Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul
Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul
Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
192 Chapter 6 Steering History<br />
<strong>and</strong> some part of the drive train in contact with the ground will have to<br />
slide or skid.<br />
Driving straight in one direction requires at least one single direction<br />
actuator. A wind-up toy is a good demonstration of this ultra-simple<br />
drive system. Driving straight in both directions requires at least one bidirectional<br />
actuator or two single-direction actuators. One of those single<br />
direction actuators can power either a steering mechanism or a second<br />
drive motor. Add one more simple single-direction motor to the wind-up<br />
toy, <strong>and</strong> it can turn to go in any new direction. This shows that the least<br />
number of actuators required to travel in any direction is two, <strong>and</strong> both<br />
can be single-direction motors.<br />
In practice, this turns out to be quite limiting, at least partly because it<br />
is tricky to turn in place with only two single direction actuators, but<br />
mostly because there aren’t enough drive <strong>and</strong> steer options to pick from<br />
to get out of a tight spot. Let’s investigate the many varieties of steering<br />
commonly used in wheeled <strong>and</strong> tracked robots.<br />
The simplest statically stable vehicle has either three wheels or two<br />
tracks, <strong>and</strong> the simplest power system to drive <strong>and</strong> steer uses only two<br />
single-direction motors. It turns out that there are only two ways to steer<br />
these very simple vehicles:<br />
1. Two single-direction motors powering a combined drive/steer wheel<br />
or combined drive/steer track with some other passive wheels or<br />
tracks<br />
2. Two single-direction motors, each driving a track or wheel (the third<br />
wheel on the wheeled layout is a passive swivel caster)<br />
The simplest version of the first steering geometry is a single-wheel<br />
drive/steer module mounted on a robot with two fixed wheels. The common<br />
tricycle uses this exact layout, but so do some automatic guided<br />
vehicles (AGVs) used in automated warehouses. Mobility is limited<br />
because there is only one wheel providing the motive force, while dragging<br />
two passive wheels. This layout works well for the AGV application<br />
because the warehouse’s floor is flat <strong>and</strong> clean <strong>and</strong> the aisles are<br />
designed for this type of vehicle. In an AGV, the drive/steer module usually<br />
has a bi-directional steering motor to remove the need to turn the<br />
drive wheel past 180° but single direction steer motors are possible.<br />
There are many versions of AGVs—the most complicated types have<br />
four drive/steer modules. These vehicles can steer with, what effectively<br />
amounts to, any common steering geometry; translate in any direction<br />
without rotating (commonly called “crabbing”), pseudo-Ackerman steer,<br />
turn about any point, or rotate in place with no skidding. Wheel modules
Change language