Radiation tolerance of robotics & telerobotics systems

In this post, a review of the main effects due to the radiation on telerobotics systems is performed in order to justify the approach I took during my research  and clarify the issues found.

Due to the radiation levels in most modern nuclear facilities or nuclear experiments are increasing, new protection measures are needed and the use of remote handling techniques becomes crucial. Different scientific facilities are mentioned here as examples, where the techniques developed in this research could be applied. These are also facilities which have been related somehow with this research. These are: CERN (Organisation Européenne pour la Recherche Nucléaire / European Organisation for Nuclear Research), ITER (International Thermonuclear Experimental Reactor) and JET (Joint European Torus).

CERN is a European organization whose purpose is to operate the world’s largest particle physics laboratory. At CERN, physicists and engineers are probing the fundamental structure of the universe by means of accelerating particles and making them to collide together at close to the speed of light [1]. The collision of high energy particles produces the liberation of -rays, neutrons, muons, etc. While at CERN the main purpose is to study the basics forms of matter, ITER and JET are fusion experiments whose aim is to prove the production of energy via atomic fusion. Both use the tokamak concept where a plasma volume made with hydrogen isotopes is confined with extremely powerful magnetic fields around the torus. JET’s primary task has been to prepare the scientific community for the construction and operation of ITER, acting as a test bed for ITER technologies and plasma operating scenarios [2]. The JET experiment is situated in Culham (UK) and ITER is being constructed in Cadarache, south France. During the fusion experiments, deuterium and tritium are forced to interact with each other, releasing helium, neutrons and energy. The fusion neutrons interact atomically with the elements of the vessel wall causing prompt and residual radiation as beta particles and gamma.

There are four main types of radiation relevant to fusion and fission represented in Figure 1 together with their penetration power [3]:

• Gamma radiation: denoted as , is electromagnetic radiation of high frequency and therefore high energy. Gamma rays are ionizing radiation and thus, biologically hazardous. They are classically produced by the decay from high energy states of atomic nuclei (gamma decay).
• Beta radiation: Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays.
• Alpha radiation: Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is generally produced in the process of alpha decay, but may be produced also in other ways and given the same name. They are a highly ionizing form of particle radiation, and have low penetration depth. They are able to be stopped by a few centimetres of air, or by the skin.
• Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce radiation. Neutrons have a 12 minute half-life so do not exist for long outside the nucleus. They are produced in abundance in Tokomaks.

Figure 1- Penetration capabilities of the main radioactive particles (Retrieved from http://www.hardhack.org.au/book/export/html/37)

The first master-slave manipulators were intrinsically tolerant to the radiation due to its mechanical nature. Nowadays, the increasing amount of electronics included in the modern robots and manipulators make them weaker under radiation conditions, and components like sensors, drives and electronic circuits have increased the sensitivity to radiation of these robots. The radiation susceptible elements of the modern manipulators are usually divided in 3 categories [4]: 1.) the actuators, including the gears and positional feedback devices (encoders, resolvers, etc.), 2.) sensors and 3) the wiring and other communication devices as additional electronic circuits, analogue to digital converters, radio links and sensors’ circuitry.

Motors

The radiation on the electric motors can produce a decrease in performance or a total failure in the system affecting mainly to the winding insulator, bearings lubricant, wiring for power and sensors, connectors and commutation electronics. The degradation can also be produced as an indirect effect of the radiation due to, e.g. the gamma heating causing too high internal temperature [4]. A radiation hardening version of a motor can be obtained by radiation hardened cables, radiation hardened grease or grease free bearings and total absence of electronics. Typically a radiation resistant of several MGy can be achieved by this method.

A study of the ITER project Japan Domestic Agency  has tested 6 commercially AC servo motors under gamma radiation, resulting in one failure after 3.47 MGy due to the radiation damage of electric insulator (Polyester) of the windings and the rest resisting more than 10 MGy. These motors were equipped with hard rad grease (GK-1) which is able to resist up to 25 MGy. For even further radiation resistance, solid lubricant called diamond like carbon can be used, this material is theoretically not affected by gamma rays.

In different results on radiation over motors are presented under a study for ITER, resulting on a radiation resistance depending strongly on the lubricant used. If the wiring insulator is chosen properly between some commons insulators for cables such as Polyimide, Polyamide or PEEK, the motor can withstand several MGy with even using standard commercial lubricants. For even more radiation resistance special grease or solid lubricants are recommended.

Force sensor for robots and radiation performance

With the increasing performance in manipulator robots and even in humanoid robots playing a fundamental role in the industry as well as in scientific areas, the use of force sensors, fundamentally in the gripper, has become necessary [5]. These sensors are used to feel the applied force upon the objects where its load will be measured. The most used force and torque sensors for robots are strain gauges based in piezoelectric effects where a Wheatstone bridge circuit is used to measure the resistance variation with the strain which is then exploited to obtain a signal proportional to the input force. But not every type of sensor is able to be used in a robotic application. Even the most sophisticated sensors which are able to measure forces and torques with 6 dof with a very small noise in the measures have to fulfil the requirements of robotic applications in terms of size, cost and the special issues of each application.

Figure 2-ATI Gamma 6-axis force torque transducer

Figure 3- Honeywell FSS1500NSB force sensor.

There are mainly three types of force sensors available in the market [6]. These are the so called load cells (see Figure 2. ATI Gamma 6-axis force torque transducer.), able to measure forces and torques in several dof and the small sensing elements based on piezoresistive transduction. This last type can be split up in two categories: sensors which use some sort of steel ball to concentrate the force to a silicon sensing element (Figure 3) and sensors that consist of piezoresistive foil layers (Figure 4).

Figure 4- FlexiForce foil sensor.

The so called silicon-sensing elements do not present a good sensitive range for remote handling applications, going up to 15 N [6] , which is not enough for most of manipulation tasks. The silicone is also sensitive to radiation, not being able to cope with more than 10 kGy [7], which indicates that this type of sensing equipment should be avoided for telerobotics operations under radiation.

Simple load cells provided by Sensy and based on strain gauges have been evaluated under radiation conditions for ITER. They do not present any signs of failure for radiation above 18 MGy, whereas than a JR3 6 axis Force/Torque sensor has been irradiated with 9 MGy finding that is not working anymore.

Several strain gauges have also been tested under radiation. These have been encapsulated in specific polymers resistant to radiation and present very good behaviour under radiation. A different source [4] explains a set of tests performed on strain gauges, load cells and 6 axis force/torque sensors, having shown that hardened versions can withstand more than 1 MGy without significant decalibration effects (EFDA, ITA Task Final Report, Rad-hard Programme: Evaluation of rad-hard COTS components for RH equipment, CMM in particular.)

It can be found that simple force sensors using basic strain gauge technology are considered useable under radiation for ITER since strain gauges withstand up to 100 MGy. The Japan agency for ITER evaluated both foil and capsule type strain gauges under radiation, with success up to at least 20 MGy.

In the AREVA recycling plant [8], a Staubli RX robot was used equipped with a hard-rad ATI force sensor. These new sensors called DeltaRad and ThetaRad Sensors manufactured by the well-known ATI Company are prepared to support more than 10 kGy. This meets the requirements of the AREVA facility in terms of radiation tolerance but would not meet the 1 MGy of ITER necessities. These sophisticated multi-axis force-moment sensors have yet to be found in a rad-hard form. Usually these involve on-board electronics, which should be avoided or replaced with a custom made rad-hard version.

Other completely different type of force sensing where a hydraulic manipulator prepared for ITER uses the difference of pressure between each hydraulic chamber in order to calculate the torque exerted in each joint. This hydraulic manipulator is prepared to support the ITER requirements for its operational area of an estimated dose rate of 300 Gy/h and the accumulated dose of 1 MGy.

An irradiation tests (Soichiro, Shigematsu. Blanket Remote Handling System R&D reports, IDM UID 3V8UDF.) have been performed on amplifier for strain gauge in order to find which amplifier on the market could be used from the point of view of radiation-hardness and a rad-hard operational amplifier HS1-5104ARH-Q provided by Interstil has been found to operate until the guaranteed value of 1 kGy without deviation and from 2 kGy up to 471 kGy with some characteristics deviating from specification.

To summarize, both traditional technology based on strain gauges and the new Flexi Force sensors present a good behaviour under radiation. The problem associated to the use of sensor technology is the amplification techniques used to convey the sensors’ output signal in a usable way and the combination of measuring elements to create a 6 dof sensor. Because it is important to place the amplification close the sensors in order to avoid noise, new approaches [9] based on encapsulating the amplification phase in a rad-hard polymer are being tested. The electronics used in these experiments are based on a rad-hard FPGA which is much more radiation resistance than conventional electronics. The shielding options becomes practical for radiation levels up to kGy, but for even more radiation resistance, the shielding becomes impractical due to its enormous dimensions. Obviously this solution requires additional research and development which increases the cost of the final solution, thus, avoiding the use of external sensing equipment in a remote handling application would be convenient.

Electrical cables and connectors

Most cables are based on polymeric insulation which can be used in areas with low background radiation level. In critical areas where the dose levels become high mineral insulation or more modern materials have to be used. Robotics sensors and actuators can be placed on the manipulator arm and on the end effector itself where higher doses are presented. The cabling connections around the manipulator have to present high level of flexibility that can be compromised due to the radiation effects in the insulation. A minimum requirement of 1 MGy [4] resistance is set, level where the cables should keep their electrical and mechanical characteristics. At those levels, flexible polymeric insulation, such as PVC (polyvinyl chloride) and PE (polyethylene) are not resistant to radiation and lose their properties at even lower values. For better behaviour under radiation, Radox (polyolefin), PEEK (polyetheretherketone) or Kapton (polymide) materials are preferred. These cables are usually more rigid and that causes greater stress at connectors. Remotely operated connectors with PEEK insulation have been shown very resistant, up to high total doses of 10 MGy [4].

Lubricants

• Grease lubricant for motors: grease lubricants are typically inexpensive in comparison with solid lubricants although major concerns of the grease lubricants are hardening due to radiation and wearing under high temperature operation. The GK-1 grease lubricant develop by the Japan Agency for ITER has been found to resist up to a limit of 10 MGy. This is not a standard lubricant since it has been modified to increase its radiation hardening properties. Other four types of grease lubricants were developed GS-1, GS-4, GS-7 and GS-13-2 (ITER, Japan Home Team. Iter Task Agreement S 72 TT 14 95-04-11 FJ: Irradiation Testing of Standard Components (T252), Japan, 1998.)
• Ball bearings: the life of the bearing strongly depends on lubrication of the retainer. Some macromolecule materials and self-lubricant alloys are promising materials as the retainer for radiation use. For this, three kind of macromolecule materials (A, B and C types) have been developed, two kinds of self-lubricant alloys (D, and E types) have been applied to the retainer and one ceramics ball bearing without retainer has also been tested (F type).
• Reduction gearbox: a reduction gearbox is combined to be used with a motor. The Harmonic Drive type reduction gear box is simple in structure and is low in amount of grease for lubrication compared with other types. Two types of Harmonic Drives which were lubricated with two different greases were tested (SK-2, standard type grease and GK-1 rad-hard grease). They were found to withstand between 20 and 30 MGy. (ITER, Japan Home Team. Iter Task Agreement S 72 TT 14 95-04-11 FJ: Irradiation Testing of Standard Components (T252), Japan, 1998.)