Servo - August 2016

62 SERVO 08.2016

exploded into flame sitting on the back seat. I have also detailed some robots in past articles that met their maker in a spontaneous conflagration.

Cylindrical lithium-ion cells may not quite have the power density and compactness, but are today’s choice for higher-end hand power tools. A lithium-ion battery has been used in the Pepper robots from the earliest prototypes to now. They are also an excellent choice for smaller robot designs. As I stated in the beginning of this section, buy the best battery that you can afford. It might seem like a no-brainer to buy some no-name import shown on Amazon or eBay, but you may end up regretting the decision.

Nickel Metal Hydride or NiMH batteries have also found favor with robotic applications, such as in robot vacuum cleaners. A NiMH battery can have two to three times the energy density of a NiCad battery and double that of a lead acid battery, and almost as much as a lithium-ion. The price was originally more than a NiCad, but is now equal and half that of a lithium-ion battery. Its main drawback is a higher self-discharge rate, but it is more environmentally friendly and not prone to overheating and starting fires. It is a good robot battery.

There are rumors that a rechargeable zinc-manganese oxide battery with far greater power density than the lithium series is about to hit the market, but we will have to wait a bit until they arrive. Also, be quite aware of your robot’s overall power system requirements as upgrading to a larger battery late in the design process can really change your robot’s overall size, center of gravity, and parts layout. Going from an 8 AH battery to a 20 AH battery is a big change. Test your drive motors and other power-hungry systems under load before selecting a battery. Remember, your battery’s discharge rating is usually determined from a 20 hour discharge curve, so discharging at a higher rate will give a lower amount of operating time. A battery discharged in 10 hours will give you less than half the watt-hour capacity than if it’s discharged over 20 hours.

Wheels for Mobile Robots

There is the old saying, “... where the rubber meets the road.” These words usually apply to cars, but wheels are an important aspect of mobile robots that are used on floors and outside on various ground surfaces. You can run simulations all day long, but the truth of how well a robot will operate is when it is operated for real.

Quite often, we are tempted to buy surplus parts for our homebuilt robot projects. This is particularly the case when we are looking around for wheels. Yes, tank-type treads and other more unique methods have been used for robot mobility, but wheels reign supreme.

Garage sales and online surplus stores often feature all sorts of wheels that have been designed for toys or scooters and similar applications that might seem like a good deal. Most home robot builders will not employ the three ball drive system used in the Pepper robot due to construction complexity and control issues.

I have a box of very nice scooter wheels that have a central set of bearings built in. At first glance, these might seem ideal for a mid-sized home robot weighing in at around 50 pounds. However, when you realize that you cannot drive such a wheel with a motor-driven shaft due to the built-in bearings, you then realize that you must mount a ring gear or pulley to the inside of the wheels to allow a motor to drive the wheel.

Another feature to consider when looking at scooter wheels is the fact that they are designed to be unpowered and just coast over streets and sidewalks. Most of these types of wheels do not have any sort of tread to give powered wheel traction. These types are best for non-driven stabilizing side wheels.

I would suggest buying the best wheels you can afford. Pneumatic types with an inner tube such as the Parallax motor mount and aluminum wheel kit shown in Figure 4 have worked very well for me. They are a bit expensive at $299 for the set of two, but extremely well built. The motors are quiet and smooth running. The wheels are precision custom machined and have pneumatic tires. They are perfect for medium to larger robots and include the machined mounting brackets. They also include a 144 count per wheel revolution set of encoders which is very useful for robot odometry.

You would spend many hours of machining and construction to re-create these motor-wheels. I have used several sets and find them indestructible. Parallax also handles several other types of wheels, including those that can be mounted directly to a standard servo. Other companies who advertise in SERVO also have good quality wheels available.