Build an Actobotics Quadruped Robot

How to make a 4-legged walking robot using Actobotics parts from ServoCity.com:

The workhorse of this robot, and what makes it relatively easy to build, are the servoblocks from Actobotics. These nifty components let you use standard hobby servos for high torque applications by providing a solid aluminum frame together with smooth bearings to take the load off of the servo's axle. And the standardized hole patterns and spacing makes connecting their other aluminum parts a breeze!

This Instructables article has grown in complexity to the extent that I will be describing the electronics & coding for this project in a Part 2 follow-up to this one.I also plan to refine and make corrections & clarifications to this particular article in the very near future.


Step 1: Actobotics' Servoblocks

Build an Actobotics Quadruped Robot

This project uses 8 of their standard size Servoblocks and 8 standard size servos. I am partial to the Hitec brand of servos, and so for this project we will use 8 of their part number 637110:

( http://www.servocity.com/html/standard_hitec_servo... )

In general we will assemble them as normally described, but we will want to leave one of the hub plates free from each servoblock to make assembly of the leg modules a little easier as we go. We'll attach it later, as you will see.

Take a look at the photo to see what comes in the Servoblocks package.

Step 2: Make 4 sets of Actuator Modules

Build an Actobotics Quadruped Robot
Build an Actobotics Quadruped Robot

Let's define one Actuator Module as one Servoblock + one Hitec standard size servo.

You'll want to make 4 sets of each of these configurations, with a set being defined as one Servoblock with the one of the hub plates remaining unattached, and another Servoblock flipped horizontally with the corresponding hub plate unattached as shown. For the sake of referencing later, let's call the Actuator Modules with the unmounted Hub Plate on the "left side" of the servo as a Left Handed Actuator Module.

The accuracy of that nomenclature is obviously somewhat ambiguous, but we will need a way to refer to the differently oriented modules later on.

Now, before you insert the Servo Spline Shafts, grab 4 of the Actuator Modules - 2 Left Handed ones and 2 Right Handed ones. Attach 2 Aluminum Beam Blocks A to the mounted Hub Plate of each. Make sure the Beam Blocks are on the side of the Hub Plate closest to the electrical leads on the servos.

Now you can go ahead and insert the Servo Spline Shaft into all 8 modules, and secure to the servo spline with the original screw that came with the servo. Be sure to keep the 4 modules with the Beam Blocks grouped together, as these will be used in the Leg Lifter Modules.

Step 3: Calibrate each Actuator Module

Build an Actobotics Quadruped Robot
Build an Actobotics Quadruped Robot

Now you should have a total of 8 Actuator Modules, with 4 of them having the hub plate removed from the left side of the servo, and 4 of them with the hub plate removed from the right side of the servo. We need to "calibrate" each Actuator Module, using the following steps:

  • Grasp the assembled module in one hand, and firmly grasp the edges of the Servo Spline Shaft with your fingertips & gently twist it in one direction until it stops.
  • Using a permanent marker or pencil place a mark along the flat top of the Servo Spline Shaft & also along the edge, as shown. Place a corresponding mark along the Ball Bearing Plate, as shown.
  • Twist the shaft in the opposite direction until it stops. Place a second mark along the Ball Bearing Plate.
  • Now rotate the shaft so it is roughly midway between the two markings on the Ball Bearing Plate and leave it there.

    Repeat these steps for all 8 Actuator Modules.

    ( NOTE: The preferred way to do this is using ServoCity's Dual Servo Driver. I have found this little gizmo to be well worth the money! http://www.servocity.com/html/dual_servo_driver.ht... . Shown in the 2nd photo for this step. )

    Step 4: Build 4 Leg Lifter Modules

    Build an Actobotics Quadruped Robot

    Now we will complete the 4 "Lifter" Leg Modules as shown in the photo.

    We will build 2 with right facing legs and 2 with left facing legs. I call the one shown a right facing Leg Lifter Module.

    So, from the previous effort, gather 2 Left Handed Actuator Modules & 2 Right Handed Actuator Modules. ( The ones with the Beam Blocks A parts attached to the mounted Hub Plate.

    Along with each Actuator Module, you also will need the following parts:

  • 2x 5 hole aluminum beams
  • 1x 7 hole aluminum beam
  • 4x 3/8" 6-32 screws
  • 1x .25" 6-32 screws
  • 1x 1.25" 6-32 screws
  • 3x Nyloc nuts Note: Screw lengths may be corrected. Please check back here again before building.

    Step 5: Leg Lifter Module - detailed views

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot

    With a picture being worth a 1000 words, here I am going to let you browse through several detailed photos of the same Leg Lifter Module. Note that the first photo is a "poor man's" exploded view.

    The next 3 are views of the bottom of the same Leg Lifter Module as viewed from the left, face-on, and then right orientations.

    Note that some of the photos show the 7-hole aluminum beam with a rubber boot on the end ... yep some of these photos were taken out of sequence from the order they are shown :-) .

    Also, the term "1/2 nylon washer" means exactly what it says: it is a nylon washer sliced in half across its circumference. ( Actually I prefer to use the metal washers from a pack of the 6-32 linkages (heavy duty 6-32 ball linkages - part # 585432). They are, of course, more expensive than the nylon washers but you can use the linkages in other projects at a later time. )

    This may look a little complicated, but I promise that once you get the first one built the next 3 will come easy for you!

    Step 6: 4 Completed Leg Lifter Modules

    Build an Actobotics Quadruped Robot

    Now you should have 4 completed Leg Lifter Modules !

    Set these aside for now & go on to the next step.

    Step 7: Complete the 4 Leg Rotator Modules

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot

    Gather the remaining 4 Actuator Modules.

    Go ahead and attach the unmounted Hub Plate to each.

    Following along in this series of photos to build up the Leg Rotator Module.

    Attach the .770 x .770 Hub Adaptor to the newly attached Hub Plate.

    Now attach one 90 Quad Hub Mount B, making sure it is rotated 90 degrees to the Hub Plate.

    Now attach one of the unmounted Hub Plates from one of the Leg Lifter Modules to the servo spline shaft.

    Repeat the above steps for the remaining 3 Actuator Modules. Remember to add one of the unmounted Hub Plates from a Leg Lifter Module to each Leg Rotator Module, as shown in the Completed Assembly photo.

    Step 8: Build the 4 Complete Leg Movement Modules

    Build an Actobotics Quadruped Robot

    Gather the 4 Leg Lifter Modules & the 4 Leg Rotator Modules.

  • Insert the free Hub Plate from one of the Leg Rotator Modules into the space left open by the unmounted Hub Plate position on one of the Leg Lifter module. Secure it with 4x 6-32 socket head screws.
  • Repeat for the remaining Lifter & Rotator modules until you have 4 Complete Leg Movement Modules. Now, for each Complete Leg Module, rotate the servo of the Rotator portion so it is 90 degrees to the servo of the Lifter portion.
  • Arrange all 4 Complete Leg Movement Modules as shown in the 2nd photo.
  • Now we need to make sure that each Complete Leg Movement Module can be rotated at least 90 degrees from the end to the side. If you can not, then you may have to reposition the servos to each other and/or remove the Lifter Module from the Rotator Module, twist it accordingly, and re-attach.

    Step 9: Mount Switches & 9vdc Battery Holder

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot

    Gather these parts:

  • 1x 3.00" Aluminum Channel
  • 2x Hitec Micro Switch Harnesses
  • 2x 1" length 6-32 socket head screws
  • 2x .375" length aluminum standoffs
  • 1x Beam Bracket E
  • 2x 9vdc Snap Fit Battery Holder (1 set)
  • 2x 6-32 nuts
  • small width electrical tape

    Flip the aluminum channel so the open side is facing up.

    You will probably want to wrap some small width electrical tape around each switch harness, as shown in the photos. Insert the screws into the Beam Bracket E, as shown. Thread the standoffs onto each screw & tighten. Slip the 2 Switch Harnesses between the standoffs, as shown. Push the screws with switch harnesses into the holes of the aluminum channel, as shown. Secure with the nuts inside the channel.

    Now insert the Snap Fit 9vdc Battery Holder into the channel holes, as shown. It's easiest if you get the bottom pegs of the bracket in place, then squeeze the top of the holder & rotate it until it is in position. Put 2 of them side by side, as shown in the photos.

    Step 10: Front & Rear Channels Assembly

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot

    You're getting close!

    Gather these parts:

  • 2x 5/8" Bore Tube Clamp Hub
  • 2x 3.00" Aluminum Channel
  • 2 to 4 6-32 socket head screws
  • 4-8 6-32 pan head screws ( or 6-32 socket head screws )

    Use 2-4 pan head screws to attach one Tube Clamp Hub to each of the 3" Channel.

    Note that the pan head screws are required for the Front Channel since we want to drop a 4 cell AA battery holder into that channel.

    Of extreme importance is insulating the ends of the battery holder from accidental contact with the metal of the aluminum channel & other parts. A couple of layers of electrical tape should do the trick. I'll add another photo ASAP showing exactly what I am taljking about.

    For the Rear Channel - the one with the switches and 9v battery - you can use either pan head or socket head screws.

    Make sure the split part of the hub is facing away from the open side of the aluminum channel. The 2nd photo shows how the clamps should be oriented to the channel.

    Loosely insert 1 or 2 of the 6-32 socket head screws into each clamp. Do not tighten yet.

    Lets refer to the channel with the switches and 9vdc battery mount the Rear Channel and the other one as the Front Channel.

    Go to the next step.

    Step 11: Almost Done !

    Build an Actobotics Quadruped Robot

    Now arrange the Front Channel, Rear Channel, 5/8" tubing, and the 4 Complete Leg Movement Modules as shown in the photo. Keeping the relative positions intact, insert the 90 Quad Hub Mount B of each Complete Leg Movement Module into the end of the channel and secure it with 4x 6-32 socket head screws.

    Now keeping the Front Channel & Rear Channel upside down on your flat work surface, along with the attached Complete Leg Movement Modules, insert the 5/8" tubing into one of the tubing clamps, and tighten the 6-32 socket head screw.

    Insert the other end of the tubing into the 2nd tubing clamp and tighten.

    You can now flip the robot over so it is sitting on its 4 legs !

    Step 12: Attach the Arduino controller to the Robot

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Show All 7 Items

    Gather these parts:

  • 2x __ length 6-32 pan head screws
  • 2x __ length 6-32 standoffs
  • 2x __ length 6-32 standoffs
  • 1x Arduino component mounting bracket
  • 90 Dual Mount A
  • 2x 6-32 socket head screws
  • Almost Completed Robot from previous step

    Attach the Arduino Component mount to the 90 Dual Mount A using the short flat head 6-32 screws, as shown.

    Insert the long 6-32 socket head screws into the through holes of the 90 Dual Mount A.

    Thread one of the shorter standoffs onto each long 6-32 socket head screw.

    Thread one of the longer standoffs onto the remaining thread of each screw.

    Next, secure the Arduino to the Arduino Component mounting bracket.

    Insert the 6-32 pan head screws through the holes of the Rear Channel, as shown. Then thread each pan head screw into the longer standoffs attached to the Arduino component mount.

    Step 13: Wiring the Electronics & Power

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot

    Arduino UNO board

    DF Robotics I/O Expansion Shield Version 7

    4x AA batteries

    1x AA 4 Battery Holder

    Further details concerning the specific connections & coding will be presented in a follow-up Part 2 article. These photos & links should at least give you a preview of how everything comes together.

    Step 14: Servo Wiring Chart

    Build an Actobotics Quadruped Robot
    Build an Actobotics Quadruped Robot

    The first graphic is a "legend" chart showing the servo channel numbers and the relative position of each corresponding servo, as viewed from the top of the robot. The black rectangles represent the Leg Rotator Modules and the blue rectangles represent the Leg Lifter Modules. I'll provide another graphic soon showing the chart next to the actual robot layout.

    The 2nd graphic is a photo showing the servo leads connected to the DF Robot I/O Expansion Shield. Note that cable 13 on the "legend chart" and cable 13 on the photo are both on the far upper left. Cable 6 on both graphics are on the far right.

    Step 15: Full Parts List

    Description Part Number UNIT PRICE LINE TOTAL


    AA Battery Tray (4 cell) Product # 57216 1x $2.95 = $2.95

    Beam Bracket E (Pair) Product # 585610 2x $1.49 = $2.98

    1.54 inch Aluminum Beam (2 pack) Product # 585404 4x $2.39 = $9.56

    2.31 inch Aluminum Beam (2 pack) Product # 585406 2x $2.59 = $5.18

    6 inch length x 5/8 inch dia. Aluminum Tubing Product # 635268 1x $3.19 = $3.19

    ServoBlocks with Hub Shaft (Standard Hitec) Product # 637110 8x $26.99 = $215.92

    Arduino Component Snap Mount Product # 585010 1x $2.19 = $2.19

    1.250 in L x 6-32 Zinc-Plated Alloy Steel Socket Head Cap Screw (25 pk) Product # 632128 1x $3.59 = $3.59

    .250 in L x 6-32 Zinc-Plated Alloy Steel Socket Head Cap Screw (25 pk) Product # 632106 2x $1.69 = $3.38

    .4375 in L x 6-32 Zinc-Plated Alloy Steel Socket Head Cap Screw (25 pk) Product # 632112 2x $2.09 = $4.18

    3.00 inch Aluminum Channel Product # 585442 2x $3.99 = $7.98

    HS-311 Servo Product # 31311S00 Rotation: Stock Direction: Stock 8x $7.99 = $63.92

    Hitec Micro Switch Harness Product # 54403 2x $5.95 = $11.90

    90 Degree Quad Hub Mount B Product # 545424 4x $4.99 = $19.96

    Hub Adaptor A Product # 545440 4x $2.99 = $11.96

    Beam Attachment Blocks (4 pack) Product # 585403 2x $4.99 = $9.98

    6-32x.375 inch Aluminum Standoffs (Round) Product # 534-3486 2x $0.34 = $0.68

    6-32x 1.00 inch Aluminum Standoffs (Round) Product # 534-3489 2x $0.70 = $1.40

    6-32 Machine Screw Nylock Nuts Product # 90631A007 12x $0.09 = $1.08

    Subtotal: $381.98

    Note: I'll try to get this list in a more presentable table format ASAP.

    Step 16: Addendum

    I've been working on this article for more than 3 weeks after hours, and frankly, I need a break !

    But I did want to get it published so that hobbyists can start thinking about this project. I will be coming back to it and adding more detail, and I have an idea on how to present the photos in a more intuitive fashion. ( I may end up embedding several short videos showing the step-by-step build process. )

    However, if you are ambitious, there is enough information here to allow you to complete the build, mechanically, for a functional 4 legged walking robot. You will probably want to add sensors and enhancements if you enjoy this basic build. The really neat thing about the Actobotics line of components is their standardization of hole patterns, so you can re-use the parts in other projects with ease.

    The electronic controller is an Arduino UNO board, along with a DF Robotics I/O Expansion Shield Version 7 to make connecting the servos much more convenient than it would be connecting directly to the UNO. Servocity.com does not carry those items, at present, so you will need to order them from a different vendor. I'll provide a few links to some soon.

    I've taking many photos, some of them out of sequence with the actual build instructions, so there are bound to be a few that are backwards or show something other than what I intended. Those I hope to correct in a timely fashion. Please contact me if you notice something or have suggestions on how to clarify this article.

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