Introduction to OOP for LabVIEW Programmers

8/10/2019 Introduction to OOP for LabVIEW Programmers

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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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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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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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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


Task Samples

Clock Raw Data

Graph

DAQ Task

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



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Measurement System


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 Data

Task

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 Data

Task

Configure

I/O

Cloc

k

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 Data

TaskConfigure

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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I/O

Samples

Clock

Raw Data

Task

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

Cloc

k

Raw Data

Task

Measurement

Class=

Measurement

Data A class contains data, plus

methods (VIs) that are allowed

to act upon and modify the data.

VIs that do not belong to the class

cannot act upon the data

A class is basically a cluster



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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 directlybundle and/or unbundle the data



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I/O

Samples

Cloc

k

Raw Data

Task

Class Data Cluster


Methods that can act upon

the class’s data cluster

Appearance 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 Data

TaskConfigure

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


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


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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Graph


Graph

I/O

Samples

Cloc

k

Raw Data

Task

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 ofintroducing a new measurement in the non-OOP example.


Graph

I/O

Samples

Clo

ck

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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Graph

I/O

Samples

Clock

RawDataTask

Measurement Type

? ? ?

As the scope of the data cluster expands, we arepassing 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 requireschanges 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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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 wirecannot be changed at run-time, as the connector pane must match exactly.



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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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The Basics of an Object Factory

AB

C Generic Measurement

Location on Disk

Where 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?”


Boolean

Temp Configure 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 auser 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 theneed 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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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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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

Configure

CurrentSource

Autozero Read





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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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All measurements use an array of hardwareMeasurement

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-5185PXI 4070Simulated Simulated Simulated



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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 theinterfaces 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 andsmall

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

• Consider using a class hierarchy to replace a massive datastructure

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

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

• Use parent classes to define the interfaces children shouldimplement

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

• 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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Introduction to OOP for LabVIEW Programmers