Tests keep tracking shipments

Axonn, a manufacturer of GPS tracking modules, needed an automated test stand to perform functional test and device programming. System integrator G Systems (www.gsystems.com) developed a system that programs the devices, simulates GPS signals, and performs power and RF measurements (figure).

The system uses a board-level fixture for functional testing and a different fixture for testing the satellite transmitter module. A digital I/O card controls relays that apply power to the module. “We begin testing with a DC power test,” said Rick Garza, engineering team lead at G Systems. A DMM card measures inrush current, active current, and sleep current. It also checks for shorts and opens.

The system applies code to the tracking device’s microcontroller so it can communicate over a serial link. The serial link lets the system configure the module and verify that the board works before moving to wireless operation and RF tests. With the module programmed, the tester can set operational parameters and run RF tests. For example, it tests transmitter output power and frequency at 3.5 GHz with the spectrum analyzer.

The tester also runs a series of electromechanical tests. A magnet, placed in proximity to the module, sets its activation bit. To test that function, the tester uses an electromagnet that receives current from a power supply through switches. Another test verifies that the module’s programmable alarms function properly.

A 32-channel digital I/O card controls status bits on the module during these tests. A multifunction data-acquisition card provides 16 digital I/O lines for relays that carry power to the module and connect it to the DMM. A power supply simulates a battery and lets the system test for low battery levels. Other digital I/O lines switch the RF signals between the unit under test (UUT) and the GPS simulator and spectrum analyzer. The PXI card’s analog I/O channels aren’t currently used.

Next, the system programs the module with production code and verifies that the module properly identifies itself and that it receives and sends the correct data.

A solenoid then performs a “thump” test on the UUT, which simulates motion. Upon receiving a bump, the module should update its location from the GPS simulator and transmit its location to its home base. This test also verifies that the module’s GPS receiver functions properly. “The GPS simulator test simulates signals from four satellites, which simulates real-world conditions” said Garza. “We adjust the simulator’s output power to test receiver sensitivity.”

Garza found that he had to change the sequence of tests from the initial order. Initially, the tester would perform the thump test early in the sequence. But because the board wasn’t fully programmed and configured at the time, the vibration from the solenoid would cause a board to reset itself. Moving the test to the end eliminated the unintentional resets. Garza also found that he needed to add relays in series with some of the system’s test pins so that only those pins needed for a test would connect to the UUT.