Trends in motion control of surgical robots

Trends in motion control of surgical robots – Cheersonic

The advantages of robotic-assisted surgery are being recognized in an increasing number of surgical fields, in an increasing number of hospitals and patients. This has driven the growth of the industry and the advancement of technology used in surgical robots. A key technology is robotic motion control, but the components involved are often invisible to the outside world, which may partly explain why they are often overlooked or misunderstood. However, they are key components of safe, smooth and accurate robotics.

“Motion control” describes systems and techniques for moving machine parts in a controlled manner. In surgical robots, these typically include motion (or servo) controllers, motors, gearboxes, and position feedback sensors. A common application is driving the shoulder, elbow, or wrist joints of robotic arms. The motor, gearbox and sensor assemblies are often referred to as actuators, and the system is sometimes referred to as a “servo” – which simply means that the feedback loop reports the actual position or velocity to compare with the command.

Motion control is not new, but the technology is developing rapidly and the trends are:

1. Digitization

This is the increasing use of fully digital control systems and the disappearance of analog signals from amplifiers, potentiometers, etc. There are many reasons for this:

Cost – Modern digital electronics are generally less expensive than analog electronics, and wiring/interconnect costs can be reduced in volume and size;
Performance – digital systems are generally less prone to EMC and thermal drift;
Familiarity – Young engineers are more accustomed to digital systems than analog circuits, so it’s no surprise that they take a digital approach to new designs.

2. Miniaturization and Net Shape

Compared to 20 years ago, most motion control components are now smaller in size with comparable performance: servo drives that used to weigh 1kg now weigh <50g; standard NEMA 34 size motors now output late 20th century power 3 times, and some optical encoder read heads have been reduced to the size of a single microchip. The miniaturization of the position sensor is due in part to advances in ASIC design and microcircuitry, while the smaller motor comes from improved magnets and better electromagnetic simulation in the design process. Technological advancements have largely enabled this, but there has also been a change in approach, from using frameless motors and frameless or packageless sensors, eliminating bulky housings and reducing their net form to essentials. 3. Direct drive Electric motors usually produce the most torque or power at higher speeds. Traditionally, gearboxes are used to reduce this speed to generate torque at low speeds. With the development of brushless DC motor technology, full torque can be produced at no or low speed, so the gearbox is no longer required. This direct drive has the advantages of no backlash, no friction losses through the gearbox, reduced maintenance, improved reliability and elimination of gearbox costs. However, direct drive is not universally applicable because the gearbox enables a smaller motor to achieve the required torque. The advantages and disadvantages of direct drive vary depending on the specific requirements, but in more and more cases, direct drive is the first choice. 4. Innovative gear system It may seem odd to describe the increased use of direct drive and then discuss the use of innovative gear systems, but recent developments mean that traditional motor and gearbox methods are not declining as rapidly as some predicted. A notable example is the use of strain wave gears (or harmonic gears), where a thin ring gear is bent to create high speed reduction (sometimes >100:1) in the form of a compact ring with near zero backlash. Other examples involve conical drives and cycloidal gear systems, methods that allow engineers to meet demanding motion control requirements at a reasonable cost.

5. Non-contact sensor

This trend has been going on for several years, and the use of potentiometers as feedback devices in the motion control of surgical robots has all but disappeared. In applications with a large number of cycles, the potentially unstable potentiometer reliability contributes to this trend, especially in restricted motion where the potentiometer track may suffer from wear problems. This means that position or velocity feedback devices are dominated by optical, magnetic, capacitive and inductive sensors that now offer measurement performance previously only achievable with very expensive sensors.

6. Simulate and customize parts

For those of us old enough to remember the transition from the drawing board to CAD, one of CAD’s purported advantages is its ability to perform engineering simulations. Back then, not many of us actually believed it… but how wrong could you be? CAD can now easily and accurately simulate actuators, mechanisms, motors, sensors, so systems can be built and tested virtually without physically building, testing, and iterating prototypes. The impact on engineering cost and lead time is huge, and nothing says it better than using custom motion control components developed to customer specifications. Traditionally, custom parts have only been economically viable in high-volume applications where engineering costs can be amortized.

7. Value-added remote diagnosis

Remote diagnostics have been around for years, but are often of little value. However, modern technology can now provide such precise data, enabling valuable information. Traditionally, position sensors only reported the displacement of the actuator, but modern measurement technology allows such high resolution, the data can be used to predict when seals will start to wear; when changes in actuator speed indicate whether lubrication is required, power is applied and activated The time delta between movements can show if wear exceeds specified limits. If such issues can be diagnosed remotely, interventions can be scheduled with the necessary parts on hand—reducing overall downtime; the impact of downtime; associated costs and minimizing the impact on patients.

Article source: MedRobot
Original author: Mark Howard

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.