Actuation means movement generation. Achieving humanlike movement is not an easy task and requires many degrees of freedom actuation systems to be used while utilizing robotics principles and watching out for safety rules. Mainly the following types of actuators are used, although other types for very specific applications also exist:

Electric motors and servos: Electric motors can run on AC or DC power and is the most common type of actuation in robots together with servos. Through electric motors a high degree of precision can be obtained and they are lighter in weight with respect to the types we list below.

Pneumatic actuators: These utilize compressed air to operate. They can offer some strength but tend to have lower precision and slower reaction time than electric motors. They are not very common but can be used in soft robotics components such as some imitations of robot muscles and in robot hands for soft pickup of things. A positive side of this type is that they can operate with low power requirements and do not generate as much heat as electric motors or hydraulic actuators.

Hydraulic actuators: Hydraulic systems use pressurized liquids which means very strong actuation can be obtained because of the fact that liquids are incompressible. Hydraulic systems are slower but more durable than pneumatic counterparts. They are affected less by temperature variations but can be harder to maintain because they also involve fluids and pumps. In humanoid robots they are also far less common than electric motors but may be used where very strong actuation is required.

No matter what type of actuators are used, there are some general principles to be followed when building humanoid robot actuation systems.

For example, the actuators need to have variable stiffness, meaning, they should be able to adapt their force output based on the feedback they obtain through their sensors, which is a critical aspect to operate safely near humans and in unpredictable environments. Series elastic actuators (SEA) include a spring between load and motor, which is another mechanism to control forces and tolerate shocks. So a variable stiffness actuator (VSA) can also include spring connected in series with the actuator (SEA), plus the stiffness control mechanism. As far as how the stiffness is controlled, this can be through mechanisms such as antagonistic actuation, preload adjustment, changing spring geometry or mechanically switched springs, which we intend to cover in a future post so please search our site if you are reading this article after a while it has been published.

Mimicking the “actuation” in humans, like how our muscles make our move possible is another way of actuation and the science and technology behind this is in development, to be able to make artificial muscles and tendons to move the robot skeleton – there are already many isolated examples and limited uses of this but we are talking about real life, commercially useful (precise, strong, durable, safe and cost effective) systems here to replace the entire conventional actuators in a humanoid robot. This will surely make dramatic shift in humanoid robot concept and in such case, we will be starting to talk about droids/androids or cyborgs, rather than humanoid robots.

By: A. Tuter

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