Team 2822 Concept

Crane

Vertical Lift · Six-bar parallelogram + level manipulator

🧪 Concept · Exploring Override 2026–27

Crane retro poster — vintage halftone illustration of a crane

// LINEAGE FROM V.1

From V.1

Spartan Hero Bot V1.0 chassis-class drivetrain and toggle architecture.

Changed

Lift → six-bar parallelogram.

Because

V2.0 exploration — level-throughout reach for cup placement precision.

Evidence

Geometric study; comparison against four-bar (Pelican) and chain-bar (Osprey) baselines.

📛 Why "Crane"

The crane bird's long neck rises straight up to strike at fish — vertical reach delivered by articulated segments stacking. The six-bar lift's compound parallelogram does exactly that: a single drive motor delivers nearly twice the vertical sweep of a four-bar at the same arm length. The construction-crane double meaning lands the engineering reference instantly — a crane in the field is literally a multi-bar boom. Bird family with Heron, Stork, Osprey, and Falcon. Predator, articulated, single-purpose: lift.

What it is Architecture Reach envelope Motor budget Torque Manipulator analysis Fleet matrix CAD starting point Build sequence Open questions Port map Build log

What this is

A six-bar parallelogram lift — effectively two stacked four-bar segments sharing a common middle arm, driven by a single motor at the bottom pivot. The end-effector platform stays level throughout the motion (parallelogram constraint) and reaches roughly 2× the vertical sweep of an equivalent-length four-bar. Highest single-mechanism vertical reach in the fleet.

Why it earns a fleet slot

Crane fills the "tall vertical reach without articulating-arm complexity" niche. Falcon's 4-DOF arm reaches further but is harder to drive; Heron's stacked four-bar + chain bar reaches similar heights but requires tuning two mechanisms. Crane is the simplest path to a 26-30″ vertical reach with a level-preserving end-effector. The construction-crane analogy makes the architecture intuitive for new team members and judges.

Architecture geometry

Side view — six-bar at three positions

Three positions: collapsed at rest (dashed gray, lift folded over chassis), mid-extension (orange, short goal scoring height), full extension (green, ~28″ vertical reach above field tile). Cyan rectangle is the manipulator at end of each lift state.

Approximate dimensions for prototyping

Subsystem	Dimension	Notes
Tower	3″ tall (chassis-top to bottom-pivot of lift)	Lower the better; minimizes 6-bar arm length needed
Lower 4-bar arms	10″ each, parallelogram height 3″	Bottom segment of the six-bar
Middle pivot link	Shared between bottom and top segments	This is what makes it a six-bar vs. just stacked four-bars
Upper 4-bar arms	10″ each, parallelogram height 3″	Top segment; identical geometry to lower
End-effector platform	3″ × 4″ × 0.090″ Al	Universal manipulator mount (same pattern as Falcon/Heron/Skimmer)
Total vertical reach	26-30″ above field tile	Tower + 2× (4-bar height) + manipulator = 3 + 24 + 4 = ~31″ at full extension
Drive method	Single 11W Red 100 RPM at the bottom pivot, 1:7 sprocket reduction + rubber-band assist (8 × #64 bands)	One motor lifts both stacked segments via the parallelogram constraint

Reach envelope vs. fleet

Crane's vertical-reach advantage is the architecture's whole pitch — it ties Heron for tallest in the fleet but with a single mechanism instead of two. This SVG overlays Crane's workspace shape against the rest of the fleet's, drawn to the same scale.

Reach envelope overlay — Crane in fleet context

All envelopes drawn to same scale (1″ = 9 px). Crane (cyan) is the tallest narrow column; Heron (lime) reaches similar height but spreads wider at the top via chain bar; Falcon (green) sweeps a semicircle but tops out lower; Osprey (orange) is the smallest envelope.

📊

Crane's distinguishing trait isn't peak reach — Heron ties it — but the simplicity of getting there. Heron's 28″ requires sequencing two mechanisms (lift + chain bar). Crane's 28″ requires moving one mechanism. For driver cognitive load and programming complexity, Crane wins by quite a lot. The trade is that Heron can also reach forward (the chain bar extends horizontally) where Crane mostly goes up.

Motor budget — with toggle allocated

Crane motor budget: Drive ........ 5 \times 11 W Blue ........ 55.0 W Six-bar lift . 1 \times 11 W Red ......... 11.0 W Manipulator .. 1 \times 5.5 W (claw or tube rotation) .. 5.5 W Toggle ....... 1 \times 5.5 W (flex wheel) ............ 5.5 W Total .................................. 77.0 W Spare .................................. 11.0 W Cap (R10a) ........................... 88.0 W Could substitute pneumatic pincers for claw/tube to save 5.5 W more, freeing 16.5 W spare \to upgrade to 6-motor drive (66 W).

Crane has the most motor headroom in the fleet (with claw or tube) because the lift itself uses only one motor. That spare can become a 6-motor drive (with pneumatic pincers manipulator) or keep two ports free for sensors.

Torque analysis — six-bar load sizing

Crane's six-bar carries the carriage + manipulator + payload through a vertical sweep with a slight horizontal arc. The worst-case torque on the drive motor is when the lift is at mid-height with the carriage extending forward — the moment arm is at maximum, and the entire stacked mass is hanging off the bottom pivot.

Worst-case load — six-bar at mid-extension, full forward sweep

Worst case is mid-extension (45° lower segment, 45° upper segment) where the cup's horizontal moment arm from the bottom pivot is at maximum. At full vertical extension the moment arm collapses to nearly zero (carriage directly over pivot). The 14″ moment arm is the design constraint.

The math

Six-bar torque demand at mid-extension: Cup payload .................. 0.5 lb at 14" ......... 7.0 lb-in Carriage + manipulator ....... 0.6 lb at 13" ......... 7.8 lb-in Upper segment self-weight .... 0.5 lb at 10" (CG) .... 5.0 lb-in Middle link .................. 0.2 lb at 7" .......... 1.4 lb-in Lower segment self-weight .... 0.5 lb at 4" (CG) ..... 2.0 lb-in Total τ_demand .............................................. 23.2 lb-in 11W Red 100 RPM with 1:7 sprocket reduction: Stall torque (output-side) .............................. 98 lb-in τ_demand / τ_stall = 23.2 / 98 = 24% \to right at thermal threshold ───────────────────────────────────── With rubber band assist (8 \times #64 @ ~10 lbf, 6" perpendicular): τ_assist (peak) = 10 \times 6 = 60 lb-in τ_assist (effective at mid-extension, geometry-derived) ~ 25 lb-in Net motor torque required: τ_net = 23.2 - 25 = ~0 lb-in (assist exceeds demand at mid-extension; motor only controls position) Motor utilization with assist: ~5% peak \to cool, smooth control through full range

💡

Crane's torque profile is friendlier than Heron's because the six-bar's effective moment arm shrinks as the lift extends vertically (cup ends up directly over the pivot at full extension). Heron's stacked architecture has the chain bar always extending past the four-bar's tip, so the moment arm is always large. Crane's natural geometry helps; Heron's hurts. Both still need rubber band assist for smooth control, but Crane's assist sizing is less critical — the design can tolerate a 50% under-sized band stack and still work.

Manipulator analysis — tube wins for Crane

Like Skimmer, Crane has only one orientation DOF (the level-preserving parallelogram constraint). The polycarb tube's rotation feature is uniquely valuable here because there's no wrist or chain bar providing alternative orientation control — this is the same reason it scored highest on Skimmer's matrix. Pneumatic pincers wins on cycle time but loses on element flexibility.

Dimension	V5 Claw	Pneumatic Pincers	Polycarb Tube
Cycle time grip → release per element	3 ~600 ms motor-limited	5 ~150 ms instant pneumatic	4 ~300 ms cinch + orient
Build difficulty hours, parts count, R-rule risk	5 stock VEX, ~2 hrs	3 custom jaws + plumbing, ~6 hrs	1 R24 fab + plumbing + drive, ~12 hrs
Programming difficulty PID + state machine	3 claw PID + lift PID + toggle	4 digital out + lift PID + toggle	3 cinch + tube rotation + lift + toggle (4 axes)
Driving ease cognitive load on driver	3 3 controls (lift, claw, toggle)	5 3 controls; pincers binary	3 4 controls (lift, cinch, rotate, toggle)
Element flexibility cup / pin / combo	3 pin OK, cup OK, combo iffy	4 all three with shaped jaws	5 all three; cinch holds combo
Vertical-reach fit deposit at 28″ height	3 claw needs precise alignment at height	3 jaws same constraint	5 tube can drop element from height (gravity-assisted release)
Total (/30)	20	24	21

🎯

Recommendation: pneumatic pincers (24/30) by a narrow margin over polycarb tube (21/30). The vertical-reach-fit row is what makes the tube competitive — depositing from a tall position favors a tube's gravity-release over a claw's precise alignment. But the build-difficulty penalty for the tube (R24 fab + plumbing + drive) is substantial. If Crane ever progresses past concept, start with pincers; tube becomes a worthwhile upgrade once basic scoring is proven.

Decision matrix — Crane vs. fleet

With each robot's recommended manipulator (pincers for Crane per the matrix above), where does Crane sit overall?

Dimension	Osprey chain bar + claw	Crane six-bar + pincers	Stork DR4B + pincers	Heron stacked + pincers
Goal coverage	3	5 tallest reach in fleet	4	5
Endgame compactness	4	2 9″ stowed; bars stack tall	5	3
Build complexity	4	3 six-bar geometry tunable but more parts than 4-bar	2	2
Driving cognitive load	4	4 3 controls (lift, pincers, toggle)	3	3
Notebook story	3	4 six-bar geometry math + construction-crane analogy	5	5
Risk of failure (higher = lower risk)	5	4 single mechanism, well-understood kinematics	2	2
Total (out of 30)	23	22	21	20

🎯

Crane scores 22/30 — tied for second in the fleet matrix, behind Osprey's 23. The simplicity premium that lets Osprey lead is exactly what Crane partially recovers: single-mechanism operation, well-understood kinematics, manageable driver controls. Crane earns its fleet slot when "tall vertical reach without two-mechanism complexity" is the strategic goal — otherwise Osprey is still the safer pick.

Reach summary table

Robot	Lift type	Vertical reach	Build hours	Status
Osprey	Chain bar (arc)	~22″	~20	Reference baseline
Skimmer	2-bar swing arm	~20″	~30	Active build (2822A)
Falcon	4-DOF arm	~25″ flexible	~40	Concept
Heron	Stacked 4-bar + chain bar	~28″	~50	Concept
Crane	Six-bar (vertical)	~28″	~35	Concept (this page)
Stork	DR4B (folding)	~26″	~40	Concept

CAD starting point

If your team commits to prototyping Crane, here are the dimensions to start with. Six-bar geometry is forgiving — small variations in bar length still produce working lifts — so consider these conservative starting points and expect to iterate by ±1″ during build.

Subassembly dimensions

CHASSIS18″ × 18″ × 4″ aluminum 1×2 box rail
DRIVE5 × 11W Blue 600 RPM, 5:3 reduction, 4″ omni
TOWER (LIFT BASE)3″ tall, centered laterally on chassis
LOWER 4-BAR ARMS10″ pivot-to-pivot, 3″ parallelogram height
MIDDLE PIVOT LINK2″ × 4″ × 0.090″ Al; shared between bottom and top segments
UPPER 4-BAR ARMS10″ pivot-to-pivot, 3″ parallelogram height (matched to lower)
CARRIAGE2″ × 3″ × 0.090″ Al with universal #4-40 hole pattern (same spec as Heron / Stork / Falcon)
SIX-BAR DRIVE11W Red 100 RPM at the bottom pivot, 1:7 sprocket reduction (12T → 84T)
RUBBER BAND ASSIST8 × #64 bands distributed across both segments (sized for cup + pin combo at full extension)
TOGGLE MECHFlex-wheel toggle on side of chassis, 5.5W half-motor; engages toggle elements at floor level during normal drive

OnShape document structure

Part Studio 1 — Chassis & Tower: chassis frame, drive motor mounts, lift-base tower, battery and brain locations.
Part Studio 2 — Lower segment: lower-arm pair, parallelogram links, drive sprocket, lower-arm pivots.
Part Studio 3 — Middle pivot + Upper segment: middle pivot link, upper-arm pair, parallelogram links, carriage plate.
Part Studio 4 — Toggle mech: flex-wheel mount, motor, support arms.
Assembly — Crane: mate all part studios + selected manipulator (use the polycarb tube model from /polycarb-tube-onshape-guide as one of the manipulator options).

Build sequence — if you commit to Crane

Phase 1 — Chassis + drivetrain (week 1). Standard rolling chassis. Drive practice can begin immediately.
Phase 2 — Tower + lower segment only (week 2). Build the tower and lower 4-bar segment. Tune the drive motor's 1:7 reduction and PID at three setpoints. Stop point check: lower segment lifts smoothly to ~13″ with rubber band assist; doesn't sag at full extension.
Phase 3 — Middle pivot link installation (week 2-3). Mount the middle pivot link to the lower segment's end. Verify the geometry holds the link level through the lower segment's sweep. Stop point check: middle link tracks parallel to the floor at every position of the lower segment.
Phase 4 — Upper segment installation (week 3). Mount the upper 4-bar segment to the middle pivot link. The upper segment's lower-arm pivots are on the middle link's top edge. This is the make-or-break phase: the upper segment's geometry must mirror the lower's exactly, or the carriage will tilt as the lift extends. Stop point check: with the lower segment held at multiple positions, the carriage stays level via the upper segment's parallelogram constraint.
Phase 5 — Manipulator + toggle integration (week 4). Mount the chosen manipulator on the carriage. Mount the flex-wheel toggle on the chassis side. Test full pickup-place cycles plus toggle activations. Stop point check: 100 cycles loader → tall goal → loader without dropping a payload.
Phase 6 — Driver practice + iteration (weeks 5–7).

⚠

Phase 4 upper-segment geometry is where six-bar builds typically fail. Tilted carriage means the manipulator drops payload off the front edge during deposit. If Phase 4 takes more than 6 hours of fiddling, the bar lengths are probably mismatched — remeasure with calipers, both segments must be identical to within 1/32″. Don't skip this check; "close enough" doesn't work for parallelogram lifts.

Open questions

Six-bar geometry verification. The vertical reach math assumes both segments are 10″ at 3″ parallelogram height. Run the 2D kinematic plot to confirm the end-effector reaches 28″+ at the geometric maximum.
Rubber-band sizing. A six-bar at full extension has the manipulator + payload at a long moment arm. Calculate the required assist torque (similar to Falcon's analysis at ~12 lb-in demand).
Tower-fold collision. When collapsed, do the upper bars clear the chassis? May need an offset tower or bent arms.
Manipulator pick. Pincers per the matrix, but the polycarb tube's gravity-release advantage at full height is worth prototyping.

Concept sketches

To be added during prototyping. iPad sketches showing the six-bar geometry at multiple positions, rubber band routing, and tower-fold collision check.

CAD exploration

To be added once CAD prototyping begins. OnShape document link with all 4 part studios.

Decision log

To be filled in. Each significant design decision and the reasoning — especially the bar-length matching tolerance from Phase 4, since that's the failure-prone subsystem.

Port map (template)

Pre-allocated port assignments for Crane's planned motor layout. If the team commits to building Crane, copy this table into robot-config.cpp and update as wired.

Port	Subsystem	Motor / sensor	Notes
1	Drive front-left	11W Blue 600 RPM	5:3 reduction · 4″ omni
2	Drive front-right	11W Blue 600 RPM	reversed
3	Drive mid-left	11W Blue 600 RPM	—
4	Drive mid-right	11W Blue 600 RPM	reversed
5	Drive back-center	11W Blue 600 RPM	OR 6th drive if pincers manipulator (Crane has 11 W spare)
6	Six-bar lift	11W Red 100 RPM	1:7 sprocket reduction · rubber band assist (8 × #64)
7	Manipulator	5.5W half-motor (claw or tube rotation) OR pneumatic out (pincers)	Pincers frees this port
8	Toggle	5.5W half-motor	Flex-wheel toggle on side of chassis
9–21	Spare	—	Reserved for sensors and post-swap expansion
ADI A	Limit switch	—	Six-bar bottom-position reference
ADI B	Pot V2	—	Six-bar position feedback (alternative to motor encoder)

Build log (template)

Each build session adds an entry: date, team members, what was attempted, what worked, what didn't, decisions made.

Template entry:
2026-MM-DD · Team members: ___ · Phase: ___
What we attempted: ___
What worked: ___
What didn't: ___
Decision made: ___
Next session focus: ___