The Ports are the main characters of the simulator hardware. They represent the trocars and the access points for the simulated minimal invasive surgical procedure, and provides the tracking of the trainees interaction to the simulation software. Collisions in the virtual world renders forces, which are sent to the ports and the trainee is immersed into the simulation with both a visual and haptic sensation. The port is a self-contained unit since it has its own control processor, sensors, motor amplifiers and power. Mount the Port on a frame structure, then plug it in with a USB-cable and it is ready to use. Its slim design allows for a close and realistic portal placement. Read more about the performance and features below.
The Port tracks the user interactions in four degrees of freedom; tool insertion, tool twist, side-to-side (yaw) and back-and-forth (pitch). The tracking range is unlimited in insertion and twist. The yaw and pitch motion range is ±90° and ±60° respectively. The tracking resolution is 5µm in insertion and 0.01° in twist, yaw and pitch.
The Port gives force feedback in all four degrees of freedom. All motors are strong and the design is free from elastic structures and transmissions, which in combination with the low inertia gives a superior dynamic response.
The maximum insertion force is 27N, and the torques in twist, yaw and pitch are 100Nmm, 760Nmm and 1000Nmm respectively.
A user can insert any Haptic Avatar Tool into a Port, which then will detect and identify the Tool shaft. The simulation software can now determine which virtual instrument representation to show and simulate, typically based on a predefined and exercise specific mapping table of Tools/Instruments. The physics engine now knows where to get the tool specific signals from (like handle opening angle) and where to send haptic forces. This solution, where the Tools and Ports are decoupled, enables a more realistic, versatile and modular training environment.
The Port communicates with the PC with USB 2.0 High-Speed (480Mbit/s), which enables loop speeds with the PC of up to 7000Hz. The internal control processor is powered by USB and starts directly when connected. The amplifiers for the motor controls are powered by internal Li-Ion batteries (16.8V) and the batteries are charged by the USB-power, which eliminates the need for external DC power supply or external battery charger. This requires a BC2.1 capable USB3.0-hub. A 6.3mm plug is available on the Ports backside for external DC power supply (or charging) if needed, e.g. in a fixed installation (15V @ 20A).
When the Port is powered up (with a USB connection) it needs to be calibrated in yaw and pitch. This is done by making a small movement around the yaw/pitch-zero angles. Thereafter, the angles are accurate and absolute with respect to the Port and to the Frame structure it is mounted on. This is only needed once after power up (and never before and between exercises). When a Tool is inserted, it is automatically calibrated upon detection, both insertion-wise and in twist. The simple calibration lets the trainee always focus on the exercise. Another advantage is that if the physical position and orientation of the port is the same as the virtual position of the access portal, a physical tool-tool collision will correspond to a virtual instrument-instrument collision, since the positions are absolute.
Internal force feedback loop
The normal force feedback loop is typically 1kHz via the PC, where the simulation program in the PC receives the positions (3 angles and the insertion length), calculates any collision forces and sends them to the Port, which actuates them. A second, complementary method, is to utilize the internal force feedback loop. To do so, the simulation provides a set of primitives, such as spheres, cylinders (capsules), toruses or triangles which all have dimensions, positions, orientations and collision material properties. These primitives can be sent once, and/or updated dynamically. The tool itself is also defined as a set of primitives (upon tool insertion and identification), and the collisions and haptic response can now be calculated internally. The typical internal loop frequency is 15kHz. This is useful when simulating hard surfaces, like bones or tool-tool collisions, and can offload the simulation program in the PC.
The Port has a mechanical interface that allows it to be quickly mounted and dis-mounted without tools. A bracket with dove tail protrusions mates with dove tail grooves on the Port. To mount the Port, position it over the bracket and push it back until a spring lock on the bracket snaps and secures the port. Dis-mounting is made by pressing the spring lock, pull the Port forward and lift it away from the bracket. With this system, Ports can be shared between one frame and another, and thereby enabling a more versatile training environment.
All structural parts in the Port are made of aluminum and thereby dissipates the heat from the motors and amplifiers effectively. The Port protects its motors from overheating with an internal thermal control system, which simulates the thermal response based on the dynamic motor loadings. The heat generated in the motor windings dissipates through the motor housings and the aluminum structures around them. When the temperature reaches a certain limit, the maximum allowed power output is gradually reduced. The temperature of the heat sink on the amplifiers is sensed and cooled when necessary with an internal fan. The figure to the left shows a step response where all motor are powered 100% from start, and then 0% at 30s into the simulation. The power output control limits the motor windings to stay below the maximum allowed temperature.
Dimensions and weight
The dimensions of the port device and its parts are shown in the drawing. The port to port distance can be as low as 50mm without mechanical interference of the gimbals.