Robotic Arm End Effector (SSME-02)
The Challenge

The customer required true absolute position feedback of several small, densely packed axes for the end effector of their minimally-invasive surgical robot.

The Solution

The innovative Searchlight™ custom encoder system was developed.  This new optical absolute encoder utilizes a VCSEL laser source, a wedge-shaped mirror and a unique, ring-shaped detector array to provide true absolute position in an extremely small, on-axis configuration.  The light source sits in the middle of the ring in a stationary location and illuminates the mirror positioned on a rotating shaft.  The mirror reflects the laser light back down on to the ring detector at a specific location.  As the shaft rotates, the reflected beam sweeps around the ring detector generating a once per revolution sinusoidal electrical signal.  This sinusoid is then interpolated 65,536 times yielding 0.0055° resolution.

The Benefit

The compact design results in a significant reduction in size and weight of each axis, the ability to pack multiple axes into a very dense configuration, as well as generous alignment tolerances between the sensor and mirror.  It also allows the encoders to be installed extremely close to motors, remaining unaffected by their magnetic fields.  True absolute position from these encoder eliminates the need to home axes upon start-up or if there were an interruption in power, mid-surgery, while the robot’s end-effectors were within the patient.  While this was not a high speed application, the detector’s 50kHz bandwidth allows this encoder to achieve speeds up to 10,000 RPM.

PCB Diameter20mm
Mirror Diameter4.5mm
OutputsCustomer-specific serial interface
Resolution65,536 CPR (19.8 arc-sec; 95.9 µrad)
Maximum Speed10,000 RPM
Alignment Tolerances+/-250µm Axial and Radial, +/-1° Tilt



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