Wheeled robots can achieve greater speeds than any other types of mobile robots, are easy to build and generally easier to control. Despite the shortcomings on some types of terrain inherent to wheeled platforms, they are widely used almost everywhere for countless applications, robotics and otherwise. In this article we try to summarize all types of wheeled platforms in use today, as well as discuss mobility aspects, advantages and shortcomings of each type of platform.
Platform mobility aspects
The most common type of vehicle in use today is the car — basically a four wheel platform with at least one drive axle, a directional axle and some form of suspension for each wheel. The majority of improvements to this type of platform have been made in the suspension and steering areas.
Modern 4 wheeled vehicles benefit from independent suspension for each wheel, although there are numerous types of suspension systems all of them consist of springs, dampers and connecting elements in certain combinations, the primary goal being to provide optimum dynamic characteristics for better control over the vehicle with respect to a myriad of factors such as terrain, vehicle type or purpose, chassis structural loads and so forth.
Suspension springs are important for proper handling of vehicles travelling at speeds over 8 meters per second, approximately 29 km/h. Below this speed springs can reduce the vehicle’s mobility because they can change the amount of pressure exerted by each wheel on the ground.
A conventional 4 wheel vehicle with independent suspension appears to touch the ground with equal force on all 4 wheels, yet wheels travelling over bumps actually support more weight and this can lead to reduced traction for light loaded vehicles. A better solution, at slow speeds, would be a means to lift some of the wheels over bumps, with respect to the chassis, this way weight distribution changes are greatly reduced. This is a characteristic of the rocker-bogie type suspension.
Pressure exerted on the soil varies between 20 and 80 kPa for all types of vehicles (the human foot exerts a pressure of around 35 kPa), this interval being so narrow because of soil types and materials. There are specialized vehicles, designed to travel over loose snow, which exert a medium pressure of only 5 kPa. As a rule of thumb, vehicles with low contact pressure will perform better on loose terrain (e.g. loose snow, sand, mud) while the ones with greater contact pressure will perform better on firm surfaces such as asphalt, hard ground and so on.
Another method to increase a mobile platform’s mobility is the possibility to change the center of mass position as needed. This can be accomplished by moving a dedicated load around the chassis or changing the position of various elements on the platform, elements heavy enough to accomplish the task (e.g. robotic arm, load carried, etc.)
How does it work? For instance when a 6-wheel robot equipped with such a system needs to cross a narrow but deep ditch, the load is moved to the back of the platform, effectively changing the position of the center of mass. The back wheels will be loaded up and the robot will not dive into the ditch, it will rather travel over it until the front wheels will touch the other side. When the robot has nearly crossed the ditch, the load will be moved forward, right before the back wheels leave the ground, loading up the front wheels. Thanks to this system the robot travels on its path barely influenced by the ditch, in which otherwise it could have remained stuck.
Wheel diameter is another important factor in a platform mobility, the greater the wheel, the larger the obstacle that can be passed by the vehicle. Considering a very simple platform, a wheel can negotiate a stair type obstacle if the height of the obstacle is less than 1/3 of the wheel diameter. In case of all-terrain 4 wheeled vehicles the height of the obstacle can be up to half the wheel’s diameter.
There are other ways to negotiate a stair-type obstacle. For instance if the actuated wheel is pushed towards the vertical wall of an obstacle larger than half the wheel’s diameter, for a certain ratio between the pushing force and load distributed to the wheel, it will climb the vertical wall. Again, this is one of the principles that make bogie-rocker suspension systems work.
For static stability at least three wheels are required, making such platforms very common. 3-wheeled robots are used in various implementations, from simple 2 motorized wheels and one fixed wheel types to complex robots, with wheels mounted on legs with 2 or even 3 degrees of freedom (DOF).
One and two wheeled platforms
Even one wheeled robots are possible. Limited in mobility they can however travel some routes with a certain degree of difficulty. The wheel is actually a sphere with a counterweight inside and movement is initiated when the counterweight is slightly offset with respect to the fulcrum, making the sphere to roll in that direction. Obstacle negotiation capabilities are limited to the obstacle geometry, sphere diameter, material and the ratio between the sphere counterweight and the mass of the robot.
Two wheeled vehicles can have wheels positioned on either side of the chassis (e.g. Segway) or inline (e.g. bicycle). Such platforms are relatively hard to use because of their inherent instability, the Segway has however an advantage in control at low speeds.
Three wheeled platforms
Back to 3 wheel robot platforms, there are several variations. The simplest 3 wheeled platform has one drive wheel, which also servers for steering purposes. Due to its limited mobility this variant is used mainly for indoor robots in controlled environments.
A method to increase stability of a 3 wheeled platform is moving traction to the two back wheels by means of a transmission which includes a differential mechanism, leaving the front wheel to handle only the steering of the platform. This design can be very efficient on smooth terrain but will handle poorly in less than ideal environments due to its lack of stability when negotiating obstacles. A variation of this design is independently motorizing the traction wheels.
Other variations, meant to increase stability at low speeds, are using the two traction wheels at the front, while the wheel in the back handles steering. Adding directional capabilities to the front motorized wheels is another possibility. Higher mobility is gained by motorizing all three wheels of the platform.
The most mobile, yet most complex system is the one in which all three wheels are directional and independently driven. This arrangement allows for movement in any direction and even turning on the spot. This ability is called holonomic motion and is very useful for mobile robots as it significantly improves mobility in rough terrain.
Four wheeled platforms
The simplest 4 wheeled mobile platform does not have a differential mechanism and all wheels have a fixed orientation with respect to the chassis, i.e. do not pivot in any way. Instead wheels on the same side of the chassis are actuated together (i.e. wheels on the same axle are actuated independently) and the direction can be changed by means of skid steering — wheels on one side receive a different amount of actuation (can even be stopped) than the wheels on the other side of the chassis, making the platform to change its orientation.
Despite inherent inefficiencies due to skidding wheels, these platforms have four wheel drive capabilities enhancing their mobility, are simple to build and extremely robust. Vehicles using such implementation are the Bobcat vehicles.
A shortcoming of such a vehicle is that when a wheel passes over an obstacle, suspending the vehicle, then another wheel will lose contact with the ground. This problem is characteristic to platforms with more than three wheels. Although good mobility does not necessarily imply having all wheels in contact with the terrain it is of best practice to use mechanisms that allow keeping all wheels on the ground.
A good method to resolve this issue is using a chassis split transversely, the two parts being connected by a passive central pivot. Such an implementation can be found in the RATLER vehicle.
Another type of vehicle is the forklift truck, which has a rigid front axle, also providing traction and a pivoting rear axle in the vertical xOz plane, with directional wheels that may or may not be motorized.
Another variant is a vehicle with a steering system on the front axle, similar to a car, and a pivoting rear axle. This platform can also have one or both axles driven.
Another type of system is a 2-section chassis, with the two sections linked by a central pivot. One axle is rigidly mounted to each section. The central joint with 2 DOF allows the sections to tilt with respect to each other in 2 planes, xOy and yOz, this way both steering and contact with the terrain of all four wheels is accomplished. Wheels actuation can be synchronized or independent.
A very efficient chassis solution for an all-terrain vehicle is employed by the Nano Rover prototype developed by Brian Wilcox and Annette Nasif at JPL.
This vehicle has an extremely high degree of mobility and it can move even when the main chassis is upside down. All wheels are motorized and are attached to legs that can pivot with respect to the chassis. This way the vehicle can change its wheelbase and ground clearance.
Five wheeled platforms
An example of a five wheeled platform is similar in construction to a three-wheeled platform, with an added motorized axle to increase traction and maximize the contact area for better weight distribution. The directional wheel is usually not motorized.
Six wheeled platforms
There are many systems and variants of platforms that employ six wheels, this is because high mobility can be achieved in this way. Using six wheels ensures good weight distribution and reduced pressure on the terrain surface, high traction and the ability to negotiate obstacles without increasing the complexity of the system too much.
The simplest six wheel vehicle does not have a suspension system and skid-steering is employed, motorizing together three wheels on each side, similar to the four-wheeled Bobcat.
An immediate advantage of a six wheel platform can be observed when skid-steering, by mounting the middle wheels on each side slightly lower than the rest, weight supported by the other four wheels is reduced, as a consequence they add less resistance in the process. A chassis balance motion from front to back will appear but it can be counteracted if taken into account by motion control algorithms.
A more complex system to counteract this shortcoming employs linear actuators for the front and rear axle wheels. These actuators can actively lift or lower the wheels independently keeping them on the ground. This system also helps negotiating an obstacle, thus increasing mobility even more.
Steering mechanisms can be added to the front and even the rear axles but this can lead to reduced mobility as the platform loses it ability to turn on the spot.
A very interesting type of platform can be found in the Alvis Stalwart amphibious military vehicle, designed to cope with any terrain in any conditions. In this vehicle all six wheels have independent, parallel-link, torsion beam suspension, all wheel drive and the first two axle wheels can steer with different steering lock.
A more simplified design, inspired by the Alvis Stalwart is the one with three independent axles, linked by two pivot joints that can rotate with respect to each other. Wheels on each side are motorized together thus the vehicle skid steers.
The Sojourner micro rover developed at JPL and employed in the Mars Pathfinder mission, back in 1997, is another example of a very high mobility six wheeled vehicle. It employs a rocker-bogie suspension system, where a bogie holding two wheels is attached to one end of the rocker arms on each side of the platform.
Generally a rocker-bogie system can lead to uneven wheel loads but this can be counteracted when using the setup found in the Sojourner rover, more exactly the bogie must be sized to be half of the rocker arm length and the chassis mounting point of the rocker arm has to be at one third of the arm’s length.
All six wheels are motorized and the front and back wheels can be independently steered to ensure high mobility. A total of 10 motors are employed for this platform setup.
Eight wheeled platforms
In some cases platforms with eight wheels can provide advantages over the six wheeled variants. The main advantage is lower ground pressure but the weight of the platform must also be taken into account.
A first example is an eight wheel platform with fixed axles, similar in operation to a six wheeled platform, with each four wheels on one side motorized together that skid steers.
Like any rigid axle platform the problem of keeping all wheels on the ground appears. To counteract this, each two wheels on one side are mounted on bogies which in turn are mounted onto axles. Steering actuators can also be added.
Another variant is to split the platform into two symmetrical sections, connected together by means of a passive joint that provides chassis flexing abilities, reducing unwanted resistance when skid-steering, or an active joint that can also handle steering by moving the sections with respect to one another in a horizontal plane. A secondary joint, that can provide vertical flexing, can be added to increase the platform’s mobility.
A variation of this method is using a spherical joint that is more robust and has reduced complexity.