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1

Describe how the interface between Petri Nets and physical systems can be achieved in cell automation.

  • Interfacing between petri nets and physical systems is achieved by linking the transitions with Inputs and places with outputs.

 

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2

Describe the basic functions common to adaptive-control systems. Discuss two types of adaptive-control systems used to control manufacturing operations

Adaptive Control (AC) for machining is defined as the on-line adjustment of process parameters for the purpose of optimising production rate, optimising quality, or minimising the cost of materials and components. The basic functions ofmost adaptive systems are:

  • Determination of unknown parameters (measured or inferred)
  • Based on these parameters make an alteration to the control strategy
  • Via the control strategy appropriately adjust existing or additional processing parameters.

A key issue for adaptive control is that of adjusting to variations in the workpiece. The diagram below illustrates differences between adaptive control and conventional (constant) control where the adaptive control is attempting to maintain constant contact force by adjusting feed.

There are two key types of adaptive control techniques:

  • Adaptive Control Constrained (ACC): These are systems that place a constraint on a process variable (e.g., force, torque, temperature). Here, ifthe thrust force and the cutting force (hence the torque) increase excessively, the system modifies the cutting speed or the feed in order to lower the cutting force to an acceptable level. Without adaptive control (or direct intervention of the operator), high cutting forces may cause tool failure or may cause the workpiece to deflect excessively resulting in workpiece dimensional accuracy and/or surface finish deterioration.
  • Adaptive Control Optimised (ACO): These are systems which optimize an operation. Optimisation may involve maximizing the material-removal rate between tool changes or improving the surface finish ofthe part. For adaptive control to be effective in manufacturing operations, quantitative relationships must be established and coded in the computer software as mathematical models. If, for instance, the tool-wear rate in a machining operation is excessive, the computer controller must be able to calculate how much of a change in speed or feed is necessary in order to reduce the tool-wear rate to an acceptable level.

 

 

3

A conveyor in a manufacturing line is controlled by a Programmable Logic Controller (PLC), which uses a momentary push button as input to turn the conveyor on when the button is pressed the first time and to turn it off when pressed again. Develop a ladder logic code to implement this logic in the PLC.

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4

Describe the role that the Programmable Logic Controller (PLC) typically plays in automated manufacturing. Include reference to typical decisions for which the PLC is responsible, and to the nature of its interfaces.

Automated manufacturing gernerally involves:

  •  one or more machines
  • producing families of parts with similar characteristics.
  • integration of both work-piece and tool handling is typical

The role of the PLC in this environment is to:

  • Coordinate the functions of different automated machines/devices
  •  Distribute operational commands to machines/devices
  •  Receive status/task complete reports from machines/devices
  •  Controls the sequence of production
  •  Manages logic decisions in production

5

In PLC operations explain the role of:

  • The PLC scanning sequence
  • An unlatching function

The PLC scanning sequence: the series of three scan cycles run to operate the PLC:

  • Step 1: INPUT SCAN
    • map external inputs to input image table
  • Step 2: PROGRAM SCAN
    • evaluate each ladder in sequence based on existing
    • write the new outputs to the output image table input image tables
  • STEP 3: OUTPUT SCAN
    • send output image table to output channels

 

An unlatching function: many inputs are momentary and to generate a permanent output it is possible to use the internal state of the PLC to generate a latch variable which permanently turns that state [and a connected output] on. An additional unlactching function ensures that the state is turned off when no longer needed:

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6

In PLC operations explain the role of:

timer and counters

state machine diagrams

Timers and Counters

  • Upward and downward counters are used to monitor the accumulation of events (e.g. parts passing a particular point). Once the accumulated counter reaches the preset value the rung (DN) becomes “true” or “on” and can be used as an alarm or as part of a resetting sequence. CD indicates that the counter is counting.
  • Timer-On (respectively Timer-Off) functions are set by the associated rung conditions becoming true (resp. false). They remain set until the accumulated time reaches a preset value. EN indicates that the timer is enabled.

State machine diagrams: a means of demonstrating the connection between different independent states of a system. Additionally links to external inputs and outputs can be added.

7

Petri Nets are used to graphically describe logic sequences in discrete event operations.

Describe, using illustrative examples where appropriate, what is meant in Petri Net design by:

  • Initial marking: an initial marking of a petri net, M0 , is the assignment of tokens to specific places in the petri net indicating the starting point of the operation modelled by the petri net.
  • Enabling and firing: a transition is enabled is the required number of input tokens are available at the places at the transition input [as designated by the weights on the input arcs joining places to the transition
  • Concurrency: refers to a situation where two events must occur simultaneously and often a single transition is used to enable and hence mark the places associated with both events.

8

Why is it important to have standardised approaches to PLC programming?

Numerous reasons including:

  •  enables re use of code/code modules/code libraries simplifies the task of plc programmers
  • enables tools for developing and analysing PLC programmes to be developed
  • enables multiple manufacturers PLC devices to be used
  • simplifies portability of code

9

Contrast three of the approaches specified by the IEC 61131-3 standard, identifying the main features and advantages in each case.

ladder logic: programming as a ladder of logic steps: simple, visual and intuitive

SFC: programming as a graphical flow chart: visually appealing, follows flow chart logic, allows nesting

Function block diagram: extending ladder like approach by adding libraries of functions: easy reuse, allows nesting

Structured text: programming language approach: similar to standard programming languages, easy for programmers to use

Instruction list: low level assembler like code: efficient coding

10

Petri nets and other modelling tools are often used in conjunction with PLC programming approaches. Why is PLC programming on its own generally insufficient for planning and developing a PLC-based control system for a complex manufacturing cell?

  • Difficult to fully capture complex logic
  • No tools to analyse properties of logic plan [feasibility, deadlock, reachability]
  • No systematic approach
  • Identify causal/concurrent relationships between operations
  • Identify possible conflicts between resource movements
  • Identify possible conflicts / priorities for multiple part productions

11

What additional factors need to be considered when a Petri Net model of a process is used to generate PLC programme code for automatic control?

Factors will include issues such as

  • Checking overall logic of the PN model for issues such as a deadlock, limits, reachability
  • Interface with the physical environment: sensing and actuation links to the production cell.
  • Communications enhancement: ensure that handshaking processes are in place for effective confirmation of instructions between PLC and machines.
  • Ensuring logic is adequate for continuous operation: allowing system to return to initial state
  • Conversion of PN model to PLC Code
  • I/O mapping of the physical devices with PLC

12

The vertical axis of the robot is to be equipped with a simple, proportional feedback controller, k, which uses a measure of the end effector position to adjust load. Draw a diagram of the closed-loop system and write a closed-loop transfer function relating load disturbance and end-effector deflection.

 

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13

Define what is meant by deadlock in a fully automated production environment and explain why it is generally undesirable

  • Deadlock represents a condition where the automated system reaches a state that it cannot move on from. This is generally due to a loop in which the requirements for the commencement of one section of the loop are conditional on the completion of another section which cannot in fact proceed before the first section has been executed.
  • Deadlock is clearly undesirable because it leads to the "freezing" of the automated process until some form of manual intervention clears the loop.

14

A robot is being used to unload parts from a machining centre. Machined parts are temporarily stored in a buffer (with a capacity of two parts) prior to being moved (by the same robot) to an assembly operation. Use a petri net to illustrate how deadlock can arise in this system. On a separate petri net show how deadlock can be avoided.

  • In this diagram transition t1 has just fired and the robot is in use loading the part to the buffer. But the two part buffer already has two parts in place and cannot accommodate a third part and hence t2 cannot fire. Similarly transition t3 cannot fire because the robot is required to do the unloading move so the process is deadlocked.
  • The petri net can be simply adjusted by shifting the arc from the buffer currently pointing at t2 to make it point to transition t1. Hence the robot cannot commence the move of parts to the buffer unless there is a free space.

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15

What are the key issues to be considered when preparing a petri net model for use in an automated machining operation?

Good responses will at least cover the following points discussed in lectures:

  • Interfacing to the Physical Systems: making sure that input / output signals to physical environment are in place. Ensure that appropriate communications / handshaking are in place to support error free automation.
  • Checking the logic of the cell control design: ensure no deadlock, unreachable states, continuous loops etc.
  • Additional Logic for Continuous Operations: ensure system has homing capability
  • Generating PLC code from Petri Nets: follow procedure to convert PN to appropriate code such as Ladder Logic.

16

Picture illustrates an automated cell for assembling a simple two part meter box. The loading operation of the cell involves a robot moving parts A and B from separate part buffers into one of two assembly jigs on a turntable before a separate robot screws them together.

An incomplete petri net model for the loading operation is given in Fig. 2. Stating any assumptions, complete the petri net model of the loading operation and demonstrate how it can be made ready to be used for the automation of the loading operation.

 

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  • Additionally handshaking procedures should be considered to ensure error safe operation of the system.
  • This would involve – for example – introducing an additional place between p1 and p2 in which the additional transition is triggered by the robot confirming that it has embarked on the loading process.
  • Further, an initialisation routline could be added to ensure the system is starting its sequence with
    •  turntable empty in both locations
    • robot in home position
    •  conveyor loading buffer empty Depending on the overall process and the degree to which the operations run in continuous mode there are many variations on this.

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17

For three robot types with different degrees of freedom, discuss the influence that the robot’s degrees of freedom will have on potential applications.

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18

Why are more flexible, ‘human-like’ robots becoming more popular in industrial robot developments?

Human like (Anthropomorphic) robots are becoming more popular in industrial robot applications because:

a) This type of robot is the most dextrous allowing it to carry out a wide variety of tasks. (Packaging, Assembly, Welding..)

b) Production systems and incorporated robots have to be as flexible possible to handle product change and customisation requirements.

c) They can have a longer operational life as they can be repurposed to a wide range of activities.

19

A consumer electronics company is looking to purchase a robot to carry out packaging at the end of a washing machine production line. The robot is required to lift a 50 kg washing machine off the assembly line and place it into an open cardboard box on the pallet line. This will require a robot to have a reach of three meters and an axis speed of one meter per second. Fig. 2 gives a chart showing the characteristics of different types of robots. Examine the information given in Fig. 2 and determine the best type of robot for this task.

Describe why you have chosen this type of robot, listing both the benefits and limitations that you have considered. What other information might you request to assist you in making your decision?

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20

Discuss the factors that need to be considered when converting a Petri Net to Ladder Logic for use in an automated manufacturing operation.

a) Ensure that the logic within the petri net is correct and it provides the correct functionality keeping in mind. (Start Conditions, Deadlocks, Conflicts, Suitability for continues operations).

b) For each of the elements within the petri net, identify related variable conditions. Transitions need to be mapped to specific external triggers [I/O Input’s and their states]. Places need to be mapped to PLC memory locations [associated variables]. Places also need to be mapped to external actuation signals [I/O Output’s and their states].

c) Petri Nets are converted into ladder logic in two phases i) Building latch logic to activate a place when transition conditions are meet. This logic is designed to unlatched when a subsequent place becomes valid. ii) Output logic is then built to check for place conditions becoming true and then firing relevant outputs. By handling input and output conditions in this way we eliminate race conditions associated to PLC scan cycles.

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