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The In-Loop Electro-Magnetic Tracker is a project to develop an open design for an electromagnetic position tracker. While we are developing this for use in robotic microsurgery, ILEMT-based designs should be useful for a wider range of purposes such as virtual reality and biomedical measurements. All source code and hardware designs are publicly available for reuse (CC_BY). This is a research project funded by the US National Institute of Health.
I am posting progress to: ILEMT2@twitter.com
- https://github.com/robmacl/ilemt_hw: Hardware design files, mainly Kicad. Also my notebook and some spice simulations.
- https://github.com/robmacl/ilemt_fpga: Verilog FPGA code and Xilinx Vivado project. This can be built using the free Vivado version.
- https://github.com/robmacl/ilemt: Labview code for signal processing and visualization, Matlab code for calibration.
Motion trackers measure the position (XYZ) and orientation (roll, pitch, yaw) of a sensor. Like motion capture systems, they are often used to measure human motion, but the emphasis is on precise measurements of one or a few sensors at a smaller scale. You can think of a position tracker as a portable coordinate system, with a fixed part that defines the coordinate system, and moving parts (sensors) that are tracked.
Some trackers work optically (using cameras), but they require an unobstructed sight-line, and are not easily scaled to kilosample/second measurement rates. An electromagnetic tracker operates using several magnetic fields created by a fixed source which are measured by the moving sensor. These fields are in the Extremely Low Frequency (ELF) range, and do not interact with the human body and most other non-metal materials, passing through unaffected.
Proprietary EMT products have been available since the 1980's. We hope ILEMT will create interest because of its open design, but the main reason for the project is to develop a tracker that has significantly better speed/noise performance than commercial products. Although commercial EMTs advertise output rates of 100's of samples/s, the actual bandwidth is often reduced to only a few Hz to control noise. The current prototype reaches 20 microns peak-to-peak noise at 1500 samples/s (300 Hz bandwidth), over a 200 mm workspace. We plan to reduce this noise level 2-3x.
The In-Loop part of ILEMT comes from the goal of supporting easy integration into real-time control loops where latency (time delay) is crucial. The open design is very valuable here because it enables application code to run on the same compute hardware as the tracker signal processing.
Aside from the engineering effort to create an open design with much better noise/speed tradeoff than existing trackers, ILEMT also has a research aim reduce the position errors caused by metal objects near the workspace. We are using a unique dual-frequency approach where each source axis emits two carriers with a wide frequency separation, near 300 Hz and 10 kHz. The high carrier provides high measurement speed, while the low carrier experiences significantly different interference from metal. We have already demonstrated that we can compensate eddy current interference from highly conductive metal (aluminum, copper) by combining the high and low rate measurements. Ferromagnetic metals (steel) will require a different approach because the ferromagnetic effects increase at the low carrier frequency.
The product of the ILEMT project will be a production ready reference design, including both fully open hardware designs and all necessary software. The reference hardware design is optimized for performance and flexibility, not cost. We expect that with single-unit distributor pricing, the cost of parts alone will be $1000, plus $300 per sensor input. When the design is ready we will crowd-fund a small production run.
It is the nature of magnetic trackers that you want to have a variety of source and sensor coil types which are adapted to particular applications. Larger sources and sensors permit longer working distances. Our source and sensor coil designs are aimed at measurement distance of 150-200 mm, but the measurement distance can be increased to the meter range by using larger coils. Off-the-shelf RFID antenna coils are quite inexpensive, and can be used for both sensor coils and some smaller sources. We are using hand-wound coils now, but will look into a small production run of custom source coils.
The mechanical constraints on coil size and mounting also varies across different uses. ILEMT supports arbitrary arrangements of the three source and sensor coils, which gives considerable packaging flexibility.