Extreme Design: Unique materials, design yield a motor than can operate in MRI’s intense field

(Editor's note: This is the next in our series of “Extreme Design” stories, where a project is focused largely on one overwhelming priority. For the previous entries, see the Other articles in this series section below, after About the author .)

Advances such as magnetic resonance imaging (MRI) have improved the surgeon's ability to see into the human body, and microsurgical techniques have evolved to the point where surgeons are reaching their physical limits in terms of both strength and precision. The next logical step is to design machines and robots to perform tasks beyond the ability of a surgeon's hands, and to have these tools perform while under the MRI, to provide visual capabilities that exceed those of the human eyes.

These efforts have traditionally failed, since the powerful magnetic field of the MRI has restricted both use of components made of metal, and those that generate a magnetic field of their own. The magnetic nature of electric motors and their metal components have made motion impossible within the MRI.

Now, with the significant developments from design teams working with ceramic motors, motion is possible without metal or magnetic properties, thus enabling the design of moving surgical devices within the MRI. In addition to the non-magnetic benefit of the ceramic motors, the designers found that their precision motion-control abilities increase the granularity with which a surgeon can work, from within an eighth of an inch using the human hand, to within the width of a hair. Using the real-time visibility into the human body provided by the MRI, these newly designed devices enable surgeons to manipulate tools at a microscopic scale and conduct surgeries that were previously difficult or impossible.

One such example is the design of the neuroArm, Figure 1 , the world's first MRI-compatible, image-guided surgical robot capable of both microsurgery and stereotaxy.

Figure 1: Ceramic motors enable precision movement of the neuroArm

(Click on image to enlarge)

A team of experts from the University of Calgary; MacDonald, Dettwiler and Associates Ltd. (MDA); and Nanomotion Inc. (Johnson Medtech group) overcame the hurdle of building a machine capable of conducting microsurgical operations safely within the strong magnetic field of an MRI system.

Using the ceramic motors from Nanomotion, the neuroArm allows surgeons to conduct microsurgical operations in the brain while the patient is in an MRI. The MRI helps the surgeon manipulate surgical tools with precise and microscopic movements in real-time. resulting in a safer and more successful surgery. The NeuroArm has sixteen HR2-1-N-3 piezoelectric motors from Nanomotion, along with an AB5 module to drive the motors. These motors are connected to six rotary joints and one linear axis.

Design Challenges
For any device to move and function inside the magnetic field of the MRI, it must be non-magnetic to avoid the creation of 'artifacts' in the image. This means that the motor, position sensors and bearing structure must all be free of magnets and magnetic material.

Using piezoceramics in a servo motor creates the opportunity to configure a non-magnetic motor. There are no windings, laminations, or magnets. However, as Nanomotion's ceramic servo motor transmits motion through the frictional contact of an alumina fingertip and ring, there is a normal force applied to the structure, perpendicular to motion.

The challenge faced by McDonald Dettwiler and Nanomotion was to define the rotary bearing configuration that offered the qualities of high stiffness/load with low friction, in materials that were completely non-magnetic. There are numerous ceramic bearings that are available to engineers today, however the vast majority are hybrids, combining ceramic and stainless steel, making them unusable.

In addition to the bearing characteristics, the robotic structure that housed the bearing needed to provide sufficient support for the bearings and maintain a stiffness of 30 N/micrometer. The servo motors also required position encoders that could function optically to control position, and be constructed of non-magnetic materials.

A close technical collaboration between McDonald Dettwiler and Nanomotion resulted in a robotic housing based on a combination of non-magnetic metals and engineered plastics. The housing was designed to sufficiently capture and preload the radial bearings which were all ceramic and thus completely non-magnetic. In addition, two sensor technologies were identified and tested, with both confirmed to have non-magnetic materials/fields.

A structure that has insufficient stiffness may allow the motor's normal force to create a preload (or compression) against the structure that will be exercised during the operation of the motor's ultrasonic standing wave. This can create vibrations in the motion structure, which may result in both poor performance and excessive wear. Ultimately, the combination of material selection and testing yielded the ability to apply Nanomotion's ceramic servo motor on each rotary joint of a six degree-of-freedom (DOF), articulated-arm robot, to create all the required motion inside the magnetic field of the MRI.

In addition to the overall materials and motor functionality, the electronics that drive the motor must be compatible with MRI. A traditional permanent-magnet motor usually results in a high-frequency pulse width modulated (PWM) signal from a digital drive. These PWM signals can interfere with the MRI and result in the generation of artifacts. Nanomotion's ceramic servo motors operate from two AC sine waves, allowing the drive to operate the motor inside the magnetic field.

In some MRI applications, such as an ultrasonic probe designed to coagulate a tumor, the structure is particularly difficult. In this instance, a bearing structure was configured entirely from engineered plastics, resulting in low stiffness. To avoid issues with vibration and wear, motors of equal size were placed opposed to each other, to balance the normal force, resulting in zero impact to the structure.

How ceramic motors work
The piezoelectric effect in piezoceramics converts an electrical field to mechanical strain. Under special electrical-excitation drive and ceramic geometry of Nanomotion motors, longitudinal extension and transverse bending oscillation modes are excited at close-frequency proximity, Figure 2 .

Figure 2: Principle of the ultrasonic standing wave motor from Nanomotion, U.S. Patent No. 5,453,653

(Click on image to enlarge)

The simultaneous excitation of the longitudinal extension mode and the transverse bending mode creates a small elliptical trajectory of the ceramic edge, thus achieving the dual mode, ultrasonic standing wave motor patented by Nanomotion, Figure 3 .

Figure 3: Construction of the piezoceramic motor shows the element, finger tip, and the spring which acts as the pre-load force.

(Click on image to enlarge)

By coupling the ceramic edge to a precision stage, the resultant driving force is exerted on the stage, causing stage movement, Figure 4 and Figure 5 .

Figure 4: Principle of motion for the Nanomotion's motor; the finger tip pushes the object sideways in either direction

(Click on image to enlarge)

Figure 5: The complete, enclosed ceramic motor installed in the system

(Click on image to enlarge)

The use of periodic driving forces at frequencies much higher than the mechanical resonance of the stage allows continuous smooth motion for unlimited travel, while maintaining high resolution and positioning accuracy typical of piezoelectric devices. Travel can be linear or rotary, depending on the coupling mechanism.

If more motive power is required, the motors are specifically built to allow for cascading: as power requirements increase, another motor is added to the system. When the driving voltage is not applied, the ceramic plate is stationary and generates holding torque on the stage. Unlike other braking devices, the holding torque of the Nanomotion motor does not cause any position shift.

The piezoceramic motor offers performance advantages over the conventional motors. Its small size and weight makes it well-suited to miniaturization. It operates with almost zero audible noise, necessary for environments sensitive to sound. In addition, since these motors do not have any magnets, they do not emit any EMI/RFI, also critical for MRI environments. Due to their precision and fast response time, they are a appropriate for applications such as surgical robots or imaging systems (scanners and electron microscopes.

The use of piezoceramics in an operational mode that generates an ultrasonic standing wave is an extremely effective motion-transmission device. The piezoceramics can generate velocities ranging from less than 1μm/sec up to 300mm/sec and operate with forces comparable to permanent-magnet motors. Additionally, from a performance perspective, while a permanent magnet motor has a time constant of a couple of milliseconds, piezoceramics have a time constant around 100 microseconds.

Additional applications
In another application, non-magnetic motors are used to rotate a probe that is generating a heat profile. The precise control of the rotational speed, closing the position loop and thermal profile via the MRI, allows healthcare professionals to completely destroy a tumor.

MRI-guided transurethral ultrasound therapy is a minimally-invasive treatment for localized prostate cancer initially developed within the Imaging Research group at Sunnybrook Health Sciences Centre in Toronto, Canada, and now undergoing commercialization as Profound Medical. This concept requires hardware that can operate inside the bore of an MR imager during imaging with no mutual interference between the two systems.

A minimally-invasive device inserted into the prostatic urethra delivers ultrasound energy to carefully selected regions of the prostate to coagulate a precise target volume with the gland. Unique control over the spatial deposition of energy is achieved by the ability to acquire real-time temperature measurements with MRI in a proprietary control system that continuously updates the energy output of the device.

The prototype MRI-compatible rotary motor is built using non-magnetic components including HR2-N series Nanomotion motors. The motors rotate a ceramic ring which provides the rotational motion of the motor. Extremely accurate rotational velocities have been achieved ranging from 5 °/minute, at up to 30 rpm, with this design. The motor is attached to a positioning fixture, and is advanced into the bore of a clinical MRI for the transurethral procedure.

Extensive testing within the MRI environment has verified the capability to operate the rotational motor during imaging with no detectable impact on image quality or signal to noise ratio (SNR). Tests have been performed at both 1.5 and 3.0 tesla, and performance of the motor in both environments is equivalent to outside the magnetic field or in a normal environment.

Additional information
1. “NeuroArm”, click here
2. “neuroArm robot performs brain surgery: A world first”, click here
3. “NeuroArm”, Bio-medicine , 2008, click here

About the author
Alan Feinstein is the President of Nanomotion Inc., a subsidiary of Johnson Electric group, developer of the patented technology using piezoceramic material to produce motion, with over 100 patents and patent applications covering the core technology, electronics, servo-control and processes; Johnson Medtech is Johnson Electric's medical products group.

Other articles in the EETimes/Planet Analog series:
1. “Extreme Design: Developing integrated circuits for -55 degC to +250 degC “, click here
2. “Extreme Design: Ultra-compact embedded computer overcomes multiple design challenges”; click here
3. “Extreme design: SuitSat pushes engineers' limits”; click here
4. “Mars lander's chem lab is NASA's MECA”; click here
5. “Engineering “ESI” instead of “CSI”?, click here

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