REVISION Control stuff Flashcards Preview

Cartesian: Big reach, low speed, high-ish payload, high repeatability

Anthropomorphic: High reach, average speed, high payload, low repeatability

Scara: high speed, average reach, high repeatability, low payload

Delta: high speed, low reach, low repeatability, low payload

• Teach mode
• Lead through
• Off-line

Teach mode

Pros

• Simple widely used technique
• No additional infrastructure required during programming

Cons

• Time consuming and repetitive
• Limited automated testing and verification

Lead through

Pros

• Mimics complex trajectories used by skilled operators

Cons

• Difficult to deal with large robots
• Inaccuracy’s in programming can’t be edited

Off-line

Pros

• Reduced down time during in programming
• Assists cell design and allows process optimisation

Cons

• Requirement for accurate CAD models of instillation
• Accuracy of robot is critical when suing off-line programming techniques

Cell automation requirements

• Receive and interpret part schedules
• Ensure the production of one or more parts is completed
• Communicate part completion reports to factory computer network
• Coordinate the functions of different automated machines
• Distribute operational commands to machines/devices
• Receive status/completion reports from machines/devices

PLC

• PLC programming provides a software environment for building sequences of logical operations.
• A PLC has a microprocessor and memory and a user interface. It is also able to connect to the network to connect to other machines

Approach to ladder logic programming

1. Determine key processing steps
2. Determine resources/equipment required
3. For each resource specify:
   1. Triggers (control inputs)
   2. Operations
   3. Pre-requisites
   4. Constraints
4. Identify allowable states for each resource
5. Using key processing steps (1) and allowable state (5)
   1. Identify single or joint states required for the process
   2. Develop state model for required operations
6. For each process state identify
   1. Required inputs from equipment
   2. Required output signals to equipment
   3. Any latching, counting, timer requirements
7. Generate equivalent ladder code to represent each state

PLC programming approaches

• Ladder logic
• Function block
• Sequential function charts
• Structured text
• Instruction set

FUNCTION BLOCKS

• Further graphical approach to representing PLC logic
• Allows for more complex functions to be developed in PLC environment

SEQUENTIAL FUNCTION CHARTS

• Graphical environment for PLC code development
• Flowchart approach to logic representation
• Allows for nesting of logic code
• Uses 3 logical elements
  • STEP
  • TRANSITION
  • FLOW LINE
• Can generate equivalent ladder code

STRUCTURED TEXT

• More formalised text based programming language
• Recognising that PLCs now have full computer capabilities
• Low level programming language similar to assembler

PC vs PLC for Cell automation

PC

• Graphical user interfaces
• Easy and open networking
• Excellent computational/memory facilities
• Relative low initial cost (for multiple applications)

PLC

• Operating systems designed for multi tasking capabilities
• Robustness of input/output handling
• Reliability
• Easy problem diagnosis/recovery

Approaches to logic (informal)

• IMMEDIATE: can execute a solution immediately
• INSPECTION: solution can be determined after examining the problem
• STRUCTURE INSPECTION: solution can be determined after following a set of checks
• PLANNING PROCEDURE: a formal set of steps is used to determine the approach to solving the problem
• ALGORITHM: An algorithm is executed which generates a solution to the logic problem

• Capture of STATE information can be cumbersome
• No explicit method for representing transition conditions between states

No formal way to express complex logic

• Conflict between operations requiring
• Representation of causality
• Ensuring events occur concurrently

Lack of analysis tools for assessing

• Feasibility of sequence of operations (e.g. avoiding deadlocks)
• Boundedness of buffers
• Reachability of required conditions
• Potential for process simplification

Petri Nets

Graphical tool which can be used for modelling, planning, control design and evaluation of manufacturing systems.

A petri net is a discrete event model which models the change of conditions before and after an event, and the factors required for that event to occur.

CAUSALITY: sequential firing of transitions via synchonisation

What is deadlock: when a system can’t get out of itself

An Approach to Petri Net Modelling

1. Determine key processing steps/states
2. Determine resources/equipment required
3. For each resource specify:     
   1. Triggers (control inputs)
   2. Operations
   3. Pre-requisites
   4. Constraints
4. Develop PN model to reflect key processing steps
5. Introduce materials handling steps into PN from 4
6. Identify allowable states for each resource and develop a PN for that resource
7. Check each PN in 5, 6 to ensure causality, conflict and concurrency conditions and that any upper bounds on capacity are reflected
8. Combine PNs from 5, 6 to form an overall system PN.

Confirmation / Handshaking

Most machines are equipped with an operational signal which can be incorporated as a confirmation flag for the cell controller (typically a PLC).

This introduces an additional state into the controller operation but greatly enhances reliability and diagnostic capability.

The process is called handshaking

Multi level view of the factory

TASK: machine, robot, conveyor

PART: machining centre, cell

PRODUCT: production line, flexible manufacturing systems, storage

ORDER: factory

LEVEL 0: the physical production process

MANUFACTURING CONTROL

LEVEL 1: sensing the production process, manipulating the process

LEVEL 2: monitoring, supervisory control and automated control of the production process

MANUFACTURING OPERATIONS MGMT.

LEVEL 3: work flow to produce desired products. Maintaining records and optimising the production process

BUSINESS PLANNING AND LOGISTICS

LEVEL 4: Establishing the basic plant schedule – production, material use, delivery etc.

Deterministic (traffic lights)

• Guaranteed flow
• Predictable
• Inefficient
• Always a chance of stopping
• Good for: heavy traffic (individual)

Non – deterministic (roundabout)

• Not guaranteed – potential bottlenecks
• Maximum use made of the intersection
• Only stop in heavy, constant traffic
• Good for light traffic, heavy traffic (total flow)

Manufacturing messaging specification

• MMS specifies the message exchange between programmable devices for computer integrated manufacturing
• MMS is a service element in ISO – the application layer
• Adherence to MMS exchanges increases the flexibility and interchangeability of different manufacturing units

Machine communications: MMS

Intra Cell communications: field bus

Between Cell communications: RFID

Factory Wide communications: Ethernet

Part Handling Systems

• Material integration between cells, between machining centres and other processing operations
• Vary in degrees of flexibility
  • Predetermined routing and sequences
  • Predetermined routing variable sequences
  • Free movement in all directions
• Selection Criteria
  • Material
  • Tasks – variety of them
  • Workpiece loads
  • Workplace size and environment
  • Cost

Part Storage Systems

• Part storage between cells, between machining centres and other processing operations to enable batch/ semi batch operations
• Vary in degrees of flexibility & control
  • FIFO buffers / accumulation buffers
  • Sorting buffers
  • Random access buffers
• Selection criteria
  • Location: in situ vs storage
  • Size: part size, volumes (control strategy)
  • Manual vs automated access

Digital manufacturing

1990s (agility, globalisation)

• The application of digital information for the enhancement of manufacturing processes

2000s (energy, emissions, sc integration)

• The application of digital information for the enhancement of manufacturing processes, supply chains

Late 2010s (distributed, small scale production, customisation, e commerce)

• The application of digital info (from multiple sources) for the enhancement of manufacturing processes, value chains, products and services

Whats new?

• Internet is ubiquitous
• Network many devices/ objects/ data sources
• Cloud computing
• Mobile, personal computing
• IT services
• Ability to share information across value chain
• Order information orientation
• Customer involvement

Not new

• Lots of sensors
• Lots of data
• Data analytics
• Simulation
• Industrial AI

A framework for enabling the integration of physical and digital resources, services and humans resources in manufacturing in real time

• Vertical integration and networked manufacturing systems
• Horizontal integration through value networks
• End to end digital integration of engineering across the entire value chain
• With real time optimisation

Industry 4.0

Internet of things

Cyber physical systems (digital twin)

Block chain

Augmented reality