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.

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.

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..

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.

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.

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)

Transitions

Each transition has the following inscriptions:

• Name (for identification). • Guard (boolean expression containing

some of the variables).

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.

Example

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

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 .

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

Firing A CPN ---- Consider

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.

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

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

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

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

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.

In this case the guard acts as a barrier The transition cannot happen because the

guard is not satisfied.

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)

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.

The following intuitive CPN is put forward to represent activities

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.

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.

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.

The Activity

Guards and Barriers

Outcomes

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.

Unfolds to become

When all the guards etc are expanded , it becomes

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.

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)

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

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.

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.

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.

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 .

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.

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.

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.

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)

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.

4 Create an Environment to Enhance Performance.

This therapeutic choice focuses on providing enriched contextual and task experiences that will enhance performance.

Examples of these environments, include playgrounds which have soft surfaces and safe equipment which allow children to climb and swing and slide and push to the limits of physical activity without an enhanced risk of injuring themselves.

Another example is a garden with raised flower beds (ref Holms et al)which allow ,say wheel chair users, tend to the plants and enjoy the experience and rewards of gardening.(Ref?)

5 Restore /Establish Capacities

There are many examples of therapeutic intervention which will restore/ establish capacities.

A physiotherapist may suggest a regime of exercise in order to build up muscle tone and strength.

A speech therapist may work out a series of vocal exercises to improve articulation.

Technology too can provide functional capacities

Augmentative communication devices such as the Message Mate 2.0 provide means to deliver speech output for communication purposes.

Indeed many AT devices fulfil similar roles of proving functional capacity, across the domains of AT products such as Communication, Activities of Daily Living, Work, Leisure, Education and so on.

The level of functionality and the potential role AT can play and how this impacts on CPN modelling of activity and AT intervention is obviously a key concern of this work and will be elaborated on in then next section.

Message Mate 20Message Mate 20

CPNS and Activity

Adapt the TAsk

3 Prevent Barriers to Task performance

5 Restore /Establish Capacities

AT and CPNs

How can the above roles be represented using CPNs

Technology Offers Alternatives Consider

We need to go from the bottom to the top of a hill

To do this by walking need stamina, leg strenght, steadiness

Or we could

Drive

Each way of going to the destination has its own requirements

CPNs allow us to model this choice

We consider AT as an alternative way of doing things if barriers are blocking our way to doing it in the usual way

Consider the problem to write i.e. generate text This can be represented by the following

CPN

However if manual dexterity isnt good then an alternative might be considered

Maybe a computer system with voice recognition

Now if some of the personal factors do not meet the conditions in the guard CPNs can model solutions.

For example

If the person cannot use a PC then taking a course might solve this problem

This can be represented as follows.