Digital Twin Series · Level 2 · Case study
Conveyor and Rotary-Table Digital Twin
Can I size the motor, gearbox and timing of an indexing table before I build it?
A conveyor feeds pallets to a servo-driven rotary table that indexes them from station to station. The twin models the mechanics and the control together: it reflects the load inertia through the gearbox, sizes the motor for the index move, finds the stopping distance on the conveyor, predicts how long the table takes to settle, and checks that the conveyor, table and robot never collide in time. The dimensions, masses and efficiencies on this page are assumptions for teaching, not data for a real machine: replace them with your own.
The motor sees the load inertia divided by the gear ratio squared, so a larger ratio makes the load feel much lighter, but the motor then has to spin faster and its own inertia costs torque. The curve has a lowest point where the reflected load inertia equals the motor inertia. Guidance on the inertia ratio differs: about 3:1 for fast, precise moves and 10:1 or more where the demands are lower, so treat the colours as a guide. The masses, radii and efficiency are assumptions; take yours from the CAD model.
The index move is simulated for the chosen profile. Torque at the motor is the inertia torque (inertia x acceleration) plus friction, divided by the gear ratio and efficiency, plus the motor rotor inertia torque. Peak torque must be inside the motor and drive limit, and the RMS torque over the whole cycle (including the dwell) inside the rated torque with a safety factor. Pick a real motor from a data sheet and check both.
Drive pull is friction times the weight of everything on the conveyor. Stopping distance is s = v squared divided by two times the deceleration, as in the formula in the case. If the stopper is closer than that distance, the pallet arrives with speed. The friction, mass and deceleration are assumptions; measure yours.
After the move stops, the drivetrain rings at its natural frequency and dies away with its damping. The pallet can be worked on once the ringing is smaller than the accuracy the process needs. Backlash and the load torque acting on the drivetrain stiffness leave a fixed error that no waiting removes. Stiffness, damping and backlash are assumptions; they need to be measured or taken from the data sheets.
The units must not overlap in the wrong way. The robot must not enter before the table has settled, and the table must not move before the robot is clear. A negative setting below means starting early, and the timeline shows the collision. The timings are the ones from the sections above plus the ones you set here.
These four charts come from the settings in the sections above. Change them there to see all four respond.
| Stage | Tools shown in the case | What they do here |
|---|
The tools are examples, not recommendations. Choose by what your team owns and what your machines and PLCs support.
The simulators are simplified teaching models. Size and select real components from manufacturer data and review safety with a qualified engineer.