Verification of Angular Motion Using NI LabVIEW and NI CompactRIO

Author(s):
Frank de Wit - SKF Group, ERC Sensor Integration

Industry:
Machines/Mechanics, Automotive

Products:
CompactRIO, LabVIEW, FPGA Module

The Challenge:
Developing a system for verifying the quality of the angle output of a sensor bearing where angle is defined as angular position between the inner- and outer- ring.

The Solution:
Using NI LabVIEW software and modular instruments to design a highly accurate system in a limited time.

The SKF Group is the leading global supplier of products, solutions, and services in the areas of rolling bearings, seals, mechatronics, services, and lubrication systems. We continually develop and test new types of bearings to stay ahead of the competition. For example, we may want to measure the indirect forces working on a bearing or investigate means of measuring the angular position and velocity of a shaft. In the research described here, we tested a new type of ball bearing equipped with position sensors and compared this with an external reference encoder to verify the quality of the sensor.

Mechanical Setup

A high mechanical precision (almost without axial or radial play) rotating element attached to a fixture can be driven by an electric motor. Its angular position is measured through a 72,000 count (optical) quadrature encoder which then serves as the reference angle. To this rotating element, a ball bearing equipped with sensors is mounted such that the inner ring is fixed to the rotating element and the outer ring to the fixture.

Electronics Setup

The electronics setup consists of an NI CompactRIO device equipped with an NI-9401 high-speed digital I/O module. Three of the module inputs are connected to the outputs of the quadrature encoder (A, B, index pulse). Another input is connected to the serial output line (RS485) of the bearing. All I/O is connected without requiring any conversion electronics.

 

LabVIEW FPGA

The FPGA is doing the previously nearly impossible job of reading these two sets of inputs simultaneously and applying a high-accuracy timestamp. The quad encoder lines are read and converted to absolute position with the help of the index pulse (zero position).  This is not an easy job, at 3,000 rpm (50Hz) there are already 50*72,000=3.6 million transitions per second to work on. A 32-bit 40 ns resolution timestamp is added to the positions using the internal FPGA reference clock. The combined position and time data is presented as a ‘bundle indicator’ to the controller interface.

In true parallel fashion, the FPGA decodes the data packets arriving at a 20 KHz rate on the 1 MB/s serial output of the bearing. A timestamp is taken at the first edge of the arriving serial data words. Again the data words are presented together with timestamp as a bundle to the controller.

The FPGA code was developed using the LabVIEW FPGA graphical environment, which proved much easier than either VHDL or the alternative interrupt routine written in C on a fast processor.

Controller

The controller has the difficult task of streaming the two asynchronous data streams to its internal RAM and later to onboard flash memory. The controller code also was developed using the LabVIEW graphical environment. First it was debugged, and later code was turned into an uploadable executable that now boots when the CompactRIO is turned on. As such it is able to function stand-alone in the setup.

User Interface

A Web page front end was then generated with a few mouse clicks and also uploaded to the CompactRIO. Because this device contains a Web server, access is available from any company computer (and to our colleagues abroad through company intranet) by simply using Web browser software and the free LabVIEW runtime engine. Measurement datasets are saved as files on the controller flash memory. These can then be accessed over the network by connecting to the FTP server on the controller or alternatively by browsing the Windows network neighborhood.

Future Expansion

We would like to use the CompactRIO to handle the speed control of the electric motor turning the bearing. We have also placed the bearing into a miniature climate chamber to do temperature tests. This could also be automated using the same CompactRIO device.

We used the LabVIEW standardized graphical programming (especially for the FPGA device), the CompactRIO controller and the FPGA and I/O modules combination to design and build a measurement setup in a timely and cost-effective way.

Author Information:
For more information on this Case Study, contact:
Frank de Wit
SKF Group, ERC Sensor Integration

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