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FEATURE MOTION CONTROL
MOTION SOLUTIONS FOR BIONICS AND ROBOT SURGERY
Robots used in applications such as bionics and robotic surgery rely on state-of-the-art motion solutions, as Portescap’s Julian Del Campo, Paul Schonhoff, and Guillaume Mougin, report
B
ionics, a term coined in the late 1950s, has grown into a multi-billion-dollar industry. Combining biology and electronics, a
recent report projects that the bionic-prosthetics market is set to grow from $1.5bn to $2.8bn by 2030, meaning an annual growth rate of over 9%. Closely allied is a corresponding growth in end effector technology development, required for the final actuation of these robotic solutions, including gripping and holding. Bionic solutions include prosthetic limbs,
providing replacements for hands, wrists, elbows or knees. Electrically-powered prosthetic limbs are driven by small DC motors, and can be controlled by either a myoelectrical signal that uses impulses from the residual limb, or a microprocessor that collects position and acceleration data from sensors to determine proper motion. Alternatively, electrically-powered exoskeletons are wearable devices that provide external support to the body, and typically involve the limbs.
MINIATURE MOTORS IN BIONICS The prosthetic limb or exoskeleton needs to feel natural for the patient, so must be low- weight. While bionic development involves light yet strong materials, such as carbon fibre and graphene, the motion systems that drive them must achieve high power density. For smaller body parts, including fingers and
elbow joints, coreless brushed DC motors are used because of their small size and weight. Here, the coreless feature optimises low mass thanks to a design that removes the traditional iron core. For larger joints, such as hips, knees or shoulders, brushless DC (BLDC) motors are used to support the increased torque requirement. Optimising power density is crucial to maintain low weight, so motor designs like Portescap’s Ultra EC brushless DC motor, which features a coil design that increases the torque- to-mass ratio, is highly beneficial.
The devices must also provide actuation and
control akin to human reactions, so motion has to be dynamic yet smooth. Low inertia, achieved through designs such as the coreless rotor, enables rapid changes in speed, and this must be combined with low cogging, giving smooth torque delivery. When brushless technology is required for higher torque, the Ultra EC motor features a slotless design, which also minimises cogging and enables a consistent magnetic field, reducing variations in torque. Bionic devices are battery-dependent, so to
extend battery life and minimise the size and weight of the battery, high motor efficiency is essential. For smaller joints, the coreless brushed DC design minimises energy dissipation by reducing the effect of eddy current losses. For larger applications, brushless motors are inherently efficient by removing mechanical commutation, but Portescap’s proprietary U coil design extends efficiency for its Ultra EC motor.
SURGICAL ROBOTS Since the first robot-assisted surgery in 1985, today’s $4.4bn surgical robot market is projected to grow at 18% up to 2030 as capabilities are enhanced by trends including AI. A key advantage of robotic surgery is extreme precision, enabling intricate procedures to be carried out with minimally invasive surgery. Like bionics, the motion systems that power surgical robots are fundamental to reach high-level precision. Portescap has extensive experience in motion control to support complex applications and is now expanding its capabilities to solve problems with frameless solutions. Frameless motors consist of un-housed rotors and stators. Some of the benefits addressed with this technology include: • Optimisation of envelope (size, weight reduction) for the specific application
• Excellent torque density and ease of integration with strain wave gearing
46 DESIGN SOLUTIONS JULY/AUGUST 2024
• Minimising backlash for precise positioning and holding • Minimising cogging for smooth motion. Although motion systems for bionics need
to be durable for everyday use, those used in robotic surgery take this up a level because of the need, in most cases, to withstand sterilisation. Portescap’s Surgical Motion Solutions features many autoclavable micro motion solutions, with its brushless slotted motors able to survive up to 1,000 autoclave cycles.
END EFFECTOR CONTROL No matter where or how a robot is applied, its end effectors are essential to pick up or grip an object, hold a tool, or push/pull an object. Commonly, end effectors are based on grippers with either parallel jaws or three fingers, plus angular motion grippers. Developments over the next decade will see the integration of machine vision with AI, enabling grippers to identify and manipulate objects with even greater accuracy, allowing them to perform multiple tasks simultaneously. To achieve these advances, electric gripper control, rather than pneumatic, is the norm, thanks to the higher degree of control and power density provided. Demand on motion systems that accentuate these advantages will continue. This will include the increasing need for precise position and force control. Dynamic motors that can rapidly accelerate will help achieve this. In addition, motors will also be equipped with advanced feedback devices, such as high-resolution encoders, to enable accurate and responsive grip adjustments.
CUSTOMISED MOTION Each sector has an array of robotic application possibilities and one motor design can’t serve all needs. To achieve the most effective motion specification, there will be a growing trend for customisation. This approach will be crucial to optimise robotic performance to specific tasks, and hence improve the patient or wearer outcome. Customisation can also streamline integration for the robot builder.
Portescap
www.portescap.com
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