Nicole KirkBlock 01 / 06

The Very Hungry Caterpillar

A two-joint robot arm dressed as the caterpillar from Eric Carle’s picture book, driven by a joystick.

EMS511U · Queen Mary University of London · Group B7

The arm

Keep scrolling to take it apart

Render of the arm: a green faceted tree on a brown trunk, the caterpillar arm reaching out to the right with its red head raised.

The tree spins. A wheel inside the canopy turns the whole arm.

The trunk is hollow and hides the electronics.

A floating motor hinge carries the arm.

The caterpillar’s head rides at the tip.

Canopy and wheel
Housing, top
Housing, base
Floating motor hinge
Body segment
Head
Analog joystick
MDF base and wooden dowels
01Canopy and wheelIntegrated rotational mechanism. The wheel gives a controlled spinning motion.
02Housing, topCompact internal housing, with room to hide the components inside.
03Housing, baseModular assembly, for ease of use.
04Floating motor hingeThe joint the arm swings from.
05Body segmentPlayful form factor, for user engagement.
06HeadThe tip of the arm.
07Analog joystickIntuitive, proportional control over both motors’ direction.
08MDF base and wooden dowelsWhat it all stands on.
  1. 01Canopy and wheelIntegrated rotational mechanism. The wheel gives a controlled spinning motion.
  2. 02Housing, topCompact internal housing, with room to hide the components inside.
  3. 03Housing, baseModular assembly, for ease of use.
  4. 04Floating motor hingeThe joint the arm swings from.
  5. 05Body segmentPlayful form factor, for user engagement.
  6. 06HeadThe tip of the arm.
  7. 07Analog joystickIntuitive, proportional control over both motors’ direction.
  8. 08MDF base and wooden dowelsWhat it all stands on.

How it moves

Coordinate frames for the arm: the base frame, link L1 turning by theta 1 about the trunk, link L2 turning by theta 2.

θ2 lifts the second link.

θ1 turns the whole arm about the trunk.

Forward kinematics

x1=(L1+L2cosθ2)cosθ1 y1=L2sinθ2 z1=(L1+L2cosθ2)sinθ1

Inverse kinematics

q1=atan2(y,x) q2=atan2(z,x2+y2)

L1 140 mm · L2 150 mm · both joints 360°

Top view of the workspace: two overlapping circles swept by link L2 around the end of link L1.
Everything the head can reach, seen from above.
The Simulink model: a target point goes through an inverse kinematics block, and the angles come back out through a forward kinematics block.
Checking it in Simulink: the target goes in through the inverse kinematics and comes back out through the forward kinematics, landing within a few millimetres.

Demo day

The finished prototype on a workshop bench, wired to a bench power supply and laptops, with a hand on the joystick.

The bench supply. On demo day it was left at 12 V instead of 6 V.

Motor 2 accelerated fast enough to rip its own wires. The robot had been fully working throughout testing. It broke half an hour before we were due to present.

A voltage saturation block in the Simulink model would have prevented this entirely.