The drive sizing question, asked properly
"What drive should I use?" doesn't have a single answer because "drive" covers three quite different technology families. Servo, stepper, and VFD overlap in capability bands and are not interchangeable in the bands where they don't. Picking by catalog number rather than by the motion profile is the source of half the field issues we get called in to fix.Servo (closed-loop, position-controlled)
Best for: precise positioning, dynamic motion profiles, high acceleration, coordinated multi-axis. Anything where the load characteristic varies and the motion has to compensate in real time.Sizing properly: inertia ratio (load reflected to motor) under 5:1 for stiff cases, under 10:1 for soft cases. Above that, gain-tuning becomes a fight, resonance shows up, and the application performs worse than a smaller, better-matched servo would.
Brands we ship most: Beckhoff AX, Siemens Sinamics S210, ABB MicroFlex, Mitsubishi MR-J5. The servo market in 2026 is mature; differentiation is in the controller integration story, not the motor itself.
Stepper (open-loop or closed-loop)
Best for: simple, repeatable point-to-point, low cost, fixed loads. Open-loop steppers are the right answer for budgets that don't justify a servo and motion profiles that don't push the speed-torque curve.Sizing properly: always with at least 50 % torque margin. Steppers stall silently — no encoder, no fault, just wrong position. Closed-loop steppers (with a glass scale or magnetic encoder) close that gap but cost approaches a small servo.
Where we use them: indexing tables at known load, label dispensers, simple positioners on test rigs. Anywhere the failure mode of "lost steps under unexpected load" is acceptable or unlikely.
VFD (variable frequency drive, induction motor)
Best for: continuous rotary motion, conveyors, fans, pumps. Anywhere the goal is "spin at speed X" rather than "position at angle θ".Sizing properly: motor + drive matched, with overload margin for break-away torque. Vector mode (sensorless or with encoder) for any application where load varies; V/Hz for the truly simple ones.
Brands: ABB ACS, Siemens Sinamics V20/G120, Yaskawa GA500. Pricing tiers and feature differences are real but rarely the deciding factor.
The decision tree
- Continuous rotary, just speed control → VFD. Always.
- Position control, ≤500 rpm, low cost, predictable load → stepper (open or closed loop based on cost/criticality).
- Position or velocity control, dynamic load, high accel, multi-axis coordination → servo.
- Stiffness or bandwidth requirement above ~50 Hz → servo, with a properly sized inertia ratio.
- Synchronous multi-axis (CNC-style) → servo on EtherCAT or Profinet IRT.
The motion profile, not the catalog spec, is the input
Sizing properly means starting from the motion profile:- What's the move time?
- What's the dwell time between moves?
- What's the load inertia? (CAD-derived, not estimated.)
- What are the friction and gravity components of torque?
- What's the duty cycle (RMS torque vs peak torque)?
Plug those into the drive selection tool. The output is the drive that will work — not the drive the catalog recommends for "your application class". We've inherited too many cells where the drive was 2x the size needed (cost waste) or 0.7x the size needed (overheats in three months).
EtherCAT vs Profinet for the bus
- EtherCAT — best-in-class cycle times, deterministic, distributed clocks. Default with Beckhoff PLCs.
- Profinet IRT — comparable performance, default with Siemens.
- Profinet RT — fine for mid-rate (4-8 ms cycles), not for tight motion sync.
- Mixed bus — avoid. Pick one and stick with it.
The last 5 % of tuning
Servo tuning is real engineering. Auto-tune gets you to 80 %. The last 20 % — notch filters on load resonance, feed-forward gains on accel, integrator anti-windup — is what makes a cell run silent and accurate at speed.What's your motion bus? And is anyone in this community using OpenServo / open-source motion control at industrial-grade?
