There are three notable ways to assemble a functional test system:
- Buy a complete custom solution, which involves significant engineering cost and presents a support challenge.
- Buy an off-the-shelf application-centered system that’s specific to one application and typically can’t be used in other systems, lacks flexibility, and often relies on proprietary software.
- Build a standardized core based on an open test platform (OTP).
An OTP system leverages a standard core built with the components that provide the basic functionality required by any test system, and then adds the components unique to the specific test system being designed. LXI-compliant (LAN Extensions for Instrumentation) components are particularly well suited for this type of application because they’re easy to build and operate in hybrid systems.
THE OTP APPROACH
The example system uses a multifunction switch/measure unit with other instruments to create a standardized core, otherwise referred to as an OTP, to build test systems (see the figure). The OTP provides 70% of a system’s total functionality; the remaining 30%, which is unique to each system, is supplied by the custom building blocks and subsystems added to the OTP.
Every functional test system includes three basic capabilities: sourcing, measuring, and switching. Sourcing provides power or stimulus to the device under test (DUT). Measuring parameters (for example, a voltage or a reaction to a stimulus) verifies that the DUT is functioning properly. Switching refers to moving the power and stimulus signals to the next test point, or switching to the next measurement setup on the same test point. The system also needs a power supply to power the DUT.
To build a cost-effective system architecture that meets its customers’ needs, the equipment and software must be flexible and easy to use. This system’s standardized core consists of a multifunction switch/measure unit with a digital multimeter (DMM) and at least one switch matrix module to build a functional test system for industrial micro programmable logic controllers (PLCs). The multifunction switch/measure unit is a versatile system core for the OTP and a solid base for integrating additional instruments and features to meet application-specific requirements.
The system software foundation can be provided by any number of software environments. For example, the Agilent TestExec SL test executive or National Instruments’ TestStand, when coupled with the OTP libraries provided by the systems integrator, create a flexible, reliable, cost-effective, and ready-to-run environment.
FUNCTIONAL TEST REQUIREMENTS FOR TESTING A MICRO PLC
Micro PLCs are small, low-cost devices used in industrial automation applications to control simple machinery or to power individual instruments in a timed sequence. A micro PLC features a set of digital and analog inputs and digital outputs. An internal controller runs an industrial-control application that’s downloaded to nonvolatile memory.
In a standard PLC, the software application creates a state machine that’s executed in real time. The micro PLC is a subset of the standard PLC that provides more I/O ports, extended software features, Fieldbus support, and motion controllers. Typically, they’re used in smaller automation applications that previously used complicated relay setups.
Testing a micro PLC in a manufacturing environment requires the test system to:
- Provide power to the PLC, measure inrush current, and ensure there are no shorts.
- Download a test application to the PLC.
- Test analog and digital inputs.
- Test digital or relay outputs at nominal load (repair and QA test scenarios would also require test under different load conditions and at different temperature settings).
- Verify operator interface (test pushbuttons and verify LCD display operation).
- Test switch outputs under load.
- Download PLC delivery software and data prior to packaging and shipment (optional).
A standalone PC is used because it costs less and is easier to upgrade than an embedded PC. In a micro PLC test system, the PC controller runs the OTP software framework and controls all of the instrumentation. LXI-compliant devices effectively operate in hybrid systems, so it works perfectly with the balance of the system, whether LXI or not. A camera, controlled via the PC’s USB port, is installed with the oscilloscope to provide an automated evaluation of the DUT. The PC’s digital I/O plug-in board controls the pneumatic vents used for adapter automation and other auxiliary functions in the test fixture. This plug-in board separates automation functionality from test and measurement functionality.
The basic core can be configured for many different applications and changed or upgraded as required.
Using a modular structure makes it easy to upgrade the system and adapt it to new applications. There are numerous plug-in modules for almost any desired functionality. An LXI-compliant multifunction switch/measure unit with a compact form factor and transaction speed compares favorably to other switch systems and is less than VXI and PXI systems. Its architecture eliminates external signal conditioning and allows the use of robust, reliable cabling.
Front-panel access to all module functions makes maintenance and debugging easier, because operators don’t have to interact with the PC to verify functionality. When the test system is connected to a LAN, the integrator can use the LXI device’s built-in Web server for remote operation and maintenance. The Web server functionality makes it possible to control and supervise switch status using a GUI. The only required software on the system controller is a Web browser. The graphical representation of the switches helps operators visualize the state of individual relays and outputs connected to terminal blocks and routed to the internal DMM.
ADDITIONAL MODULES TO THE CORE SYSTEM OTP
- Dual 16 x 4 armature matrix, 300 V/1 A, 30 MHz. The first 16 channels of the first matrix are instrumentation channels; the others are DUT channels.
- 28-channel Form C (1 A) and 4-channel Form A (5 A) switch module, 300 V. The Form C and Form A channels switch PLC loads and provide switching resources for auxiliary hardware residing in the test fixture.
- 64-bit digital I/O, programmable thresholds with programmable polarity and pattern memory enable the stimulus of the PLC’s digital inputs and provide programming control over the digital outputs.
- 4-channel isolated digital-to-analog converter ±16 V/±20 mA, 16-bit resolution, 200-kHz update rate, 500-kpoint waveform memory that stimulates the DUT’s analog inputs with static voltages, as well as with defined signal slopes stored in the waveform memory.
The mainframe provides 100-W power-supply modules with four independent and fully programmable voltages and currents.
- Compact form factor (1U) with effective airflow design for cooling.
- Ethernet/LAN (LXI-C), USB, and GPIB interfaces standard.
- High data throughput on all interfaces and less than 1-ms command processing time over all interfaces.
In the micro PLC system, the power supply provides the following voltage sources:
- DUT supply power.
- Reference voltage for open collector digital outputs.
- Adapter auxiliary supply for sensors and USB camera.
Power supplies (750- and 1500-W versions) with models ranging from 6 to 600 V, providing maximum current at the voltage level needed.
- Compact form factor with effective airflow design for cooling
- Low cost
- Ethernet/LAN (LXI-C), USB and GPIB interfaces standard
In a micro-PLC application, the power supply is used to test the PLC’s switch outputs under high load current.
The OTP architecture and software support other instruments. For example, a function/arbitrary waveform generator, a universal counter, or a mixed signal oscilloscope can be used, which means the core of this system can be used in applications over and above the micro PLC application.
OTP PROGRAMMING INTERFACE STANDARDS
Ethernet/LAN is inexpensive and easy to implement, provides fast throughput, and allows remote access to the system. LXI Class B and Class A add trigger and synchronization functionality to the LAN standard, making it even more suited for instrument control. The OTP architecture uses LXI-based instrumentation wherever possible so that all core instruments are controlled through Ethernet/LAN. Additional instruments may be programmed using LAN, GPIB, USB, or RS-232 interfaces.
I/O libraries that typically ship with LXI devices provide one user interface to access all standard PC interfaces such as Ethernet/LAN, USB, or RS-232, as well as test-and-measurement interfaces like LXI, GPIB, or VXI. These libraries provide interface debugging tools and the flexibility to use either drivers or native instrument commands for controlling each instrument.
When specifying components for a core system, the fixture interfaces are critical. Fixture interfaces should be specified for more than 20,000 fixture changes without maintenance.
OPEN SYSTEM SOFTWARE
Often more than 50% of the cost of implementing a test system is related to software engineering. One key to reducing software development costs is to use commercially available test sequencers coupled with a modular software approach that maximizes code reuse. A successful software strategy includes:
- Choosing programming languages and instruments designed to be “open.”
- Using commercial “off-the-shelf” applications for the intended software environment.
- Using defined software layers APIs to provide docking points for customized functions.
- Reusing instrument code via test step libraries.
- Fast test-sequence execution that meets real-world requirements, including comprehensive user interfaces, reporting functionality, and integration into automated-manufacturing environments.
The test executive includes a runtime engine that interprets and executes test sequences, evaluates test results against the limits defined in the test sequence, and provides APIs for control by the operator interface at runtime. The API also supplies status information to the operator interface, as well as a test sequence development environment.
Test sequences define the individual tests used to evaluate a DUT and include test parameters, limits, and the order of execution of test steps. The integrated test-sequence development environment allows test engineers to focus on defining test-sequence content and test methodologies rather than developing instrument-control code, data handling, and other low-level programming tasks.
In a micro-PLC system, switch handlers control complex topologies that are defined by a switching topology editor. The user defines the switch route to be used for a given test. At runtime, the software closes all relays on the switch path prior to execution of the test and opens them after execution of the test. OTP-based systems are delivered with switch control software and predefined switch topologies to make it easier to develop test sequences.
Instrument drivers encapsulate instrument complexity and provide programmatic access to the instruments without having to learn the underlying SCPI syntax.
DATA REPORTING AND ARCHIVING
The OTP software architecture separates data reporting and archiving from the individual test sequences and utilizes the APIs provided by the test sequencer. The test executive takes care of collecting data, displaying results, and storing them. The OTP approach enforces consistent data handling for all sequences and helps standardize test data formats throughout the test floor.
OPERATING SYSTEM AND INTERFACE LIBRARY
The OTP software foundation is built on Microsoft Windows. All instruments are programmed using the VISA interface driver framework, which also supports interfaces such as FireWire (VXI), USB, RS-232, and GPIB (IEEE 488.2). The standard system core can be extended to provide test capability for a variety of other industries and applications. This core could work with other applications as well:
- RF and microwave measurements and switching (aerospace/defense, telecommunications, automotive)
- Serial communication interfaces (industrial automation, automotive)
- Higher voltages and currents (white goods, automotive, and industrial automation industries)
- Optical inspection and mechanical stimulus (applications in all industries)
This system used a multifunction switch/measure unit and open software standards to create a standardized open test platform that made it easier to develop and support custom functional test systems. Whether developing a test system in house or using a system integrator, the basic principles outlined here can be used to create a test system that meets a range of needs.