Robotics: Design Basics: Physical Design

work version. still needs a whole lot of work. and spell checking ;-)

Designing a robot requires to find a balance between size (mostly weight), motor power and battery power. These three elements are connected with each other (more battery power increases the weight of the robot and requires stronger motors) and finding the "perfect" balance requires a lot of tweaking and experimenting. Try to describe heavy components in output/weight (e.g. motors: torque/Kg; batteries: mAh/Kg) and pick the one that gives the highest value.
Using light materials brings down the weight significantly (aluminum instead of steel). Building a frame out of light metal and using plastic plates as surfaces would be a lot lighter than using metal plates.
There are other ways to build a robot than to cut and drill your own aluminum plates. Toys like Lego Technic and Meccano, although expensive, are an alternative when you don't have the ability to cut and drill your own parts. Especially Meccano (or better: the cheap imitations) can be useful even when you do make your own parts. There is something convenient about having a collection of parts with standard holes and sizes.

When you start your design, first decide how big you want it to be. Don't think about exact sizes, comparing it to the size of an object ("the size of a shoe box") will be sufficient. Make an estimated about the weight of the complete robot and pick your motors and wheels. Keep in mind you need high torque and low speed. A bare DC-motor has high speed and low torque, adding a gear reduction will solve this. Motors with reduction gears are also available. The speed of your motor and the size of your wheels determine how fast your robot will be able to move.

For example: RB-35 is a motor with a 1:50 reduction. It makes 120 RPM or 2 rounds per second. Lets pick a wheel with a diameter of 20cm (radius R = 10cm). This wheel has an circumference of 2 x PI x R = 2 x 3.14 x 10 = 62,8cm. This means in one turn the wheel moves 62.8cm. When we mount this wheel on the motor it'll turn twice every second and therefor move 2 x 62.8cm = 125.6cm. So it's speed would be 125.6cm/second or 1.256m/s.
In reality this speed is going to be a little lower as the motor turns 120RPM without a load. But even an 1m/s is pretty fast for indoor robots. You'll probably use PWM or other methodes to slow it down.

Pick your batteries. Make sure you have enough power to keep the motors and all the electronics running for a sufficient amount of time and keep some reserve for future additions. Compare the weight of the batteries and motors you've chosen to what you had planned. You might need to go over this part again (picking different motors and/or batteries).Keep in mind that your robot's body has a significant weight.

Wheeled platforms can have any number of wheels. Most commonly are 3, 4 and 6 wheeled vehicles(excluding wheels used for feedback). Other numbers are also possible, but can be hard (1 or 2 wheels) or have superfluous wheels (5 or more than 6, except for large robots or train-like robots). Basically there are 2 types of wheels: powered wheels and unpowered wheels. The first are powered by the motors and are used to move the robot forwards (or backwards). Unpowered wheels are used to keep the robot in balance.

Turning can be accomplished in several different ways:

• Tank-like Turning: Move one wheel forwards, the other backwards. The robot turns around within a small circle which center lays in between the 2 powered wheels.

• Differential Steering: Same steering system as used in cars. Harder to implement since the inner and outer wheels need to turn to different angles.

• 3 wheeled platforms can use the single unpowered wheel to turn.

Tracked platforms use tracks similar to tanks. This kind of propulsion is only useful on loose sand and mutt. On concrete or indoor it's less effective than wheels.

Walkers are robots that use legs instead of wheels or tracks. These robots are harder to build than wheeled robots and can be a nice challenge for an experienced builder. Walkers are designed to imitate how animals (or humans) move.

The hardest

These walkers are imitations of insects. Many of these move 3-legs-at-a-time. (again for static balance)

Note: Static balance means the construction is at all time in balance. This means that if the robot would stop moving at any time while moving it wouldn't fall over. In contrast there is Dynamic Balance. This means that the robot is only in balance when it completes its step. If it's stopped in the middle of its step it would fall over. Although this might sound like a bad thing, dynamic balance allows much faster and smoother movement, but requires sensors to sense balance.

• Use low power leds. Always. This drastically reduces how much current your circuits consume. A normal Led consumes around 15mA. A modern µcontroller consumes about the same.

• Use CMOS ICs instead of classic TTL. Again this reduces current usage. But pay attention to their sensitivity for static electricity when soldering them.

• Use good quality IC-sockets (or better don't use them at all). They're worth there price.

• Avoid using IC-sockets on sensitive circuits (high speed digital, clock signal and analog signals). The moving robot can shake those ICs loose over time.

• LEDs are very practical to make slow digital signals visible,adding them on some signal lines can be interesting for testing purposes, however they do increase power consumption. Removing them when your circuit works correctly can make it more power efficient (replace led with a wire, replace resistor with one with a higher value).

• See if your µcontrollers can run with a lower clock speed. The higher the clock speed, the more they consume.

• Use your µcontrollers sleep functions whenever possible.

• Learn to make PCBs. it's not that hard and it makes your electronics look more professional. Don't trow away your breadboard, pcb aren't very practical for prototyping.

• If you're up to it: build your circuits in SMD components on PCBs. This reduces both size and weight. However soldering SMD ICs isn't easy. SO-packages aren't too hard for an experienced builder. Smaller packages are almost impossible to do by hand. Get good soldering tools before you attempt this. And practice, practice, practice,...

• buy a breadboard. There invaluable for designing and testing circuits.

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