a robot

Robots with legs, what's going on?

More and more robots are being built without legs, why is this happening?

Recently it’s been a trend where manufacturers are building their social robots without legs, gone are the days of smaller robots with legs like Nao, Robots recently have been moving towards larger wheeled bases, Robots such as Pepper, Sanbot and Amy Robots have been leading the social robotics industry and they all have wheeled bases in common.

There are a number of advantages of walking robots, but why are wheeled robots taking over?


Batteries for robotsLegged robots use a significant amount of power, for a legged humanoid robot to be able to stand up on its own, Power to over 10 motors is needed at all times. Without this, the stability of the robot is compromised.

XRay of robot to show number of motorsWhen you start to increase the power requirement of the robot you encounter two problems, firstly, the size and location of the battery. Battery technology has developed dramatically over the last 10 years however batteries are still large and heavy. Putting large, heavy batteries in the robot can interfere with the centre of balance, or even worse, require you to upgrade to even more powerful motors to be able to support the weight of the battery.

For a robot such as Pepper, which has a wheeled base, the robot can stand up straight and not require any power to the motors at all. This is because of the wider, more stable base and the ability to lock the hip motor in a central position which means even the hip motor can be disengaged. The only time the motors are needed to be powered is when the robot makes any movements that require the hips or the knee motors.


Navigation has been a limiting factor in social robotics for several years, the sensors required to understand the environment and the software that is needed to make use of this is extremely complicated.

When mapping an environment, it’s much easier to manually create an accurate map of the environment and using accurate wheel encodes as one of the primary sensors for the navigation.

This allows you to setup the map, tell the robot where it is within the map and using the wheel encoders it can accurately keep track of its location.

Walking robots cannot make use of wheel encoders to keep track of distance travelled and so far, there is no accurate way to do this with just the motors.  This means that you have to use alternative sensors such as laser range finders and sonar which both increase cost and complexity of the robot.

Motor Prices

dynamixel Servo Motors for robots

Robots such as Romeo, by Softbank Robotics, or Asimo require extremely powerful, extremely expensive motors to be able to support the weight of the robot enough to walk. Romeo, who is the cheaper of the two, weighs in at 36.6 kilograms and requires RE40 motors for the legs, which cost over £500 each. With 22 degrees of freedom the price of the motors alone adds up extremely quickly.

With smaller robots such as Nao, this is less of a problem but as soon as you start to scale the robots up to sizes closer to Asimo or Romeo the power requirement of the motors starts to dramatically increase.

Legged robot examples

Romeo Robot by Softbank Robotics

Romeo, Softbank Robotics

Originally revealed in 2009 Romeo is a 1.4 metre tall humanoid robot designed to be used as an assistive robot for use in care situations. Now, 10 years later, they’re still having substantial issues powering him and he does not currently have his own battery power supply. Currently the only way to power the robot is via external power cables. So far only a handful of Romeo robots have been sold and they have only been sold to research groups with prices are rumoured over $400,000.

Atlas robot by boston Dynamics

Atlas, Boston Dynamics

Released in 2016 atlas is currently the most dynamic robot available. Capable of running, jumping and even backflipping atlas has a very wide range of movements. One of the major differences between Atlas and more traditional robots such as Romeo is Atlas uses hydraulic actuators instead of traditional servos. Hydraulics give Atlas much more power however one of the major disadvantages of this is the robots movements are extremely loud and disruptive. In addition to this Atlas currently weighs over 75kg and has not been made commercially available.

 Asimo Robot by Honda

Asimo, Honda

Asimo is the worlds most advanced humanoid robot, Capable of playing football, using the stairs, jumping and running. However the most substantial drawback is the price… Asimo has previously been rented for $150,000 per month and requires its own 24/7 technician. No Asimo robots have been sold and the project has now been retired so the company could focus on using Asimos technology for more practical use cases.

Is this starting to change?

With Robots like Ubtech’s Walker due to be released in the next 18 months, maybe things are starting to change. Motor prices are starting to get cheaper and battery technology is always getting better, but will Ubtech be the first company to be able to release a commercially available, affordable, legged robot?