Drawbridge Task

Torque

Introduction to Torque

Why is it easier to push a door open at the handle rather than the hinge? This lesson dives into Torque (also known as the moment of force)—the measure of a force's tendency to produce rotation. We will explore how the magnitude of force, the angle of application, and the distance from the pivot point all work together to create rotational motion.

PowerPoint: Covers key definitions, the torque formula (T = F x D), and real-world examples like wrenches and seesaws.
Worksheet: Practice problems for calculating torque and determining rotational equilibrium.

Don't forget to watch the videos above to help your understanding. Teacher Notes

Torque Presentation

Truss Structures

What is a Truss?

Trusses are structural elements that can carry loads with relatively long spans compared to beams. Trusses are characterized by having tensions and compression members. These structures are often used in roof, floor and bridge structures.

Different kids of truss

Triangles are the strongest shape. We will look at different truss designs (Pratt, Warren, Howe).

Information about Trusses

See how far you can get at polybride, keep tabs as a class who gets the furtherest. Make sure you use the skills that you have learned above to build your bridges. We are only going to do this for one lesson

Teacher note: If this is blocked at your school see if you can find one elsewhere or get it unblocked for this lesson

LINK to the GAME

Bridge Build

Welcome to the build phase! Over the last few lessons, we explored the physics of Torque (how force causes rotation) and the engineering behind Truss Structures (how triangles distribute stress). Now, we are putting that theory into action.

In this project, you are engineering a moving machine. Your challenge is to design and construct a bridge that is rigid enough to hold its shape, but lightweight enough for a servo motor to lift. You must carefully balance structural integrity against the limited torque of your motor—if the bridge is too heavy or the center of mass is too far from the pivot, the physics won't work!

The Brains (Servo & Code)

Before building the bridge, we must ensure our motor works and understand its limits.

1. Wiring the Servo Connect your servo motor to your microcontroller board using dupont cables.

Brown/Black Wire: Ground (GND)
Red Wire: Power (5V or VCC)
Orange/Yellow Wire: Signal (Pin specified in code)

Servo Code For Picaxe

Upload the Code Copy and paste the code provided below into your IDE. Upload it to the board.

Option A: Simple Test (Up and Down)

This moves the bridge immediately. It's jerky but good for testing if your wiring works.

In these sets of code above, I used the values 100 and 200.

PICAXE servos usually work between 75 (far left) and 225 (far right).
150 is the exact center.
Once you build the bridge, you will need to adjust these numbers slightly so the bridge lays perfectly flat (maybe 95?) and stands perfectly straight (maybe 180?).

Option B: Smooth "Hydraulic" Motion (Best for Bridges)

Real drawbridges don't snap open instantly. This code uses a loop to move the bridge slowly. It looks much cooler and puts less stress on your glue joints.

Other Microcontrollers code

Test the Movement

You should see the servo "horn" (the white plastic arm) rotate. If it jitters or doesn't move, check your power connections.

The Rule: A standard micro-servo is not very strong. It cannot lift a heavy log, but it can lift a lightweight truss!

The Anchor (The Start Piece)

Goal: Create a secure connection point between the motor and the wood.

Start by preparing the critical link between your code and your structure. Take the plastic servo "horn" (the small arm included with your motor) and glue it firmly to a single, high-quality popsicle stick to create your Start Piece.

Once the glue is dry, ensure your servo is programmatically set to the 0-degree position, then screw the horn onto the motor gear so the stick points horizontally.

Finally, run your code to test the movement; the stick should lift smoothly without hitting the table or the motor body.

Engineering Note: The Moment Arm As you build outward from this start piece, physics works against you. A 10-gram weight placed 2.5 cm from the servo is easy to lift. That same 10-gram weight placed 25 cm away is 10x harder to lift.

The Span (Choose Your Design)

Goal: Design and build a bridge structure attached to your anchor.

Before you glue, you must choose a truss design. This is a trade-off: More sticks = More weight. You need to balance structural strength with the ability of your motor to actually lift it. If you are using a SG90 Microservo you are able to lift about

Option A: The Warren Truss (Recommended)

The Look: A series of equilateral triangles (looks like Ws).
Pros: Uses the fewest sticks. Extremely lightweight.
Cons: Slightly less rigid if joints aren't glued perfectly.
Verdict: Best for this project. It is light enough for the servo to lift easily.

Option B: The Pratt Truss

The Look: Vertical beams with diagonal braces slanting down toward the center.
Pros: Very strong and rigid. Good for holding heavy weights on the bridge.
Cons: Uses more sticks (verticals + diagonals).
Verdict: Risky. It might be too heavy for the servo unless you are very careful with glue.

Build Instructions

Sketch: Draw your chosen pattern on paper 1:1 scale.
Frame: Lay out your top and bottom chords (the long horizontal sticks).
Fill: Cut and glue your pattern (Triangles or Verticals) between the chords.
Integrate: Glue your finished truss onto the Start Piece from the anchor.

Don't double-layer your sticks unless absolutely necessary. A single layer with strong joints is much lighter!

Final Assessment

The Weight Test

Place weights in the center of the bridge while it is closed. Does it hold the required load?

The Operation Test

Run your PICAXE program.

Does the warning light come on?
Does the bridge lift fully?
Does it close gently?

Student Reflection

Torque Analysis: Did your servo struggle? If you had to do it again, would you make the bridge lighter or the lever arm shorter? (put any answers in your research document)
Code Review: Paste your PICAXE code in your research document and explain how it works.
Photo: Put a photo of your bridge into your research document

Placeholder image

Things to consider over the project

PICAXE Power: Servos drain a lot of current. If the PICAXE chip resets when the servo moves, the batteries are sagging. Solution: Use a separate battery pack for the servos (connecting the Grounds together) OR ensure fresh batteries are used.

The Torque Trap: you will likely build heavy bridges that the small servos (like an SG90) cannot lift.(1.98 kg/cm)

Calculate the torque. If the bridge is too heavy, they must add a Counterweight behind the pivot point to help the servo!
Extend the bridge beams behind the pivot point. Add weight (washers, clay, heavy bolts) to this rear section.

Encourage students to cut small triangles or rectangles out of card or thin wood. Glue these "plates" over the joints where truss members meet. This increases the surface area for the glue and drastically improves strength.

Advanced concepts for experts

Tension vs. Compression (The Drawbridge Flip)

This is a great high-level concept to check with students.

Bridge Closed (Static Span): When the bridge is resting across the gap, the Top Chord is in compression (squeezing) and the Bottom Chord is in tension (stretching).
Bridge Opening (Cantilever): As soon as the bridge lifts, it becomes a cantilever. The forces flip! The Top Chord is now in tension and the Bottom Chord is in compression.
Ensure the truss design has diagonal bracing that can handle both forces, or the bridge might twist when it lifts.

Page updated

Google Sites

Report abuse