Choosing Actuators for Surgical Robotic Joints

Discover how choosing the right actuators for surgical robotic joints enhances precision and control in complex procedures. Learn about the benefits of compact torque motors and how FiberPrinting&#; technology optimizes size, power, and customization for next-generation surgical robotics.

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Robotic surgery is becoming increasingly popular because it enables surgeons to perform complex procedures with more precision and control, leading to a variety of patient benefits. The initial scepticism around having a &#;robot operate on me&#; continues to reduce, thanks, in part, to the emergence of other robotic solutions in our daily lives whether it be a humanoid serving your drink in a bar or a mobile robot delivering your groceries.

A key element influencing the overall performance and capabilities of the surgical robot is the design and make-up of its joints. Due to different design priorities, there are variations in most current systems on the market. This is reflected in the technical specifications and aesthetic design. Although there are these variations, there are some aspects of the system that nearly all manufacturers are trying to improve including size, safety, speed and power consumption (efficiency).

This paper reviews different design approaches for the robotic joints and introduces a new technology that is being increasingly adopted by surgical robotic manufacturers.

Fig. 1. Example of a surgical robot.

Actuators in Surgical Robots

There are several types of actuation technologies used in surgical robotic systems. There are also several companies that promote off-the-shelf actuators with fixed specifications. The off-the-shelf solutions can be useful for robotic manufacturers producing an initial prototype or first-generation system as they can reduce the development time. However, in the long-term, such solutions are usually designed out as the manufacturers look to reach a higher level of integration to meet a more specific set of specifications aligned to required performance and unique selling points of the system.  

What&#;s in a robot joint?

A robot joint is made up of several components that enable controlled movement. The types of components can vary but typically include a motor, a gear/reduction in form of a gearbox, encoders, bearings, eventually torque sensors, a brake and a servo drive.  

This article will focus on the primary mechanical components: the motor, gears/reduction and brake.

Gears

In a robot joint, gears are used primarily for transmitting torque and controlling the motion of the joint. Gear ratio refers to the relationship between the number of teeth on two meshing cogs or the relative speeds of rotation between those gears. It indicates the extent to which a motor&#;s torque is increased or reduced.

The gear ratio is typically expressed as the ratio between the number of teeth on the driven gear (the gear receiving power) to the number of teeth on the driving gear (the gear providing power), or it can be represented as the ratio of their rotational speeds.

&#;There are different technologies available for the selection of a gear for a robot joint, and the selection will depend on the desired specifications for the robotic application. Robotic solutions in surgery are typically serial kinematics. In serial kinematic solutions the torque required in the bigger axes is quite elevated and therefore the gear ratios tend to be very high. In these applications strain wave gears are typically used.

Motors

One of the factors impacting gear selection is the type of motor technology used in the joint. The strain wave technology is most common when using frameless motors as the driving technology, which is the motor technology selected when compactness is important.

When using other motor types, other gear technologies are more suitable, like the very common integration of worm gears with BLDC motors.

Frameless torque motors provide many design advantages with regards to compactness, controllability, weight reduction and performance. Torque motors can also provide a higher torque than regular BLDC motors, which allows for a lower gear ratio or, in some instances, no gearing at all (see later). The lower the gear ratio the easier it is to back drive as well as improved efficiency, greater sensitivity and transparency (i.e. the ability to sense torque with the current applied to the motor).

Brakes

One of the challenges of reducing the gear ratio is related to the brakes. Serial kinematics require a brake in almost all axes, with some exceptions sometimes like the end effectors or axes that only move horizontally (perpendicular to gravity). If the gear ratio is reduced, the brakes would have to be able also to produce a higher braking torque which, depending on the technology used in the brakes, could mean also bigger sizes and increased weight. This would ultimately contradict or work against the aimed weight and size reduction of motors and gears.

 Electromagnetic brakes, however, have improved in recent years and very light brakes can now provide a high braking torque while also being scalable and customizable.

Using Direct Drives in Surgical Robots

Not all axes of a surgical robot joint require geared actuators. An end effector or the &#;wrist&#; can also be solved by using a torque motor with no reduction. A direct drive solution in these types of applications can provide several benefits in terms of size and weight reduction, high dynamic performance and lower cost. See A Guide to Torque Motors to learn more.

If torque motors offer such benefits, one might ask why manufacturers do not design a whole robot with no gears, and just use direct drives. Some parallel kinematics have shown that this design is feasible, however, for serial kinematics this solution is uncommon, due to the lack of robustness that such a system would have, which is an issue for systems that require high stability.

Precision Torque Motors with FiberPrinted&#; Technology

Integrating torque motors into a geared actuator, requires careful consideration. A growing trend in the surgical robotics industry and other precision applications, is to use precision torque motors manufactured with FiberPrinted&#; technology.

FiberPrinting&#; is an industrial winding method that allows for continuous winding of stator mats for ironless and slotless motors.This method maximizes copper fill factor and enables precise customization of size & stator configuration, resulting in the optimal combination of torque density and precision in a thin and lightweight form factor.

These ultra-thin motors allow for a complete integration inside a gearbox (concept expressed in Figure 3) and their coreless design also enables the torque motors to operate at higher speeds.

Further information is available in this article: Slotless Motors vs. Slotted Motors: What is the difference?.

Fig. 2. Concept sketches of motor integration inside a gearbox: in-runner motor (a), out-runner motor (b).

The ability that FiberPrinting&#; provides to integrate compact solutions results in several benefits, not only to the mechatronics designers, but also the surgical robot manufacturers and end-user surgeons. As shown in Figure 3 below.

Fig. 3. Advantages of FiberPrinted&#; motors and how they can impact the design of robots and the use of such.

&#;

If you are going to build robot hardware, it is important to choose the right robot components. It will be good if you have read my previous post about robotics, different types of robots and choosing the right robot sensors. These posts will give you a fundamental concept of robots, robotics, and sensors.

In this post, you can see a quick way to choose the right actuators/motors for your robot. There are different categories of robot actuators, you can see an introduction of different kinds of actuators used in robots first and then you can see which type of actuators can be used in different kinds of robots. This will help you to choose the right actuators for your robot.

1.

Different types of Robot Actuators

The robot actuators can classify based on how they move the output motor shaft and which energy they transform to make the move. According to the movement of the actuator shaft, we can simply classify the actuators as

1. Linear Actuators

   : The shaft of the linear actuators will only move in a linear fashion.

2. Rotary actuators

   : The shaft of the rotary actuator will only rotate in an axis.

The linear and rotary actuators can be classified based on the energy they use to move the shaft of the motors. Here is a list of actuator classes that use different energy to create movement.

1.1

Hydraulic actuators

The hydraulic actuators are used in robots handling heavy loads. These actuators can produce very high force if we compared them with other actuators. These actuators are deployed where higher speed, accuracy, and stability are required.

These actuators have a cylinder and piston arrangement which is shown in the following figure. The chamber is filled with hydraulic fluid. The pressure applied to the fluid will push the piston, and that will move the actuator output shaft. The hydraulic actuators can convert the piston movement into linear and rotary movements.

Example of hydraulic actuator and a its cross section.

1.1.1 Advantages of Hydraulic actuator

1. Easy to control and accurate
2. Simpler and easier to maintain
3. Constant torque or force regardless of speed changes
4. Easy to spot leakages of system
5. Less noise

1.1.2 Disadvantages of Hydraulic actuator

1. Proper maintenance is required
2. Expensive
3. Leakage of the fluid creates environmental problems
4. Wrong hydraulic fluid for a system can damage the components

A common example of a hydraulic actuator system: JCB

Hydraulic actuators in JCB (marked as red)

The Boston Dynamics WildCat robot is one of the example robot working with hydraulic actuators.

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Boston Dynamics WildCat robot

1.2

Pneumatic actuators

As you have seen in hydraulic actuators, they use a hydraulic fluid in the cylinder in order to move the piston. The pressure applied to the fluid will move the piston. But in pneumatic actuators, instead of hydraulic fluid, compressed air is moving the piston.

Similar to hydraulic actuators, it can produce linear and rotary movements.

Pneumatic actuator

When compared to hydraulic actuators, here are the advantages and disadvantages of Pneumatic actuators.

1.2. 1 Advantages of Pneumatic actuators

1. Clean, less pollution to the environment
2. Inexpensive
3. Safe and easy to operate

1.2. 2 Disadvantages of Pneumatic actuators

1. Loud and noisy
2. Lack of precision controls
3. Sensitive to vibrations

Here is an example of a bionic soft arm robot that is made of Pneumatic actuators. The robot is made by a robotics company called Festo.

BionicSoftArm: Modular pneumatic lightweight robot

Here is the video of BionicSoftArm

Festo Bionic arm

1.3 Electric actuators

The commonly used actuators in robotics are electric actuators. This actuator converts electric energy into linear or rotary motion.

The electric actuator can be AC/DC actuators. Mostly, robots are using DC actuators.

Here are the advantages and disadvantage of electric actuators

1.3.1

Advantages of electric actuators

1. These actuators offer the highest precision among other actuators.
2. It can be easily network and can easily program. They offer immediate feedback for diagnostic and maintenance.
3. They provide complete control on motion profiles and can include an encoder to control the velocity, position, and torque.
4. Less noise compared to hydraulic and pneumatic actuators
5. No fluid leak, so fewer environmental hazards.

1.3.2

Disadvantages of electric actuators

1. The initial cost of the electrical actuator is higher
2. Unlike pneumatic and hydraulic actuators, these actuators are not suitable for all environments.
3. There are overheating, wear and tear issues are there compared to pneumatic and hydraulic actuators.
4. The actuator&#;s parameters are fixed, so to change torque, speed, etc to a different level, actuators should replace.

1.3.3

Different types of DC actuators

Let&#;s see different types of DC actuators used in robots.

Examples of DC actuators

1.3.3.1 DC Motors: A dc motor will have &#;+&#; and &#;-&#; negative terminal. The output of the dc motors shaft will start to spin if we supply DC voltage to the motor terminals. The speed of the motor shaft can be adjusted based on the voltage across the motor terminals.
Here are some examples of DC motors that you can buy.

1.3.3.2 DC Gear Motor: Adding a gearbox on DC motors can increase the shaft torque and reduce the motor speed. DC Gear motors consist of a DC motor attached with a gear system with an output shaft.
Here are some examples of the DC gear motor that you can buy

1.3.3.3 Servo Motors: The servo motors consist of a DC motor plus gear system plus a servo control circuit. The servo control circuit can able to rotate the gear shaft with a specific angle. The computer inside the robot can command the servo motor to rotate at a specific angle using PWM signals. There are different types of servo motors, the normal servos are called RC servos. There are analog servos and digital servos. Normal RC servos are analog servos.

Here are some examples of RC servo motors

Here is a video of the working principle of normal RC servo motor

There are smart digital servos available in the market. These can give you the current position of servo, speed, torque, temperature, etc as feedback. The digital servos are working by sending/receiving data packets. The control of digital servos is done by sending data packets. One of the examples of digital servos is Dynamixel from ROBOTIS.

Working of Dynamixel actuators

1.3.3.4 Stepper Motors: The stepper motors are DC motors that can move in discrete steps. This motor is having multiple sets of coils organized in groups called &#;phases&#;. The motor will rotate in each step at a time when you trigger each phase in a sequence. The stepper motors are used where high precision in movement is required.

Here are a few examples of stepper motor you can purchase

Here is a detailed working video of stepper motors.

Working of stepper motor

1.3.3.5 BLDC Motors: BLDC motors are quite popular nowadays and used in many robotic applications. The BLDC motor stands for Brushless DC motor. The main difference between BLDC and DC motors is, ordinary DC motors work using a commutator, which is touching the brushes in the armature, but there is no commutator and brushes in BLDC. Instead of brushes, it uses an electronic commutation.

Here are some examples of BLDC motors that you can purchase

BLDC motors are using in robotics widely, here are some videos which are demonstrating its working.

1.3.3.6 Harmonic Drives: Harmonic drive, which is the brand name of strain wave gear trademarked by Harmonic Drive company and invented in . It is very popular in robotics applications. The working of harmonic drives is demonstrated in the following video

1.3.3.7 Linear DC actuators: These dc motors can convert the DC voltage to linear motion just like pneumatic and hydraulic actuators you have already read. There is a lot of robotic application using linear actuators. Here are some videos to show the working of these motors.

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