Drivetrain

Original text by Gareth Branwyn

The drivetrain is sort of like the robot analog of legs and feet on a person -- sorta. When you talk about drivetrains, you're obviously talking about the wheels or legs or other direct movement components on a robot, and then the motors that actuate that movement, and any gearing system that translates the rotation of the motors into the proper speed and power (torque) that you need to move your bot. And then there's the issue of getting power to this movement system. That's usually included in discussions of robot drivetrains.

And then there's also control. Most robots have something called a motor controller, or "motor driver." This is usually a secondary board or chip that sits between the microcontroller unit (MCU) and the robot's motors. It's dedicated to controlling the speed and direction of the (usually) two motors. You'll frequently hear people talking about an H-bridge motor controller. This simply refers to a popular motor controller circuit configuration, which is designed to control the forward and backward operations, and varying speeds of two motors -- its configuration sort of looks like the capital letter "H."

Wheels ¶ 

Wheels used for bots can be lots of different sizes and made from various materials. In CoasterBot builds, CDs themselves can be wheels, with rubber band material used for tires. You can use mini-CDs too. There are a number of types of commercial wheels available, from Jameco, Solarbotics, and elsewhere. So-called SumoBot wheels are great on most lightweight hobby robots. They're relatively lightweight plastic and have wide rubber tires that offer good traction. In BEAM robotics, it's common to angle the motor shafts of DC motors and put heat-shrink tubing or rubber right onto the shaft to make it into a tiny-diameter wheel. My Mousey the Junkbot uses this technique.

In the MAKE Forums, Matt Mets brought up another idea: using buttons as wheels. This is something you can explore on truly mini-robotic platform.

Motors ¶ 

Motors are a fascinating aspect of robot engineering and the subject could fill books (and certainly has). The main types of motors you'll likely encounter in the scale of robot design we're dealing with here are DC motors, servomotors, and stepper motors.

DC motors are the most common types of motors found in toys, appliances, power tools, etc. These motors come in two main flavors, brushless and brushed (Wikipedia has good entries on both of them that explain, and illustrate, the basics).

Servomotors actually have a DC motor inside of them. A servomotor is basically a DC motor, a gearbox, and a control circuit. The control circuit is used to send signals to the motor for precise positioning within a 180° arc. Check out Tod Kurt's Servomotor Primer here on Make:Projects, which provides all the basics on servos. To use a servo as basically a DC motor and gearbox, you can remove the control circuit and the mechanical stop that prevents it from 360° rotation. See how to hack servomotors here.

Stepper motors are actually a type of brushless motor. They're commonly found in printers, 3D fabbers, and other machines that require precise positioning and holding of that position.They are called steppers because they step through a series of positions of the rotor, and those steps can be precisely controlled. Stepper motors tend to be heavier than DC motors, require more fusing to control, and tend to be slower than other DC motors of a similar size/class. The Wikipedia page has an animation that simply illustrates how a stepper functions.

Pager motors are actually just a tiny type of brushed or brushless DC motor with an "eccentric weight" on it, an off-kilter hunk of metal on the shaft that makes the motor shake when powered. These motors are great for making really tiny robot drivetrains or for using in vibrobots.

Gears ¶ 

Gearing can be a very complicated and tricky business. The idea is to use a collection of smaller and larger gears to change the speed at which the motor turns into the speed and torque you want from your wheels. So, through the use of gearing, you can get either a faster speed or higher power (torque) out of your motors. For robots in this contest, you might be able to get away with direct-drive from the motor shafts of your DC motors to the wheels (if the robot is heavy enough to add resistance to the drivetrain). Your best bet is likely going to be using hacked servomotors (for continuous rotation), which have a nice built-in gearbox. If you do decide you need more power than these simpler options can provide, you can look into a kit such as the excellent Tamiya gearboxes that Jameco sells, which allow you to select from four different gear ratios, for each wheel.

Power ¶ 

Powering your drivetrain for an autonomous robot likely means batteries (although BEAM robots use solar cells and capacitors for collecting and delivering short bursts of power). For your motors, you're probably going to use a holder of AA or AAA batteries, either disposable alkaline or nickel-metal hydride (NiMH) rechargeable. There are also battery packs, frequently lithium-ion polymer (LiPo), used in R/C vehicles. These are all dry-cell type batteries. There are also sealed lead-acid (SLA) batteries commonly used in more power-hungry robots (such as larger bots and high-power combat robots). It may sound like a joke, but to educate yourself on batteries, get thee to Battery University . It's an online repository of pretty much everything you always wanted to know about batteries but were afraid to ask.