FEATURE MEDICAL EQUIPMENT & DEVICES
Encoders drive up performance in surgical hand tools
Ever more sophisticated products are needed for the most challenging procedures
Andrew Sharp, Lead Design Engineer, Portescap, examines the benefits and trade-offs of different feedback options, enabling designers to develop surgical hand tools that are capable of addressing the most demanding motion profiles
D
esigners of surgical hand tools are constantly developing sophisticated
products for the most challenging procedures. As a result, the motion profiles for these tools are becoming increasingly complex, requiring increased quality and quantity of feedback data in a closed loop system. With brushless DC (BLDC) motors, feedback options for sensing a motor’s position include Hall sensors, an encoder or sensorless feedback using software algorithms. For surgical hand tools, for speed, acceleration motion, precise angular position control, exact velocity control, smooth torque control and increased safety through rotor position locking, an encoder is the best solution. Potential applications include robotic actuation or fine-tuned tightening control, where the output shaft of the motor needs to be precisely rotated from one angle to another. The feedback loop for this control system must provide enough resolution to command a rotation from 0-90 degrees without overshooting the target. Precise velocity control is required in applications such as arthroscopic shavers, where an oscillation profile is characterised by timed changes
28 APRIL 2021 | IRISH MANUFACTURING
in velocity. Leadscrew-driven linear motion assemblies are further potential applications, where the rapid movement of a load is critical.
For applications such as reaming or precision tightening, smooth torque control is a key requirement. Typically, field-orientated-control (FOC) systems are used to provide smooth torque control over a wide speed range, and these control systems require the high fidelity position feedback that is best provided by an encoder. Finally, there are surgical hand tools
that require safety features for shutdown routines or to protect an operator from a blade or bit. For example, if a sharp blade is required to remain within a safety shroud or stay unexposed to the surgeon, the position information from an encoder can define either a keep-out region or a target ‘home’ position to return to during specified situations.
MAIN DESIGNS When it comes to the choice of encoder technology, optical and magnetic are the two main categories. Optical encoders require a light source and sensor with a wheel between the two with transparent
and opaque sections, usually along multiple tracks to generate the rotational pulses. Within magnetic encoders, there are magneto-resistive and Hall-effect based technologies. In autoclavable applications, with steam, fluids and potential debris, magnetic topologies offer a small and robust option. Then there are absolute and incremental encoders, where the absolute encoder feeds back the absolute angle of the rotor position, with respect to a reference point, and thus always know the absolute position of the rotor, even after power loss. In contrast, an incremental encoder will only provide an output when the rotor incrementally changes position. It can sense direction, but is not reporting position referenced to an index point. As a result, the feedback system loses track of the true position of the rotor when powered off. Resolution defines the precision capability of an encoder. In an incremental encoder the resolution represents the angular value of a single pulse. Resolution is often given in terms of the number of pulses in a single mechanical rotation. This definition is nearly the same in an absolute encoder but just defines the granularity of the angular sensing capability and is not tied to a single pulse. Accuracy in an encoder represents the capability to correctly report the actual angular position of the rotor. It allows a system designer to understand the margin of error in the reported angle and to build in an allowable margin in the drive system.
Portescap
www.portescap. com
INTEGRATION Key goals when designing the surgical hand tool design are compactness and light weight. Optical encoders, typically mounted on the rear side of a BLDC motor, can require a shaft extension and add axial length as well as increased overall diameter. In contrast, an integrated magnetic encoder can often meet the feedback requirements for a drive while remaining within the motor’s maximum outer diameter, sealing all sensing components internal to the motor body, and minimising the axial length growth. As a fully- contactless sensing solution, and with no optical wheel to get damaged or obfuscated, the electronics can be sealed and protected against the autoclave environment.
/ IRISHMANUFACTURING
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