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NI LabVIEW Makes a SCADA System for Vacuum Deposition

Author(s):

N. D. "Buck" Smith, Cal-Bay Systems, Inc.

Industry:

Industrial Controls/ Devices/ Systems

Product:

LabVIEW

The Challenge:

Creating a process-control system for a new piece of production equipment that applies coatings to plastic films via vacuum deposition.

The Solution:

Using National Instruments LabVIEW and the Enterprise Connectivity Toolkit to create a system that applies four layers to a roll of plastic film in one pass through the coater, twice the capacity of the existing coater.


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Comparison of Time and Position Logging

The Need for Full-Featured SCADA Capability
The new equipment applied four layers to a roll film in one pass through the coater, twice the capacity of the existing coater. The existing coater was known as NV1; the new four-layer coater was designated NV2. The existing coater used a simple NI LabVIEW application to log process data and provide some limited supervisory control and data acquisition (SCADA) capability. The new coater, NV2, needed to have full-featured SCADA capability with a higher level of automation.

For NV2, several improvements over NV1 were required. These improvements were:

  1. Increased digital control of system settings
  2. Trend logging and display by position instead of time
  3. Ability to save, recall, edit, and run product recipes
  4. Integration of vacuum-control and deposition-control data into one SCADA system

Digital Control
A key improvement was more digital control of more process variables. We purchased mass flow controllers (MFCs) and power supplies with PROFIBUS interfaces. LabVIEW drivers for the power supplies and MFCs were developed and debugged. As a result, we achieved digital control of process set points and digital acquisition of measured data.

Based on our experience with NV1, we knew it would be important in the larger system to graph correlated process data versus length along the roll. We recorded the quality data for a section of the film some time interval after that section was exposed to the electric field created by the power supply. The time interval varied as the speed of Web movement changed.

This meant that trend data recorded versus time did not give an intuitive view of what was happening in the process. Most off-the-shelf trend logging software records the various data channels, or tags, versus time. In order to recreate trend data versus length with this data structure, the system required a series of queries and calculations including:

  1. Recording roll positions for a period of time during which the length of roll desired to view was moving through the process.
  2. Extracting times for all the roll positions for which trend data was desired. This could involve several time intervals because rolls are often processed back and forth many times to apply many coatings to a single film. Each time through the coater is designated with a pass number.
  3. Making separate queries for each tag at the times required in order to build arrays of tag data correlated by position.

With this approach, we added programming, debugging, and computational overhead with little corresponding value. The alternative approach was to log data by position. Fortunately, the LabVIEW Enterprise Connectivity Toolkit provided a powerful interface to common databases. We created database tables to record each tag data point with position and pass number. With this data structure, one query could return arrays of trend data by roll position. We selected Microsoft SQL Server as the database engine for recording data. The interactive query builder in SQL Server Enterprise Manager made building and debugging queries quite efficient.

Trend data was displayed in a VI that showed six graphs with a common x-axis. The VI displayed the data on two monitors stacked one over the other. This display gave the operator much better visualization of cause and effect in the process compared to NV1. To make data logging and display more efficient, we applied a fence-post and dead band algorithm to each tag. To improve data integrity, we developed a LabVIEW routine to automate backup of the trend data using SQL Server command line utilities.

The system facilitated switching between products through the use of product recipes. A product recipe consisted of all configuration parameters required to make a certain product. After the operator selected a product recipe, the system configured the automatically controlled components of the coater accordingly. The system then prompted the operator to configure those items that were not under machine control.

The user was able to select different control schemes for each zone using product recipes. Control schemes specified process variables and controlled outputs for the deposition process. Depending on the control scheme selected, the system dynamically loaded plug-in VIs and performed PID and other closed-loop control algorithms on parameters such as gas flow, voltage, and power.

Vacuum and Deposition Control Integration
The manufacturer of the NV2 system supplied the vacuum control system, which used Siemens PLCs to handle interlocks and pump control. As a result, the system safely achieved high vacuum conditions in the large volume required to handle rolls that were up to 54 in. wide and 24 in. in diameter. A man-machine interface (MMI) PC provided a graphic display of the vacuum control status. The vacuum control system supplied key process data, including Web position and speed. The MMI PC made this data available via the Windows DDE protocol. After some testing, a reliable data interface was created with a LabVIEW shim application that ran on the MMI PC. This LabVIEW application acquired data via DDE and used networked file transfer to pass this data to the SCADA PCs.

LabVIEW is a powerful tool for SCADA systems. It incorporated many data acquisition and control devices into a single, integrated user interface. Using the NV2 SCADA system, a single operator could control four coating processes with increased precision and repeatability.

For more information, contact:
N.D. “Buck” Smith
Principal
Cal-Bay Systems, Inc.
Tel (415) 258-9400
Fax (415) 258-9288
E-mail: bsmith@calbay.com