Customer Solutions

PXI-Based Test System Performs Production Testing of Cam-Position Sensors

Author(s):

Gary Sweezy, MicroCraft Corporation

Industry:

Automotive

Product:

Data Acquisition, GPIB & Instrument Control, High-Speed Digital I/O, LabVIEW, Modular Instruments, PXI/CompactPCI, Serial

The Challenge:

Developing a flexible, off-the-shelf production test system for testing automotive cam-position sensors.

The Solution:

Building a PXI-based test system using DAQ, Digital I/O, GPIB, serial instrumentation, and LabVIEW.

Introduction
Since the 1970s, improvements in fuel efficiency have been a major objective of the automotive industry. As a result, engine design has changed to include components intended to improve engine performance and to decrease fuel consumption. One such component is cam-position sensors. Invensys Corporation in Shelby, NC, manufactures cam-position sensors that are used in diesel engines to provide feedback on piston position. This feedback is used to provide real-time optimization of engine firing and burn, hence, improving fuel efficiency.

The sensor is a high-precision, hall-effect sensor that outputs an analog pulse train corresponding to the transition pattern of a target disk mounted on the engine cam gear. The target pattern consists of metal/air transitions which determine piston position throughout each engine stroke. The output voltage and transition ratio of each sensor must be tested thoroughly to ensure that it meets or exceeds acceptable quality and performance standards.

Invensys’ prior test system incorporated custom designed circuit boards and hardware. Consequently, locating replacement parts was becoming increasingly difficult since some of the components had become obsolete. Furthermore, the existing test system software lacked the flexibility to adapt to changing test needs and requirements. Invensys contracted MicroCraft Corporation, a National Instruments Select Integrator, to design and build a more flexible tester. MicroCraft chose to use LabVIEW in conjunction with off-the-shelf NI hardware to provide a maintainable and configurable system capable of changing with expanding needs.

System Architecture
MicroCraft began by selecting a NI PXI industrial computer running Windows NT to provide a friendly and robust operating platform. Next we added a NI PXI-5102 digital oscilloscope module for capturing the UUT waveform. We selected the PXI-5102 because it offers a reliable means for high-speed waveform capture at about half the cost of most digitizing oscilloscopes. In addition, with the readily available instrument drivers, MicroCraft quickly developed the data acquisition algorithm without the need for designing and writing complicated counter timer processes.

For proper inspection, the sensor output must be analyzed at varying engine RPMs and air gap settings. The air gap setting represents the sensor's position in reference to the target. An increase in the air gap causes signal intensity to diminish, as does an increase in RPMs. A stepper motor controlling a linear slide is used to adjust the air gap by positioning the cam sensor in relation to the target. A servo motor is used to turn the cam gear, thereby controlling RPMs. MicroCraft selected a Compumotor 2-axis motor controller to control both the stepper and servo motors.

In addition to RPM and air gap setting, the test unit’s output voltage is influenced by changes in reference and supply voltages. These two voltages are configurable via two GPIB-programmable power supplies controlled through the built-in IEEE 488.2 interface on the PXI industrial computer. Mechanical actuation of the test fixture (UUT positioning, safety guards, etc.) is controlled pneumatically using a PXI-2565 SPST switch card. The PXI-2565 optically isolates all output which decreases the noise distortion and feedback prominent in many motion control applications. Digital safety limits and feedback are monitored using a PXI-6508 96-channel digital I/O card. Again isolation was critical and optically isolated, so solid-state relay modules are used.

System Software
The test software is written in National Instruments LabVIEW. Together MicroCraft and Invensys chose LabVIEW in an effort to decrease development time and simplify maintainability of the source code.
We began writing the software well before completion of the mechanical fixture. With the modular VI/subVI architecture of LabVIEW, we tested and debugged code quickly and efficiently while the LabVIEW DAQ VIs and instrument drivers shaved several weeks off our development time. In addition, LabVIEW’s powerful graphical user interface allowed us to develop a configurable and versatile program for test engineers and an intuitive, easy-to-operate user interface for system operators.
Features of the program include:

• Diagnostic mode
• Run time menus
• Multi-level security
• Calibration utilities
• Configurable test sequencing and profile generation
• Production statistics
• Data logging
  
  

Operation
After manual loading, the part is automatically positioned and inspected, using fiber optic proximity sensors, for unit accessories which include an o-ring and mounting bracket. Next, the electrical output is tested by acquiring and analyzing the sensor output with the PXI-5120 digital oscilloscope at user-specified combinations of RPM, air gap, and voltage settings. Acquisition is post triggered by the servo encoder so that exactly one revolution of the target is captured at each setting. The acquired pulse train is verified for transitions, voltage level, duty cycle, and sync ratios. Passed units are automatically date stamped and each unit is deposited in a bin according to its pass/fail status.

Unit test data and statistical information for the current production run are prominently displayed on the front panel. Data for each day’s production is stored in an ASCII data log file and can be imported to other applications for statistical analysis. With multilevel access, test engineers easily modify test parameters and design custom tests within software. In addition, with standard initialization (.ini) files, custom test profiles are saved and automatically loaded for future use. Finally, with diagnostic and calibration utilities, quick and easy troubleshooting and calibration of system hardware are possible.

Results
With LabVIEW and a PXI industrial computer, MicroCraft and Invensys met a very aggressive completion schedule with the threat of a failure in the existing system looming. The flexibility of the new test system has provided Invensys not only with a production test system, but with a system capable of performing as an analysis tool for engineering runs.

For more information, contact:

Gary Sweezy

MicroCraft Corp.

3209-154 Gresham Lake Rd.

Raleigh, NC 27615

Tel: (919) 872-2272 ext. 25

Fax: (919) 872-5822

E-mail: gary.sweezy@microcraftcorp.com

Web: www.microcraftcorp.com

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