BUILDING A HEAVY-DUTY
SIXLEGGED WALKING ROBOT
SIXLEGGED WALKING ROBOT
The strange and unique contraption shown in Fig. 24-1 walks on six legs and turns corners
with an ease and grace that belies its rather simple design. The Walkerbot design
described in this chapter is for the basic frame, motor, battery system, running gear, and
legs. You can embellish the robot with additional components, such as arms, a head, as well
as computer control. The frame is oversized (in fact, it’s too large to fit through some inside
doors!), and there’s plenty of room to add new subsystems.
The only requirement is that the weight doesn’t exceed the driving capacity of the
motors and batteries and that the legs and axles don’t bend. The prototype Walkerbot
weighs about 50 lb. It moves along swiftly and no structural problems have yet occurred.
Another 10 or 15 lb could be added without worry.
The completed Walkerbot frame measures 18 in wide by 24 in long by 12 in deep. Construction
is all aluminum, using a combination of 
-by-
-by-
-in channel stock and 1-by-
1-by--in angle stock
-by--by--in channel stock and 1-by-
1-by--in angle stockBuild the bottom of the frame by cutting two 18-in lengths of channel stock and two
24-in lengths of channel stock, as shown in Fig. 24-2 (refer to the parts list in Table 24-1).
Miter the ends with a 45° angle. Attach the four pieces using 1-by-
-in flat angle irons and secure them with 3-by--in bolts and nuts. For added strength, use four bolts on each
corner.
-by--in flat angle irons and secure them with 3-by--in bolts and nuts. For added strength, use four bolts on each
corner.
4 |
24-in lengths  -by- -by- -in aluminum channel stock |
4 |
18-in lengths -by--by--in aluminum channel stock |
4 |
12-in lengths 1-by-1-by- -in aluminum angle stock |
8 |
1½-by-  -in flat angle iron |
4 |
24-in lengths 1-by-1-by- -in aluminum angle stock |
2 |
17  -in lengths 1-by-1-by--in aluminum angle stock |
Misc. |
 stove bolts, nuts, tooth lock washers, as needed |
In the prototype Walkerbot, many of the nuts and bolts were replaced with aluminum pop
rivets in order to reduce the weight. Until the entire frame is assembled, however, use the bolts
as temporary fasteners. Then, when the frame is assembled, square it up and replace the bolts
and nuts with rivets one at a time. Construct the top of the frame in the same manner.
Connect the two halves with four 12-in lengths of angle stock, as shown in Fig. 24-3.
Secure the angle stock to the frame pieces by drilling holes at the corners. Use 
-by--in
bolts and nuts initially; exchange for pop rivets after the frame is complete. The finished
frame should look like the one diagrammed in Fig. 24-4
-by--in
bolts and nuts initially; exchange for pop rivets after the frame is complete. The finished
frame should look like the one diagrammed in Fig. 24-4
Complete the basic frame by adding the running gear mounting rails. Cut four 24-in
lengths of 1-by-1-by--in angle stock and two 17
-in lengths of the same angle stock. Drill 
-in holes in four long pieces as shown in Fig. 24-5. The spacing between the sets of holes
is important. If the spacing is incorrect, the U-bolts won’t fit properly.
-in angle stock and two 17-in lengths of the same angle stock. Drill -in holes in four long pieces as shown in Fig. 24-5. The spacing between the sets of holes
is important. If the spacing is incorrect, the U-bolts won’t fit properly.
Refer to Fig. 24-6. When the holes are drilled, mount two of the long lengths of angle
stock as shown. The holes should point up, with the side of the angle stock flush against the
frame of the robot. Mount the two short lengths on the ends. Tuck the short lengths immediately
under the two long pieces of angle stock you just secured. Use -by--in bolts and
nuts to secure the pieces together. Dimensions, drilling, and placement are critical with
these components. Put the remaining two long lengths of drilled angle stock aside for the
time being.
-by--in bolts and
nuts to secure the pieces together. Dimensions, drilling, and placement are critical with
these components. Put the remaining two long lengths of drilled angle stock aside for the
time being.
Figure 24-6 The motor mount rails secured to the robot. a. The long rail mounts 2 in from the bottom of the frame (the holes drilled earlier point up);
b. the short end crosspiece rail mounts 2in from the bottom of the frame.
in from the bottom of the frame (the holes drilled earlier point up);
b. the short end crosspiece rail mounts 2in from the bottom of the frame.You’re now ready to construct and attach the legs. This is probably the hardest part of the
project, so take your time and measure everything twice to assure accuracy. Cut six 14-in
lengths of 
-by- 
-by--in aluminum channel stock. Do not miter the ends. Drill a hole with
a #19 bit in from one end (the top); drill a -in hole 4
in from the top (see Fig. 24-7; refer
to the parts list in Table 24-2). Make sure the holes are in the center of the channel stock.
-by- -by--in aluminum channel stock. Do not miter the ends. Drill a hole with
a #19 bit in from one end (the top); drill a -in hole 4 in from the top (see Fig. 24-7; refer
to the parts list in Table 24-2). Make sure the holes are in the center of the channel stock.
6 |
14-in lengths  -by- -by--in aluminum channel stock |
6 |
6-in lengths -by--by--in aluminum channel stock |
6 |
Roller bearings |
6 |
Steel electrical covers (4 -in diameter) |
6 |
5-in hex-head carriage bolt |
6 |
2-by- -in flat mending iron |
6 |
1  -in 45° “Ell” Schedule 40 PCV pipe fitting |
Misc. |
 and stove bolts, nuts, tooth lock washers, locking nuts, flat washers,
as needed. -in Schedule 40 PVC cut to length (see text) |
With a -in bit, drill out the center of six 4-in-diameter circular electric receptacle plate
covers. The plate cover should have a notched hole near the outside, which is used to
secure it to the receptacle box. If the cover doesn’t have the hole, drill one with a -in bit
in from the outside edge. The finished plate cover becomes a cam for operating the up
and down movement of the legs.
-in bit, drill out the center of six 4-in-diameter circular electric receptacle plate
covers. The plate cover should have a notched hole near the outside, which is used to
secure it to the receptacle box. If the cover doesn’t have the hole, drill one with a -in bit
in from the outside edge. The finished plate cover becomes a cam for operating the up
and down movement of the legs.Assemble four legs as follows: attach the 14-in-long leg piece to the cam using a -in
length of Schedule 40 PVC pipe and hardware, as shown in Fig. 24-8. Be sure the ends of the pipe are filed clean and that the cut is as square as possible. The bolt should be tightened
against the cam but should freely rotate within the leg hole.
Figure 24-8 Hardware detail for the leg cam. a. Complete cam and leg; b. exploded view. Note that two of the legs use a 2-in piece of PVC and a 3-in
bolt.
Assemble the remaining two legs in a similar fashion, but use a 2-in length of PVC pipe
and a 3-in stove bolt. These two legs will be placed in the center of the robot and will stick
out from the others. This allows the legs to cross one another without interfering with the
gait of the robot. The bearings used in the prototype were -in-diameter closet door rollers.
-in-diameter closet door rollers.Now refer to Fig. 24-9. Thread a 5-by--in 20 carriage bolt through the center of the
cam, using the hardware shown. Next, install the wheel bearings to the shafts, 1-in from
the cam. The 1-in-diameter bearings are the kind commonly used in lawn mowers and
are readily available. The bearings used in the prototype had -in hubs. A -to -in
reducing bushing was used to make the bearings compatible with the diameter of the
shaft.
-in 20 carriage bolt through the center of the
cam, using the hardware shown. Next, install the wheel bearings to the shafts, 1-in from
the cam. The 1-in-diameter bearings are the kind commonly used in lawn mowers and
are readily available. The bearings used in the prototype had -in hubs. A -to -in
reducing bushing was used to make the bearings compatible with the diameter of the
shaft.
Install 3-in-diameter 30 tooth #25 chain sprocket (another size will also do, as long as
all the leg mechanism sprockets in the robot are the same size). Like the bearings, a reducing
bushing was used to make the -in ID hubs of the sprockets fit on the shaft. The exact
positioning of the sprockets on the shaft is not important at this time, but follow the spacing
diagram shown in Fig. 24-10 as a guide. You’ll have to fine-tune the sprockets on the
shaft as a final alignment procedure anyway.
-in-diameter 30 tooth #25 chain sprocket (another size will also do, as long as
all the leg mechanism sprockets in the robot are the same size). Like the bearings, a reducing
bushing was used to make the -in ID hubs of the sprockets fit on the shaft. The exact
positioning of the sprockets on the shaft is not important at this time, but follow the spacing
diagram shown in Fig. 24-10 as a guide. You’ll have to fine-tune the sprockets on the
shaft as a final alignment procedure anyway.
Once all the legs are complete, install them on the robot using U-bolts. The 1-in-wide
by 2-in-long by -in 20 thread U-bolts fit over the bearings perfectly. Secure the U-bolts
using the -in 20 nuts supplied.
-in-wide
by 2-in-long by -in 20 thread U-bolts fit over the bearings perfectly. Secure the U-bolts
using the -in 20 nuts supplied.Refer to Fig. 24-11 for the next step. Cut six 6-in lengths of -by--by--in aluminum
channel stock. With a #19 bit, drill holes in from the top and bottom of the rail. With a nibbler
tool, cut a 3-in slot in the center of each rail. The slot should start in from one end.
-by--by--in aluminum
channel stock. With a #19 bit, drill holes in from the top and bottom of the rail. With a nibbler
tool, cut a 3-in slot in the center of each rail. The slot should start in from one end.
Alternatively, you can use a router, motorized rasp, or other tool to cut the slot. In any
case, make sure the slot is perfectly straight. Once cut, polish the edges with a piece of 300
grit wet-dry Emory paper, used wet. Use your fingers to find any rough edges. There can be
none. This is a difficult task to do properly, and you may want to take this portion to a sheet
metal shop and have them do it for you (it’ll save you an hour or two of blister-producing
nibbling!). An alternative method, which requires no slot cutting, is shown in Fig. 24-12. Be
sure to mount the double rails exactly parallel to one another.
Mount the rails using -by-2-in bolts and nuts. Make sure the rails are directly above
the shaft of each leg or the legs may not operate properly. You’ll have to drill through both
walls of the channel in the top of the frame.
-by-2-in bolts and nuts. Make sure the rails are directly above
the shaft of each leg or the legs may not operate properly. You’ll have to drill through both
walls of the channel in the top of the frame.The rails serve to keep the legs aligned for the up-and-down piston-like stroke of the
legs. Attach the legs to the rails using -by-1-in bolts. Use nuts and locking nuts fasteners
as shown in Fig. 24-13. This finished leg mechanism should look like the one depicted in
Fig. 24-14. Use grease or light oil to lubricate the slot. Be sure that there is sufficient play
between the slot and the bolt stem. The play cannot be excessive, however, or the leg may
bind as the bolt moves up and down inside the slot. Adjust the sliding bolt on all six legs for
proper clearance.
-by-1-in bolts. Use nuts and locking nuts fasteners
as shown in Fig. 24-13. This finished leg mechanism should look like the one depicted in
Fig. 24-14. Use grease or light oil to lubricate the slot. Be sure that there is sufficient play
between the slot and the bolt stem. The play cannot be excessive, however, or the leg may
bind as the bolt moves up and down inside the slot. Adjust the sliding bolt on all six legs for
proper clearance.
Figure 24-13 Cam slider hardware detail. a. Complete assembly; b. exploded view.
Note that the center legs have 2-in bolts.
Figure 24-14 The slider cam and hardware. The slot must be smooth and free of burrs, or the leg will snag.
Drill small pilot holes in the side of six 45° 1-in PVC pipe elbows. These serve as the
feet of the legs. Paint the feet at the point if you wish. Using #10 wood screws, attach a 2-
by--in flat mending iron to each of the elbow feet. Drill -in holes 1 in from the bottom
of the leg. Secure the feet onto the legs using -by--in 20 machine bolts, nuts, and lock
washers. Apply a 3-in length of rubber weather strip to the bottom of each foot for better
traction. The leg should look like the one in Fig. 24-15. The legs should look like the one
in Fig. 24-16. A close-up of the cam mechanism is shown in Fig. 24-17.
-in PVC pipe elbows. These serve as the
feet of the legs. Paint the feet at the point if you wish. Using #10 wood screws, attach a 2-
by--in flat mending iron to each of the elbow feet. Drill -in holes 1 in from the bottom
of the leg. Secure the feet onto the legs using -by--in 20 machine bolts, nuts, and lock
washers. Apply a 3-in length of rubber weather strip to the bottom of each foot for better
traction. The leg should look like the one in Fig. 24-15. The legs should look like the one
in Fig. 24-16. A close-up of the cam mechanism is shown in Fig. 24-17.
Figure 24-15 PVC plumbing fittings used as feet. The feet use a flat mending iron. Add pads or rubber to the bottom of the feet as desired.
The motors used in the prototype Walkerbot were surplus finds originally intended as the
driving motors in a child’s motorized bike or go-cart. The motors have a fairly high torque
at 12 V DC and a speed of about 600 r/min. A one-step reduction gear was added to bring
the speed down to about 230 r/min. The output speed is further reduced to about 138
r/min by using a drive sprocket. For a walking machine, that’s about right, although it could
stand to be a bit slower. Electronic speed reduction can be used to slow the motor output
down to about 100 r/min. You can use other motors and other driving techniques as long
as the motors have a (prereduced) torque of at least 6 lb-ft and a speed that can be reduced
to 140 r/min or so.
Mount the motors inside two 6-by-1-in mending plate Ts. Drill a large hole, if necessary,
for the shaft of the motor to stick through, as shown in Fig. 24-18 (refer to the parts
list in Table 24-3). The motors used in the prototype came with a 12-pitch 12-tooth nylon
gear. The gear was not removed for assembly, so the hole had to be large enough for it to
pass through. The 30-tooth 12-pitch metal gear and 18-tooth -in chain sprocket were
also sandwiched between the mending plates.
-by-1-in mending plate Ts. Drill a large hole, if necessary,
for the shaft of the motor to stick through, as shown in Fig. 24-18 (refer to the parts
list in Table 24-3). The motors used in the prototype came with a 12-pitch 12-tooth nylon
gear. The gear was not removed for assembly, so the hole had to be large enough for it to
pass through. The 30-tooth 12-pitch metal gear and 18-tooth -in chain sprocket were
also sandwiched between the mending plates.
Figure 24-18 Motor mount details. a. Drilling guide for the mending T; b. the motor and drive gear-sprocket mounted with two mending Ts.
4 |
6 -in galvanized mending plate T |
4 |
3-in galvanized mending plate T |
2 |
Heavy-duty gear-reduction DC motors |
12 |
3 -in-diameter 30-tooth #15 chain sprocket |
4 |
28 -in-length #25 roller chain |
12 |
2 -by-1-by--in 20 U-bolts, with nuts and tooth lock washers |
12 |
1 -in O.D. to -in ID bearing |
Misc. |
Reducing bushings (see text) |
The -in shaft of the driven gear and sprocket is free running. You can install a bearing
on each plate, if you wish, or have the shaft freely rotate in oversize holes. The sprocket
and gear have -in ID hubs, so reducing bushings were used. The sprocket and gear are
held in place with compression. Don’t forget the split washers. They provide the necessary
compression to keep things from working loose.
-in shaft of the driven gear and sprocket is free running. You can install a bearing
on each plate, if you wish, or have the shaft freely rotate in oversize holes. The sprocket
and gear have -in ID hubs, so reducing bushings were used. The sprocket and gear are
held in place with compression. Don’t forget the split washers. They provide the necessary
compression to keep things from working loose.Before attaching the two mending plates together, thread a 28-in length of #25 roller
chain over the sprocket. The exact length can be one or two links off; you can correct for
any variance later on. Assemble the two plates using by 3-in bolts and nuts and lock
washers. Separate the plates using 2-in spacers.
-in length of #25 roller
chain over the sprocket. The exact length can be one or two links off; you can correct for
any variance later on. Assemble the two plates using by 3-in bolts and nuts and lock
washers. Separate the plates using 2-in spacers.Attach the two 17
-in lengths of angle bracket on the robot, as shown in Fig. 24-19.
The stock mounts directly under the two end pieces. Use -in-by- bolts and nuts to secure
the crosspieces into place. Secure the leg shafts using 1-in bearings and U-bolts.
-in lengths of angle bracket on the robot, as shown in Fig. 24-19.
The stock mounts directly under the two end pieces. Use -in-by- bolts and nuts to secure
the crosspieces into place. Secure the leg shafts using 1-in bearings and U-bolts.
Mount the motor to the newly added inner mounting rails using 3-by--in mending
plate Ts. Fasten the plates onto the motor mount, as shown in Fig. 24-20, with by
-in bolts and nuts. Position the shaft of the motor approximately 7 in from the back of
the robot (you can make any end of the shaft the back; it doesn’t matter). Thread the roller
chain over the center sprocket and the end sprocket. Position the motor until the roller
chain is taut. Mark holes and drill. Secure the motor and mount to the frame using by
-in bolts and nuts. Repeat the process for the opposite motor. The final assembly should
look like Fig. 24-21.
-in mending
plate Ts. Fasten the plates onto the motor mount, as shown in Fig. 24-20, with by
-in bolts and nuts. Position the shaft of the motor approximately 7 in from the back of
the robot (you can make any end of the shaft the back; it doesn’t matter). Thread the roller
chain over the center sprocket and the end sprocket. Position the motor until the roller
chain is taut. Mark holes and drill. Secure the motor and mount to the frame using by
-in bolts and nuts. Repeat the process for the opposite motor. The final assembly should
look like Fig. 24-21.
Figure 24-21 Drive motor attached to the Walkerbot, with drive chain joining the motor to the leg shafts.
Thread a 28-in length of #25 roller chain around sprockets of the center and front legs.
Attach an idler sprocket 7 in from the front of the robot in line with the leg mounts. Use
a diameter as close to 2 in as possible for the idler; otherwise, you may need to shorten or
lengthen the roller chain. Thread the roller chain around the sprocket, and find a position
along the rail until the roller chain is taut (but not overly tight). Make a mark using the center
of the sprocket as a guide and drill a -in hole in the rail. Attach the sprocket to the
robot. Figs. 24-22 through 24-24 show the motor mount, idler sprocket, and roller chain
locations.
-in length of #25 roller chain around sprockets of the center and front legs.
Attach an idler sprocket 7 in from the front of the robot in line with the leg mounts. Use
a diameter as close to 2 in as possible for the idler; otherwise, you may need to shorten or
lengthen the roller chain. Thread the roller chain around the sprocket, and find a position
along the rail until the roller chain is taut (but not overly tight). Make a mark using the center
of the sprocket as a guide and drill a -in hole in the rail. Attach the sprocket to the
robot. Figs. 24-22 through 24-24 show the motor mount, idler sprocket, and roller chain
locations.
The Walkerbot is not a lightweight robot, and its walking design requires at least 30 percent
more power than a wheeled robot. The batteries for the Walkerbot are not trivial. You have
a number of alternatives. One workable approach is to use two 6-V motorcycle batteries,
each rated at about 30 AH. The two batteries together equal a slimmed-down version of a
car battery in size and weight.
You can also use a 12-V motorcycle or dune buggy battery, rated at more than 20 AH.
The prototype Walkerbot used 12-AH 6-V gel-cell batteries. The amp-hour capacity is a bit
on the low side, considering the 2-A draw from each motor, and the planned heavy use of
electronics and support circuits. In tests, the 12-AH batteries provided about 2 h of use
before requiring a recharge.
There is plenty of room to mount the batteries. A good spot is slightly behind the center
legs. By offsetting the batteries a bit in relation to the drive motors, you restore the center
of gravity to the center of the robot. Of course, other components you add to the robot can
throw the center of gravity off. Add one or two articulated arms to the robot, and the weight
suddenly shifts toward the front. For flexibility, you might want to mount the batteries on a
sliding rail, which will allow you to shift their position forward or back depending on the
other weight you add to the Walkerbot.
You can test the operation of the Walkerbot by temporarily installing the wired control box
you built earlier for the other, more basic robots that consists of two DPDT switches wired
to control the forward and backward motion of the two legs. But before you test the
Walkerbot, you need to align its legs. The legs on each side should be positioned so that
either the center leg touches the ground or the front and back legs touch the ground.
When the two sets of legs are working in tandem, the walking gait should be as shown in
Fig. 24-25. This gait is the same as an insect’s and provides a great deal of stability. To
turn, one set of legs stops (or reverses) while the other set continues. During this time, the
tripod arrangement of the gait will be lost, but the robot will still be supported by at least
three legs.
Figure 24-25 The walking gait. a. The alternating tripod walking style of the Walkerbot, shared with
thousands of crawling insects; b. the positioning of the legs for proper walking (front and back legs in synchrony;
middle leg 180 degrees out of sync). The middle leg doesn’t hit either the front or back leg because it
is further from the body of the robot.
An easy way to align the legs is to loosen the chain sprockets (so you can move the legs
independently) and position the middle leg all the way forward and the front and back legs
all the way back. Retighten the sprockets, and look out for misalignment of the roller chain
and sprockets. If a chain bends to mesh with a sprocket, it is likely to pop off when the robot
is in motion.
During testing, be on the lookout for things that rub, squeak, and work loose. Keep your
wrench handy and adjust gaps and tighten bolts as necessary. Add a dab of oil to those parts
that seem to be binding. You may find that a sprocket or gear doesn’t stay tightened on a
shaft. Look for ways to better secure the component to the shaft, such as by using a
setscrew or another split lock washer. It may take several hours of tuning up to get the robot
working at top efficiency.
Once the robot is aligned, run it through its paces by having it walk over level ground,
step over small rocks and ditches, and navigate tight corners. Keep an eye on your watch
to see how long the batteries provide power. You may need to upgrade the batteries if they
cannot provide more than an hour of operation.
The Walkerbot is ideally suited for expansion. Fig. 24-26 shows an arm attached to the
front side of the robot. You can add a second arm on the other side for more complete dexterity.
Attach a dome on the top of the robot, and you’ve added a head on which you can
attach a video camera, ultrasonic ears and eyes, and lots more. Additional panels can be
added to the front and back ends; attach them using hook-and-loop (such as Velcro) strips.
That way, you can easily remove the panels should you need quick access to the inside of
the robot.
To learn more about . . . |
Read |
|
Working with metal |
Chapter 10, “Building a Metal Platform” |
|
Robot locomotion styles, including wheels, treads, and legs |
Chapter 18, “Principles of Robot Locomotion” |
|
Using DC motors |
Chapter 20, “Working with DC Motors” |
|
Additional locomotion systems based on the Walkerbot frame |
Chapter 25, “Advanced Locomotion Systems” |
|
Constructing an arm for the Walkerbot |
Chapter 27, “Build a Revolute Coordinate Arm” |