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National Instruments LabVIEW: A Programming Environment for Laboratory Automation and Measurement .
National Instruments LabVIEW is a graphical programming language that has its roots in automation control and data acquisition. Its graphical representation, similar to a process flow diagram, was created to provide an intuitive programming environment for scientists and engineers. The language has matured over the last 20 years to become a general purpose programming environment. LabVIEW has several key features which make it a good choice in an automation environment. These include simple network communication, turnkey implementation of common communication protocols (RS232, GPIB, etc.), powerful toolsets for process control and data fitting, fast and easy user interface construction, and an efficient code execution environment. We discuss the merits of the language and provide an example application suite written in-house which is used in integrating and controlling automation platforms.
Keywords: NI LabVIEW; graphical programming; system integration; instrument control; component based architecture; robotics; automation; static scheduling; dynamic scheduling; database Introduction
Cytokinetics is a biopharmaceutical company focused on the discovery of small molecule therapeutics that target the cytoskeleton. Since inception we have developed a robust technology infrastructure to support our drug discovery efforts. The infrastructure provides capacity to screen millions of compounds per year in tests ranging from multiprotein biochemical assays that mimic biological function to automated image-based cellular assays with phenotypic readouts. The requirements for processing these numbers and diversity of assays have mandated deployment of multiple integrated automation systems. For example, we have several platforms for biochemical screening, systems for live cell processing, automated microscopy systems, and an automated compound storage and retrieval system. Each in-house integrated system is designed around a robotic arm and contains an optimal set of plate-processing peripherals (such as pipetting devices, plate readers, and carousels) depending on its intended range of use. To create the most flexible, high performance, and cost-effective systems, we have taken the approach of building our own systems in-house. This has given us the ability to integrate the most appropriate hardware and software solutions regardless of whether they are purchased from a vendor or engineered de novo, and hence we can rapidly modify systems as assay requirements change.
To maximize platform consistency and modularity, each of our 10 automated platforms is controlled by a common, distributed application suite that we developed using National Instruments (NI) LabVIEW. This application suite described in detail below, enables our end users to create and manage their own process models (assay
scripts) in a common modeling environment, to use these process models on any automation system with the required devices, and allows easy and rapid device reconfiguration. The platform is supported by a central Oracle database and can run either statically or dynamically scheduled processes. NI LabVIEW Background
LabVIEW, which stands for Laboratory Virtual Instrumentation Engineering Workbench is a graphical programming language first released in 1986 by National Instruments (Austin, TX). LabVIEW implements a dataflow paradigm in which the code is not written, but rather drawn or represented graphically similar to a flowchart diagram Program execution follows connector wires linking processing nodes together. Each function or routine is stored as a virtual instrument (VI) having three main components: the front panel which is essentially a form containing inputs and controls and can be displayed at run time, a block diagram where the code is edited and represented graphically, and a connector pane which serves as an interface to the VI when it is imbedded as a sub-VI.
The top panel (A) shows the front panel of the VI. Input data are passed through “Controls” which are shown to the left. Included here are number inputs, a file path box, and a general error propagation cluster. When the VI runs, the “Indicator” outputs on the right of the panel are populated with output data. In this example, data include numbers (both as scalar and array), a graph, and the output of the error cluster. In the bottom panel (B) the block diagram for the VI is shown. The outer case structure executes in the “No Error” case (VIs can make internal errors or if called as a sub-VI the caller may propagate an error through the connector pane).Unlike most programming languages, LabVIEW compiles code as it is created thereby providing immediate syntactic and semantic feedback and reducing the time required for development and testing.2 Writing code is as simple as dragging and dropping
functions or VIs from a functions palette onto the block diagram within process structures (such as For Loops, or Case Structures) and wiring terminals (passing input values, or references). Unit testing is simplified because each function is separately encapsulated; input values can be set directly on the front panel without having to test the containing module or create a separate test harness. The functions that generate data take care of managing the storage for the data.
NI LabVIEW supports multithreaded application design and executes code in an inherently parallel rather than sequential manner; as soon as a function or sub-VI receives all of its required inputs, it can begin execution. In Figure 1b, all the sub-VIs receive the array input simultaneously as soon as the For Loop is complete, and thus they execute in parallel. This is unique from a typical text-based environment where the control flows line by line within a function. When sequential execution is required, control flow can be enforced by use of structures such as Sequences, Events, or by chaining sub-VIs where output data from one VI is passed to the input of the next VI.
Similar to most programming languages, LabVIEW supports all common data types such as integers, floats, strings, and clusters (structures) and can readily interface with external libraries, ActiveX components, and .NET framework. As shown in Figure 1b, each data type is graphically represented by wires of different colors and thickness. LabVIEW also supports common configuration management applications such as Visual SourceSafe making multideveloper projects reasonable to manage.Applications may be compiled as executables or as Dynamic Link Libraries (DLLs) that execute using a run-time engine similar to the Java Runtime Environment. The development environment provides a variety of debugging tools such as break-points, trace (trace), and single-step. Applications can be developed using a variety of design patterns such as Client-Server, Consumer-Producer, and