In this tutorial, I will teach you about suspension systems, and a new type of suspension system I invented for robots that's very simple to make. (so give me credit and link back if you use it!)
But first, enjoy the video!
What is a Suspension System?
your robot travels at high speed on rough terrain
your robot has more than 3 wheels
your robot experiences high frequency shock
your robot is micro-sized
Disadvantages of a Suspension System
A perfect example of a very complex suspension is the one here below designed by Honda. It has a large number of parts and requires a very complex mathematical analysis to design it:
The example below is a pic of oil-filled shocks used on an RC truck. It also requires a decent number of parts, although mathematically its rather simple:
To answer your question, you can buy these springs at any RC website:
My Invention: Single Part Suspension System
To demonstrate my suspension system, I used it on my Experimental Robot Platform. Here is what it looks like in CAD:
And in real life:
Look carefully at the large round wheels on the left, and the orange wheels on the right. They both use my single part suspension system design.
Lets start with the simpler design - the long beams attached to the small wheels. These beams are designed to deflect to be perfectly horizontal under the full weight of the robot. When the terrain under it changes, the deflection changes to conform with it. This is what the finished design looks like:
But its more complicated than it looks! You can't just guess a design and it'll 'just work'. Of course for simple designs, you can use basic stress analysis equations for bending beams. However I wanted more complex features, so I used FEA to simulate it. FEA isn't very reliable under large deflections, so it took me two iterations/experiments to get it right. The manufacturer of the delrin material I used also didn't release product specs, so I had to guess it based on typical values. I also didn't know the exact weight distribution of my robot to use in FEA, so I used the center of mass and weight calculator in my CAD software to get a good approximation of it.
Here is what the beam looks like in FEA:
To make it I used CNC to cut out the profile. This made it easy to produce a whole bunch at once.
And then to screw it into my robot base, I drilled two holes into the top of each. I just used my tabletop drill press, with an attached X-Y table and vice clamp. Make sure it doesn't flex while drilling!
Wheel Suspension System
A major problem that you will encounter when CNC'ing the flexible suspension systems is that they also flex when you are machining them! They also tend to vibrate in a mill, causing rough surfaces on your parts and scary loud noises. I designed the wheel with this in mind, but it still had serious problems. Do your best to secure the entire part during machining! If you have the option, a laser cutter would be a much better choice for these designs.
Here is what the completed wheel looks like:
A perfectly cut wheel has very little friction. To improve traction, around the edges of the wheel I added a rubber material as described in my ERP updates documentation.
Sag is when over time the 'spring' in your suspension permanently settles into another position. On my 3rd iteration, I noticed serious sagging within just days of building the system. It was like putting a fat guy on a motor bike - you better believe the shocks will wear out!
There are several ways to design around sag. You can either stiffen up the spring and have it designed for 25%-75% more weight, setup an actively controlled spring modifier using something like servos, or set up some type of calibration device that you can tweak as the sag gets worse. To keep it simple I went with the first option - making it stiffer.
So finally on the 4th design, with a stiffer design, it lasted about a month before sag became noticeable. Its still working now, but at some point I'll have to swap them out for new ones. Keeping the robot weight off the suspension system during storage helps it last much longer. I'm sure certain materials will sag less too, yet still flex as needed, but I didn't investigate any further. I didn't want to over design it =P
Spring Damper Control
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