ADVANCED ROBOT
LOCOMOTION SYSTEMS
LOCOMOTION SYSTEMS
Two drive wheels aren’t the only way to move a robot across the living room or workshop
floor. Here, in this chapter, you’ll learn the basics of applying some unique drive
systems to propel your robot designs, including a stair-climbing robot, an outdoor tracked
robot, and even a six-wheeled Buggybot.
There is something exciting about seeing a tank climb embankments, bounding over huge
boulders as if they were tiny dirt clods. A robot with tracked drive is a perfect contender for
an automaton that’s designed for outdoor use. Where a wheeled or legged robot can’t go,
the tracked robot can roll in with relative ease. Tracked robots, using metal tracks just like
tanks, have been designed for the Navy and Army and are even used by many police and
fire departments. The all-terrain ability and ruggedness of metal tracked robots made them
the design of choice during rescue efforts after the 9/11 attacks.
Using a metal track for your personal robot is decidedly a bad idea. A metal track will be
too heavy and much too hard to fabricate. For a home-brew robot, a rubber track is more
than adequate. You can use a large timing belt, even an automotive fan belt, for the track
or a large rubber O ring (like the one used to drive your vacuum cleaner).
Another alternative that has been used with some success is rubber wetsuit material.
Most diving shops have long strips of the rubber lying around that they'll sell or give to you.
You can mend the rubber using a special waterproof adhesive. You can glue the strip
together to make a band, then glue small rubber cleats onto the band. Fig. 25-1 shows the
basic idea.
The drive train for a tracked robot must be engineered for the task of driving a track. A
two-wheel differentially driven robot (like the wooden and metal platform robots presented
earlier) can be used as a base for a tracked robot. Install three free-turning large pulleys and
three small drive pulleys (one driven on each side of the robot), as diagrammed in Fig. 25-2.
The track fits inside the groove of the pulleys, so it won’t easily slip out.
Figure 25-2 Two ways to add track drive to the Walkerbot
presented in Chapter 22. a. Track roller arrangement for
good traction and stability but relatively poor turning radius.
b. Track roller arrangement for good turning radius, but hindered
traction and stability.
To propel the robot, you activate both motors so the tracks move in the same direction
and at the same speed. To steer, you simply stop or reverse one side (the same as the basic
differentially driven robot). For example, to turn left, stop the left track. To make a hard left
turn, reverse the left track.
Using dual motors to effect propulsion and steering is just one method for getting your
robot around. Another approach is to use a pivoting wheel to steer the robot. The same
wheel can provide power, or power can come from two wheels in the rear (the latter is
much more common). The arrangement is not unlike golf carts, where the two rear wheels
provide power and a single wheel in the front provides steering. See Fig. 25-3 for a diagram
of a typical steering-wheel robot. Fig. 25-4 shows a detail of the steering mechanism.
The advantage of a steering-wheel robot is that you need only one powerful drive motor.
The motor can power both rear wheels at once as shown in Fig. 25-5, but this isn't recommended
for a reason that anyone who is aware of car drivetrains understands. With the two wheels turning together, there is a lot of friction when the robot wants to turn because
both wheels are locked together even though they will be turning at different speeds as the
robot changes direction.
Figure 25-5 Converting the Walkerbot (from Chapter 22) into a six-wheeled,
all-terrain Buggybot.
The solution to this dilemma in an automobile is a gear system known as a differential,
which allows the wheels to turn at different speeds when the car is changing direction. Finding
or making a differential for a robot is a daunting challenge but there are two simple solutions
to the dilemma. The first is to drive only one of the two rear wheels and let the other
turn freely. This way the other wheel will turn at the appropriate rate for the current
motion. The second solution is to let both rear wheels turn freely and independently and
drive the turning wheel.
The steering-wheel motor needn’t be as powerful since all it has to do is swivel the wheel
back and forth a few degrees. The biggest disadvantage of steering-wheel systems is the
steering! You must build stops into the steering mechanisms (either mechanical or electronic)
to prevent the wheel from turning more than 50° or 60° to either side. Angles
greater than about 60° cause the robot to suddenly steer in the other direction. They may
even cause the robot to lurch to a sudden stop because the front wheel is at a right angle to
the rear wheels.
The servo mechanism that controls the steering wheel must know when the wheel is
pointing forward. The wheel must return to this exact spot when the robot is commanded
to forge straight ahead. Not all servo mechanisms are this accurate. The motor may stop
one or more degrees off the center point, and the robot may never actually travel in a
straight line. A good steering motor, and a more sophisticated servo mechanism, can
reduce this limitation.
A number of robot designs with steering-wheel mechanisms has been described in other
robot books and on various web pages. Check out Appendix A, "Further Reading," and
Appendix C, "Robot Information on the Internet," for more information
A variation on the tracked robot is the six-wheeled rugged terrain cart (also known as a Buggybot),
shown in Fig. 25-5. The larger the wheels the better, as long as they aren’t greater
than the centerline diameter between each drive shaft.
Pneumatic wheels are the best choice because they provide more bounce and handle
rough ground better than hard rubber tires. Most hardware stores carry a full assortment of
pneumatic tires. Most are designed for things like wheelbarrows and hand dollies. Cost can
be high, so you may want to check out the surplus or used industrial supply houses.
Steering is accomplished as with two-wheeled or tracked differentially driven robots. The
series of three wheels on each side act as a kind of track tread, so the vehicle behaves much
like a tracked vehicle.
The maneuverability isn't as good as with a two-wheeled robot, but you can still turn the
robot in a radius a little longer than its length. Sharp turns require you to reverse one set of
wheels while applying forward motion to the other.
As early as 1938, scientists observed that certain metal alloys, once bent into odd shapes,
returned to the original form when heated. This property was considered little more than a
laboratory curiosity because the metal alloys were weak, difficult and expensive to manufacture,
and they broke apart after just a couple of heating/cooling cycles.
Research into metals with memory took off in 1961, when William Beuhler and his team
of researchers at the U.S. Naval Ordnance Laboratory developed a titanium-nickel alloy
that repeatedly displayed the memory effect. Beuhler and his cohorts developed the first
commercially viable shape-memory alloy, or SMA. They called the stuff Nitinol, a fancy-sounding
name derived from Nickel Titanium Naval Ordnance Laboratory.
Since its introduction, Nitinol has been used in a number of commercial products–;but
not many. For example, several Nitinol engines have been developed that operate with only
hot and cold water. In operation, the metal contracts when exposed to hot water and
relaxes when exposed to cold water. Combined with various assemblies of springs and
cams, the contraction and relaxation (similar to a human muscle) cause the engine to move.
Other commercial applications of Nitinol include pipe fittings that automatically seal
when cooled, large antenna arrays that can be bent (using hot water) into most any shape
desired, sunglass frames that spring back to their original shape after being bent, and an
anti-scald device that shuts off water flow in a shower should the water temperature exceed
a certain limit.
Regular Nitinol contracts and relaxes in heat (in air, water, or other liquid). That limits the
effectiveness of the metal in many applications where local heat can't be applied.
Researchers have attempted to heat the Nitinol metal using electrical current in an effort to
exactly control the contraction and relaxation. But because of the molecular construction of
Nitinol, hot spots develop along the length of the metal, causing early fatigue and breakage.
In 1985, a Japanese company, Toki, unveiled a new type of shape-memory alloy specially
designed to be activated by electrical current. Toki's unique SMA material, trade-named
BioMetal, offers all of the versatility of the original Nitinol, with the added benefit of
near-instant electrical actuation. BioMetal and materials like it—Muscle Wire from Mondo-Tronics or Flexinol from Dynalloy—have many uses in robotics, including novel locomotive
actuation. From here on out this family of materials will be referred to as shape-memory
alloy, or simply SMA.
At its most basic level, SMA is a strand of nickel titanium alloy wire. Though the material
may be very thin (a typical thickness is 0.15 mm–;slightly wider than a strand of human
hair), it is exceptionally strong. In fact, the tensile strength of SMA rivals that of stainless
steel: the breaking point of the slender wire is a whopping 6 lb. Even under this much
weight, SMA stretches little. In addition to its strength, SMA also shares the corrosion resistance
of stainless steel.
Shape-memory alloys change their internal crystal structure when exposed to certain
higher-than-normal temperatures (this includes the induced temperatures caused by passing an electrical current through the wire). The structure changes again when the alloy is
allowed to cool. More specifically, during manufacture the SMA wire is heated to a very
high temperature, which embosses or memorizes a certain crystal structure. The wire is
then cooled and stretched to its practical limits. When the wire is reheated, it contracts
because it is returning to the memorized state.
Although most SMA strands are straight, the material can also be manufactured in spring
form, usually as an expansion spring. In its normal state, the spring exerts minimum tension,
but when current is applied the spring stiffens, exerting greater tension. Used in this
fashion, an SMA becomes an "active spring" that can adjust itself to a particular load, pressure,
or weight.
Shape-memory alloys have an electrical resistance of about 1 Ω/in. That's more than
ordinary hookup wire, so SMAs will heat up more rapidly when an electrical current is
passed through them. The more current passes through, the hotter the wire becomes and
the more contracted the strand. Under normal conditions, a 2- to 3-in length of SMA is
actuated with a current of about 450 mA. That creates an internally generated temperature
of about 100 to 130C; 90C is required to achieve the shape-memory change. Most SMAs
can be manufactured to change shape at most any temperature, but 90C is the standard
value for off-the-shelf material.
Excessive current should be avoided. Why? Extra current causes the wire to overheat,
which can greatly degrade its shape-memory characteristics. For best results, current should
be as low as necessary to achieve the contraction desired. Shape–;memory alloys will contract
by 2 to 4 percent of their length, depending on the amount of current applied. The
maximum contraction of typical SMA material is 8 percent, but that requires heavy current
that can, over a period of just a few seconds, damage the wire.
Shape-memory alloys need little support paraphernalia. Besides the wire itself, you need
some type of terminating system, a bias force, and an actuating circuit.
Terminating system The terminators attach the ends of the SMA wires to the support
structure or mechanism you are moving. Because SMAs expand as they contract, using glue
or other adhesive will not secure the wire to the mechanism. Ordinary soldering is not recommended
as the extreme heat of the soldering can permanently damage the wire. The
best approach is to use a crimp-on terminator. These and other crimp terminators are available
from companies that sell shape-memory alloy wire (either in the experimenter’s kit or
separately).
You can make your own crimp-on connectors using 18-gauge or smaller solderless crimp
connectors (the smaller the better). Although these connectors are rather large for the thin
0.15 mm SMA, you can achieve a fairly secure termination by folding the wire in the connector
and pressing firmly with a suitable crimp tool. Be sure to completely flatten the connector.
If necessary, place the connector in a vise or use a ball peen hammer to flatten it all
the way.
Bias force Apply current to the ends of an SMA wire and it just contracts in air. To be
useful, the wire must be attached to one end of the moving mechanism and biased (as shown in Fig. 25-6) at the other end. Besides offering physical support, the bias offers the
counteracting force that returns the SMA wire to its limber condition once current is
removed from the strand.
Actuating circuit SMAs can be actuated with a 1.5-V penlight battery. Because the circuit
through the SMA wire is almost a dead short, the battery delivers almost its maximum
current capacity. But the average 1.5-V alkaline penlight battery has a maximum current
output of only a few hundred milliamps, so the current is limited through the wire. You can
connect a simple on/off switch in line with the battery, as detailed in Fig. 25-7, to contract
or relax the SMA wire.
Figure 25-7 A simple switch in series with a 1.5-V penlight battery forms a simple SMA driving circuit. The low current delivered by the penlight battery prevents damage to the SMA wire.
The problem with this setup is that it wastes battery power, and if the power switch is left
on for too long, it can do some damage to the SMA strand. A more sophisticated approach
uses a 555 timer IC that automatically shuts off the current after a short time. The schematic in
Fig. 25-8 shows one way of connecting a 555 timer IC to turn off power to a
length of SMA 0.1 s after the button (S1) is opened. Table 25-1 provides a parts list for the
555 SMA circuit.
Figure 25-8 A 555 timer IC is at the heart of an ideal driving circuit for SMA wire. The
555 removes the current shortly after you release activating switch S1.
IC1 |
555 timer |
Q1 |
2N2222 NPN transistor |
R1 |
47K resistor |
R2 |
27K resistor |
R3 |
1K resistor |
C1 |
3.3 μF polarized electrolytic capacitor |
Misc. |
Momentary SPST switch, SMA wire |
In operation, when you press momentary switch S1 current passes through the wire and
it contracts. Release S1 immediately, and the SMA stays contracted for an extra fraction of
a second, then releases as the 555 timer shuts off. Since the total ON time of the 555
depends on how long you hold S1 down, plus the 
-of-a-second delay, you should depress
the switch only momentarily.
-of-a-second delay, you should depress
the switch only momentarily.
With the SMA properly terminated and actuated, it's up to you and your own imagination
to think of ways to use it in your robots. Fig. 25-9 shows a typical application using an SMA
wire in a pulley configuration. Apply current to the wire and the pulley turns, giving you
rotational motion. A large-diameter pulley will turn very little when the SMA tenses up, but
a small-diameter one will turn an appreciable distance.
Fig. 25-10 shows a length of SMA wire used in a lever arrangement. Here, the metal
strand is attached to one end of a bell crank. On the opposite end is a bias spring. Applying
juice to the wire causes the bell crank to move. The spot where you attach the drive arm
dictates the amount of movement you will obtain when the SMA contracts.
Figure 25-10 The bell crank changes the contraction of the SMA for sideways movement of the lever. The spring enables the bell crank to return to its original position after the current is
removed from the wire.
SMA wire is tiny stuff, and you will find that the miniature hardware designed for model
R/C airplanes is most useful for constructing mechanisms. Most any well-stocked hobby store will stock a full variety of bell cranks, levers, pulleys, wheels, gears, springs, and other
odds and ends to make your work with SMA more enjoyable.
The operation of shape-memory alloys probably seems like they are the perfect material to
simulate the operation of muscles, eliminating the need for motors, hydraulics, and pneumatics
in many robots. Before you get visions of building robots that can walk or manipulate
objects like humans do, there are a few caveats about SMAs that you should be aware of.
First off, SMAs contract relatively slowly when current is applied and can take a very long
time to cool and relax when current is turned off. Coupled with this is the all-or-nothing
behavior of SMAs; they are either fully contracted or fully relaxed. It may be possible to specify
an amount of contraction, but this would require closed loop control of a power PWM
applied to the current and would be difficult to design. These two issues can compound each
other, resulting in a robot part that is slow to respond and when it does it goes too far.
SMAs require a disproportionate amount of power when compared to electric motors or
R/C servos. Along with this extra power, they cannot provide as much force as an electric
motor or servo and when they are used in a robot, you will find that they cannot carry batteries.
Any robot you design with SMAs will have a tether wire running to it.
Finally, SMAs are surprisingly difficult to work with. The ideal method of attaching them
to structures is to use a crimped or flattened tube, as discussed previously with the SMA
being held under light tension when crimping the attachment tube. You will find your first
attempts at working with SMA to not work very well, but over time, you will gain the knack
and will find that you can work with SMA quickly and efficiently.
These issues will make SMAs seem a lot less attractive than what you might have thought
before you read this section, but by keeping these concerns in the back of your mind you
can create prototype robots quickly and efficiently. Chances are you had some plans for a
very special robot that could take advantage of the different properties of SMAs and you still
can, but recognize that your SMA-based robot will have to be built very lightly and probably
smaller than the final robot that you want to build. This is actually the advantage of SMA; it
can be used to create prototype robot structures quickly and efficiently. Along with allowing
you to create prototype structures in just a few moments, its slow operation will allow you
to observe the action of the mechanical system and learn what is the correct way to
sequence it.
To learn more about . . . |
Read |
|
Selecting a motor for driving a robot |
Chapter 19,, "Choosing the Right Motor" |
|
Using DC motors for robot locomotion |
Chapter 20,, "Working with DC Motors" |
|
Building Walkerbot, a six–;legged mobile walking robot |
Chapter 24,, "Build a Heavy-Duty Six-Legged Walking Robot" |