Mobility Tutorial

Well, you gotta be able to move around the arena right? This tutorial covers some of the common ways you can build and wire your robot for easy driving. Of course, you can do just about anything you want to make your bot move. You don't even need to use wheels if you don't want to. But for now, we'll just be talking about wheeled robots. There are numerous possibilities for how you set up your wheels: 2 wheels, 4 wheels, powered steering unit, wheels kept internal or placed external, etc. You'll probably discover many variations on your own. I will get you started with a basic 2 wheeled bot to get you started.

Note: For this tutorial, we won't cover some of the basic concepts like how to create a chassis. If you don't know how to use the workshop to build a chassis and place components, you might want to read the first tutorial on basic construction before reading this one.

2-Wheeled Bots

Two wheeled bots are one of the easiest types of robots to build and wire. This is a common design seen in AI bots like Stinger, Wide Load, Flame Chopper, and LugNut. The basic principle is to set up two spin motors facing the opposite direction from one another and attach wheels to those motors. The steering technique is accomplished through wiring. Each motor is turning either clockwise or counterclockwise. When the wheels rotate together the bot goes forward, when they turn in opposite directions the bot steers left or right. Lets build a quick one.

In the workshop, build a new robot and use any chassis shape, but make sure it is large enough to hold two spin motors, a battery, and control board. My shape is shown below.

Go to the components menu and place two spin motors on the baseplate facing opposite directions. Remember that holding the SHIFT key while placing a component will let you rotate it. I used the Red Bird motor here, but you can use another spin motor if you want. Just make sure the axles point outward from the center of the bot. Although the body of a spinmotor cannot pass through the walls of the chassis, the axles can. This helps you to place wheels externally.

Note: If you wanted, you could keep the motors and axles completely inside the chassis so that when you put a wheel on the axle it is guarded inside the walls of the chassis. You need to ensure there is enough room for the motor/axle AND the wheel, of course.

The next step is simply to place wheels on the axles of the spin motors. You will find that the various wheels have different diameters and some may not be large enough to use with certain spin motor setups. For example, the mini wheel is too small to use with the Z-Tek motor because it can't reach the ground.

If I test my robot now, the front end will fall to the ground since it has no support to balance it. You might want this effect for some designs, but for this example, lets add a Balance Caster component to keep the front of the bot level. I picked the 20 cm version of the caster. Some components have styles like this to help fine-tune your robot. Note that the caster component is placed on the bottom of the baseplate. You won't see it if you are viewing from the top.

Go ahead and put a battery and controlboard somewhere inside the bot. We will need these if we want to be able to wire and drive the bot.

Go to the Wiring setup screen. We will be using two analog controls for this robot. Remember that analog controls have two inputs: one positive and one negative. This is ideal for driving setups because we have opposing directions. So we will use one analog control for the Forward-Reverse directions and another analog control for Left-Right steering.

It is frequently helpful to give names to your controls. For the first analog input, you could call it "Forward-Back." Click the input boxes and assign two keys from your keyboard that will be used for control input. My example uses W and S. Or you might press two directions on a gamepad or joystick if you are using that style of input.

Now you need to wire the Forward-Back control to your spinmotors. Rotate your 3D view so you can easily see the two spin motors, and so that UP would represent a forward direction for your bot to drive. With the control selected in the grid, click on the right spin motor. You will get a dialog box asking how to wire the motor. For the right motor, wire it so that positive is clockwise and negative is counter-clockwise. Click OK when you have done this.

You have wired one motor for Forward-Back, now wire the other one. With the same Forward-Back analog control still selected in the grid, click the left spin motor. This motor will be wired opposite of the right motor, since it faces the opposite direction. Wire this one so that positive is counter-clockwise and negative is clockwise.

If you think about it, it makes sense: pressing the forward input key (my example is "W") sends positive input to the control. Positive is wired so that the right motor spins clockwise and the left motor spins counter-clockwise. Imagine the wheels spinning in this fashion and you can see that the bot would drive forward. Pressing the other input key ("S" in my case) sends negative input which spins the wheels the opposite direction causing the bot to drive in reverse. Try it out by going to the test garage (click "Test Robot") if you want to see how the bot drives so far.

OK. We have forward and reverse. Now we need to be able to steer the robot. Steering is easy to set up. Drag another analog control onto the controller grid and name it "Left-Right" and assign some input keys. Since I used W and S for forward/reverse, I might choose A and D for left and right. Then, with that analog control selected, you will wire each spin motor identically: positive is clockwise, negative is counter-clockwise. Again, think about what is happening here. Pressing the "Left" input sends a positive control signal. Positive is wired so that each motor spins clockwise. If the right motor spins clockwise, it will push the right side of the bot forward. If the left motor spins clockwise, it will push the left side backward. When the right side goes forward and the left side goes backward, the resulting motion is a rotation of the entire bot chassis in the left direction.

If it helps, here is a diagram of your wiring. It shows how each analog control is wired to each spin motor.

Remember that terms like "left" and "right" are dependent on your viewpoint. This assumes you are looking at the bot from the top down and that UP in the view would match the robot's forward direction.

4-Wheeled Bots

The good news here is that once you understand the principles of the 2-wheeled wiring, a 4-wheeled bot is nearly identical. Instead of just one motor facing each direction, set up two on each side of the bot. Motors on the right side of the bot are wired the same as one another. Motors on the left side are wired the same as one another.

You have the added option here of using 4-wheel drive or 2-wheel drive, and 4-wheel steering or 2-wheel steering. With a lightweight bot and powerful spin motors, you might find that having all 4 motors wired for steering is too powerful and your bot spins too fast. In this case you could use all four motors for driving forward and reverse, but use only the front two motors for steering.

With the diagram below, assuming up is the forward direction of the robot, see if you can wire the two analog controls to the four spin motors to get this 4-wheeled bot to drive properly. The motors and controls are labeled and the solution is given following the illustration.

Solution:

Control 1 | Motor A | Positive = CCW Negative = CW
Control 1 | Motor B | Positive = CCW Negative = CW
Control 1 | Motor C | Positive = CW Negative = CCW
Control 1 | Motor D | Positive = CW Negative = CCW
Control 2 | Motor A | Positive = CW Negative = CCW
Control 2 | Motor B | Positive = CW Negative = CCW
Control 2 | Motor C | Positive = CW Negative = CCW
Control 2 | Motor D | Positive = CW Negative = CCW

Powered Steering Component

The Powered Steering component is a single component that you place on your chassis baseplate to make it easier to set up and wire the bot for driving. The component has two axles with rotating hubs, and wiring connections for forward, reverse, left, and right. It typically needs to be used on a four-wheeled robot because it mimics the type of steering on a car, with moveable rotors for directing the angle of the wheels. This is different from the style of steering you used above where you wired one motor to go forward and the other to go in reverse in order to steer the robot. This part of the tutorial will walk you through using the Powered Steering component.

Build a chassis that would be suitable for a four-wheeled design. Something sort of rectangular would work, but remember that you can make just about any shape you want as long as you can fit the components you need into it.

With your chassis ready, go to the components screen and pick the mechanics category. You will see the Powered Steering component in the list. This component comes in three styles: narrow, medium, and wide, depending on the size of your chassis. For the one I have built here, the narrow style will fit best. Click to attach the component and place it in the front of your chassis. Notice the blue triangular arrows on top of the component. These indicate the forward direction, so place the component so the arrows point towards the front of the bot.

The hexagonal hubs on each side are just like axles of a spin motor. Go ahead and put wheels on these. You'll see they pop into place easily.

To make this a four-wheeled bot, we need some wheels in the back of the robot. You could place spin motors back there, but let's make this example as easy as possible and use generic axle mounts. Axle mounts are unpowered axles that can rotate freely--useful for extra support wheels. These are found in the extenders category. Put one on each side in the back, as my example shows.

Your bot is nearly built. Pop a few wheels on these axle mounts, then add a battery and control board.

You may notice that the axle mounts are at a slightly different height than the powered steering hubs. For that matter, each spin motor is at a different height and each wheel is a different diameter, so mixing and matching components may result in bots that are not perfectly level, but that's ok. Remember you can also raise components up off the baseplate to adjust how high they are positioned.

For wiring, we will again use two analog controls. Remember how to drag these onto your controller? Label one of them "Forward-Back" and the other "Left-Right" if you want to remember what each one is for. Assign keys or joystick inputs to these controls.

The actual wiring part is easy. Start with the "Forward-Back" control, select it, and click on the Powered Steering component in the 3d view. In the popup dialog, wire positive to "Forward" and negative to "Backward." Then click OK.

Now, select the "Left-Right" analog control and again click on the Powered Steering unit. This time, wire positive to "Left" and negative to "Right." Piece of cake, eh?

That's all there is to it. Try it out in the test garage and you'll see that you can drive forward and reverse, and steer the wheels left and right. Similar to a car, left and right by themselves simply rotate the wheel angles. You have to be driving forward or backward while steering if you want to turn.

There are many variations you can do with the powered steering component. You could combine it with spin motors for added power. You could use two powered steering components, one in front and one in the rear. You could steer from the back of the bot. And so forth.

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