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Data Acquisition System for Torque Measurement |
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| The Challenge
To develop system for performing endurance test for a shaft by acquiring Torque, Speed and Temperature in real time. The endurance test duration was in months.
The Solution
Using the National Instruments PCI, SCXI Platforms, NI DAQmx and LabVIEW Development environment to create a highly flexible, scalable and powerful test system.
Introduction
Our client, a leading R&D organization, specializes in developing automobile components. For testing their engine components they have various test benches. A typical four stroke engine drives its output power to a crankshaft. Every two revolutions of the crankshaft constitute the four strokes of the engine. If a torque measuring device is connected to the crankshaft the torque signature can be seen to repeat after every two revolutions. The signature itself depends on the characteristics of the engine. An ideal engine would reproduce the same torque signature every two revolutions. Practically the torque signature remains within a certain percentage band of reference. The endurance test performed monitors that the torque signature remains in the allowed band for the entire duration of test.
The temperature of the engine also needed to be monitored at around twenty different points for the entire duration of test. It the torque signature or one of the temperature values crossed the specified limits the system needed to automatically shut down and protect the engine.
System Design
An overview of the complete system is shown in figure 1 below.
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| Fig 1 Schematic Representation of system |
The system basically acquired three types of parameters
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Torque is measured from field through HBM Flange [Non contact type torque sensor] which gives a voltage output; this is acquired through NI-PCI-6220 card. |
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Speed is measured by tachometer from field and it is acquired through NI PCI-6220. |
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Temperature is measured through SCXI-1102 and PCI-6220. |
Using torque and speed signal software will plot the graph of torque verses revolution, Acquired temperature channels are also plotted on the screen. While acquiring if input crosses the predefined limits at any time then system gives digital output to perform shut off operation through SCXI-1163 and PCI-6220
Software Features:
The software developed in LabVIEW provides following features:
Configure: This is for configuring test parameters.
- Flexibility for configuring channels for torque, speed, temperatures.
- Flexibility for configuring limits for torque and temperatures.
- Flexibility for configuring update rate for temperature, type of thermocouple, assigning names for channels.
- Flexibility for configuring logging duration of data, EU conversion for torque, number of pulses per rotation for Tachometer.
Acquire:
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| Fig 2 Acquisition Design |
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The catch in the acquisition loop is plotting exactly two revolution torque data.
This is the most critical part in the system. To compare two signals [Actual and Reference band] and to plot a graph which should exactly fit on the screen, the number of points in the array must be same for every two revolution. But speed and torque both the signal are varying continuously during the test, so number of points for torque and speed data will also vary accordingly. To overcome this problem re-sampling technique was used. Our custom algorithm would interpolate or extrapolate the number of points and give fixed number of points on the fly during the entire duration of the test.
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As the load connected to the shaft is varied amplitude value for the torque also changes, which will tend to move the signature outside the specified band. To avoid this problem acquired input is passed through a normalizing algorithm. |
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Another challenge was continuous logging data for long durations of time. Since the test duration was in months it was impractical to store all the test data. Only data before a failure needed to be stored. This was implemented by cyclically writing to a specified files. So the point acquisition stopped or failure occurred the last segments of data will be available for user to view. The data file size can be reconfigured during acquisition, user has facility to store maximum 30 minutes of data. |
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Another interesting feature is changing limits for temperature and the reference band for torque during acquisition |
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Acquisition Panel is displayed in Figure 3.
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| Fig 3 Acquisition Panel |
It has following features:
- Reconfigure torque, temperature limits, data logging file size.
- Apply reconfigured limits
- View temperature graph with individual channel selection facility.
- Start, stop, exit test.
- Go for offline during online, printing the image.
Offline:
This to view torque and temperature data for already performed tests. Since logging is done circularly reading is also performed circularly. The data is display section to section for easy for user to understand. In one section 1 minute data or the data logged between two intervals is displayed. Offline module has following features
- Ability to view whole data section to section.
- View temperature graph with individual channel selection facility.
- Save torque, temperature and test parameter data in text format.
- Print option.
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| The offline panel is shown in figure 4. |
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| Fig 4 Offline Panel |
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Conclusion: We were able to design and develop highly accurate and reliable system that enabled our client to efficiently test his test bench. The seamless integration of NI hardware and software ensured quick development. The customer was delighted at the overall performance of the system as it ideally suited his R&D requirements and is now a true believer in the power of virtual instrumentation. |
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