Integrating Measurements with Vision and Motion

Many of today's fast-growing industries, such as optoelectronics, semiconductor, and life sciences, require a combination of motion, vision, and measurements to meet system requirements. By integrating measurement and automation devices with platforms such as LabVIEW and PXI, you can reliably pass data between image acquisition, data acquisition, and motion control devices. RTSI technology, which is embedded in the PXI backplane, is the key to achieving total integration between your devices.

RTSI Technology

RTSI stands for Real-Time System Integration bus, a dedicated high-speed digital bus designed to facilitate systems integration by employing high-speed and real-time communication between National Instruments devices. Using RTSI, data acquisition boards can share high-speed digital signals with motion control, image acquisition, or digital I/O boards with no external cabling and without consuming bandwidth on the host bus, an advantage when several devices are competing for PCI bandwidth. The RTSI bus also has built-in switching, so you can route signals to and from the bus instantly through software. RTSI is available on PCI boards through an internal 34-pin connector; signals are shared via a ribbon cable inside the PC enclosure. RTSI cables are available for chaining two, three, four, or five boards together.

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PXI Technology

PXI modules require no cabling for synchronization because the backplane handles all RTSI functions. Electrically, PXI adds numerous instrumentation extensions to the CompactPCI specification to improve timing and control of instruments and signals. The first addition is a system clock, which is simply a 10 MHz reference clock distributed to each device slot through equal-length traces. Multiple boards can use this common timebase for synchronization of events. This is very useful for high channel-count applications, such as noise and vibration analysis. The next enhancement is an eight-line trigger bus. These trigger lines link all PXI slots in a bus segment so multiple devices can interact and control each other's events through hardware. Also, PXI defines a special star trigger slot, which provides an independent dedicated line for each of the 13 device slots on a single backplane. When a star trigger controller is plugged into this slot, it can control, monitor, or route triggers among peripheral slots with very low skew (within 1 ns). The final electrical extension is a local bus. It is defined to the right and left of each slot and is composed of 13 lines available to each device slot for private communication between adjacent slots. Pairs of peripherals can pass analog or digital signals back and forth using the local bus.

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Learn more about PXI.

Synchronization Considerations

The star trigger is a high-performance trigger signal that can synchronize all the modules in a chassis. You can also synchronize the modules using the normal PXI trigger bus, but the star trigger offers increased performance, specifically a propagation delay of no more than 5 ns and an inter-device delay of no more than 1 ns. Slot 2 is dedicated for the star trigger controller. The other devices can access the trigger signal generated by the slot 2 controller through the backplane. If the star trigger is not required, you can use slot 2 as a standard device slot. The standard PXI trigger bus can perform the same functionality from any device slot on the backplane, but the latency will be higher.

There are other considerations when using RTSI lines for synchronization on a PCI device. A clock pulse signal, such as a DAQ scan clock, will be immediately seen on the slave board (minus a small delay because of propagation across a 2" piece of wire). Shared triggers appear on RTSI lines immediately; however, if the pulse is used to trigger an acquisition on another device, there might be a small delay on the order of 2 timebase clock cycles after the trigger pulse is received.

Application of Synchronization with Measurements, Vision, and Motion

A perfect example of synchronization is aligning optoelectronic devices such as an optical fiber to a transmitter. In order to maximize the amount of light and thus energy passed from the optical fiber to the component, nanometer accuracy is required. A vision system in conjunction with motion control is typically used to perform a coarse alignment, a general alignment which has accuracy in the micron range. A more precise alignment in the nanometer range is then achieved by using data acquisition together with motion control. A common technique uses the scan clock from the DAQ device to perform a high-speed capture of the position on the motion controller. Both of these signals are available through the API for synchronization purposes. This technique acquires optical-power measurements at a rate determined by the scan clock on a DAQ device and simultaneously collects position data from the encoders on the stage. This is called time-based position measurements and is much more accurate and faster than using a traditional move, stop, and measure approach.


Another example of synchronization involves characterizing bacterial colonies. Scientists typically test new compounds in hundreds of reagents to determine the properties of a compound. By synchronizing motion and vision, scientists can automate the process of inspecting the hundreds of wells that contain bacterial colonies. DAQ triggers a fluorescence light to illuminate specimens. Fluorescence shows which bacterial colonies have "expressed" the desired properties. Motion control then moves an x-y stage in a predetermined raster scan and sends breakpoints to the vision system. Images are acquired by an asynchronous reset camera at distinct points and the image processing is performed to characterize each colony.

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The 1, 2, and 3 of Light Measurements with DAQ

Conclusion

Integration and measurement synchronization are key components in many large machines. Many engineers and scientists assume that simply connecting one component to another will meet specifications. Ideally, this should be the case, but with so many different components from many different vendors, integration becomes a much more important issue. Although most applications require, at minimum, a basic level of integration such as connectivity, more and more systems require high-speed and real-time synchronization found using NI devices with RTSI and PXI technology.

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