Questions about robot actuators

Hi to all, I’m new to this forum ( but not to robotics ). I’m no pro but just a hobbyst, however ( being a programmer ) I have a basic knowledge of physics and other stuff necessary to build robots.

I have built some simple robots ( some crawlers and a bioloid, plus some wheeled robots ) and I have always used dc motors and servomotors. But I’m trying to build something more interesting, a two legged humanoid the size of a child to experiment with AI and machine learning in robots.

I have made my homeworks and researched a bit the topic of actuators. What I have seen is that hobbysts use servos ( obviously ), but pros use custom built motors, linear actuators with dc motors, linear motors ( the ones that work like the maglev ), pneumatics, hydraulics, and so on.

For various reasons I don’t think that hydraulic or pneumatic actuators are a good practical choice, so the only remaining realistic actuator type is the electric motor.

I have seen that the universities and labs around the world use frameless dc brushless motors ( torque motors ) + harmonic gears or custom built linear actuator with springs and other mechanisms to implement compliant joints.

But there is another type of electrical actuator, the so called linear motor ( also called direct drive linear motor ). I’m talking about this thing

This type of motor is fast, has great peak force, no bearings, pulleys, gears, etc… It seems a really good choice, but no humanoid or legged robot in existence uses them. Why? Is there a catch?


Some of the manufacturing equipment we have here at Pololu use them, but I have never played with them, so my experience with linear motors is very limited. WIth that disclaimer out of the way, I suspect linear motors are not very good in terms of output per weight. This isn’t much of a problem if the motor is going to mounted inside a heavy, stationary industrial machine, but the power-to-weight ratio of an actuator is a big concern for mobile robots. Also, I suspect linear motors might be prohibitively expensive for typical mobile robotics applications.

- Ben