Motorised Folding Hip-Wing System

Scope and Purpose

This project is a prototype motorised folding wing system for a cosplay costume.

The goal is to build a pair of lightweight wings mounted near the hips that can:

Sit securely on a belt/harness structure.
Rotate outward from the hips using vertical hinges.
Fold open and closed using a cable-and-pulley mechanism.
Be driven by a 12V DC worm gear motor.
Be controlled by an Arduino Nano R4 and BTS7960 motor driver.

This is a working prototype, not a finished production mechanism. The electronics are relatively straightforward. The main engineering risk is mechanical: mounting the wings securely, choosing the correct cable attachment point, and preventing the belt/wing structure from twisting under load.

Project Priorities

Because this is being built under a short deadline, the priority order is:

Make the wings wearable.
Make the wings structurally secure.
Make the wings rotate outward from the hips.
Make the wings open and close under motor power.
Improve neatness, speed, and cosmetics only after the mechanism works.

A functioning prototype is more important than a perfect mechanism.

System Overview

flowchart LR
    Battery[12V Battery] --> Fuse[Fused Positive Line]
    Fuse --> Switch[Main Power Switch]

    Switch --> ArduinoVIN[Arduino Nano R4 VIN]
    Switch --> DriverPower[BTS7960 Motor Power]

    ArduinoVIN --> Arduino[Arduino Nano R4]
    Arduino --> DriverLogic[BTS7960 Logic Pins]

    DriverPower --> Driver[BTS7960 Motor Driver]
    Driver --> Motor[JGY-370 12V Worm Gear Motor]

    Motor --> Drum[6mm Bore Timing Pulley Used as Drum]
    Drum --> Cable[Bike Cable]
    Cable --> Pulleys[Guide Pulleys]
    Pulleys --> Linkage[Wing Linkage]

    Belt[Tactical Belt + Suspenders] --> HipPlate[Outer Rigid Hip Plate]
    HipPlate --> Hinges[Vertical Hinges]
    Hinges --> WingRoot[Wing Root]
    WingRoot --> Linkage

Materials and Components

Mechanical Components

Wing and Frame

Existing folding wing frame
White tube / pipe wing spars
Metal mending plates / strips
Bolts, washers, and nuts for pivots
Nyloc nuts or threadlocker
EVA foam or costume covering material

Belt and Hip Mounting

LUUFAN-style tactical MOLLE belt or equivalent
Shoulder strap / suspenders if available
BAKI commercial plastic cutting board, 18 x 12 x 0.5 inch, used as rigid HDPE hip-plate material
Large penny washers
M5 or M6 bolts
M5 or M6 nyloc nuts
Optional smaller inner backing plates
Optional rear brace:
- aluminium flat bar,
- steel mending strip,
- plywood strip,
- or spare HDPE strip.

Hinges and Rotation

Two small vertical hinges per wing if possible
Alternatively, one stronger vertical hinge per wing
Bolts, washers, and nyloc nuts for hinge mounting
Physical hard stops for open and closed wing positions
Rubber bumpers or washer stacks for stops if available

Cable and Pulley System

Bike cable / braided steel cable
Cable ferrules / crimps
Cable thimbles
Cable clamps if needed
30mm nylon U-groove ball-bearing pulleys
Heavier steel pulleys for prototyping if already available
20T 6mm bore aluminium timing pulley used as temporary motor drum
Small screws/bolts for pulley mounting

Electrical Components

Arduino Nano R4
BTS7960 / IBT-2 motor driver
JGY-370 12V DC worm gear motor, 6 RPM
12V battery source
Fuse holder
Suitable fuse
Main power switch
Push button
Limit switches
LEDs for testing
220Ω resistors for LED testing
22 AWG stranded silicone wire for signal wiring
Heavier wire for motor power, ideally 18–20 AWG
Heat shrink tubing
Terminal blocks or connectors
Solder
Soldering iron
Multimeter

Mechanical Build

Mechanical Design Principle

The belt does not directly carry the wing hardware.

The belt holds a rigid structure against the body. The rigid structure carries the hinge, motor, pulleys, and wing load.

The correct structural chain is:

Body / hips
↓
Tactical belt + suspenders
↓
Outer rigid hip plate
↓
Vertical hinge
↓
Wing root bracket
↓
Folding wing frame

The belt is for wearing the mechanism.

The rigid hip plate is what the mechanism actually mounts to.

Belt and Hip Structure

Use the plastic cutting board as the rigid hip-plate material.

Cut the board into two outer hip plates.

Suggested size per plate:

Height: 200-250mm
Width: 100-150mm
Thickness: 0.5 inch / about 12.7mm

Do not make the plates too small. Larger plates spread the load better and provide more room for:

hinge mounting,
motor mounting,
pulley mounting,
cable routing,
belt bolts.

The plates should sit on the outside of the belt.

Outside of body

[ hinge / motor / pulley hardware ]
          ↓
[ outer plastic hip plate ]
          ↓
[ tactical belt fabric ]
          ↓
[ large penny washers or inner backing plate ]
          ↓
[ nyloc nuts ]

Inside of body

Cutting the Hip Plates

Recommended tool:

Jigsaw
+
fine-tooth wood/plastic blade

Acceptable alternatives:

Hacksaw
Fine-tooth hand saw
Oscillating multi-tool

Avoid:

Stanley knife
Angle grinder
Circular saw unless experienced

Cut rough rectangles first. Do not waste time shaping them elegantly until the mechanism works.

Suggested process:

Mark two rectangles on the cutting board.
Clamp the board securely.
Cut slowly with a jigsaw or hacksaw.
Round sharp edges with sandpaper or a file.
Drill holes only after the plates are cut.

Attaching Hip Plates to the Belt

Each hip plate should be bolted through the belt.

Use four bolts per hip plate if possible.

Outer hip plate:

┌────────────────────┐
│  O              O  │
│                    │
│                    │
│  O              O  │
└────────────────────┘

Recommended hardware:

M5 or M6 bolts
Large penny washers
Nyloc nuts

Bolt stack:

Outside of body

Bolt head
↓
Outer plastic hip plate
↓
Tactical belt fabric
↓
Large penny washer or inner backing plate
↓
Nyloc nut

Inside of body

A smaller inner backing plate is better than washers alone because it spreads the load over more belt material.

Best version:

Outer large hip plate
↓
Belt
↓
Smaller inner backing plate
↓
Nuts

Minimum acceptable version:

Outer large hip plate
↓
Belt
↓
Large penny washers
↓
Nuts

Do not screw the hinge directly into the belt fabric.

Optional Rear Brace

If time allows, connect the left and right hip plates across the back.

Left hip plate ───── rear brace ───── right hip plate

This makes the belt behave more like a rigid waist frame.

Useful materials:

Aluminium flat bar
Steel mending strip
Plywood strip
Spare HDPE strip

This is strongly recommended if the wings are heavy or long, because it reduces twisting and sagging.

Hinge System for Outward Rotation

The wing should rotate outward from the hip using a vertical hinge.

The hinge pin must be vertical, like a door hinge.

Top view:

Body / hip
   │
   │ outer hip plate
   │
   O  vertical hinge pin
    \
     \
      wing swings outward

Use two hinges per wing if possible.

Side view:

Outer hip plate:

┌────────────────────┐
│  [ upper hinge ]   │
│                    │
│  [ lower hinge ]   │
│                    │
└────────────────────┘

Two hinges resist twisting better than one hinge.

The hinge structure should be:

Outer hip plate
↓
Vertical hinge
↓
Wing root bracket
↓
Folding wing frame

Hinge Stops

The hinge needs hard stops.

Without stops, the wing can rotate too far and damage the cable, linkage, belt, or wearer.

Minimum stops:

Closed stop:
Prevents the wing folding too far into the body.

Open stop:
Prevents the wing rotating too far outward.

Simple emergency stop options:

Bolt used as a stop peg
Rubber bumper
Washer stack
Small metal bracket
Wood/plastic block

The motor should never be allowed to pull indefinitely against a hard stop. Limit switches should eventually stop the motor before or at the stop point.

Pulley and Cable System

Cable Drive Principle

The motor does not directly move the wing.

The motor rotates a drum. The drum winds the cable. The cable pulls a moving linkage point on the wing. That movement opens the wing.

flowchart LR
    Motor[JGY-370 Motor] --> Drum[Timing Pulley Used as Drum]
    Drum --> Cable[Bike Cable]
    Cable --> RootPulley[Root Guide Pulley]
    RootPulley --> OptionalPulley[Optional Guide Pulley]
    OptionalPulley --> AttachPoint[Cable Attachment Point]
    AttachPoint --> Linkage[Wing Linkage]
    Linkage --> WingOpen[Wing Opens]

The most important rule:

Pulleys guide the cable.
The cable endpoint moves the wing.

Do not attach the cable at every bolt.

Do not put a pulley at every joint just because there is a bolt there.

Motor Drum

Use the 20T 6mm bore aluminium timing pulley as a temporary drum.

It should fit the JGY-370 motor shaft if the motor shaft is 6mm.

The timing pulley must be secured to the motor shaft using its grub screws.

Motor shaft
↓
6mm bore timing pulley
↓
Grub screws tightened onto shaft

The grub screws secure the pulley to the motor shaft.

They do not secure the bike cable to the pulley.

Securing the Bike Cable to the Drum

The cable must be fixed to the drum so that the drum winds the cable rather than slipping underneath it.

Do not rely on friction alone.

Prototype methods:

Method A: Drill-and-clamp

Drill small hole in pulley flange
↓
Pass bike cable through hole
↓
Clamp or crimp cable
↓
Wrap cable around pulley

Method B: Crimped loop

Bike cable
↓
Loop around a small bolt/hole on the drum
↓
Ferrule/crimp secures loop
↓
Cable wraps around pulley

Method C: Emergency temporary method

Cable through hole
↓
Zip tie or small clamp
↓
Temporary test only

Hot glue or epoxy can help for a temporary prototype, but it should not be trusted as the only load-bearing cable connection.

Pulley Placement

Start with the minimum number of pulleys.

Per wing, begin with:

Motor drum on fixed hip plate
↓
Root guide pulley on fixed hip plate
↓
Optional guide pulley if needed
↓
Cable attachment point on moving linkage

Use pulleys only where needed to:

redirect the cable,
stop the cable rubbing on the frame,
improve cable alignment,
prevent sharp cable bends.

Do not add unnecessary pulleys. Every pulley adds friction, weight, and another possible failure point.

Cable Attachment Point on the Wing

The cable should terminate on a moving inner/root linkage, not automatically on the wing tip.

The likely best area is:

Lower moving linkage near the wing root

However, this must be proven manually.

Use string first.

Manual test:

Close the wing.
Tie string to a candidate linkage point.
Pull the string from the direction the motor/pulley would pull.
Watch whether the wing unfolds smoothly.
Try another point if it binds or only moves the wrong section.
Mark the best point.
Only then attach the real bike cable.

Correct attachment point:

Pulling it opens the first/root section.
The linkage opens smoothly.
The required force feels manageable.
The wing does not twist badly.
The cable does not rub sharply.

Cable Attachment to Linkage

Preferred linkage-side attachment:

Bike cable
↓
Thimble
↓
Crimped loop
↓
Bolt through metal tab/control horn
↓
Moving linkage

If needed, add a small metal tab or control horn to the linkage.

A control horn is simply a small lever or tab that gives the cable a better pulling point.

Do not clamp the moving cable under multiple nuts along the route. The cable must slide freely through guides and pulleys. Only the final cable end should be fixed to the moving linkage.

Electrical Build

Electrical Design Principle

The Arduino controls the motor driver.

The Arduino does not power the motor directly.

flowchart LR
    Button[Button] --> Arduino[Arduino Nano R4]
    OpenLimit[Open Limit Switch] --> Arduino
    ClosedLimit[Closed Limit Switch] --> Arduino

    Arduino --> BTSLogic[BTS7960 Logic Inputs]

    Battery[12V Battery] --> Fuse[Fuse]
    Fuse --> Switch[Main Switch]
    Switch --> BTSMotorPower[BTS7960 Motor Power]
    Switch --> ArduinoVIN[Arduino VIN]

    BTSMotorPower --> BTS[BTS7960]
    BTS --> Motor[JGY-370 Motor]

Single Battery Power System

A separate Arduino battery module is not required for this design.

Use one 12V battery source.

Power split:

12V battery positive
↓
Fuse
↓
Main switch
↓
Split to:
  - Arduino Nano R4 VIN
  - BTS7960 B+

Common ground:

12V battery negative
↓
Common ground
↓
Connect to:
  - Arduino GND
  - BTS7960 B- / power ground
  - BTS7960 logic GND

Important:

All grounds must be common.
The motor must not be powered from the Arduino.
The BTS7960 handles motor current.
The Arduino only sends control signals.

Power Wiring Diagram

flowchart TB
    BatteryPlus[Battery +] --> Fuse[Fuse]
    Fuse --> Switch[Main Power Switch]

    Switch --> NanoVIN[Arduino Nano R4 VIN]
    Switch --> BTSPlus[BTS7960 B+]

    BatteryMinus[Battery -] --> GroundBus[Common Ground]
    GroundBus --> NanoGND[Arduino GND]
    GroundBus --> BTSMinus[BTS7960 B-]
    GroundBus --> BTSLogicGND[BTS7960 Logic GND]

Fuse Placement

The fuse should be placed on the battery positive line before the circuit splits.

Battery +
↓
Fuse
↓
Main switch
↓
Arduino + motor driver

Do not place the fuse only on the Arduino side. The motor driver and motor are the higher-current parts of the system.

Use a fuse appropriate to the motor current and wire size. Do not automatically use a huge fuse just because it fits the holder.

BTS7960 Wiring

Typical BTS7960 logic pins:

VCC
GND
RPWM
LPWM
R_EN
L_EN
R_IS
L_IS

Suggested Arduino wiring:

Arduino 5V  -> BTS7960 VCC
Arduino GND -> BTS7960 GND

Arduino D5  -> BTS7960 RPWM
Arduino D6  -> BTS7960 LPWM
Arduino D7  -> BTS7960 R_EN
Arduino D8  -> BTS7960 L_EN

Unused for first prototype:

R_IS
L_IS

Motor power side:

Battery + -> BTS7960 B+
Battery - -> BTS7960 B-

Motor wire 1 -> BTS7960 M+
Motor wire 2 -> BTS7960 M-

If the motor direction is reversed, either swap M+ and M- or invert the forward/reverse logic in code.

Button Wiring

Use the Arduino internal pull-up resistor.

Button leg 1 -> Arduino D2
Button leg 2 -> Arduino GND

With INPUT_PULLUP:

Button not pressed = HIGH
Button pressed     = LOW

Limit Switch Wiring

Use two limit switches:

Open limit switch
Closed limit switch

Simple prototype wiring:

Open limit switch:
One side -> Arduino D3
Other side -> GND

Closed limit switch:
One side -> Arduino D4
Other side -> GND

Use INPUT_PULLUP.

Logic:

Switch not pressed = HIGH
Switch pressed     = LOW

Recommended final improvement:

Use normally-closed limit switch wiring so that a broken wire can be detected as a fault.

For the rushed prototype, normally-open switches using INPUT_PULLUP are acceptable if tested carefully.

Test Code

Button-Controlled LED Test

Use this before connecting the motor.

const int BUTTON_PIN = 2;
const int LED_PIN = 5;

void setup()
{
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
}

void loop()
{
  const bool isPressed = digitalRead(BUTTON_PIN) == LOW;

  digitalWrite(LED_PIN, isPressed ? HIGH : LOW);
}

Expected behaviour:

Button not pressed -> LED off
Button pressed     -> LED on

Basic BTS7960 Motor Test

Use this before connecting the motor to the wing.

const int L_EN = 8;
const int LPWM = 6;
const int R_EN = 7;
const int RPWM = 5;

void stopMotor()
{
  analogWrite(RPWM, 0);
  analogWrite(LPWM, 0);
}

void runForward(const int speed)
{
  analogWrite(RPWM, speed);
  analogWrite(LPWM, 0);
}

void runReverse(const int speed)
{
  analogWrite(RPWM, 0);
  analogWrite(LPWM, speed);
}

void setup()
{
  pinMode(L_EN, OUTPUT);
  pinMode(LPWM, OUTPUT);
  pinMode(R_EN, OUTPUT);
  pinMode(RPWM, OUTPUT);

  digitalWrite(L_EN, HIGH);
  digitalWrite(R_EN, HIGH);

  stopMotor();
}

void loop()
{
  runForward(128);
  delay(3000);

  stopMotor();
  delay(1000);

  runReverse(128);
  delay(3000);

  stopMotor();
  delay(1000);
}

Expected behaviour:

Motor turns one direction for 3 seconds
Motor stops for 1 second
Motor turns the other direction for 3 seconds
Motor stops for 1 second

Integration Build

Correct Build Order

Do not connect everything at once.

Build in this order:

flowchart TB
    A[1. Cut hip plates] --> B[2. Bolt hip plates to belt]
    B --> C[3. Mount hinges to hip plates]
    C --> D[4. Attach wing roots to hinges]
    D --> E[5. Check outward wing rotation by hand]
    E --> F[6. Mount motor to fixed hip plate]
    F --> G[7. Fit timing pulley drum to motor]
    G --> H[8. Test motor and drum without wing]
    H --> I[9. Route string through pulley path]
    I --> J[10. Pull wing open by hand]
    J --> K[11. Replace string with bike cable]
    K --> L[12. Motorised cable pull test]
    L --> M[13. Add limit switches]
    M --> N[14. Full off-body open/close test]

Step 1: Build the Belt Structure

Cut two outer hip plates from the plastic cutting board.
Smooth sharp edges.
Put the belt on the wearer or a dummy form.
Mark where the plates should sit on the hips.
Drill four mounting holes per plate.
Drill matching holes through the belt.
Bolt each plate to the outside of the belt.
Use large washers or inner backing plates on the inside.

The plates should feel rigid and should not flap around independently.

Step 2: Add Hinges for Outward Rotation

Mount two vertical hinges to each outer hip plate if possible.
Make sure hinge pins are vertical.
Attach the wing root bracket/frame to the moving side of the hinges.
Test the wing rotating outward by hand.
Add a hard stop for the closed position.
Add a hard stop for the open/outward position.

The hinge should rotate like a door.

If the wing sags or twists badly, increase spacing between the two hinges or add a rear brace between hip plates.

Step 3: Mount the Motor

Mount the motor to the fixed outer hip plate, not to the moving wing if possible.

Preferred layout:

Outer hip plate
├── motor + drum
├── root guide pulley
├── vertical hinges
└── cable route to moving linkage

The motor should stay fixed relative to the belt.

The wing should rotate on the hinge.

Step 4: Fit the Drum

Slide the 6mm bore timing pulley onto the motor shaft.
Tighten the grub screws.
Check the pulley does not wobble.
Check the pulley does not slip on the shaft.
Rotate the motor slowly and watch the pulley.

Do not attach the wing cable yet.

Step 5: Test Cable Winding Without the Wing

Before using the real wing:

Attach string or temporary cable to the drum.
Run the motor briefly.
Check whether the drum winds the cable.
Check whether the cable slips.
Check whether the cable bunches up.

Fix cable slipping before connecting the wing.

Step 6: Manual Wing Pull Test

This is the most important mechanical test.

Use string first.

String from hand
↓
Root guide pulley
↓
Candidate attachment point on moving linkage

Pull by hand.

The correct attachment point is the one that opens the wing with the least binding.

Do not assume the wing tip is correct.

Likely starting point:

Lower moving linkage near the root

If it does not work, test another nearby moving linkage point.

Step 7: Replace String With Bike Cable

Once the manual string test works:

Create a crimped loop on the linkage end.
Attach the loop to a bolt, tab, or control horn.
Route the cable through the guide pulleys.
Attach the motor end to the drum.
Wind cable onto the drum by hand first if possible.
Keep the cable path clean and untangled.

Step 8: Motorised Wing Test

Run the motor at low PWM first.

Start with:

PWM speed: 70-100

Increase only if required.

Keep one hand near the power switch.

Stop immediately if:

cable slips,
wing binds,
belt twists badly,
motor stalls,
pulley bends,
hinge pulls away from plate,
cable starts cutting into plastic or fabric.

Step 9: Add Limit Switches

Add limit switches after the movement works.

Suggested locations:

Closed limit:
Pressed when the wing is fully folded.

Open limit:
Pressed when the wing reaches the desired open/outward position.

The motor should stop when either limit switch is reached.

Do not rely only on timing.

Recommended Control Logic

Final control should use a state machine.

stateDiagram-v2
    [*] --> Closed
    Closed --> Opening: Button pressed
    Opening --> Open: Open limit switch pressed
    Open --> Closing: Button pressed
    Closing --> Closed: Closed limit switch pressed
    Opening --> Fault: Timeout or emergency stop
    Closing --> Fault: Timeout or emergency stop
    Fault --> Closed: Manual reset

Recommended states:

CLOSED
OPENING
OPEN
CLOSING
FAULT

The motor should stop when:

the open limit switch is triggered,
the closed limit switch is triggered,
the main switch is turned off,
the motor runs too long,
the mechanism binds,
any unsafe condition occurs.

Safety Notes

Electrical Safety

Fuse the battery positive line.
Use a main power switch.
Do not power the motor from the Arduino.
Use the BTS7960 for motor current.
Connect Arduino GND and BTS7960 GND together.
Use thicker wire for motor power than for signal wiring.
Keep motor wiring and signal wiring tidy.
Do not use an oversized fuse.

Mechanical Safety

Test off-body first.
Keep fingers away from the linkage during powered tests.
Add hard stops.
Add limit switches.
Cover sharp cable ends.
Use nyloc nuts or threadlocker.
Check every pivot before powering the system.
Do not wear the mechanism until it has completed repeated off-body tests.

Wearable Safety

Before wearing:

Check the belt does not dig into the wearer.
Check the hip plates do not have sharp edges.
Check inner bolts/nuts are covered or padded.
Check the wings cannot hit the wearer.
Check the main power switch is reachable.
Check the battery is secured.
Check no cables can catch clothing, hair, or skin.

Current Minimum Viable Build

For the 2-day deadline, the minimum viable version is:

Tactical belt + suspenders
↓
Outer HDPE hip plates
↓
Vertical hinges
↓
Wing frame rotates outward
↓
Motor fixed to hip plate
↓
Timing pulley used as drum
↓
Bike cable routed through guide pulley
↓
Cable pulls moving linkage
↓
Arduino controls BTS7960
↓
BTS7960 drives motor

Minimum success condition:

The wings can be worn.
The wings can rotate outward from the hips.
The wings can open and close under controlled motor power.
The system can be stopped safely.

If motorisation fails under the deadline, the fallback is:

Keep the belt, hip plates, hinges, and wing frame.
Use manual cable pull or manual deployment.
Prioritise visual appearance for the costume.

Current Remaining Risks

The remaining high-risk areas are mechanical:

The correct cable attachment point has not been fully proven.
The wing may require more or less cable travel than expected.
The belt may twist without a rear brace.
The wing may sag if the hinge spacing is too small.
The motor may be too slow for dramatic deployment.
The timing pulley may work as a temporary drum but is not an ideal final cable drum.
The cable may slip if not properly fixed to the drum.
The mechanism may need elastic, spring, or manual assistance to close reliably.

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