Rad-hard closed-loop motor control system for ITER Remote Handling
ITER is a first-of-kind Tokamak fusion nuclear plant and its related remote maintenance also represents a significant novelty. During the ITER lifetime, components that operate near the hot plasma will have to be replaced due to erosion and damage. Because of the level of radioactivity, soon after the start of the Deuterium-Tritium pulses, these operations will be carried out by means of full Remote Handling (RH) procedures. Remote maintenance is required inside the Vacuum Vessel, the Neutral Beam Cell, and the Hot Cell. ITER Remote Handling equipment operates in high radiation environments in and around the ITER machine.
In general, the radiation sensitive electronic boards are centralized in control cubicles that are located in green zones and connect to the actuators and sensors through long stretches of cables. However, it is forecasted the need of some front-end electronics that will be located close to those actuators and sensors. In particular for Divertor Remote Handling System (DRHS) it is estimated that these components will face gamma radiation up to 300 Gy/h (in-vessel) and a total dose of 1MGy.
In this business case, MAGICS Instruments has adopted a full-custom approach to achieve the following objectives:
- Design and manufacture radiation-hardened ASICs for sensor readout, motor control, and bus communication;
- Produce a test vehicle to assess the electrical performance and radiation hardness of the ASICs;
- Define the assessment method, design and construct the test rig to determine the suitability, through qualification methods, of the potential use of these ASICs inside ITER remote handling system.
Most of today’s commercial off-the-shelf (COTS) electronics are not specified to meet the demanding requirements of advanced nuclear applications requiring MGy-level total ionizing dose tolerance. They can only maintain their functionality in these radioactive environments through shielding with at least 10 centimeters of tungsten or lead. This leads to heavy and bulky solutions, making the design, installation and replacement of these electronic solutions complex and expensive.
Therefore, the development of custom tailored MGy tolerant integrated solutions becomes favorable for use in these environments. It will not only reduce shielding but will make it possible to place electronics close to front-end sensor transducers and actuators in a radiation environment.
A Gamma and Neutron radiation tolerant closed-loop motor control system has been designed for this task. The system consists of five rad-hard system-on-chip ASICs:
- A resolver/LVDT to digital converter to read out angle information from a resolver or linear distortion information from a LVDT.
- A resistive bridge sensor signal conditioning ASIC to read out sensors such as RTD, thermocouple, and strain gauge.
- A 24V 10-channel limit switch conditioning ASIC to read the status of limit switches connected to it.
- A 24V 10-channel relay driver ASIC to drive high-side solid-state or mechanical relays.
- A bus communication ASIC to implement the BiSS interface (bidirectional/serial/synchronous) protocol, the SPI master protocol, and the RS485 bus transceiver.
The system is capable of transmitting sensor data over 150 meters cable at 2 Mbit/s. It has several advantages. First, the analog signal can be digitized close to the sensor. In this way, signal degradation over long cables to the control room through noise/interference can be avoided. Secondly, multiple sensor signals can be transmitted digitally over a few signal wires. Hereby the number of cables is greatly reduced in the umbilical going to the control room. Thirdly, introduction of customized integrated circuits in these extreme environments allows for more advanced robotic or remote handling solutions, hence creating more degrees of freedom for system integrators.
The main benefit of the rad-hard closed-loop motor control solution, discussed here, is that it can be directly employed in a MGy-level accumulated dose gamma radiation as well as neutron radiation environment. It can digitize and multiplex sensor readout values early in the signal chain. Hereby the sensor values are not distorted by external interferences on a long transmission cable. Moreover it allows reading out and digitizing multiple sensors (pressure and temperature sensors, accelerometers, thermocouples, angular resolvers and LVDTs). Hence these instrumentation modules can be used to follow the trend to digital in the nuclear industry. Without the need for shielding, integrating all electronic components on one single chip greatly improves the reliability of an electronic solution and reduces the risk of any interconnection failures, since the amounts of cable connections, connectors, external components and soldering are significantly reduced.
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