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High Performance Weighing with PWM3
SP, the Swedish National Testing and Research Institute, breaks new ground in high-performance weighing with a custom-built OLTOM-system.
Background
Ordinary high-precision balances operate in air. However air buoyancy and convection currents have negative effects on the accuracy.
Mass is the only unit still defined by a physical standard, the kilogram standard weight in Paris. Therefore precise weighing of physical standards is of the utmost importance for scientific and engineering purposes.
More precise measurements can be made by lowering the air-pressure and thus minimizing the mentioned inaccuracies.
Vacuum chamber containing balance and weight changer. OLTOM PWM3 system unit is visible to the right.
How PWM3 Helped
The previous system at SP used an old system interface, requiring a personal computer with several serial ports. In addition it was hard to maintain, program, and upgrade.
Converting the system to a OLTOM PWM3-based system saved several cards compared to the previous solution. In addition, only one serial cable is required between the personal computer running the measurement software and the PWM3 unit.
Also users report smoother movement, as well as less vibration and noise in the stepper motors used in the weight changer. The better handling is due to PMW3's use of extra smooth modified half steps instead of full steps.
The OLTOM PWM3-system was also easier to interface to SP's in-house measuring software, developed in Visual Basic. An OLE DLL (Object Linking and Embedding Dynamic Link Library) provides an easy interface to other measuring and statistics programs.
As the current and stepper characteristics are programmable, it's also easy to experiment with different motors and movement profiles.
Finally, the PWM3 bus architecture makes future expansion easy: SP can just add more cards for control and data gathering to the system. The I2C-compatible bus can support hundreds of devices, and up to five PWM3 cards can be connected to one bus. Advanced error detection and corrections ensure that command and data packets always are delivered, even in a noisy environment.
System unit containing an OLTOM PWM3 card, an OLTOM custom I/O card, and the power supply.
System Architecture
The system consists of a high performance weight-changing balance in a vacuum chamber.
Special "space" class motors and lubricants are used in the vacuum chamber. The heat build-up and evaporation from ordinary lubricants would otherwise make the system unusable for precision weighing.
A PWM3-card together with an I/O card controls the motors for the weight changer. The I/O card also interfaces to the balance control and instrumentation unit via a serial port. Interlock switches connected to the I/O card prevent the PWM3 for rotating or moving the weights unless the weights are in the proper position.
Data from the balance is collected by the I/O card, forwarded over the PWM3 I2C-based serial bus to the PWM3 card, which finally forwards it to the personal computer.
A personal computer handles the actual data gathering and measurement sequences. SP uses an application developed in-house for the data gathering and statistical analysis.
Both the PWM3 and the I/O software was specially modified to fit the application requirements.
More Information
For more information about OLTOM Engineering or the PWM stepper controllers, contact OLTOM:
For more information about SP:
R&D at the Mass Section at SP, http://www.sp.se/metrology/mass/eng/research.htm.
Read about SP's research in Ny Teknik (in Swedish), http://www.nyteknik.se/pub/pub26_3.asp?art_id=1726.