Designing the Right Motor to Enhance Surgical Robot Precision

Paul Schonhoff, Portescap

A surgical robot’s precision is critical to the success of the surgery — and to the outcome of the patient. Advances in motion system technology will go a long way toward improving surgical robotic precision, unlocking opportunities for previously high-risk procedures, enabling new minimally invasive surgical techniques, reducing damage to tissue and facilitating patient recovery times.

Central to the motion system that will make these and other benefits possible is the electric motor. However, not all motors are created equal — especially where surgical robots are concerned.

For example, it’s important to choose motors that have a zero-cogging brushless DC design and slotless structure, both of which improve robotic control and responsiveness. Coupled with the right specifications related to the power density, encoder and autoclavable capabilities, this brushless DC motor has the potential to redefine existing standards of precision and performance in robotic surgeries.

To explore this topic further, the following article highlights the steps designers should take when specifying their motor and motion system for use in surgical robot applications.

How Robotic Precision Enhances Patient Outcomes

Improving the precision of surgical robots has the potential to increase the effectiveness of the surgery, advance patient outcomes and enable procedures that were previously considered too high risk. Better tool control would reduce the risk of damage to vital organs and arteries close to the surgical site. Similarly, smaller incisions would minimize the area of damage to healthy tissue. Even for robotic surgical procedures considered “standard,” enhancing precision would improve patient recovery. The less invasive the surgery, the less time it would take someone to heal. There would also be less scarring from smaller incisions, reducing the risk of future complications.

The Central Role of the Motion System

Ongoing developments to improve the precision of surgical robots tend to focus on the end effectors that hold and operate tools like blades and grinders. Central to the end effectors’ performance is the motion system that drives and controls the components — specifically, the motor that controls the end effector’s speed and position. In order to deliver smooth torque, which is essential to the robot’s precision, the motor must overcome cogging, or the periodic variation in torque resulting in ripples during rotation.

While cogging can result in jerky motion, it can also reduce the motor’s responsiveness to control commands. Delays in achieving the end effector’s desired position or trajectory will inhibit haptic feedback and decrease the robot’s control. Due to this outcome, it is critical to specify a motor that can deliver as close to zero cogging as possible in order to optimize precision and achieve real-time responsiveness during robotic surgeries.

A Closer Look at Zero-Cogging Motor Designs

Brushless DC (BLDC) motors are preferred when it comes to smooth torque delivery. A BLDC motor uses electronics to achieve commutation — the process of switching the direction of current flow in the motor’s coils to maintain continuous rotor rotation. Electronic commutation can also include integrated Hall sensors to optimize feedback and control of the electromagnetic circuit. This design is typically smoother compared to the mechanical method used by brushed DC motors, where brushes must make physical contact with the rotating commutator.

A slotless motor design also minimizes the cogging effect. Traditionally, the stator, which is responsible for generating the motor’s electromagnetic field, has slots to accommodate the copper windings and provide a path for the magnetic flux. Alternatively, in a slotless BLDC motor design, the windings are distributed evenly, creating a more symmetrical, continuous structure. This design enhances the uniformity of the magnetic force distribution, leading to smoother motor operation. Together, a BLDC and slotless motor design can eliminate virtually all cogging.

Power Density and Encoder Considerations

Motion systems must be small to fit the compact envelope of a surgical robot’s end effector, all while delivering the required torque. Optimizing the torque-to-mass ratio will ensure the motor can provide sufficient rotational force to overcome the delaying effect caused by residual cogging. Minimizing the motor’s inertia can also improve response times. Optimizing the motor’s dynamism not only improves precision, but also helps achieve real-time control. It is important to consider the motor’s design and materials to reduce the mass of the rotor. In addition, motor cooling techniques can minimize the heat-induced resistance that could otherwise increase the inertia.

Combined with a BLDC motor design, encoder performance is critical when it comes to control over the motor’s position and speed. The encoder provides feedback to the controller on the motor’s actual rotation characteristics to ensure more precise control. High-resolution encoder capabilities can enhance this performance further. Magnetic encoders — which sense the motor’s magnetic fields to identify position and speed — are preferred for surgical robots. This type of encoder improves operational reliability during surgeries compared to optical designs, where splashes and ingress of debris can obscure the sensor.

Autoclave Capabilities

A surgical robot’s end effector may operate either inside of, or in extremely close proximity to, an incision during surgical procedures; because of this, both the end effector and the motion system driving it must be sterile to ensure patient safety and well-being. This sterilization is accomplished through an autoclave, which eliminates bacteria and microorganisms via high-pressure saturated steam (see Figure 1).

It’s critical that micro-motion system be able to fully withstand the sterilization process. Portescap has been at the forefront of innovating autoclavable motion solutions, with its brushless DC slotted motors and controllers guaranteed for a minimum of 1,000 autoclave procedures.

Autoclavable encoders from Portescap are designed to withstand at least 2,000 autoclave cycles, while gearheads are rigorously tested to endure 3,000 cycles or more. The method of design and sealing with a high IP rating, along with continuous innovation in materials development, ensures the durability of these components.

Customization

While the need for control precision and durability during sterilization processes are common requirements for motion systems in surgical robots, OEMs usually have their own specific design specifications — including torque and speed profiles, as well as control characteristics. The form of the host robot may also require a specific mechanical integration, which could include a custom footprint or frameless motor design.

To maximize the benefits of this approach, it’s advantageous to involve the motion team as early as possible in the robot design process (see Figure 2). Providing full visibility into the requirements from the outset can speed up development and minimize the need for rework or modifications at a later date. Keep in mind, while a standard catalog motor can save time initially, later design adaptations will extend the project length beyond that of the custom approach. Instead, having a motion system that’s tailored to the surgical robot’s specific operational parameters removes the need to compromise. This custom approach will achieve the optimal balance of motion performance and design integration ease and speed.

This article was written by Paul Schonhoff, Senior Industry Manager, Portescap, West Chester, PA. For more information, visit here .

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