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Biomedical Instrumentation Company: Development of New Embedded Bio-medical Product
The Challenge:
To develop a cost-effective user-friendly test system that can apply and monitor various stimulus signals on a clinical analyzer. Studies on these clinical analyzers enable scientists to design the best algorithm for a final embedded biomedical instrument which uses these sensors.
The Solution:
V I Engineering, a premier National Instruments Alliance Member, has developed a customized test system using LabVIEW, that combines stimulus signal editing, data acquisition and signal analysis. With this system, researchers can extensively test different samples under various test conditions allowing the best test algorithm to be selected for the final embedded product. Using National Instruments NI-488.2 and NI-VXI drivers, V I Engineering has developed a control program for GPIB and VXI instruments using LabVIEW. The program's graphical user interface allows the user to easily configure relatively complicated test procedures.
Introduction
A test system has been developed for scientists in a Biomedical Instrumentation company that manufactures instruments which include clinical sensors. The customer requirements included a system capable of testing and studying the characteristics of these sensors - i.e. monitor and analyze the response of the monitors to different electrical stimuli. The system requirements included having up to four channels running simultaneously and independent of each other. Both DC and AC stimulus signals are applied to the sensor samples. For AC stimulus, both the amplitude and phase responses are studied. The response signals are acquired continuously with different stimulus signal segments applied for a few minutes at a time. Signal conditioning is also required to amplify the sample signals which are usually in the microvolt range to a readable millivolt range.
V I Engineering has developed a PC-based test system for this research. The customer had inherited legacy hardware which was required be re-used in the new system. Even though the instruments were products from different manufacturers - including VXI cards, GPIB instruments and custom interface circuits, LabVIEW was selected as the ideal platform to integrate these instruments into a customized solution. The broad array of drivers available in LabVIEW, the ease of writing new driver functions, and the ease of developing user-friendly interfaces all made LabVIEW the development platform of choice. A PCI-MXI-2 card and a VXI-MXI-s interface board provided the control of VXI arbitrary function generators and digital I/O ports. A Stanford research 830 lock-in amplifier measures the phase and amplitude responses of the samples. The communication between the PC and SR830 is through GPIB with a National Instruments AT-GPIB/TNT board. A custom built signal conditioning board ensures the accuracy of low signal measurements.
System Specifications
The sensors are electro-chemical cells that develop potential across the electrodes, depending on the voltage applied and the electrochemical composition between them. The potential developed is typically in the range of pico-micro volts requiring the instrumentation to be very sensitive as well as provide accurate results. The test system has to calibrate itself, then apply varying voltage profiles across the electrode in different modes and study the continuous response of the test cells to these different stimuli in terms of phase and amplitude. The system requirements include supporting varying test conditions such as the introduction of blood or solution drops on the cell. This enables scientists to study changes in response patterns.
The E 1340A arbitrary function generator is used to generate independent waveforms on each of the four channels, at different amplitude and frequency. Four such arbitrary generators are used in the VXI chassis. Four Stanford Research 830 lock-in amplifiers are used to measure both the amplitude and phase response of the samples. With the built-in buffers, continuous data acquisitions can be performed without any gap between blocks of data. With the lock-in technology, the small biomedical signals can be measured with high accuracy. Digital outputs are used to enable/disable various control signals within the custom-made instrumentation amplifier circuit.
In order to achieve independent operation for each channel, separate HP E1340A arbitrary function generators are utilized. The PC and VXI modules communicate through a MXI bus. Both the read and write functions are handled through VXI register-based programming.
Software
The system is comprised of four major modules:
- Calibration Screen
Calibration of the system involves calibrating the four custom-made instrumentation amplifier circuits and the arbitrary waveform generators. This includes the offset calibration of the arbitrary waveform generator. Gain calibration is performed by measuring and computing current gain values of the four circuits. The data is stored in a tab-delimited ACSII text file and is used while a test is running, to produce an accurate voltage across the electrodes.
- Test Sequence Editor
Within the test, different stimuli (AC/DC) need to be applied to the samples. Depending on the kinds of stimuli applied, different digital gates are opened or closed to provide the proper gain values for the measurements. The setup is performed by a graphical user interface which provides an easy editing and overview of all of the stimulus signals used in the test. Each stimulus profile is saved as a distinct ACSII file. The screen shown in Figure 1 provides the user the ability to create new stimulus files, view existing stimulus files, edit existing files and save them.
Specific conditions such as open circuit and drop detect can be set on each of the segments. The drop detect condition enables a segment to be terminated when a drop (solution or blood) is electrically detected as having been placed in the electrochemical circuit within the sensor under study. Error checking is also provided to check any invalid parameter entries. All of the stimulus signal sequences can be saved and loaded, simplifying the test setup process.
- Automatic Test Execution
The screen shown in Figure 3 runs a test using the selected stimulus file, gain resistor and electrode configuration. The start/stop of each channel is independent of the other channels. The start/stop of saving data can be manually triggered by clicking on the buttons or automatically controlled by the program itself by checking the preset conditions specified in the test sequence. Both the acquired data and the test segment information is displayed on the screen in real-time. The current measurement is continuously updated on the sweep chart for that channel.
- Independent Y-Axis Graph Display
During the test, both the amplitude and phase responses of the samples are recorded. LabVIEW's advanced graphical display function allows the program to easily display different signals with their own Y-Axis scales. The scales of the signals can be changed independently which easily align the signals and find the time relations
between them.
Post Processing/Data Display
The Post Processing/Data Display screen displays collected data and analyzes the results. In an earlier version of this system, this utility included provisions for adding different software filters to the raw data and viewing the results. This provides the product designer an understanding of the effects that different hardware filters can have on the results and allows the designer to apply this information to the final embedded product.
Conclusion
The stimulation test system developed by V I Engineering provids the researchers in a Biomedical Instrumentation company the opportunity to extensively test their samples with various stimulus signals. With the graphical user interface provided by LabVIEW, the relatively complicated test setup and execution can be accomplished in an intuitive and straightforward manner. This greatly reduces the cost and improves the efficiency in the new bio-medical instrument development process.
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