Build a Real Robot – Part 4 – Selecting Wheels

This is part four of an ongoing series about building a REAL robot. You can read Part 1, Part 2, and Part 3 if you need to bring yourself up to speed.

Introduction

In this installment of the Build REAL Robot series, I will be discussing an elementary yet important aspect of robot design – choosing and arranging wheels for your robot.  Although wheels may seem to be a simple subject there is actually a lot more to them than you might suspect.

I will also show you the wheels that I’m using for my DB1 robot and, for those of you who are following along on this build, I’ll give you the part numbers for both the wheels and the hardware that I used to mount them to the robot chassis.

Finally, we will look at the types of wheels available for robotics, as well as a number of mounting arrangements that you can use that involve Omni wheels.

So let’s get rolling!

Wheels

One of the most basic of mechanical devices, the wheel has been around in one form or another since about 6500 BC. It’s likely that we will continue to use wheels for many millennia to come.

Robots, of course, have not been around for anywhere near that time, even fictional robots. And some robots don’t even use wheels, tractor treads or legs are other methods of locomotion you could use.

But wheels are simple to use and are the basis of many robot designs.  

The DB1 Robot uses four wheels. The two front wheels are standard wheels and are driven by motors, one per wheel. The rear wheels are Omni wheels and are idler wheels, they are not driven by motors but instead can turn freely.

Wheel Specifications

There are a number of different specifications you need to consider when choosing wheels for your robot. Three of the key ones are as follows:

Wheel Size

The size of the wheel is an obvious parameter to consider, and it can affect the design of many elements of your robot. The chassis design, the motor speed ratings and the motor controller programming are all affected by the size of the wheels.

The clearance of the robot above the surface is also related to the size of the wheels. An indoor robot, like DB1, only needs an inch or so of clearance. A robot designed to run outdoors should have several inches of clearance to allow for small rocks and uneven terrain.

When we speak of the wheel size we usually are referring to its diameter, not its radius (which is half the diameter).  The circumference of the wheel is the diameter multiplied by pi (about 3.1415926), and the circumference determines how far the robot will travel on one revolution of the wheel.    

DB1 uses 4-inch diameter wheels, so every revolution of the wheel moves the chassis about 12.56 inches or 31.9 cm.

Wheel Width

The width of the wheel can affect the steering, especially when using skid steering. Skid steering is when you steer the robot by altering the speed of its wheels, DB1 uses this technique as do most hobbyist robots due to its simplicity.

The wider the wheel the less accurate the skid steering will be. Other forms of steering, such as rack-and-pinion, do not suffer from this limitation.

However,  a wider wheel can support more mass, as more of the wheel contacts the surface.

The wheels used in DB1 are 1-inch, or 25 mm, wide.

Wheel Tread

The concept of the wheel or tire tread is familiar to anyone who owns a motor vehicle or bicycle. The tread can affect what surfaces the robot can be used on.

A rubberized or soft tread can offer better grip on smooth surfaces, an important point if the robot will be run on tile or hardwood flooring.

A thicker or patterned tread can be useful for an outdoor design, especially one that is meant to be used “off road”.

Types of Wheels for Robots

There are several different types of wheels available for robots, each with their own advantages and disadvantages.

Standard Wheels

These are, of course, the most common types of wheels – when you think of a wheel this is the type you usually think of.

They are simple devices that offer two degrees of freedom, meaning that they can travel forward or in reverse but not side to side.

As they are the most common type of wheel they are available with a multitude of mounting arrangements.

They can be used as drive wheels, wheels that are directly or indirectly powered by a motor. They can also be used as idler wheels, wheels the spin freely and move along with the robot.

There is a wide selection of tire treads for these wheels, and they can be used indoors or outdoors.

The front wheels on DB1 are standard wheels.

Caster and Ball Wheels

These types of wheels can move in many degrees of freedom, in other words, they can travel in just about any direction.

They are not used as drive wheels, instead, they function as idler wheels. In many cases, such as with those inexpensive “robot car bases” that I have used in many previous projects, they serve as a support wheel.

Caster and ball wheels are best suited for indoor designs or outdoor designs that will be restricted to running on paved surfaces.

Mecanum Wheels

These wheels are probably the coolest looking wheels, they are about as far away from the original caveman wheels as you can get!

These are multi-directional wheels, capable of many degrees of freedom. They can be used as either drive or idler wheels, although most designs use them as drive wheels.

Mecanum wheels consist of a wheel with a number of rollers, mounted at a 45-degree angle. This allows them to turn forward or on an angle.

Because of their unique design, it is common to require different wheels for the left and right side of the chassis, with the rollers at different angles.

Mecanum wheels are available for both indoor and outdoor applications. They are generally fairly expensive.

Omni Wheels

Omni Wheel are like Mecanum wheels except their rollers are mounted at a 90-degree angle. This gives them four degrees of freedom – forward, backward, left and right.  They can also travel at an angle with a bit of resistance.

Omni wheels can be used as either drive or idler wheels. The DB1 Robot uses Omni wheels as idler wheels.

While they can be used outdoors Omni wheels are best suited for indoor designs.

Omni Wheel Mounting Arrangements

If you elect to use Omni wheels in your robot design you can choose from a number of different wheel configuration patterns.

Here are a few that you might consider:

2 Motors & 2 Omni Wheels

The first two patterns use two standard wheels and to Omni wheels.  Both of these designs also use two motors to drive the standard wheels. The Omni wheels are used as idler wheels.

The first layout is the one used in DB1. It consists of two standard wheels at the front and two Omni wheels behind them.

This is one of the simplest and least expensive configurations. You could also substitute casters for the two Omni wheels if you wish to reduce costs.

This design is easy to control by manipulating the speed of the two motors. It does, however, exhibit some drag on turns, however, this is not too severe.  It has a very low drag going forward.

This second design also uses the same parts, with the position of the standard and Omni wheels changed.

This design offers excellent control as the two drive motors are in the center of the chassis. Because the wheels are all on the same radius it experiences no drag on turns.

However, depending upon the surface, this design can have some drag when moving forward. On a smooth surface it is not noticeable, however on a surface like a carpet the Omni wheels will not be as efficient running at a 90-degree angle.

4 Motors & 2 Omni Wheels

This design is similar to the first design, the difference being that now the Omni wheels are powered.

It shares the same advantages and disadvantages as the first design and offers higher torque. It is, however, a bit more difficult to control. It also consumes more power naturally, due to the two additional motors.

4 Motors & 4 Omni Wheels

The final two designs use four motors and four Omni wheels. They are the most expensive designs presented here, due to the cost of both the Omni wheels and the extra motors.

This design shares a lot of the characteristics of the first design, however,  it experiences less drag on turns due to the use of four Omni wheels.  

The four motors provide more torque, this is at the expense of consuming more power.  It also has no actual “front” and “Back”.

A variation on this design would only use two motors and have the rear Omni wheels act as idler wheels.  That would reduce the cost while maintaining the advantage of low drag on turns.

This final design is pretty wild, in some respects, it resembles the second configuration.

This design offers the advantage of no drag on turns, it also is the only design that can move 90 degrees (or virtually any angle actually) without turning.

The extra motors add to the expense, however, in normal running situations only two are powered so there is no torque advantage. When moving at an angle all four motors are powered at lower speeds.

Wheels used in DB1

The DB1 design uses two 4-inch standard wheels and two 4-inch Omni wheels.

Actually, it really uses four Omni wheels as each idler wheel consists of two individual Omni wheel assemblies.

The two front wheels are mounted onto hubs. Hubs are devices that attach to shafts, so each hub attaches to a ¼ inch D-shaft using a set screw on the hub.  The use of a D-shaft as opposed to a round shaft allows the set screw to get a firmer grip.

The ¼ inch shafts are attached to the gearmotor using a 6mm to ¼ inch shaft coupler, we have already seen that when we discussed motors. A ¼ inch inside diameter bearing holds the assembly in place on the Actobotics channel, it has a ½ inch outside diameter that fits perfectly in the holes in the channeling.

The rear Omni wheels are assembled using two Omni wheels mounted so the rollers are staggered. A 1/8 inch spacer separates the two wheels to keep the rollers from binding.

The Omni wheel assembly is then attached to a hub, which is attached to a ¼ inch D-shaft. This is a longer shaft and it uses the same bearing to hold it in place that the front wheels use. On the other side of the channel, a pillow block with a ¼ inch bearing is used to complete the support, The shaft is terminated with a ¼ inch screw collar.

Here is a table that lists all of the parts I used, along with links to them.

QTY	Description	Part Number
2	4-inch wheels	595410
4	4-inch Omni wheel assemblies	595671
2	⅛” Hub Spacers	545372
4	Set Screw D-Hubs	545579
2	¼ inch x 1.5-inch D-shafts – for front wheels	634064
2	¼ inch x 3-inch D-shafts – for Omni wheels	634078
4	Beveled ¼ inch ID bearings, ½ inch OD	535198
2	1/4″ Ball Bearing Quad Pillow Block	535130
2	¼ inch screw collars	6432K12
4	¼ inch spacers	633104
8	6-32 x 0.5” Screws (for front wheels)	632114
8	6-32 x 0.25” Screws (to hold pillow blocks)	632106
8	6-32 x 1.25”  Screws (for Omni wheels)	632128

Conclusion

So that about wraps it up for wheels. It also warps it up for the chassis assembly. Hopefully, between the articles and videos,  you have enough information to replicate this design, or to build a similar design.

Next time we meet we will go over the electronics that I will be using on the base unit.

I hope you are enjoying the series, if you have any questions please write them in the comments below.