Home / Applications / Optical focus and alignment with 48 V DC servo drives
Technical note
Optical focus and alignment with 48 V DC servo drives
A classic autofocus-on-contact case, and one where the drive's current-loop speed decides how much overshoot you see.
What the motion looks like
A Z stage that travels until the optic touches or the signal crosses a threshold, then holds position. The stage is usually small, fast and highly sensitive to vibration.Does it need force control?
Focus-by-contact needs the drive to detect the contact point and stop there with a defined, small force. The interesting number is not peak force but overshoot — how far past the contact point the axis travels before the loop catches it.What to look at in the specification
| Parameter | Value and why it matters |
|---|---|
| Current control frequency | 20 kHz, maximum 40 kHz — sets how quickly overshoot is caught |
| Current sampling | 12-bit standard, 16-bit on -P — sets resolution of the contact detection |
| Contact force | typically around 10 g with roughly ±1 g variation |
| ZR accuracy | rotational ±0.01°, linear ±2 μm repeatability |
A selection checklist you can send to your engineer
Send this list to whoever is doing the mechanical work. If the answers come back with specific numbers rather than adjectives, the drive selection is half done — and so is the quotation.- Current control frequency
20 kHz, maximum 40 kHz — sets how quickly overshoot is caught - Current sampling
12-bit standard, 16-bit on -P — sets resolution of the contact detection - Contact force
typically around 10 g with roughly ±1 g variation - ZR accuracy
rotational ±0.01°, linear ±2 μm repeatability
Common mistakes I see
- Optimising for peak force and ignoring overshoot. Two systems with the same peak force can behave completely differently if one overshoots two microns and the other overshoots twenty.
- Adding mass to the stage to stiffen it and then wondering why the force loop got slower. The current loop does not care how stiff the mechanics are; the loop speed does not change, but the resonance does.
Cycle time and settling
The cycle consideration in a focus stage is that focus is usually the last thing in a longer sequence, and any overshoot in the Z axis shows up as a focus error rather than as a visible fault. This makes the settling behaviour more important than the raw travel speed. If focus is performed on every unit, the approach has to be repeatable, not just fast — a stage that settles differently from unit to unit needs a different focus strategy upstream. A fast, consistent current loop gives you a consistent contact event, which is what makes the upstream logic simple.Where I would start
Starting point: UF -P on a ZR or compact linear stage.. That is a starting point, not a finished selection — the deciding inputs are the motor model, the bus interface, the payload or stroke, the axis count and the ambient temperature. The parameter reference lists what each family covers, and the documents library has the cable tables and application notes your engineer will want.Key takeaways
- Where I would start: UF -P on a ZR or compact linear stage.
- Compare the current-loop and sampling specifications, not the force headline — the force number alone does not decide these applications.
- If the axis never touches the workpiece, say so. Position-control criteria are the right ones to judge on.
- Send the checklist to your mechanical engineer before choosing a model — most wrong selections start as an incomplete brief.
Common questions
Is a 48 V DC servo drive the right choice for optical focus and alignment?
It depends on whether the axis makes contact and on the thermal environment. My starting recommendation for optical focus and alignment is UF -P on a ZR or compact linear stage. — but the motor model, payload, bus type and ambient decide it properly. Send me those four and I will give you a specific answer.
Do I need force control for this application?
Focus-by-contact needs the drive to detect the contact point and stop there with a defined, small force. The interesting number is not peak force but overshoot — how far past the contact point the axis travels before the loop catches it.
Can one drive cover several axes in this machine?
For synchronised axes on one bus, the G-series multi-axis units cover pairs and six-axis groups over EtherCAT. If the axes are independent, separate drives are usually simpler and cheaper. Tell me the axis count and which axes must move together.
Which communication interface should I specify?
EtherCAT if you have multiple axes that must move together, or if you need STO. Modbus-RTU or CANopen for simple point-to-point axes where the motion is self-contained. Pulse/direction when you are retrofitting into an existing controller. One warning worth repeating: STO exists on EtherCAT models only, so if your safety case needs it, that decision forces the bus choice.
How do I validate the choice before committing to a quantity?
Run the drive on your own part, not on a test piece. The ForceControl Application note in the documents library describes the setup, and the current-loop and sampling figures in the parameter reference tell you what to look for in the force signal at the moment of contact. If you would rather have someone look at your data with you, send me the traces.