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.
SOFTWARE FOUNDATION
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).
HARDWARE ARCHITECTURE
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.
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