p6

p7 p8

t1 t3

t2 t4

• Add waitstates (W)beforesynchronizedtransitions tomodeloperationcompletion

Logic control for automatic cycle

• Directly generated from timing bar chart• Guaranteed to be live, safe, reversible

1

2

3

4

5

6

7

Transfer Bar Module

Cradle Module

Clamp Module

Mill 1 Module

Mill 2 Module

Advance Clamp

Return Clamp

Rapid Advance Positionong Slide

Reset Main Slide

Feed Main Slide

Decel


CradleModule

Transfer BarModule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21



0.7

0.3

0.30.3

2.5

0.5

0.30.3

0.5

2.5

19.6

15.4

22.2

63.1

50.0

Servo

29.6

20.5

36.5Servo


2.9

3.8

1.0

1.0

4.9

6.5

1.5

0.5

1.5

2.1

Servo

1.3

Servo

0.6

0.9

9.7

0.6

9.0





Advance Transfer





Return Transfer



Lower Rotate

Advance Grippers

Raise Rotate

Return Rails


Return Grippers


Advance Rails

6.3

2.2

4.7

8.3

1.7

3.9

1.0

2.3

2.8

1.0

2.85

0.5

2.0

1.0

2.0

2.0

ModuleMill 1

RotatingTableModule

Clamp 1

Reconfiguration of logic control

• Add a new millto transfer line

• New logic formill & clamp

• Modify transferbar logic tosynchronizewith clamp

• Shaded logicunchanged

Mill 1Transfer

Bar CradleRotating

Table Clamp 4 Mill 4Clamp 1

Modular structure of control

• Transfer bar synchronizes all modules• Limited communication• Ease of modification

Cradle &Clamp Modulefor Mill 1&2

RotatingTable

Clamp Modulefor Mill 3

Clamp Modulefor Mill 4

Transfer BarModule

Mill 1

Mill 2

Mill 3

Mill 4

Module

Module

Module

Module

Outline


• Background• Automatic cycle logic control• Multi-mode logic control

– What happens when things go wrong?

• Implementation and future work

Operator interaction with control

• Manual mode– Single station fine operation control

• Hand mode– Normal operation cycle without overlapping– Reverse sequences possible

Unclamp All Stations

RaiseTransfer

Advance Transfer

Lower Transfer

ClampAll Stations

Cycle All Machining

Stations

ReturnTransfer

Cycle Rotating

Table Station

• Operating:– Activate actuator

• Completed stop:– Deactivate actuator– Set internal variable to 1

• Incomplete stop:– Deactivate actuator– Announce fault

• Sensors trigger transitions between states• Internal variable used for coordination

with other stations

Three states for each operation

O

C

Events

Events

IC

Internal variables to synchronize

• Synchronization conditions depend oncontrol mode

• Decouple each control module

O

C

O

C

O

O

C

C

A1

A2

O

C

O

C

O

O

C

C

A1

A2

A1B1

B1

B2

B1

B2

Reversible operation module

• Normal and fault recovery operations• Fault recovery:

– Restart from fault position– Return to initial position

of operation, then restart

• Faults may occur infault recovery operation

• Transitions triggered byoperator commands advance (a) and return (r) in manual mode

NormalOperation

Fault-recoveryOperation

O

C

OIC

C

a

IC

rr

a

a

r

a

r

sN and (r or a)

sF and (r or a)

Irreversible operation module

• Metal-removal operations (i.e. milling)• Only contains normal operation block• Fault recovery:

– Restart from fault position– Return to home position

of machining station and restart

• Transitions triggered byoperator commands advance (a) and return (r)in manual mode

sN and a

Normal Operation

O

Chome position

IC

aa

Combining control modes

• Same set of states for all modes• Transition conditions depend on active

control mode• Superposition preserves liveness,

safeness, reversibility propertiest1

p1 p2p1 p2t2

t3

p1 p2t2

p1 p2t3

p1 p2t1

t

Auto

Hand

Manualt = (t1^Auto) (t2^Hand) (t3^Manual)

^^

Algorithm to build logic controller

1. Assign module for each operation2. Normal operation cycle ordered as in

timing bar chart– Mill example: Advance, Feed, Return

O

C

OIC

C

IC O

C

OIC

C

ICO

C

IC

Algorithm to build logic control

3. Reverse sequences for repeatable stepsin hand mode

4. Fault recovery sequences for irreversibleoperations

Theorem: Station control moduleconstructed according to algorithm islive, safe, and reversible Petri net.

Mode decision control logic

• Mode chosen by operator from input panel• Mode decision control logic is live, safe,

reversible

Station-Auto

Station-Manual

Each Station

Master/Start Master/Stop

HandAuto E-Stop

Main ConsoleStation-Manual

Station-Auto

Station-Manual

(Any)

System Stop(Home Position)

Stand-

Master/Start

Master/Stop

Hand

Auto

Manual E-Stop

E-Stop

Hand

AnyFault

AnyFaultAuto

(All)

by

Auto

Modular logic controller

• Operation causality condition to ensurewell-ordered set of operations

• Station logic controllers with modedecision control logic

Theorem: Resulting controller isguaranteed to be live, safe, reversible

Outline



– To the factory floor

Implementation in PLC

Sequentialfunctioncharts (SFC)– IEC 1131-3

standardlanguage

– Based on Petrinets

– One-onetranslation

Simple Place Simple Step

Initial Place Initial Step

Simple Transition Simple Transition



Macro Place Macro Step

Marked Graph SFC/Grafcet

Industrial implementation

• US Patent applied for, 1998• Current cooperation with Lamb Technicon

on Cummins Engine project• Evaluating implementation needs

– Machine services– Safety and gate interlocks– Reusability– PLC platform dependence– User interface

• Commercialization potential

Future of reconfigurable control

• Unified framework for continuous anddiscrete control for machining systems– Modular structures for reconfigurability– Mathematical basis for verification– Integrated diagnostics– Automatic fault detection and recovery

• Software tools for control design/analysis– Interface with mechanical design software– Automatic control code generation

Documents

University of Michigantilbury/logiccontrol/talks/... · 2006-10-25 · Acknowledgments • Joint work with Euisu Park and Pramod Khargonekar of University of Michigan • Funding