Tutorial May 7, 2024 · 4 min read

Effortless Auto-tuning of Servo Motors — Delta B3 & A3

Tuning servos has always been a frustrating part of CNC builds — especially with older models like the JMC and Delta ASD-M series. But the newer Delta B3 and A3 servos have changed the game completely.

https://www.youtube.com/watch?v=aI-yvvGZ3u4

Why servo tuning used to be painful

If you’ve tuned hobby-grade servos before — JMC, Delta ASD-M, or similar — you know the drill. You’re reading parameter tables, guessing at starting values, running motion profiles, watching for overshoot or oscillation, tweaking a gain, running again. It’s the kind of tuning loop where you can easily burn a whole evening and still not be happy with the result.

It’s also a tax on anyone building their first CNC. Servos absolutely beat steppers on dynamic performance, but the tuning overhead has historically been a real argument for sticking with steppers on a first build.

What’s actually different with the Delta B3 / A3

During my MightyMill build, I encountered a Delta B3 servo for the Z axis. Initially apprehensive — based on past experiences with older Delta drives — I was pleasantly surprised to find a streamlined, one-button solution for auto-tuning. The drive runs its own motion profile, characterises the load, and comes back with a working set of gains.

Auto-tuning a Delta B3 / A3 servo

The simplest possible view of the auto-tune procedure on a Delta B3 or A3 servo drive.


  1. Mount and wire the motor normally
    Mechanically install the servo on the axis and wire the drive as you normally would. Auto-tune characterises the load, so the motor has to be connected to what it will actually be driving.

  2. Enable auto-tune on the drive
    Enter the drive’s auto-tune mode (one button on the B3 / A3 series). This puts the drive in a state where it can command its own motion profile for characterisation.

  3. Let the drive run its characterisation move
    The drive runs a short motion profile back and forth to measure inertia, friction, and response. Keep the axis clear and watch for anything binding.

  4. Accept the resulting gains
    The drive returns a set of tuning parameters that are ready to use. On the B3 / A3 this was good enough out of the box for my Z axis — no manual loop tuning needed.

FAQ


In my experience the newer Delta B3 and A3 series have genuine, useful auto-tuning — one-button, done. The older Delta ASD-M series does not work like this; you’re back to manual loop tuning.

For a typical DIY CNC axis, no — the auto-tune result was good enough out of the box on my MightyMill’s Z axis. If you’re chasing the absolute limits of dynamic performance or your mechanics are unusual, you can refine from there, but you start from a working baseline instead of nothing.

Don’t. Auto-tune characterises the load — inertia, friction, backlash — and those only exist when the motor is coupled to the axis it’s going to drive. Tuning on a free shaft gives you gains for a free shaft, not for the machine.

Stepper vs servo is a separate question — I wrote a full breakdown here. The short version: modern servos like the B3 / A3 remove the “tuning is too hard” argument against going servo on a hobby build.

Nothing was broken about them — they just require you to tune the position and velocity loops yourself. That’s a real time investment, and it’s an unforgiving step if you’re new to servo systems. Modern drives like the B3 / A3 hide all of that behind auto-tune.

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