Increasing Patient Safety Through Precision Robotics

An inherent risk of any surgery is uncontrolled or unwarranted motion—by robot or human hand—during a procedure, where even a slight misstep can be catastrophic. A surgeon’s hand is stable to roughly 100 microns, while a surgical robot is stable to roughly 25 microns. Many medical robots today are built to both aerospace and medical standards in order to guarantee quality control. Ensuring patient safety is always the top priority in the design. External brakes, haptic feedback hand controls, no-go zones, motion scaling, tremor filters, and battery backups have all been utilized on medical robots to increase patient safety.

Zero Backlash Brakes

New technologies are being developed to manage complementary issues that became apparent during the design and development of precision robotics. For example, one problem is the presence of backlash, or lost motion, caused by small gaps between a mechanism’s parts. Standard spring set power-off brakes, similar to those used in the original PUMA 560, are readily available and common in robotics, but generally have splined or hex-shaped hubs. These brakes experience considerable backlash because the design requires the hub and rotor (the friction disc in this case) to “float.” In order for the drive hub to float, sufficient clearances are required which consequently create backlash in the brake. (See Figures 3 and 4)

One solution to the issue of backlash is the electromagnetic permanent magnet power off brake (PMB). Electromagnetic PMBs have true zero backlash (no free play, no lost motion), making them ideal for robotic surgery equipment. Essentially, PMBs lock joints into place with absolutely zero radial movement.

In place of a “spring” that creates the normal force to transmit torque (as in Spring Engaged Brakes) the PMBs use Permanent Magnets to create the normal force. When the brake is energized, there is a reverse magnetic flux path created and the “return spring” disengages the braking surface, so when the power is “on” the brake is released. The “return” spring force is extremely low compared to the spring force required to transmit torque in a Spring Engaged Brake, therefore the PM brakes are very quiet in comparison—an important consideration in the operating room.

Permanent magnet brakes are also highly customizable, due to simple configuration and fewer parts, making them well suited for the numerous design iterations needed in order to engineer the best robotic surgery system possible. Other issues in robotic medical applications can also be resolved with the use of a PMB. PMBs are compact in size and have a low profile as well as high torque versus body size. Integral in the design of PMBs is a large inner diameter, which allows wires and cables to be run through the bodies.

PMB brakes operate at low voltage and have low current draw (to release). This is especially important if the equipment is portable or battery operated. PMB brakes are also environmentally friendly and UL, CSA, and RoHS compliant.

This article was written by Craig Harvey, Sales Engineer, and Rocco Dragone, Senior Sales/Application Engineer, at SEPAC, Inc., Elmira, NY. For more information, Click Here.