Robotically controlled surgical instruments are often used in minimally invasive medical procedures. One architecture for such instruments includes an effector or tool such as forceps, a cutting tool, or a cauterizing tool mounted on a wrist mechanism at the distal end of an extension, sometimes referred to herein as the main tube of the instrument. During a medical procedure, the effector and the distal end of the main tube can be inserted into a small incision or a natural orifice of a patient to position the effector at a work site within the body of the patient. The wrist mechanism can then be used to move and operate the effector when performing the desired procedure at the work site. Cables or similar structures extending through the main tube of the instrument can connect the wrist mechanism to a transmission sometimes referred to herein as a backend mechanism. For robotic control of the effector and wrist mechanism, the backend mechanism is motor driven, and a processing system may be used to provide a user interface for a doctor to control the instrument.
The wrist mechanism generally provides specific degrees of freedom for movement of the effector. For example, for forceps or other grasping tools, the wrist may be able to change the pitch, yaw, and grip of the effector. More degrees of freedom could be implemented through the wrist, but implementing three degrees of freedom in the wrist and implementing other degrees of freedom such as roll or insertion/extraction through movement of the main tube generally provides the motions necessary for performing medical procedures.
A conventional architecture for a wrist mechanism in a robotically controlled medical instrument uses cables to turn a capstan in the wrist mechanism and thereby rotate the portion of the wrist mechanism that is connected to the capstan. For example, a wrist mechanism can include three capstans for rotations about a pitch axis, a yaw axis, or a grip axis. Each capstan can be controlled using two cables that are attached to the capstan so that one side pays out cable while the other side pulls in an equal length of cable. With this architecture, three degrees of freedom call for a total of six cables extending from the wrist mechanism back along the length of the main tube to the backend mechanism of the instrument. Efficient implementation of a wrist mechanism can be complicated because the cables must be carefully routed through the wrist mechanism to maintain cable tension throughout the range of motion of the wrist mechanism and to minimize the interactions (or coupling effects) of one rotation axis upon another. Further, pulleys are generally needed to reduce cable friction, which extends instrument life and permits operation without excessive forces being applied to the cables or other structures in the wrist mechanism. Cauterizing tools present a further complication for an instrument with a wrist because cauterizing instruments require delivery of electrical power to a movable effector. For example, a bipolar cauterizing tool requires delivery of opposite polarity voltages, room for movement of conductors, and effective isolation of electrical voltages.
A wrist mechanism is desired that can be operated with a small number of cables to facilitate miniaturization of the instrument and reduce costs of the instrument. A low part count for the wrist mechanism is desired to reduce manufacturing cost. Further, the wrist mechanism should allow electrical isolation suitable for a cauterizing instrument and have an efficient backend mechanism.