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MILI project

 

Contact Information

Country: United Kingdom

Year Submitted: 2017

University: University of Manchester

Team: Tobias Carrigan, Ben Forrest, Hector Andem, Kaiyu Gui, Lewis Johnson (2017)

Faculty Advisers: Prof. Rob Sloan, Prof. Barry Lennox

Main Contact Email Address: tobydante@gmail.com

 

Project Information

Microwave In-Line Inspection (MILI)

mili.png

 

Description

The MILI project is a team project aiming to create an in-line inspection robot that travels inside pipelines to inspect and evaluate their structural integrity. The robot uses a novel method of microwave reflectometry to scan the wall of non-metallic pipelines, such as HDPE, for defects. This team project has been completed over a full academic year from October 2016 to June 2017. 

 

Products

Software:

LabVIEW 2015, LabVIEW Advanced Plotting Toolkit, Keysight IO Libraries, Keysight Command Expert

Measurement hardware:   

Keysight 'FieldFox' N9918A Vector Network Analyser, 3x Avago HEDS-9140-B00 rotary incremental encoders

Motor hardware:

5:1 geared stepper motor, 3x brush DC motors

Control hardware:

Raspberry Pi, 3x PIC18FxxK80-series microcontrollers

 

 

Background

Inspection is an important procedure for pipeline operators to ensure the safety and integrity of pipelines, especially those transporting hazardous substances such as oil and gas. A structural failure (burst) of an oil or gas pipe can have severe environmental ramifications, while the failure of a water distribution pipe can still cost the operator on average $10,000 for a small pipe and $1.7 million for a larger pipe [1] [2].

 

Pipelines made of non-metallic materials such as HDPE are becoming increasingly used in industries including water and natural gas distribution [3] because of their superior flexibility and corrosion resistance compared to steel pipelines [4]. As with most kinds of materials, defects occur in HDPE pipes that can cause a failure if left unchecked. Examples of short-term defects are mechanical damage and improperly-formed butt fusion welds. The most common long-term failure mode in HDPE is the progression of slow crack growth (SCG), which is characterised by longitudinal cracks on the outer pipe wall that gradually grow over time, usually with very little deformation around it [4].

 

Non-destructive testing (NDT) techniques are employed to inspect pipelines during both installation and service. It is often very inconvenient to inspect pipelines externally if they are underground or undersea, so in-line inspection robots that travel inside the pipe are used in these cases. Currently, in-line inspection devices are available that can inspect metallic pipes. However, these use non-destructive testing techniques that are unsuitable for non-metallic pipes, such as magnetic flux leakage and eddy current testing.  

 

Microwave reflectometry is an emerging NDT technique that has been demonstrated to identify defects in HDPE and other non-metallic materials more reliably than other NDT techniques [5]. It is performed by sending a continuous microwave beam into the material and measuring the variations in the back-scattered beam caused by defects in the material. These measurements are taken at each of a grid of points over the pipe surface to create a raster scan image in which defects can be identified. To date, no in-line inspection device has been produced that uses microwave reflectometry. 

 

The Challenge

  1. Investigate the use of microwave reflectometry to inspect HDPE pipes for defects.
  2. Create an in-line inspection robot that can travel inside a HDPE pipe and scan the pipe wall for defects using microwave reflectometry. 

 

The Solution

The MILI project team created a functioning inspection robot that comprises of several interlinked subsystems that together scan the pipe and plot the measurements onto a 2D image. 

 

 

Description of robot's subsystems

The main subsystem that performs the measurements is the microwave reflectometry system. It comprises of a Keysight N9918A vector network analyser (VNA) connected by a precision coaxial cable to an open-ended waveguide probe clamped onto the robot's rotary head, and operates in the 18–26.5 GHz band. 

 

The microwave subsystem works with the robotics and localisation subsystems that together accurately position the probe. A PC computer outside the robot running NI LabVIEW 2015 is connected by a serial cable to a Raspberry Pi on the robot (labelled 'MCU' on the block diagram), which in turn controls the robotics and localisation subsystems. The robot itself is made up of a hexagonal chassis fitted with three motorised traction wheels, three encoder wheels and a rotary head at one end. The sets of wheels are sprung against the pipe wall at a 120° angular spacing, with the encoder wheels fitted just behind the rotary head. The microwave probe is clamped onto the rotary head, which is rotated by a geared stepper motor. The traction motors and encoders are connected to a PIC microcontroller with a PID control loop that controls the robot's (and hence the probe's) linear position along the pipe as set by the Raspberry Pi. A second PIC microcontroller controls the rotary head, which sets the microwave probe's angular position.

 

Robot.pngAnnotated image of prototype in-line inspection robot, showing microwave probe on right-hand side

SBD.png

 Block diagram showing the subsystems inside and outside the robot vehicle

 

Process of microwave reflectometry

To perform a measurement, the network analyser transmits a continuous microwave signal out of the probe, which penetrates into the pipe wall. The signal partially reflects at any boundary between different permittivities, such as at the pipe surfaces. The network analyser measures the magnitude and phase difference of the reflected signal picked up by the probe, and compares this with the signal it transmitted to calculate a complex value of reflectivity (S11). Any defect in the pipe wall varies the reflected signal and hence the reflectivity measurement because it is a discontinuity in permittivity, and this effect is the enabling factor for the use of microwave reflectometry to detect defects. In order to produce a 2D image of the pipe, the system repeatedly takes measurements while rotating the rotary head to scan each 'slice' of the pipe, and the robot moves forward by a predetermined step width between slices. 

probe_vna1.png

Microwave reflectometry setup showing network analyser transmitting towards a material under
test and receiving reflected waves via a probe antenna

 

LabVIEW scan control and measurement acquisition program

NI LabVIEW is used to control the network analyser and the robot in order to perform a scan on the pipe. In LabVIEW, GUIs, sequences and functionality to process and display results can be assembled in very little time compared to other programming languages. Communication with the network analyser is facilitated over an Ethernet LAN connection using the Keysight IO libraries and Command Expert, which contain an API for LabVIEW in the form of a Command Expert sequence block. A sequence block sends list of SCPI commands and queries to the network analyser that set parameters and acquire data. 
NDT-BD.png

 Connection diagram of microwave subsystem

 

The sequence used to perform a scan is orchestrated by the LabVIEW VI on the PC.  

The VI performs the following steps:

  1. Position the robot to its starting position
  2. Arm the network analyser's external trigger
  3. Command the rotary head to rotate at a constant speed through 90 degrees. The rotary head's controller sends a trigger pulse over a dedicated cable to the network analyser once the rotary head reaches a constant speed, and the network analyser then takes 401 reflectivity measurements at regular intervals over the same duration as the head's rotation. 
  4. Move the robot forward by a step width of approximately 2 mm
  5. Rotate the rotary head quickly back to its starting position
  6. Acquire the measurements from the network analyser (while the robot is repositioning)

The VI contains four continuous loops: the first loop translates presses of the action buttons into state FGVs (functional global variables). The second loop sequences the scanning process according to the above steps. The third loop, the 'serial control and acquisition loop,' communicates with the robot and the network analyser, and the action this loop performs is determined by reading the 'operation' FGV, which is written to by the first two loops. The fourth loop processes and displays the measurements as a 2D image on the front panel. One of the actions in the third loop acquires the scan data from the VNA and appends them into an 'array of scan clusters' FGV. The fourth loop updates the image whenever new measurements are appended into this FGV. 

 

Results

A specimen of HDPE pipe of outer diameter 160 mm, inner diameter 140.4 mm and wall thickness 9.8 mm was prepared by cutting shallow notches and holes onto an area of its outer surface as illustrated in the defect map below. This pipe and the scanning process can be seen via the video link below. The last (rightmost) notch and hole are 1 mm deep, which meets the Plastic Pipe Institute's recommendation of the maximum defect depth at which the pipe is safe to operate - 10% of the pipe wall's thickness. The robot was positioned in the pipe, placing the probe just left of the large hole, and a scan of a 90 degree sector of the pipe was performed under the defects. 

 

This scan produced the colour-grade image shown below the defect map. It can be seen that the large hole and the notch right of it appear very clearly on the image. The smallest hole and notch, both of which are 1 mm deep, can also be seen as relatively faint indications on the right hand side of the image. There are some artefacts on the image caused by inaccuracies in the scanning process: the vertical dark red lines are caused by a temporary variation in the standoff distance between the probe and the pipe surface, which should be properly controlled in the next iteration of this project. 

Defect map.png

Defect map showing the approximate location and size of defects on the HDPE pipe specimen 

test11 2D deskew 2.png

 Scan image showing the defects on the HDPE pipe specimen

  

Conclusion

A prototype in-line inspection robot has been created and demonstrated to successfully identify a series of defects in HDPE pipe using the NDT technique of microwave reflectometry. This technology has the potential of monitoring the condition of pipelines, enabling the pipeline operator to schedule maintenance more effectively to meet safety requirements. 

 

 

Comments and additional revisions that could be made

The microwave in-line inspection robot is currently an alpha-level prototype designed purely to demonstrate the use of microwave reflectometry NDT inside non-metallic pipes. In order to be deployed into an in-service pipeline, a number of improvements are required:

  • A network analyser was used to take the reflectometry measurements. It was placed outside the pipe since it is too large to fit inside it. The next step is to develop a miniaturised and ruggedised microwave subsystem to performs the measurements, that can be physically integrated into the robot. Following from that, advanced signal processing techniques such as Synthetic Aperture Radar should be implemented to produce sharper images and prevent material outside the pipe interfering with the measurements. 
  • A mechanism should be employed that maintains a constant standoff distance between the probe and the pipe wall. The prototype does not accurately control this standoff distance.
  • All components of the robot in general should be ruggedised to withstand the conditions inside the pipe in which it will be deployed. 

 

Link to Video

https://www.youtube.com/watch?v=CqSDMg5rVb0

 

Comments
RER
NI Employee (retired)
on

Congratulations team. This is an impressive, innovative project - and which could see real usage in industry! 

Rich Roberts
Senior Marketing Engineer, National Instruments
Connect on LinkedIn: https://www.linkedin.com/in/richard-roberts-4176a27b/
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