Legged Robots: Actuators Comparation - MAB Robotics

How to choose the proper actuator for a legged robot? In all mobile machines movement execution is the most important task. If we want to control precisely what our robot does, we have to choose a proper actuator. Robot’s “muscles” are touching the world and giving acceleration, which is necessary to move its body.

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Choosing a proper actuator is always a challenge. That challenge appears in all kinds of robots, but it’s especially difficult in legged robots. These machines have many actuated degrees of freedom. All joints must keep desired position, velocity, and torque/force. Additionally, while walking and running, legs cyclically switch between phases of swinging and supporting the robot body, which causes big impact loads. For those reasons, we have to be sure that desired movement computed by the control system is immediately and precisely executed, and the actuator can withstand shock loads.

Good suited actuator for legged robots should be characterized by the following factors:

• high torque density — when constructing robots, we often try to do them as compact and dynamic as possible, so the proper choice is actuator, which uses as small volume and mass as it is possible, and generates as much torque as possible. Also, heavy actuator will drastically reduce robot’s dynamics, overall performance and increase energy consumption, what is very important if we are using battery as power supply

• low mechanical impedance — it’s not an intuitive quantity, but very important. It tells us about relation about external torques/forces and resulting motor velocity. With high mechanical impedance, the drive is very stiff — is not easy to move the output shaft. External torques/forces are damped. If we use only mass and volume as factors, it will lead us directly to drives, which consist of small motors and gears with high reduction ratios(frequently in range 100:1–300:1). That combination limits overall dynamics as achievable velocities are limited. Also, in case of legged robot gear teeth are often cracking, as during walking we have continuously repeating impacts while touchdowns.

• high force transparency — it’s a factor connected with low mechanical impedance. When our drive doesn’t have“obstacles” on the output shaft(like a gearbox, friction, and other resistance to motion) we can “observe” impacts by measuring currents in the drive. All external forces/torques have significant influence on motor velocity and we achieve backdrivability. In case of walking robot, that information can be used for detecting contact between foot and ground without external force/torque sensor

• precise motion controller — reliable motion control requires a proper motor controller, which is modulating currents in the most optimal way. In good robotics actuator user can control position, velocity, torque, and often mechanical impedance(spring-damper mode)

• high-bandwidth interface — it’s important to get feedback from the drive as frequently as possible, and send desired motion commands too. In legged robots reliable control in real time requires high-bandwidth, low-latency control loops (usually 500–Hz)

Now we know the factors, which let us imagine an ideal actuator. It of course doesn’t exist in the real world, so let’s look at available approximations. Below I present some solutions, which show balance between parameters described above.

That leads us to four main categories of drives used for actuating legged robots across the world.

DIRECT DRIVE DD

First, we have direct drives. They usually are just a big diameter, flat BLDC motor. As in motors torque production is proportional to volume, it’s reasonable to keep thickness to diameter ratio low. To properly control the movement, we need a motor controller — when we connect the proper piece of electronics to the motor we get a servo actuator. The advantages are simplicity, low mass, very low mechanical impedance and good reliability. The only thing between motor and environment are bearings, so the force transparency is high. That lets us precisely estimate torques from currents. Also, absence of gears which can be destroyed let us throw a robot from 2 meters safely or kick them. The disadvantage is quite small torque density — all torque production is pure electromagnetic, so motors become hot when the robot is moving dynamically for a long time.

Example of direct drive actuator:

In this case, it’s a drone motor stacked with MAB Drive. In MAB Robotics, we have used DD in one of our prototypes built for Engineering Thesis:

QUASI DIRECT DRIVE (QDD)

Structurally they are very similar to direct drives, but there is an additional component — a gearbox. Usually a planetary gearbox with up to 10:1 reduction is used. That composition of motor with gearbox in this range of ratio has much better torque density than direct drive, as torque is multiplied. Overall dynamics are still good. We have slightly increased mechanical impedance, but we can still detect contacts without external sensors, just by measuring motor current. Drive control electronics are exactly the same as in the previous case — we still have a BLDC motor, so all we need is a three phase inverter with an encoder.

Advantage of that solution is much higher torque density without big mass increase. Force transparency is still acceptable. The only disadvantages are increased drive complexity — we have to connect the motor with the gearbox. It has to be precisely machined and assembled.

QDDs is approach that we are using in our newest prototype:

In this case, the drive is a T-Motor AK80 module with our custom controller.

Quasi-direct drive, due to their medium simplicity and excellent performance in small scale legged robotics, are becoming the most popular actuators.

SERIES ELASTIC ACTUATOR (SEA)

It’s more advanced than QDD. In this case, we have a motor, high ratio gearbox(frequently about 100:1), usually harmonic, and series springs.

A concept construction we can see below:

Photo: Motion Control Lab/http://control.dgist.ac.kr/web/research/rotary-type-series-elastic-actuator/

SEA has excellent torque density, and is able to monitor output torque by measuring torsional spring deflection with encoder. By connecting the motor with spring, mechanical impedance is tuned physically. As we have precise torque sensing, estimation torque from current is no longer necessary. It’s a sophisticated actuator, ideally suited for legged robots, but very difficult to manufacture and very expensive. Off the shelf solutions are for example ANYdrive used in ANYmal B robot from ANYbotics:

Photo: ETH Zürich/https://rsl.ethz.ch/robots-media/actuators/anydrive.html

HYDRAULIC ACTUATORS

Last category are hydraulic actuators. Their torque density is the highest. Principle of their work is other than described above solutions — instead of electromagnetic torque, we have extremely high pressure and cylinders connected with precise servo valves and encoders. As torque production is big, and mass small, it’s ideal for dynamic legged robots. The best example is Atlas from Boston Dynamics, which is able to run, jump, and even do a backflip:

Hydraulic actuators also have disadvantages. They are operating at high pressure, so all valves and cables have to be extremely strong. Also they require regular inspections. All malfunctions with pressure in the range of tens, or hundreds of bars are very dangerous for people around. Also, precise control of fluid flow in a closed loop is not an easy task. That’s the reason that technology is very expensive, and probably in the future will be replaced by electric drives at all.

SUMMARY

There are a few approaches that let us build high dynamic articulated legged robots. Depending on the budget, project needs and desired performance we can choose solutions, which are optimal for our machine. If you are interested in motor control technology, legged robots or robots in general, feel free to contact us.

Author: Łukasz Antczak

CEO and Co-founder at MAB Robotics

LinkedIn: https://www.linkedin.com/company/mabrobotics

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.

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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.

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

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

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.

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.

Here is the video of BionicSoftArm

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.

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.

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.

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

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