I worked on a product using the Polaroid Piso distance sensors. 1980's. Problem was the ringing in the piso unit required a separate sender and a receiver for up close sensing, which is what I was trying to do. Figured out was not a big enough market to continue on the idea. As to fast, heavy duty, accurate movers, Voice coils work great. Use a linear transformer as the sensor. Was the way we build disk drives in the 1970's early 80's. The magnet on a CDC 200 megabyte drive probably weighs 25-30 pounds total.
There were some reasons that solenoid-type actuators weren't ideal. I think it was the response rate with the weight of the actuator required for the job, but it's been too long for me to trust my memory.
I see that Takisawa is still making their line of oval-piston machines (TPS Series). Maybe there's something on their website about the actuator:
A voice coil actuator is not a solenoid in the usual sense. Voice coil actuators can exhibit fairly linear force vs current over a fair range of motion, while solenoid force is a highly nonlinear function of current and distance from closed. They tend to "snap shut" when actuated, are best-suited for two-position situations like valve actuators. A voice coil actuator requires a magnet while a solenoid does not.
Right, I forgot about how they work. But rest assured that the piston manufacturers have tried about everything, and that combination of programmability, adequate force, and speeds high enough for productive turning of aluminum pistons has been a son of a gun to accomplish. All sorts of electromagnetic devices have been tried.
Right. Don reminded me of that. But as I said to him, the biggest car manufacturers and lathe builders have all tried everything they can think of. We're talking about a manufacturing issue that's worth many tens of millions, if not hundreds of millions, of dollars. There are an awful lot of pistons made and, until the EPA relented, even Homelite was exploring our machines for making pistons for string-trimmer engines. All of the car manufacturers, all over the world, are caught up in it. They want programmable machines.
If Takisawa is still selling their TPS machines, it seems likely that no one has succeeded in building anything better. And they all have some engineer or another who knows how a voice coil works.
See US Patents 5,085,109 and 5,313,694. The first uses a hydraulic servo system, the second uses a servo motor of some kind. Lots of other details are given. Referenced and referencing patents may also be instructive.
Voice coils tend to be limp, long travel things. They're great for machines that need to completely isolate one part from another, but that's the opposite of what you want in a lathe.
You may be able to do it with a voice coil driving a lever, with a flexure at the other end and the tool holder a lot closer to the flexure than the coil. It'd be one involved set of work to get it working, and to make it fast enough to keep up with the spindle.
Right. Voice coil actuators are quick but they're not stiff or high-force devices. That said, the shakers used to vibration-test aerospace hardware to 20 G's and beyond were voice coil actuated. Several feet in diameter, water cooled. Helluva ride!
Voice coils are can also be short travel and very precise. I designed disk drives and head actuators buy and large are voice coils. The positioning track to track can be short or far and keeping the head on track is very precise.
They are precise only because they get feedback from the heads and "servo tracks" written on one surface (at least the ones in sealed hard drives, though I think that the old 5MB removable platter ones used an an optical sensor to determine position, because in those days every surface was precious.
But a voice coil _by itself_ is infinitely floppy. Cut the power -- or just supply it with a constant current -- and it displays zero spring rate, zero damping rate, just a constant force.
It's just what you want for a disk drive -- speedy, no hysteresis, rigidity doesn't matter much because the whole assembly is mechanically quiet, etc.
(take apart a hard drive today and you'll find a voice coil, with some Really Strong rare earth magnets providing a field)
But this is quite the opposite of what you need on a lathe. To get the rigidity from the voice coil itself you need an exceedingly high bandwidth on your control loop, and to develop lots of force you need a physically large coil with lots of inductance -- which is going to cause all sorts of electrical difficulties.
Not having tried this I couldn't say for sure, but _having_ worked on voice coil actuated control loops that shove a 30 pound gimbal around, I can say that you're probably going to be at least two orders of magnitude short of nirvana with just a voice coil, sensor, and 'the usual' electronics.
Thanks for the links to the patents. Those systems are more like what I was imagining. I was surprised that the magnetostrictive actuators Ed mentioned can give a great enough distance of travel. To what degree are pistons typically elliptical, relative to their diameter? Is it visible with the naked eye?
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.