This 3D-Printed Micro Manipulator Can Achieve Accurate Sub-Micron Movement in All Three Axes

Movement at really, really small scales is difficult to achieve, as the limitations in conventional machine design, physics, and manufacturing precision result in error that exceeds the desired resolution. Traditional three-axis kinematics, even when built well with high-end components, generally stop being practical around the 1-10 micron scale. So, how can one build a machine capable of sub-micron movement? Diffraction Limited pulled it off with this very clever 3D-printed manipulator.

To get a sense of the scale we’re talking about, a single red blood cell’s diameter is about 7.5 microns. Sub-micron movement is an incredibly difficult task that requires specialized equipment that would stretch the budget of even a well-funded lab. And yet, Diffraction Limited was able to achieve that at a cost that hobbyists can afford.

This is possible through clever engineering, the use of affordable 3D-printed parts, and the selection of off-the-shelf components. The manipulator is a parallel-arm design that can move the end effector in X, Y, and Z. It does that with enough precision that it could, quite literally, draw a detailed picture on one of the aforementioned 7.5-micron red blood cells.

There were two big breakthroughs behind that achievement: the elimination of all backlash and slop, and very high-resolution closed-loop feedback.

Diffraction Limited used standard, inexpensive stepper motors and carefully chosen H-bridge drivers that allow for thousands of microsteps. Each stepper pushes and pulls the end effector through rods with ball joints at both ends, so there isn’t any backlash. Friction on the balls is minimal and the end effector doesn’t weigh much, so there is very little force to cause deflection.

The closed-loop feedback is especially impressive. A 21-bit magnetic encoder on each axis monitors an array of magnets in alternating orientation and that ultimately enables a resolution of hundreds of thousands of points per revolution. That does require calibration and is best for relative positioning, rather than absolute positioning, but that is acceptable for this manipulator’s intended use cases.

A Raspberry Pi Pico 2 board, with an RP2030 microcontroller, generates the stepper motor control pulses with precise timing according to the feedback from the magnetic encoders. The firmware was programmed to distribute tasks across the RP2030’s cores to maintain timing. It accepts an abridged set of g-code commands and Diffraction Limited created a basic Python program, with GUI, to provide those.

This manipulator has many applications, such as microscopy and microfabrication. Diffraction Limited demonstrates the capabilities with a microscope and even with fiber optic positioning, both of which are very impressive.

And if you want to build your own Open Micro Manipulator, all of the files you need are available on GitHub.