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Motion-sensor tester runs from a browser

By Sean O’Leary and Jed Marti, Artis- March 1, 2009

Automated test stations don’t always need proprietary test tools. Using a microcontroller, a PC, and free software tools, we developed an automated test station to test and align motion sensors. Test engineers can operate the system from any Web browser in the company network and can also check the status of a unit under test or retrieve data on a previously tested sensor module.

The motion-sensor module is an optical sensor that uses illuminating LEDs and focused detectors to sense moving objects. The module uses one bank of LEDs to illuminate a target and another bank of 32 detectors to sense motion. The sensor’s output is a bit vector indicating the current state of each detector. The outputs of many such sensors are assembled into a profile of the moving object for identification by a pattern-matching system.

Figure 1.  A server PC communicates to an 8051 microcontroller that operates a motion table and makes measurements.

To function properly, the sensor module’s LEDs and detectors need to be mechanically aligned, something that isn’t practical to do by hand. Thus, we built an automated system that provides consistent alignment and also tests each sensor’s signal-conditioning and communications lines. The block diagram in Figure 1 shows the main parts of the tester.

Using a motion table to move a sensor module in front of a known-good detector array, the test station verifies that each LED functions and is properly aligned. The system tests the 32 detectors by moving a known-good LED in front of the detector array under test. Signal-conditioning circuits in the test station boost the detector’s output signal to a range detectable by an ADC (analog-to-digital converter) in the 8051 microcontroller. The ADC digitizes the signals and the station verifies the module’s data transfer.

For the movable platform, we use a commercial x-axis table (Figure 2). A stepper motor positions the table in increments of 0.005 in. based on direction bits and clock pulses from the microcontroller.

Figure 2.  A linear motion table aligns the sensor arrays to LEDs and detectors in the test fixture.

We also designed a fixture that holds the sensor module during alignment, makes electrical connections, and provides access to the LED and detector arrays. Spring-loaded pins make the electrical connections to the sensor module. Holes in the fixture (Figure 3) let the sensors under test send and receive light from the LEDs and detectors as the position table moves them.

Figure 3.  A test fixture contains holes that pass light between 32 sensors and a test board.

Using a Web-based system

The circuits that interface with the motor driver are located on the station’s printed-circuit board (Figure 4), which also contains the limit sensors, the drivers for the optical devices, and the circuits for the sensor module. The 8051 has a 10-bit ADC, 8 kbytes of RAM, 24 I/O pins, and two UARTs (universal asynchronous receiver-transmitters). An external memory chip extends the total memory to 64 kbytes. The 8051’s serial port communicates between the test-system hardware and a PC running Apache Web Server under Linux. This approach has several advantages:

Figure 4.  Known-good LEDs in the test fixture transmit light to a sensor module under test, and known-good detectors detect the light that the module transmits.

• The Apache Web Server software is easy to set up, secure, and free; it runs under Linux or Windows.

• Connecting the server to a serial port through a CGI-BIN (common gateway interface-binary) script is easy. A small C program on the server sends a command to the test system to start the test. The program also interprets commands and retrieves content such as bit-mapped images and HTML text. Linux C and C++ compilers are free.

• The 8051 microcontroller doesn’t need to support a file system, Internet protocol, or other Web interfaces.

The test system's embedded microcontroller connects to the server through a server’s serial port. Test results, images, and text are stored in a data directory on the server for immediate display or recall.

The server sends a command over the serial port that tells the station to execute the test. Once the server sends the start command, the station code runs through an entire test. As the microcontroller collects data, it generates tables, charts, and bit-mapped images. Once an image is completed, the microcontroller sends it to the server through the CGI-BIN program.

A simple welcome screen from the Web server tells the user when to steer the browser to the address assigned to the station. Anyone on the corporate network has access to this start-up screen. From the main Web page, a user can:

• see a statistics page indicating the number of devices tested and examine a summary of the results;

• view reports, by serial number, for devices whose testing has been completed; and

• initiate the test protocol.

Once testing is complete, the server displays the test results for all 32 sensors as a color graph on the Web page. This report, including all the graphs, is generated by the microcontroller and subsequently transferred to the server. This Web page contains tables and graphs detailing the results of the test as well as any messages that indicate errors or anomalies.

The Web-based system did introduce one obstacle that we had to overcome. When the user initiates the test protocol, the station opens a new Web page and begins writing to it. As tests complete, the server adds additional information to the Web page. We encountered a problem regarding dead time in communicating with the Web page. Web pages can time out in four to five minutes if they don’t receive new information. Because one test took six minutes to complete, we wrote some code to ping the browser periodically to prevent timeouts, and this lets the system complete the page.

Even though our test system does not permit motion beyond program limits, it is always wise to include limit sensors when building a motion-controlled tester. Limit sensors indicate that the table is nearing the end of its travel. We added limit sensors to allow the microcontroller to cut power when the table reaches the end of its travel. In fact, those sensors proved useful during software development when the table went too far.

Overall, we have been very happy with the results from our testers. While the first time through this approach presented certain challenges, we now have a test-setup template that we can reuse when designing new test stations. In the long run, our system has reduced development time as well as training time and overall costs.