Festo PLC's, Fluidic muscles, Python and Blender

Over the last month I graduated University First! and was employed to try and finish of a project that had been lying around the uni un-touched for a year. The concept of the project was/is [REDACTED FOR SECRACY].

A Fluidic Muscle, for thoes of you who want to know, is essentially a long rubber tube that is sealed at one end. As you pump air into it, it gets wider, and shorter. As you release air, it gets thinner and longer. Essentially, It opperates in reverse to how you would excpect a standard piston to work. Because of its elastic / bendy nature, it should alow for a sort of 'automatic' suspension to be added to the rig.


The CPX PLC I was using had 6 proportional valves connected to it, these were hard-wired to 6 output words on the PLC, to change the length of one of the muscles you simply change the output words to a value (0=long, 6000=short). WHile you can do that from within the FST software, the CPX offers several ways of connecting too it. As the documentation for how that works is a bit naff, I thaught I would highlight some of the key features. The FST PLC's offer a command interface. This allows you to send commands formatted in the following way: [command][word]=[value]

So to Modify the 64th Output Word

"mow64=100"... simple!


Oops did no one tell you Festo is German? Too bad you loose.

Modify Output Word = mow

Modifizieren Ausgang Wort = maw

The correct CI would be "maw64=100"

I knew I should have studdied harder in Mr Heales German lessons...

To TCP or not to TCP

So we know what to send but how do we send it? Well there are two main ways:


Http offers us the ability to perform a 'get' request on the server: Limitations: Soooooo many. This way sucks. Dont use it, Its slow and... I mean really? http get as a means of updating a plc?


TCP Rocks... I can accsess it in a beautiful terminal, simply telnet in with the IP and port 991 telnet 991 Then send any command you like, as many as you like (but if you pause for 60s you will be booted off, so keep them coming) Limitations: From my trials and errors, I believe the minimum time required to connect, send a command, and then boot out of there is about 0.1s. That might not seem like a lot to you and me, but actually this caused me a fair amount of headache. My Game engine ran at about 40fps, and it meant that the 0.1s the telnet server took to respond, caused a massive slow-down in framerate. Obviously, this could be solved with a combination of threading and other tricks, but it is still a limitation. I will talk more about this later.


Blender is my engine of choice for a couple of reasons. 1) It is fully capable of doing what I need it to do 2) Im familiar with it, so I will have a quicker time prototyping 3) Python support built in, and a huge comminity of people to fall back on when it all goes wrong. - However - Blender threading is nothing short of awfull, and there seem to be massive problems with telnet from within the blender python interface.

Sadly the blog post ends here, Unfortunalty development on this project has halted somewhat, and so I never got further than a simple blender link to the cpx, Hopefully the project will be resurected in the new year, but untill then, thats all folks.


So... several months on and I have re-visited this project, Hopefully I can post some photos and such soon, but for the moment let me jump on in and explain how I programmed it.


UDP is the fast ugly version of TCP. It essentially works in exactly the same way, you send messages to an IP address, but there is no handshaking. There is no confirmation that the message has arrived. This basically makes it the programing equivilent of shouting at someone, rather than having a conversation. But as this project has no feedback control, this is fine for our needs. This is actually especially helpful for blender, as the python in blender runs on a "per frame" basis, and the TCP handshaking took precious milliseconds, slowing the game-engine down.

Now my game-engine runs super- fast. The code below shows how I structured the sending of commands.

Essentially I store the desired muscle pressures in an array, and then itterate through them, spitting them out with the festo required "mawXX=YY\r\n" format

The Blender code is a little more complex, and requires a byte-conversion before sending the data via UDP (this is to do with legacy python versions)

I havent included the entier script. It will be open sourced later on, but for now, this is how the coms protocol works.


So... several months on again!

Part of this work was used on the Nuerosis project

By Calum Knott

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