This creates a challenge similar to building a very detailed model
ship inside a glass bottle.
As robots shrink in size, minimizing friction between mechanical components becomes increasingly important. For flexible
cable-driven instruments, the complete actuation mechanism
(from flexure to actuators) should be designed to minimize friction between the actuator and the instrument tip. Achieving low
friction is necessary to produce a fast-actuating, highly responsive
system. This is particularly important in surgical robotics, as the
system must be able to keep up with the motions of the surgeon’s
hands. If the system does not have enough bandwidth to faithfully recreate the movements of the surgeon, its operation will be
slow and confusing to use.
One solution to low-friction flexures we identified is to use
rolling elements, like pin joints, to build the flexure instead of
sliding ones (see Figure 2). This is because one component rolling upon another will resist motion significantly less than if they
were sliding against each other. However, using rolling elements
can come at the cost of losing the linear behavior of the flexure
mechanism, especially if the design of the rolling curve is limited
by manufacturing techniques. This means cable pairs used to
articulate each of the steering degrees of freedom of the flexure
need to be pulled and pushed at different rates because the total
cable length varies throughout the range of motion. In order
to maintain internal cable tension, the cable length needs to be
compensated in some way. One approach is to use one actuator
per cable, which results in two actuators per degree of freedom,
but this increases the size and cost of the system.
Solving this non-linear cable lengthening problem begins with
analyzing the kinematics of the flexure and the requirements
imposed by this non-linear behavior. This is the first step in designing a mechanism that produces a linear approximation of the
cables’ required motion profile. Numerical simulation and optimization techniques can be employed to optimize the geometry of
the mechanism until the error between the linear and non-linear
motions is reduced to an acceptable level. We used this approach
to optimize a gimbal-type mechanism employed to drive the
actuation cables of the flexible arm shown above.
Making precision components at this scale in a cost-effective
manner is also a challenge. Techniques like precision micromachining can achieve and maintain the required tolerances of the
Figure 2 – Flexible micromanipulator arm with rolling links.
(Image Credit: Dr. Rodrigo Zapiain)
Figure 3 – The kinematics of the driving mechanism were optimized using numerical simulation. (Image Credit: Dr. Rodrigo Zapiain)