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TWI, InnoTecUK, and ANSYS develop ATEX Encoder

Introduction

TWI has worked with InnoTecUK and Ansys to develop an ATEX certified encoder that is suitable for applications in an explosive atmosphere, see Figure 1.

The ATEX encoder is based on MEMS hall sensor technology, allowing it to be physically smaller than conventional designs and, as it operates through magnetic coupling, requires virtually zero torque to operate.

The encoder provides regular electrical pulses relative to a rotary movement, which can then be used to determine the angular or linear distance travelled. The encoder can be used to monitor the position of a device relative to a set datum point.

ATEX certification is an EU directive designed to prevent explosions in areas where explosive atmospheres may be present. Any equipment used in an explosive or possibly explosive environment, such as oil and gas applications, must be ATEX certified.

The encoder was designed so that it is affordable, compact and ready for direct integration into high volume OEM equipment. Because it is a non-contact device, it can work at high-speeds with precision in harsh industrial environments such as those associated with oil and gas, coal mining, pharmaceutical, food industries, etc. The design negates the need for seals or bearings, ensuring long term reliability, friction free operation and simple installation.

Demonstration of Capability

To demonstrate the accuracy and repeatability of ATEX encoder, a stepper motor was programmed to rotate a drive shaft 100,000 cycles in one direction and then 100,000 cycles in the opposite direction, the setup is shown in Figure 2. At one end of the drive shaft was the ATEX encoder and at the other end a COTS encoder. The output from both encoders was collected and reviewed. Both the encoders registered exactly 100,000 cycles at the halfway point and both returned to 0 at the end of the test, see Figure 3, demonstrating equivalence between the two encoders.

Operational Variables

The ATEX encoder technical parameters were established through a series of independent studies, one virtual (Ansys) and one physical (TWI), to determine:

  • Maximum separation between ATEX encoder and magnetic actuator
  • Maximum off-set distance from the centreline of the ATEX encoder and centreline of the actuator magnet
  • Maximum off-set angle between the centreline of the ATEX encoder and centreline of the actuator magnet
  • Effect of different case materials

The setup for the physical separation investigation is shown in Figure 4 and the results of the modelling are shown in Figure 5. The outcome from both studies agreed within 1%, allowing TWI to replace further physical testing with virtual tests. These results were then used to set the operational parameters of the ATEX encoder relative to the actuator magnet.

It was found that the results from the Ansys Maxwell model matched the practical test results. The good correlation between the model and practical results provided confidence in the modelling and allowed practical experimentation to be replaced by simulation, this significantly reduces the test time whilst extending the number of scenarios that could be investigated. A variety of magnet materials available in Ansys Maxwell with the MDS (Materials Data for Simulation) add-on, allowed the partners to explore the effectiveness of the magnet encased by different materials. Given the anticipated harsh operating environments, selection of the right material was important. Ansys Granta Selector was used to downselect appropriate materials based on durability, cost and supply factors.

The following operational parameters were determined:

  • Separation distance between the encoder and magnetic actuator should be < 1.5 mm
  • The centre line of the magnet needs to be in line with the recess, should be within 2°
  • The case material can be manufactured from brass or stainless steel without compromising the encoder operation

This information was supplied to an independent certification body and the encoder was awarded the designation IECEx BAS 21.0038 (SGS21ATEX0106).

Figure 1. The ATEX encoder
Figure 1. The ATEX encoder
Figure 2. Operational setup
Figure 2. Operational setup
Figure 3. The output from the ATEX and COTS encoders over 100,000 cycles
Figure 3. The output from the ATEX and COTS encoders over 100,000 cycles
Figure 4. Testing set-up of the ATEX encoder
Figure 4. Testing set-up of the ATEX encoder
Figure 5. Modelling of the test set-up in Figure 1
Figure 5. Modelling of the test set-up in Figure 1

Practical Testing in an Operational Environment

Project partner InnoTecUK were able to test the encoder on a submersible robot called ‘Nautilus,’ which was designed for inspection of oil storage tank bottom plates. It was found that the accuracy of the encoder was comparable to the accuracy of the on-board encoders built into the submersible. This series of tests demonstrated the ATEX encoder can be integrated into a submersible system and produce accurate and repeatable results.

 

Conclusion

The key features of the design:

  • IECEx BAS 21.0038 (SGS21ATEX0106)
  • ATEX Zone 0 (gas) operation
  • Portable compact size, 300g
  • Contactless encoder (frictionless design)
  • High accuracy to up to ±0.5°
  • Low torque requirement
  • Corrosion resistance brass casing
  • Fully submersible, IP68

 

Project Partners

InnoTecUK is a dynamic and progressive research and innovation company specialising in the development and commercialisation of high-quality and high-value technical robotic and automation solutions to address real-world market challenges in industries such as oil and gas, renewable energy, offshore, aerospace and rail, among others.

Ansys is the global leader in engineering simulation, helping the world's most innovative companies deliver radically better products to their customers. By offering the best and broadest portfolio of engineering simulation software, Ansys help them solve the most complex design challenges and engineer products - limited only by imagination. 

 

The ATEX Encoder Project has received funding from Innovate UK under No 105604.

Figure 6. The ATEX encoder on the Nautilus submersible inspection robot
Figure 6. The ATEX encoder on the Nautilus submersible inspection robot
Avatar James Kern Senior Project Leader

James graduated from Sheffield University with a B.Eng and PhD in 1992. He worked as a metallurgist at Sandberg consultant engineers, a Senior Technologist at Rolls Royce and as Chief Materials Scientist at APPH (now Heroux Devtek), before joining TWI in 2014. He first specialised in hot forming and bonding processes before transferring to the NDT Group.

He has successfully lead numerous collaborative (Horizon 2020 and Innovate UK) and single client projects, leading research activities and developing manufacturing prototypes. Current areas of interest include radiation effects on materials and sensors, Geothermal Energy, ATEX, manufactured defects for calibration purposes and shearography.

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