BONUS ONLINE ARTICLE
The following is an extension of the full article that appears in the Winter 2005 edition of Robot magazine.
USING GYROS FOR SPINNER STABILITY
Early on in building spinners, I discovered that spinning a large percentage of your weight allowance makes it harder to control the robot, and controlling your robot is very important! I have tried many different RC gyros in my robots, but none have worked as well as the Futaba GY240. With variable gain and both heading hold and rate mode this tiny, but stable, and powerful little piezo gyro is perfect for all sizes of robots. The Futaba GY240 is a single piece unit, which makes it easier and simpler to install than those gyros with separate sensors and amplifiers. This gyro is small enough for a featherweight robot and yet powerful enough for the biggest of the super-heavyweights. Yes, even the baddest Big Daddy of them all, the 340 pound Shrederator-Supreme uses one of these little gyros in it!
First, you MUST have a tank steering robot with on-board channel mixing to use one of these single axis gyros. Single stick steering leaves your other hand open for weapons activation! Many of the electronic speed controls used in combat robotics already have channel mixing built in, but channel mixing is still uncommon in the hobby gear used in some smaller classes. I have found that for smaller bots the IFI Robotics Victor 883 Electronic Speed controller is a great match.
Because there is a separate controller for each motor, we need an elevon mixer to control the two drive trains (moving the stick forward and back causes the bot to move forward or back; side-to-side movements cause the treads to move in alternate directions for turning).
We set the gyro up in RATE mode because there is less chance of a “robot runaway” and once all the adjustments are made in rate mode then we should just be able to flip it into heading-hold mode and it should work fine. First adjust the driveline so that it responds appropriately without the gyro installed. Get the mixer working right, the motors turning the right way and exponential and travel limits set in your transmitter the way you like them.
Now place your robot on blocks that will keep the drive wheels from touching anything… we don’t want it running away on its own just yet! OK, ready? We are going to put the gyro in the steering circuit to the mixer so that it can correct the steering. You’ll unplug the steering cable from the radio and plug it into the output port on the gyro. Then take the input cable on the gyro and plug it into the receiver steering port you just removed the other wire from.
If the gyro is correcting the wrong way you will get what robot jocks like to call the death-spin. That happens when the gyro is ADDING yaw command instead of countering it. Here’s how you check for that potentially dangerous situation even before the wheels touch the road. With the robot up on blocks, using your hands not the radio, turn the robot counter-clockwise about its center. The wheels on the left side of the robot should spin forward and the wheels on the right should spin backwards. If you turn the robot clockwise then the wheels on the left side of the robot should spin backwards and the wheels on the right should spin forward. If that is working correctly then you are ready to put the rubber down!
Now place the bot on the ground and we will set the gain. After the robot’s wheels are touching the ground and not using radio control of course, push the nose sideways a little bit. The robot should ‘fight’ the turn. If the robot fights so much that it starts to oscillate (shiver back and forth) then your gain is set too high. If it doesn’t really fight too much at all then the gain is too low. I usually turn the gain up in the bot until it JUST starts to shiver back and forth a little then I turn the gain down until it just stops. When I drive my bots a really fast turn will give me just a hint of a shiver when it stops. That makes the robot respond nice and crisply to my commands and yet causes it to fight valiantly against unwanted turns. The Futaba GY240 has all the controls up on top where they are easy to get to which makes this gyro very easy to adjust. Now you know the inside secrets for stabilizing spinners!
COMPUTING KINETIC ENERGY
How much do you need to spin and how fast? Here’s some tips from our friend, physics, that will show you how to store as much kinetic energy as possible in your spinning system while spending the least amount of your weight budget as possible.
In a linear movement the kinetic energy stored is given by the formula KE =½MV², Where the Kinetic Energy equals the <M>ass in motion times the square of the <V>elocity divided by 2. In a rotational movement the concept is the same, but since the motion is rotational some of the parameters mean something a little different KE = ½Iw², Where the Kinetic Energy equals the moment of inertia about the axis if rotation (I) times the rotational velocity (w) squared. Divided by two.
The thing that interests us spinner-builders about the physics in this formula is that we will get the most bang for the buck by spinning less mass faster than by spinning more mass. How is that, you ask? Well, our bang is how much kinetic energy we can store up before we impart it into our opponent and our bucks are how much weight we have to work with. The KE of a spinning mass increases linearly with respect to the moment of inertia (more on this in a minute), but increases exponentially with respect to rotational velocity (2 times I = 2KE, but 2 times w = 4KE). So what does all that mean to us spinner builders? Simply this; It is more efficient (in terms of your weight budget) to increase the RPM than it is to increase the weight of the spinner.
OK, what about the moment of inertia, why are we spinner builders interested in this? The moment of inertial (I) is calculated by integrating all the little point ‘inertias’ in an object and adding them all together. Each little chunk of material contributes to I according to this formula; I = mr². So each little piece of our spinning mass contributes its own little part to the overall moment of inertia. Again, why do we care? Well, we still want to hit as hard as we can by spending as little of our weight budget as possible. By understanding the physics of this situation we can see that the inertia increases exponentially with the distance from the rotational center. That means we want to put the most mass we can furthest from the axis of rotation. We want our spinning weight to be as far from the center as possible.
So you never thought you would use those physics formulas again after leaving school huh? Well, if you want to build the best fighting robot possible then you must spend your weight allowance as effectively as you can… and to do that physics is your friend!
Words by Brian Nave