The ideal surgical viewing system would provide a high quality image of the retina while taking up a minimal amount of space below the microscope and requiring little or no additional effort from the surgeon to use it.
Typically, they're accessories to the standard surgical microscope. The MERLIN, for example, is positioned between the lower end of the microscope and the patient's eye. It consists of two parts, the lens positioning unit (LPU), holding the surgical lens, and the condensing lens assembly (CLA) with a condensing lens. The surgical lens modifies the optical system of the microscope such that it shows the retina. The condensing lens shortens the focal length of the microscope objective lens by about 1 in, which allows the surgeon to move the LPU in and out of the optical path without having to move the scope up or down. In combination, the two components allow the surgeon to quickly change between corneal views (without the LPU in place) and retinal views (with the LPU in place) without making any adjustments to the microscope. Conventional viewing systems are either completely manual or require the surgeon to use a foot switch to move the condensing lens into position. With the help of a controller and a Precistep micro stepper motor from FAULHABER, the MERLIN performs this task automatically as soon as the surgeon begins to swing the LPU into position.
One of the key focuses of the design team was to keep the depth occupied by the MERLIN as small as possible. “Doctors are very sensitive to the height of the microscope above the patient”, says Bill Hudgins, manager of product engineering at Volk. “They don’t want us changing that too much. To fit into the existing enclosure, we really needed a small motor that we could put a gearhead on to get the speed and torque that we needed. In this area, the drive experts of FAULHABER's sister company MICROMO in the U.S. helped us a great deal."
Because of size constraints, it wasn't feasible to use a servo motor and encoder. Instead, Volk chose a 15 mm stepper motor in combination with a controller board. Stepper motors tend to be highly deterministic – command a step, get a step. The microcontroller determines the direction and the number of pulses applied to translate the condensing lens in and out of the optical path.
The electromechanical design process wasn't entirely straightforward. Translating the lens required more torque than the micro stepper motor could supply. Instead, the team added a gearhead to convert speed into sufficient torque to move the lens. The viewing system needed to perform, first and foremost, but it also had to be reliable. They’re expected to last for years, so the whole design had to be a robust. The gearhead connects to a pulley and a belt drive actually moves the lens carriage. An optical sensor built into the bottom of the CLA detects the position of the LPU and the controller sends the command to the stepper motor.