The advantages of robot-assisted surgery are recognised in an increasing number of surgical areas, by ever more hospitals and by prospective patients. This is driving the sector’s growth as well as the technologies that are used in surgical robotics. A key technology is robot motion control but the components involved are often invisible to the outside world and this might partly explain why they are often overlooked or misunderstood. Nevertheless, they are critical components for safe, smooth and accurate robotics.
‘Motion control’ describes the systems and techniques that move parts of a machine in a controlled manner. In surgical robotics these typically include a motion (or servo) controller, a motor, a gearbox and a position feedback sensor. A common application is driving the shoulder, elbow or wrist joints of a robot arm. The motor, gearbox and sensor assembly is often referred to as an actuator and the system is sometimes referred to as a ‘servo’ – which simply means a feedback loop reports actual position or speed for comparison to the command.
Motion control is not new but the technology is developing quickly and the trends are:-
- Digitalisation. This is the increasing use of fully digital control systems and the disappearance of analogue signals from amplifiers, potentiometers etc. There are a number of reasons for this:-
- cost – modern digital electronics are often cheaper than analogue equivalents and cabling/interconnection costs can be reduced in quantity and specification
- performance – digital systems are typically less prone to EMC and thermal drift
- familiarity – younger engineers are more accustomed to digital systems than they are analogue circuits so it’s no surprise they take a digital approach to new designs.
- Miniaturisation & Nett Form Factor. Most motion control components now come in ever smaller sizes for equivalent performance than they did 20 years ago:- a servo drive that previously weighed 1kg now weighs <50grams; a standard NEMA 34 size motor now outputs 3x as much power as it did at the turn of the century and the read head of some optical encoders has reduced to the size of a single microchip. Miniaturisation of position sensors can be partly attributed to advances in ASIC design and microcircuitry, while smaller motors come from improved magnets and better electromagnetic simulation during design. Technical advances have enabled much of this but there is also a change of approach from the use of frameless motors and frameless or unpacked sensors which eradicate bulky housings so that their nett form is reduced to the bare essentials. Robotic surgery generates some highly specific and unusual space constraints for components and so miniaturisation is not necessarily required but rather that the unusual demands can simply be accommodated and nowhere better can this be seen in some of the custom sensors or motors (such as arc or segment formats) now available.
- Direct Drives. Motors typically generate most torque or power at higher speeds. Traditionally, gearboxes are used to reduce this speed to produce torque at low speed. As brushless DC motor technology has developed, it is possible to produce full torque at no or low speed so a gearbox is no longer needed. Such direct drives offer advantages of no backlash, no friction losses through the gearbox, reduced maintenance, improved reliability and elimination of gearbox cost. However, direct drives are not universally applicable since a gearbox enables a smaller motor to achieve the required torque. The pros and cons of direct drives vary depending on specific requirements but in an increasing number of instances a direct drive is a preferred option.
- Innovative Gear Systems. It may seem curious to describe increasing use of direct drives and then discuss the use of innovative gear systems but recent developments have meant that the traditional motor and gearbox approach has not dwindled as fast as some predicted. A notable example is the use of strain wave gears (or harmonic gearing) in which a thin annular gear flexes to produce high speed reductions (sometimes >100:1) in a compact, annular form with near zero backlash. Other examples involve cone drives and cycloidal gear systems and such approaches have enabled engineers to meet demanding motion control requirements at reasonable cost.
- Non-contact Sensors. This trend has been underway for some years now and the use of potentiometers as feedback devices in motion control for surgical robotics has almost disappeared. The trend is fuelled by a possibly unfair reputation of potentiometer reliability in applications with a high number of cycles – particularly over restricted movements where the potentiometer’s track might suffer wear problems. This has meant that position or speed feedback devices are dominated by optical, magnetic, capacitive and inductive sensors which nowadays offer measurement performance previously only attainable with very costly sensors.
- Simulation & Custom Parts. For those of us old enough to remember the transition from drawing boards to CAD, one of the advantages claimed for CAD was its ability to carry out engineering simulations. Back in the day, not many of us really believed it…. but how wrong can you be? CAD now enables easy and precise simulation of actuators, mechanisms, motors, sensors so that systems can be virtually built and tested without the need for prototypes to be physically built, tested and iterated. The effect on engineering cost and lead-time has been dramatic and nowhere is this better illustrated than with the use of custom motion control components developed to a customer’s specification. Traditionally, custom parts were only economically feasible for high volume applications where the engineering costs could be amortised. With the advent of ever better simulation packages, the prospect of bespoke motors, gearboxes and sensors has meant that custom parts are a viable option for robotic motion control.
- Added Value Remote Diagnostics. Remote diagnostics has been around for years but has often been of marginal value. However, modern techniques now deliver such precise data that valuable information is possible. Traditionally, a position sensor just reported an actuator’s displacement but modern measurement techniques allow such high resolution that the data can be used to predict when a seal has started to wear; when variations in actuator speed indicate if lubrication is required and the time delta between applying power and starting motion can show if wear has passed specified limits. If such problems can be diagnosed remotely, interventions can be scheduled with the necessary parts in hand – thus reducing total downtime; the impact of the downtime; the associated costs and minimising impact on patients.