Motors and Motion Control Technologies in Orthopedic Surgical Robotics

Oct 15, 2023

Motors found inside orthopedic tools normally serve as the primary power source for driving devices like bone saws, drills or shavers. In these variable speed applications, motors need to be exceptionally small in diameter, lightweight & capable of running at high speed.

John Chandler, Control Systems Director | FAULHABER MICROMO

Several key roles exist for motor, and motion control technology in the field of orthopedic surgery. The applications in this field are demanding and diverse. Fundamental applications range from providing primary power for surgical tools, to position control of robotic joints, to generating force feedback in haptic interface devices. Other critical applications for motor and motion control technology involve pumping, ventilating and dosing, to name just a few. Each of these unique functions requires a highly specialized motor, and motion control solution.

Motors found inside orthopedic tools normally serve as the primary power source for driving devices like bone saws, drills or shavers. In these variable speed applications, motors need to be exceptionally small in diameter, lightweight, and capable of running at high speed. They also need to be capable of running at a high power level continuously. Unique environmental requirements found in surgical applications drives the need for exotic materials, lubricants, bearing technology, autoclavability, tight tolerances and strict quality control. FAULHABER offers our BP4, BHS, BHT and BA series brushless motor products, all specifically designed for use in surgical tool applications. FAULHABER also pairs these specialized motors with optimized control products supporting closed loop speed regulation, sensorless operation, and tuned power delivery optimized for these slotless, high-speed, power dense motors.

An entirely different set of performance requirements emerges for motor and motion control technology when surgical tools need to be robotically controlled, or actively supported during an orthopedic procedure. Here, positioning motors, or "servomotors", are required to produce high torque, but not necessarily high continuous power. In this case, a larger diameter, relatively low speed motor works best. For servo positioning, a motor's "power to weight ratio" is often less critical, but it's "torque to inertia ratio" is typically more critical. This is because motors applied in servo mechanisms need to hold position, but they also need to be highly dynamic when changing position, or reacting to a change in load when holding position. To support high dynamics, servomotors are fitted with high-resolution encoders that provide the increased level of feedback necessary for more sophisticated servo amplifier control. High pole count FAULHABER BXT motors, in a magnetically laminated out-runner configuration, excel in this situation. The availability of BXT motors, complemented by an extensive range of GPT planetary gearheads, high-resolution optical, magnetic and absolute encoders, as well as miniature intelligent servo amplifiers, make sourcing of a fully optimized servo system possible, when collaborating with FAULHABER as a vertically integrated supplier.

Unlike the variable speed motors used to power surgical tools, or servomotors used in robotics, motors applied in haptic interfaces play a completely different role in orthopedic surgery. In fact, motors used in haptic devices actually function more as generators than they do as motors. This is because the purpose of a haptic interface is to produce a reference signal for the robotic control system to follow, but also to simultaneously provide a realistic sense of force, or resistance, to the surgeon performing the procedure remotely. In a haptic device, motor speed and power are less critical, but instead, sensitivity and fidelity are paramount. Motors used in haptic interfaces must have very low inertia; else, the surgeon will feel an unnatural flywheel effect when interacting with the device. Also, the motors used in haptic devices are normally ironless DC, or slotless brushless AC, because this type of motor provides a very constant level of torque for a fixed level of current, at any angular position. As such, these motors are referred to as "Zero Cogging" motors. With adequate controls in place, they allow an orthopedic surgeon to remotely sense, or feel the procedure and not feel some unwanted parasitic effect of the motor's geometry. Coupling a zero cogging motor to the surgeon's hands through precision gearing, like the type of gearing found inside a precision mechanical Swiss watch, provides the surgeon with a sense of touch that feels completely natural. FAULHABER offers zero cogging motors with optimized gearing for haptic applications that are truly sensitive, smooth and forceful. In fact, they are just what the doctor ordered!

One of the primary technology drivers in advancing orthopedic surgical treatment is the miniaturization of motor technology used in surgical tools and end effectors. Given smaller motors, design engineers are able to produce surgical tools that are more ergonomic. A more ergonomic tool design reduces fatigue for the surgeon. Alternatively, if a tool or end effector is robotically controlled, then a smaller tool size and weight subsequently reduces the size, mass, and cost of all robotic structural elements needed to support it. Simply put, whether hand held or robotically controlled, it really pays to have a small, powerful motor inside a surgical tool or end effector.

Making a motor small is challenging, but increasing its output power at the same time, is twice as challenging. This is because for a fixed level of output power, a motor's surface temperature will increase if its surface area, or housing size, decreases. Delivering an increased level of power though a smaller motor housing means that even more heat must dissipate across this reduced surface area, increasing temperature even further. So optimally managing the motor's thermal design becomes a critical factor in the design of electric motors. Characterizing the thermal mass and thermal resistivity between various elements inside a motor has a very large impact on its ability to perform in an orthopedic application.

The design of high performance electric motors is complex. Configuring a motor's magnetic geometry to minimize loss, while maximizing torque production, is critical. Maximizing copper fill, while maintaining the structural integrity of a coil operating at high temperature in a slotless design, is especially challenging. Pushing the limits of all mechanical elements requires detailed analysis of stress, strain and wear. Electrical insulating materials must be selected for high dielectric strength and for efficient thermal properties. Designs need to function quietly at high speed and deliver smooth torque. Designs need to be manufacturable and they need to meet demanding medical quality standards. Designs need to be safe to use in orthopedic applications.

FAULHABER continuously advances motor technology in this market by leveraging advanced multiphysics design tools, by taking advantage of advancements in the material science, and by continually improving the art of manufacturing these energetic electromagnetic machines. As a vertically integrated supplier in this market, FAULHABER is also able to leverage key investments in motor technology and infrastructure to manufacture complete motion control systems for applications found in surgical robotics.

Simply put, motor and motion control systems need to be custom engineered for each unique surgical end use case. Each unique profile shape or surface found at the working end of an orthopedic surgical tool operates most efficiently at a specific level of speed and torque. Understanding this speed and torque represents the starting point for designing a custom motor or motion control solution. With this starting point, a thermal analysis can be performed to determine physically how large a motor is likely to be, what type of motor might work best, and whether a geared or direct drive approach might be indicated.

Beyond just sizing for power, it is equally critical to consider what quality of motion needs to be delivered to facilitate the end procedure. This normally leads to a discussion of resolution, accuracy and repeatability of the required motion. This information helps determine what feedback sensor is best; optical, magnetic, linear, rotary, incremental, absolute, or sensorless. Discussion of motion quality, will also directly lead to a discussion of bearing types, configuration options, lubricants, and some thought about where best to locate feedback sensors if required. Quality of motion will also lead to a discussion of required bandwidth, stiffness, and smoothness of control. When the best solution starts to take focus, consideration can be then be given to how to interconnect selected elements, optimizing them for performance, manufacturability, quality and cost. Finally, consideration can be given to how best to enclose the elements for each end use case.

When FAULHABER is contacted at the conceptualization phase of a major new project, or product development effort, we respond by pulling together an interdisciplinary team of engineers representing key steak holders in technology, design, manufacturing and quality, to host an open round table discussion with our customer's design team. We know from experience that the best outcomes and best fit solutions result when, together with the customer, we look at the design and manufacturing challenge holistically, and together we drive to the best final solution.

Surgeons fundamentally focus on procedures to improve patient outcomes. Medical device manufactures, employing design engineers, focus on developing the tools and equipment needed to perform these surgical procedures. FAULHABER is a vertically integrated motor and motion control supplier serving these medical device manufactures. Our customers, medical device engineers, of course are very interested in the highest performing motor and motion control technology. But more commercially in scope, medical device manufactures are most often seeking capable suppliers to collaborate with, to bring a new technology to market that takes advantage their unique intellectual property (IP). Typically, their IP centers on the application of perhaps some new imaging capability, a novel surgical procedure, or might be even be related to a breakthrough in information technology. And yet the surgical device or system required to bring this critical IP to market contains a complex motion control subsystem consisting of; motors, gearing, bearings, couplings, lead screws, lubricants, feedback sensors, and drive electronics. All of these electro-mechanical elements need to work together as a system, consistently and flawlessly, to deliver a critical patient outcome. For medical device manufactures, success or failure depends heavily on executing a new product design with experienced engineers, validating it properly, and then being able to ramp production while meeting demanding quality control standards.

Vertically integrated motion control suppliers like FAULHABER have the component technologies, engineering and manufacturing experience needed to optimize this key subsystem design. Vertically integrated suppliers, who manufacture all of the key component technologies in house, are able to control supply and take end-to-end system responsibility for a subsystem, by optimizing the design integration of all critical components, and by applying best practices in each end use case. For medical device manufacturers, motion control system suppliers certified to ISO13485 are most qualified to assist in bringing a new product to market.

Miniaturization of motor technology used in surgical tools to lower impact on patients and surgeons will continue be a diving factor. Robotics in orthopedic surgery will become more commonplace. A robot-assisted operation will be possible for nearly all orthopedic procedures. Today, more than 70 companies worldwide offer systems for a wide range of procedures. For example, procedures on the spinal column, the knee, the hips, in the abdomen, in neurosurgery, in the ear, nose and throat area.

Devices used in the operating room have different designs depending on their purpose. Form factors for these devices range from very voluminous, multi-armed systems to systems that are no larger than a beverage can. While the former is used for complex minimally invasive procedures, the latter system is used simply to accurately hold instruments at an optimal orientation. Successful systems integration of motor and motion control technology is at the heart of this medical trend. Safety standards will continue to evolve and improve for these systems. Patients will benefit from this technological revolution.

About John ChandlerJohn Chandler studied Electrical Engineering at the University of Michigan and has worked in the electronic drives and motion control industry for over 34 years. John's application experience is extensive and he is focused on working with advanced OEM development teams in the medical device industry, emphasizing a collaborative and interdisciplinary approach

Since 1961, FAULHABER MICROMO has partnered with OEMs to deliver high precision, high performance, custom micro motion system solutions to markets such as medical, robotics and automation in North America. FAULHABER MICROMO's tradition of innovation started decades ago in Germany. The groundbreaking invention of the FAULHABER coreless winding started it all for a market that produces millions of motors today. How can the FAULHABER MICROMO team help you deliver your next innovation to market first?Learn more about MICROMO's solutions for the most demanding applications, our diverse motion products and technologies, online ordering, Engineering and R&D teams, Clean Room Assembly, Machining Center and other services at our Clearwater, FL facility at https://www.faulhaber.com.

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