Hydraulic actuators

This is a continuation of my previous posts about actuators.

What about hydraulics? Hydraulics and pneumatics have a lot in common, but instead of using gas to push around pistons, in hydraulics a liquid is used instead. In practice this means two things. For one, the cushioning effect which is due to the compressible nature of gases is gone. Hydraulic actuators will be rigid in comparison. Second, the fluid used as a medium for transporting the force will need to be recycled, so when in pneumatics could just release the "used" air out, in hydraulics you have to guide that stream of liquid back to a tank. So what is the main advantage? The amount of power you can put into one tiny actuator is enormous. It really is the pump that decides the amount of force you can swing around with, the cylinders stay pretty much the same.
Hydraulic pump with tank
Double acting hydraulic cylinder

However there still are some drawbacks. The return hosing I mentioned means alot more hoses everywhere, the rigid nature of hydraulics may not suit your application, and you still have the whole inertia/delay/sluggishenss of the system due to the fact that you are relying on fluid running around in tubes. So hydraulics in my robot? We will have to see...

This concludes my series on actuators, however I will post more on actuators when I get to test some of them. I need to figure out what I really want to use in my robot.


Pneumatic actuators

This is a continuation of my previous post about actuators.

Pneumatic actuators have the following benefits over electromechanical actuators: They can be made to require less space, and it is often easier to direct their force where it's actually needed. This is especially so with the Air muscles, which are soft and bendable. They also have natural cushioning that may be desirable when building a robot that needs to be gentle with its surroundings (or in my case, impact and shock resistant).

Pneumatic cylinder

Air muscle
Air compressor
Solenoid valves on manifoil

On the negative side however, controlling the force of pneumatic actuators will be much harder, since you rely on the flow of gases in tubes which will inevitably add some delay to the whole control loop. Also having top precision is also very hard, especially for air muscles due to their very soft nature. And finally, they come with some baggage. You will need an air compressor on board your robot, or a solid supply of gas under pressure, and each actuator will need 1 or 2 dedicated solenoid valves for controlling air flow. With many actuators (36 in my worst case scenario) this ads up to a huge manifoil with a carnage of tubes perturbing in every direction.

So pneumatic actuators in my project? Possibly...

My next post will cover hydraulic actuators.


Electromagnetic actuators

This is a continuation of my previous posts about actuators.

Electromechanical actuators are often very simple to work with. You connect them to a controller chip that feeds it with the right voltage, and apply a gearbox with the correct gear ratio to get the speed/torque needed for your application, simple as that. In most cases you just throw in a servo and you are instantly satisfied. They are fast, efficient and accurate.
Regular DC Motor
Stepper motor
Servo motor

Rotary motor encoder
DC Motor with Gearbox
Solenoid actuator

However what bothers me about them is that the motors will continuously pull current from your precious power source. In my project this is a big no-no, since I am aiming for very long power life. One way to avoid this constant power tapping is to use a very high gear ratio or a clutch mechanism that will effectively make the actuator stay locked in place when not active. However this can quickly add complexity and cost to the solution. Another problem with using motors directly is that you need to facilitate the means to get position feedback yourself by either using a rotary encoder or some other 100% reliable sensor. If you use a stepper motor or a servo, you get around this problem, but they are very expensive. Another problem is where to put them. In many small robots, the servos actually make up the limb entirely (as seen in the construction of the excellent A-Pod robot by Zenta), but for my project that is not desirable. I don't want the motors sticking out of the limbs either. I want the limbs to fully contain its actuators in an elegant manner. So electromechanical actuators? Maybe...

Next post will cover the pneumatic part of the actuator spectrum.


What is a good actuator for my robot?

Deciding which kind of actuator to use has bothered me for quite some time now. I would like to use this post to create a summary of what options are out there with pros and cons.

But first a definition. What exactly is an actuator? Actuators are used to move stuff. In my case I need actuators to move the legs of my robot. As you can se in the list below, there are a lot of different kinds of actuators to choose from:

  • Electromechanical
    • DC motor
      • Gears
      • Lead screws
      • Servos
    • Stepper motor
    • Solenoid
      • Spring return
      • Single acting 
      • Double acting 
  • Pneumatic
  • Hydraulic
    • Cylinder
      • Spring return
      • Single acting 
      • Double acting
In my next posts I will cover the pros and cons of each type of actuator in more detail.



It begins with a pile of junk...

As promised this will be a short post just summarizing the status right now. Where do I start of?

A few pictures are in order:

My living room has recently become a robot-workshop. 

My living room table is quite effective as a white board!

My computer desk. This is where I will work with software and modeling.

My workbench. Right now its just a big pile of junk.

My stash. Thoroughly sorted, although that is not at all apparent in this picture...

Build a robot? No way!

I finally decided to build a robot!

And i decided that I would blog/vlog the whole endeavor from the start. In this very first post I will try to summarize the scope of the project. What do I really want to build?
  • I want a fairly large robot that is robust to shock, impact and and that can be used outdoors in all sorts of weather and temperatures (I live in the rainiest city in Norway, so that's a given).
  • I would like the robot to last something like 24 hours on one charge without being dependent on any external sources of power.
  • I want the robot to be able to act on it's own to the largest degree possible, that is, I will strive to make it autonomous.
  • I want it to have legs. I like spiders, but having 8 legs has no obvious benefits while adding cost. 6 legs will provide a similar degree of freedom, and it works for insects so I guess it should work for my robot too. In other words, I am making a hexapod.
There all done!

Seriously though, if you were paying any attention at all, I just described a robot so complex that even NASA would be reluctant to build it. This project will take a lot of time and effort, and will most likely never complete, but hopefully it will provide me with the kind of entertainment and hard-earned knowledge that I am addicted to.

So over to what I'll need to get started.
  • I need a hardware design.
  • I need a software program to drive the robot .
  • I need tools to craft the robot body.
  • I need materials to create the robot body from.
First things first: the hardware design. The plan is to create a computer model of the robot body that is refined to the point where it is ready for prototyping. In the prototyping stage the design will be tested against reality and refined some more until I find it's ready for freezing. An essential part of this is sourcing the parts with the right properties to actually being able to implement the plans. This will be a huge equation of a bazillion variables that I will have to juggle for a long time until it sits right.

Then there is the software. I work as a developer and writing software is a second nature to me (first nature)? But still, the software that goes into a mobile walking autonomous robot is not beginner material in any way. Reliable and robust real-time calculation of multiple DOF limb motion for 6 legs, processing of sensor data from countless rotary encoders, the IMU, and real-time processing of several video sources for stereo vision, and we haven't even started with all the subsystems required for NLP, AGI, SLAM....

Then I will need a set of tools to build this robot, and a place to do it. I have a few tools already, but I will probably need a lot more for this project to get anywhere.

Finally I will need to get hold of the necessary materials for prototyping and building the robot.


Now that you know what we are up against, I'll let you ponder until the next post, in which I will give you some pictures of the current status.

PS: I decided to name the robot "Devol" after the father of industrial robotics, George Devol.