Moving Coil Acctuator

[Fishman-1]

Hi I have a number of high precision encoders with voice coil actuators attached. They came from the semiconductor industry for mapping out hard drives. Initially I aimed at just using the encoders but am now wondering if I can also make use of the actuators. What sort of drive would I need to control the position of the actuator using the encoder for feedback? There are two wires from the voice coil side and putting a voltage across these causes the actuator to move.

Would a normal low current brushed servo drive do?

Thanks

Barry

[vger]

A servo drive will be what you need. The coil is most likely a fairly low impedance. Any idea what the resolution is on the encoders? You will probably not get much driving force from them. Positioning accuracy will depend on the encoder. They should be very fast in response.

Steve

[Fishman-1]

Hi Steve, encoder grating period is 5 micron 2V pp differential. I have a number of interpolator options up to 200x pulse quad decode. In theory the resolution is quite good. However, the stroke is more limited (30 degrees). The positioners are a couple of MicroE and Canon items both giving sub arcsecond resolution with appropriate electronics. Getting any useful data from the manufaturers has been quite poor so I have had to result to experimentation and back tracing the encoder leads to get basic pinouts.

Should I be using an AC brushless with two phases connected or DC brushless servo drive? I have a Copley ACM-090-09 (90V, 10A max) which can be configured for wake and shake commutation and is a three phase drive?

Sorry I am fine with encoders and normal brushless DC drives but not so good with AC and more exotic motors although keen to experiment and learn. I guess a linear motor is the same as a moving coil system is the same as a Voice Coil motor?

Barry

[mike_Kilroy]

Wow, 27 million questions in 1 day! All good ones at that!

Let's take them one at a time.....

Yes, consider it same as linear moving coil motor but only built in an arc to make ot rotary. exact same principal.

consider it exactly same as an audio speaker moving coil - it is the same again! just apply dc one polarity and it makes a magnetic field that moves it one way against the built in magnet in the motor; reverse dc polarity and it moves the other way! same thing. In fact, u could prob hook it up to your stereo and 'see' ur music as motion (and sound)!

from post 1, yes, a normal dc brush motor drive is indeed what u need. BUT BIG CAVEAT/CAUTION: Inductance will be VERY low - 99% probability WAY too low for ANY dc brush (or brushless) drive you will find (one that works is our Kollmorgen KXA drive). But the good news is you can easily cheat and just add a say 1mh inductor in series with one motor lead and that will make the drive happy - buy it from digikey if u want.

Can u use an ac (brushless) drive? sure, if the mfgr states how to run dc brush type motors with it (u gotta fake out the commutation feedback and then use the riht 2 of 3 output leads). No wake and shake cuz u aint commutating. amc mostly shows how to run dc motors with ac or brushless drives. any will do but you dont want to take hours figuring it out yourself...

Another caution: without steel in the coil, it willhave a very low thermal time constant - ie., it will heat up very quickly. so if u stick too much current into it it will likely burn up in like 20 seconds (from ur description)....

u say it is 30 degrees or rotary motion; means it is simply designed as a dc motor MINUS the commutator and brushes! If you wire just one set of coils in a dc brush motor directly to a drive u get the same thing. angle is dependent on motor design (no of poles etc). interesting to know?

I question (but do not say u r wrong!!) the 2Vpp sine encoder output; 99.9% of sine encoders are 1Vpp; I never heard of a 2Vpp but that does not mean u r wrong! scoping it would show for sure. I mention this cuz any servo drive u find - not special made for this unit of course - that accepts sine encoder feedback will be 1Vpp. No drive today even takes the old Hiedenhain std of 11ua level, and only a special one we modify from Germany accepts the special hi res Indramat 200mVpp sine encoder output....

It is really cool to consider the 5um/sine x200 = .006um/pulse (after quadrature) output!!! Can u imagine this???? If u BLOW on it, it will change 2 um! that scale is amazing! We supply servos for coordinate measurement machines that work in the sub um arena. Just a few degrees change in a room can change the machine location by .002"!!!! But I digress (sorry, this is fun!)

Barry, I keep going back to ur email to see what u say next. Ur last comment is regarding dc brushless vs ac again. I think I commented on that up top but one more comment is in order. brushless dc and ac ARE THE SAME THING! both are 3 phase ac voltage & currents put into the 3 phase motor! some people call them brushless dc, others call them brushless ac. same animals! today some consider brushless dc as motors with magnets inside (synchronous motors) & motors with no magnets (squelral -sorry on spelling but Ii am an engineer - cage rotors) ac servos. in either case, control is very similar so u shud consider them the same for this meaning.

Enjoy!

[Fishman-1]

Hi Mike, thanks

Both the MicroE and the Canon positioners have encoders that are a little strange and customized to fit a very much niche high precision application which uses their own electronics. I only came accross them by chance as I was looking for a high resolution rotary encoder to track small changes in rotational velocity and position for a mount I am building. The project is a hobby in my "retirement" so I did not have commercial levels of funding to buy into the day to day systems out there.

Both are however very interesting bits of kit. The MicroE houses a very compact almost on chip encoder with a track covering roughly 30 degrees. The Canon makes use of a number of optical prisms to bend the laser through the mechanics of the system but has a 360 degree coverage. Both equate to around 88,000 transitions per revolution prior to any interpolation. The bearing housings on both are pretty robust. The raw transitions are quite stable under non ideal real world conditions (kitchen table) when viewed through a scope. I plan to keep interpolation as low as possible for my end use and can vary this in my interpolation electronics.

They are used to map out the tracks on disc drives and are the same as used in in commercial hard discs but bigger and more robust. Lots of chips out there but aimed at hard disc control functions and with control of both the spindle motor and actuator built in. Too much additional cluuter for my end use.

Barry