Motors take on Cancer

The Project: A multi-leaf collimator device needed to offer greater levels of accuracy than any similar product on the market.

The Solution: Multiple devices were incorporated, including 180 motors, to shape gamma ray radiation therapy beams, controlled by one interlinked motion control system.

 When it came to designing a multi-leaf collimator (MLC) device, ViewRay took on the challenge to produce an end product that would be more accurate than others on the market. ViewRay collaborated with Maxon Precision Motors for critical components, including a custom motherboard, motors, encoders, gearheads, and individual motor control modules, according to an engineering team member.

ViewRay Incorporated is a privately held medical device company developing advanced radiation therapy technology for the treatment of cancer. The ViewRay system provides continuous soft-tissue imaging during treatment, using MRI-guided radiotherapy, so that clinicians are able to see where the actual radiation dose is being delivered and adapt to changes in the patient’s anatomy.

The EPOS2, brushless DC motor controllers are easy to install and implement using one of several major interfaces, plus are compact enough to keep the system’s footprint small.

The EPOS2, brushless DC motor controllers are easy to install and implement using one of several major interfaces, plus are compact enough to keep the system’s footprint small.

Overall, the ViewRay system includes five sub-systems, which are seamlessly integrated to provide optimal patient care. The major subsystems are: real time MR imaging, treatment planning, dose prediction and optimisation, real time soft-tissue targeting, and remote review and approval. The treatment delivery is performed in a split-magnet MRI system with rotating gantry assembly to position three shielded Cobalt-60 sources with the three multileaf collimator’s.

The imaging in other radiotherapy technologies took place before or after treatment, not while the beam was on. This was a limitation in providing therapy because targeting could not be adjusted dynamically. Soft-tissue motion often allowed a tumours position to shift during treatment, causing soft tissue damage.

ViewRay’s MRI-guided Radiotherapy System is designed using 180 motors and controllers for accurate gamma ray delivery.

ViewRay’s MRI-guided Radiotherapy System is designed using 180 motors and controllers for accurate gamma ray delivery.

The ViewRay system solved this problem by using a combination of MR imaging and radiation therapy delivery technologies. With continuous soft-tissue imaging, during treatment and with the beam on, ViewRay tools can refine the target and re-optimise the dose while the patient is on the treatment table. This was the first MRI-guided Radiotherapy System produced.

According to the engineering team, “The MLC motor control system is a critical portion of the system. We knew it would also be one of the more challenging to design because of proximity to the MRI magnet, and volume constraints due to the gantry configuration.” The system uses three gamma ray sources, mounted in separate shielded heads. For ViewRay, the double-focused MLC is designed to sharpen field edges to produce penumbra comparable to conventional accelerators, so that clinicians can treat patients with greater confidence.

The team chose Maxon’s EPOS2 Module 36/2 compact digital positioning controllers for their small form factor, which allowed them to package 60 channels of motion control for each collimator. Thirty modules can fit on a single custom motherboard, and there are two motherboards per collimator. Each ViewRay system required three of these collimator’s, one for each of the three heads used in the system.

EPOS controllers are compact, fully digital, smart motion controllers designed for use as plug-in modules in customer-specific motherboards, and can be used for single-axis or multi-axis motion control systems. Each EPOS module implements a flexible and highly efficient power stage, and drives a brushed DC motor with digital encoder. They are especially designed to operate as a slave node in a CANopen network. They have a maximum current output capability of two amps, running from an eleven to thirty-six volt DC power supply.

In order to interface with the controllers, the ViewRay team used the resident EPOS2 firmware, which provided simple to implement operating modes and monitoring functions required by the control system. What made things easier was that the controllers are CANopen compatible, which allowed the design team to define up to 127 device addresses on a single bus segment.

This CAD drawing of one of the collimators, shows the motors extending from one of the sides.

This CAD drawing of one of the collimators, shows the motors extending from one of the sides.

Each of the collimator’s contains sixty leaves that are arranged in two opposing banks of thirty. Since there are three collimator’s, the device uses 180 EPOS controllers as well as 180 motor/encoder/gearhead assemblies. The MLCs are mounted on the gantry system to provide collimation of the three gamma radiation sources with respect to the target. While the gantry is moving into position, each collimator leaf is positioned according to the treatment plan. To position each leaf, CANopen bus commands are directed to the associated node consisting of a motor, encoder, gearhead, and controller. The result is a precisely collimated shape that matches the treatment plan.

The 180 motors used in the system are RE16 brushed DC, 4.5 Watt motors. Each motor operates at speeds up to 11,000 rpm. Along with the motors are MEnc13 Hall-effect encoders for precise motor control and GP16A gearheads with 19:1 reduction, providing adequate torque for positioning speeds of 1.0 cm per second.

The EPOS Studio software package provided an easy-to-learn, easy-to-use development environment for proof of concept and end-product development activities, according to engineering. Due to MRI magnetic field effects, the MLC controllers had to                  be  remote from the motors, and housed in a convection-cooled chassis.

www.maxonmotor.com.au

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About maxon motor Australia

maxon motor is the world’s leading supplier of high-precision drive systems. More than 40 years’ experience, constant innovation, top-quality products and competent customer service make maxon motor a reliable partner in the drive technology sector. We are driven by your specific requirements. The product range incorporated in the modular system is comprehensive: Brushed and Brushless DC motors with the unique ironless maxon winding Flat motors with iron cores Planetary gears, spur gears and special gears Sensors (encoders, DC tachos, resolvers) Servo amplifiers, position controllers and control electronics High-tech CIM and MIM components Customer specific drive solutions. maxon motor is a Swiss company headquartered in Sachseln (Central Switzerland) that employs more than 2000 staff worldwide. With 14 sales companies based in over 30 countries, sales partners in another 10 countries and with six production sites, maxon motor are able to provide locally-based and competent customer service. maxon motor helps provide innovative solutions at competitive prices for numerous applications in various markets, such as Medical technology, Industrial automation and robotics, Aerospace industry, Automotive industry, Communication, Consumer applications, Inspection and Instrumentation and Safety engineering. maxon motor Australia is based in Mt Kuring-Gai NSW and our staff will be happy to help you with all your DC motor and technical enquiries. www.maxonmotor.com.au

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