How to Integrate a PLC with LabVIEW and TestStand for Production Test

In the blog post "DV vs. PV," we talked about the shift from flexible lab setups to robust, automated production test systems. A critical component in many of these systems is a Programmable Logic Controller (PLC), which handles the physical automation of the test fixture things like opening and closing clamps, activating pneumatic presses, or turning on lights. While LabVIEW and TestStand are excellent for the test sequence and data acquisition, they often need to communicate with a PLC to control the real-world actions of the test fixture.

This is the intention behind this post: to show you how to seamlessly integrate a PLC into your test architecture using National Instruments (NI) software. We'll explore why this is so important and how you can make it happen.

The Role of the PLC in Production Validation

A PLC is a rugged, industrial computer designed for control and automation. In a production test environment, the PLC is the workhorse that manages the physical test fixture.

• Reliability: PLCs are built to be extremely reliable and operate in harsh industrial environments. They handle the low-level, high-speed I/O of the fixture without interruption.

• Safety: PLCs are often the first line of defense for safety, ensuring that a test fixture's clamps won't close on an operator's hand or that a high-voltage test doesn't activate at the wrong time.

• Physical Control: The PLC directly controls motors, pneumatic valves, sensors, and other hardware components that make up the test station. It takes the "Test Fixtures" concept from our previous article and brings it to life.
  

Integrating the PLC: A Two-Part System

A modern production test system is not a single entity; it's a collaborative effort between the PLC and the test software.

1. The PLC's Job: The PLC manages all physical I/O on the fixture. Its program, often written in ladder logic, is responsible for the state of the fixture itself. For example, it confirms that a part is in place, commands the clamps to close, and verifies they are fully sealed.

2. The Test Software's Job: LabVIEW or TestStand, running on a separate computer, acts as the "brain." It is responsible for the overall test sequence. It communicates with the PLC to send commands ("Close clamps," "Enable power"), reads measurement data from instruments, analyzes the data, and logs the results.
   

Communication: The Bridge Between PLC and PC

For this two-part system to work, the PLC and the PC running LabVIEW or TestStand need to communicate. NI software provides several ways to do this, but one of the most common and robust methods is through industrial communication protocols.

A great example is using a communication standard like Ethernet/IP. Both the PLC and the PC can be configured to "talk" to each other over this protocol. A LabVIEW program or a TestStand test step can send a command to the PLC's memory address, and the PLC can respond with a status update, such as "Clamps are closed."

Why This Integration Is So Powerful

Separating the PLC's control over the fixture from the PC's control over the test sequence and data is a best practice in production automation.

• Simplified Debugging: If the test sequence fails, you know the issue is likely in the software or instrumentation. If the fixture itself doesn't move correctly, you can troubleshoot the PLC independently.

• Increased Reliability: The PLC can continue to manage safety and critical fixture states even if the PC-based test sequence crashes.

• Faster Development: You can develop the PLC code and the LabVIEW/TestStand sequence in parallel, speeding up your time to market.
  

This architecture is a perfect example of how the "Test Software" and "Test Fixtures" mentioned in our previous article come together to create a reliable and efficient Production Validation system.