Responding to
>>36430 from the diy motor thread.
>Maybe we should consider a more unconventional design thats more similar to how muscles work
I just want to say upfront that I’m not trying to be critical of your idea, I really appreciate outside the box thinking, since it’s exactly what we need to push robowaifu development forward. On the topic of your design, If the magnet is fixed and the coil moves, the coil will experience a force, but it may not move efficiently because electromagnetic forces generally require a closed magnetic circuit for strong actuation. In your design, the permanent magnet at the bottom has a magnetic field extending outward. The coil generates its own magnetic field when current flows through it. However, the fields don’t have a clear return path, meaning most of the flux will escape into free space rather than interacting efficiently. The force might not be strong enough to move the coil significantly unless very high currents are used. The spring’s placement directly below the coil could also interfere with the electromagnetic forces. Solenoid-based actuators are notoriously difficult for precise movement due to their binary nature (typically fully on or off), high hysteresis, and nonlinear force response. But based on your drawing it's far more like a Linear Voice Coil Motor except your coil doesn't surround the return spring. Look into Linear Voice Coil Motor or Voice Coil Actuators. Typically these have no torque rating and instead have a linear force rating. VCA/LCVM generally produce a low linear force, but they excel in applications requiring high precision, fast response times. They are largely unsuitable for applications requiring high rotational torque. (I assume you're wanting to drive a linkage to rotate limbs via the linear force).
The rest of my post will be about magnetic linear actuators and more specifically linear synchronous motors (LSAs). These LSAs are largely how maglev trains and modern roller coasters work. These systems rely on electromagnetic forces to generate linear motion directly, eliminating the need for mechanical conversion from rotary motion. Industrial automation often uses them for contactless linear transport. These are really unlike any other commonly used actuator, such as a ball screw, timing belt, or rack and pinion, they provide high precision, high velocity, high force and long travel. They are pretty complicated and extraordinary expensive devices. These systems are extremely useful in automation, but largely no one ever thinks of them or even knows about them outside of maglev trains.
See
https://www.youtube.com/watch?v=ICN2iO3nbiQ for the pic in action and see
https://www.instructables.com/DIY-IRONLESS-LINEAR-SERVO-MOTOR/ for more information on the pic.
Looks amazing right? While LSAs excel in high-speed automation (maglev trains, CNC automation), they face significant challenges in humanoid robotics due to power inefficiency, control complexity, and limited precision for small-range movements. Below is my experience with them.
Using LSAs as linear actuators is a really intriguing idea, I played around with the idea many years ago as linear actuators in a CNC shop before there was really any large interest in humanoid robotics. I've been trying to build a good humanoid robot for many years (since the 90's) and the problem has always been good ways to translate motion with low power. At the shop I was tasked with building some automation and they had a few LSAs lying around unused and I thought they were pretty cool and thought they could be used as linear actuators, so I messed around with them on my lunches. I built a LSA bicep/forearm and wrote some pages of notes on my experiences with them. (I wish I had some pictures but this was before there were cameras on phones, lol.) Here's some of my observed limitations with using LSAs as linear actuators.
The main problem with them for humanoid robotics is multi-faceted. LSAs require significant magnetic field strength and large electromagnets or permanent magnet arrays to function efficiently, making them bulky compared to other options. The amount of current required for high-torque, low-speed operation (needed for humanoid joints) is substantial, leading to high power consumption. The coils in the actuator generate heat due to resistive losses, requiring cooling systems, which further increases size and complexity. LSAs are often water-cooled in automation systems, at least the ones I used were. Unlike geared motors, an LSA requires continuous power to hold a position. If power is lost, the joint loses all resistance, making it impractical for static postures without a mechanical brake. Furthermore, Since LSAs operate on an electromagnetic field, they constantly draw power to maintain a position, unlike other kinds of motors which can maintain positions with mechanical resistance due to gearing. LSAs are great at high-speed travel over long distances, like maglev systems. But humanoid robots require fine, precise movements in small ranges (gripping, balancing, small joint corrections). Magnetic force distribution in LSAs doesn't favor short-range precision, making them inefficient for fine motor control. LSAs require precise real-time control of magnetic fields to achieve stable, smooth motion. Unlike traditional servos (which only need PID control), LSAs need a dynamic magnetic field generation system, leading to increased CPU load and additional sensor requirements. Limited Back-Drivability: Unlike geared motors, LSAs resist passive motion, which can make them problematic for compliance-based movements in humanoid robots (soft grasping, adapting to external forces). The lack of small precise movements, limited back-drivability, and high power requirements is a real killer here. Also unlike rotational motor-based humanoid robots (where actuators are largely modular), LSAs require custom electromagnetic configurations per joint, making them hard to scale for general use.