Vision, communication, and control

Aug. 1, 2009
Several vision communications and control tools are improving connections to operator interfaces, PLCs, and automation equipment for faster and more accurate inspection.

Communication is key in relationships — and that includes those between the operator of an inspection system, its process controls, and the vision equipment used to inspect that process. New vision-software advancements are improving the integration through intuitive control interfaces and more efficient closed-loop information exchanges.

Human-machine interface

Designing an inspection process is possible with some vision software interfaces. Here, newer configurable controls allow custom creation of control panels with image displays and data charts to match those of the application at hand. Inspection results display in realtime.

Figure 1

Data transmission

Engineers can then quickly build human-machine interfaces by linking data from attached vision devices (for example, smart cameras that perform both image acquisition and processing) to the software running on a PC or other machine controller. In this way, operators monitor and control inspections with a panel catered to specific application needs, and only necessary process elements are displayed and available for modification.

Vision systems must also take process information and send it to controls and other automated equipment — a capability that has become increasingly important. Traditionally, vision systems only activated discrete outputs upon “seeing” defective parts. However, as resolution and speed have increased, many vision systems are now used for quantitative or dimensional measurements; these supply numerical data as well as discrete I/O.

Common control protocols, such as Modbus (serial or Ethernet) and Ethernet I/P can transmit this data to control systems. However, much automation equipment and many manufacturers work off different protocols. For this reason, many vision systems communicate in multiple languages and share process information with several equipment varieties. To this end, Ethernet is one of the most common and preferred communications protocols. However, not every processor has an Ethernet port — and even if one does, it may not support the required protocol.

One alternative is the use of serial ports. Nearly every PLC, industrial computer, scale, HMI, and legacy device has at least one standard serial port, and users can leverage this feature to facilitate communications using some of the latest protocols.

For example, a vision-based positioning application may employ a smart camera mounted to a laterally variable extruder head to obtain images and identify a dark line on a continuous white web. The camera then sends positional data to a PLC, proportional to the deviation of the line from the center of the camera's field of view or FOV. The PLC calculates a correction signal to the motor driving the extruder head, creating a true closed-loop system by keeping the extruder precisely centered over the line as it moves from side to side.

Another example: Imagine a seam tracking application. To ensure that the sewing machine stitches the correct product in the proper location, a smart camera tracks a seam as it is sewn and communicates position data to a PLC, which in turn conveys messages to the machine to adjust its position when necessary.

In this example, setting up the actual vision application is relatively simple. The trouble is finding an efficient, reliable way to transmit camera-collected data to the PLC using Ethernet protocols. One common method is effective but inconvenient: 12 discrete outputs from the vision camera form a 12-bit word. This passes to a digital-to-analog converter or DAC, and then the output is sent to an analog input card in the PLC. Finally, the processor scales the analog input to the required range.

Figure 2

What is the main disadvantage? Best resolution is limited by the number of available camera outputs — 12. (Resolution in this case is not camera system resolution — which is the FOV divided by pixel width — but rather, the smallest increment of information that can be communicated to the PLC with this arrangement.) To illustrate, here resolution for a FOV of 4 in. is that divided by 212 — 4 in./4,096, or about 1 mil — 0.001 in. This is acceptable for some but not all applications.

In contrast, 16-bit systems yield 0.0625-mil resolution, which is 16 times more precise. These also eliminate the DAC and free an analog input for a simpler and more dependable system.

How does it work? To facilitate communications through a PLC that allows reads and writes via an unused serial port, a serial communication tool (sometimes part of vision-programming software) is dropped into the PLC logic. From there, operators simply link the required data so that it can be sent out via the serial port, and adjust port parameters so that the data is formatted in a way that allows the PLC to decode the message.

Indeed, serial communication is generally regarded as too slow for some applications. However, with modern Universal Asynchronous Receiver Transmitter or UART chips, today's speeds can reach 115,200 baud and beyond — in the sub-millisecond range for small units of data. Because the fastest vision inspections take a few milliseconds to complete, this is easily fast enough for most applications, and reliable enough for closed-loop systems.

Some vision software also uses the Microsoft ActiveX platform for greater application flexibility. This feature allows operators to easily integrate machine vision inspections into process control systems. Here, control systems can load vision programs, put a smart camera online, monitor inspection data, and then act on inspection results by controlling pass and fail outputs. The interaction may include archiving images and analyzing inspection results for traceability and quality purposes.

In addition, the interface allows operators to write programs in the Visual Basic development system or other Microsoft programming languages to supply data directly from a smart camera to other devices, such as quality-control monitoring systems, advantageous where a PC controls the automation process.

An interface utilizing the ActiveX platform also allows programmers to link a PC to the process data collected by a smart camera. The PC is then able to store the information so that it can be displayed, manipulated, or transferred as needed.

Using this same interface, PC-to-camera communication is accomplished through industry-standard Web-service protocols. These protocols provide robust communications without locking up application programs if the signal is lost — for example, if a network disruption occurs in the field or during a power cycle.

Setup-saavy software

Utilizing the ActiveX platform, vision software can monitor communications and reestablish failed communications without disturbing other automation equipment. Once this occurs, if the camera has returned online but has been rebooted, the operator's program is reestablished. These “lost” and “found” events are also sent to the application program, so that all equipment is kept in the loop about these occurrences.

Network and connectivity news

Serial connection: Faster and USB ready

ActiveX and Visual Basic are registered trademarks of Microsoft Corp. For more information on the topics covered in this article, call (952) 996-9500 or visit Advanced Industry Support Inc. is online at

CiA promotes CANopen over Bluetooth

Some vision software includes setup tools to help engineers design vision inspection, guiding them through common vision tasks. Here, Impact Software Suite 8.3 with smart-camera technology simplifies inspection setup for machine vision. The software includes a program manager with more than 120 tools including code reading, pattern find, blob analysis, circle gauge, line find, and subpixel gauging. A preconfigured operator runtime display panel is built within the manager so information such as the inspection date and pass/fail results are displayed while the camera inspects. Even users with minimal programming experience can efficiently create inspection designs.

Now new chips are making serial communications faster and more connected than ever. Last month, Exar Corp., Fremont, Calif., released UARTs XR21V1410, 12, and 14, with a 12-Mbps serial data rate. They are suitable for embedded automation systems and other handheld systems. The units have a USB-2.0 slave interface and large FIFO sizes (to optimize overall data throughput) and a multi-drop mode. Exar also manufactures serial transceivers (RS-232, 485, multi-protocol) wireless chipsets, and UART-transceiver combinations.

The CAN in Automation (CiA) international user and manufacturer group has now established CANopen via Bluetooth. In their inaugural July meeting, experts agreed to map CANopen communications (such as service and process data objects) to lower Bluetooth layers. “The scope is not limited to Bluetooth. Other wireless communication technologies are welcome, too,” says Reiner Zitzmann of CiA. Harm-Peter Krause, CANopen via Bluetooth chairman, invites all interested parties to join in the standardization. For more information, visit

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