MIY STUDIO · COBOT PROJECT 01
Building a 6-DOF Collaborative Robot from Scratch
#01 — Requirements & Joint Torque Sizing
Load modeling & the first actuator-sizing loop.
Design → Calculate → Build → Test → Repeat
6 DOF · 600 mm
Motion envelope
1.0 kg · 48 VDC
Load & power target
BLDC · CAN-FD · ROS2
Control stack target
01 / PROJECT INTENT
A robot as a complete engineering system
My starting point is mechanical robot design. This project expands that skill into actuator sizing, electronics, embedded control, ROS2, and system-level validation.
The target is not a one-off DIY arm. It is a traceable engineering portfolio where each major design choice can be tied back to a requirement, a calculation, or a test.
Make every design decision explainable.
02 / GEOMETRY
Start with the envelope before the components
I first defined a simple link envelope so actuator sizing could begin before detailed CAD.
Upper arm
250 mm
Forearm
250 mm
Wrist + tool
100 mm
Total reach
600 mm
Sizing dimensions, not frozen production geometry.
03 / LOAD MODEL
Mass distribution matters more than payload alone
The first model uses estimated masses and center-of-mass positions. These will later be replaced by CAD mass properties and measured values.
Component
Mass
COM from Shoulder
Upper arm
1.0 kg
0.125 m
Forearm
0.8 kg
0.375 m
Wrist
0.6 kg
0.525 m
Payload
1.0 kg
0.600 m
04 / STATIC TORQUE
The reference axis changes the answer
The basic relation is T = Σ mi g ri. The distance ri is always measured from the joint axis currently being analyzed.
SHOULDER
13.15 Nm
All downstream links
plus the payload.
ELBOW
6.03 Nm
Downstream forearm, wrist
and payload.
PAYLOAD ONLY
5.89 Nm
Payload at full reach.
Arm mass excluded.
05 / DYNAMIC TORQUE
Faster motion gets expensive quickly
For a 90° move with a symmetric triangular velocity profile, angular acceleration follows α = 4θ / t2, and the peak speed is ωmax = αt / 2.
1.0 s MOVE
α = 6.28 rad/s2
ωmax ≈ 30 rpm
Tinertia = 4.16 Nm
17.31 Nm
Preliminary peak
0.5 s MOVE
α = 25.13 rad/s2
ωmax ≈ 60 rpm
Tinertia = 16.66 Nm
29.81 Nm
Preliminary peak
Why the jump? Halving move time makes α 4× larger because α ∝ 1/t2. Since Tinertia = Iα, the dynamic torque component rises by the same factor.
06 / ENGINEERING TOOL
Turn the equations into something reusable
I converted the first sizing model into Python so link dimensions, masses, and move time can be changed without repeating every hand calculation.
- Gravity torque
- Link inertia
- Triangular motion profile
- Preliminary peak joint torque
Not yet included: multibody coupling, RMS torque, thermal model, reflected inertia, reducer limits.
SIZING PIPELINE / PYTHON
01 Mass + geometry
02 Gravity torque
03 Link inertia
04 Motion profile
05 Inertia torque
06 Preliminary joint peak
NEXT / ACTUATOR ARCHITECTURE
From joint requirement to motor + reducer
The next design loop converts joint-side torque and speed into motor-side requirements and real component candidates.
01
Gear ratio & efficiency
02
Motor torque & speed
03
Kt / Ke / current / voltage
04
Thermal & continuous load
All values shown here are preliminary engineering estimates for early-stage sizing and will be refined as CAD mass properties, selected component data, and test results become available.