Scale Model Animation 4 – Critter Guidance, Sensors, & Fun
Sensors Tell You Where To Go
To understand how the Critter is guided, one first needs to understand a great little sensor called a Hall Effect Device. Two types I use are the AH180-PL-A (the one with leads) and the TCS20DLRLF (the surface mount device- SMD) both pictured below:
Digikey.com and Mouser.com are good sources for all sorts of Hall Effect sensors. Be careful though, there are several variations. One with an “omnipole” characteristic will activate with either the north or south pole of a magnet. This is the kind I look for. Either of these will react (turn on) in the presence of a magnetic field. So they switch “on” when a magnet is near and switch off when the magnet (and its field) goes away. Simple, right? Yes, very simple. Now for the best part--- you can obtain tiny rare earth (read incredibly strong) magnets from many sources including your local Radio Shack.
I’ll come back to why this is important later, but some of you want to know how this is used in the Critter Guidance system. You will see from the picture below and the accompanying video that below the black plastic top, that much like the Faller car system (a great innovation in my opinion) a slit is cut into the foam core (I was not very neat with this because I ripped it up and re-tried different wire thicknesses and sources) and a iron/steel/ferrous wire was inserted to determine the path of travel for the Critter.
I started with an old wire coat hanger, by the way, and it worked great! Since then in the model shipbuilding section of hobby stores I found black annealed spools of wire, some of which work great (some are not attracted to a magnet, so take one along with you when you look for it). I found these in 19GA, 21GA, and 24GA wire. All work here, but bigger is better here (smaller GA number).
Now here’s where allot of the guesses were close but not quite accurate (the readers of this blog are amazing observers and modelers, if I may say so). Many people found the white “shoe” on the bottom of the Critter, some deduced it held a magnet—all correct! The “shoe” was a small piece of styrene freely pivoting on one end (the far end from the magnet) and was dragged from SIDE TO SIDE as the Critter moved along the embedded wire under the black plastic top.
The wire had no signal emission, and there were no markings whatsoever on the plastic for guidance. BUT, in the Critter chassis above the shoe on each left and right side, there was a Hall Effect device (a TCS20DLRLF-two in all). When the Critter deviated from the course set by the wire below, the shoe moved under one sensor, the sensor turns on, and the controller in the Critter detected the change, and simply drove the treads to turn until the sensor switched off! Neat and simple! You may get a better of what’s going on in this video:
A comment: This is a very simple variation of a “line following algorithm” used by all kinds of robots! Hence, The Critter bridges the worlds of Model Railroading, Animation, and Robotics. Maybe I should say I’m working on Scale Model Robotics? Micro-Robotics? Could this be the draw for a new generation of scale modelers? However, as opposed to what some usually think of when they think of robots and robotics, scale model animation needs for things to move very slowly, not fast, and typically, small scale models don’t have the power, volume, and control sophistication for complex systems. But the sensors and techniques used even in the large sale world might point the way here.
It should be clear that there is no mechanical linkage as in the clever Faller car system for steering. There really can’t be since this is a tracked vehicle anyway. What was amazing to me is what happens in the turns. My first “track” or “course” was a decreasing radius spiral following several “wiggles” left and right. I thought I would measure how sharp a radius in the turn I could take. Originally, I turned the Critter by stopping one track, and driving the other to pivot around the stopped track. This worked, but sure enough on the decreasing radius turn it eventually went off course. Now the control system freaks reading this will immediately pounce here and proclaim that I could adjust speed and timing to make this work. They would be right in theory, but in practice it is much more difficult for a couple of reasons: the Hall Effect sensors exhibit slight hysteresis—if they might turn on at 0.4 inches from a magnet but then they might turn off at 0.6 inches away; also, you can only make the tracks go so slow before friction stops them. Nonetheless, the pivoting turn worked. But I have seen a tread crawler pivot in place by driving one track forwards and the other backwards—why not here? Well, this yields the amazing capability to (almost) make a 90 degree turn following the embedded wire. In the teaser Critter video (number 2) you can see the Critter make some relatively sharp turns. Believe it or not, I am hoping to have it either navigate through a forest, city streets, or a quarry. The treads also allow it to climb very easily. The magnet shoe is tapered on the front so it can go over uneven surfaces. By the way, I left the shoe white and long to make it easier to see in the videos—that was my hint to you all. It could be hidden by shortening it and painting it black—or better by using black styrene to fabricate it.
A Switch with More Value Than Meets the Eye
The Hall Effect Sensor is likely known to some, but not to most modelers. It is a three terminal device. In my case, it is powered by 3.5 volts (+ & - are two connections; the AH180-PL-A can be powered with 5Volts) and it has a single output pin that will pull the line low (connect it to +3.5 to 5Volts through a 4.7K resistor) when it detects a magnetic field—so it’s a magnetic switch. Big deal you say? Maybe, but consider the possibilities. These switches can be turned on by incredibly small magnets. Take a look in the picture here:
All of these magnets are incredibly small and cheap! Other than the Radio Shack (RS) one, the others were purchased from MagnetsandMagnets.com and they also are on ebay. The bigger and stronger the magnet, the greater the distance away from the sensor it will switch. If you have a moving apparatus (say a turntable) you can detect multiple positions with one sensor by secreting incredibly tiny magnets at all relevant positions (say track alignments) with no mechanical connections or contacts—pretty neat if you start thinking about it. I glue one of each type magnet to a small stick and use it to determine the orientation of the magnet (pole position) and to pick them up before they “launch” off my workbench.
The Critter is going to be equipped with the AH180-PL-A to perform another critical function: absolute position reporting. What good is that you say? Well, the Critter guidance system I described keeps it on its path- and even works well (I’m still amazed at that!), but you need to get The Critter to stop and “work” along the way. I’m pretty sure that with another “downward looking” sensor, this time with a magnet embedded in the surface (or to the side), it will be able to detect “milestones” along the way. The controller could then pause, turn, raise or lower the boom or hook, etc. and perform some “useful” function on my layout.
So there’s more to come for The Critter.
‘Hope you like it! Your terrific (and relevant) suggestions are always welcome.
Have fun! 
Best Regards,
Geoff Bunza
