Fuel Cell Testing with the NI PXI-4070 FlexDMM

Publish Date: Sep 06, 2006 | 4 Ratings | 4.25 out of 5 |  PDF

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Fuel cell testing is one example of an emerging application that pushes the limits of the traditional measurement approach. Initially, engineers used fuel cells as a clean source of electrical energy in specialized applications such as aerospace and aeronautics. With recent innovations, fuel cells are becoming potential power sources for automobiles, residences, portable electronics, and other electrical power applications. The research and commercialization of fuel cells is a challenging electrical test application requiring new combinations of measurements that are not addressed by fundamental instrumentation.

For instance, the accurate measurement of the voltage generated by each individual cell is one of the most important requirements in these fuel cell test systems. Though each cell may generate no more than 1 V, multiple cell membranes are stacked together to yield higher voltages and currents, and high-performance stacks have hundreds of cells. Accurate characterization of a fuel cell stack requires a multichannel measurement of a small voltage in the presence of a large common-mode voltage, up to several hundred volts. Making these high common-mode voltage measurements can prove hazardous to the test equipment, to the unit under test, and perhaps to the operator.

Isolation Improves Measurement Quality
Isolation allows engineers to safely measure a small voltage in the presence of a large common-mode signal. It has the added advantage of improving the accuracy of the measurement by eliminating the need for external signal conditioning. The three advantages of isolation are:

1. Improved rejection - Isolation increases the ability of the measurement system to reject common-mode voltages. Common-mode voltage, a source of measurement error, is the signal that is present or "common" to both the positive and negative input of a measurement device, but is not part of the signal to be measured. Common-mode voltages are often several hundred volts on a fuel cell.

2. Improved safety - By creating an insulation barrier, isolation allows floating measurements while protecting against large transient voltage spikes. A properly isolated measurement circuit can generally withstand spikes greater than 2 kV.

3. Improved accuracy - Isolation improves measurement accuracy by physically preventing ground loops. Ground loops, a common source of error and noise, are the result of a measurement system having separate grounds at different potentials.

Fuel cell test systems also must accurately monitor and control hundreds of measurements, including the flow, temperature, pressure, power, noise, and humidity of the fuel cell stack. For example, engineers monitor temperature to optimize variables in the electrochemical process to increase power output. Thermocouples and resistance temperature detectors (RTDs) are common sensors for monitoring both the stack temperature and the temperature of the incoming reactant gas streams.

Hence, a typical fuel cell test system requires high-resolution, high-voltage input, isolation, and waveform acquisition capability. The built-in signal conditioning of the traditional multimeter makes them appropriate for making many of these measurements. However, traditional multimeters provide limited measurement throughput and no waveform acquisition capability. Engineers therefore require an additional instrument, a high-speed digitizer, to make the required waveform measurements, such as measuring the noise ripple on an individual membrane. This approach has proved expensive because it requires multiple instruments and suffers from limited throughput due to the lack of integration between the two measurement products. A more cost effective solution is a system based solely on a low-voltage, high-speed digitizer. However, this solution requires engineers to provide the required isolation and attenuation through external proprietary signal conditioning, which can be unreliable and offer limited performance.

NI PXI-4070 FlexDMM Deliver Increased Flexibly and Performance
Multifunction instruments, like the NI PXI-4070 FlexDMM meet the challenges of fuel cell testing and other emerging applications. The NI PXI-4070 FlexDMM’s 7-digit multimeter capability is ideal for making all of the voltage, power, and sensor measurements with the best voltage stability and noise performance available in a digital multimeter today. For fuel cell testing and other higher throughput applications, the FlexDMM has a maximum single-point DC (voltage, current, and resistance) reading rate of 10 kS/s at 4½ digits (15 bits).

The NI PXI-4070 FlexDMM's 1.8 MS/s isolated digitizer capability allows engineers to acquire both AC and DC-coupled voltage and current waveforms up to 300 V and 1 A. By using flexible measurement software like NI LabVIEW with the isolated digitizer capability, engineers can analyze transients, noise ripple on a fuel cell membrane, or other non-repetitive high-voltage AC waveforms in both the time and frequency domain. Engineers can vary the resolution of the FlexDMM from 10 to 23 bits by simply changing the sample rate. The NI PXI-4070 FlexDMM offers a single application programming interface (API) that allows engineers to quickly program the device as both a traditional multimeter and isolated digitizer in a single NI LabVIEW program, as shown in Figure 1.

Engineers can quickly increase the channel count of these fuel cell test systems because the multifunction instruments are designed to work well with all multiplexer/matrix switch modules. In particular, the NI PXI-4070 FlexDMM integrates seamlessly with National Instruments switch modules, such as the NI PXI-2503 multiplexer and the NI SCXI-1129 high-density matrix. When engineers use the FlexDMM in conjunction with these NI switch modules and NI Switch Executive switch management software, they can measure thousands of channels with a single DMM.

Figure 1: NI PXI-4070 FlexDMM Operating as a Multimeter and Digitizer

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