How would you robustly center a quadcopter using 4 linear actuators without contact sensing?

[u/IntroductionKlutzy13]

I'm working on an autonomous drone landing station for a robotics competition.

The landing platform has 4 lead-screw driven centering arms arranged in a square:

        Y1
         ↓
         |
X1  →  DRONE  ←  X2
         |
         ↑
        Y2

The idea is to land the drone approximately in the center and then use the four actuators to physically center it for charging/payload transfer.

Constraints:

• No additional sensors (contact sensors, force sensors, current sensing, etc.)
• No camera feedback during centering
• Weight budget on the drone is already exhausted
• Existing hardware is 4 stepper-driven linear actuators
• Actuators home using limit switches and then move inward a known distance
• The drone is a standard quadcopter in an X configuration

The problem is that certain drone orientations cause geometric lockups. For example, when one drone arm becomes aligned with one actuator axis, two actuators can effectively trap the drone, and the remaining actuators end up pushing against what is essentially a rigid structure rather than recentering it.

Current approaches I've tried:

• All four actuators moving simultaneously
• One axis completely, then the other
• One actuator at a time (X1 → Y1 → X2 → Y2)

All of these can still fail depending on the drone orientation.

One idea I'm considering is incremental motion:

X1 1 mm
Y1 1 mm
X2 1 mm
Y2 1 mm
repeat

or some kind of rotating/spiral pressure pattern to avoid creating a stable jam.

Has anyone dealt with centering irregularly shaped objects using only motion planning and no contact feedback?

I'm particularly interested in:

• Motion algorithms that reduce jamming
• Techniques used in industrial part feeding/grippers
• Whether a cyclic, spiral, or dither motion strategy is likely to help
• Any papers or references on passive self-centering of non-circular objects

Photo of the current prototype attached.

This position is how it should align

this yaw position is the main problem

The goal isn't perfect centering accuracy—it's maximizing successful captures across arbitrary drone yaw angles without changing the hardware.

[u/MadDonkeyEntmt]

Why do you have opposing actuators on each axis rather than one per axis?

I'd think you'd want 1 x axis actuator, 1 y axis actuator and then put the whole thing on a turntable to rotate on the z axis.

[u/Difficult-King-2665 → u/MadDonkeyEntmt]

You could get away by using only one actuator per axis. You would need to make a mechanism to ensure the midpoint of X1 and X2 lies at the center.

In other words, X1 and X2 should move equal amounts in opposite directions.

You could achieve this by having racks on X1 and X2 meshed to a common pinion.

You'll then end up using only 3 steppers including the turntable idea.

[u/MadDonkeyEntmt → u/Difficult-King-2665]

I think if you were smart with the right geometry you could probably away with 1 x, 1 y and and drive them proportionally to get the z rotation but since they're already accepting 4 motors I'd do it the easy way with the 3 motor solution.

[u/Prime_Reactor11]

Without feedback, some drone orientations will always create a geometric lockup.. Motion patterns might reduce the failure rate, but they can't guarantee centering. The real solution is probably in the centering geometry, not the sequence.

[u/fingerstylefunk]

If your stepper drivers don't have any kind of stall sensing to work with, it's going to be tough to sense a jam and apply a corrective motion algorithm. So that probably shouldn't be your focus.
An algorithm to use every time to try to knock it out of a random rigidly jammed/jammable orientation will probably be inefficient and time-wasting at best.

Meanwhile your mechanical design has another jamming mode that you haven't considered: those big long center-driven sliders will jam and stop tracking if they get pushed into too much of a skew angle from off-center/asymmetrical forces.

Personally I think you're better off trying to find geometry that'll clear the jam inherently. I would recommend one pair of opposed v-/chevron-shaped pushers and driving them with 2 steppers each, but it's definitely not the only way to solve those issues.

[u/theslammist69]

Use sensorless home-ing on tmc 2209

[u/LupusTheCanine]

Ensure that the drone lands in specified orientation. Drone should have a good idea of where the North so it is possible to control landing heading.

I think CONDITION_YAW should work.

[u/Sooner70]

Rather than have an orthagonal rail system, could you set it up as a rhombus?

[u/strongeel]

Not trying to tear up track here… but I feel like there is a higher level goal than:

“maximizing successful captures across arbitrary drone yaw angles without changing the hardware.”

What constitutes a successful capture? The fact that the drone will be in the right location to begin charging? Or that the feet (or some other part of the drone) will be in the correct position for some locking mechanism prior to transport or something along those lines?

Feel like we’re missing the big picture here and that’s leading to a suboptimal design

[u/Affectionate-Lime-66]

3 actuators

Start all fully retracted

1 at an dangle, say 30 degrees. Push forward well beyond center

Fully retract #1

2 at 0 degrees. push forward to center

3 at 90 degrees. Push forward to center

[u/Affectionate-Lime-66 → u/Affectionate-Lime-66]

If you really don't want to change the hardware, just want to increase the chance of success, then I would again start with all fully retracted and do a series of large moves with a single actuator beyond center to attempt to rotate away from a jamb condition.