Matthew Dean, head of medical business unit, maxon UK & Ireland, explains how force feedback optimises control in robotic surgery, and why it matters.
In robotic surgery, real-time tactile information is relayed back to the surgeon. This haptic, life-like perception of contact forces at the instrument tip enables intuitive manipulation, aiding precision and minimising the risk of unintended damage during surgical procedures. This capability is vital to the surgeon’s control, enabling the combination of robotic precision and motion scaling alongside the surgeon’s dexterity, experience, and decision-making.
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The feature that enables the real-time interaction from the robot’s instrument is force feedback, achieved by using an electric motor to resist or assist movement based on the loads experienced. Brushless DC (BLDC) motors are the technology choice as they optimise precise, smooth, and repeatable motion. Their electronic commutation enhances accurate low-speed torque regulation, essential for the delicate manipulation of surgical tools, while contact-free commutation improves reliability and achieves the cleanliness required within operating theatres.
To optimise the simulation of the human physical touch, force feedback must feel natural to the surgeon and present smoothness, rather than discrete steps. As a result, motor technology that’s key to achieve this is the ironless winding, which can be integrated to BLDC motors used in force feedback applications.
Preventing cogging torque
A crucial design consideration within motors for precision applications is cogging torque, a resistance that pulls the motor shaft to certain angular positions. As force feedback is often achieved by having a motor coupled directly to the joystick or finger trigger, any cogging from the motor will immediately be felt as a low pulsing or vibration. When micron-level, hair’s width precision is required, cogging can decrease control precision.
The cogging torque challenge is inherent to the iron-core motor design. As the rotor’s permanent magnets interact with the slotted iron teeth of the stator, points of preferred magnetic alignment are created. When the rotor moves between these positions, the varying magnetic attraction produces resistance, creating a cogging effect experienced as a small torque ripple.
Instead, in coreless motors, also known as ironless designs, cogging torque is eliminated as there are no iron stator teeth for the rotor magnets to align with. Instead, a self-supporting copper winding structure is used in place of the iron teeth, allowing the magnetic field to act through a uniform air gap free of discrete slots. As a result of the coreless design, the magnetic attraction remains smooth and continuous throughout rotation.
Optimising torque density
As well as optimising precision and enhancing the natural feel of force feedback, it’s also important for the motor to maximise torque density, maintaining a compact footprint with low mass. A larger motor will increase inertia, reducing the responsiveness crucial to force feedback, and its greater footprint can make the surgeon’s interface bulkier and more difficult to control.
While the iron-core motor introduces cogging, an advantage of the design is greater torque density for a given diameter as its laminated steel stator concentrates and increases magnetic flux, resulting in higher torque.
Increasing pole count
Instead, to enhance the torque density of a coreless BLDC motor, a higher pole-count is the answer, which means increasing the number of permanent magnet pairs within a motor. A conventional motor has a single pair of magnets in opposing north and south position, while a 4pole motor includes two pole pairs of magnets, and the number of poles can increase further.
Increasing the number of poles generates higher torque by raising the magnetic field interactions between the rotor’s permanent magnets and the stator windings over each electrical cycle, resulting in more continuous force generation around the full 360-degree rotation. Performance can be further enhanced through space-efficient winding techniques that maximise the copper fill.
Adding pole pairs does not inherently increase motor size or weight as a higher pole count can be achieved by redistributing magnet segments within the same air gap and housing. While increasing pole-pairs means a reduction in speed, this doesn’t negatively impact a force feedback application, which typically operates at lower rotational speeds.
The architecture of a coreless BLDC motor also eliminates the losses associated with iron-cored motors including magnetic hysteresis and eddy currents. These losses result in higher heat generation and reduced energy efficiency, and can also lead to decreased control precision.
Motion engineering support
Across the world, the use of robotic surgery will continue to grow, and precise and smooth force feedback is a key feature that will optimise patient outcomes. In combination with high-resolution encoders, coreless BLDC motor technology will be integral to the development of these systems.
Achieving this level of performance depends on close collaboration with motion engineering expertise from the earliest stages of system design, ensuring that motor selection, control architecture, and mechanical integration are optimised as a drive system. This integrated engineering approach can also enhance the efficiency of development and accelerate time to market.
