Welcome to Our 4th Dispatch
Move your bot!
This week we'll talk about robot chassis and drivetrains, i.e. the structure that holds all of our other robot subsystems and the motors, gears, and wheels that make our robot go. For more discussion on this topic, check out the On Chassis and Drive Trains topic in the MAKE Forums.
And, as always, if you have any questions, you can ask us in the Forums or email us.
Gareth and Matt
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Anatomy of a Drivetrain
By Gareth Branwyn
The drivetrain is sort of like the robot analog of legs and feet on a person -- sorta. On a person, it gets complicated trying to decide all of the systems actually involved in making a person move. Obviously there's the skeletal system and musculature of the legs and feet. And then, of course, the nervous system. Oh and then there's the power provided by the burning of calories, and the circulation of nutrition, which pretty much gets into all of the remaining systems of the body.
When you talk about drivetrains, you're obvously talking about the wheels or legs or other direct movement components, 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 a 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 the circuit design, which is designed to control the forward and backward operations, and speeds of two motors, and its configration sort of looks like the capital letter "H." We'll discuss motor controllers during the Control Systems phase (next week). This week, we'll focus on wheels, gears, motors, and their power system.
The Boe-Bot/SumoBot wheel from Parallax.
– Most of you are likely to be using wheels of some sort in your CoasterBot design. We know at least one person who's planning a vibrobot, so their mobility will basically come from little micro-jumps on some sort of legs or pins or what have you. But likely, you're looking at wheels. Wheels can be lots of different sizes and made from various materials. CDs themselves can be wheels, with rubber band material 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. They're relatively lightweight plastic and have relatively 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.
Mousey the Junkbot, illustrating the motor shaft AS the wheel technique, here with some Lego tubing as the tire, sanded to provide more traction.
In the Forums, Matt Mets brought up another idea: using buttons as wheels. This is something you can explore, especially those who've been talking about using 3.5" CDs and making a 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 enounter in the scale of robot design we're dealing with here are DC motors, servomotors, and stepper motors.
Typical three-wire servomotor, as illustrated in Tod Kurt's Servo Primer in MAKE Volume 19.
- 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, linked here).
- Servomotors actually have a DC motor inside them. A servomotor is basically a DC motor, a gearbox, and a control circuit. The control circuit is used to send signal to the motor for precise positioning within a 180° arc. See the link to the piece from MAKE below, by Tod Kurt, 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 Matt's article here in the newsletter.
- 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.
A solar-powered "vibrobot" that uses the vibrations of a pager motor to make the bot jump around chaotically on its (unpowered) metal legs.
– 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 continous 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.
– 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.
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By Tod E. Kurt
Illustration from Tod's "Servomotor Primer," showing the parts of a servo and the control sequence [Read position (pot)-->Drive motor (control electronics)-->Make motion (motor)-->Couple rotation (gear train)-->].
R/C servomotors (or just “servos”) are packed with technology, including a DC motor, gear train, sensor, and control electronics. They are a kind of servomechanism. Servomechanisms use a feedback control loop to adjust how a mechanism functions. One example of a servomechanism is a thermostat-controlled heating system. The temperature-sensing thermostat is the feedback-providing sensor, and the heating element is the output. The heater gets turned on and off based on the temperature sensing.
For R/C servos, the sensor input is a potentiometer (or “pot” for short) that’s used to measure the amount of rotation from the output motor. Control electronics read the electrical resistance of the pot and adjust the speed and direction of the motor to spin it toward the commanded position. Figure A shows an exploded view of a standard servo and the workings of the servo’s closed feedback control loop.
This is an excerpt from the "Servomotors Primer" that appeared in MAKE Volume 19. You can download a free PDF of the entire article here. It's highly recommended, a clear and full-featured introduction to using servos.
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By Matt Mets
If you're building your robot using the Jameco Robot Parts Bundle, you may have noticed that the included servo motors can only be turned 180° before they come to a mechanical stop. To use them as drive motors, we'll need to modify them to work in continuous rotation mode.
There are three major steps to the process: removing the built-in controller board, wiring the motor up directly to power cables, and cutting off a small plastic end stop. To do this, you will need a soldering iron, small Phillips screwdriver, and a pair of diagonal cutters. Check out Karl Demuth's excellent set of instructions to learn how, and chime in on the MAKE Forums if you run into any issues.
So, why does the kit include servo motors if you just have to turn around and modify them? Well, regular DC motors spin too quickly to use without gearing them down
, and it turns out that servos usually have a good quality gearbox built into them. Also, they are available in small packages, making them easy to fit onto a tiny robot. You can buy servo motors that come pre-modified to work in continuous rotation mode, however regular servos are usually cheaper and easier to find, and once you learn the process you will be able to use any of them in your bot.
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CDs as Building Material
By Matt Mets
Chris Kern mills out a support CD for his robot, Curl.
The main contest rule is that you have to use CD material as a main structural component in your bot. Using a circular disc (or series of discs) as your base is a great way to go, but what if you want to get more creative? Here are some techniques you can use to re-work those discs into new shapes:
Cutting: Use sharp scissors to cut, or score with a knife and then break carefully (try putting the piece in a vice).
Drilling: Use a standard drill, and cut slowly. Make sure your bit is sharp!
Milling: CDs can be milled, if you have the equipment. Regular metalworking tools should work fine.
Thermoforming: Use heat to form the plastic to a mold. CDs are made of thermoplastic, which can be handled in this manner; however, the metal data layer can cause issues. See last week's newsletter for a longer discussion on this topic.
Plastic welding: To "weld" two pieces of CD together, use a solvent such as acetone, or a specially made plastic glue. This will chemically bond them together, making a very strong joint. Try using a syringe to apply the chemical precisely.
Filing: Sharp edges? Use a smooth hand file to round out the edges, making them safe for handling.
Safety Note: Make sure to wear safety googles and work gloves when cutting CD material. A dust mask is smart, too, to prevent breathing in the nastiness that cutting CDs can generate.
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