Production Control and Quality Assurance Using BridgeVIEW and DAQ

Author(s):
Matthew Twomey - University of Washington

Industry:
University/Education

Products:
LabVIEW,

The Challenge:
Economically controlling multistage production with a quality assurance test designed for use by inexperienced technicians.

The Solution:
Developing a PC-based system using National Instruments BridgeVIEW and data acquisition (DAQ) boards to control production hardware.

Introduction
At the University of Washington, as part of the Atlas Muon Detector Collaboration*, we required a production system to manufacture and test particle drift tubes. We needed a control system for this production line to move a carriage to a series of precision positions, tension a wire to a precise value, and find the tension of a wire suspended inside a sealed tube. The control software needed to manage these complex tasks while requiring little user intervention once an experienced engineer had set up and calibrated the system.

Some of the steps required to produce a tube are not automated; the system needed to prompt the persons assembling the tube to perform the manual actions. The system needed to collect and store data during the assembly process to perform quality assurance and statistical analysis on each tube in real time and on each run of tubes at a later time.

With the BridgeVIEW built-in database, Citadel, and the use of tags, we knew that we could log data from the National Instruments PCI-MIO-16E-4 DAQ board quickly and easily. BridgeVIEW was the obvious choice for the control and interface software, and with the multifunction capabilities of the DAQ board, we found it a perfect choice for our system controller.

Production Control
We needed the system to assemble tubes to exacting tolerances of 1 or 2 mils. These tolerances, and the varying lengths of the tubes, require exact positioning. The system uses a linear actuator with a 9-ft travel to put the carriage in position; the user then follows the online instructions, hitting the return key after the completion of each step.

Before the system performs any automated step, it notifies the user, who must confirm the action. Using case statements in BridgeVIEW, we created as part of our human-machine interface (HMI) a state machine to run a sequence of events or jump to another case depending on the user’s selections.

Using a hidden string control (Sequence Name), we hook up local variables to the case input as read local indicators, while we set write local controls inside the case. We usually imbed this series of diagrams inside a while loop, so that each iteration of the while loop moves to the next state.

For the case diagrammed in the figure, the sequence of events is Introduction, Middle, and Conclusion. By using this method, we construct a series of frames that contain subVI calls to the appropriate action needed. As diagrammed, the first frame is a login Prompt, and the next frame calls a subVI to find the home switch. The program only calls the first two steps and the last step once during the execution of the program; by using the state machine, the program can skip directly to the third step and loop the series of steps in the middle, as shown in the figure.

At a certain point in the construction, the system uses a second actuator to tension a 50-micron wire stretched inside the tube, while receiving feedback from a tension meter. Once the system has tensioned the wire, it crimps the wire to the end-plug. Finally, it performs an immediate quality-assurance test to ensure that the tube passes a minimum requirement. The system compares the actual tension - measured by the tension meter - to a calculated tension based on the resonant frequency of the wire inside the tube.

In earlier product tests we used a lock-in amplifier to calculate the tension. However, lock-in amplifiers not only are expensive and relatively slow, but also cannot be controlled by computer. Using them requires an extensively trained assembly crew. We found our solution in a National Instruments DAQ board and do the following to calculate the tension:

• Run a magnetic field through the tube near its center
• Electronically drive the wire so it oscillates
• Examine the voltage on the wire from its motion in a magnetic field
• Amplify the voltage so it is in a range where the DAQ board can digitize it
  

If the two tension values agree within an allowed error, the tube passes the minimum test requirements and the system sends it on for further testing. If the tube fails this test, the production line stops and the system runs an analysis to discover why the failure occurred. In the development of this prototype production system, we examined ways to control the two actuators we needed to move the platform and tension the wire. For this simple motion application, we decided to use two stepper drivers and the DAQ board, which contained counter/timers that could control the stepper drivers, thus eliminating the need for an intelligent controller. (Intelligent controllers, such as those manufactured by National Instruments, are required when using closed-loop control for complex motion profiles.)

Using basic counter/timer functions and the digital I/O lines on the DAQ board, we found it easy to create a rudimentary control for one actuator. The ability to create a program and later call it as a subVI proved a very useful tool during development of the HMI. We used some simple analog switch integrated circuits governed by a control bit to create a multiplexer that selects which of the two actuators to use.

Integration for the Future
The control system we designed is inexpensive and expandable. With BridgeVIEW, it is easy to access new hardware and program new functional modules. Production time for this control system was much less than that of conventional programming methods because National Instruments software and hardware are so closely integrated.

Because unskilled users must operate this system, changes to the user interface may need to happen rapidly as the system grows. With BridgeVIEW, the programmer can quickl customize the user interface, while the BridgeVIEW security features ensure that only users with the correct access level can make such changes.
The BridgeVIEW development system lends itself to unique solutions such as our resonant frequency test system. Converting a mechanical system to a virtual system quickly saves time and money. Instead of purchasing an intelligent controller for stepper motors and a lock-in amplifier to calculate tension, we needed to buy only one DAQ board, which meant saving thousands of dollars. These monetary savings are impressive and the virtual system also improves accuracy, performance, and functionality. In fact, the virtual production system has many more features that we plan to use in the future.

For more information, contact:

Matthew S. Twomey

University of Washington Physics Dept.

Physics/Astronomy Bldg.

Box 351560

Seattle, WA 98195-1560

Tel: (206) 543-6113

Fax: (206) 543-1104

E-mail: saukeye@u.washington.edu

* Supported in part by NSF Grant PHY-9722468

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