14 June 2011
Turning off the shelf components into application specific data acquisition solutions
By definition, off the shelf modular instruments are designed to be general purpose, programmable and flexible enough to handle a variety of input ranges and types, speeds and functions. At first sight, these instruments may seem ready to go. However, 100% COTS is generally inadequate for complex, commercialised measurement systems.
Steve Krebs, director of engineering with KineticSystems, said: "With modular instruments, there is this idea that you can buy pieces from vendors A, B and C, stick them together in a system and that'll be it. But once you put these modules in a chassis, there can be issues with interoperability, input ranges, synchronisation, signal amplitude or conditioning."
Although some customisation can be handled by the customer's engineering staff, modifications can be time consuming and expensive. As a result, it makes sense for manufacturers to partner with instrument providers willing to semi customise COTS products.
KineticSystems recently completed an automotive component testing application that upgraded a data acquisition system used for testing accelerator pedal assemblies. The customer required a COTS solution that gave the same performance as existing instrumentation, but at lower cost.
The test process uses temperature controlled chambers to simulate worst case component environments. The pedals are mechanically cycled 24 hours a day for up to several months, while the data acquisition system records the position and monitors the motion profile to ensure the pedal is performing as designed. In some instances, stress is also measured on the component under test.
The original ATE system was proprietary and no longer supported. While the system was updated with VXI components from KineticSystems, it was still largely proprietary.
"Every time the company purchased a new system, it made incremental improvements, but never came up with a standardised solution it could duplicate," says Krebs. "Now, it was interested in a more economical solution that took less real estate and on which it could standardise globally."
KineticSystems supplied four modular off the shelf instruments in a rack mounted enclosure. Each CP246 is a flexible eight channel CompactPCI/PXI module with signal conditioning and a/d conversion.
The customer also wanted to perform more frequent calibration of the instruments in its metrology labs. While calibration is normally performed annually at KineticSystems' facility, calibrating in house would minimise downtime and expense. In response, Kinetic Systems developed a standalone software application to perform periodic calibration with pre- and post calibration report generation for traceability.
The customer also required the ATE to perform ad hoc data acquisition without having to write any code. This was achieved through KineticSystems' configurable data acquisition software, SoftView, which provides access to all of an instrument's capabilities and features through a point and click GUI.
KineticSystems further customised SoftView to include the ability to specify modules as master/slave to allow for simultaneous acquisition of multiple transducers signals between multiple modules.
One of the advantages of purchasing a COTS module is to reduce cost and time to market, while allowing a company to focus on system design, rather than instrument manufacturing. This was the case for a US manufacturer of laser instrument systems.
Although commercial digitisers were available, the company had been developing its own digitiser. When the product started to take off, it decided to incorporate an off the shelf digitiser into the system. After some research, a 12bit high speed digitiser from GaGe was selected.
Almost immediately, it was clear the on board fpga was not large enough for the application, so GaGe worked with the customer to add another larger pin compatible fpga. The manufacturer then realised its digitiser featured low speed a/d and d/a I/O to capture analogue temperature measurements, activate motors and turn various devices on and off.
To add these elements, GaGe partnered with KineticSystems to deliver a two board solution – the digitiser and a PXI DAQ data acquisition board, modified to operate from a USB port, that provided the additional functionality.
The manufacturer also needed to address low power requirements. Because the laser instruments had no ready access to power, they operated from solar powered rechargeable batteries. This created a strict power budget.
Working with the customer, GaGe removed unnecessary components to reduce power consumption by 50%. "Because our board is general purpose, we had multiple input ranges and multiple channels, but the customer only required one," says GaGe's Andrew Dawson. "We removed a channel and the amplifiers, which resulted in a significant reduction in the power required."
Another element was the duration of the trigger pulse used to activate the laser beam. Although adequate for most applications, the GaGe board's standard trigger pulse was too short, causing the laser to activate intermittently. Adding an external circuit would have solved the problem, but the customer rejected that on cost grounds. GaGe's engineers modified the card to extend the duration of the trigger pulse.
In a similar example, a US manufacturer of emc systems required an 8bit high speed digitiser, but with more memory than available on the largest GaGe digitiser. GaGe solved the problem with a two card solution, with each card's memory designed to fill in succession.
Timing delays between the cards meant a special triggering signal was added to activate the second card when the first was full. GaGe provided a procedure to work out the delay between the two cards and provided custom software that could be used to align data acquisition between the two cards through a visual representation of the signals.
The application also required a subtle adjustment to the sampling rate in order to cover the entire em frequency spectrum. According to Nyquist, the sampling rate must be at least twice the highest analogue frequency for completely accurate a/d conversion of the signal.
While the GaGe card sampled at 2Gsample/s, the customer wanted to identify signal frequencies at up to 1Gsample/s. To build in flexibility, the customer specified a sampling rate from 2.1 to 2.2Gsample/s.
GaGe's digitisers have an integral 10MHz reference input that, multiplied by 200, supports 2Gsample/s. GaGe modified the fixed reference to be adjustable between 9 and 11MHz, allowing the sampling rate be tuned between 90 and 110% of 2GHz and a sampling rate from 1.8 to 2.2Gsample/s.
The previous solution, despite its higher cost, used a spectrum analyser that scanned at specific frequencies over the course of an event. This meant a non compliant signal could be missed as the equipment scanned the various frequencies. GaGe's solution provides 100% coverage, capturing all the data at all frequencies during an 8s event of interest.
According to Krebs, there is a limit to customisation that should be expected from COTS products. "Once you get to the point of having a vendor create a new circuit board, you are really designing a fully custom product," says Krebs. "The expense goes up and you are no longer reaping the advantages of COTS modules."
The bottom line is modular instrument manufacturers need to be able to deliver the best of both worlds: a level of customisation at the price and timing of COTS products.
Jeff Elliott is a US based technical writer.
Author
Jeff Elliott
Supporting Information
Downloads
34462\P41-42.pdf
Websites
http://www.gage-applied.com/
http://www.kscorp.com/
Companies
Gage Applied Inc
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