Pololu Sample Project: Simple Hexapod Walker User manual

Sample Project: Simple
Hexapod Walker
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Materials and Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Sequencing the Hexapod Gait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Using a Script for Obstacle Avoidance . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Suggested Modications and Improvements . . . . . . . . . . . . . . . . . . . . . . 20
7. Conclusion and Community . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
Page 1 of 21

1. Introduction
Six-legged locomotion is a simple, robust system of walking that is very popular both in
the animal kingdom and among robotics hobbyists. Robot hexapods range from simple one-
motor toys to advanced platforms with 18 or more servos. This tutorial shows you how to
build a very simple autonomous hexapod robot using just three servos. The 2"-high hexapod
is capable of walking forward and backward, and can turn left and right. Two forward-
looking distance sensors provide obstacle avoidance. The brain of the hexapod is the Pololu
Micro Maestro [http://www.pololu.com/catalog/product/1351], a 6-servo controller that can read
inputs and play motion sequences in a stored script.
See the Micro Maestro User’s Guide [http://www.pololu.com/docs/0J40] for complete
documentation on the Micro Maestro.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
1. Introduction Page 2 of 21

Parts you will need to build the hexapod robot.
2. Materials and Tools
Parts list:
Quantity Part # Part Notes
1 1351
Pololu Micro
Maestro Partial
Kit
Get the kit version so that you can solder in your
own wires for the most compact possible robot.
3 1053 Sub-Micro Servo
3.7g Generic
These generic servos provide the lowest
possible cost and weight, but you may substitute
other servos, such as the Power HD sub-micro
servo HD-1440A, to customize the design.
2 1134
Pololu Carrier
with Sharp
GP2Y0D810Z0F
Digital Distance
Sensor 10cm
This is a tiny distance sensor with a long enough
range to keep your hexapod out of trouble.
1 1171 Battery Pack: 4.8
V, 150mAh
This battery pack will provide enough power at
about 5 V to power the hexapod for ve or ten
minutes.
1 1168
2.5 mm Shrouded
Male Connector:
2-Pin, Right Angle
The polarized connector lets you connect the
battery pack safely to the Micro Maestro.
3 – “Jumbo” paper clip Used to form the legs of the hexapod. These
should be 6" long when unfolded.
Tools required:
• Soldering iron and solder
• Hot glue gun
• Wire stripper
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
2. Materials and Tools Page 3 of 21

• Long-nose pliers
• Diagonal cutter
• Some wire for connecting the parts
Most of these parts are available in the Tools [http://www.pololu.com/catalog/category/5] section
of the Pololu web site. A hot glue gun is available at most craft stores for a few dollars.
Update: The case for sub-micro servo 3.7g generic [http://www.pololu.com/catalog/
product/1053] has changed slightly since this sample project was written. Versions
shipping now have solid black cases instead of transparent blue ones and a portion
of the top plane on the opposite side of the output shaft is now slanted rather than
completely at, but they still work as simple hexapod robot actuators as described
in this project. The pictures below show the two version side by side, with the old
version on the left and the new version on the right.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
2. Materials and Tools Page 4 of 21

3. Construction
Step 1: Attach the battery connector.
Using a pair of pliers, ip the leads on the battery connector to the other side.
Bending the leads on the battery connector to
the other side.
Verify that this allows you to plug in the battery as shown below, with the black wire
connected to ground and the red wire connected to BAT, then solder in the connector.
Soldering a power connector to the Micro
Maestro.
Step 2: Set up the Maestro for self power.
With your battery disconnected, attach a wire (red) from the positive terminal of the battery
connector to VIN. Take care not to short or damage any of the components on the board.
Now, with the battery plugged in, your Maestro should be powered-up and slowly ashing its
yellow LED, indicating that it is waiting to detect the baud rate on the serial communication.
You will not be using serial communication for this project, so you need to disable baud rate
detection in the next step.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 5 of 21

Self power and a battery connector on the Micro
Maestro.
Step 3: Verify that the Maestro and servos are functioning.
The Maestro Control Center is used for conguration and control of the Micro Maestro,
for testing, debugging, scripting, and more. See Section 4 of the Micro Maestro User’s
Guide [http://www.pololu.com/docs/0J40] for complete instructions on using the Maestro
Control Center.
Launch the Maestro Control Center and congure your Maestro for “USB Dual Port” mode.
The yellow LED should now be mostly o, ashing very briey about once per second. On
the Status tab, enable servo ports 0, 1, and 2, and the yellow LED will start double-blinking,
indicating that some ports are active.
Next, using a piece of male header (included with the Maestro), temporarily connect a servo
to port 0. Make sure to connect the wires correctly, with the brown or black wire connected
to ground. You should hear a short high-pitched whine as the servo activates, moving to and
holding a position in the middle of its range. After a fraction of a second, when the servo
has reached its position, it should be silent. Move the slider from 1000 μs to 2000 μs to test
the motion of the servo. Test all three ports and all three servos before continuing with the
assembly.
Congure each of the servo ports to “Go to” 1500 μs on startup. This will make it easier to
align the legs later on.
Congure ports 3, 4, and 5 to be inputs. This is important, since you will be connecting
sensors later and want to avoid shorting them out!
Disconnect the battery before continuing.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 6 of 21

A sub-micro servo with mounting
tabs clipped o.
Step 4: Construct the body by gluing the servos
together.
Remove the mounting tabs from all three servos with
diagonal cutters. The tabs are not needed for this project and
can interfere with the motion of the servos.
Next, join the servos with a few dabs of hot glue as shown
below. You do not need much glue to hold them securely! Try
to align the corners precisely to make at surfaces for
mounting the other parts.
Try to align the corners.
Clip the servo cables, leaving at least 2" (more if you are less experienced with soldering).
Strip a small amount of wire from the end of each cable.
Cut and strip the servo leads, leaving about 2” of wire.
Step 5: Solder the servos and sensors to the Micro Maestro.
This step requires patience and care. A second pair of hands could be very useful.
Use solder to tin the leads of the servos so that they can be connected initially without
additional solder. Looking at the pictures below for reference, place the Maestro on the back
side of the body and place the middle wires over the front of the Maestro and into the holes
for channel 1. Holding the wires in these holes, pull the Maestro away from the body, then
touch the soldering iron to each connection so that the small amount of solder on the wires
melts and holds them in place. You should now be able to add more solder to each of the
connections, until the holes are lled and the wires are held securely. Check carefully for
loose strands of wire, which could cause shorts.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 7 of 21

Digital distance sensors with
trimmed carrier boards.
Continue, soldering the right servo to channel 0 and the left servo to channel 2, so that the
servos are arranged in the same relative positions as the ports.
Connecting the servos to ports
0, 1, and 2 on the Micro Maestro.
Cut the sensor boards with a rotary tool, grinding wheel,
diagonal cutters, or a jeweler’s saw, removing the part
containing the unneeded mounting hole, so that they are as
small as possible. (Make sure you do not cut any traces.)
Then solder them to a cable so that you can connect them to
the Maestro. The example below uses a 4-wire ribbon cable,
sharing the power and ground connections for the two
sensors. Ribbon cable makes the assembly relatively clean,
but you can use whatever wire you have available. Look
ahead in the instructions to see where the sensors are going
to go, and make sure that you have a long enough cable.
Think about how to keep the wires close to the body and out
of the way of the legs and servos.
Soldering the sensors to a four-pin cable.
Solder the sensor power and ground to +5V and ground on the Maestro, and connect the
outputs of the right and left sensors to channels 3 and 4, respectively. Note that we use +5V
instead of the battery voltage so that the Maestro channels will never see higher voltages
– and another benet is that the sensors will work under USB power, without the battery
plugged in.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 8 of 21

Soldering the sensor cable on to the Maestro.
You now have a complete electrical assembly. Plug in the batteries, and the sensors should
become active, turning on their red LEDs whenever they detect an object within 10 cm. With
the Maestro control center, you should be able to see the input value change from 255, when
no object is present, to a low value of 40 or so, when an object is detected. If the LEDs are
always on, you probably forgot to set the ports to inputs in Step 3.
Step 6: Construct the legs.
Unfold the paper clips into straight pieces of wire. Pliers make ugly dents in the metal, so
try to use your ngers and the edge of a table to do this.
Straighten the paper clips as much as possible.
The wires should be six inches long. To make the front and back legs, fold two of them into
1.5" sections, with 90° angles between the sections, like this:
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 9 of 21

The front and back legs of the hexapod. The segments at the end
should be 1.5” in length.
Fold the third piece into an M shape, with sections of length 1.25", 1.75", 1.75", and 1.25",
like this:
The middle legs of the hexapod. The segments at the end should
be 1.25” in length.
Hot-glue the legs onto servo horns. Use a straight horn for the middle legs and cross-shaped
or round horns for the front and back legs.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 10 of 21

Gluing the hexapod legs to servo horns.
With battery power connected, so that the servos hold their neutral positions, put the horns
onto the servos so that the legs are as close as possible to neutral positions, as shown in the
picture. Fix them in place with the included screws, holding the legs as you tighten them so
that you do not apply torque to the fragile servo gears. Glue the Maestro to the back side of
the servos, ush with the bottom.
Attaching the legs and Maestro to the servos.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 11 of 21

Important: Never apply torque to the legs with your hands, attempt to prevent
them from moving, or backdrive them. Servo gears can be easily broken, so they
should only ever move under their own power. Use the Maestro Control Center to
experiment with dierent positions, instead of forcing the servos.
Step 7: Attaching the battery and sensors.
Glue the battery to the front of the hexapod, ush with the bottom, so that there will be as
much clearance as possible. Make sure that the middle legs have plenty of room to turn left
and right without hitting the battery. Glue the sensors to the top of the battery, angled to
the left and right.
The assembled hexapod, front view.
Take care that the wiring does interfere with the motion of the middle legs. You might want
to route yours dierently. Use small drops of hot glue as necessary to hold the wires in place.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 12 of 21

The assembled hexapod, top view.
Step 8: Final touches.
Use the Maestro Control Center to nd the neutral positions (where the legs are as
symmetrically arranged as possible) as well as their safe minimums and maximums. Set the
neutral positions as the “Go to” values for each servo, and set Min and Max values so that
your hexapod will never destroy itself. Adjust the angles of the wires so that all six feet touch
the ground in the neutral position, probably by bending the front and back legs to more than
90°.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 13 of 21

The assembled hexapod, side view.
One nal, optional thing that you might want to do is to add a dab of hot glue to each foot,
so that the metal is less likely to scratch up your work surface. Your hexapod is now ready
to be programmed!
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
3. Construction Page 14 of 21

4. Sequencing the Hexapod Gait
Gaits
Now that you have constructed your hexapod, it is time to make it walk. A method of walking
forward with legs is called a gait, and animals or robots with many degrees of freedom have
a variety of gaits available – humans can walk, run, hop, or skip; horses can walk, trot,
canter, or gallop, and so on. Your hexapod is so simple that it has just one possible gait for
forward motion, called the tripod gait.
In the tripod gait, your hexapod has three feet in contact with the ground at all times: the
front and back legs on one side and the middle leg on the other side. The angle of the middle
servo determines which side is up and which side is down. It achieves forward motion by
pushing those feet backwards against the ground while the other feet move forward through
the air. Then the hexapod shifts its weight to the other three feet and moves forward in the
same way. By continuously shifting its weight using the middle legs, then moving the raised
feet forward, it walks forward.
Walking forward with the Maestro Control Center
You can easily assemble motion sequences using the Maestro Control Center’s sequencer
feature. For documentation on the sequencer, see Section 4.c of the Micro Maestro
User’s Guide [http://www.pololu.com/docs/0J40].
Use the controls on the Status tab of the Maestro Control center to move your hexapod
to each of these four positions, pressing the “Save Frame” button after each, to save a
sequence.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
4. Sequencing the Hexapod Gait Page 15 of 21

Frame 0
right front & back,
left middle legs
touching the
ground
Frame 1
down legs move
back, front legs
move forward
Frame 2
weight shifted to
the other tripod
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
4. Sequencing the Hexapod Gait Page 16 of 21

Frame 3
down legs move
back, front legs
move forward
In the screenshots of the Maestro Control Center, you can see that the servos are always
either at their minimum or maximum values, which you should have congured to be safe
values that do not cause the servo to strain. Your numbers might be slightly dierent from
the ones shown here. If you have assembled the servos in a dierent conguration or
connected them to dierent ports, you will, of course, have dierent very settings for each
frame.
On the Sequence tab, you should now have four frames saved. Select “Play in a loop” and
play the sequence to see your hexapod walk. Rename your sequence to “forward” before
continuing.
Walking forward autonomously
Click the button “Copy Sequence to Script” to convert your sequence into a script that can
be saved on the Maestro. If you select the “Run script on startup” option on the Script tab
and apply settings, your hexapod will automatically start to walk. You can now disconnect it
from USB and allow it to walk on its own.
Reconnect USB and click “Stop Script” or disable the “Run script on startup” option to get
it to stop walking.
Backwards and turning gaits
On the status tab, start again with Frame 0 but go through the sequence of motions in
reverse to get backwards walking: Frame 0, Frame 3, Frame 2, then Frame 1. Save this
sequence under the name “backwards”, and test that it moves your hexapod backwards.
Turning is dierent. To create turning sequences, you will need to move all front and back
legs forward or backward together, instead of moving the two sides in opposite directions.
Try it out, and save two more sequences: “right” and “left”, verifying that they turn the
hexapod right and left.
You are now ready to program your hexapod to avoid obstacles.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
4. Sequencing the Hexapod Gait Page 17 of 21

5. Using a Script for Obstacle Avoidance
The Micro Maestro has an internal scripting language that can store sequences, read
sensors, and link everything together to form intelligent behaviors, making your hexapod
truly autonomous. For complete documentation on the scripting language, see Section 6 of
the Micro Maestro User’s Guide [http://www.pololu.com/docs/0J40].
Once you have set up all of your basic gaits, erase your script, then click the “Copy All
Sequences to Script” button on the Sequence tab. This button adds a set of subrountines
that you can use to access the various gaits from within a script.
You can then call these subroutines from your script. For example,
begin
forward
left
repeat
will cause the hexapod to repeatedly step forward then turn left, over, and over. You will
probably want to take into account the sensor readings, which can be accessed using the
GET_POSITION command. Here is an example of a very simple program that uses the sensor
readings to try to avoid objects – customize this to get the behavior you want!
start:
# back up if both sensors see an object
left_sensor right_sensor logical_and
if back back back goto start endif
# back up and turn right if the left sensor sees an object
left_sensor if back right right right goto start endif
# back up and turn left if the right sensor sees an object
right_sensor if back left left left goto start endif
# otherwise, if there is nothing ahead, walk forward
forward
goto start
# returns true if the left sensor sees an object
sub left_sensor
4 get_position 512 less_than
return
# returns true if the right sensor sees an object
sub right_sensor
3 get_position 512 less_than
return
### Sequence subroutines: ###
# back
sub back
100 4992 5312 5056 frame_0_1_2 # Frame 0
120 7168 6976 frame_0_2 # Frame 1
100 6528 frame_1 # Frame 2
120 4992 5056 frame_0_2 # Frame 3
return
# forward
sub forward
100 7168 5312 6976 frame_0_1_2 # Frame 1
120 4992 5056 frame_0_2 # Frame 2
100 6528 frame_1 # Frame 3
120 7168 6976 frame_0_2 # Frame 0
return
# left
sub left
100 7168 5312 5056 frame_0_1_2 # Frame 0
150 4992 6976 frame_0_2 # Frame 1
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
5. Using a Script for Obstacle Avoidance Page 18 of 21

100 6528 frame_1 # Frame 2
150 7168 5056 frame_0_2 # Frame 3
return
# right
sub right
100 4992 5312 6976 frame_0_1_2 # Frame 1
120 7168 5056 frame_0_2 # Frame 2
100 6528 frame_1 # Frame 3
120 4992 6976 frame_0_2 # Frame 0
return
sub frame_0_1_2
2 servo 1 servo 0 servo delay
return
sub frame_0_2
2 servo 0 servo delay
return
sub frame_1
1 servo delay
return
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
5. Using a Script for Obstacle Avoidance Page 19 of 21

6. Suggested Modications and Improvements
Here are some ideas for improvements or modications that could be made to the hexapod
design:
• More complicated scripted behaviors – help your hexapod get out of stuck situations
more reliably.
• More robust sensor readings. The example code only read the sensors once after each
sequence – can you do better than that, and detect obstacles sooner?
• Alternative servos – the entire design could be scaled up.
• A power switch so that the battery does not have to be unplugged over and over.
•Boost regulation [http://www.pololu.com/catalog/product/791] for consistent power from
a more compact (lithium?) battery.
•QTR sensors [http://www.pololu.com/catalog/product/958] for line following or table-edge
detection.
• Light sensors for light-seeking or dark-seeking behaviors.
Sample Project: Simple Hexapod Walker © 2001–2010 Pololu Corporation
6. Suggested Modications and Improvements Page 20 of 21