Biological pulses

Transmitters used in scientific research that implant into animals ranging from mice to cattle. A transmitter receives analog signals from sensors, digitizes sensor data, and transmits measurements by pulsing a carrier. Each transmitter can accept up to 10 sensor types.

Develop a test system that automates testing. The system must measure electrical properties such as power consumption, coil voltage, noise, crosstalk, and pulse duration on 12 models of transmitters. System software must easily adapt to testing of new models.

• GE Druck: pressure meter. www.druck.com.
• Keithley Instruments: digital multimeter. www.keithley.com.
• National Instruments: PXI chassis with system controller, digital I/O card, oscilloscope card, and counter/timer card; software includes a graphical programming language and test executive. www.ni.com.
• Pragmatic Instruments (now Tegam): arbitrary waveform generator. www.tegam.com.

An automated system tests transmitters used to monitor an animal’s vital signs.

When Transoma Medical decided it needed an automated test system, the company turned to VI Engineering (www.viengineering.com). Ayse Guven developed the system while working for VI Engineering. Since delivering two systems in early 2004, Guven left VI Engineering for Transoma, where she now works as a senior test engineer.

Before Guven developed the automated test system, test operators at Transoma used handheld DMMs to measure current consumption when the DUT transmitted (10–20 mA) and when idle (100–200 nA). To verify that a transmitter worked, operators used AM radios, listening for beeps that indicated the pulse durations—a laborious, subjective process.

With the automated system, a transmitter broadcasts a pulsed carrier in which the pulses represent digital data from sensors inside the animal. One of those sensors performs an electrocardiogram (ECG). An arbitrary waveform generator (AWG) excites the ECG input. A PXI counter/timer card measures the duration of the pulses. If a transmitter produces pulse durations that are out of tolerance, an operator will replace those components in the transmitter that adjust pulse durations. A pressure meter monitors the ambient pressure surrounding the DUT.

Guven notes that the system makes more accurate and faster measurements than were possible manually. For example, the system measures a DUT's crosstalk. Crosstalk results when signals from one sensor input leak into an adjacent channel's input, thus causing an unwanted change in pulse duration.

To conserve the life of the batteries that power them, the transmitters use a magnetically activated on-off switch. The oscilloscope measures the voltage across a coil to verify that the voltage is sufficient to activate the switch.

Each of the 12 models of transmitter requires a custom test fixture that connects signals to the tester. Relays in the fixtures route the AWG's output signal to the DUT, and they can simulate test conditions such as open circuits . Other relays apply power to the DUT. A PXI digital I/O card controls those relays.

With 12 transmitter models in production prior to development of the test systems, Transoma needed a flexible test system. Using a test executive, Guven selects the order of the tests and report test results. A 13th model is now in production. Because of the system's modular design, Guven expects to quickly configure it to meet the test requirements of the new model.

The automated systems do away with operator subjectivity. They improve test accuracy and repeatability to levels not possible by manual methods. The bench DMM also improves accuracy of power-consumption measurements over handheld meters. Automation cut testing time by 57% over manual methods while providing more test coverage.