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Using OOP in Measurement Systems

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Common Problem: Software becomes difficult to maintain over time…

Initial investment in software

architecture, training and processes

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Why OOP is an Answer

When you want to build scalable, extensible systems.

Translation: OOP concepts, when used correctly, decrease the risk and effort required to add functionality to an existing system. They do this, in part, by:- Minimizing the scope of changes that have to be introduced to an existing system to add new functionality

- Restricting access to data to methods that have explicitly been permitted access

- Making it possible to add and load new behaviors dynamically that implement existing interfaces

- Enable reuse and extension of pre-existing functionality

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Configure Acquire Measure

The journey to OOP starts sooner than you think…

Conceptually, many LabVIEW applications start with a simple, synchronous set of operations (many times all on a single diagram)

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Configure Acquire Measure

Start Task Read Samples Stop Task

Savvy programmers will often use SubVIs to wrap API calls to things like device drivers or loading configuration from a file

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Configure Acquire Measure

Task Samples

Clock Raw Data

Graph

DAQ Task

These SubVIs typically expect certain inputs to do their jobs – many times these inputs are the output of previous operations, or a single input is shared by multiple VIs in this sequence

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Measurement System

Configure Acquire Measure

Task Samples

Clock Raw Data

Graph

DAQ Task

As the application becomes more sophisticated, it’s common to want to iterate on certain tasks within a loop..

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Measurement System

Configure Acquire Measure

I/O Samples

Clock Raw Data

Graph

DAQ Task

…or perhaps the entire system

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Measurement System

Configure

Task

Clock

Configure Event Case

Eventually, automation requires that we be able to programmatically control the order of operations and/or restart the system, which requires a state-machine-like

pattern that passes data between states

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Measurement System

Acquire Event Case

Acquire

Samples

Raw Data

The scope of data these operations has access to can be seen by examining the shift registers in a loop

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Measurement System

Measure Event Case

Measure

Graph

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Measurement System

Event Case

I/O

Samples

Clock

Raw DataTask

As the functionality grows, so too does the scope of data. Eventually it becomes helpful to contain all of this data in a cluster, giving us a very clearly defined data-scope for this

process

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Measurement System

Event Case

I/O

Samples

Clock

Raw DataTask

Configure

I/O

Clock

Task

The methods within this state machine retrieve the information they need from the cluster and update values as

necessary

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Measurement System

Event Case

I/O

Samples

Clock

Raw DataTask

Configure

Unbundling and bundling data can be encapsulated by the VI, giving the user a clean, simple top-level interface (the type

definition)

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Configure Acquire Measure

I/O

Samples

Clock

Raw DataTask

Graph

This simple illustration shows how these three operations we started with now just act upon the data within the cluster

These VIs are explicitly coupled to this data

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I/O

Samples

Clock

Raw DataTask

Measurement Class=

Measurement Data A class contains data, plus

methods (VIs) that are allowedto act upon and modify the data.

VIs that do not belong to the classcannot act upon the data

A class is basically a cluster

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Configure Acquire Measure

Graph

Measurement

The object wire can be passed into any VI that has the class on the connector pane, but only VIs that belong to the class can directly bundle and/or unbundle the data

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I/O

Samples

Clock

Raw DataTask

Class Data Cluster

Configure Acquire Measure

Methods that can act upon the class’s data

clusterAppearance in Project

Class constant

These VIs are now owned by the class. They are transported as a cohesive library of code

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DemonstrationCreating a New Class

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Measurement System

Event Case

I/O

Samples

Clock

Raw DataTask

Configure

Returning to our basic system, this implementation using a clsuter effectively becomes…

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Measurement System

Case

ConfigureMeasurement

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Measurement System

Case

ConfigureMeasurement

What if you want a different definition of how these methods should act?

In this example, consider the different ways in which a ‘measurement’ might be implemented

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Measurement

Temp Strain

These are children of the measurement class

Finite measurement of a single channel

Applies stimuli before acquiring value

Inheritance Allows Descendant Classes to Modify, Extend and Add Functionality of a Parent

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DemonstrationDefine inheritance and view the class hierarchy diagram

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Measurement System

Case

ConfigureMeasurement

What if you want a different definition of how these methods should act?

In this example, consider the different ways in which a ‘measurement’ might be implemented

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Measurement System

Case

Start Task Read Samples Stop Task

AcquireTemp

‘Acquire’ is Dynamically Dispatched

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Measurement System

Case

StimulateOutput

Sweep Inputs

Acquire

Start AI Task

Start AO Task

Strain

‘Acquire’ is Dynamically Dispatched

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Strain.lvclass:Acquire.vi

Temp.lvclass:Acquire.vi

Resistance.lvclass:Acquire.vi

Understanding Dynamic Dispatch

2610

Acquire

Strain

Temp

Resistance

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DemonstrationIllustrate dynamic dispatch

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Configure Acquire Measure

Graph

Configure Acquire Measure

Graph

I/O

Samples

Clock

Raw DataTask

Measurement Type

? ? ?

Q: Isn’t this the same thing as using case structures inside these VIs ?

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A: It’s conceptually similar, but there are extremely important differences…(NO)

To understand the difference, first consider the impact of introducing a new measurement in the non-OOP example.

Configure Acquire Measure

Graph

I/O

Samples

Clock

Raw DataTask

Measurement Type

? ? ?

2. We have to modify all of these VIs

1. We have to add a new type to the enum

3. We probably have to add new elements to this data cluster

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Configure Acquire Measure

Graph

I/O

Samples

Clock

Raw DataTask

Measurement Type

? ? ?

As the scope of the data cluster expands, we are passing data into large segments of code inside the cases that should not have access to it. Our data is not protected

I/O2

Trigger

DIO

Introducing a new measurements requires changes within the VIs, as well as the calling code (sometimes referred to as a framework). This makes code very costly to maintain and brittle

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Sibling Classes Have Unique Data Scope

Measurement

Strain Temp

I/O

Bridge Type

Task

Thermocouple

Samples

Data unique to these specific measurements, plus ‘exposed’ data of parent measurement

Data that every measurement needs to

have

excitation

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Configure Acquire Measure

Graph

I/O

Samples

Clock

Raw DataTask

Measurement Type

? ? ?

One of the biggest differences: this new functionality has to be added at edit time.

What if you want to load a new measurement into your calling system at run-time?

I/O2

Trigger

DIO

Yes, you can dynamically load the VIs called by these VIs, but you have to have pre-defined the data they have access to. The data in the cluster wire cannot be changed at run-time, as the connector pane must match exactly.

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Configure Acquire Measure

Graph

At edit-time, LabVIEW shows us the wire of the parent class we have said will be passed along this wire (in this example: Measurement.lvclass)

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Configure Acquire Measure

Graph?

We may pass any child of this class down this wire at run-time. Dynamically dispatched methods will execute the copy belonging to the run-time instance.

LabVIEW can load a new child at run-time. The class will bring its methods and it’s data cluster into memory when loaded. This makes it possible to add functionality without modifying calling code. The code to load a child class is referred to as a Factory.

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The Basics of an Object Factory

AB

C Generic Measurement

Location on DiskWhere Measurements

Classes are Stored

Objects Loaded Into Memory

A B C

Parent

Children

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Group Exercise“About that ‘Graph’ output… what if my measurements output different data types?”

Configure Acquire Measure

Boolean

TempConfigure Acquire Measure

Graph

Strain

XDynamically dispatched VIs must have the same connector pane. You cannot

have a different data type output on ‘Measure.’ So how do we solve this problem?

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Q: How can an instance of the measure method return information that is appropriate and specific to the measurement class it belongs to?

Measure

Consider the following requirements:

• We have to pass different data-types at-run time out of the measure method

• We always know a specific measurement will return a certain type of data

• We need to be able to display that data to a user in an appropriate format

• We need to pass the data back to a framework that implements a pre-defined interface for operations like ‘dispaly,’ ‘save,’ etc…

Is a variant a valid solution? • We can pass any data-type on a variant wire• How do we know how to display the data on a variant wire to a

user?• If we pass the variant to our calling code, how will it know how to

save it to disk or display it to the user?

?

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Use a Class Hierarchy

Measurement Result

Strain Result Temp Result Resistance Result

Defines methods all results should be able to define, such as ‘Save,’ or ‘Display’

Each has a unique private data cluster to store the result of a measurement and defines how that data is stored or displayed using dynamically dispatched methods that override the interface defined by the parent ‘Measurement Result’

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General Best-Practice: Don’t Use Variants

Variants are typically used when different types of data have to travel down a single wire. Anytime you feel the need to do this, consider replacing the wire that would be a variant with a class hierarchy.

Why? Classes still enforce strict data-types at edit-time, thereby ensuring no run-time errors. Variants do not, and therefore increase the likelihood of run-time errors.

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Configure Acquire Measure

But I’d like to..• run the same measurement class on different

machines, which have different hardware• continue development on a machine with no

access to hardware• be able to add a new device of a certain type

without modifying the measurement

All measurements will need to use hardware.

Measurement Class

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Configure Acquire Measure

A measurement is defined assuming certain classes of devices are available, but without knowing exactly which instrument.

Temp

DMM Class

In this example, this measurement strategy uses a DMM. It assumes an instance of a DMM implements a pre-defined interface, without knowledge of how that interface is implemented

ConfigureCurrent Source

Autozero Read

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DMM Class

Agilent 34401a

PXI-4070

Simulated DMM

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Configure Acquire Measure

Other measurements may require different classes of hardware, or perhaps multiple devices (ie: stimulus/response measurements), but we can’t change the connector pane of methods we want to override (like Acquire.vi).

Measurement Class

The objects passed along this wire must all be children of the same parent class?

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Configure Acquire Measure

All measurements use an array of hardware

Measurement Class

But different classes of hardware would definitely not implement the same interface

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Sample Hardware Class Hierarchy

Hardware

ScopePower Supply

DMM Generator

Simulated

34401aPXI-4110

PXIe-5185

PXI 4070Simulate

dSimulate

dSimulate

d

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Configure Acquire Measure

Methods can cast hardware objects to specific children at edit-time using the ‘to more specific’ primitive

Measurement Class

DMM

SCOPE

The dark blue wire can be passed into the interfaces for the specific device

classes

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DemonstrationUse these concepts in a real system

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Summary of Most Important Concepts• Always be thinking about data scope – keep it cohesive and small

• Classes create define data scope and a set of functions that are allowed access to data

• Consider using a class hierarchy to replace a massive data structure

• Dynamic dispatch allows child classes to override a parent’s method and reuse others

• Dynamic dispatch occurs at run-time, whereas polymorphism occurs at edit-time

• Use parent classes to define the interfaces children should implement

• If you find yourself using a lot of variants, consider a class hierarchy

• And finally, classes are not as big of a leap as you might think – we hope you agree after this presentation!

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