MOCpages : Share your LEGO® creations
LEGO models my own creation MOCpages toys shop Advanced PackBot (APB)Robotics
Welcome to the world's greatest LEGO fan community!
Explore cool creations, share your own, and have lots of fun together.  ~  It's all free!
Advanced PackBot (APB)
An Mindstorms NXT-based PackBot under RF remote control adding a new mode of locomotion and other major enhancements to the LNE PackBot developed by Brian Davis.
About this creation
Please feel free to look over the images and skip the verbiage.

On this page:
Intro
About the PackBot
Brian Davis' LNE
Modeling goals
Beginning of annotated MOC photos
Obstacle sequence with flippers leading
Obstacle sequence with flippers trailing
Obstacle sequence with right tracks only
Holonomic wheels
Automatic control consistency
Table of features and stats




Introduction
The starting point for this MOC was the "LEGOŽ NXT Explorer" (LNE for short) -- an excellent rendition of iRobot's famous PackBot developed by Mindstorms master Brian Davis. I call my adaptation of his design the "Advanced PackBot" (APB) because it adds several major enhancements yielding substantial improvements in functionality and drivability.



About the PackBot
The real PackBot is a small, rugged, portable, all-terrain, multi-mission tracked robotic vehicle developed by the iRobot Corporation for use by military personnel, firefighters, search-and-rescue and HazMat teams, and bomb squads, among others. In its most common role as a remote sensing (telepresence) platform, its primary mission is to go where no man or canine can or should go and return video, audio, or other kinds of data to its handlers. For example, PackBots carried out the initial reconnaissance and radiation measurements inside the Fukushima nuclear power plant damaged in the great Mw 9.0 Tohoku earthquake of 11Mar2011. You may have seen them in action in news footage of rescue efforts in the rubble left by major earthquakes or terrorist attacks.

PackBots generally come with deck-mounted lights, driving, rear-view, and upward-looking video cameras, a microphone, and a speaker. Optional task-specific deck-mounted appendages include a 2 meter-long, 12-DOF (degree of freedom) folding robotic arm that can be fitted with various cameras, sensors, and manipulators. Some military versions have been weaponized or equipped to sniff out and dig up improvised explosive devices. The first vehicle to carry the name "PackBot" was a small military version that could be carried in a backpack and lobbed over a wall or through a window to see what's on the other side, as shown here. (PackBots are designed to take 400 G impacts.) PackBots are semi-autonomous, in that they're primarily operated by human handlers via game-style controllers but can also right themselves if they land belly-up after a fall. (A similar NASA robot can also climb stairs on its own).

Just add flippers
For my money, the most remarkable thing about the PackBot by far is its totally outside-the-box mobility system, and that's the focus of this MOC. The PackBot can move and hold itself in so many ways that talking about them is a real challenge. Establishing a few definitions will help. Hereafter, I'll refer to a mode of locomotion as simply a mode and a particular arrangement of the PackBot's flippers, body, and rear tracks in space as a posture. A maneuver is a motion that takes the PackBot from one posture or mode to another without serving locomotion directly. The folks at iRobot seem to lump modes, maneuvers, and postures under the word "pose", but I think that only adds to the confusion.

Like most tracked vehicles, the PackBot has 2 independently driven main tracks. The body in between has a readily identifiable deck (back, top), belly (bottom), front, and rear and spends most of its time belly-down.

Unlike all previous tracked vehicles, however, the PackBot also has a pair of lever-like tracked flippers mounted on axles coaxial with the main track front sprockets. Each flipper track rolls in unison with the main track on the same side. Meanwhile, the flipper arms are free to rotate in unison through any angle at any rate in either direction. The flippers are long enough and powerful enough to lift either end of the body off the ground. They're used primarily to lift the front end onto obstacles in climbing mode, ease it down from ledges, and bridge gaps while the main tracks provide most of the propulsion.



But the PackBot's flippers turn out to be much more versatile than that. For example, they can (i) stabilize a standing PackBot when the robotic arm is in use, (ii) change its center of gravity as needed, and (iii) right a belly-up PackBot by lifting its rear end up and over its front end. The PackBot travels mostly in driving mode (belly down, flippers off the ground) to reduce friction, but the flippers can also hug the ground when a need for added traction or reduced ground pressure on loose surfaces arises. This extended mode can also be helpful on steep inclines (60° maximum).

Sea lions and fur seals as biological analogs
The PackBot's flippers endow it with modes, maneuvers, and postures never before seen in tracked vehicles, but these talents are nothing new to sea lions and fur seals (collectively, the eared seals or otariids). These agile, all-terrain marine mammals spend a good bit of time on land. Despite their short limbs and low ground-clearance, eared seals easily scamper over obstacles taller than their shoulders, and several species actually prefer rocky shorelines over low-relief beaches for both haul-outs and rookeries. When climbing over an obstacle like a boulder, they lift and lower their bodies mostly with their fore-flippers while pushing themselves forward primarily with their pelvis and hind-flippers. Eared seals also pull themselves along over soft surfaces like wet sand and mud with a butterfly-like "swimming" motion utilizing both fore-flippers in unison. Finally, they can improve their maneuverability on land by pivoting on their fore-flippers so as to de-weight their hindquarters. Sea lions can be trained to take this a step further and balance on their fore-flippers with their hind-flippers held high so as to be able to turn on a dime.

The PackBot can do all of these things, and for much the same reasons. Indeed, that's the key to its ability to keep moving through rubble and over high-relief and loose surfaces with minimal ground clearance. I have yet to find a reference stating that the PackBot's designers drew inspiration from the terrestrial locomotion of eared seals, but the similarities are striking.



Brian Davis' LEGOŽ NXT Explorer (LNE)
The LNE captured all of the PackBot's modes of locomotion, but its mobility system differed in 2 main ways: (i) Its flipper arm axles were mounted above rather than coaxial with the front main track sprockets, and (ii) it replaced each of the PackBot's main tracks with a pair of shorter tracks mounted rigidly inline. The former limited the traction the flippers could contribute in driving and climbing modes, but the latter had little impact on the LNE's performance WRT real PackBots.

After watching the LNE videos below, I knew I had to build my own version. (All of these clips are entertaining, but "Packbot Escape" best illustrates the versatility of the PackBot/LNE mobility system.) Davis' pictorial LNE instructions on Brickshelf provided a good starting point.
PackBot escape
LNE on point
LNE: LEGO PackBot
LNE: Old Technic vs. the New Technic
LNE vs. the Empire



Modeling goals
This MOC is basically a functioning PackBot-like mobility system that could easily be turned into a genuine remote-sensing platform by adding suitable sensors. After putting my first LNE-based PackBot through its paces, I decided to add several major features of my own:
  • Mounted protective wheels on top of the NXT and a spring-loaded rear rollbar with a holonomic wheel on the chassis (i) to allow the APB to get around equally well with its belly up or down, and (ii) to lessen shocks to the NXT when landing belly-up.

  • Added a power-transmitting articulation between the LNE's middle and rear tracks (together, the main tracks) and spring-loaded the rear tracks (i) to improve belly-down ground contact in rough terrain, and (ii) to lessen shocks to the NXT when landing belly-down.

  • Added several front-end rollers (i) to improve maneuverability when balancing on flippers, and (ii) to smooth out transitions between belly-down to belly-up attitudes.

  • Programmed a PS/2 software interface taking full advantage of PS/2 controller features in NXT-G.

  • Improved drivability by adding a rear rollbar-activated touch sensor to maintain control consistency automatically.

  • Ruggedized the chassis, especially around the NXT and the flipper arm turntable mounts.
These goals were eventually realized in the MOC presented here. Automatic control consistency (ACC) was one of the last features added but arguably the most important. Without it, I found the APB very hard to drive -- mainly because all the controls produce opposite effects when the APB goes belly-up. I still have to think like a trained sea lion to take full advantage of its many modes and maneuvers, but my driving got a lot better after implementing ACC.

The ruggedization really paid off when the APB went to the Denver ComiCon in 2013. Kids passing by the DENLUG area were allowed to drive several MOCs brought for that purpose, and the APB was one of them. I'm proud to report that it stood up to nearly 10 hours of hard use with nary a complaint.


Locomotion- and maneuver-wise, the LNE and APB can do everything a real PackBot does, but the APB adds a unique and very useful wolf-spider mode (belly-up, flippers down) to the menu. In wolf-spider mode, the flipper track tips can swing the APB's tail around very quickly on a largely hidden rear holonomic wheel. This is by far the APB's most maneuverable mode. Wolf spiders are quick and fiesty little critters that do whatever it takes to keep perceived threats directly in front of them at a comfortable distance. The APB can do the same quite well in this mode.




Photos and text

Top view for scale. Light-colored floor tiles were 305 mm (12 in) squares before corner cuts.

At far right is the APB's ThrustMaster PS/2 controller. The PS/2 receiver (with lit red LED) to its immediate left plugs into an NXT-PS/2 controller interface sensor hidden under the holonomic wheel.

The APB operates entirely under remote control. The joysticks on the controller adjust the corresponding track speeds independently and progressively. With the right joystick pressed down, the left joystick adjusts the left track speed and sets the right track speed to the same value. Pressing the left joystick down and working the right joystick does the same thing. Most of the remaining controls are presets. The cross-shaped silver control accesses full-speed presets for straight forward, straight backward, and spinning motions. The 2 left front buttons are track speed governors respectively limiting actual track speeds to 55% and 75% of the speeds specified by the other track control(s) active at the time. The "Triangle" and "X" buttons on the right are respectively full-speed forward and reverse flipper presets, with the right front buttons serving as their governors. (Since there are no progressive flipper speed controls, the governors are important for delicate flipper maneuvers.) The "SE" button sounds a klaxon, while the "SE" button kills the NXT program and the APB along with it. The remaining controls don't affect the APB per se.


Side view in driving mode (belly down, flippers off the ground). Driving mode is fine for straight-line forward and backward locomotion but poor for maneuvering due the friction involved in skidding the main tracks. Note the holonomic wheel on the spring-loaded rear rollbar above and behind the NXT. The NXT wheels at center serve to keep the face of the NXT embedded processor off the ground.




The next 5 photos show the APB flipping from driving mode to a belly-up attitude. The APB toppled onto its back just after the 4th photo. The front rollbars (5x7 quarter-ellipse liftarms) and associated rollers supported its front end smoothly throughout the flip. The spring-loaded rear rollbar bearing the APB's holonomic wheel cushioned the landing.

This maneuver leaves the APB in the immobile sunning posture (belly up, flippers off the ground) shown in the 5th photo. In sunning posture, the APB rests only on its unpowered holonomic and front NXT wheels. Lowering the flippers onto the ground in either direction is all it takes to get moving again. Eared seals often sleep and sunbathe in similar postures -- hence the name.













The next 6 photos show the APB going from sunning posture to wolf-spider mode by rotating its flippers forward and downward.















The next 2 photos could be thought of as snapshots taken during swimming mode (belly down, flippers rotating continuously to pull the body forward, usually with help from the tracks). This mode can be very useful when moving forward over obstacles and loose surfaces.

These photos could also be seen as depicting a push-up mode (belly down, flippers down) that finds a good bit of transient use when negotiating obstacles. You'd think that the greatly reduced main track contact area would make push-up mode more maneuverable than driving mode, but the improvement is minor at best.








Obstacle sequence with flippers leading
The next 13 photos show the APB overcoming an obstacle in the forward direction. Note how the main track articulation (between the middle and rear tracks) helps to maintain traction.






























Obstacle sequence with flippers trailing
The next 10 photos show the APB tackling the same obstacle in reverse -- i.e., with flippers trailing. The approach is in C-mode (see below). The main track articulations were particularly helpful here. Backing over the obstacle allows the flippers to do a better job of easing the APB back down onto the floor.


























Obstacle sequence with right tracks only
The next 6 photos show the APB overcoming the same obstacle going forward again -- this time with only the right tracks. No problem.
















The remaining photos show the APB in various modes and postures. Close-ups of various key features and additional comments are coming soon. NB: From here on out, comments will refer to the photo(s) directly above them.




The front rollbars (5x7 quarter-ellipses) and front rollers seen here were carefully sized, positioned, and reinforced to smooth out the front-down phases of maneuvers between belly-up and belly-down attitudes.




C-mode (front end down, flippers and rear tracks pointing toward belly) is so named becasue the flippers, body, and rear tracks form the letter "C". C-mode isn't just a transient posture. It's a bona fide mode of locomotion that favors stability over maneuverability while offering a good mix of both. It's not as maneuverable as wolf-spider mode or even Z-mode, but it's much more maneuverable than driving mode. It's also a good lead-in for backing over obstacles, as seen above. Better yet, it's rather menacing look makes it an excellent choice for intimidating other MOCs. I almost called it "Godzilla mode".


The APB can move with its belly leading or trailing in C-mode.




Two more views of wolf-spider mode.






You could think of Z-mode (front end down, flippers pointed away from belly) as a transient posture passed through when maneuving between driving and wolf-spider modes, but it's also a bona fide mode of locomotion. As such, it favors maneuverability over stability but, like C-mode, offers a very useful blend of both. The range of Z-mode attitudes stable during locomotion is fairly broad, but it took a good bit of fiddling with weight distribution to make that happen. Z-mode is also an excellent lead-in if you feel a need to body-slam something with your APB.





Holonomic wheels

The main axle of the holonomic wheel -- by definition, a wheel designed to roll in any direction, regardless of main axle orientation -- is mounted between the ends of the rear rollbar.


The holonomic wheel itself is at bottom center here. Think of this photo as a snapshot of the wheel first touching down as the APB approaches a belly-up attitude. Then imagine how the wheel would respond to forward or backward locomotion of the APB or side-to-side motions of its rear end.

My first thought was to use a caster on the rear rollbar, but casters tend to snag on surface irregularities and need to be pulled into alignment with the desired direction of travel in the likely event that they land the wrong way. Using a large holonomic wheel instead effectively eliminated these problems.


The holonomic wheel consists of 9 subwheel pairs staggered as shown. Since only 1 subwheel contacts the ground at a time, successive subwheel-ground contacts are roughly 20° apart.


Turns out that a 14x4 mm smooth tire (59895 or 3139) spontaneously adopts the well-suited asymmetric profile seen here when forced -- and I do mean forced -- onto an 11x8 mm wheel with center groove (42610). (I discovered this quite by accident.) This construction makes for smoother rolling when the APB's direction of travel has a forward or backward component.


Close-ups of the right protective NXT wheels and upper front rollers at top, right front rollbars (5x7 quarter-ellipse at image right), and right flipper at bottom right. The NXT flipper motor drives the 8-tooth pinion meshing with the outer gear ring of the large Technic turntable. The resulting 7:1 reduction yields a happy mix of very high flipper torque and easily controlled flipper rotation rate. That's all Davis' doing.


Close-up of track motor gearing. I prefer the combination of speed and torque resulting from this 1:1 direct drive over that of Davis' original 1.67:1 reduction.


Close-up of right main track showing articulation between middle and rear tracks.


Small Technic turntables and belt drives used to transmit power through main track articulations. The 30.4x14 VR wheels (2994) make great pulleys. Their deep central groves hold the blue silicone belts in firmly place.


Close-up of rear rollbar and rear track shock absorbers.



Automatic control consistency

Oblique view of deck with NXT embedded processor at upper left and NXT touch sensor used to maintain control consistency toward opposite corner.


When the APB enters a belly-down attitude, the spring-loaded rear rollbar presses and holds the ACC touch sensor. This triggers the NXT to play a sonar sound and restore its default responses to PS/2 controller inputs.


Conversely, when the holonomic wheel lands as the APB approaches a belly-up attitude, it releases the ACC touch sensor by pushing the rear rollbar toward the rear. At this point, the NXT plays a klaxon sound and reverses all of its responses to PS/2 controller inputs. By keeping belly-up responses consistent with belly-down responses, this automatic reversal greatly improves drivability.


ThrustMaster PS/2 receiver plugged into Mindsensors PSP-Nx-v3 NXT-PS/2 controller interface sensor.


NXT screen shot showing information displayed during APB operation.






Table of features and stats


Overall dimensions:
[] mm ([] LU, [] in) in LxWxH with flippers forward

Overall weight:
[] kg ([] lb)

Construction:

Studless throughout

Mobility system:

Flipper-assisted differential drive with 3 rubber tracks on each side -- 1 for flipper and 1 each for articulated middle and rear tracks

Middle to rear track power transfer:

Belt drives built around small Technic turntables

Locomotion modes:

Many (see above)

Embedded processor:

NXT with LEGOŽ v. 1.31 firmware

Embedded processor protection:

Front rollbars and rollers; spring-loaded middle-rear track articulation; spring-loaded rollbar with holonomic wheel; NXT-mounted wheels

Programming:

Original PS/2 interface in NXT-G

Motors:

3 NXTs in all -- 1 for each trackset, 1 for flippers

Sensors:

2 -- Mindsensors PSP-Nx-v3 PS/2 receiver interface sensor for remote control; NXT touch sensor to determine attitude

Remote control:

RF via ThrustMaster PS/2 controller and receiver and PS/2 receiver interface sensor

Autonomous functions:

None

Electrical power:

Rechargable NXT lithium ion battery

Modified LEGOŽ parts:

None

Non-LEGOŽ parts:

Mindsensors PS/2 receiver interface sensor and Flexi-Cables. (NB: Link is to the PSP-Nx-v4 sensor page. The discontinued PSP-Nx-v3 was used here.)

Credits:

Based on LNE design by Brian Davis





Comments

 I made it 
  November 12, 2015
Quoting Family Vuurzoon Very impressive! You added lots of photos to demonstrate the complex working principle. Would it be possible to make a video too? Regards,
Thank you very much. Will do as soon as I get someone to help me with the videography. If only I had 3 hands!
 I like it 
  November 12, 2015
Very impressive! You added lots of photos to demonstrate the complex working principle. Would it be possible to make a video too? Regards,
 I like it 
  November 20, 2014
Whao!
 I made it 
  March 11, 2014
Quoting matt rowntRee Awesome explanation there Jeremy. Metal is forgiving in that it is easy to repair WHEN I mess it up. ;) I've noticed in my past engineering experiments with Lego that I was more frustrated with the gears more than anything. In the end, I sort of gave up on the whole shebang and concentrated with brick on brick. Keep 'em coming, I love seeing engineering feats that really push the material to it's limits. To me, THAT'S true Art. Thank you Jeremy!
I'm been meaning to post a gear tutorial for a while, and you got me off the dime here. It's up and about 99% complete now. Hope it helps.
 I like it 
  March 10, 2014
quite a feat! I love robots, I have 2 nxts, but, did nt do much with them yet, you re quite an inspiration, keep at it!
  March 8, 2014
Awesome explanation there Jeremy. Metal is forgiving in that it is easy to repair WHEN I mess it up. ;) I've noticed in my past engineering experiments with Lego that I was more frustrated with the gears more than anything. In the end, I sort of gave up on the whole shebang and concentrated with brick on brick. Keep 'em coming, I love seeing engineering feats that really push the material to it's limits. To me, THAT'S true Art. Thank you Jeremy!
 I made it 
  March 8, 2014
Quoting matt rowntRee You have officially freaked me out with your genius here! The holonomic wheel is truly inspired. Everything about this is inspired. I love how you explain everything in detail, it shows a wealth of understanding in the functionality and physical engineering involved. I am humbled by your work and have to wonder why you choose Lego as your medium. Not that it is a poor choice, but I would think that steel with nuts and bolts and welds would suit your purposes better. ABS has certain limits in its capacities as a viable structural material. Are the stresses negligible in creations like this to where it doesn't matter? Apologies if my questions seem base or rudimentary, but I am a metal worker and look at structure and stresses with an acute eye. Just simply a curiosity of mine. An interesting build with some innovative and outstanding techniques!
First, thanks for the very kind words. You should know that I envy your metal working skills because (i) metal's by no means an easy medium, (ii) I don't have a place to put all the necessary machine tools, (iii) I wouldn't know how to use them even if I did, and most of all, (iv) I'm the world's worst fabricator, hands-down. Metal, wood, plastic -- you name it, and I can screw it up royally. Hence, the unmatched modularity, precision, and parts selection of the LEGO(R) construction system makes it a natural outlet for my inner engineer. Then there's my wife, who's made it quite clear that there will be no unsightly excavators or haul trucks or trebuchets parked out front, and that research vessels and Mars rovers are out of the budget, period. The good news is threefold: (i) Over the years, I've found that with enough patience and preparation (mostly research into how similar real gizmos work and why they look the way they do), I can model just about anything that catches my fancy =to my satisfaction= with LEGO(R). (ii) The inherent limitations -- and there are amazingly few when you make the proper scaling adjustments -- generally feel a lot more like interesting challenges than tiresome frustrations. (iii) A willingness to modify parts and use non-LEGO(R) components here and there generally takes care of the worst frustrations and impasses. Yes, I run into the limited strength and frictional properties of ABS plastic all the time. Hence I also have to look at structure and stresses with an acute eye. And I yearn for positive tension fasteners like nuts and bolts. Turns out, though, that there are almost always acceptable work-arounds with ABS if you pick the right scale. For example, the bogie arms on my Mars rover still flexed and twisted too much for my taste after doubling them, but they turned out to be just stiff enough at 1:12. The rocker axles have always been the big problem. The hull and front bogies all want to do their own things, even at rest, and all that conflict translates into large and constantly changing torques and bending moments on the rocker axles. That's most evident in the wheel spread seen when the rover goes forward. I'd stoop to metal versions of LEGO(R) axles in a heart beat if I could get my hands them at comparable cost. Then I wouldn't have to put up with marginally acceptable kludges like pulling the front bogies together with rubber bands. Mechanical MOCs will always have their imperfections, but it's usually quite gratifying to see how well they actually work when I've done everything in my power to make up for all the limitations. (That's especially true of this rover and the trebuchet.) Granted, that often requires some knowledge of engineering and physics as they apply to LEGO(R), but certainly nothing that a metal worker wouldn't have to know. Most of all, though, it just takes a lot of patience and trial and error. I'm sure that metal working takes just as much patience, but the luxury of endless trial and error must surely be cost-prohibitive. In the end, however, the lion's share of the credit for whatever successes I may have had really goes to LEGO(R). Their engineering prowess with ABS and the genius behind the LEGO(R) metric never ceases to amaze.
 I like it 
  March 8, 2014
Holy Cow! Those are some serious computer and engineering skills. So impressive.
 I like it 
  March 7, 2014
You have officially freaked me out with your genius here! The holonomic wheel is truly inspired. Everything about this is inspired. I love how you explain everything in detail, it shows a wealth of understanding in the functionality and physical engineering involved. I am humbled by your work and have to wonder why you choose Lego as your medium. Not that it is a poor choice, but I would think that steel with nuts and bolts and welds would suit your purposes better. ABS has certain limits in its capacities as a viable structural material. Are the stresses negligible in creations like this to where it doesn't matter? Apologies if my questions seem base or rudimentary, but I am a metal worker and look at structure and stresses with an acute eye. Just simply a curiosity of mine. An interesting build with some innovative and outstanding techniques!
 I like it 
  March 7, 2014
Excellent job!! Very good use of technic!! 5/5
 I like it 
  March 7, 2014
Great!! (I will watch it later to see the pics ;-) )
 
By Jeremy McCreary
Add to my favorite builders

12
people like this. See who.

3,734 visitors
11 comments
Added March 7, 2014
 


LEGO models my own creation MOCpages toys shop Advanced PackBot (APB)Robotics


You Your home page | LEGO creations | Favorite builders
Activity Activity | Comments | Creations
Explore Explore | Recent | Groups
MOCpages is an unofficial, fan-created website. LEGO® and the brick configuration are property of The LEGO Group, which does not sponsor, own, or endorse this site.
©2002-2017 Sean Kenney Design Inc | Privacy policy | Terms of use