Building The Lynxmotion BRAT

Exploring Humanoid Robotics Has Never Been Easier

Story and photos by Tom Atwood and Harry Mueller

This pictorial shows the assembly of Lynxmotion’s, www.lynxmotion.com, new BRAT, an entry level bipedal robot that is a milestone robot. With a starting price of less than $300, BRAT offers an affordable yet sophisticated entry into the humanoid robotics field. Use Lynxmotion’s powerful sequencing software and you will be able to program the coordinated movement of up to 32 servos (the BRAT biped uses only six servos). The BRAT is a powerful testament to the flexibility of the “servo erector set” product line developed by Lynxmotion. Harry Mueller, who reviews the BRAT in our Winter 2006 issue, notes:

“BRAT, an acronym for Bipedal Robotic Articulating Transport, is the latest creation to come whirring out of the Lynxmotion robotics lab. It promises to be a great addition to the entry-level humanoid robot class. This robotic incarnation can trace its development back to its distant cousin Tao-Pie-Pie of New Zealand, through to SixPac, as described in the Fall 2006 issue of Robot magazine, and up to the first inspirational glimmerings of Jim Frye of Lynxmotion, who created the wonderfully expandable BRAT package.

“In the feature article on the BRAT published in the Winter 2006 issue of Robot, we examine two versions, a bare-bones basic kit and a full-blown “Tarzan-on-steroids” BRAT capable of autonomous operation in addition to great feats of strength. The main difference between the versions is the type of servo used; the basic kit comes with the Hitec HS-422 servos while my hopped up BRAT uses Hitec 5645 digital servos. In addition, my autonomous BRAT uses the optional bot board with an Atom Pro microprocessor. Obstacle detection is achieved with a Sharp GP2D12 analog infrared ranger…

“A number of optional configurations are available for BRAT using different servos and SES components and a few such versions are shown on the Lynxmotion website (www.lynxmotion.com). Once your BRAT has been configured Lynxmotion provides three ways of implementing control.

• Just use the free terminal program available for the SSC-32.


• Purchase and use the powerful Sequencer software with the SSC-32.


• Get a bot board with processor for untethered operation. The Bot Board can also be piggybacked onto the SSC-32 servo controller, in addition to being used alone.”

I assembled the entry level BRAT kit to shed light on the easy steps involved. Putting this research biped together was enormous fun―here are some highlights of the assembly process.

The autonomous BRAT, with infra-red obstacle avoidance, built by Harry Mueller. The parts are very well organized and are sealed in discrete, well-labeled plastic bags. One of the nice things about the modular Servo Erector Set parts series is that individual brackets come with a standard allotment of screws, nuts, washers and the like, typically more than you’ll need. This put a smile on my face because the assortment of nuts and bolts in my spare parts bin grew as I built the BRAT! The black anodized aluminum parts almost chime when you put them together, which added to the fun (it feels like you are working with fine metal pieces, something reminiscent of medieval armor). I used bare metal 256 screws when I anchored the multi-purpose brackets to the feet to show the attachment points (black screws come with the kit). In the second step, L brackets are bolted to C brackets. The instructions illustrate building the left leg. As you build it, simultaneously assemble the right leg as a mirror image. Multi-purpose brackets are then bolted to the L brackets. Next, 3mm ID x 8mm OD flanged ball bearings are used to connect the multi-purpose brackets on the feet to the assemblies. Note the bearing and lock washer. Once assembled, the partial leg assemblies can pivot on the bearings. There is a little play in the joint; this is normal and everything becomes very stable when servos are installed. Two C brackets are joined. C brackets are then attached to the leg assembly, and a multi-purpose bracket is attached to the other side of the C bracket. These are bearing-supported, pivoting joints. Next the U-channel, which will carry the battery, is bolted in place on each leg assembly. The parts are face down and the U-channel is bolted to the left leg. Both legs are connected to the U-channel; the BRAT is face down. We are ready to install the first Hitec HS-422 servo. The plastic rivets are easy to use and do a great job attaching the servos to the brackets. Just two self-tapping screws are used to mount the servo wheel to the bracket. I used a Servo Lab from FMA Direct to center the servos before installation. This is important as you want all the parts to be in proper alignment when the servos are at neutral. Proper alignment results in a better performing robot and makes it easier to share and use projects created by other BRAT owners. Note the placement of the rivets. Servos have been installed in both ankle joints. The knee servos have been installed. The hip servos are mounted. It’s beginning to look like a walker! Use tie wraps to secure the servo cabling. This shows the first of several tie-downs needed. The power switch has been attached. I later changed the plug from the Tamiya connector shown to an Anderson Power Pole (my workbench standard). The SSC-32 has been installed with the pins and surface mounted chips on the exterior. This is temporary because after you have trouble-shot the installation, the SSC-32 is flipped over to protect the components. Carefully thread the servo leads (pen is pointing to the lead path through the frame). Carefully strip and insert the power leads into the VS1 or VS2 terminal. I used the VS2 (both will work with the jumpers properly set). Make sure no tiny strand of wire is poking out of the connection; that could cause a short (and let the dreaded magic smoke out). Note the position of the jumpers. These positions are specified on page two of the SSC-32 Ver. 2.0 manual. Read the first two pages of that manual carefully; it describes the layout of the board. The finished entry level BRAT with the cable in place to connect the SSC-32 to the serial port on my Dell Inspiron laptop. I bought a serial to USB cable from Radio Shack, and had no problem hooking up the BRAT. I did have to toggle through a couple of Com ports to find one that recognized the SSC-32, but this only took seconds. After connecting to the serial port and applying power, the green light on the SSC-32 illuminates. You then run the Visual Sequencer program. Make sure you have the latest version. The instructions note that the robot might move swiftly and be prepared. I “knew” all of my servos were properly centered, but when I told the software to center all servos by clicking on the “All=1.5mS” button, my BRAT came alive and kicked me! I had installed one servo 90 degrees out of phase! You are now ready to set the throw limits and centers on your servos. Here, an ankle servo is set to a 45 degree deflection. Using the Visual Sequencer program, you can then build coordinated, timed sequences of moves that define how your BRAT walks and moves. For more detail on what can be done with the Sequencer, download the sequencer manual at: www.lynxmotion.com/images/data/seq01h.pdf. This screen grab shows the visual depiction of BRAT’s servos in the Sequencer program. Imagine how this would look if you added additional servos to build this biped into a full-featured humanoid! Once Harry Mueller added the bot board to his BRAT (see Winter 2006 Robot), switching between the Sequencer and the Atom Pro became something of a chore. A nice fix is to hook up all the pins required to set both the DB9 enable and the TTL serial comm to a 4PST top slide switch. That way you can just flip a couple of switches to go from using the Sequencer to sending things to the Atom Pro and back. Jim from Lynxmotion pointed out that the baud rate can be set permanently to 38.4K for both functions without affecting performance. Harry’s autonomous BRAT with digital servos executes a Karate kick. Exploring this software has not only been a hoot, it’s been an inspiration. The power of this software is enormous and one can easily envision “growing” the BRAT to include additional joints and limbs. Want to build your own Robo-One humanoid robot? Start with a BRAT and add a torso, head and arms. See the Winter 2006 issue of Robot to read Harry Mueller’s evaluation of the autonomous Lynxmotion BRAT powered by digital servos. The entry level Biped BRAT Combo Kit for PC (serial port, Brushed or Black Anodized brackets) includes the Sequencer software, SSC-32 Servo Controller, Serial Data cable and four Hitec HS-422 servos. Price: $286.09

For more information, contact Lynxmotion, Inc., www.lynxmotion.com, (866) 512-1024.