Coloured Petri Nets

1. Coloured Petri Nets • The purpose of this discussion is to develop a representation scheme for AT systems. • The domain of reference for AT systems is rich and wide ranging with many factors to consider. • Any representation of this world of AT , must be able to embody these many factors and be able to mark the changes which inform AT outcomes.

2. The Weaknesses of Traditional Petri Nets • Their primary aim is to represent the dynamic aspects of system behaviour and because of this they do not have anything other than very simple capacity to represent entities of the domain of application. • Data representation is limited to tokens which are indistinguishable from each other. • Clearly this is inadequate for representing AT systems.

3. Coloured Petri Nets • In order to represent data representing the context of AT a richer scheme is required. • This extended data definition capacity is offered by an extension of traditional Petri nets known as Coloured Petri Nets. • Coloured Petri Nets have got their name because they allow the use of tokens that carry data values and can hence be distinguished from each other – • This is in contrast to traditional Petri nets, which have only one kind of token . These tokens by convention are drawn as black, “uncoloured” dots..

4. Coloured Petri Nets • Coloured Petri Nets combine the strength of Petri nets with the strength of programming languages. • Petri nets provide the primitives for the description of the synchronisation of concurrent processes, while programming languages provide the primitives for the definition of data types and the manipulation of data values.

5. Characteristics of Coloured Petri Nets • Like traditional Petri Nets Coloured Petri Nets consist of Places Transitions, connected by Arcs. (forming a bi-partite graph). • They are a combination of text and graphics.

6. Places • Places are specified with the following inscriptions: • • Name (for identification). • • Colour set (specifying the type of tokens which may reside on the place). • • Initial marking (multi-set of token colours)

7. Transitions • Each transition has the following inscriptions: • • Name (for identification). • • Guard (boolean expression containing some of the variables).

8. Arcs • Each arc has the following inscriptions: • • Arc expression (containing some of the variables). • When the arc expression is evaluated it yields a multi-set of token colours.

9. Example

10. Marking • Place P2 is empty. • The marking at P1 consists of 2 tokens of type integer whose value is 3 and • 2 tokens of type integer whose value is 8. • On T1 is the guard X > 5. This is a barrier to T1 happening, in that T1 will only pass if the assignment to X under the occurrence in question is greater than 5. • As we will see these guards on transitions will play an important part in representing the barriers to the performance of activities . • There are 2 places P1 and P2. and a transition T1

11. Transition Guard • On T1 is the guard X > 5. This is a barrier to T1 happening, in that T1 will only pass if the assignment to X under the occurrence in question is greater than 5. • As we will see these guards on transitions will play an important part in representing the barriers to the performance of activities .

12. The Arc Expression • On the arc between P1 and T1 is the atomic expression consisting of a variable X to which may be bound one of the input tokens in P1. Such a binding is called an occurrence. • The expression on the output arc from T1 to P2 represents the state change which takes place across transition T1. In this example it embodies an increment of 1 on the variable X

13. Firing A CPN ---- Consider

14. In this example • On place P1 there are 5 colour tokens. of type Integer. There are three tokens with value 8 and two with value 5. • On the arc between place P1 and Transition T1 there is a variable expression X. Tokens from place P1 can be taken from P1 and bound to this variable expression.

15. X is bound to one of the Integer tokens with value 8

16. Guards on the expression • The expression X > 5 at transition T1 is known as a guard and must be satisfied by the incoming token values. • If the incoming tokens do not satisfy the guard then the transition cannot happen. • In this case X has value 8 which is > 5 so the transition can take place

18. If the guard is satisfied, then the transition may happen and the token is passed to the output place P2 . The token is modified with those changes that are specified in the expression on the output arc. • In this case this expression is X + 3. which specifies that 3 is to be added to the token bound to X

19. X is assigned the value 8 + 3A token with value 11 is output

20. Occurrence • The passage of tokens across transitions from place to place through this process of binding, satisfying guards and modifying data is called an occurrence. • The occurrence will be blocked if the guard is not satisfied by the incoming tokens. • So if an input token with value 5 was bound to X then the transition could not occur. • The guard is a barrier to the transition happening.

22. In this case the guard acts as a barrier • The transition cannot happen because the guard is not satisfied.

23. CPNS and AT systems. • At one level Assistive technology is used by somebody in some context to do something. • That is it is characterised by a person performing an occupation in some environment using technology to assist them. (PEO). • It is about a human performing an activity with Assistive technology.(HAAT)

24. How can CPNS model people performing activities.. It is necessary to: • Represent features of the person and the factors that impact on their capacity and performance of the activity. • Characterise the activity in terms of its requirements on the person or barriers to its performance. • Represent the outcomes or changes that take place as a result of the activity being performed.

25. The following intuitive CPN is put forward to represent activities

26. In other words …… • The activity is represented by a transition. The personal Features and factors relevant to the activity are represented by a set of tokens at the input place. • Barriers to the performance of the activity are represented as guards on the transition. • Outcomes of the activity are captured by the specified state change of the transition.

27. Sub- elements of activity CPN • 1: The set of tokens at PLACE 1 representing the person performing the activity and the factors that influence their ability and capacity to perform that activity. • 2: The Activity itself at TRANSITION 1 • 3: The guards and barriers to performing that activity at TRANSITION 1 • 4: The outcomes of making a cup of tea.

28. Tokens at PLACE 1 • The set of tokens at PLACE 1 representing the person performing the activity and the factors that influence their ability and capacity to perform that activity.

30. The Activity

31. Guards and Barriers

32. Outcomes

33. Tasks and Subtasks • CPNs can represent the sub tasks of a task , the steps of an algorithm, the subroutines of a routine. For example consider.

34. Unfolds to become

35. When all the guards etc are expanded , it becomes

37. Role of AT in Activity Performance • So what does AT do. • In terms of Activity, what contribution can AT make to the overall performance of the activity.

38. AT devices are many and varied in terms of their application . • Reviews of AT databases such as Abledata and Eastin (refs) show that AT devices have application in many environments, such as the Home/Local Environment, the Community, in Education and employment and for Leisure/Social/Recreation. • Furthermore AT devices vary considerably in terms of their functionality and what they can contribute to Activity Performance (Ref)

39. Different ways AT can contribute to activity performance. • 1: AT can perform that activity autonomously. • Example robotic vacuum cleaner. • 2: AT can perform the activity semi-autonomously. • e.g. standard vacuum cleaner • 3: AT can facilitate the performance of activity by removing one or more barrier to that activity. • For example if products are out of reach then we can use a mechanical grip

40. 4: AT can enable the performance of an activity by adapting the environment in which activity is performed. e.g. Having lower work surfaces can facilitate performance of kitchen based activities for a wheel chair bound person. • 5: At can be used to restore or establish capacity within an individual with respect to Activity Performance.For example an automatic can opener could provide someone with the ability to open cans and bottles, thereby allowing them to participate in food preparation.

41. Categories of Assistive Technology • Assistive technology may be high or low tech. High technology includes devices which incorporate electronics or microchips to perform some function. • Low technology does not require a power source. An example of high technology is a computer. An example of low technology would be a pair of eyeglasses or a hand operated reaching device.

42. Categories of AT • Hard technology is technology which is tangible versus soft technology which cannot be seen. An example of hard technology is a calculator. An example of soft technology is a computer program which assists with cognition, decision making, and concept formation. • Appliances are examples of AT which do an entire task alone. Tools, on the other hand are AT’s which assist an individual in task performance. • Adaptive/assistive technology is used to help an individual versus rehabilitative/educational technology is used to teach an individual. (Cook & Hussey, 1996; Trefler & Hobson,1997). • Minimal technology, i.e. a wrist support for eating, supports an individual in task performance whereas maximal technology performs a task for the individual, i.e. a robotic arm feeding an individual.

43. Role of AT • From the preceding discussion it is clear that the term AT encompasses many many different kinds of technologies which impact on the AT user in numerous ways. For example we have technology, such as a robotic vacuum cleaner (ref) which can perform a task relatively autonomously to wall mounted handles and grips which of themselves do not perform any task but do facilitate the performance of tasks by enhancing the environment in which the task is performed . • Similarly technology such as artificial prostheses do not perform a task by themselves but restore bodily structure and capacity to the individual which in turn enable the individual to perform some activity .

44. Some technologies , such as a pencil grip, impact positively on the objects involved in the task or activity. Yet other technologies such as audio alarms provide extra functionality to task objects which provide process information for task monitoring.

45. AT as Intervention • The roles different technologies play reflect to some degree, different intervention strategies which someone like an OT can make to increase occupational performance.

47. 1: Adapting the Task • When the Task method is altered the same task objects are used in the same environment but the method of performing the task is altered to make the task feasible given the persons circumstances. Examples of this kind of intervention, include one handed techniques (ref) for tasks normally requiring two hands e.g. one handed dressing or typing. • Mastering one handed methods require the capacity to learn and practice is a necessary component of this process.

48. 2: Adapting the Environment • This intervention emphasizes selecting and implementing an environment that enables the person to perform with current skills and abilities (Dunn et al) • Examples of environmental adaptation include making changes to a home , to facilitate a wheelchair user perform every day activities. For example a bathroom could be adapted by the provision of a higher toilet to facilitate easier transfers, grab bars introduced for standing pivot transfers and recessed plumbing and drawers for easier access to the sink. (ref)

49. 3 Prevent Barriers to Task performance • Therapeutic interventions can prevent the occurrence or evolution of barriers to performance in context. • Family members could be trained in a hierarchy of assists, for example verbal cues, that can help maintain the remaining skills that a person with a progressive dementing illness still has. • (Rogers et al 2000) • Of particular interest to this dissertation is the role assistive technology can play. • Technology can help prevent barriers resultant from discrepancies in task requirements and personal capacity. Consider a standard keyboard. • If someone with poor targeting abilities is repeatedly hitting two keys at once , then a key guard can prevent this difficulty by placing raised borders around each key button, which make it impossible to hit more than one key at a time.