Your robotic problems solved here.
To post comments on this article and find any associated files and/or
downloads, go to www.servomagazine.com/index.php/magazine/issue/2017/12.
A. I feel your pain! I could talk about robot navigation all day. Driving in a straight line is the most basic requirement to get from A to B, yet
it’s surprisingly difficult.
You didn’t mention exactly which robot kit you
have, but it sure sounds like a traditional 2WD
differentially-steered chassis. These are very
maneuverable and able to spin in place (a.k.a., zero
turning radius). These are simple enough for
beginners to understand, and maneuverable enough
for “real” robots like Roomba. eBay is rife with
inexpensive kits such as shown in Figure 1
From my perspective, these cheap kits are the
bane of robotics, and may do more damage than
good. Not real “smart cars,” since they cause a lot of
frustration. IF you can get both wheels turning at the
same speed, you’ll travel in a straight line, but the
odds are against you.
First, most of these kits use gearmotors with a
48:1 gear ratio. These are way too fast, and not
enough torque for the large wheels included. They slow
down significantly when any resistance is encountered. The
robot is easily thrown off course as each wheel hits little
bumps along the way and bogs down.
This chassis would benefit from using gearmotors with
more reduction (120:1 or even 256:1), which go slower and
have higher torque to maintain more consistent speed.
Second — and most important — there is simply no
guarantee that two separate motors will turn at the same
speed, which is absolutely required to go straight. The
slightest difference in gear friction or motor commutator
conductivity will cause RPM differences.
You can temporarily match your motor speeds by
various means, but without some
way of electronically verifying
their position, they will eventually
drift out of sync and the robot
will veer off course.
Plenty of other robots suffer
from this same problem. For
many of us, building a Parallax
Boe-Bot (Figure 2) kit was our
gateway into robotics, and a rite
of passage. This classic BASIC
Stamp 2-powered robot debuted
in 1998 and is still sold today. It
comes with a top-mounted
components, and a fantastic
curriculum which teaches many
aspects of electronics,
programming, and robotics. With
a MILLION sold (many used in school programs), the Boe-Bot is far and away the world’s most popular robot.
It uses continuous rotation servomotors (a.k.a., CR
servos). These are essentially DC gearmotors with a built-in
motor controller in a standard form factor housing.
Although these allow fine motor speed control, they are still
open loop systems which don’t supply any feedback about
how fast or how far they have rotated. There is no
guarantee of wheel synchronization.
Measuring distances and turns remains challenging;
usually based on timing delays, experimentation, and
prayers (!) for constant speed, in spite of ever-changing
variables such as battery voltage and rolling resistance. In
fact, a fair amount of the Boe-Bot coursework has to do with
experimentally calibrating the
software to allow predictable
The terms “odometry” and
“dead reckoning” relate to using
precise motion control of the
wheels for blind navigation in
order to move a desired distance
in a specified direction. Easy to
say, harder to do!
Parallax hosts a lively forum
that is full of some really smart,
friendly, and enthusiastic people.
There’s an active Robotics
category, and there’s always
something interesting going on.
One of the longest-running
SERVO 12.2017 9